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R&S®FPS-K91
WLAN Measurements
User Manual
(;ÚãÁ2)
1176.8551.02 ─ 08
User Manual
Test & Measurement
Page 2
This manual applies to the following R&S®FPS models with firmware version 1.50 and higher:
●
R&S®FPS4 (1319.2008K04)
●
R&S®FPS7 (1319.2008K07)
●
R&S®FPS13 (1319.2008K13)
●
R&S®FPS30 (1319.2008K30)
●
R&S®FPS40 (1319.2008K40)
The following firmware options are described:
●
R&S FPS-K91 WLAN 802.11a/b/g (1321.4191.02)
●
R&S FPS-K91ac WLAN 802.11ac (1321.4210.02)
●
R&S FPS-K91n WLAN 802.11n (1321.4204.02)
●
R&S FPS-K91p WLAN 802.11p (1321.4391.02)
© 2017 Rohde & Schwarz GmbH & Co. KG
Mühldorfstr. 15, 81671 München, Germany
Phone: +49 89 41 29 - 0
Fax: +49 89 41 29 12 164
Email: info@rohde-schwarz.com
Internet: www.rohde-schwarz.com
Subject to change – Data without tolerance limits is not binding.
R&S® is a registered trademark of Rohde & Schwarz GmbH & Co. KG.
Trade names are trademarks of their owners.
The following abbreviations are used throughout this manual: R&S®FPS is abbreviated as R&S FPS.
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R&S®FPS-K91
1.1 About this Manual......................................................................................................... 5
1.2 Typographical Conventions.........................................................................................6
2.1 Starting the WLAN Application....................................................................................8
2.2 Understanding the Display Information......................................................................8
3.1 WLAN I/Q Measurement (Modulation Accuracy, Flatness and Tolerance)............11
3.2 Frequency Sweep Measurements............................................................................. 47
Contents
Contents
1 Preface.................................................................................................... 5
2 Welcome to the WLAN Application...................................................... 7
3 Measurements and Result Displays...................................................11
4 Measurement Basics........................................................................... 54
4.1 Signal Processing for Multicarrier Measurements (IEEE 802.11a, g (OFDM), j, p)
...................................................................................................................................... 54
4.2 Signal Processing for Single-Carrier Measurements (IEEE 802.11b, g (DSSS))... 61
4.3 Signal Processing for MIMO Measurements (IEEE 802.11ac, n)............................ 67
4.4 Channels and Carriers................................................................................................76
4.5 Recognized vs. Analyzed PPDUs.............................................................................. 76
4.6 Demodulation Parameters - Logical Filters.............................................................. 77
4.7 Basics on Input from I/Q Data Files...........................................................................78
4.8 Triggered Measurements........................................................................................... 79
4.9 WLAN I/Q Measurements in MSRA Operating Mode............................................... 83
5 Configuration........................................................................................85
5.1 Multiple Measurement Channels and Sequencer Function.................................... 85
5.2 Display Configuration.................................................................................................87
5.3 WLAN I/Q Measurement Configuration.....................................................................87
5.4 Frequency Sweep Measurements........................................................................... 149
6 Analysis.............................................................................................. 154
7 I/Q Data Import and Export................................................................155
7.1 Import/Export Functions.......................................................................................... 155
7.2 How to Export and Import I/Q Data..........................................................................157
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8.1 How to Determine Modulation Accuracy, Flatness and Tolerance Parameters for
8.2 How to Determine the OBW, SEM, ACLR or CCDF for WLAN Signals.................162
9.1 Measurement Example: Setting up a MIMO measurement................................... 163
10 Optimizing and Troubleshooting the Measurement....................... 170
10.1 Optimizing the Measurement Results..................................................................... 170
10.2 Error Messages and Warnings................................................................................ 171
11 Remote Commands for WLAN 802.11 Measurements....................173
11.1 Common Suffixes......................................................................................................173
11.2 Introduction............................................................................................................... 174
Contents
8 How to Perform Measurements in the WLAN Application............. 160
WLAN Signals............................................................................................................160
9 Basic Measurement Examples..........................................................163
11.3 Activating WLAN 802.11 Measurements.................................................................179
11.4 Selecting a Measurement......................................................................................... 183
11.5 Configuring the WLAN IQ Measurement (Modulation Accuracy, Flatness and Tol-
erance)....................................................................................................................... 190
11.6 Configuring Frequency Sweep Measurements on WLAN 802.11 Signals........... 251
11.7 Configuring the Result Display................................................................................255
11.8 Starting a Measurement........................................................................................... 275
11.9 Retrieving Results.....................................................................................................280
11.10 Analysis..................................................................................................................... 324
11.11 Status Registers........................................................................................................327
11.12 Deprecated Commands............................................................................................ 331
11.13 Programming Examples (R&S FPS WLAN application)........................................ 333
Annex.................................................................................................. 339
A Annex: Reference...............................................................................339
A.1 Sample Rate and Maximum Usable I/Q Bandwidth for RF Input.......................... 339
A.2 I/Q Data File Format (iq-tar)......................................................................................343
List of Remote Commands (WLAN)................................................. 349
Index....................................................................................................358
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1 Preface
Preface
About this Manual
1.1 About this Manual
This WLAN User Manual provides all the information specific to the application. All
general instrument functions and settings common to all applications and operating
modes are described in the main R&S FPS User Manual.
The main focus in this manual is on the measurement results and the tasks required to
obtain them. The following topics are included:
●
Chapter 2, "Welcome to the WLAN Application", on page 7
Introduction to and getting familiar with the application
●
Chapter 3, "Measurements and Result Displays", on page 11
Details on supported measurements and their result types
●
Chapter 4, "Measurement Basics", on page 54
Background information on basic terms and principles in the context of the measurement
●
Chapter 5, "Configuration", on page 85 and Chapter 6, "Analysis", on page 154
A concise description of all functions and settings available to configure measurements and analyze results with their corresponding remote control command
●
Chapter 7.1, "Import/Export Functions", on page 155
Description of general functions to import and export raw I/Q (measurement) data
●
Chapter 8, "How to Perform Measurements in the WLAN Application",
on page 160
The basic procedure to perform each measurement and step-by-step instructions
for more complex tasks or alternative methods
●
Chapter 10, "Optimizing and Troubleshooting the Measurement", on page 170
Hints and tips on how to handle errors and optimize the test setup
●
Chapter 11, "Remote Commands for WLAN 802.11 Measurements", on page 173
Remote commands required to configure and perform WLAN measurements in a
remote environment, sorted by tasks
(Commands required to set up the environment or to perform common tasks on the
instrument are provided in the main R&S FPS User Manual)
Programming examples demonstrate the use of many commands and can usually
be executed directly for test purposes
●
Chapter A, "Annex: Reference", on page 339
Reference material
●
List of remote commands
Alpahabetical list of all remote commands described in the manual
●
Index
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Preface
Typographical Conventions
1.2 Typographical Conventions
The following text markers are used throughout this documentation:
Convention Description
"Graphical user interface elements"
KEYS Key names are written in capital letters.
File names, commands,
program code
Input Input to be entered by the user is displayed in italics.
Links Links that you can click are displayed in blue font.
"References" References to other parts of the documentation are enclosed by quota-
All names of graphical user interface elements on the screen, such as
dialog boxes, menus, options, buttons, and softkeys are enclosed by
quotation marks.
File names, commands, coding samples and screen output are distinguished by their font.
tion marks.
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2 Welcome to the WLAN Application
Welcome to the WLAN Application
The R&S FPS WLAN application extends the functionality of the R&S FPS to enable
accurate and reproducible Tx measurements of a WLAN device under test (DUT) in
accordance with the standards specified for the device. The following standards are
currently supported (if the corresponding firmware option is installed):
●
IEEE standards 802.11a
●
IEEE standards 802.11ac (SISO + MIMO)
●
IEEE standards 802.11b
●
IEEE standards 802.11g (OFDM)
●
IEEE standards 802.11g (DSSS)
●
IEEE standards 802.11j
●
IEEE standards 802.11n (SISO + MIMO)
●
IEEE standards 802.11p
The R&S FPS WLAN application features:
Modulation measurements
●
Constellation diagram for demodulated signal
●
Constellation diagram for individual carriers
●
I/Q offset and I/Q imbalance
●
Modulation error (EVM) for individual carriers or symbols
●
Amplitude response and group-delay distortion (spectrum flatness)
●
Carrier and symbol frequency errors
Further measurements and results
●
Amplitude statistics (CCDF) and crest factor
●
FFT, also over a selected part of the signal, e.g. preamble
●
Payload bit information
●
Freq/Phase Err vs. Preamble
This user manual contains a description of the functionality that is specific to the application, including remote control operation.
Functions that are not discussed in this manual are the same as in the Spectrum application and are described in the R&S FPS User Manual. The latest version is available
for download at the product homepage
http://www2.rohde-schwarz.com/product/FPS.html.
Installation
You can find detailed installation instructions in the R&S FPS Getting Started manual
or in the Release Notes.
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Welcome to the WLAN Application
Understanding the Display Information
2.1 Starting the WLAN Application
The WLAN measurements require a special application on the R&S FPS.
Manual operation via an external monitor and mouse
Although the R&S FPS does not have a built-in display, it is possible to operate it interactively in manual mode using a graphical user interface with an external monitor and
a mouse connected.
It is recommended that you use the manual mode initially to get familiar with the instrument and its functions before using it in pure remote mode. Thus, this document
describes in detail how to operate the instrument manually using an external monitor
and mouse. The remote commands are described in the second part of the document.
For details on manual operation see the R&S FPS Getting Started manual.
To activate the WLAN application
1. Select the MODE key.
A dialog box opens that contains all operating modes and applications currently
available on your R&S FPS.
2. Select the "WLAN" item.
The R&S FPS opens a new measurement channel for the WLAN application.
The measurement is started immediately with the default settings. It can be configured
in the WLAN "Overview" dialog box, which is displayed when you select the "Overview"
softkey from any menu (see Chapter 5.3.1, "Configuration Overview", on page 88).
2.2 Understanding the Display Information
The following figure shows a measurement diagram during analyzer operation. All
information areas are labeled. They are explained in more detail in the following sections.
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Welcome to the WLAN Application
Understanding the Display Information
1
2
3
4
5
1 = Channel bar for firmware and measurement settings
2 = Window title bar with diagram-specific (trace) information
3 = Diagram area with marker information
4 = Diagram footer with diagram-specific information, depending on result display
5 = Instrument status bar with error messages, progress bar and date/time display
MSRA operating mode
In MSRA operating mode, additional tabs and elements are available. A colored background of the screen behind the measurement channel tabs indicates that you are in
MSRA operating mode.
For details on the MSRA operating mode see the R&S FPS MSRA User Manual.
Channel bar information
In the WLAN application, the R&S FPS shows the following settings:
Table 2-1: Information displayed in the channel bar in the WLAN application
Label Description
"Sample Rate Fs" Input sample rate
"PPDU / MCS Index / GI"
"PPDU / MCS Index / GI+HELTF"
IEEE 802.11a, ac, g (OFDM), j, n, p, ax:
The PPDU type, MCS index and guard interval (GI) used for the analysis
of the signal; Depending on the demodulation settings, these values are
either detected automatically from the signal or the user settings are
applied.
WLAN 802.11ax only: PPDU type, MCS index, guard interval (GI), and
high-efficiency long training field (HE-LTF) used for the analysis of the signal
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Welcome to the WLAN Application
Understanding the Display Information
Label Description
"PPDU / Data Rate" WLAN 802.11b:
The PPDU type and data rate used for the analysis of the signal; Depending on the demodulation settings, these values are either detected automatically from the signal or the user settings are applied.
"Standard" Selected WLAN measurement standard
"Meas Setup" Number of Transmitter (Tx) and Receiver (Rx) channels used in the mea-
surement (for MIMO)
"Capt time / Samples" Duration of signal capture and number of samples captured
"Data Symbols" The minimum and maximum number of data symbols that a PPDU may
have if it is to be considered in results analysis.
"PPDUs" [x of y (z)] For statistical evaluation over PPDUs (see "PPDU Statistic Count / No of
PPDUs to Analyze" on page 137):
<x> PPDUs of totally required <y> PPDUs have been analyzed so far.
<z> PPDUs were analyzed in the most recent sweep.
In addition, the channel bar also displays information on instrument settings that affect
the measurement results even though this is not immediately apparent from the display
of the measured values (e.g. transducer or trigger settings). This information is displayed only when applicable for the current measurement. For details see the
R&S FPS Getting Started manual.
Window title bar information
For each diagram, the header provides the following information:
Figure 2-1: Window title bar information in the WLAN application
1 = Window number
2 = Window type
3 = Trace color
4 = Trace number
6 = Trace mode
Diagram footer information
The diagram footer (beneath the diagram) contains the start and stop values for the
displayed x-axis range.
Status bar information
Global instrument settings, the instrument status and any irregularities are indicated in
the status bar beneath the diagram. Furthermore, the progress of the current operation
is displayed in the status bar. Click on a displayed warning or error message to obtain
more details (see also .
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3 Measurements and Result Displays
Measurements and Result Displays
WLAN I/Q Measurement (Modulation Accuracy, Flatness and Tolerance)
The R&S FPS WLAN application provides several different measurements in order to
determine the parameters described by the WLAN 802.11 specifications.
For details on selecting measurements, see "Selecting the measurement type"
on page 85.
● WLAN I/Q Measurement (Modulation Accuracy, Flatness and Tolerance).............11
● Frequency Sweep Measurements.......................................................................... 47
3.1 WLAN I/Q Measurement (Modulation Accuracy, Flatness and Tolerance)
The default WLAN I/Q measurement captures the I/Q data from the WLAN signal using
a (nearly rectangular) filter with a relatively large bandwidth. The I/Q data captured with
this filter includes magnitude and phase information. That allows the R&S FPS WLAN
application to demodulate broadband signals and determine various characteristic signal parameters in just one measurement. Modulation accuracy, spectrum flatness, center frequency tolerance and symbol clock tolerance are only a few of the characteristic
parameters.
Other parameters specified in the WLAN 802.11 standard require a better signal-tonoise level or a smaller bandwidth filter than the I/Q measurement provides and must
be determined in separate measurements (see Chapter 3.2, "Frequency Sweep Mea-
surements", on page 47).
● Modulation Accuracy, Flatness and Tolerance Parameters....................................11
● Evaluation Methods for WLAN IQ Measurements.................................................. 20
3.1.1 Modulation Accuracy, Flatness and Tolerance Parameters
The default WLAN I/Q measurement (Modulation Accuracy, Flatness,...) captures the
I/Q data from the WLAN signal and determines all the following I/Q parameters in a
single sweep.
Table 3-1: WLAN I/Q parameters for IEEE 802.11a, ac, g (OFDM), j, n, p
Parameter Description
General measurement parameters
Sample Rate Fs Input sample rate
PPDU Type of analyzed PPDUs
MCS Index Modulation and Coding Scheme (MCS) index of the analyzed PPDUs
*) the limits can be changed via remote control (not manually, see Chapter 11.5.9, "Limits", on page 244);
in this case, the currently defined limits are displayed here
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Measurements and Result Displays
WLAN I/Q Measurement (Modulation Accuracy, Flatness and Tolerance)
Parameter Description
Data Rate Data rate used for analysis of the signal
(IEEE 802.11a only)
GI Guard interval length for current measurement
Standard Selected WLAN measurement standard
Meas Setup Number of Transmitter (Tx) and Receiver (Rx) channels used in the measure-
ment
Capture time Duration of signal capture
Samples Number of samples captured
Data Symbols The minimum and maximum number of data symbols that a PPDU can have if
it is to be considered in results analysis
PPDU parameters
Analyzed PPDUs For statistical evaluation of PPDUs (see "PPDU Statistic Count / No of PPDUs
to Analyze" on page 137): <x> PPDUs of the required <y> PPDUs have been
analyzed so far. <z> indicates the number of analyzed PPDUs in the most
recent sweep.
Number of recognized
PPDUs (global)
Number of analyzed
PPDUs (global)
Number of analyzed
PPDUs in physical channel
TX and Rx carrier parameters
I/Q offset [dB] Transmitter center frequency leakage relative to the total Tx channel power
Gain imbalance [%/dB] Amplification of the quadrature phase component of the signal relative to the
Quadrature offset [°] Deviation of the quadrature phase angle from the ideal 90° (see Chap-
I/Q skew [s] Delay of the transmission of the data on the I path compared to the Q path
PPDU power [dBm] Mean PPDU power
Crest factor [dB] The ratio of the peak power to the mean power of the signal (also called Peak
MIMO Cross Power [dB]
Number of PPDUs recognized in capture buffer
Number of analyzed PPDUs in capture buffer
Number of PPDUs analyzed in entire signal (if available)
(see Chapter 3.1.1.1, "I/Q Offset", on page 15)
amplification of the in-phase component (see Chapter 3.1.1.2, "Gain Imbal-
ance", on page 15)
ter 3.1.1.3, "Quadrature Offset", on page 16).
(see Chapter 3.1.1.4, "I/Q Skew", on page 17)
to Average Power Ratio, PAPR).
*) the limits can be changed via remote control (not manually, see Chapter 11.5.9, "Limits", on page 244);
in this case, the currently defined limits are displayed here
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Measurements and Result Displays
WLAN I/Q Measurement (Modulation Accuracy, Flatness and Tolerance)
Parameter Description
Center frequency error
[Hz]
Symbol clock error [ppm] Clock error between the signal and the sample clock of the R&S FPS in parts
CPE Common phase error
Stream parameters
BER Pilot [%] Bit error rate (BER) of the pilot carriers
EVM all carriers [%/dB] EVM (Error Vector Magnitude) of the payload symbols over all carriers; the
EVM data carriers [%/dB] EVM (Error Vector Magnitude) of the payload symbols over all data carriers;
EVM pilot carriers [%/dB] EVM (Error Vector Magnitude) of the payload symbols over all pilot carriers;
Frequency error between the signal and the current center frequency of the
R&S FPS; the corresponding limits specified in the standard are also indicated*)
The absolute frequency error includes the frequency error of the R&S FPS and
that of the DUT. If possible, synchronize the transmitter R&S FPS and the DUT
using an external reference.
