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Keysight Technologies
Making Custom OFDM Measurements
Using the Keysight 89600 VSA Software
with Option BHF
Application Note
This application note describes
how to configure the Keysight
Technologies Inc. 89600 VSA
software with Option BHF
custom OFDM modulation
analysis to analyze and
troubleshoot new, custom,
proprietary, generic, or non-
standard OFDM signals.
Example files are developed
and are available for further
examination in the 89600
VSA software's demo signal
library and Help text. The
only hardware required for
demonstration is a PC.
Introduction
OFDM (Orthogonal Frequency-Division Multiplexing) signals
have gained great acceptance in communications systems due
Table of Contents
to their inherent noise immunity and transmission efficiency. But
troubleshooting and analyzing these systems, particularly for new, Introduction ..........................2
custom, generic, or non-standard OFDM formats, is difficult. The
89600 VSA software software with Option BHF custom OFDM
modulation analysis provides many data traces and tables to help Demodulation
the OFDM designer, including those developing multi-user or Configuration .......................4
MIMO systems.
Multi-carrier modulation schemes, such as OFDM, represent sig- Advanced Topics ................14
nificant challenges for those seeking to verify PHY-layer charac-
teristics. Most OFDM signals are designed to an open commercial
standard, and so are the tools that test them. While traditional Summary ............................21
spectrum analysis can measure simpler parameters such as fre-
quency, power level, and spectral mask, the more in-depth quality Appendix A .........................22
measurements require special capabilities found only in vector
signal analysis tools flexible enough to work with custom OFDM
systems. Appendix B .........................24
The 89600 VSA software with Option BHF custom OFDM modula-
tion analysis provides an easy-to-use, text file-driven approach to Additional Resources .........25
describing generic and proprietary OFDM signals requiring that no
special software be developed.
Once the custom OFDM signal is properly described, the 89600
VSA software's troubleshooting capability can be brought to
bear on it, including displaying error traces, such as EVM by
time or frequency, IQ constellations, tabular data such as OFDM
data burst info displaying EVM, power, modulation format, and
resource units assigned for preamble pilot, and data subcarriers.
Error summary information per channel provides information on
EVM (pilot, data, and preamble EVM), frequency, symbol clock,
and common pilot errors, as well as IQ errors such as offset,
quadrature errors, gain imbalance and time skew.
Statistical information such as CCDF and CDF traces is also
available on these signals, even before demodulation begins. In
addition, the 89600 VSA software can operate with data taken
from over 30 supported waveform acquisition platforms, including
simulation software, logic analyzers, oscilloscopes, and signal
analyzers. This allows you to examine your custom OFDM signals
anywhere in your block diagram. Automate design verification
tests using SCPI or any supported .NET language.
OFDM overview
Figure 1. OFDM is a multi-carrier scheme where closely spaced carriers overlap. Nulls
in each carrier's spectrum land at the center of all other carriers for zero inter-carrier
interference.
OFDM uses a multicarrier scheme to achieve transmission efficiencies (data rate
per Hz of bandwidth) similar to traditional, single-carrier schemes (QPSK, QAM,
etc.), but with better immunity to common channel impairments. It does so by
clocking many carriers simultaneously, but at proportionately slower symbol
rates compared to single carrier modulation (SCM) schemes.
The multi-carrier scheme is robust in the presence of single-frequency interfer-
ers and noise, because (unlike SCM) the loss of an individual carrier (or sev-
eral) is not fatal to the entire transmission. OFDM is also tolerant of multipath;
spectral dropouts only affect a limited number of carriers, and the OFDM signal
structure lends itself to strong equalization schemes, which can further reduce
the effects of multipath.
It can be made even more tolerant to multipath with the addition of more chan-
nels and MIMO signal processing techniques. Using OFDM allows some impor-
tant assumptions to be made about the signal for MIMO operation. For example,
the modulation bandwidth for each sub-carrier is small enough to assume the RF
path can be represented by a single complex coefficient.
Multi-carrier signals such as OFDM offer useful benefits for many digital com-
munications applications, but with a tradeoff in signal and design complexity.
OFDM signals are subject to the same sorts of design problems as any vector-
modulated signal, but these can be difficult to uncover and troubleshoot without
OFDM-specific signal analysis tools.
Try the 89600 VSA software, for free
Download the 89600 VSA software and use it free for 14 days to make mea-
surements with your analysis hardware, or use our recorded demo signals
by selecting File> Recall> Recall Demo> Custom OFDM > on the 89600 VSA
software toolbar.
The signals and configuration files shown in this application note
are all available there.
www.keysight.com/find/89600_trial
3
Demodulation Configuration
Before selecting the custom OFDM mode, ensure that the 89600 VSA software's
basic acquisition parameters are set correctly for the signal under test. Enter the ap-
propriate center frequency, span, input range, etc., and verify that the entire signal is
visible in a spectrum display. Information on setting up the VSA software is available
in the Help text.
Once these basic parameters are entered, you can use the 89600 VSA software to
perform CCDF and various other power, time, and frequency measurements, even
before you begin custom OFDM demodulation.
The OFDM examples in this document are based on an 802.11a/g OFDM signal. The
configuration developed in this application note can be used with the 89600 VSA
software example signal found at File> Recall> Recall Demo> Custom OFDM >
CustomOFDM_80211a.htm. Note that no hardware besides the PC is needed to run
or view these signals in demo mode. Further, when setting up measurements for your
own system, you can make measurements on software-based signals such as those
developed in MATLAB, or properly formatted .txt files. See the 89600 VSA software
Help text topic on "Supported File Formats" for more information.
The 89600 VSA software Option BHF custom OFDM demodulation software is config-
ured with the following three steps:
1. Set up the signal for basic spectrum analysis and then enter the high-level FFT
parameters for the OFDM signal under test
2. Describe the detailed signal structure of the 89600 VSA software, in the form of
3-4 configuration files that define each subcarrier in each symbol
3. Configure the measurement setup, e.g. synchronization type, result length, etc.
4
Step 1: Enter the basic signal After recalling the basic signal, select MeasSetup > Measurement Type > Custom
OFDM, and then MeasSetup > Custom OFDM Demod Properties. In the Format
and FFT parameters
section of the Format tab properties dialog box, enter the five numeric values shown
in Figure 2. These values are specific to the signal under test, and will normally be
found in the early part of its technical standard.
Notes: