R&S® FSV-K91/91n/91ac/91p
WLAN TX Measurements
Operating Manual
(;ÚÚa2)
1176.7649.02 ─ 06
Operating Manual
Test & Measurement
This manual describes the following R&S®FSV/FSVA options:
●
R&S FSV-K91 (1310.8903.02)
●
R&S FSV-K91n (1310.9468.02)
●
R&S FSV-K91ac (1310.8926.02)
●
R&S FSV-K91p (1321.3314.02)
This manual describes the following R&S FSVA/FSV models with firmware version 3.20 and higher:
●
R&S®FSVA4 (1321.3008K05)
●
R&S®FSVA7 (1321.3008K08)
●
R&S®FSVA13 (1321.3008K14)
●
R&S®FSVA30 (1321.3008K31)
●
R&S®FSVA40 (1321.3008K41)
●
R&S®FSV4 (1321.3008K04)
●
R&S®FSV7 (1321.3008K07)
●
R&S®FSV13 (1321.3008K13)
●
R&S®FSV30 (1321.3008K30)
●
R&S®FSV40 (1321.3008K39/1321.3008K40)
It also applies to the following R&S®FSV models. However, note the differences described in Chapter 1.4,
"Notes for Users of R&S FSV 1307.9002Kxx Models", on page 9.
●
R&S®FSV3 (1307.9002K03)
●
R&S®FSV7 (1307.9002K07)
●
R&S®FSV13 (1307.9002K13)
●
R&S®FSV30 (1307.9002K30)
●
R&S®FSV40 (1307.9002K39/1307.9002K40)
© 2016 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 the owners.
The following abbreviations are used throughout this manual: R&S®FSV/FSVA is abbreviated as R&S FSV/FSVA.
R&S® FSV-K91/91n/91ac/91p
Contents
1 Preface.................................................................................................... 5
1.1 Documentation Overview............................................................................................. 5
1.2 Conventions Used in the Documentation...................................................................6
1.3 How to Use the Help System........................................................................................7
1.4 Notes for Users of R&S FSV 1307.9002Kxx Models.................................................. 9
2 Introduction.......................................................................................... 10
3 WLAN TX Measurements.....................................................................11
3.1 Introduction to WLAN 802.11A, AC, B, G, J, N & P TX Tests.................................. 12
3.2 Basic Measurement Examples...................................................................................13
3.3 Signal Processing of the IEEE 802.11a Application................................................ 21
Contents
3.4 Signal Processing of the IEEE 802.11b Application................................................ 29
3.5 802.11b RF Carrier Suppression................................................................................35
3.6 IEEE 802.11n/ac MIMO Measurements......................................................................36
3.7 Signal Field Measurement (IEEE 802.11ac, n (SISO+MIMO)).................................. 39
3.8 Optimized Signal Levels.............................................................................................43
3.9 Measurement Result Types........................................................................................44
3.10 Measurement Settings and Result Displays.............................................................50
4 Instrument Functions WLAN TX Measurements...............................62
4.1 Softkeys of the WLAN TX Menu.................................................................................63
4.2 General Settings Dialog Box (K91)............................................................................87
4.3 Demod Settings Dialog Box....................................................................................... 97
4.4 Softkeys of the Sweep Menu – SWEEP key ...........................................................110
4.5 Softkeys of the Trace Menu – TRAC key................................................................ 111
4.6 Softkeys of the Marker Menu – MKR key (WLAN)..................................................112
4.7 Softkeys of the Marker To Menu – MKR-> key....................................................... 112
4.8 Softkeys of the Lines Menu – LINES key................................................................ 113
4.9 Softkeys of the Input/Output Menu for WLAN Measurements..............................114
5 Remote Commands for WLAN TX Measurements.......................... 116
5.1 Notation......................................................................................................................117
5.2 ABORt Subsystem.................................................................................................... 120
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5.3 CALCulate:BURSt Subsystem (WLAN)...................................................................120
5.4 CALCulate:LIMit Subsystem (WLAN)......................................................................121
5.5 CALCulate:MARKer Subsystem (WLAN)................................................................ 138
5.6 CONFigure Subsystem (WLAN)...............................................................................145
5.7 DISPlay Subsystem (WLAN).................................................................................... 163
5.8 FETCh Subsystem (WLAN)...................................................................................... 168
5.9 FORMat Subsystem.................................................................................................. 176
5.10 INITiate Subsystem................................................................................................... 176
5.11 INPut Subsystem.......................................................................................................177
5.12 INSTrument Subsystem (WLAN)............................................................................. 180
5.13 MMEMory Subsystem (WLAN).................................................................................181
5.14 SENSe Subsystem (WLAN)...................................................................................... 182
5.15 STATus Subsystem (WLAN).................................................................................... 205
Contents
5.16 TRACe Subsystem (WLAN)......................................................................................209
5.17 TRIGger Subsystem (WLAN)....................................................................................218
5.18 UNIT Subsystem (K91)..............................................................................................221
5.19 Status Reporting System (Option R&S FSV-K91).................................................. 222
List of Commands..............................................................................229
Index....................................................................................................235
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1 Preface
Preface
Documentation Overview
1.1 Documentation Overview
This section provides an overview of the R&S FSVA/FSV user documentation. You find
it on the product page at:
www.rohde-schwarz.com/product/FSV > "Downloads" > "Manuals"
Quick start guide
Introduces the R&S FSVA/FSV and describes how to set up and start working with the
product. Includes basic operations, typical measurement examples, and general information, e.g. safety instructions, etc.. A printed version is delivered with the instrument.