See R&S FPS user manual > Instrument setup > External reference
per million (ppm), i.e. the symbol timing error; the corresponding limits specified in the standard are also indicated *)
If possible, synchronize the transmitter R&S FPS and the DUT using an external reference.
See R&S FPS user manual > Instrument setup > External reference
corresponding limits specified in the standard are also indicated*)
the corresponding limits specified in the standard are also indicated*)
the corresponding limits specified in the standard are also indicated*)
*) the limits can be changed via remote control (not manually, see Chapter 11.5.9, "Limits", on page 244);
in this case, the currently defined limits are displayed here
Table 3-2: WLAN I/Q parameters for IEEE 802.11b or g (DSSS)
Parameter Description
Sample Rate Fs Input sample rate
PPDU Type of the analyzed PPDU
Data Rate Data rate used for analysis of the signal
SGL Indicates single measurement mode (as opposed to continuous)
Standard Selected WLAN measurement standard
Meas Setup Number of Transmitter (Tx) and Receiver (Rx) channels used in the measure-
ment
Capture time Duration of signal capture
No. of Samples Number of samples captured (= sample rate * capture time)
No. of Data Symbols The minimum and maximum number of data symbols that a PPDU can have if
it is to be considered in results analysis
PPDU parameters
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Measurements and Result Displays
WLAN I/Q Measurement (Modulation Accuracy, Flatness and Tolerance)
Parameter Description
Analyzed PPDUs For statistical evaluation of PPDUs (see "PPDU Statistic Count / No of PPDUs
to Analyze" on page 137): <x> PPDUs of the required <y> PPDUs have been
analyzed so far. <z> indicates the number of analyzed PPDUs in the most
recent sweep.
Number of recognized
PPDUs (global)
Number of analyzed
PPDUs (global)
Number of analyzed
PPDUs in physical channel
Peak vector error Peak vector error (EVM) over the complete PPDU including the preamble in %
PPDU EVM EVM (Error Vector Magnitude) over the complete PPDU including the pream-
I/Q offset [dB] Transmitter center frequency leakage relative to the total Tx channel power
Gain imbalance [%/dB] Amplification of the quadrature phase component of the signal relative to the
Quadrature error [°] Measure for the crosstalk of the Q-branch into the I-branch (see "Gain imbal-
Center frequency error
[Hz]
Number of PPDUs recognized in capture buffer
Number of analyzed PPDUs in capture buffer
Number of PPDUs analyzed in entire signal (if available)
and in dB; calculated according to the IEEE 802.11b or g (DSSS) definition of
the normalized error vector magnitude (see "Peak Vector Error (IEEE
Method)" on page 19);
The corresponding limits specified in the standard are also indicated *)
ble in % and dB
(see Chapter 3.1.1.1, "I/Q Offset", on page 15)
amplification of the in-phase component (see Chapter 3.1.1.2, "Gain Imbal-
ance", on page 15)
ance, I/Q offset, quadrature error" on page 65).
Frequency error between the signal and the current center frequency of the
R&S FPS; the corresponding limits specified in the standard are also indicated*)
The absolute frequency error includes the frequency error of the R&S FPS
and that of the DUT. If possible, synchronize the transmitter R&S FPS and the
DUT using an external reference.
See R&S FPS user manual > Instrument setup > External reference
Chip clock error [ppm] Clock error between the signal and the chip clock of the R&S FPS in parts per
million (ppm), i.e. the chip timing error; the corresponding limits specified in
the standard are also indicated *)
If possible, synchronize the transmitter R&S FPS and the DUT using an external reference.
See R&S FPS user manual > Instrument setup > External reference
Rise time Time the signal needs to increase its power level from 10% to 90% of the
maximum or the average power (depending on the reference power setting)
The corresponding limits specified in the standard are also indicated *)
Fall time Time the signal needs to decrease its power level from 90% to 10% of the
maximum or the average power (depending on the reference power setting)
The corresponding limits specified in the standard are also indicated *)
Mean power [dBm] Mean PPDU power
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Measurements and Result Displays
WLAN I/Q Measurement (Modulation Accuracy, Flatness and Tolerance)
Parameter Description
Peak power [dBm] Peak PPDU power
Crest factor [dB] The ratio of the peak power to the mean power of the PPDU (also called Peak
to Average Power Ratio, PAPR).
The R&S FPS WLAN application also performs statistical evaluation over several
PPDUs and displays one or more of the following results:
Table 3-3: Calculated summary results
Result type Description
Min Minimum measured value
Mean/ Limit Mean measured value / limit defined in standard
Max/Limit Maximum measured value / limit defined in standard
3.1.1.1 I/Q Offset
An I/Q offset indicates a carrier offset with fixed amplitude. This results in a constant
shift of the I/Q axes. The offset is normalized by the mean symbol power and displayed
in dB.
Figure 3-1: I/Q offset in a vector diagram
3.1.1.2 Gain Imbalance
An ideal I/Q modulator amplifies the I and Q signal path by exactly the same degree.
The imbalance corresponds to the difference in amplification of the I and Q channel
and therefore to the difference in amplitude of the signal components. In the vector diagram, the length of the I vector changes relative to the length of the Q vector.
The result is displayed in dB and %, where 1 dB offset corresponds to roughly 12 %
difference between the I and Q gain, according to the following equation:
Imbalance [dB] = 20log (| GainQ |/| GainI |)
Positive values mean that the Q vector is amplified more than the I vector by the corresponding percentage. For example, using the figures mentioned above:
0.98 ≈ 20*log10(1.12/1)
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WLAN I/Q Measurement (Modulation Accuracy, Flatness and Tolerance)
Figure 3-2: Positive gain imbalance
Negative values mean that the I vector is amplified more than the Q vector by the corresponding percentage. For example, using the figures mentioned above:
-0.98 ≈ 20*log10(1/1.12)
Figure 3-3: Negative gain imbalance
3.1.1.3 Quadrature Offset
An ideal I/Q modulator sets the phase angle between the I and Q path mixer to exactly
90 degrees. With a quadrature offset, the phase angle deviates from the ideal 90
degrees, the amplitudes of both components are of the same size. In the vector diagram, the quadrature offset causes the coordinate system to shift.
A positive quadrature offset means a phase angle greater than 90 degrees:
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Measurements and Result Displays
WLAN I/Q Measurement (Modulation Accuracy, Flatness and Tolerance)
Figure 3-4: Positive quadrature offset
A negative quadrature offset means a phase angle less than 90 degrees:
Figure 3-5: Negative quadrature offset
3.1.1.4 I/Q Skew
If transmission of the data on the I path is delayed compared to the Q path, or vice
versa, the I/Q data becomes skewed.
The I/Q skew results can be compensated for together with Gain Imbalance and Quad-
rature Offset (see "I/Q Mismatch Compensation" on page 118).
3.1.1.5 I/Q Mismatch
I/Q mismatch is a comprehensive term for Gain Imbalance, Quadrature Offset, and I/Q
Skew.
Compensation for I/Q mismatch is useful, for example, if the device under test is
known to be affected by these impairments but the EVM without these effects is of
interest. Note, however, that measurements strictly according to IEEE 802.11-2012,
IEEE 802.11ac-2013 WLAN standard must not use compensation.
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Measurements and Result Displays
WLAN I/Q Measurement (Modulation Accuracy, Flatness and Tolerance)
3.1.1.6 RF Carrier Suppression (IEEE 802.11b, g (DSSS))
Standard definition
The RF carrier suppression, measured at the channel center frequency, shall be at
least 15 dB below the peak SIN(x)/x power spectrum. The RF carrier suppression shall
be measured while transmitting a repetitive 01 data sequence with the scrambler disabled using DQPSK modulation. A 100 kHz resolution bandwidth shall be used to perform this measurement.
Comparison to IQ offset measurement in the R&S FPS WLAN application
The IQ offset measurement in the R&S FPS WLAN application returns the current carrier feedthrough normalized to the mean power at the symbol timings. This measurement does not require a special test signal and is independent of the transmit filter
shape.
The RF carrier suppression measured according to the standard is inversely proportional to the IQ offset measured in the R&S FPS WLAN application. The difference (in
dB) between the two values depends on the transmit filter shape. Determine it with a
reference measurement.
The following table lists the difference exemplarily for three transmit filter shapes
(±0.5 dB):
Transmit filter – IQ-Offset [dB] – RF-Carrier-Suppression [dB]
Rectangular 11 dB
Root raised cosine, "α" = 0.3 10 dB
Gaussian, "α" = 0.3 9 dB
3.1.1.7 EVM Measurement
The R&S FPS WLAN application provides two different types of EVM calculation.
PPDU EVM (Direct Method)
The PPDU EVM (direct) method evaluates the root mean square EVM over one PPDU.
That is the square root of the averaged error power normalized by the averaged reference power:
Before calculation of the EVM, tracking errors in the measured signal are compensated
for if specified by the user. In the ideal reference signal, the tracking errors are always
compensated for. Tracking errors include phase (center frequency error + common
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phase error), timing (sampling frequency error) and gain errors. Quadrature offset and
gain imbalance errors, however, are not corrected.
The PPDU EVM is not part of the IEEE standard and no limit check is specified. Nevertheless, this commonly used EVM calculation can provide some insight in modulation
quality and enables comparisons to other modulation standards.
Figure 3-6: I/Q diagram for EVM calculation
Peak Vector Error (IEEE Method)
The peak vector error (Peak EVM) is defined in section 18.4.7.8 "Transmit modulation
accuracy" of the IEEE 802.11b standard. The phase, timing and gain tracking errors of
the measurement signal (center frequency error, common phase error, sampling frequency error) are compensated for before EVM calculation.
The standard does not specify a normalization factor for the error vector magnitude. To
get an EVM value that is independent of the level, the R&S FPS WLAN application normalizes the EVM values. Thus, an EVM of 100% indicates that the error power on the
I- or Q-channels equals the mean power on the I- or Q-channels, respectively.
The peak vector error is the maximum EVM over all payload symbols and all active
carriers for one PPDU. If more than one PPDU is analyzed the Min / Mean / Max columns show the minimum, mean or maximum Peak EVM of all analyzed PPDUs. This
can be the case, for example, if several analyzed PPDUs are in the capture buffer or
due to the PPDU Statistic Count / No of PPDUs to Analyze setting.
The IEEE 802.11b or g (DSSS) standards allow a peak vector error of less than 35%.
In contrary to the specification, the R&S FPS WLAN application does not limit the measurement to 1000 chips length, but searches the maximum over the whole PPDU.
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3.1.2 Evaluation Methods for WLAN IQ Measurements
The captured I/Q data from the WLAN signal can be evaluated using various different
methods without having to start a new measurement or sweep. Which results are displayed depends on the selected evaluation.
Result display windows
All evaluations available for the selected WLAN measurement are displayed in SmartGrid mode.
To activate SmartGrid mode, do one of the following:
●
Select the "SmartGrid" icon from the toolbar.
●
Select the "Display Config" button in the configuration "Overview" (see Chapter 5.2,
"Display Configuration", on page 87).
●
Press the MEAS CONFIG hardkey and then select the "Display Config" softkey.
To close the SmartGrid mode and restore the previous softkey menu select the
"Close" icon in the right-hand corner of the toolbar, or press any key.
MIMO measurements
When you capture more than one data stream (MIMO measurement setup, see Chap-
ter 4.3, "Signal Processing for MIMO Measurements (IEEE 802.11ac, n)",
on page 67), each result display contains several tabs. The results for each data
stream are displayed in a separate tab. In addition, an overview tab is provided in
which all data streams are displayed at once, in individual subwindows.
The selected evaluation method not only affects the result display in a window, but also
the results of the trace data query in remote control (see TRACe[:DATA]
on page 308).
The WLAN measurements provide the following evaluation methods:
AM/AM.......................................................................................................................... 21
AM/PM.......................................................................................................................... 22
AM/EVM........................................................................................................................22
Bitstream.......................................................................................................................23
Constellation................................................................................................................. 25
Constellation vs Carrier.................................................................................................27
EVM vs Carrier..............................................................................................................28
EVM vs Chip................................................................................................................. 29
EVM vs Symbol.............................................................................................................29
FFT Spectrum............................................................................................................... 30
Freq. Error vs Preamble................................................................................................31
Gain Imbalance vs Carrier............................................................................................ 32
Group Delay..................................................................................................................33
Magnitude Capture........................................................................................................34
Phase Error vs Preamble..............................................................................................34
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Phase Tracking............................................................................................................. 35
PLCP Header (IEEE 802.11b, g (DSSS)...................................................................... 35
PvT Full PPDU..............................................................................................................37
PvT Rising Edge........................................................................................................... 38
PvT Falling Edge...........................................................................................................39
Quad Error vs Carrier....................................................................................................39
Result Summary Detailed............................................................................................. 40
Result Summary Global................................................................................................ 41
Signal Field................................................................................................................... 43
Spectrum Flatness........................................................................................................ 46
AM/AM
This result display shows the measured and the reference signal in the time domain.
For each sample, the x-axis value represents the amplitude of the reference signal and
the y-axis value represents the amplitude of the measured signal.
The reference signal is derived from the measured signal after frequency and time synchronization, channel equalization and demodulation of the signal. The equivalent time
domain representation of the reference signal is calculated by reapplying all the impairments that were removed before demodulation.
The trace is determined by calculating a polynomial regression model of a specified
degree (see "Polynomial degree for curve fitting" on page 143) for the scattered measurement vs. reference signal data. The resulting regression polynomial is indicated in
the window title of the result display.
Note: The measured signal and reference signal are complex signals.
This result display is not available for single-carrier measurements (IEEE 802.11b, g
(DSSS)).
Remote command:
LAY:ADD? '1',RIGH,AMAM, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:AM:AM[:IMMediate] on page 184
Polynomial degree:
CONFigure:BURSt:AM:AM:POLYnomial on page 268
Results:
TRACe[:DATA], see Chapter 11.9.4.1, "AM/AM", on page 313
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AM/PM
This result display shows the measured and the reference signal in the time domain.
For each sample, the x-axis value represents the amplitude of the reference signal.
The y-axis value represents the angle difference of the measured signal minus the reference signal.
This result display is not available for single-carrier measurements (IEEE 802.11b, g
(DSSS)).
Remote command:
LAY:ADD? '1',RIGH,AMPM, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:AM:PM[:IMMediate] on page 184
Querying results:
TRACe[:DATA], see Chapter 11.9.4.2, "AM/PM", on page 313
AM/EVM
This result display shows the measured and the reference signal in the time domain.
For each sample, the x-axis value represents the amplitude of the reference signal.
The y-axis value represents the length of the error vector between the measured signal
and the reference signal.
The length of the error vector is normalized with the power of the corresponding reference signal sample.
This result display is not available for single-carrier measurements (IEEE 802.11b, g
(DSSS)).
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Remote command:
LAY:ADD? '1',RIGH,AMEV, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:AM:EVM[:IMMediate] on page 184
Querying results:
TRACe[:DATA], see Chapter 11.9.4.3, "AM/EVM", on page 313
Bitstream
This result display shows a demodulated payload data stream for all analyzed PPDUs
of the currently captured I/Q data as indicated in the "Magnitude Capture" display. The
bitstream is derived from the constellation diagram points using the 'constellation bit
encoding' from the corresponding WLAN standard. See, for example, IEEE Std.
802.11-2012 'Fig. 18-10 BPSK, QPSK, 16-QAM and 64-QAM constellation bit encoding'. Thus, the bitstream is NOT channel-decoded.
For multicarrier measurements (IEEE 802.11a, ac, g (OFDM), j, n, p), the results are
grouped by symbol and carrier.
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Figure 3-7: Bitstream result display for IEEE 802.11a, ac, g (OFDM), j, n, p standards
For MIMO measurements (IEEE 802.11 ac, n), the results are grouped by stream,
symbol and carrier.
Figure 3-8: Bitstream result display for IEEE 802.11n MIMO measurements
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For single-carrier measurements (IEEE 802.11b, g (DSSS)) the results are grouped by
PPDU.
Figure 3-9: Bitstream result display for IEEE 802.11b, g (DSSS) standards
The numeric trace results for this evaluation method are described in Chapter 11.9.4.4,
"Bitstream", on page 313.
Remote command:
LAY:ADD? '1',RIGH, BITS, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:STATistics:BSTReam[:IMMediate] on page 188
Querying results:
TRACe[:DATA], see Chapter 11.9.4.4, "Bitstream", on page 313
Constellation
This result display shows the in-phase and quadrature phase results for all payload
symbols and all carriers for the analyzed PPDUs of the current capture buffer. The
Tracking/Channel Estimation according to the user settings is applied.
The inphase results (I) are displayed on the x-axis, the quadrature phase (Q) results on
the y-axis.
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Figure 3-10: Constellation result display for IEEE 802.11n MIMO measurements
The numeric trace results for this evaluation method are described in Chapter 11.9.4.6,
"Constellation", on page 315.
Remote command:
LAY:ADD? '1',RIGH, CONS, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:CONSt:CSYMbol[:IMMediate] on page 184
Querying results:
TRACe[:DATA], see Chapter 11.9.4.6, "Constellation", on page 315
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Constellation vs Carrier
This result display shows the in-phase and quadrature phase results for all payload
symbols and all carriers for the analyzed PPDUs of the current capture buffer. The
Tracking/Channel Estimation according to the user settings is applied.
This result display is not available for single-carrier measurements (IEEE 802.11b, g
(DSSS)).
