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Make Custom OFDM Measurements with Keysight 89600B VSA Software Option BHF 5990-6824EN c20140708 [26]


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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:
                                    



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