Online help
The online help offers quick, context-sensitive access to the complete information for
the base unit and the software options directly on the instrument.
Operating manuals
Separate manuals for the base unit and the software options are provided for download:
●
Base unit manual
Contains the description of the graphical user interface, an introduction to remote
control, the description of all SCPI remote control commands, programming examples, and information on maintenance, instrument interfaces and error messages.
Includes the contents of the Quick Start Guide.
●
Software option manuals
Contain the description of the specific functions of an option. Basic information on
operating the R&S FSVA/FSV is not included.
The online version of the user manual provides the complete contents for immediate
display on the internet.
Service manual
Describes the performance test for checking the rated specifications, module replacement and repair, firmware update, troubleshooting and fault elimination, and contains
mechanical drawings and spare part lists.
The service manual is available for registered users on the global Rohde & Schwarz
information system (GLORIS, https://gloris.rohde-schwarz.com).
Instrument security procedures manual
Deals with security issues when working with the R&S FSVA/FSV in secure areas.
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Basic safety instructions
Contains safety instructions, operating conditions and further important information.
The printed document is delivered with the instrument.
Data sheet and brochure
The data sheet contains the technical specifications of the R&S FSVA/FSV. It also lists
the options and their order numbers as well as optional accessories.
The brochure provides an overview of the instrument and deals with the specific characteristics.
Release notes and open source acknowledgment (OSA)
The release notes list new features, improvements and known issues of the current
firmware version, and describe the firmware installation.
The open source acknowledgment document provides verbatim license texts of the
used open source software.
Preface
Conventions Used in the Documentation
See www.rohde-schwarz.com/product/FSV > "Downloads" > "Firmware"
Application notes, application cards, white papers, etc.
These documents deal with special applications or background information on particular topics, see www.rohde-schwarz.com/appnotes.
1.2 Conventions Used in the Documentation
1.2.1 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
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.
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-
tion marks.
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Preface
How to Use the Help System
1.2.2 Conventions for Procedure Descriptions
When describing how to operate the instrument, several alternative methods may be
available to perform the same task. In this case, the procedure using the touchscreen
is described. Any elements that can be activated by touching can also be clicked using
an additionally connected mouse. The alternative procedure using the keys on the
instrument or the on-screen keyboard is only described if it deviates from the standard
operating procedures.
The term "select" may refer to any of the described methods, i.e. using a finger on the
touchscreen, a mouse pointer in the display, or a key on the instrument or on a keyboard.
1.2.3 Notes on Screenshots
When describing the functions of the product, we use sample screenshots. These
screenshots are meant to illustrate as much as possible of the provided functions and
possible interdependencies between parameters.
The screenshots usually show a fully equipped product, that is: with all options installed. Thus, some functions shown in the screenshots may not be available in your particular product configuration.
1.3 How to Use the Help System
Calling context-sensitive and general help
► To display the general help dialog box, press the HELP key on the front panel.
The help dialog box "View" tab is displayed. A topic containing information about
the current menu or the currently opened dialog box and its function is displayed.