The x-axis represents the carriers. The magnitude of the in-phase and quadrature part
is shown on the y-axis, both are displayed as separate traces (I-> trace 1, Q-> trace 2).
Figure 3-11: Constellation vs. carrier result display for IEEE 802.11n MIMO measurements
The numeric trace results for this evaluation method are described in Chapter 11.9.4.7,
"Constellation Vs Carrier", on page 316.
Remote command:
LAY:ADD? '1',RIGH, CVC, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:CONSt:CCARrier[:IMMediate] on page 184
Querying results:
TRACe[:DATA], see Chapter 11.9.4.7, "Constellation Vs Carrier", on page 316
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EVM vs Carrier
This result display shows all EVM values recorded on a per-subcarrier basis over the
number of analyzed PPDUs as defined by the "Evaluation Range > Statistics". The
Tracking/Channel Estimation according to the user settings is applied (see Chap-
ter 5.3.7, "Tracking and Channel Estimation", on page 116). The minimum, average
and maximum traces are displayed.
This result display is not available for single-carrier measurements (IEEE 802.11b, g
(DSSS)).
Figure 3-12: EVM vs carrier result display for IEEE 802.11n MIMO measurements
The numeric trace results for this evaluation method are described in Chap-
ter 11.9.4.10, "EVM Vs Carrier", on page 316.
Remote command:
LAY:ADD? '1',RIGH, EVC, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:EVM:ECARrier[:IMMediate] on page 185
Querying results:
TRACe[:DATA], see Chapter 11.9.4.10, "EVM Vs Carrier", on page 316
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EVM vs Chip
This result display shows the error vector magnitude per chip.
This result display is only available for single-carrier measurements (IEEE 802.11b, g
(DSSS)).
Since the R&S FPS WLAN application provides two different methods to calculate the
EVM, two traces are displayed:
●
"Vector Error IEEE" shows the error vector magnitude as defined in the IEEE
802.11b or g (DSSS) standards (see also "Error vector magnitude (EVM) - IEEE
802.11b or g (DSSS) method" on page 66)
●
"EVM" shows the error vector magnitude calculated with an alternative method that
provides higher accuracy of the estimations (see also "Error vector magnitude
(EVM) - R&S FPS method" on page 65).
Remote command:
LAY:ADD? '1',RIGH, EVCH, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:EVM:EVCHip[:IMMediate] on page 185
CONFigure:BURSt:EVM:ESYMbol[:IMMediate] on page 185
Querying results:
TRACe[:DATA], see Chapter 11.9.4.11, "EVM Vs Chip", on page 317
EVM vs Symbol
This result display shows all EVM values calculated on a per-carrier basis over the
number of analyzed PPDUs as defined by the "Evaluation Range > Statistics" settings
(see "PPDU Statistic Count / No of PPDUs to Analyze" on page 137). The Tracking/
Channel Estimation according to the user settings is applied (see Chapter 5.3.7,
"Tracking and Channel Estimation", on page 116). The minimum, average and maxi-
mum traces are displayed.
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Figure 3-13: EVM vs symbol result display for IEEE 802.11n MIMO measurements
This result display is not available for single-carrier measurements (IEEE 802.11b, g
(DSSS)).
Remote command:
LAY:ADD? '1',RIGH, EVSY, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:EVM:ESYMbol[:IMMediate] on page 185
Querying results:
TRACe[:DATA], see Chapter 11.9.4.12, "EVM Vs Symbol", on page 317
FFT Spectrum
This result display shows the power vs frequency values obtained from an FFT. The
FFT is performed over the complete data in the current capture buffer, without any correction or compensation.
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Figure 3-14: FFT spectrum result display for IEEE 802.11n MIMO measurements
The numeric trace results for this evaluation method are described in Chap-
ter 11.9.4.13, "FFT Spectrum", on page 318.
Remote command:
LAY:ADD? '1',RIGH, FSP, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:SPECtrum:FFT[:IMMediate] on page 187
Querying results:
TRACe[:DATA], see Chapter 11.9.4.13, "FFT Spectrum", on page 318
Freq. Error vs Preamble
Displays the frequency error values recorded over the preamble part of the PPDU. The
minimum, average and maximum traces are displayed.
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Remote command:
LAY:ADD? '1',RIGH,FEVP, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:PREamble[:IMMediate] on page 186
CONFigure:BURSt:PREamble:SELect on page 186
Querying results:
TRACe[:DATA], see Chapter 11.9.4.9, "Error Vs Preamble", on page 316
Gain Imbalance vs Carrier
Displays the minimum, average and maximum gain imbalance versus carrier in individual traces. For details on gain imbalance, see Chapter 3.1.1.2, "Gain Imbalance",
on page 15.
Remote command:
LAY:ADD? '1',RIGH,GAIN, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:GAIN:GCARrier[:IMMediate] on page 186
Querying results:
TRACe[:DATA], see Chapter 11.9.4.8, "Error Vs Carrier", on page 316
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Group Delay
Displays all Group Delay (GD) values recorded on a per-subcarrier basis - over the
number of analyzed PPDUs as defined by the "Evaluation Range > Statistics" settings
(see "PPDU Statistic Count / No of PPDUs to Analyze" on page 137.
All 57 carriers are shown, including the unused carrier 0.
This result display is not available for single-carrier measurements (IEEE 802.11b, g
(DSSS)).
Figure 3-15: Group delay result display for IEEE 802.11n MIMO measurements
Group delay is a measure of phase distortion and defined as the derivation of phase
over frequency.
To calculate the group delay, the estimated channel is upsampled, inactive carriers are
interpolated, and phases are unwrapped before they are differentiated over the carrier
frequencies. Thus, the group delay indicates the time a pulse in the channel is delayed
for each carrier frequency. However, not the absolute delay is of interest, but rather the
deviation between carriers. Thus, the mean delay over all carriers is deducted.
For an ideal channel, the phase increases linearly, which causes a constant time delay
over all carriers. In this case, a horizontal line at the zero value would be the result.
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The numeric trace results for this evaluation method are described in Chap-
ter 11.9.4.14, "Group Delay", on page 318.
Remote command:
LAY:ADD? '1',RIGH, GDEL, see LAYout:ADD[:WINDow]? on page 258
Or:
CONF:BURS:SPEC:FLAT:SEL GRD, see CONFigure:BURSt:SPECtrum:
FLATness:SELect on page 187 and CONFigure:BURSt:SPECtrum:FLATness[:
IMMediate] on page 188
Querying results:
TRACe[:DATA], see Chapter 11.9.4.14, "Group Delay", on page 318
Magnitude Capture
The Magnitude Capture Buffer display shows the complete range of captured data for
the last sweep. Green bars at the bottom of the Magnitude Capture Buffer display indicate the positions of the analyzed PPDUs.
A blue bar indicates the selected PPDU if the evaluation range is limited to a single
PPDU (see "Analyze this PPDU / Specified PPDU / PPDU to Analyze" on page 136).
Figure 3-16: Magnitude capture display for single PPDU evaluation
Numeric trace results are not available for this evaluation method.
Remote command:
LAY:ADD? '1',RIGH, CMEM, see LAYout:ADD[:WINDow]? on page 258
Querying results:
TRACe[:DATA], see Chapter 11.9.4.15, "Magnitude Capture", on page 319
Phase Error vs Preamble
Displays the phase error values recorded over the preamble part of the PPDU. A minimum, average and maximum trace is displayed.
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Remote command:
LAY:ADD? '1',RIGH,PEVP, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:PREamble[:IMMediate] on page 186
CONFigure:BURSt:PREamble:SELect on page 186
Querying results:
TRACe[:DATA], see Chapter 11.9.4.9, "Error Vs Preamble", on page 316
Phase Tracking
Displays the average phase tracking result per symbol (in radians).
This result display is not available for single-carrier measurements (IEEE 802.11b, g
(DSSS)).
Remote command:
LAY:ADD? '1',RIGH,PTR, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:PTRacking[:IMMediate] on page 186
Querying results:
TRACe[:DATA], see Chapter 11.9.4.16, "Phase Tracking", on page 319
PLCP Header (IEEE 802.11b, g (DSSS)
This result display shows the decoded data from the PLCP header of the PPDU.
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This result display is only available for single-carrier measurements (IEEE 802.11b, g
(DSSS)); for other standards, use Signal Field instead.
Figure 3-17: PLCP Header result display for IEEE 802.11b, g (DSSS) standards
The following information is provided:
Note: The signal field information is provided as a decoded bit sequence and, where
appropriate, also in human-readable form beneath the bit sequence for each PPDU.
Table 3-4: Demodulation results in PLCP Header result display (IEEE 802.11b, g (DSSS))
Result Description Example
PPDU Number of the decoded PPDU
A colored block indicates that the PPDU was successfully decoded.
Signal Information in "signal" field
The decoded data rate is shown below.
Service Information in "service" field
<Symbol clock state> /<Modulation format> / <Length extension
bit state>
where:
<Symbol clock state>: Locked / - <Modulation format>: see Table 4-1
<Length extension bit state>: 1 (set) / - - (not set)
PSDU Length Information in "length" field
Time required to transmit the PSDU
CRC Information in "CRC" field
Result of cyclic redundancy code check: "OK" or "Failed"
PPDU 1
01101110
11 MBits/s
00100000
Lock/CCK/- -
000000000111100
0
120 µs
111010011100111
0
OK
Remote command:
LAY:ADD? '1',RIGH, SFI, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:STATistics:SFIeld[:IMMediate] on page 188
Querying results:
TRACe[:DATA], see Chapter 11.9.4.18, "Signal Field", on page 320
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PvT Full PPDU
Displays the minimum, average and maximum power vs time diagram for all PPDUs.
Figure 3-18: PvT Full PPDU result display for IEEE 802.11a, ac, g (OFDM), j, n, p standards
Figure 3-19: PvT Full PPDU result display for IEEE 802.11n MIMO measurements
For single-carrier measurements (IEEE 802.11b, g (DSSS)), the PVT results are displayed as percentage values of the reference power. The reference can be set to
either the maximum or mean power of the PPDU.
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Figure 3-20: PvT Full PPDU result display for IEEE 802.11b, g (DSSS) standards
Remote command:
LAY:ADD:WIND '2',RIGH,PFPP see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:PVT:SELect on page 187
CONFigure:BURSt:PVT[:IMMediate] on page 186
Querying results:
TRACe[:DATA], see Chapter 11.9.4.17, "Power Vs Time (PVT)", on page 319
PvT Rising Edge
Displays the minimum, average and maximum power vs time diagram for the rising
edge of all PPDUs.
Figure 3-21: PvT Rising Edge result display
Remote command:
LAY:ADD:WIND '2',RIGH,PRIS see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:PVT:SELect on page 187
CONFigure:BURSt:PVT[:IMMediate] on page 186
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Querying results:
TRACe[:DATA], see Chapter 11.9.4.17, "Power Vs Time (PVT)", on page 319
PvT Falling Edge
Displays the minimum, average and maximum power vs time diagram for the falling
edge of all PPDUs.
Figure 3-22: PvT Falling Edge result display
Remote command:
LAY:ADD:WIND '2',RIGH,PFAL see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:PVT:SELect on page 187
CONFigure:BURSt:PVT[:IMMediate] on page 186
Querying results:
TRACe[:DATA], see Chapter 11.9.4.17, "Power Vs Time (PVT)", on page 319
Quad Error vs Carrier
Displays the minimum, average and maximum quadrature offset (error) versus carrier
in individual traces. For details on quadrature offset, see Chapter 3.1.1.3, "Quadrature
Offset", on page 16.
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Remote command:
LAY:ADD? '1',RIGH,QUAD, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:QUAD:QCARrier[:IMMediate] on page 187
Querying results:
TRACe[:DATA], see Chapter 11.9.4.8, "Error Vs Carrier", on page 316
Result Summary Detailed
The detailed result summary contains individual measurement results for the Transmitter and Receiver channels and for the bitstream.
This result display is not available for single-carrier measurements (IEEE 802.11b, g
(DSSS)).
Figure 3-23: Detailed Result Summary result display for IEEE 802.11n MIMO measurements
The "Result Summary Detailed" contains the following information:
Note: You can configure which results are displayed (see Chapter 5.3.10, "Result Con-
figuration", on page 141). However, the results are always calculated, regardless of
their visibility.
Tx channel ("Tx All"):
●
I/Q offset [dB]
●
Gain imbalance [%/dB]
●
Quadrature offset [°]
●
I/Q skew [ps]
●
PPDU power [dBm]
●
Crest factor [dB]
Receive channel ("Rx All"):
●
PPDU power [dBm]
●
Crest factor [dB]
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●
MIMO cross power
●
Center frequency error
●
Symbol clock error
●
CPE
Bitstream ("Stream All"):
●
Pilot bit error rate [%]
●
EVM all carriers [%/dB]
●
EVM data carriers [%/dB]
●
EVM pilot carriers [%/dB]
For details on the individual parameters and the summarized values, see Chap-
ter 3.1.1, "Modulation Accuracy, Flatness and Tolerance Parameters", on page 11.
Remote command:
LAY:ADD? '1',RIGH, RSD, see LAYout:ADD[:WINDow]? on page 258
Querying results:
FETCh:BURSt:ALL:FORMatted? on page 285
Result Summary Global
The global result summary provides measurement results based on the complete signal, consisting of all channels and streams. The observation length is the number of
PPDUs to be analyzed as defined by the "Evaluation Range > Statistics" settings. In
contrast, the detailed result summary provides results for each individual channel and
stream.
For MIMO measurements (IEEE 802.11 ac, n), the global result summary provides the
results for all data streams, whereas the detailed result summary provides the results
for individual streams.
Figure 3-24: Global result summary for IEEE 802.11a, ac, g (OFDM), j, n, p standards
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WLAN I/Q Measurement (Modulation Accuracy, Flatness and Tolerance)
Figure 3-25: Global result summary for IEEE 802.11b, g (DSSS) standards
The "Result Summary Global" contains the following information:
Note: You can configure which results are displayed (see Chapter 5.3.10, "Result Con-
figuration", on page 141). However, the results are always calculated, regardless of
their visibility.
●
Number of recognized PPDUs
●
Number of analyzed PPDUs
●
Number of analyzed PPDUs in entire physical channel, if available
IEEE 802.11a, ac, g (OFDM), j, n, p standards:
●
Pilot bit error rate [%]
●
EVM all carriers [%/dB]
●
EVM data carriers [%/dB]
●
EVM pilot carriers [%/dB]
●
Center frequency error [Hz]
●
Symbol clock error [ppm]
IEEE 802.11b, g (DSSS) standards:
●
Peak vector error
●
PPDU EVM
●
Quadrature offset
●
Gain imbalance
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●
Quadrature error
●
Center frequency error
●
Chip cock error
●
Rise time
●
Fall time
●
Mean power
●
Peak power
●
Crest power
For details on the individual results and the summarized values, see Chapter 3.1.1,
"Modulation Accuracy, Flatness and Tolerance Parameters", on page 11.
Remote command:
LAY:ADD? '1',RIGH, RSGL, see LAYout:ADD[:WINDow]? on page 258
Querying results:
All values in result summary table:
FETCh:BURSt:ALL:FORMatted? on page 285
Signal Field
This result display shows the decoded data from the "Signal" field of each recognized
PPDU. This field contains information on the modulation used for transmission.
This result display is not available for single-carrier measurements (IEEE 802.11b, g
(DSSS)); use PLCP Header (IEEE 802.11b, g (DSSS) instead.
Figure 3-26: Signal Field display for IEEE 802.11n
The signal field information is provided as a decoded bit sequence and, where appropriate, also in human-readable form, beneath the bit sequence for each PPDU.
The currently applied user-defined demodulation settings are indicated beneath the
table header for reference. Since the demodulation settings define which PPDUs are to
be analyzed, this logical filter can be the reason if the "Signal Field" display is not as
expected.
Table 3-5: Demodulation parameters and results for Signal Field result display (IEEE 802.11a, g
Parameter Description
Format PPDU format used for measurement (not part of the IEEE 802.11a, g (OFDM), p
CBW Channel bandwidth to measure (not part of the signal field, displayed for conven-
Rate / Mbit/s Symbol rate per second
(OFDM), j, p)
signal field, displayed for convenience; see "PPDU Format to measure"
on page 121)
ience)
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Parameter Description
R Reserved bit
Length / Sym Human-readable length of payload in OFDM symbols
P Parity bit
(Signal) Tail Signal tail (preset to 0)
Table 3-6: Demodulation parameters and results for Signal Field result display (IEEE 802.11ac)
Parameter Description
Format PPDU format used for measurement (not part of the IEEE 802.11ac signal field,
displayed for convenience; see "PPDU Format to measure" on page 121)
MCS Modulation and Coding Scheme (MCS) index of the PPDU as defined in IEEE
Std 802.11-2012 section "20.6 Parameters for HT MCSs"
BW Channel bandwidth to measure
0: 20 MHz
1: 40 MHz
2: 80 MHz
3: 80+80 MHz and 160MHz
L-SIG Length / Sym Human-readable length of payload in OFDM symbols
STBC Space-Time Block Coding
0: no spatial streams of any user have space time block coding
1: all spatial streams of all users have space time block coding
GI Guard interval length PPDU must have to be measured
1: short guard interval is used in the Data field
0: short guard interval is not used in the Data field
Ness Number of extension spatial streams (N
(sounding)" on page 133)
CRC Cyclic redundancy code
Table 3-7: Demodulation parameters and results for Signal Field result display (IEEE 802.11n)
Parameter Description
Format PPDU format used for measurement (not part of the IEEE 802.11n signal field,
displayed for convenience; see "PPDU Format to measure" on page 121)
MCS Modulation and Coding Scheme (MCS) index of the PPDU as defined in IEEE
Std 802.11-2012 section "20.6 Parameters for HT MCSs"
CBW Channel bandwidth to measure
0: 20 MHz or 40 MHz upper/lower
1: 40 MHz
, see "Extension Spatial Streams
ESS
HT-SIG Length / Sym Human-readable length of payload in OFDM symbols
The number of octets of data in the PSDU in the range of 0 to 65 535
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Parameter Description
SNRA Smoothing/Not Sounding/Reserved/Aggregation:
Smoothing:
1: channel estimate smoothing is recommended
0: only per-carrier independent (unsmoothed) channel estimate is recommended
Not Sounding:
1: PPDU is not a sounding PPDU
0: PPDU is a sounding PPDU
Reserved: Set to 1
Aggregation:
1: PPDU in the data portion of the packet contains an AMPDU
0: otherwise
STBC Space-Time Block Coding
00: no STBC (NSTS = NSS)
≠0: the difference between the number of space-time streams (NSTS) and the
number of spatial streams (NSS) indicated by the MCS
GI Guard interval length PPDU must have to be measured
1: short GI used after HT training
0: otherwise
Ness Number of extension spatial streams (N
(sounding)" on page 133)
CRC Cyclic redundancy code of bits 0 to 23 in HT-SIG1 and bits 0 to 9 in HT-SIG2
Tail Bits Used to terminate the trellis of the convolution coder. Set to 0.
