R&S®FSW-K70
Measuring the BER and the EVM for
Signals with Low SNR
Application Sheet
Signals with low signal-to-noise ratios (SNR) often cause bit errors during demodulation, so that modulation accuracy values such as the error vector magnitude (EVM) may not be determined correctly.
This application sheet describes:
●
The significance of the Bit Error Rate (BER) parameter for signal analysis
●
How the R&S FSW VSA application calculates the BER and determines bit errors
●
How this knowledge can be used to obtain correct modulation accuracy results
(;ÜOÔ2)
1178.3170.02 ─ 01
Application Sheet
Test & Measurement
R&S®FSW-K70
1 Introduction............................................................................................ 2
2 How to Create Known Data Files.......................................................... 3
3 BER Measurement in the R&S FSW VSA application.........................6
4 Tips for Improving Signal Demodulation in Preparation for the
5 Measurement Example - Measuring the BER.................................... 14
6 Additional Information.........................................................................16
Contents
Contents
Known Data File..................................................................................... 8
1 Introduction
What is the Bit Error Rate (BER)?
In signal analysis, a bit error occurs when a false symbol decision is made during
demodulation. That is: the demodulated symbol does not correspond to the transmitted
symbol. Bit errors often occur when demodulating highly distorted signals or signals
with low signal-to-noise ratios (SNR).
Bit errors due to noise cause additional points in a constellation diagram, creating a
"cloud" around the ideal symbol point. If these points exceed the defined symbol decision boundaries for the symbol mapping of the used modulation, wrong symbol decisions are made and the number of bit errors increases.
Figure 1-1: Constellation diagram with "clouds" of bit errors caused by noise
The bit error rate (BER) is calculated by comparing the transmitted bits with the determined symbol decision bits in the measured signal. The number of falsely detected bits
is then set in relation to the total number of analyzed bits:
2Application Sheet 1178.3170.02 ─ 01
R&S®FSW-K70
How to Create Known Data Files
BER = falsely detected bits / number of analyzed bits
How do you detect bit errors in the signal and calculate the bit error rate?
Obviously, knowledge of the originally transmitted data is required. There are two possible methods of calculating the bit error rate:
●
With a signal generator: The data transmitted to the DUT by the signal generator
is also sent to a BER calculator. The measured, demodulated data from the analyzer is also sent to the BER calculator. The BER calculator can then compare the
data and provide the results.
Figure 1-2: BER measurement using the signal generator data
●
Without a signal generator: The information on the expected input signal is provided to the analyzer in advance. The analyzer then compares the measured,
demodulated data with the available reference information and can calculate the
BER itself.
Figure 1-3: BER measurement without a generator, using the R&S FSW VSA application
If knowledge of the expected input signal is available, the R&S FSW VSA application
provides BER results without the need of connecting a signal generator. The required
information consists of a set of all possible data sequences of the input signal. These
data sequences are stored in a Known Data file in a predefined format, and then loaded in the R&S FSW VSA application.
2 How to Create Known Data Files
The R&S FSW VSA application provides an auxiliary tool on the R&S FSW (free of
charge) to create Known Data files from existing data. You can copy this tool to a PC
and work with it locally, as well.
This section describes how to create known data files using this tool.
3Application Sheet 1178.3170.02 ─ 01
R&S®FSW-K70
How to Create Known Data Files
It is essential that no bit errors occur during this recording. To obtain stable results
before recording known data sequences refer to Tips for Improving Signal Demodula-
tion in Preparation for the Known Data File.
If it is not possible to ensure stable demodulation in order to record known data
sequences, you can also create the Known Data files yourself. Use any XML editing
tool you like, following the rules of the syntax defined by Rohde & Schwarz (see the
R&S FSW VSA application user manual). Before loading the file to the R&S FSW VSA
application, make sure the syntax of your file is valid.
To create a Known Data file using the recording tool for sequences
1. Import or input data for which stable demodulation results are available to the R&S
FSW VSA application. If necessary, adapt the demodulation settings until the
requested results are obtained (see Tips for Improving Signal Demodulation in
Preparation for the Known Data File).
2. Do one of the following:
● Start the "R&S Recording Tool for Sequences" from the Windows task bar on
the R&S FSW.
● Execute the file
C:\Program Files (x86)\Rohde-Schwarz\FSW\<version>\
RecordingToolforSequences.EXE on the R&S FSW.
The "R&S Recording Tool for Sequences" window is displayed.
3. Define the VISA resource string required to connect to the instrument:
● If the tool is started directly from the instrument: enter TCPIP::localhost
(default).
4Application Sheet 1178.3170.02 ─ 01
R&S®FSW-K70
How to Create Known Data Files
● If the tool is started from a computer: enter the TCPIP address or name of the
computer in the VISA resource string (e.g. TCPIP::123.4.5.6).
4. In the tool window, select "Run".
The tool records the demodulated data sequences. The following result information
is provided by the tool during recording:
● Analyzed Sequences: number of data sequences analyzed since the tool was
started
● Different Sequences: number of unique sequences detected in the measured
data
● Last New Sequence Found: time that has passed since the most recent
unique sequence was detected
● Throughput: current data processing speed of the tool
Note that while the tool is running, the R&S FSW is set to remote mode, that is: the
manual interface is not available. As soon as the tool is closed, the remote mode is
automatically deactivated.
Tip: Sometimes the recording tool window is moved to the background and is no
longer visible behind the R&S FSW firmware window.
In this case, select the on-screen keyboard key, then press the "Windows" key
twice. Select the tool icon in the task bar to bring it back to the foreground.
Figure 2-1: On-screen keyboard key
5. Stop the tool by selecting "Stop" when one of the following situations occurs:
● All known possible sequences have been detected
● A significant amount of time has passed to assume no more sequences will be
detected
6. ● If the results are acceptable, select "Store for K70" to store a valid XML file with
the recorded data sequences.
A file selection dialog box is displayed in which you can select the storage location and file name.
You can also add an optional comment to the file.
● Otherwise, reset the tool to start a new recording, possibly after changing the
demodulation settings or input data.
7. Close the tool window to return to normal operation of the R&S FSW VSA application.
The created XML file can now be loaded in the R&S FSW VSA application.
5Application Sheet 1178.3170.02 ─ 01
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