Main Content

Recovery Procedure for an 802.11ax Packet

Open Live Script

This example shows how to detect a packet and decode the payload bits in a received IEEE® 802.11ax™ (Wi-Fi 6) waveform. The receiver recovers the packet format parameters from the preamble fields to decode the data field and the MAC frame.

Introduction

In an 802.11ax packet, the L-SIG, HE-SIG-A, and HE-SIG-B preamble fields signal transmission parameters to the receiver [1]:

  • The L-SIG field contains the transmission time of the packet.

  • The HE-SIG-A field contains common transmission parameters for HE MU users and all transmission parameters for HE SU and HE EXT SU packets.

  • The length information in the L-SIG field, the modulation type, and the number of OFDM symbols in the HE-SIG-A field determine the HE packet format.

  • The HE-SIG-B field contains resource unit (RU) allocation information and the transmission parameters for the users in an HE MU packet.

This example shows how to recover these transmission parameters from an HE MU packet within a generated waveform. This example can also recover HE SU and HE EXT SU packets. The example assumes knowledge of only the channel bandwidth. The receiver obtains the other transmission parameters from the decoded L-SIG, HE-SIG-A, and HE-SIG-B preamble fields. You then use these parameters to decode the HE-Data field. You can use this example to perform the following analyses:

  • Recover and display the waveform of the detected packet.

  • Recover and display the spectrum of the detected packet.

  • Display the constellation of the equalized data symbols for all spatial streams.

  • Measure the error vector magnitude (EVM) of each field.

  • Detect an A-MPDU and determine the frame check sequence (FCS) for the recovered MAC frame.

  • Display the EVM per data symbol and spatial stream averaged over subcarriers.

  • Display the EVM per data subcarrier and spatial stream averaged over symbols.

Waveform Generation

For this example, you synthesize a 802.11ax HE MU waveform. To synthesize an HE MU waveform, you create an HE MU format configuration object wlanHEMUConfig. The synthesized waveform is impaired by a 2x2 TGax indoor fading channel, additive white Gaussian noise (AWGN), and carrier frequency offset.

Note that the wlanHEMUConfig object is used at the transmitter side only. The receiver uses an HE recovery configuration object wlanHERecoveryConfig. The receiver sets the unknown properties of the HE recovery configuration object after decoding the information bits in the L-SIG, HE-SIG-A, and HE-SIG-B fields. The helper function helperSigRecGenerateWaveform generates the impaired waveform by:

  • Creating a random payload of MSDUs for the MAC frame, which is encoded into an HE MU packet

  • Passing the waveform through a TGax indoor fading channel model

  • Adding carrier frequency offset (CFO) and AWGN to the waveform

Alternatively, you can capture a waveform by using Instrument Control Toolbox™ to acquire I/Q data from a wide range of instruments and software-defined radio (SDR) platforms. If you capture an off-the-air waveform using an SDR, see the Recover and Analyze Packets in 802.11 Waveform example.

Define a mixed OFDMA and MU-MIMO configuration for an HE MU packet. The allocation index 17 defines two 52-tone RUs, with one user in each RU, and one 106-tone RU. The 106-tone RU has two users in an MU-MIMO configuration. The waveform to process is stored in the variable rx.

cfgMU = wlanHEMUConfig(17);
cfgMU.NumTransmitAntennas = 2;

% Configure RU 1 and user 1
cfgMU.RU{1}.SpatialMapping = 'Direct';
cfgMU.User{1}.STAID = 1;
cfgMU.User{1}.APEPLength = 1e3;
cfgMU.User{1}.MCS = 5;
cfgMU.User{1}.NumSpaceTimeStreams = 2;
cfgMU.User{1}.ChannelCoding = 'LDPC';

% Configure RU 2 and user 2
cfgMU.RU{2}.SpatialMapping = 'Fourier';
cfgMU.User{2}.STAID = 2;
cfgMU.User{2}.APEPLength = 500;
cfgMU.User{2}.MCS = 4;
cfgMU.User{2}.NumSpaceTimeStreams = 1;
cfgMU.User{2}.ChannelCoding = 'BCC';

% Configure RU 3 and user 1
cfgMU.RU{3}.SpatialMapping = 'Fourier';
cfgMU.User{3}.STAID = 3;
cfgMU.User{3}.APEPLength = 100;
cfgMU.User{3}.MCS = 2;
cfgMU.User{3}.NumSpaceTimeStreams = 1;
cfgMU.User{3}.ChannelCoding = 'BCC';

% Configure RU 3 and user 2
cfgMU.User{4}.STAID = 4;
cfgMU.User{4}.APEPLength = 500;
cfgMU.User{4}.MCS = 3;
cfgMU.User{4}.NumSpaceTimeStreams = 1;
cfgMU.User{4}.ChannelCoding = 'LDPC';

% Specify propagation channel
numRx = 2; % Number of receive antennas
delayProfile = 'Model-D'; % TGax channel delay profile

% Specify impairments
noisePower = -40; % Noise power to apply in dBW
cfo = 62e3; % Carrier frequency offset Hz

% Generate waveform
rx = helperSigRecGenerateWaveform(cfgMU,numRx,delayProfile,noisePower,cfo);

Packet Recovery

To recover a packet, you process the waveform by following these steps:

  1. Create an HE recovery configuration object, wlanHERecoveryConfig. After decoding the preamble fields in the waveform, the receiver updates the object properties.

  2. Detect and synchronize the packet.

  3. Extract and demodulate the L-LTF. The demodulated L-LTF symbols do not include tone rotation for each 20 MHz segment as described in [2], Section 21.3.7.5.

  4. Use the demodulated L-LTF symbols for channel and noise estimates.

  5. Use the time-domain signal containing samples equivalent to four OFDM symbols that immediately follow the L-LTF to determine the HE packet format. Update the packet format in the wlanHERecoveryConfig object.

  6. Demodulate the L-LTF. The demodulated L-LTF symbols include tone rotation for each 20 MHz segment as described in [2], Section 21.3.7.5. Use the L-LTF channel estimates (with tone rotation) to decode the pre-HE-LTF fields.

  7. Extract the L-SIG and RL-SIG fields. Estimate the channel on an extra four subcarriers per subchannel in the L-SIG and RL-SIG fields. Update the L-LTF channel estimates to include the channel estimates on the extra subcarriers.

  8. Determine the length of the packet in microseconds from the information bits in the L-SIG field.

  9. Update the recovery configuration object after decoding the HE-SIG-A field. In an HE MU packet, the HE-SIG-A field contains the common transmission parameters. In an HE SU or HE EXT SU packet, the HE-SIG-A field contains all the transmission parameters.

  10. For an HE MU packet format, decode the HE-SIG-B field. For a non-compressed SIGB waveform the HE-SIG-B common field is decoded first followed by the HE-SIG-B user field. For a compressed SIGB waveform only the HE-SIG-B user field is decoded.

  11. For an HE MU format without SIGB compression, recover the RU allocation and user transmission parameters from the HE-SIG-B field. For a compressed SIGB waveform, determine the RU allocation information from the HE-SIG-A field and the user transmission parameters from the HE-SIG-B user field bits.

  12. Create the wlanHERecoveryConfig object using the recovered transmission parameters for each user after HE-SIG-B decoding.

  13. Extract and demodulate the HE-LTF field. Use the demodulated symbols for channel estimation of the subcarriers allocated to the user of interest. Use the MIMO channel estimates to decode the HE-Data field.

  14. Extract the HE-Data field and recover the PSDU bits using the wlanHERecoveryConfig object for each user.

  15. Detect the A-MPDU within the recovered PSDU and check the FCS for the recovered MAC frame.

Recovery Parameter Setup

To recover the received packet from the waveform, create an HE recovery configuration object, wlanHERecoveryConfig. The recovery object stores the recovered information from the L-SIG, HE-SIG-A, and HE-SIG-B preamble fields. After decoding the preamble fields, the receiver updates the unknown properties of the object. The following code configures the object and variables for processing.

chanBW = cfgMU.ChannelBandwidth; % Assume channel bandwidth is known
sr = wlanSampleRate(cfgMU); % Sample rate

% Specify pilot tracking method for recovering the data field. When
% recovering 26-tone RUs only phase tracking is used as both time and phase
% tracking algorithm is susceptible to noise.
phaseTracking = true;
timeTracking = true;

% Create an HE recovery configuration object and set the channel bandwidth
cfgRx = wlanHERecoveryConfig;
cfgRx.ChannelBandwidth = chanBW;

% The recovery configuration object is used to get the start and end
% indices of the pre-HE-SIG-B field.
ind = wlanFieldIndices(cfgRx);

% Setup plots for the example
[spectrumAnalyzer,timeScope,ConstellationDiagram,EVMPerSubcarrier,EVMPerSymbol] = helperSigRecSetupPlots(sr);

% Minimum packet length is 10 OFDM symbols
lstfLength = double(ind.LSTF(2));
minPktLen = lstfLength*5; % Number of samples in L-STF

rxWaveLen = size(rx,1);

Front-End Processing

The front-end processing uses a while loop for packet detection, coarse carrier frequency offset correction, timing synchronization, and fine carrier frequency offset correction. To detect a packet, the sample offset searchOffset indexes into rx. If synchronization fails for the first packet, the searchOffset is incremented to move beyond that packet. This process repeats until a packet is successfully synchronized.

searchOffset = 0; % Offset from start of waveform in samples
while (searchOffset + minPktLen) <= rxWaveLen
    % Packet detection
    pktOffset = wlanPacketDetect(rx,chanBW,searchOffset);

    % Adjust packet offset
    pktOffset = searchOffset + pktOffset;
    if isempty(pktOffset) || (pktOffset + ind.LSIG(2) > rxWaveLen)
        error('** No packet detected **');
    end

    % Coarse frequency offset estimation and correction using L-STF
    rxLSTF = rx(pktOffset+(ind.LSTF(1):ind.LSTF(2)), :);
    coarseFreqOffset = wlanCoarseCFOEstimate(rxLSTF,chanBW);
    rx = frequencyOffset(rx,sr,-coarseFreqOffset);

    % Symbol timing synchronization
    searchBufferLLTF = rx(pktOffset+(ind.LSTF(1):ind.LSIG(2)),:);
    pktOffset = pktOffset+wlanSymbolTimingEstimate(searchBufferLLTF,chanBW);

    % Fine frequency offset estimation and correction using L-STF
    rxLLTF = rx(pktOffset+(ind.LLTF(1):ind.LLTF(2)),:);
    fineFreqOffset = wlanFineCFOEstimate(rxLLTF,chanBW);
    rx = frequencyOffset(rx,sr,-fineFreqOffset);

    % Timing synchronization complete: packet detected
    fprintf('Packet detected at index %d\n',pktOffset + 1);

    % Display estimated carrier frequency offset
    cfoCorrection = coarseFreqOffset + fineFreqOffset; % Total CFO
    fprintf('Estimated CFO: %5.1f Hz\n\n',cfoCorrection);

    break; % Front-end processing complete, stop searching for a packet
end
Packet detected at index 404

Estimated CFO: 62017.7 Hz

% Scale the waveform based on L-STF power (AGC)
gain = 1./(sqrt(mean(rxLSTF.*conj(rxLSTF))));
rx = rx.*gain;

Packet Format Detection

To determine the packet format, the receiver uses the time-domain samples equivalent to the four OFDM symbols immediately following the L-LTF [1 Figure 27-63]. Extract and demodulate the L-LTF. For format detection, the demodulated L-LTF symbols must not include tone rotation for each 20 MHz segment as described in [2], Section 21.3.7.5. Use the demodulated L-LTF symbols for channel and noise estimation. Detect the packet format using the L-LTF channel (without tone rotation) and noise power estimates.