, see "Extension Spatial Streams
ESS
The values for the individual demodulation parameters are described in Chapter 5.3.8,
"Demodulation", on page 120. The following abbreviations are used in the "Signal
Field" table:
Table 3-8: Abbreviations for demodulation parameters shown in "Signal Field" display
Abbreviation in "Signal
Field" display
A1st Auto, same type as first PPDU
AI Auto, individual for each PPDU
M<x> Meas only the specified PPDUs (<x>)
D<x> Demod all with specified parameter <y>
Parameter in "Demodulation" settings
The Signal Field measurement indicates certain inconsistencies in the signal or discrepancies between the demodulation settings and the signal to be analyzed. In both
cases, an appropriate warning is displayed and the results for the PPDU are highlighted orange - both in the "Signal Field" display and the "Magnitude Capture" display. If
the signal was analyzed with warnings the results – indicated by a message - also contribute to the overall analysis results.
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PPDUs detected in the signal that do not pass the logical filter, i.e. are not to be included in analysis, are dismissed. An appropriate message is provided. The corresponding PPDU in the capture buffer is not highlighted.
The numeric trace results for this evaluation method are described in Chap-
ter 11.9.4.18, "Signal Field", on page 320.
Remote command:
LAY:ADD? '1',RIGH, SFI, see LAYout:ADD[:WINDow]? on page 258
Or:
CONFigure:BURSt:STATistics:SFIeld[:IMMediate] on page 188
Querying results:
TRACe[:DATA], see Chapter 11.9.4.18, "Signal Field", on page 320
Spectrum Flatness
The Spectrum Flatness trace is derived from the magnitude of the estimated channel
transfer function. Since this estimated channel is calculated from all payload symbols
of the PPDU, it represents a carrier-wise mean gain of the channel. We assume the
cable connection between the DUT and the R&S FPS adds no residual channel distortion. Then the "Spectrum Flatness" shows the spectral distortion caused by the DUT,
for example the transmit filter.
This result display is not available for single-carrier measurements (IEEE 802.11b, g
(DSSS)).
The diagram shows the relative power per carrier. All carriers are displayed, including
the unused carriers.
In contrast to the SISO measurements in previous Rohde & Schwarz signal and spectrum analyzers, the trace is no longer normalized to 0 dB, that is: scaled by the mean
gain of all carriers.
For more information, see Chapter 4.3.6, "Crosstalk and Spectrum Flatness",
on page 75.
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Frequency Sweep Measurements
Figure 3-27: Spectrum flatness result display for IEEE 802.11n MIMO measurements
The numeric trace results for this evaluation method are described in Chap-
ter 11.9.4.19, "Spectrum Flatness", on page 320.
Remote command:
LAY:ADD? '1',RIGH, SFL, see LAYout:ADD[:WINDow]? on page 258
Or:
CONF:BURS:SPEC:FLAT:SEL FLAT (see CONFigure:BURSt:SPECtrum:
FLATness:SELect on page 187) and CONFigure:BURSt:SPECtrum:
FLATness[:IMMediate] on page 188
Querying results:
TRACe[:DATA], see Chapter 11.9.4.19, "Spectrum Flatness", on page 320
3.2 Frequency Sweep Measurements
As described above, the WLAN IQ measurement captures the I/Q data from the WLAN
signal using a (nearly rectangular) filter with a relatively large bandwidth. However,
some parameters specified in the WLAN 802.11 standard require a better signal-tonoise level or a smaller bandwidth filter than the I/Q measurement provides and must
be determined in separate measurements.
Parameters that are common to several digital standards and are often required in signal and spectrum test scenarios can be determined by the standard measurements
provided in the R&S FPS base unit (Spectrum application). These measurements are
performed using a much narrower bandwidth filter, and they capture only the power
level (magnitude, which we refer to as RF data) of the signal, as opposed to the two
components provided by I/Q data.
Frequency sweep measurements can tune on a constant frequency ("Zero span measurement") or sweep a frequency range ("Frequency sweep measurement")
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Frequency Sweep Measurements
The signal cannot be demodulated based on the captured RF data. However, the
required power information can be determined much more precisely, as more noise is
filtered out of the signal.
The Frequency sweep measurements provided by the R&S FPS WLAN application are
identical to the corresponding measurements in the base unit, but are pre-configured
according to the requirements of the selected WLAN 802.11 standard.
For details on these measurements see the R&S FPS User Manual.
MSRA operating mode
Frequency sweep measurements are not available in MSRA operating mode.
For details on the MSRA operating mode see the R&S FPS MSRA User Manual.
The R&S FPS WLAN application provides the following frequency sweep measurements:
3.2.1 Measurement Types and Results for Frequency Sweep Measurements
The R&S FPS WLAN application provides the following pre-configured frequency
sweep measurements:
Channel Power ACLR...................................................................................................48
Spectrum Emission Mask..............................................................................................49
Occupied Bandwidth..................................................................................................... 50
CCDF............................................................................................................................ 51
Channel Power ACLR
Channel Power ACLR performs an adjacent channel power (also known as adjacent
channel leakage ratio) measurement according to WLAN 802.11 specifications.
The R&S FPS measures the channel power and the relative power of the adjacent
channels and of the alternate channels. The results are displayed in the Result Summary.
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Frequency Sweep Measurements
For details see Chapter 5.4.1, "Channel Power (ACLR) Measurements", on page 150.
Remote command:
CONFigure:BURSt:SPECtrum:ACPR[:IMMediate] on page 189
Querying results:
CALC:MARK:FUNC:POW:RES? ACP, see CALCulate<n>:MARKer<m>:FUNCtion:
POWer<sb>:RESult? on page 301
Spectrum Emission Mask
Access: "Overview" > "Select Measurement" > "SEM"
Or: MEAS > "Select Measurement" > "SEM"
The Spectrum Emission Mask (SEM) measurement determines the power of the
WLAN 802.11 signal in defined offsets from the carrier and compares the power values
with a spectral mask specified by the WLAN 802.11 specifications. The limits depend
on the selected bandclass. Thus, the performance of the DUT can be tested and the
emissions and their distance to the limit be identified.
Note: The WLAN 802.11 standard does not distinguish between spurious and spectral
emissions.
For details see Chapter 5.4.2, "Spectrum Emission Mask", on page 151.
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Frequency Sweep Measurements
Figure 3-28: SEM measurement results
Remote command:
CONFigure:BURSt:SPECtrum:MASK[:IMMediate] on page 189
Querying results:
CALCulate<n>:LIMit<li>:FAIL? on page 298
TRAC:DATA? LIST, see TRACe[:DATA] on page 308
Occupied Bandwidth
The Occupied Bandwidth (OBW) measurement determines the bandwidth in which a
certain percentage of the total signal power is measured. The percentage of the signal
power to be included in the bandwidth measurement can be changed; by default settings it is 99 %.
The occupied bandwidth is indicated as the "Occ BW" function result in the marker
table; the frequency markers used to determine it are also displayed.
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Frequency Sweep Measurements
For details, see Chapter 5.4.3, "Occupied Bandwidth", on page 152.
Remote command:
CONFigure:BURSt:SPECtrum:OBWidth[:IMMediate] on page 189
Querying results:
CALC:MARK:FUNC:POW:RES? OBW, see CALCulate<n>:MARKer<m>:FUNCtion:
POWer<sb>:RESult? on page 301
CCDF
The CCDF (complementary cumulative distribution function) measurement determines
the distribution of the signal amplitudes. The measurement captures a user-definable
number of samples and calculates their mean power. As a result, the probability that a
sample's power is higher than the calculated mean power + x dB is displayed. The
crest factor is displayed in the Result Summary.
For details see Chapter 5.4.4, "CCDF", on page 153.
Figure 3-29: CCDF measurement results
Remote command:
CONFigure:BURSt:STATistics:CCDF[:IMMediate] on page 190
Querying results:
CALCulate<n>:MARKer<m>:Y? on page 325
CALCulate<n>:STATistics:RESult<t>? on page 305
3.2.2 Evaluation Methods for Frequency Sweep Measurements
The evaluation methods for frequency sweep measurements in the R&S FPS WLAN
application are identical to those in the R&S FPS base unit (Spectrum application).
Diagram ........................................................................................................................52
Result Summary ...........................................................................................................52
Marker Table ................................................................................................................52
Marker Peak List .......................................................................................................... 53
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Diagram
Displays a basic level vs. frequency or level vs. time diagram of the measured data to
evaluate the results graphically. This is the default evaluation method. Which data is
displayed in the diagram depends on the "Trace" settings. Scaling for the y-axis can be
configured.
Remote command:
LAY:ADD? '1',RIGH, DIAG, see LAYout:ADD[:WINDow]? on page 258
Results:
Result Summary
Result summaries provide the results of specific measurement functions in a table for
numerical evaluation. The contents of the result summary vary depending on the
selected measurement function. See the description of the individual measurement
functions for details.
Remote command:
LAY:ADD? '1',RIGH, RSUM, see LAYout:ADD[:WINDow]? on page 258
Marker Table
Displays a table with the current marker values for the active markers.
Remote command:
LAY:ADD? '1',RIGH, MTAB, see LAYout:ADD[:WINDow]? on page 258
Results:
CALCulate<n>:MARKer<m>:X on page 304
CALCulate<n>:MARKer<m>:Y? on page 325
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Marker Peak List
The marker peak list determines the frequencies and levels of peaks in the spectrum or
time domain. How many peaks are displayed can be defined, as well as the sort order.
In addition, the detected peaks can be indicated in the diagram. The peak list can also
be exported to a file for analysis in an external application.
Remote command:
LAY:ADD? '1',RIGH, PEAK, see LAYout:ADD[:WINDow]? on page 258
Results:
CALCulate<n>:MARKer<m>:X on page 304
CALCulate<n>:MARKer<m>:Y? on page 325
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4 Measurement Basics
Measurement Basics
Signal Processing for Multicarrier Measurements (IEEE 802.11a, g (OFDM), j, p)
Some background knowledge on basic terms and principles used in WLAN measurements is provided here for a better understanding of the required configuration settings.
4.1 Signal Processing for Multicarrier Measurements (IEEE 802.11a, g (OFDM), j, p)
This description gives a rough view of the signal processing when using the R&S FPS
WLAN application with the IEEE 802.11a, g (OFDM), j, p standards. Details are disregarded in order to provide a concept overview.
Abbreviations
a
l,k
EVM
k
EVM Error vector magnitude of current packet
g Signal gain
Δf Frequency deviation between Tx and Rx
l Symbol index l = {1 ... nof_Symbols}
nof_symbols Number of symbols of payload
H
k
k Channel index k = {–31 ... 32}
Symbol at symbol l of subcarrier k
Error vector magnitude of subcarrier k
Channel transfer function of subcarrier k
K
mod
ξ Relative clock error of reference oscillator
r
l,k
Modulation-dependent normalization factor
Subcarrier of symbol l
● Block Diagram for Multicarrier Measurements........................................................54
● Literature on the IEEE 802.11a Standard............................................................... 61
4.1.1 Block Diagram for Multicarrier Measurements
A diagram of the significant blocks when using the IEEE 802.11a, g (OFDM), j, p standard in the R&S FPS WLAN application is shown in Figure 4-1.
First the RF signal is downconverted to the IF frequency fIF. The resulting IF signal rIF(t)
is shown on the left-hand side of the figure. After bandpass filtering, the signal is sam-
pled by an analog to digital converter (ADC) at a sample rate of fs1. This digital
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Measurement Basics
Signal Processing for Multicarrier Measurements (IEEE 802.11a, g (OFDM), j, p)
sequence is resampled. Thus, the sample rate of the downsampled sequence r(i) is the
Nyquist rate of fs3 = 20 MHz. Up to this point the digital part is implemented in an ASIC.
Figure 4-1: Block diagram for the R&S FPS WLAN application using the IEEE 802.11a, g (OFDM), j, p
standard
In the lower part of the figure the subsequent digital signal processing is shown.
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kl
phasephasej
klkl
neHgaKr
kl
common
l
kl
,
(
,mod
)t iming(
,
)(
,
Measurement Basics
Signal Processing for Multicarrier Measurements (IEEE 802.11a, g (OFDM), j, p)
Packet search and timing detection
In the first block the packet search is performed. This block detects the long symbol
(LS) and recovers the timing. The coarse timing is detected first. This search is implemented in the time domain. The algorithm is based on cyclic repetition within the LS
after N = 64 samples. Numerous treatises exist on this subject, e.g. [1] to [3].
Furthermore, a coarse estimate Δ
of the Rx-Tx frequency offset Δf is derived
coarse
from the metric in [6]. (The hat generally indicates an estimate, e.g. is the estimate of
·Δ
x.) This can easily be understood because the phase of r(i)
r* (i + N) is determined
by the frequency offset. As the frequency deviation Δf can exceed half a bin (distance
between neighboring subcarriers) the preceding short symbol (SS) is also analyzed in
order to detect the ambiguity.
After the coarse timing calculation the time estimate is improved by the fine timing
calculation. This is achieved by first estimating the coarse frequency response
(LS)
Ĥ
,
k
where k = {–26.. 26} denotes the channel index of the occupied subcarriers. First the
FFT of the LS is calculated. After the FFT calculation the known symbol information of
the LS subcarriers is removed by dividing by the symbols. The result is a coarse estimate
Ĥ
of the channel transfer function. In the next step, the complex channel impulse
k
response is computed by an IFFT. Then the energy of the windowed impulse response
(the window size is equal to the guard period) is calculated for each trial time. Afterwards the trial time of the maximum energy is detected. This trial time is used to adjust
the timing.
Determing the payload window
Now the position of the LS is known and the starting point of the useful part of the first
payload symbol can be derived. In the next block this calculated time instant is used to
position the payload window. Only the payload part is windowed. This is sufficient
because the payload is the only subject of the subsequent measurements.
In the next block the windowed sequence is compensated by the coarse frequency
estimate
Δ
. This is necessary because otherwise inter-channel interference (ICI)
course
would occur in the frequency domain.
The transition to the frequency domain is achieved by an FFT of length 64. The FFT is
performed symbol-wise for each symbol of the payload ("nof_symbols"). The calculated FFTs are described byr
●
l = {1 .. nof_symbols} as the symbol index
●
k = {–31 .. 32} as the channel index
with:
l,k
In case of an additive white Gaussian noise (AWGN) channel, the FFT is described by
[4], [5]
Equation 4-1: FFT
with:
●
K
: the modulation-dependant normalization factor
mod
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lrests
common
l
dlTfNNphase
/2
)(
Measurement Basics
Signal Processing for Multicarrier Measurements (IEEE 802.11a, g (OFDM), j, p)
●
a
: the symbol of subcarrier k at symbol l
l,k
●
gl: the gain at the symbol l in relation to the reference gain g = 1 at the long symbol
(LS)
●
Hk: the channel frequency response at the long symbol (LS)
●
phasel
(common)
: the common phase drift phase of all subcarriers at symbol l (see
Common phase drift)
●
phase
l,k
(timing)
: the phase of subcarrier k at symbol l caused by the timing drift (see
Common phase drift)
●
n
: the independent Gaussian distributed noise samples
l,k
Phase drift and frequency deviation
The common phase drift in FFT is given by:
Equation 4-2: Common phase drift
with
●
Ns = 80: the number of Nyquist samples of the symbol period
●
N = 64: the number of Nyquist samples of the useful part of the symbol
●
Δ f
: the (not yet compensated) frequency deviation
rest
●
dϒ l: the phase jitter at the symbol l
In general, the coarse frequency estimate Δ
Therefore the remaining frequency error Δf
(see Figure 4-1) is not error-free.
coarse
represents the frequency deviation in r
rest
l,k
not yet compensated. Consequently, the overall frequency deviation of the device
under test (DUT) is calculated by:
Δf = Δ
coarse
+ Δf
rest
The common phase drift in Common phase drift is divided into two parts to calculate
the overall frequency deviation of the DUT.
The reason for the phase jitter dγ l in Common phase drift may be different. The nonlinear part of the phase jitter may be caused by the phase noise of the DUT oscillator.
Another reason for nonlinear phase jitter may be the increase of the DUT amplifier
temperature at the beginning of the PPDU. Note that besides the nonlinear part the
phase jitter, dγ l also contains a constant part. This constant part is caused by the fre-
quency deviation Δ f
not yet compensated. To understand this, keep in mind that the
rest
measurement of the phase starts at the first symbol l = 1 of the payload. In contrast,
the channel frequency response Hk in FFT represents the channel at the long symbol
of the preamble. Consequently, the frequency deviation Δ f
not yet compensated
rest
produces a phase drift between the long symbol and the first symbol of the payload.