For standard Windows dialog boxes (e.g. File Properties, Print dialog etc.), no contextsensitive help is available.
► If the help is already displayed, press the softkey for which you want to display
help.
A topic containing information about the softkey and its function is displayed.
If a softkey opens a submenu and you press the softkey a second time, the submenu
of the softkey is displayed.
Contents of the help dialog box
The help dialog box contains four tabs:
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●
"Contents" - contains a table of help contents
●
"View" - contains a specific help topic
●
"Index" - contains index entries to search for help topics
●
"Zoom" - contains zoom functions for the help display
To change between these tabs, press the tab on the touchscreen.
Navigating in the table of contents
●
To move through the displayed contents entries, use the UP ARROW and DOWN
ARROW keys. Entries that contain further entries are marked with a plus sign.
●
To display a help topic, press the ENTER key. The "View" tab with the corresponding help topic is displayed.
●
To change to the next tab, press the tab on the touchscreen.
Navigating in the help topics
●
To scroll through a page, use the rotary knob or the UP ARROW and DOWN
ARROW keys.
●
To jump to the linked topic, press the link text on the touchscreen.
Preface
How to Use the Help System
Searching for a topic
1. Change to the "Index" tab.
2. Enter the first characters of the topic you are interested in. The entries starting with
these characters are displayed.
3. Change the focus by pressing the ENTER key.
4. Select the suitable keyword by using the UP ARROW or DOWN ARROW keys or
the rotary knob.
5. Press the ENTER key to display the help topic.
The "View" tab with the corresponding help topic is displayed.
Changing the zoom
1. Change to the "Zoom" tab.
2. Set the zoom using the rotary knob. Four settings are available: 1-4. The smallest
size is selected by number 1, the largest size is selected by number 4.
Closing the help window
► Press the ESC key or a function key on the front panel.
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Preface
Notes for Users of R&S FSV 1307.9002Kxx Models
1.4 Notes for Users of R&S FSV 1307.9002Kxx Models
Users of R&S FSV 1307.9002Kxx models should consider the following differences to
the description of the newer R&S FSVA/FSV 1321.3008Kxx models:
●
Functions that are based on the Windows7 operating system (e.g. printing or setting up networks) may have a slightly different appearance or require different settings on the Windows XP based models. For such functions, refer to the Windows
documentation or the documentation originally provided with the R&S FSV instrument.
●
The R&S FSV 1307.9002K03 model is restricted to a maximum frequency of
3 GHz, whereas the R&S FSVA/FSV1321.3008K04 model has a maximum frequency of 4 GHz.
●
The bandwidth extension option R&S FSV-B160 (1311.2015.xx) is not available for
the R&S FSV 1307.9002Kxx models. The maximum usable I/Q analysis bandwidth
for these models is 28 MHz, or with option R&S FSV-B70, 40 MHz.
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2 Introduction
This document contains all information required for operation of an R&S FSVA/FSV
equipped with Application Firmware R&S FSVA/FSV. It covers operation via menus
and the remote control commands for WLAN measurements.
This option is not available for R&S FSVA/FSV 40 model 1307.9002K39.
This part of the documentation consists of the following chapters:
●
Chapter 3.2, "Basic Measurement Examples", on page 13
Describes the measurement setup for WLAN TX measurements.
●
Chapter 4, "Instrument Functions WLAN TX Measurements", on page 62
Describes the overall instrument functions and provides further information
●
Chapter 4.1, "Softkeys of the WLAN TX Menu", on page 63
Shows all softkeys available in the "WLAN" menu. This chapter also refers to the
remote control commands associated with each softkey function.
●
Chapter 5, "Remote Commands for WLAN TX Measurements", on page 116
Describes all remote control commands defined for the power meter measurement.
Introduction
This part of the documentation includes only functions of the Application Firmware
R&S FSV-K91/91n/91ac/91p. For all other descriptions, please refer to the description
of the base unit.
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R&S® FSV-K91/91n/91ac/91p
3 WLAN TX Measurements
The R&S FSV-K91/91n/91ac/91p application extends the functionality of the
R&S FSVA/FSV signal analyzer to enable wireless LAN TX measurements in accordance with IEEE standards 802.11 a, ac, b, g, j, n & p (assuming the required firmware
option is installed).