rxLLTF = rx(pktOffset+(ind.LLTF(1):ind.LLTF(2)),:);
lltfDemod = wlanLLTFDemodulate(rxLLTF,chanBW);
lltfChanEst = wlanLLTFChannelEstimate(lltfDemod,chanBW);
noiseVar = wlanLLTFNoiseEstimate(lltfDemod);

disp('Detect packet format...');
Detect packet format...

rxSIGA = rx(pktOffset+(ind.LSIG(1):ind.HESIGA(2)),:);
pktFormat = wlanFormatDetect(rxSIGA,lltfChanEst,noiseVar,chanBW);
fprintf('  %s packet detected\n\n',pktFormat);
  HE-MU packet detected

% Set the packet format in the recovery object and update the field indices
cfgRx.PacketFormat = pktFormat;
ind = wlanFieldIndices(cfgRx);

L-LTF Channel Estimate

Demodulate the L-LTF and perform channel estimation. The demodulated L-LTF symbols include tone rotation for each 20 MHz segment as described in [2], Section 21.3.7.5. The receiver uses the L-LTF channel estimates (with tone rotation) to equalize and decode the pre-HE-LTF fields.

lltfDemod = wlanHEDemodulate(rxLLTF,'L-LTF',chanBW);
lltfChanEst = wlanLLTFChannelEstimate(lltfDemod,chanBW);

L-SIG and RL-SIG Decoding

The L-SIG field determines the receive time, or RXTIME, of the packet. The receiver calculates RXTIME using the length bits of the L-SIG payload. The receiver recovers the L-SIG and RL-SIG fields to estimate the channel at the extra subcarriers in these fields. It then updates the lltfChanEst channel estimates to include the channel estimates at the extra subcarriers in the L-SIG and RL-SIG fields. Using the estimates of the channel and the noise power from the L-LTF, the receiver decodes the L-SIG payload. After L-SIG decoding, update the L-SIG length property in the wlanHERecoveryConfig object.

disp('Decoding L-SIG... ');
Decoding L-SIG... 

% Extract L-SIG and RL-SIG fields
rxLSIG = rx(pktOffset+(ind.LSIG(1):ind.RLSIG(2)),:);

% OFDM demodulate
helsigDemod = wlanHEDemodulate(rxLSIG,'L-SIG',chanBW);

% Estimate CPE and phase correct symbols
helsigDemod = wlanHETrackPilotError(helsigDemod,lltfChanEst,cfgRx,'L-SIG');

% Estimate channel on extra 4 subcarriers per subchannel and create full
% channel estimate
preheInfo = wlanHEOFDMInfo('L-SIG',chanBW);
preHEChanEst = wlanPreHEChannelEstimate(helsigDemod,lltfChanEst,chanBW);

% Average L-SIG and RL-SIG before equalization
helsigDemod = mean(helsigDemod,2);

% Equalize data carrying subcarriers, merging 20 MHz subchannels
[eqLSIGSym,csi] = wlanHEEqualize(helsigDemod(preheInfo.DataIndices,:,:), ...
                                 preHEChanEst(preheInfo.DataIndices,:,:),noiseVar,cfgRx,'L-SIG');

% Decode L-SIG field
[~,failCheck,lsigInfo] = wlanLSIGBitRecover(eqLSIGSym,noiseVar,csi);

if failCheck
    disp(' ** L-SIG check fail **');
else
    disp(' L-SIG check pass');
end
 L-SIG check pass

% Get the length information from the recovered L-SIG bits and update the
% L-SIG length property of the recovery configuration object
lsigLength = lsigInfo.Length;
cfgRx.LSIGLength = lsigLength;