Therefore, this phase drift appears as a constant value ("DC value") in dϒ l.
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lkNNphase
skl
/2
)timing(
,
Measurement Basics
Signal Processing for Multicarrier Measurements (IEEE 802.11a, g (OFDM), j, p)
Tracking the phase drift, timing jitter and gain
Referring to the IEEE 802.11a, g (OFDM), j, p measurement standard, chapter
17.3.9.7 "Transmit modulation accuracy test'' [6], the common phase drift phasel
mon)
must be estimated and compensated from the pilots. Therefore this "symbol-wise
(com-
phase tracking'' is activated as the default setting of the R&S FPS WLAN application
(see "Phase Tracking" on page 118).
Furthermore, the timing drift in FFT is given by:
Equation 4-3: Timing drift
with ξ: the relative clock deviation of the reference oscillator
Normally, a symbol-wise timing jitter is negligible and thus not modeled in Timing drift.
However, there may be situations where the timing drift has to be taken into account.
This is illustrated by an example: In accordance to [6], the allowed clock deviation of
the DUT is up to ξ
= 20 ppm. Furthermore, a long packet with 400 symbols is
max
assumed. The result of FFT and Timing drift is that the phase drift of the highest subcarrier k = 26 in the last symbol l = nof_symbols is 93 degrees. Even in the noise-free
case, this would lead to symbol errors. The example shows that it is actually necessary
to estimate and compensate the clock deviation, which is accomplished in the next
block.
Referring to the IEEE 802.11a, g (OFDM), j, p measurement standard [6], the timing
drift phase
(timing)
is not part of the requirements. Therefore the "time tracking" is not
l,k
activated as the default setting of the R&S FPS WLAN application (see "Timing Error
Tracking" on page 118). The time tracking option should rather be seen as a powerful
analyzing option.
In addition, the tracking of the gain gl in FFT is supported for each symbol in relation to
the reference gain g = 1 at the time instant of the long symbol (LS). At this time the
coarse channel transfer function Ĥ
This makes sense since the sequence r
fer function Ĥ
(LS)
before estimating the symbols. Consequently, a potential change of
k
(LS)
is calculated.
k
'
is compensated by the coarse channel trans-
l,k
the gain at the symbol l (caused, for example, by the increase of the DUT amplifier
temperature) may lead to symbol errors especially for a large symbol alphabet M of the
MQAM transmission. In this case, the estimation and the subsequent compensation of
the gain are useful.
Referring to the IEEE 802.11a, g (OFDM), j, p measurement standard [6], the compensation of the gain gl is not part of the requirements. Therefore the "gain tracking" is not
activated as the default setting of the R&S FPS WLAN application (see "Level Error
(Gain) Tracking" on page 118).
Determining the error parameters (log likelihood function)
How can the parameters above be calculated? In this application the optimum maximum likelihood algorithm is used. In the first estimation step the symbol-independent
parameters Δ f
and ξ are estimated. The symbol-dependent parameters can be
rest
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Measurement Basics
Signal Processing for Multicarrier Measurements (IEEE 802.11a, g (OFDM), j, p)
neglected in this step, i.e. the parameters are set to gl = 1 and dγ = 0. Referring to
FFT, the log likelihood function L must be calculated as a function of the trial parame-
ters Δ
and . (The tilde generally describes a trial parameter. Example: is the trial
rest
parameter of x.)
Equation 4-4: Log likelihood function (step 1)
The trial parameters leading to the minimum of the log likelihood function are used as
estimates Δ
and . In Log likelihood function (step 1) the known pilot symbols a
rest
l,k
are read from a table.
In the second step, the log likelihood function is calculated for every symbol l as a function of the trial parameters
and dl:
l
Equation 4-5: Log likelihood function (step 2)
Finally, the trial parameters leading to the minimum of the log likelihood function are
used as estimates ĝl and .
This robust algorithm works well even at low signal to noise ratios with the Cramer Rao
Bound being reached.
Compensation
After estimation of the parameters, the sequence r
is compensated in the compensa-
l,k
tion blocks.
In the upper analyzing branch the compensation is user-defined i.e. the user determines which of the parameters are compensated. This is useful in order to extract the
influence of these parameters. The resulting output sequence is described by: γ
'
δ,k
.
Data symbol estimation
In the lower compensation branch the full compensation is always performed. This
separate compensation is necessary in order to avoid symbol errors. After the full compensation the secure estimation of the data symbols â
is performed. From FFT it is
l,k
clear that first the channel transfer function Hk must be removed. This is achieved by
59User Manual 1176.8551.02 ─ 08
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Measurement Basics
Signal Processing for Multicarrier Measurements (IEEE 802.11a, g (OFDM), j, p)
dividing the known coarse channel estimate Ĥ
(LS)
calculated from the LS. Usually an
k
error free estimation of the data symbols can be assumed.
Improving the channel estimation
In the next block a better channel estimate Ĥ
(PL)
of the data and pilot subcarriers is
k
calculated by using all "nof_symbols" symbols of the payload (PL). This can be accomplished at this point because the phase is compensated and the data symbols are
known. The long observation interval of nof_symbols symbols (compared to the short
interval of 2 symbols for the estimation of Ĥ
(LS)
) leads to a nearly error-free channel
k
estimate.
In the following equalizer block, Ĥ
resulting channel-compensated sequence is described by γ
choose the coarse channel estimate Ĥ
free channel estimate Ĥ
mate Ĥ
(LS)
is used, a 2 dB reduction of the subsequent EVM measurement can be
k
(PL)
k
(LS)
is compensated by the channel estimate. The
k
(LS)
(from the long symbol) or the nearly error-
k
''
. The user may either
δ,k
(from the payload) for equalization. If the improved esti-
expected.
According to the IEEE 802.11a measurement standard [6], the coarse channel estima-
(LS)
tion Ĥ
(from the long symbol) has to be used for equalization. Therefore the default
k
setting of the R&S FPS WLAN application is equalization from the coarse channel estimate derived from the long symbol.
Calculating error parameters
In the last block the parameters of the demodulated signal are calculated. The most
important parameter is the error vector magnitude of the subcarrier "k" of the current
packet:
Equation 4-6: Error vector magnitude of the subcarrier k in current packet
Furthermore, the packet error vector magnitude is derived by averaging the squared
EVMk versus k:
Equation 4-7: Error vector magnitude of the entire packet
Finally, the average error vector magnitude is calculated by averaging the packet EVM
of all nof_symbols detected packets:
Equation 4-8: Average error vector magnitude
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Measurement Basics
Signal Processing for Single-Carrier Measurements (IEEE 802.11b, g (DSSS))
This parameter is equivalent to the "RMS average of all errors": Error
of the IEEE
RMS
802.11a measurement commandment (see [6]).
4.1.2 Literature on the IEEE 802.11a Standard
4.2 Signal Processing for Single-Carrier Measurements
[1] Speth, Classen, Meyr: ''Frame synchronization of OFDM systems in frequency selective fading
channels", VTC '97, pp. 1807-1811
[2] Schmidl, Cox: ''Robust Frequency and Timing Synchronization of OFDM", IEEE Trans. on Comm.,
Dec. 1997, pp. 1613-621
[3] Minn, Zeng, Bhargava: ''On Timing Offset Estimation for OFDM", IEEE Communication Letters,
July 2000, pp. 242-244
[4] Speth, Fechtel, Fock, Meyr: ''Optimum receive antenna Design for Wireless Broad-Band Systems
Using OFDM – Part I", IEEE Trans. On Comm. VOL. 47, NO 11, Nov. 1999
[5] Speth, Fechtel, Fock, Meyr: ''Optimum receive antenna Design for Wireless Broad-Band Systems
Using OFDM – Part II", IEEE Trans. On Comm. VOL. 49, NO 4, April. 2001
[6] IEEE 802.11a, Part 11: WLAN Medium Access Control (MAC) and Physical Layer (PHY) specifi-
cations
(IEEE 802.11b, g (DSSS))
This description gives a rough overview of the signal processing concept of the WLAN
802.11 application for IEEE 802.11b or g (DSSS) signals.
Abbreviations
ε timing offset
Δ"f" frequency offset
ΔΦ phase offset
ĝ
I
ĝ
Q
Δĝ
Q
ĥs(v) estimated baseband filter of the transmit antenna
ĥr(v) estimated baseband filter of the receive antenna
ô
I
ô
Q
r(v) measurement signal
estimate of the gain factor in the I-branch
estimate of the gain factor in the Q-branch
accurate estimate of the crosstalk factor of the Q-branch in the I-branch
estimate of the IQ-offset in the I-branch
estimate of the IQ-offset in the I-branch
ŝ(v) estimate of the reference signal
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Measurement Basics
Signal Processing for Single-Carrier Measurements (IEEE 802.11b, g (DSSS))
ŝn(v) estimate of the power-normalized and undisturbed reference signal
ARG{...} calculation of the angle of a complex value
EVM error vector magnitude
IMAG{...} calculation of the imaginary part of a complex value
PPDU protocol data unit - a burst in the signal containing transmission data
PSDU protocol service data unit- a burst in the signal containing service data
REAL{...} calculation of the real part of a complex value
● Block Diagram for Single-Carrier Measurements....................................................62
● Calculation of Signal Parameters............................................................................64
● Literature on the IEEE 802.11b Standard............................................................... 67
4.2.1 Block Diagram for Single-Carrier Measurements
A block diagram of the measurement application is shown below in Figure 4-2. The
baseband signal of an IEEE 802.11b or g (DSSS) wireless LAN system transmit
antenna is sampled with a sample rate of 44 MHz.
The first task of the measurement application is to detect the position of the PPDU
within the measurement signal r1(v). The detection algorithm is able to find the begin-
ning of short and long PPDUs and can distinguish between them. The algorithm also
detects the initial state of the scrambler, which is not specified by the IEEE 802.11
standard.
If the start position of the PPDU is known, the header of the PPDU can be demodulated. The bits transmitted in the header provide information about the length of the
PPDU and the modulation type used in the PSDU.
Once the start position and the PPDU length are fully known, better estimates of timing
offset, timing drift, frequency offset and phase offset can be calculated using the entire
data of the PPDU.
At this point of the signal processing, demodulation can be performed without decision
error. After demodulation the normalized (in terms of power) and undisturbed reference
signal s(v) is available.
If the frequency offset is not constant and varies with time, the frequency offset and
phase offset in several partitions of the PPDU must be estimated and corrected. Additionally, timing offset, timing drift and gain factor can be estimated and corrected in
several partitions of the PPDU. These corrections can be switched off individually in
the demodulation settings of the application.
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Measurement Basics
Signal Processing for Single-Carrier Measurements (IEEE 802.11b, g (DSSS))
Figure 4-2: Signal processing for IEEE 802.11b or g (DSSS) signals
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Measurement Basics
Signal Processing for Single-Carrier Measurements (IEEE 802.11b, g (DSSS))
Once the normalized and undisturbed reference signal is available, the transmit
antenna baseband filter (Tx filter) is estimated by minimizing the cost function of a
maximum-likelihood-based estimator:
Equation 4-9: transmit antenna baseband filter (Tx filter) estimation
where:
: the oversampled measurement signal
: the normalized oversampled power of the undisturbed reference signal
: the observation length
: the filter length
: the variation parameters of the frequency offset
: the variation parameters of the phase offset
: the variation parameters of the IQ-offset
: the coefficients of the transmitter filter
4.2.2 Calculation of Signal Parameters
The frequency offset, the phase offset and the IQ-offset are estimated jointly with the
coefficients of the transmit filter to increase the estimation quality.
Once the transmit filter is known, all other unknown signal parameters are estimated
with a maximum-likelihood-based estimation, which minimizes the cost function:
Equation 4-10: Cost function for signal parameters
where:
The unknown signal parameters are estimated in a joint estimation process to increase
the accuracy of the estimates.
: the variation parameters of the gain used in the I/Q-branch
: the crosstalk factor of the Q-branch into the I-branch
: the filtered reference signal of the I/Q-branch
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I
QQ
g
gg
imbalanceGain
QQ
gjgARGErrorQuadrature
2
22
22
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2
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oo
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Measurement Basics
Signal Processing for Single-Carrier Measurements (IEEE 802.11b, g (DSSS))
The accurate estimates of the frequency offset, the gain imbalance, the quadrature
error and the normalized I/Q offset are displayed by the measurement software.
Gain imbalance, I/Q offset, quadrature error
The gain imbalance is the quotient of the estimates of the gain factor of the Q-branch,
the crosstalk factor and the gain factor of the I-branch:
Equation 4-11: Gain imbalance
The quadrature error is a measure for the crosstalk of the Q-branch into the I-branch:
Equation 4-12: Quadrature error (crosstalk)
The normalized I/Q offset is defined as the magnitude of the I/Q offset normalized by
the magnitude of the reference signal:
Equation 4-13: I/Q offset
At this point of the signal processing all unknown signal parameters such as timing offset, frequency offset, phase offset, I/Q offset and gain imbalance have been evaluated
and the measurement signal can be corrected accordingly.
Error vector magnitude (EVM) - R&S FPS method
Using the corrected measurement signal r(v) and the estimated reference signal ŝ(v),
the modulation quality parameters can be calculated. The mean error vector magnitude
(EVM) is the quotient of the root-mean-square values of the error signal power and the
reference signal power:
Equation 4-14: Mean error vector magnitude (EVM)
Whereas the symbol error vector magnitude is the momentary error signal magnitude
normalized by the root mean square value of the reference signal power:
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1
0
2
)(
ˆ
)(
ˆ
)(
)(
N
v
vs
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REAL
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ˆ
N
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N
o
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Measurement Basics
Signal Processing for Single-Carrier Measurements (IEEE 802.11b, g (DSSS))
Equation 4-15: Symbol error vector magnitude
Error vector magnitude (EVM) - IEEE 802.11b or g (DSSS) method
In [2] a different algorithm is proposed to calculate the error vector magnitude. In a first
step the IQ-offset in the I-branch and the IQ-offset of the Q-branch are estimated separately:
Equation 4-16: I/Q offset I-branch
Equation 4-17: I/Q offset Q-branch
where r(v) is the measurement signal which has been corrected with the estimates of
the timing offset, frequency offset and phase offset, but not with the estimates of the
gain imbalance and I/Q offset
With these values the gain imbalance of the I-branch and the gain imbalance of the Qbranch are estimated in a non-linear estimation in a second step:
Equation 4-18: Gain imbalance I-branch
Equation 4-19: Gain imbalance Q-branch
Finally, the mean error vector magnitude can be calculated with a non-data-aided calculation:
Equation 4-20: Mean error vector magnitude
The symbol error vector magnitude is the error signal magnitude normalized by the
root mean square value of the estimate of the measurement signal power:
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2
22
2
2
err
2
1
IMAG
2
1
REAL
2
1
QI
QQII
gg
govrgovr
vV
ˆˆ
ˆˆ
)(
ˆˆ
)(
)(
Measurement Basics
Signal Processing for MIMO Measurements (IEEE 802.11ac, n)
Equation 4-21: Symbol error vector magnitude
The advantage of this method is that no estimate of the reference signal is needed, but
the I/Q offset and gain imbalance values are not estimated in a joint estimation procedure. Therefore, each estimation parameter disturbs the estimation of the other parameter and the accuracy of the estimates is lower than the accuracy of the estimations
achieved by transmit antenna baseband filter (Tx filter) estimation. If the EVM value is
dominated by Gaussian noise this method yields similar results as Cost function for
signal parameters.
The EVM vs Symbol result display shows two traces, each using a different calculation
method, so you can easily compare the results (see "EVM vs Symbol" on page 29).
4.2.3 Literature on the IEEE 802.11b Standard
4.3 Signal Processing for MIMO Measurements (IEEE
[1] Institute of Electrical and Electronic Engineers, Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) specifications, IEEE Std 802.11-1999, Institute of Electrical and
Electronic Engineers, Inc., 1999.
[2] Institute of Electrical and Electronic Engineers, Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) specifications: Higher-Speed Physical Layer Extensions in the
2.4 GHz Band, IEEE Std 802.11b-1999, Institute of Electrical and Electronic Engineers, Inc., 1999.
802.11ac, n)
For measurements according to the IEEE 802.11a, b, g standards, only a single transmit antenna and a single receive antenna are required (SISO = single in, single out).
For measurements according to the IEEE 802.11ac or n standard, the R&S FPS can
measure multiple data streams between multiple transmit antennas and multiple
receive antennas (MIMO = multiple in, multiple out).
As opposed to other Rohde & Schwarz signal and spectrum analyzers, in the
R&S FPS WLAN application, MIMO is not selected as a specific standard. Rather,
when you select the IEEE 802.11ac or n standard, MIMO is automatically available. In
the default configuration, a single transmit antenna and a single receive antenna are
assumed, which corresponds to the common SISO setup.
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Measurement Basics
Signal Processing for MIMO Measurements (IEEE 802.11ac, n)
Basic technologies
Some basic technologies used in MIMO systems are introduced briefly here.
For more detailed information, see the following application notes, available from the
Rohde & Schwarz website:
1MA142: "Introduction to MIMO"
1MA192: 802.11ac Technology Introduction
MIMO systems use transmit diversity or space-division multiplexing, or both. With
transmit diversity, a bit stream is transmitted simultaneously via two antennas, but
with different coding in each case. This improves the signal-to-noise ratio and the cell
edge capacity.
For space-division multiplexing, multiple (different) data streams are sent simultaneously from the transmit antennas. Each receive antenna captures the superposition of
all transmit antennas. In addition, channel effects caused by reflections and scattering
etc., are added to the received signals. The receiver determines the originally sent
symbols by multiplying the received symbols with the inverted channel matrix (that is,
the mapping between the streams and the transmit antennas, see Chapter 4.3.2, "Spa-
tial Mapping", on page 69).
Using space-division multiplexing, the transmitted data rates can be increased significantly by using additional antennas.