The following topics are described in this section:
3.1 Introduction to WLAN 802.11A, AC, B, G, J, N & P TX Tests.................................. 12
3.2 Basic Measurement Examples...................................................................................13
3.2.1 Setting Up the Measurement........................................................................................ 13
3.2.2 Performing the Main Measurement...............................................................................14
3.2.3 Setting up a MIMO measurement................................................................................. 14
3.3 Signal Processing of the IEEE 802.11a Application................................................ 21
3.3.1 Understanding Signal Processing of the IEEE 802.11a Application............................. 22
WLAN TX Measurements
3.3.2 Literature to the IEEE 802.11a Application................................................................... 29
3.4 Signal Processing of the IEEE 802.11b Application................................................ 29
3.4.1 Understanding Signal Processing of the IEEE 802.11b Application............................. 30
3.4.2 Literature of the IEEE 802.11b Application................................................................... 35
3.5 802.11b RF Carrier Suppression................................................................................35
3.6 IEEE 802.11n/ac MIMO Measurements......................................................................36
3.6.1
3.9.1 IQ Impairments..............................................................................................................44
3.9.2 EVM Measurement....................................................................................................... 48
3.9.3 Rise/Fall Time Measurement........................................................................................ 50
3.10 Measurement Settings and Result Displays.............................................................50
3.10.1 Measurement Settings.................................................................................................. 51
3.10.2 Result Summary List.....................................................................................................54
Trigger Synchronization Using an R&S®FS-Z11 Trigger Unit....................................... 37
3.7 Signal Field Measurement (IEEE 802.11ac, n (SISO+MIMO)).................................. 39
3.8 Optimized Signal Levels.............................................................................................43
3.9 Measurement Result Types........................................................................................44
3.10.3 Result Display Graph.................................................................................................... 59
3.10.4 Title Bar Information......................................................................................................61
3.10.5 Status Bar Information.................................................................................................. 61
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WLAN TX Measurements
Introduction to WLAN 802.11A, AC, B, G, J, N & P TX Tests
3.1 Introduction to WLAN 802.11A, AC, B, G, J, N & P TX Tests
The use of an R&S FSVA/FSV spectrum analyzer enables the accurate and reproducible TX measurement of a wireless LAN device under test (DUT) in accordance with the
standards specified for the device. The following test conditions are supported:
Modulation formats:
●
IEEE 802.11j (10 MHz)
– BPSK (3 & 4.5 Mbps)
– QPSK (6 & 9 Mbps)
– 16QAM (12 & 18 Mbps)
– 64QAM (24 & 27 Mbps)
●
IEEE 802.11a, j & g (OFDM), p
– BPSK (6 & 9 Mbps)
– QPSK (12 & 18 Mbps)
– 16QAM (24 & 36 Mbps)
– 64QAM (48 & 54 Mbps)
●
IEEE 802.11b & g (single carrier mode)
– DBPSK (1 Mbps)
– DQPSK (2 Mbps)
– CCK (5.5 & 11 Mbps)
– PBCC (5.5, 11 & 22 Mbps)
●
IEEE 802.11ac (SISO and MIMO)
– 16QAM
– 64QAM
– 256QAM
For IEEE 802.11n (MIMO) the modulation and data rates depend on the MCS
index
●
IEEE 802.11n (OFDM), (SISO and MIMO)
– 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)
For IEEE 802.11n (MIMO) the modulation and data rates depend on the MCS
index
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SISO vs MIMO
For most WLAN measurements, a single transmitter and a single receiver are used
(SISO = single in, single out). For measurements according to the IEEE 802.11n or ac
standards, the R&S FSVA/FSV can measure multiple data streams between multiple
transmitters and multiple receivers (MIMO = multiple in, multiple out).
For MIMO the modulation and data rates depend on the MCS index.
Modulation measurements
●
Constellation diagram
●
Constellation diagram for each OFDM carrier
●
I/Q offset and I/Q imbalance
●
Carrier and symbol frequency errors
●
Modulation error (EVM) for each OFDM carrier or symbol
●
Amplitude response and group-delay distortion (spectral flatness)
Further measurements and results
●
Amplitude statistics (CCDF) and crest factor
●
Transmit spectrum mask
●
FFT, also over a selected part of the signal, e.g. preamble
●
Payload bit information
●
Freq/Phase Err vs. Preamble
WLAN TX Measurements
Basic Measurement Examples
3.2 Basic Measurement Examples
This section provides step-by-step instruction for working through an ordinary measurement. The following steps are described:
1. Chapter 3.2.1, "Setting Up the Measurement", on page 13
2. Chapter 3.2.2, "Performing the Main Measurement", on page 14
In this example, a DUT using IEEE 802.11a is be used. The DUT is connected to the
R&S FSVA/FSV using the RF input of the R&S FSVA/FSV. The DUT generates a signal modulated using 16QAM.