To reduce the correlation between the propagation paths, the transmit antenna can
delay all of the transmission signals except one. This method is referred to as cyclic
delay diversity or cyclic delay shift.
The basis of the majority of the applications for broadband transmission is the OFDM
method. In contrast to single-carrier methods, an OFDM signal is a combination of
many orthogonal, separately modulated carriers. Since the data is transmitted in parallel, the symbol length is significantly smaller than in single-carrier methods with identical transmission rates.
Signal processing chain
In a test setup with multiple antennas, the R&S FPS is likely to receive multiple spatial
streams, one from each antenna. Each stream has gone through a variety of transformations during transmission. The signal processing chain is displayed in Figure 4-3,
starting with the creation of the spatial streams in the transmitting device, through the
wireless transmission and ending with the merging of the spatial streams in the receiving device. This processing chain has been defined by IEEE.
The following figure shows the basic processing steps performed by the transmit
antenna and the complementary blocks in reverse order applied at the receive
antenna:
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Measurement Basics
Signal Processing for MIMO Measurements (IEEE 802.11ac, n)
Figure 4-3: Data flow from the transmit antenna to the receive antenna
4.3.1 Space-Time Block Coding (STBC)
The coded bits to be transmitted are modulated to create a data stream, referred to as
a spatial stream, by the stream parser in the transmitting device under test (see Fig-
ure 4-3).
The Space-Time Block Encoder (STBC) implements the transmit diversity technique
(see "Basic technologies" on page 68). It creates multiple copies of the data streams,
each encoded differently, which can then be transmitted by a number of antennas.
To do so, the STBC encodes only the data carriers in the spatial stream using a matrix.
Each row in the matrix represents an OFDM symbol and each column represents one
antenna's transmissions over time (thus the term space-time encoder). This means
each block represents the same data, but with a different coding. The resulting blocks
are referred to as space-time streams (STS). Each stream is sent to a different Tx
antenna. This diversity coding increases the signal-to-noise ratio at the receive
antenna. The pilot carriers are inserted after the data carriers went through the STBC.
Thus, only the data carriers are decoded by the analyzer to determine characteristics
of the demodulated data (see also Figure 4-6).
In order to transmit the space-time streams, two or more antennas are required by the
sender, and one or more antennas are required by the receive antenna.
4.3.2 Spatial Mapping
The Spatial Encoder (see Figure 4-3) is responsible for the spatial multiplexing. It
defines the mapping between the streams and the transmit antennas - referred to as
spatial mapping - or as a matrix: the spatial mapping matrix.
In the R&S FPS WLAN application, the mapping can be defined using the following
methods:
●
Direct mapping: one single data stream is mapped to an exclusive Tx antenna
(The spatial matrix contains "1" on the diagonal and otherwise zeros.)
●
Spatial Expansion: multiple (different) data streams are assigned to each antenna
in a defined pattern
●
User-defined mapping: the data streams are mapped to the antennas by a userdefined matrix
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4
1
44
11
4
1
.
.
4.,..1.,
..
..
4.,..1.,
.
.
StreamSTS
StreamSTS
STSTxSTSTx
STSTxSTSTx
StreamTx
StreamTx
Measurement Basics
Signal Processing for MIMO Measurements (IEEE 802.11ac, n)
User-defined spatial mapping
You can define your own spatial mapping between streams and Tx antennas.
For each antenna (Tx1..4), the complex element of each STS-stream is defined. The
upper value is the real part of the complex element. The lower value is the imaginary
part of the complex element.
Additionally, a "Time Shift" can be defined for cyclic delay diversity (CSD).
The stream for each antenna is calculated as:
4.3.3 Physical vs Effective Channels
The effective channel refers to the transmission path starting from the space-time
stream and ending at the receive antenna. It is the product of the following components:
●
the spatial mapping
●
the crosstalk inside the device under test (DUT) transmission paths
●
the crosstalk of the channel between the transmit antennas and the receive antennas
For each space-time stream, at least one training field (the (V)HT-LTF) is included in
every PPDU preamble (see Figure 4-4). Each sender antenna transmits these training
fields, which are known by the receive antenna. The effective channel can be calculated from the received (and known) (V)HT-LTF symbols of the preamble, without knowledge of the spatial mapping matrix or the physical channel. Thus, the effective channel
can always be calculated.
Figure 4-4: Training fields (TF) in the preamble of PPDUs in IEEE 802.11n standard
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Measurement Basics
Signal Processing for MIMO Measurements (IEEE 802.11ac, n)
The effective channel is sufficient to calculate the EVM, the constellation diagram and
the bitstream results of the measured signal, so these results are always available.
The physical channel refers to the transmission path starting from the transmit
antenna streams and ending at the receive antenna. It is the product of the following
components:
●
the crosstalk inside the device under test (DUT) transmission paths
●
the crosstalk of the channel between the transmit antennas and the receive antennas
The physical channel is derived from the effective channel using the inverted spatial
mapping matrix Q:
H
= H
phy
-1
Q
eff
Thus, if the spatial mapping matrix cannot be inverted, the physical channel cannot be
calculated. This may be the case, for example, if the signal contains fewer streams
than Rx antenna signals, or if the spatial matrix is close to numerical singularity.
In this case, results that are based on the transmit antenna such as I/Q offset, gain
imbalance and quadrature offset are not available.
Crosstalk in estimated channels
Note that the estimated channel transfer function contains crosstalk from various sources, for example:
●
from the transmission paths inside the DUT
●
from the connection between the analyzer and the DUT
●
from the analyzer itself
The crosstalk from the analyzer can be neglected. If the analyzer and DUT are connected by cable, this source of crosstalk can also be neglected. For further information on
crosstalk see Chapter 4.3.6, "Crosstalk and Spectrum Flatness", on page 75.
4.3.4 Capturing Data from MIMO Antennas
The primary purpose of many test applications that verify design parameters, or are
used in production, is to determine if the transmitted signals adhere to the relevant
standards and whether the physical characteristics fall within the specified limits. In
such cases there is no need to measure the various transmit paths simultaneously.
Instead, they can either be tested as single antenna measurements, or sequentially
(with restrictions, see also Chapter 4.3.4.1, "Sequential MIMO Measurement",
on page 73). Then only one analyzer is needed to measure parameters such as error
vector magnitude (EVM), power and I/Q imbalance.
Measurements that have to be carried out for development or certification testing are
significantly more extensive. In order to fully reproduce the data in transmit signals or
analyze the crosstalk between the antennas, for example, measurements must be performed simultaneously on all antennas. One analyzer is still sufficient if the system is
using transmit diversity (multiple input single output – MISO). However, space-division
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Measurement Basics
Signal Processing for MIMO Measurements (IEEE 802.11ac, n)
multiplexing requires two or more analyzers to calculate the precoding matrix and
demodulate the signals.
The R&S FPS WLAN application provides the following methods to capture data from
the MIMO antennas:
●
Simultaneous MIMO operation
The data streams are measured simultaneously by multiple analyzers. One of the
analyzers is defined as a master, which receives the I/Q data from the other analyzers (the slaves). The IP addresses of each slave analyzer must be provided to
the master. The only function of the slaves is to record the data that is then accumulated centrally by the master.
(Note that only the MIMO master analyzer requires the R&S FPS-K91n or ac
option. The slave analyzers do not require a R&S FPS WLAN application.)
The number of Tx antennas on the DUT defines the number of analyzers required
for this measurement setup.
Tip: Use the master's trigger output (see Chapter 4.8.5, "Trigger Synchronization
Using the Master's Trigger Output", on page 82) or an R&S Z11 trigger box (see
Chapter 4.8.6, "Trigger Synchronization Using an R&S FS-Z11 Trigger Unit",
on page 82) to send the same trigger signal to all devices.
The master calculates the measurement results based on the I/Q data captured by
all analyzers (master and slaves) and displays them in the selected result displays.
●
Sequential using open switch platform
The data streams are measured sequentially by a single analyzer connected to an
additional switch platform that switches between antenna signals. No manual interaction is necessary during the measurement. The R&S FPS WLAN application
captures the I/Q data for all antennas sequentially and calculates and displays the
results (individually for each data stream) in the selected result displays automatically.
A single analyzer and the Rohde & Schwarz OSP Switch Platform is required to
measure the multiple DUT Tx antennas (the switch platform must be fitted with at
least one R&S®OSP-B101 option; the number depends on the number of Tx
antennas to measure). The IP address of the OSP and the used module (configuration bank) must be defined on the analyzer; the required connections between
the DUT Tx antennas, the switch box and the analyzer are indicated in the MIMO
"Signal Capture" dialog box.
For important restrictions concerning sequential measurement see Chap-
ter 4.3.4.1, "Sequential MIMO Measurement", on page 73.
●
Sequential using manual operation
The data streams are captured sequentially by a single analyzer. The antenna signals must be connected to the single analyzer input sequentially by the user.
In the R&S FPS WLAN application, individual capture buffers are provided (and
displayed) for each antenna input source, so that results for the individual data
streams can be calculated. The user must initiate data capturing for each antenna
and result calculation for all data streams manually.
For important restrictions concerning sequential measurement see Chap-
ter 4.3.4.1, "Sequential MIMO Measurement", on page 73.
●
Single antenna measurement
The data from the Tx antenna is measured and evaluated as a single antenna
(SISO) measurement ("DUT MIMO configuration" = "1 Tx antenna").
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4.3.4.1 Sequential MIMO Measurement
Sequential MIMO measurement allows for MIMO analysis with a single analyzer by
capturing the receive antennas one after another (sequentially). However, sequential
MIMO measurement requires each Tx antenna to transmit the same PPDU over time.
(The PPDU content from different Tx antennas, on the other hand, may be different.) If
this requirement can not be fulfilled, use the simultaneous MIMO capture method (see
Chapter 4.3.4, "Capturing Data from MIMO Antennas", on page 71).
In addition, the following PPDU attributes must be identical for ALL antennas:
●
PPDU length
●
PPDU type
●
Channel bandwidth
●
MCS Index
●
Guard Interval Length
●
Number of STBC Streams
●
Number of Extension Streams
Thus, for each PPDU the Signal Field bit vector has to be identical for ALL antennas!
Figure 4-5: Basic principle of “Sequential MIMO Measurement” with 2 receive antennas
Note that, additionally, the data contents of the sent PPDU
same for each Tx antenna, but this is not checked. Thus, useless results are returned if
different data was sent.
To provide identical PPDU content for each Tx antenna in the measurement, you can
use the same pseudo-random bit sequence (PRBS) with the same PRBS seed (initial
bit sequence), for example, when generating the useful data for the PPDU.
4.3.5 Calculating Results
When you analyze a WLAN signal in a MIMO setup, the R&S FPS acts as the receiving device. Since most measurement results have to be calculated at a particular stage
in the processing chain, the R&S FPS WLAN application has to do the same decoding
that the receive antenna does.
payloads must also be the
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The following diagram takes a closer look at the processing chain and the results at its
individual stages.
Figure 4-6: Results at individual processing stages
Receive antenna results
The R&S FPS WLAN application can determine receive antenna results directly from
the captured data at the receive antenna, namely:
●
PPDU Power
●
Crest factor
For all other results, the R&S FPS WLAN application has to revert the processing
steps to determine the signal characteristics at those stages.
Transmit antenna results (based on the physical channel)
If the R&S FPS WLAN application can determine the physical channel (see Chap-
ter 4.3.3, "Physical vs Effective Channels", on page 70), it can evaluate the following
results:
●
Channel Flatness (based on the physical channel)
●
Group Delay (based on the physical channel)
●
I/Q Offset
●
Quadrature Offset
●
Gain Imbalance
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Space-time stream results (based on the effective channel)
If the application knows the effective channel (see Chapter 4.3.3, "Physical vs Effective
Channels", on page 70), it can evaluate the following results:
●
Channel Flatness (based on the effective channel)
●
Group Delay (based on the effective channel)
●
EVM of pilot carriers
●
Constellation of pilot carriers
●
Bitstream of pilot carriers
Spatial stream results
If space-time encoding is implemented, the demodulated data must first be decoded to
determine the following results:
●
EVM of data carriers
●
Constellation diagram
●
Bitstream
The pilot carriers are inserted directly after the data carriers went through the STBC
(see also Chapter 4.3.1, "Space-Time Block Coding (STBC)", on page 69). Thus, only
the data carriers need to be decoded by the analyzer to determine characteristics of
the demodulated data. Because of this approach to calculate the EVM, Constellation
and Bitstream results, you might get results for a different number of streams for pilots
and data carriers if STBC is applied.
4.3.6 Crosstalk and Spectrum Flatness
In contrast to the SISO measurements in previous Rohde & Schwarz signal and spectrum analyzers, the spectrum flatness trace is no longer normalized to 0 dB (scaled by
the mean gain of all carriers).
For MIMO there may be different gains in the transmission paths and you do not want
to lose the relation between these transmission paths. For example, in a MIMO transmission path matrix we have paths carrying power (usually the diagonal elements for
the transmitted streams), but also elements with only residual crosstalk power. The
power distribution of the transmission matrix depends on the spatial mapping of the
transmitted streams. But even if all matrix elements carry power, the gains may be different. This is the reason why the traces are no longer scaled to 0 dB. Although the
absolute gain of the Spectrum Flatness is not of interrest, it is now maintained in order
to show the different gains in the transmission matrix elements. Nevertheless, the limit
lines are still symmetric to the mean trace, individually for each element of the transmission matrix.
By default, full MIMO equalizing is performed by the R&S FPS WLAN application.
However, you can deactivate compensation for crosstalk (see "Compensate Cross-
talk(MIMO only)" on page 119). In this case, simple main path equalizing is performed
only for direct connections between Tx and Rx antennas, disregarding ancillary trans-
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Recognized vs. Analyzed PPDUs
mission between the main paths (crosstalk). This is useful to investigate the effects of
crosstalk on results such as EVM.
4.4 Channels and Carriers
In an OFDM system such as WLAN, the channel is divided into carriers using FFT /
IFFT. Depending on the channel bandwidth, the FFT window varies between 64 and
512 (see also Chapter 4.6, "Demodulation Parameters - Logical Filters", on page 77).
Some of these carriers can be used (active carriers), others are inactive (e.g. guard
carriers at the edges). The channel can then be determined using the active carriers as
known points; inactive carriers are interpolated.
4.5 Recognized vs. Analyzed PPDUs
A PPDU in a WLAN signal consists of the following parts:
(For IEEE 802.11n see also Figure 4-4)
●
Preamble
Information required to recognize the PPDU within the signal, for example training
fields
●
Signal Field
Information on the modulation used for transmission of the useful data
●
Payload
The useful data
During signal processing, PPDUs are recognized by their preamble symbols. The recognized PPDUs and the information on the modulation used for transmission of the
useful data are shown in the "Signal Field" result display (see "Signal Field"
on page 43).
Not all of the recognized PPDUs are analyzed. Some are dismissed because the
PPDU parameters do not match the user-defined demodulation settings, which act as
a logical filter (see also Chapter 4.6, "Demodulation Parameters - Logical Filters",
on page 77). Others may be dismissed because they contain too many or too few
payload symbols (as defined by the user), or due to other irregularities or inconsistency.
Dismissed PPDUs are indicated as such in the "Signal Field" result display (highlighted
red, with a reason for dismissal).
PPDUs with detected inconsistencies are indicated by orange highlighting and a warning in the "Signal Field" result display, but are nevertheless analyzed and included in
statistical and global evaluations.
The remaining correct PPDUs are highlighted green in the "Magnitude Capture" buffer
and "Signal Field" result displays and analyzed according to the current user settings.
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Demodulation Parameters - Logical Filters
Example:
The evaluation range is configured to take the "Source of Payload Length" from the
signal field. If the power period detected for a PPDU deviates from the PPDU length
coded in the signal field, a warning is assigned to this PPDU. The decoded signal field
length is used to analyze the PPDU. The decoded and measured PPDU length
together with the appropriate information is shown in the "Signal Field" result display.
4.6 Demodulation Parameters - Logical Filters
The demodulation settings define which PPDUs are to be analyzed, thus they define a
logical filter. They can either be defined using specific values or according to the first
measured PPDU.
Which of the WLAN demodulation parameter values are supported depends on the
selected digital standard, some are also interdependant.
Table 4-1: Supported modulation formats, PPDU formats and channel bandwidths depending on
Standard Modulation formats PPDU formats Channel bandwidths
IEEE 802.11a,
g (OFDM), j, p
IEEE 802.11ac 16QAM
standard
BPSK (6 Mbps & 9 Mbps)
QPSK (12 Mbps &
18 Mbps)
16QAM (24 Mbps &
36 Mbps)
64QAM (48 Mbps &
54 Mbps)
64QAM
256QAM
1024QAM
Non-HT
Short PPDU
Long PPDU
VHT
5 MHz, 10 MHz, 20 MHz
20 MHz*), 40 MHz*), 80 MHz*),
160 MHz
*)
*)
IEEE 802.11b,
g (DSSS)
*)
: requires R&S FPS bandwidth extension option, see Chapter A.1, "Sample Rate and Maximum Usable
I/Q Bandwidth for RF Input", on page 339
DBPSK (1 Mbps)
DQPSK (2 Mbps)
CCK (5.5 Mbps &
11 Mbps)
PBCC (5.5 Mbps &
11 Mbps)
Short PPDU
Long PPDU
22 MHz
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Basics on Input from I/Q Data Files
Standard Modulation formats PPDU formats Channel bandwidths
IEEE 802.11n SISO:
BPSK (6.5, 7.2, 13.5 &
15 Mbps)
QPSK (13, 14.4, 19.5,
21.7, 27, 30, 40,5 &
45 Mbps)
16QAM (26, 28.9, 39, 43.3,
54, 60, 81 & 90 Mbps)
64QAM (52, 57.8, 58.5, 65,
72.2, 108, 121.5, 135, 120,
135 & 150 Mbps)
MIMO:
depends on the MCS index
*)
: requires R&S FPS bandwidth extension option, see Chapter A.1, "Sample Rate and Maximum Usable
I/Q Bandwidth for RF Input", on page 339
HT-MF (Mixed format)
HT-GF (Greenfield format)
20 MHz*), 40 MHz
*)
4.7 Basics on Input from I/Q Data Files
The I/Q data to be evaluated in a particular R&S FPS application can not only be captured by the application itself, it can also be loaded from a file, provided it has the correct format. The file is then used as the input source for the application.
For example, you can capture I/Q data using the I/Q Analyzer application, store it to a
file, and then analyze the signal parameters for that data later using the Pulse application (if available).
The I/Q data must be stored in a format with the file extension .iq.tar. For a detailed
description see Chapter A.2, "I/Q Data File Format (iq-tar)", on page 343.
An application note on converting Rohde & Schwarz I/Q data files is available from the
Rohde & Schwarz website:
1EF85: Converting R&S I/Q data files
As opposed to importing data from an I/Q data file using the import functions provided
by some R&S FPS applications (e.g. the I/Q Analyzer or the R&S FPS VSA application), the data is not only stored temporarily in the capture buffer, where it overwrites
the current measurement data and is in turn overwritten by a new measurement.
Instead, the stored I/Q data remains available as input for any number of subsequent
measurements. Furthermore, the (temporary) data import requires the current measurement settings in the current application to match the settings that were applied
when the measurement results were stored (possibly in a different application). When
the data is used as an input source, however, the data acquisition settings in the current application (attenuation, center frequency, measurement bandwidth, sample rate)
can be ignored. As a result, these settings cannot be changed in the current application. Only the measurement time can be decreased, in order to perform measurements
on an extract of the available data (from the beginning of the file) only.
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When using input from an I/Q data file, the RUN SINGLE function starts a single measurement (i.e. analysis) of the stored I/Q data, while the RUN CONT function repeatedly analyzes the same data from the file.
Sample iq.tar files
If you have the optional R&S FPS VSA application (R&S FPS-K70), some sample
iq.tar files are provided in the C:/R_S/Instr/user/vsa/DemoSignals directory
on the R&S FPS.
Pre-trigger and post-trigger samples
In applications that use pre-triggers or post-triggers, if no pre-trigger or post-trigger
samples are specified in the I/Q data file, or too few trigger samples are provided to
satisfy the requirements of the application, the missing pre- or post-trigger values are
filled up with zeros. Superfluous samples in the file are dropped, if necessary. For pretrigger samples, values are filled up or omitted at the beginning of the capture buffer,
for post-trigger samples, values are filled up or omitted at the end of the capture buffer.
4.8 Triggered Measurements
In a basic measurement with default settings, the measurement is started immediately.
However, sometimes you want the measurement to start only when a specific condition
is fulfilled, for example a signal level is exceeded, or in certain time intervals. For these
cases you can define a trigger for the measurement. In FFT sweep mode, the trigger
defines when the data acquisition starts for the FFT conversion.
An "Offset" can be defined to delay the measurement after the trigger event, or to
include data before the actual trigger event in time domain measurements (pre-trigger
offset).
For complex tasks, advanced trigger settings are available:
●
Hysteresis to avoid unwanted trigger events caused by noise
●
Holdoff to define exactly which trigger event will cause the trigger in a jittering signal
● Trigger Offset.......................................................................................................... 79
● Trigger Hysteresis...................................................................................................80
● Trigger Drop-Out Time............................................................................................80
● Trigger Holdoff........................................................................................................ 81
● Trigger Synchronization Using the Master's Trigger Output................................... 82
● Trigger Synchronization Using an R&S FS-Z11 Trigger Unit..................................82
4.8.1 Trigger Offset
An offset can be defined to delay the measurement after the trigger event, or to include
data before the actual trigger event in time domain measurements (pre-trigger offset).
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Pre-trigger offsets are possible because the R&S FPS captures data continuously in
the time domain, even before the trigger occurs.
See " Trigger Offset " on page 106.
4.8.2 Trigger Hysteresis
Setting a hysteresis for the trigger helps avoid unwanted trigger events caused by
noise, for example. The hysteresis is a threshold to the trigger level that the signal
must fall below on a rising slope or rise above on a falling slope before another trigger
event occurs.
Example:
In the following example, the second possible trigger event is ignored as the signal
does not exceed the hysteresis (threshold) before it reaches the trigger level again on
the rising edge. On the falling edge, however, two trigger events occur as the signal
exceeds the hysteresis before it falls to the trigger level the second time.
Trigger level
Trigger
hysteresis
T
T
T
T
Figure 4-7: Effects of the trigger hysteresis
See
" Hysteresis " on page 107
4.8.3 Trigger Drop-Out Time
If a modulated signal is instable and produces occasional "drop-outs" during a burst,
you can define a minimum duration that the input signal must stay below the trigger
level before triggering again. This is called the "drop-out" time. Defining a dropout time
helps you stabilize triggering when the analyzer is triggering on undesired events.
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T
T T
Drop-Out
Figure 4-8: Effect of the trigger drop-out time
See " Drop-Out Time " on page 106.
Drop-out times for falling edge triggers
If a trigger is set to a falling edge ( "Slope" = "Falling" , see " Slope " on page 107) the
measurement is to start when the power level falls below a certain level. This is useful,
for example, to trigger at the end of a burst, similar to triggering on the rising edge for
the beginning of a burst.
If a drop-out time is defined, the power level must remain below the trigger level at
least for the duration of the drop-out time (as defined above). However, if a drop-out
time is defined that is longer than the pulse width, this condition cannot be met before
the final pulse, so a trigger event will not occur until the pulsed signal is over!
T
T
T
Figure 4-9: Trigger drop-out time for falling edge trigger
For gated measurements, a combination of a falling edge trigger and a drop-out time is
generally not allowed.
4.8.4 Trigger Holdoff
The trigger holdoff defines a waiting period before the next trigger after the current one
will be recognized.
Drop-Out
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Frame 1
T
TT
T
Frame 2
Holdoff
Figure 4-10: Effect of the trigger holdoff
See " Trigger Holdoff " on page 107.
4.8.5 Trigger Synchronization Using the Master's Trigger Output
For MIMO measurements in which the data from the multiple antennas is captured
simultaneously by multiple analyzers (see "Simultaneous Signal Capture Setup"
on page 109, the data streams to be analyzed must be synchronized in time. One possibility to ensure that all analyzers start capturing I/Q data at the same time is using the
master's trigger output functionality.
The R&S FPS has variable input/output connectors for trigger signals. If you set the
master's TRIGGER 2 INPUT/OUTPUT connector to "Device Triggered" output, and
connect it to the slaves' trigger input connectors, the master R&S FPS sends its trigger
event signal to any connected slaves. The slaves are automatically configured to use
the trigger source "External" . The master itself can be configured to use any of the following trigger sources:
●
External
●
I/Q Power
●
IF Power
●
RF Power
●
Power Sensor
4.8.6 Trigger Synchronization Using an R&S FS-Z11 Trigger Unit
For MIMO measurements in which the data from the multiple antennas is captured
simultaneously by multiple analyzers (see "Simultaneous Signal Capture Setup"
on page 109, the data streams to be analyzed must be synchronized in time. The R&S
FS-Z11 Trigger Unit can ensure that all analyzers start capturing I/Q data at the same
time. Compared to using the master's trigger out function, using the Trigger Unit pro-
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vides a more accurate synchronization of the slaves. However, it requires the additional hardware.
The Trigger Unit is connected to the DUT and all involved analyzers. Then the Trigger
Unit can be used in the following operating modes:
●
External mode: If the DUT has a trigger output, the trigger signal from the DUT
triggers all analyzers simultaneously.
The DUT's TRIGGER OUTPUT is connected to the Trigger Unit's TRIG INPUT
connector. Each of the Trigger Unit's TRIG OUT connectors is connected to one of
the analyzer's TRIGGER INPUT connectors.
●
Free Run mode: This mode is used if no trigger signal is available. The master
analyzer sends a trigger impulse to the Trigger Unit to start the measurement as
soon as all slave analyzers are ready to measure.
The NOISE SOURCE output of the master analyzer is connected to the Trigger
Unit's NOISE SOURCE input. Each of the Trigger Unit's TRIG OUT connectors is
connected to one of the analyzer's TRIGGER INPUT connectors. When the master
analyzer sends a signal to the Trigger Unit via its NOISE SOURCE output, the Trigger Unit triggers all analyzers simultaneously via its TRIGGER OUTPUT.
●
Manual mode: a trigger is generated by the Trigger Unit and triggers all analyzers
simultaneously. No connection to the DUT is required.
Each of the Trigger Unit's TRIG OUT connectors is connected to one of the analyzer's TRIGGER INPUT connectors. A trigger signal is generated when you press
(release) the TRIG MANUAL button on the Trigger unit.
Note: In manual mode you must turn on the NOISE SOURCE output of the master
analyzer manually (see the manual of the analyzer)!
A Trigger Unit is activated in the Trigger Source Settings. The required connections
between the analyzers, the trigger unit, and the DUT are visualized in the dialog box.
The NOISE SOURCE output of the master analyzer must be connected to the Trigger
Unit's NOISE SOURCE input for all operating modes to supply the power for the Trigger Unit.
For more detailed information on the R&S FS-Z11 Trigger Unit and the required connections, see the "R&S FS-Z11 Trigger Unit Manual".
4.9 WLAN I/Q Measurements in MSRA Operating Mode
The R&S FPS WLAN application can also be used to analyze I/Q data in MSRA operating mode.
In MSRA operating mode, the IEEE 802.11b and g (DSSS) standards are not supported.
In MSRA operating mode, only the MSRA Master actually captures data; the MSRA
applications receive an extract of the captured data for analysis, referred to as the
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application data. For the R&S FPS WLAN application in MSRA operating mode, the
application data range is defined by the same settings used to define the signal capture in Signal and Spectrum Analyzer mode. In addition, a capture offset can be
defined, i.e. an offset from the start of the captured data to the start of the analysis
interval for the WLAN I/Q measurement.
Data coverage for each active application
Generally, if a signal contains multiple data channels for multiple standards, separate
applications are used to analyze each data channel. Thus, it is of interest to know
which application is analyzing which data channel. The MSRA Master display indicates
the data covered by each application, restricted to the channel bandwidth used by the
corresponding standard, by vertical blue lines labeled with the application name.
Analysis interval
However, the individual result displays of the application need not analyze the complete data range. The data range that is actually analyzed by the individual result display is referred to as the analysis interval.
In the R&S FPS WLAN application the analysis interval is automatically determined
according to the selected channel, carrier or PPDU to analyze which is defined for the
evaluation range, depending on the result display. The analysis interval can not be edited directly in the R&S FPS WLAN application, but is changed automatically when you
change the evaluation range.
Analysis line
A frequent question when analyzing multi-standard signals is how each data channel is
correlated (in time) to others. Thus, an analysis line has been introduced. The analysis
line is a common time marker for all MSRA slave applications. It can be positioned in
any MSRA slave application or the MSRA Master and is then adjusted in all other slave
applications. Thus, you can easily analyze the results at a specific time in the measurement in all slave applications and determine correlations.
If the marked point in time is contained in the analysis interval of the slave application,
the line is indicated in all time-based result displays, such as time, symbol, slot or bit
diagrams. By default, the analysis line is displayed, however, it can be hidden from
view manually. In all result displays, the "AL" label in the window title bar indicates
whether the analysis line lies within the analysis interval or not:
●
orange "AL": the line lies within the interval
●
white "AL": the line lies within the interval, but is not displayed (hidden)
●
no "AL": the line lies outside the interval
The analysis line is displayed in the following result displays.
●
Magnitude Capture
●
Power vs Time
●
EVM vs Symbol
For details on the MSRA operating mode see the R&S FPS MSRA User Manual.
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5 Configuration
Configuration
Multiple Measurement Channels and Sequencer Function
The default WLAN I/Q measurement captures the I/Q data from the WLAN signal and
determines various characteristic signal parameters such as the modulation accuracy,
spectrum flatness, center frequency tolerance and symbol clock tolerance in just one
measurement (see Chapter 3.1, "WLAN I/Q Measurement (Modulation Accuracy, Flat-
ness and Tolerance)", on page 11)
Other parameters specified in the WLAN 802.11 standard must be determined in separate measurements (see Chapter 5.4, "Frequency Sweep Measurements",
on page 149).
The settings required to configure each of these measurements are described here.
Selecting the measurement type
► To select a different measurement type, do one of the following:
● Select the "Overview" softkey. In the "Overview", select the "Select Measurement" button. Select the required measurement.
● Press the MEAS key. In the "Select Measurement" dialog box, select the
required measurement.
● Multiple Measurement Channels and Sequencer Function.................................... 85
● Display Configuration..............................................................................................87
● WLAN I/Q Measurement Configuration...................................................................87
● Frequency Sweep Measurements........................................................................ 149
5.1 Multiple Measurement Channels and Sequencer Function
When you activate an application, a new measurement channel is created which determines the measurement settings for that application. These settings include the input
source, the type of data to be processed (I/Q or RF data), frequency and level settings,
measurement functions etc. If you want to perform the same measurement but with different center frequencies, for instance, or process the same input data with different
measurement functions, there are two ways to do so:
●
Change the settings in the measurement channel for each measurement scenario.
In this case the results of each measurement are updated each time you change
the settings and you cannot compare them or analyze them together without storing them on an external medium.
●
Activate a new measurement channel for the same application.
In the latter case, the two measurement scenarios with their different settings are
displayed simultaneously in separate tabs, and you can switch between the tabs to
compare the results.
For example, you can activate one WLAN measurement channel to perform a
WLAN modulation accuracy measurement, and a second channel to perform an
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Multiple Measurement Channels and Sequencer Function
SEM measurement using the same WLAN input source. Then you can monitor all
results at the same time in the "MultiView" tab.
The number of channels that can be configured at the same time depends on the available memory on the instrument.
Only one measurement can be performed on the R&S FPS at any time. If one measurement is running and you start another, or switch to another channel, the first measurement is stopped. In order to perform the different measurements you configured in
multiple channels, you must switch from one tab to another.
However, you can enable a Sequencer function that automatically calls up each activated measurement channel in turn. This means the measurements configured in the
channels are performed one after the other in the order of the tabs. The currently
active measurement is indicated by a
the individual channels are updated in the corresponding tab (as well as the "MultiView") as the measurements are performed. Sequencer operation is independent of
the currently displayed tab; for example, you can analyze the SEM measurement while
the modulation accuracy measurement is being performed by the Sequencer.
For details on the Sequencer function see the R&S FPS User Manual.
symbol in the tab label. The result displays of
The Sequencer functions are only available in the "MultiView" tab.
Sequencer State .......................................................................................................... 86
Sequencer Mode ..........................................................................................................86
Sequencer State
Activates or deactivates the Sequencer. If activated, sequential operation according to
the selected Sequencer mode is started immediately.
Remote command:
SYSTem:SEQuencer on page 280
INITiate<n>:SEQuencer:IMMediate on page 278
INITiate<n>:SEQuencer:ABORt on page 277
Sequencer Mode
Defines how often which measurements are performed. The currently selected mode
softkey is highlighted blue. During an active Sequencer process, the selected mode
softkey is highlighted orange.
"Single Sequence"
Each measurement is performed once, until all measurements in all
active channels have been performed.
"Continuous Sequence"
The measurements in each active channel are performed one after
the other, repeatedly, in the same order, until sequential operation is
stopped.
This is the default Sequencer mode.
Remote command:
INITiate<n>:SEQuencer:MODE on page 278
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WLAN I/Q Measurement Configuration
5.2 Display Configuration
The measurement results can be displayed using various evaluation methods. All evaluation methods available for the R&S FPS WLAN application are displayed in the evaluation bar in SmartGrid mode when you do one of the following:
●
Select the "SmartGrid" icon from the toolbar.
●
Select the "Display Config" button in the "Overview".
●
Select the "Display Config" softkey in any WLAN menu.
Then you can drag one or more evaluations to the display area and configure the layout as required.
Up to 16 evaluation methods can be displayed simultaneously in separate windows.
The WLAN evaluation methods are described in Chapter 3, "Measurements and Result
Displays", on page 11.
To close the SmartGrid mode and restore the previous softkey menu select the
"Close" icon in the righthand corner of the toolbar, or press any key.
For details on working with the SmartGrid see the R&S FPS Getting Started manual.
5.3 WLAN I/Q Measurement Configuration
Access: MODE > "WLAN 802.11"
WLAN 802.11 measurements require a special application on the R&S FPS.
When you activate the R&S FPS WLAN application, an I/Q measurement of the input
signal is started automatically with the default configuration. The "WLAN" menu is displayed and provides access to the most important configuration functions.
The "Span", "Bandwidth", "Lines", and "Marker Functions" menus are not available for
WLAN I/Q measurements.
Multiple access paths to functionality
The easiest way to configure a measurement channel is via the "Overview" dialog box.
Alternatively, you can access the individual dialog boxes from the corresponding menu
items, or via tools in the toolbars, if available.
In this documentation, only the most convenient method of accessing the dialog boxes
is indicated - usually via the "Overview".
● Configuration Overview...........................................................................................88
● Signal Description................................................................................................... 90
● Input and Frontend Settings....................................................................................91
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● Signal Capture (Data Acquisition).........................................................................101
● Slave Application Data (MSRA) ........................................................................... 115
● Synchronization and OFDM Demodulation...........................................................115
● Tracking and Channel Estimation......................................................................... 116
● Demodulation........................................................................................................120
● Evaluation Range..................................................................................................135
● Result Configuration..............................................................................................141
● Automatic Settings................................................................................................ 148
● Sweep Settings..................................................................................................... 148
5.3.1 Configuration Overview
Access: all menus
Throughout the measurement channel configuration, an overview of the most important
currently defined settings is provided in the "Overview".