3.2.1 Setting Up the Measurement
1. Activate the "WLAN" mode using the MODE > "WLAN" keys.
2. Press the "FREQ" key once to select and open the Demod Settings Dialog Box and
to activate the frequency input field.
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WLAN TX Measurements
Basic Measurement Examples
3. Activate "Auto Demodulation" (see "Auto Demodulation (IEEE 802.11n, SISO)"
on page 100) to use the content of the signal inherent field to detect the modulation type automatically.
3.2.2 Performing the Main Measurement
●
Select single sweep measurements by pressing the RUN SINGLE hardkey.
●
Select continuous measurements by pressing the RUN CONT hardkey.
During the measurement, the status message "Running" is displayed.
Leveling is done automatically.
Measurement results are updated once the measurement has completed. The
results are displayed in graphical form. The display can be toggled to a tabular list
of measurement points by pressing the "Display" softkey (in the "WLAN" menu or
"Trace" menu).
3.2.3 Setting up a MIMO measurement
For this example a 2 Tx MIMO DUT according to IEEE 802.11n is used.
1. The MIMO DUT is connected to the analyzers according to the following setup:
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WLAN TX Measurements
Basic Measurement Examples
2. Connect the external reference REF OUT of the SMU with the external reference
REF IN of the analyzers. Switch on the external reference for both analyzers in the
spectrum analyzer base system.
3. Connect the marker output of the SMU with the EXT TRIGGER input of the ana-
lyzers.
4. Either connect the "Path A RF/Baseband" connector with one analyzer and the
"Path B RF/Baseband" connector with the other analyzer, or use the air interface
with appropriate antennas.
5. Connect the master and the slave anaylzer via LAN according to the figure above.
As an alternative, it is sufficient to connect master and slave with a cross LAN
cable. The analyzer with the R&S FSV-K91n option can be used as master. The
slave analyzer does not require a WLAN option.
6. Setup the SMU to generate a 2 Tx IEEE 802.11n (MIMO) signal.
For the SMU "Baseband A" select the "IEEE 802.11n …" option. This opens the
"IEEE 802.11n WLAN A" dialog.
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WLAN TX Measurements
Basic Measurement Examples
7. Select the "Transmission Bandwidth" 40MHz.
In the "IEEE 802.11n WLAN A" dialog, press the "Frame Block Configuration …"
button to open the "IEEE 802.11n WLAN A: Frame Blocks Configuration" dialog.
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WLAN TX Measurements
Basic Measurement Examples
8. Select "Antennas" 2.
In the "IEEE 802.11n WLAN A" dialog, press the "Frame Block Configuration …"
button to open the "IEEE 802.11n WLAN A: Frame Blocks Configuration" dialog.
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WLAN TX Measurements
Basic Measurement Examples
9. Select "Tx Mode" HT-40MHz.
Press the "PPDU Config …" button to open the "IEEE 802.11n WLAN A: PPDU
Configuration for Frame Block 1" dialog.
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WLAN TX Measurements
Basic Measurement Examples
10. Select "Spatial Streams" 2 and "Space Time Streams" 2.
Return to the "IEEE 802.11n WLAN A" dialog.
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WLAN TX Measurements
Basic Measurement Examples
11. Check "Configure Baseband B from Baseband A". This will generate a IEEE
802.11n conform Tx 2 signal for path B of the SMU.
12. Toggle the "State" to On and make sure "RF/A Mod A" and "RF/B Mod B" are
switched on.
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WLAN TX Measurements
Signal Processing of the IEEE 802.11a Application
13. Using the "Graphics | Power Spectrum" display shows the power spectrum for both
antennas.
14. Now set up the spectrum analyzer with the R&S FSV-K91n option to perform the
WLAN MIMO measurements. Start the R&S FSV-K91n application.
15. Select "Standard" IEEE 802.11n (MIMO).
Set the "RF Frequency" the DUT is transmitting.
16. Set "Trigger Mode" to "External".
Select the "STC/MIMO" tab in the "General Settings" dialog box.
17. Select "DUT MIMO configuration" 2 Tx Antennas.
18. Set the "IP Address" of the slave in the "MIMO Measurement Setup" table and turn
the "State" of the slave to ON.
3.3 Signal Processing of the IEEE 802.11a Application
This description gives a rough view of the IEEE 802.11a application signal processing.
Details are disregarded in order to get a concept overview.
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●
Chapter 3.3.1, "Understanding Signal Processing of the IEEE 802.11a Application",
on page 22
●
Chapter 3.3.2, "Literature to the IEEE 802.11a Application", on page 29
Abbreviations
WLAN TX Measurements
Signal Processing of the IEEE 802.11a Application
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]
K
mod
ξ relative clock error of reference oscillator
r
l,k
3.3.1 Understanding Signal Processing of the IEEE 802.11a Application
symbol at symbol l of subcarrier k
error vector magnitude of subcarrier k
channel transfer function of subcarrier k
modulation-dependent normalization factor
subcarrier of symbol l
A diagram of the interesting blocks is shown in Figure 3-1. First the RF signal is down
converted to the IF frequency fIF = 96 MHz. The resulting IF signal rIF(t) is shown on the
left-hand side of the figure. After bandpass filtering, the signal is sampled by an Analog
to Digital Converter (ADC) at a sampling rate of fs1 = 128 MHz. This digital sequence is
resampled. Thus the sampling rate of the down sampled sequence r(i) is the Nyquist
rate of fs3 = 20 MHz. Up to this point the digital part is implemented in an ASIC.
In the lower part of the figure the subsequent digital signal processing is shown. 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 from
coarse
the metric in [6]. (The hat generally indicates an estimate, e.g. is the estimate of x.)
This can easily be understood because the phase of r(i) Δ r* (i + N) is determined by
the frequency offset. As the frequency deviation Δf can exceed half a bin (distance
between neighboring sub-carriers) 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 Ĥ
(LS)
, with
k
k = [–26, 26] denoting the channel index of the occupied sub-carriers.
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First the FFT of the LS is calculated. After the FFT calculation the known symbol information of the LS sub-carriers is removed by dividing by the symbols. The result is a
coarse estimate Ĥk of the channel transfer function.
In the next step the complex channel impulse response is computed by an IFFT. Next
the energy of the windowed impulse response (the window size is equal to the guard
period) is calculated for every trial time. Afterwards the trail time of the maximum
energy is detected. This trial time is used to adjust the timing.
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
would occur in the frequency domain.
WLAN TX Measurements
Signal Processing of the IEEE 802.11a Application
. This is necessary because otherwise inter channel interference (ICI)
course
23Operating Manual 1176.7649.02 ─ 06
R&S® FSV-K91/91n/91ac/91p
l,k
r''
1
measurement
k
H
of
parameters
k
H
(PL)
k
H
channel
estimation
l,k
a
WLAN TX Measurements
Signal Processing of the IEEE 802.11a Application
pilots + data
r(i)
= 20MHz
s3
f
l,k
r'
estimate
data symbols
data
user defined
compensation
l
, d
rest
f
l
g
full
compensation
l
FIR
S2
·kT
IF
-j
e
l,k
r
Resampler
=128MHz
ADC
f
s1
FFT
frequency
estimation
N = 64
compensation
of
gain, frequency, time
l,k
a
pilot
table
~~~
f
coarse
)
(LS
k
H
payload
window
timing
LS
(t)
IF
r
1.coarse timing
packet search:
2.fine timing
Figure 3-1: Signal processing of the IEEE 802.11a application
24Operating Manual 1176.7649.02 ─ 06
R&S® FSV-K91/91n/91ac/91p
kl
phasephasej
klkl
neHgaKr
kl
common
l
kl
,
(
,mod
)timing(
,
)(
,
The transition to the frequency domain is achieved by an FFT of length 64. The FFT is
performed symbol-wise for every of the "nof_symbols" symbols of the payload. The
calculated FFTs are described by r
●
l = [1, nof_symbols] as the symbol index
●
k = [–31, 32] as the channel index
In case of an additive white Gaussian noise (AWGN) channel the FFT is described by
[4], [5]
Equation 3-1: Equation (10)
with:
●
k
: the modulation-dependant normalization factor
mod
●
a
: the symbol of sub-carrier 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)
(common)
●
l
tion (11))
●
phase
l,k
Equation (11))
●
n
: the independent Gaussian distributed noise samples
l,k
WLAN TX Measurements
Signal Processing of the IEEE 802.11a Application
with:
l,k
: the common phase drift phase of all sub-carriers at symbol l (see Equa-
(timing)
: the phase of sub-carrier k at symbol l caused by the timing drift (see
The common phase drift in Equation (10) is given by:
Equation 3-2: Equation (11)
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
802.11a application) is not error-free. Therefore the remaining frequency error Δf
represents the frequency deviation in r
not yet compensated. Consequently, the over-
l,k
(see) Signal processing of the IEEE
coarse
rest
all frequency deviation of the device under test (DUT) is calculated by:
Δf = Δ
coarse
+ Δf
rest
The only motivation for dividing the common phase drift in Equation (11) into two parts
is to be able to calculate the overall frequency deviation of the DUT.