The "Overview" not only shows the main measurement settings, it also provides quick
access to the main settings dialog boxes. The indicated signal flow shows which
parameters affect which processing stage in the measurement. Thus, you can easily
configure an entire measurement channel from input over processing to output and
analysis by stepping through the dialog boxes as indicated in the "Overview".
The available settings and functions in the "Overview" vary depending on the currently
selected measurement. For frequency sweep measurements see Chapter 5.4, "Fre-
quency Sweep Measurements", on page 149.
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For the WLAN I/Q measurement, the "Overview" provides quick access to the following
configuration dialog boxes (listed in the recommended order of processing):
1. "Select Measurement"
See "Selecting the measurement type" on page 85
2. "Signal Description"
See Chapter 5.3.2, "Signal Description", on page 90
3. "Input/ Frontend"
See and Chapter 5.3.3, "Input and Frontend Settings", on page 91
4. "Signal Capture"
See Chapter 5.3.4, "Signal Capture (Data Acquisition)", on page 101
5. "Synchronization / OFDM demodulation"
See Chapter 5.3.6, "Synchronization and OFDM Demodulation", on page 115
6. "Tracking / Channel Estimation"
See Chapter 5.3.7, "Tracking and Channel Estimation", on page 116
7. "Demodulation"
See Chapter 5.3.8, "Demodulation", on page 120
8. "Evaluation Range"
See Chapter 5.3.9, "Evaluation Range", on page 135
9. "Display Configuration"
See Chapter 5.2, "Display Configuration", on page 87
To configure settings
► Select any button in the "Overview" to open the corresponding dialog box.
Preset Channel
Select the "Preset Channel" button in the lower left-hand corner of the "Overview" to
restore all measurement settings in the current channel to their default values.
Do not confuse the "Preset Channel" button with the PRESET key, which restores the
entire instrument to its default values and thus closes all channels on the R&S FPS
(except for the default channel)!
Remote command:
SYSTem:PRESet:CHANnel[:EXEC] on page 182
Select Measurement
Selects a measurement to be performed.
See "Selecting the measurement type" on page 85.
Specifics for
The channel may contain several windows for different results. Thus, the settings indicated in the "Overview" and configured in the dialog boxes vary depending on the
selected window.
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Select an active window from the "Specifics for" selection list that is displayed in the
"Overview" and in all window-specific configuration dialog boxes.
The "Overview" and dialog boxes are updated to indicate the settings for the selected
window.
5.3.2 Signal Description
Access: "Overview" > "Signal Description"
Or: MEAS CONFIG > "Signal Description"
The signal description provides information on the expected input signal.
Standard........................................................................................................................90
Frequency..................................................................................................................... 90
Tolerance Limit..............................................................................................................90
Standard
Defines the WLAN standard (depending on which WLAN options are installed). The
measurements are performed according to the specified standard with the correct limit
values and limit lines.
Many other WLAN measurement settings depend on the selected standard (see Chap-
ter 4.6, "Demodulation Parameters - Logical Filters", on page 77).
Note: In MSRA operating mode, the IEEE 802.11b and g (DSSS) standards are not
supported.
Remote command:
CONFigure:STANdard on page 190
Frequency
Specifies the center frequency of the signal to be measured.
Remote command:
[SENSe:]FREQuency:CENTer on page 195
Tolerance Limit
Defines the tolerance limit to be used for the measurement. The required tolerance
limit depends on the used standard:
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"Prior IEEE 802.11-2012 Standard"
Tolerance limits are based on the IEEE 802.11 specification prior to
2012.
Default for OFDM standards (except 802.11ac).
"In line with IEEE 802.11-2012 Standard"
Tolerance limits are based on the IEEE 802.11 specification from
2012.
Required for DSSS standards. Also possible for OFDM standards
(except 802.11ac).
"In line with IEEE 802.11ac standard"
Tolerance limits are based on the IEEE 802.11ac specification.
Required by IEEE 802.11ac standard.
Remote command:
CALCulate<n>:LIMit<li>:TOLerance on page 191
5.3.3 Input and Frontend Settings
Access: "Overview" > "Input/Frontend"
Or: MEAS CONFIG > "Input/Frontend"
The R&S FPS can analyze signals from different input sources and provide various
types of output (such as noise or trigger signals).
Importing and Exporting I/Q Data
The I/Q data to be analyzed for WLAN 802.11 can not only be measured by the WLAN
application itself, it can also be imported to the application, provided it has the correct
format. Furthermore, the analyzed I/Q data from the WLAN application can be exported for further analysis in external applications.
See Chapter 7.1, "Import/Export Functions", on page 155.
Frequency, amplitude and y-axis scaling settings represent the "frontend" of the measurement setup.
● Input Source Settings..............................................................................................91
● Output Settings....................................................................................................... 93
● Frequency Settings................................................................................................. 94
● Amplitude Settings.................................................................................................. 95
● Y-Axis Scaling.......................................................................................................100
5.3.3.1 Input Source Settings
Access: "Overview" > "Input/Frontend" > "Input Source"
The input source determines which data the R&S FPS will analyze.
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The default input source for the R&S FPS is "Radio Frequency" , i.e. the signal at the
RF INPUT connector of the R&S FPS. If no additional options are installed, this is the
only available input source.
The Digital I/Q input source is currently not available in the R&S FPS WLAN application.
● Radio Frequency Input............................................................................................92
Radio Frequency Input
Access: "Overview" > "Input/Frontend" > "Input Source" > "Radio Frequency"
Radio Frequency State ................................................................................................ 92
Input Coupling ..............................................................................................................92
Impedance ................................................................................................................... 93
YIG-Preselector ............................................................................................................93
Radio Frequency State
Activates input from the RF INPUT connector.
Remote command:
INPut:SELect on page 193
Input Coupling
The RF input of the R&S FPS can be coupled by alternating current (AC) or direct current (DC).
AC coupling blocks any DC voltage from the input signal. This is the default setting to
prevent damage to the instrument. Very low frequencies in the input signal may be distorted.
However, some specifications require DC coupling. In this case, you must protect the
instrument from damaging DC input voltages manually. For details, refer to the data
sheet.
Remote command:
INPut:COUPling on page 192
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Impedance
For some measurements, the reference impedance for the measured levels of the
R&S FPS can be set to 50 Ω or 75 Ω.
Select 75 Ω if the 50 Ω input impedance is transformed to a higher impedance using a
75 Ω adapter of the RAZ type. (That corresponds to 25Ω in series to the input impedance of the instrument.) The correction value in this case is 1.76 dB = 10 log (75Ω/
50Ω).
Remote command:
INPut:IMPedance on page 193
YIG-Preselector
Activates or deactivates the YIG-preselector, if available on the R&S FPS.
An internal YIG-preselector at the input of the R&S FPS ensures that image frequen-
cies are rejected. However, the YIG-preselector can limit the bandwidth of the I/Q data
and adds some magnitude and phase distortions. You can check the impact in the
Spectrum Flatness and Group Delay result displays.
Note that the YIG-preselector is active only on frequencies greater than 8 GHz. Therefore, switching the YIG-preselector on or off has no effect if the frequency is below that
value.
Remote command:
INPut:FILTer:YIG[:STATe] on page 193
5.3.3.2 Output Settings
Access: INPUT/OUTPUT > "Output"
The R&S FPS can provide output to special connectors for other devices.
For details on connectors, refer to the R&S FPS Getting Started manual, "Front / Rear
Panel View" chapters.
How to provide trigger signals as output is described in detail in the R&S FPS User
Manual.
Noise Source Control....................................................................................................94
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Noise Source Control
The R&S FPS provides a connector (NOISE SOURCE CONTROL) with a 28 V voltage
supply for an external noise source. By switching the supply voltage for an external
noise source on or off in the firmware, you can activate or deactivate the device as
required.
External noise sources are useful when you are measuring power levels that fall below
the noise floor of the R&S FPS itself, for example when measuring the noise level of an
amplifier.
In this case, you can first connect an external noise source (whose noise power level is
known in advance) to the R&S FPS and measure the total noise power. From this
value you can determine the noise power of the R&S FPS. Then when you measure
the power level of the actual DUT, you can deduct the known noise level from the total
power to obtain the power level of the DUT.
Remote command:
DIAGnostic:SERVice:NSOurce on page 195
5.3.3.3 Frequency Settings
Access: "Overview" > "Input/Frontend" > "Frequency"
Center Frequency ........................................................................................................ 94
Center Frequency Stepsize ..........................................................................................94
Frequency Offset ..........................................................................................................95
Center Frequency
Defines the center frequency of the signal in Hertz.
Remote command:
[SENSe:]FREQuency:CENTer on page 195
Center Frequency Stepsize
Defines the step size by which the center frequency is increased or decreased using
the arrow keys.
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When you use the rotary knob the center frequency changes in steps of only 1/10 of
the span.
The step size can be coupled to another value or it can be manually set to a fixed
value.
"= Center"
Sets the step size to the value of the center frequency. The used
value is indicated in the "Value" field.
"Manual"
Defines a fixed step size for the center frequency. Enter the step size
in the "Value" field.
Remote command:
[SENSe:]FREQuency:CENTer:STEP on page 196
Frequency Offset
Shifts the displayed frequency range along the x-axis by the defined offset.
This parameter has no effect on the instrument's hardware, or on the captured data or
on data processing. It is simply a manipulation of the final results in which absolute frequency values are displayed. Thus, the x-axis of a spectrum display is shifted by a
constant offset if it shows absolute frequencies, but not if it shows frequencies relative
to the signal's center frequency.
A frequency offset can be used to correct the display of a signal that is slightly distorted
by the measurement setup, for example.
The allowed values range from -100 GHz to 100 GHz. The default setting is 0 Hz.
Note: In MSRA mode, this function is only available for the MSRA Master.
Remote command:
[SENSe:]FREQuency:OFFSet on page 197
5.3.3.4 Amplitude Settings
Access: "Overview" > "Input/Frontend" > "Amplitude"
Amplitude settings determine how the R&S FPS must process or display the expected
input power levels.
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Reference Level Settings..............................................................................................96
└ Reference Level Mode....................................................................................97
└ Reference Level..............................................................................................97
└ Signal Level (RMS).........................................................................................97
└ Shifting the Display (Offset)............................................................................ 97
└ Unit .................................................................................................................97
└ Setting the Reference Level Automatically (Auto Level).................................98
RF Attenuation.............................................................................................................. 98
└ Attenuation Mode / Value ...............................................................................98
Using Electronic Attenuation ........................................................................................98
Input Settings................................................................................................................ 99
└ Preamplifier (option B22/B24).........................................................................99
└ Input Coupling ................................................................................................99
└ Impedance ..................................................................................................... 99
Reference Level Settings
The reference level defines the expected maximum signal level. Signal levels above
this value may not be measured correctly, which is indicated by the "IF OVLD" status
display.
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Reference Level Mode ← Reference Level Settings
By default, the reference level is automatically adapted to its optimal value for the current input data (continuously). At the same time, the internal attenuators and the preamplifier are adjusted so the signal-to-noise ratio is optimized, while signal compression, clipping and overload conditions are minimized.
In order to define the reference level manually, switch to "Manual" mode. In this case
you must define the following reference level parameters.
Remote command:
CONF:POW:AUTO ON, see CONFigure:POWer:AUTO on page 198
Reference Level ← Reference Level Settings
Defines the expected maximum signal level. Signal levels above this value may not be
measured correctly, which is indicated by the "IF OVLD" status display.
This value is overwritten if "Auto Level" mode is turned on.
Remote command:
DISPlay[:WINDow<n>]:TRACe<t>:Y[:SCALe]:RLEVel on page 199
Signal Level (RMS) ← Reference Level Settings
Specifies the mean power level of the source signal as supplied to the instrument's RF
input. This value is overwritten if "Auto Level" mode is turned on.
Remote command:
CONFigure:POWer:EXPected:RF on page 199
Shifting the Display (Offset) ← Reference Level Settings
Defines an arithmetic level offset. This offset is added to the measured level irrespective of the selected unit. The scaling of the y-axis is changed accordingly.
Define an offset if the signal is attenuated or amplified before it is fed into the R&S FPS
so the application shows correct power results. All displayed power level results will be
shifted by this value.
Note, however, that the Reference Level value ignores the "Reference Level Offset". It
is important to know the actual power level the R&S FPS must handle.
To determine the required offset, consider the external attenuation or gain applied to
the input signal. A positive value indicates that an attenuation took place (R&S FPS
increases the displayed power values) , a negative value indicates an external gain
(R&S FPS decreases the displayed power values).
The setting range is ±200 dB in 0.01 dB steps.
Remote command:
DISPlay[:WINDow<n>]:TRACe<t>:Y[:SCALe]:RLEVel:OFFSet on page 199
Unit ← Reference Level Settings
The R&S FPS measures the signal voltage at the RF input.
The following units are available and directly convertible:
●
dBm
●
dBmV
●
dBμV
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Remote command:
CALCulate<n>:UNIT:POWer on page 198
Setting the Reference Level Automatically (Auto Level) ← Reference Level Settings
Automatically determines the optimal reference level for the current input data. At the
same time, the internal attenuators and the preamplifier are adjusted so the signal-tonoise ratio is optimized, while signal compression, clipping and overload conditions are
minimized.
In order to do so, a level measurement is performed to determine the optimal reference
level.
Remote command:
CONFigure:POWer:AUTO on page 198
RF Attenuation
Defines the attenuation applied to the RF input.
Attenuation Mode / Value ← RF Attenuation
The RF attenuation can be set automatically as a function of the selected reference
level (Auto mode). This ensures that no overload occurs at the RF INPUT connector
for the current reference level. It is the default setting.
By default and when no (optional) electronic attenuation is available, mechanical
attenuation is applied.
In "Manual" mode, you can set the RF attenuation in 1 dB steps (down to 0 dB). Other
entries are rounded to the next integer value. The range is specified in the data sheet.
If the defined reference level cannot be set for the defined RF attenuation, the reference level is adjusted accordingly and the warning "limit reached" is displayed.
NOTICE! Risk of hardware damage due to high power levels. When decreasing the
attenuation manually, ensure that the power level does not exceed the maximum level
allowed at the RF input, as an overload may lead to hardware damage.
Remote command:
INPut:ATTenuation on page 199
INPut:ATTenuation:AUTO on page 200
Using Electronic Attenuation
If the (optional) Electronic Attenuation hardware is installed on the R&S FPS, you can
also activate an electronic attenuator.
In "Auto" mode, the settings are defined automatically; in "Manual" mode, you can
define the mechanical and electronic attenuation separately.
Note: Electronic attenuation is not available for stop frequencies (or center frequencies
in zero span) above 7 GHz.
In "Auto" mode, RF attenuation is provided by the electronic attenuator as much as
possible to reduce the amount of mechanical switching required. Mechanical attenuation may provide a better signal-to-noise ratio, however.
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When you switch off electronic attenuation, the RF attenuation is automatically set to
the same mode (auto/manual) as the electronic attenuation was set to. Thus, the RF
attenuation can be set to automatic mode, and the full attenuation is provided by the
mechanical attenuator, if possible.
The electronic attenuation can be varied in 1 dB steps. If the electronic attenuation is
on, the mechanical attenuation can be varied in 5 dB steps. Other entries are rounded
to the next lower integer value.
If the defined reference level cannot be set for the given attenuation, the reference
level is adjusted accordingly and the warning "limit reached" is displayed in the status
bar.
Remote command:
INPut:EATT:STATe on page 201
INPut:EATT:AUTO on page 201
INPut:EATT on page 200
Input Settings
Some input settings affect the measured amplitude of the signal, as well.
The parameters "Input Coupling" and "Impedance" are identical to those in the "Input"
settings, see Chapter 5.3.3.1, "Input Source Settings", on page 91.
Preamplifier (option B22/B24) ← Input Settings
Switches the preamplifier on and off. If activated, the input signal is amplified by 20 dB.
If option R&S FPS-B22 is installed, the preamplifier is only active below 7 GHz.
If option R&S FPS-B24 is installed, the preamplifier is active for all frequencies.
Remote command:
INPut:GAIN:STATe on page 201
Input Coupling ← Input Settings
The RF input of the R&S FPS can be coupled by alternating current (AC) or direct current (DC).
AC coupling blocks any DC voltage from the input signal. This is the default setting to
prevent damage to the instrument. Very low frequencies in the input signal may be distorted.
However, some specifications require DC coupling. In this case, you must protect the
instrument from damaging DC input voltages manually. For details, refer to the data
sheet.
Remote command:
INPut:COUPling on page 192
Impedance ← Input Settings
For some measurements, the reference impedance for the measured levels of the
R&S FPS can be set to 50 Ω or 75 Ω.
Select 75 Ω if the 50 Ω input impedance is transformed to a higher impedance using a
75 Ω adapter of the RAZ type. (That corresponds to 25Ω in series to the input impedance of the instrument.) The correction value in this case is 1.76 dB = 10 log (75Ω/
50Ω).
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Remote command:
INPut:IMPedance on page 193
5.3.3.5 Y-Axis Scaling
Access: "Overview" > "Amplitude" > "Scale" tab
The individual scaling settings that affect the vertical axis are described here. These
settings are window-specific.
Range .........................................................................................................................100
Ref Level Position ...................................................................................................... 100
Range
Defines the displayed y-axis range in dB.
The default value is 100 dB.
Remote command:
DISPlay[:WINDow<n>]:TRACe<t>:Y[:SCALe] on page 202
Ref Level Position
Defines the reference level position, i.e. the position of the maximum AD converter
value on the level axis in %, where 0 % corresponds to the lower and 100 % to the
upper limit of the diagram.
Remote command:
DISPlay[:WINDow<n>]:TRACe<t>:Y[:SCALe]:RPOSition on page 202
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