25Operating Manual 1176.7649.02 ─ 06
R&S® FSV-K91/91n/91ac/91p
lkNNphase
skl
/2
)timing(
,
The reason for the phase jitter dγ l in Equation (11) 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 burst. 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
measurement of the phase starts at the first symbol l = 1 of the payload. In contrast the
channel frequency response Hk in Equation (10) represents the channel at the long
symbol of the preamble. Consequently, the frequency deviation Δ f
sated produces a phase drift between the long symbol and the first symbol of the pay-
load. Therefore, this phase drift appears as a constant value ("DC value") in dϒ l.
Referring to the IEEE 802.11a measurement standard Chapter 17.3.9.7 "Transmit
modulation accuracy test'' [6], the common phase drift phasel
ted and compensated from the pilots. Therefore this "symbol-wise phase tracking''
(Tracking Phase) is activated as the default setting of the R&S FSV-K91/91n.
Furthermore, the timing drift in Equation (10) is given by:
WLAN TX Measurements
Signal Processing of the IEEE 802.11a Application
not yet compensated. To understand this, keep in mind that the
rest
not yet compen-
rest
(common)
must be estima-
Equation 3-3: Equation (12)
with ξ: the relative clock deviation of the reference oscillator
Normally, a symbol-wise timing jitter is negligible and thus not modeled in Equation
(12). 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
max
is assumed. The result of Equation (10) and Equation (12), is that the phase drift of the
highest sub-carrier 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 measurement standard [6], the timing drift phase
(timing)
l,k
is not part of the requirements. Therefore the "time tracking" (Tracking Time) is not
activated as the default setting of the R&S FSV-K91/91n. The time tracking option
should rather be seen as a powerful analyzing option.
In addition, the tracking of the gain gl in Equation (10) 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.
26Operating Manual 1176.7649.02 ─ 06
R&S® FSV-K91/91n/91ac/91p
Referring to the IEEE 802.11a measurement standard [6], the compensation of the
gain gl is not part of the requirements. Therefore the "gain tracking"' (Tracking Gain) is
not activated as the default setting of the R&S FSV-K91/91n.
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
neglected in this step, i.e. the parameters are set to gl = 1 and dγ = 0. Referring to
Equation (10), the log likelihood function L must be calculated as a function of the trial
parameters
the trial parameter of x.)
WLAN TX Measurements
Signal Processing of the IEEE 802.11a Application
and ξ are estimated. The symbol dependent parameters can be
rest
and . (The tilde generally describes a trial parameter. Example: is
Equation 3-4: (13a)
The trial parameters leading to the minimum of the log likelihood function are used as
estimates
and . In (13a) the known pilot symbols a
are read from a table.
l,k
In the second step, the log likelihood function is calculated for every symbol l as a function of the trial parameters and :
Equation 3-5: (13b)
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.
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
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 Equation
l,k
27Operating Manual 1176.7649.02 ─ 06
R&S® FSV-K91/91n/91ac/91p
packetsnof
counter
counterEVM
packetsnof
EVM
_
1
2
)(
_
1
26
)0(26
2
52
1
kk
k
EVMEVM
symbolsnof
l
klklk
aKr
symbolsnof
EVM
_
1
2
,mod
''
,
_
1
(10) it is clear that first the channel transfer function Hk must be removed. This is ach-
ieved by dividing the known coarse channel estimate Ĥ
Usually an error free estimation of the data symbols can be assumed.
In the next block a better channel estimate Ĥ
calculated by using all nof_symbols symbols of the payload (PL). This can be accom-
plished 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
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)
k
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 FSV-K91/91n is equalization from the coarse channel estimate
derived from the long symbol.
WLAN TX Measurements
Signal Processing of the IEEE 802.11a Application
(LS)
calculated from the LS.
k
(PL)
of the data and pilot sub-carriers is
k
(LS)
) leads to a nearly error-free channel
k
(LS)
is compensated by the channel estimate. The
k
(LS)
(from the long symbol) or the nearly error-
(PL)
(from the payload) for equalization. If the improved esti-
k
k
''
. The user may either
δ,k
is used, a 2 dB reduction of the subsequent EVM measurement can be
In the last block the measurement variables are calculated. The most important variable is the error vector magnitude of the sub-carrier "k" of the current packet:
Equation 3-6: (14)
Furthermore, the packet error vector magnitude is derived by averaging the squared
EVMk versus k:
Equation 3-7: (15)
Finally, the average error vector magnitude is calculated by averaging the packet EVM
of all nof_symbols detected packets:
Equation 3-8: (16)
This parameter is equivalent to the so-called "RMS average of all errors": Error
RMS
of
the IEEE 802.11a measurement commandment (see [6], ).
28Operating Manual 1176.7649.02 ─ 06
R&S® FSV-K91/91n/91ac/91p
WLAN TX Measurements
Signal Processing of the IEEE 802.11b Application
3.3.2 Literature to the IEEE 802.11a Application
3.4 Signal Processing of the IEEE 802.11b Application
[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 Receiver 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 Receiver 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: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY)
specifications
This description gives a rough overview of the signal processing concept of the IEEE
802.11b application.
●
Chapter 3.4.1, "Understanding Signal Processing of the IEEE 802.11b Application",
on page 30
●
Chapter 3.4.2, "Literature of the IEEE 802.11b Application", on page 35
Abbreviations
ε timing offset
Δ"f" frequency offset
ΔΦ phase offset
ARG{...} calculation of the angle of a complex value
EVM error vector magnitude
ĝ
I
ĝ
Q
Δĝ
Q
ĥs(v) estimated baseband filter of the transmitter
ĥr(v) estimated baseband filter of the receiver
ô
I
ô
Q
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
r(v) measurement signal
ŝ(v) estimate of the reference signal
29Operating Manual 1176.7649.02 ─ 06
R&S® FSV-K91/91n/91ac/91p
ŝn(v) estimate of the power normalized and undisturbed reference signal
REAL{...} calculation of the real part of a complex value
IMAG{...} calculation of the imaginary part of a complex value
WLAN TX Measurements
Signal Processing of the IEEE 802.11b Application
3.4.1 Understanding Signal Processing of the IEEE 802.11b Application
A block diagram of the measurement application is shown below in Figure 3-2. The
baseband signal of an IEEE 802.11b wireless LAN system transmitter is sampled with
a sampling rate of 44 MHz.
The first task of the measurement application is to detect the position of the bursts
within the measurement signal r1(v). The detection algorithm is able to find the posi-
tions of the beginning of short and long bursts and can distinguish between them. The
algorithm also detects the initial state of the scrambler. This is required if IEEE 802.11
signals should be analyzed, because this standard does not specify the initial state of
the scrambler.
With the knowledge of the start position of the burst, the header of the burst can be
demodulated. The bits transmitted in the header provide information about the length
of the burst and the modulation type used in the PSDU.
After the start position and the burst length is fully known, better estimates of timing offset, timing drift, frequency offset and phase offset can be calculated using the entire
data of the burst.
At this point of the signal processing a demodulation can be performed without decision error. After demodulation the normalized and undisturbed reference signal s(v) is
available.
If the frequency offset is not constant and varies with time, the frequency- and phase
offset in several partitions of the burst must be estimated and corrected. Additionally,
timing offset, timing drift and gain factor can be estimated and corrected in several partitions of the burst. These corrections can be separately switched off in the "Demod
Settings" menu.
30Operating Manual 1176.7649.02 ─ 06
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