% Measure EVM of L-SIG symbols
EVM = comm.EVM;
EVM.ReferenceSignalSource = 'Estimated from reference constellation';
EVM.Normalization = 'Average constellation power';
EVM.ReferenceConstellation = wlanReferenceSymbols('BPSK');
rmsEVM = EVM(eqLSIGSym);
fprintf(' L-SIG EVM: %2.2fdB\n\n',20*log10(rmsEVM/100));
 L-SIG EVM: -36.78dB

% Calculate the receive time and corresponding number of samples in the
% packet
RXTime = ceil((lsigLength + 3)/3) * 4 + 20; % In microseconds
numRxSamples = round(RXTime * 1e-6 * sr);   % Number of samples in time

fprintf(' RXTIME: %dus\n',RXTime);
 RXTIME: 536us

fprintf(' Number of samples in the packet: %d\n\n',numRxSamples);
 Number of samples in the packet: 10720

Given the RXTIME and corresponding number of samples within rx, plot and display the waveform and the spectrum of the received packet.

sampleOffset = max((-lstfLength + pktOffset),1); % First index to plot
sampleSpan = numRxSamples + 2*lstfLength; % Number samples to plot
                                          % Plot as much of the packet (and extra samples) as we can
plotIdx = sampleOffset:min(sampleOffset + sampleSpan,rxWaveLen);

% Configure timeScope to display the packet
timeScope.TimeSpan = sampleSpan/sr;
timeScope.TimeDisplayOffset = sampleOffset/sr;
timeScope.YLimits = [0 max(abs(rx(:)))];
timeScope(abs(rx(plotIdx,:)));
release(timeScope);

% Display the spectrum of the detected packet
spectrumAnalyzer(rx(pktOffset + (1:numRxSamples),:));
release(spectrumAnalyzer);

HE-SIG-A Decoding

The HE-SIG-A field contains the transmission configuration of an HE packet. Decode the HE-SIG-A field using estimates of the channel and the noise power from the L-LTF field.

disp('Decoding HE-SIG-A...')
Decoding HE-SIG-A...

rxSIGA = rx(pktOffset+(ind.HESIGA(1):ind.HESIGA(2)),:);
sigaDemod = wlanHEDemodulate(rxSIGA,'HE-SIG-A',chanBW);
hesigaDemod = wlanHETrackPilotError(sigaDemod,preHEChanEst,cfgRx,'HE-SIG-A');

% Equalize data carrying subcarriers, merging 20 MHz subchannels
preheInfo = wlanHEOFDMInfo('HE-SIG-A',chanBW);
[eqSIGASym,csi] = wlanHEEqualize(hesigaDemod(preheInfo.DataIndices,:,:), ...
                                 preHEChanEst(preheInfo.DataIndices,:,:), ...
                                 noiseVar,cfgRx,'HE-SIG-A');
% Recover HE-SIG-A bits
[sigaBits,failCRC] = wlanHESIGABitRecover(eqSIGASym,noiseVar,csi);

% Perform the CRC on HE-SIG-A bits
if failCRC
    disp(' ** HE-SIG-A CRC fail **');
else
    disp(' HE-SIG-A CRC pass');
end
 HE-SIG-A CRC pass

% Measure EVM of HE-SIG-A symbols
release(EVM);
if strcmp(pktFormat,'HE-EXT-SU')
    % The second symbol of an HE-SIG-A field for an HE EXT SU packet is
    % QBPSK.
    EVM.ReferenceConstellation = wlanReferenceSymbols('BPSK',[0 pi/2 0 0]);
    % Account for scaling of L-LTF for an HE EXT SU packet
    rmsEVM = EVM(eqSIGASym*sqrt(2));
else
    EVM.ReferenceConstellation = wlanReferenceSymbols('BPSK');
    rmsEVM = EVM(eqSIGASym);
end
fprintf(' HE-SIG-A EVM: %2.2fdB\n\n',20*log10(mean(rmsEVM)/100));
 HE-SIG-A EVM: -35.85dB

Interpret Recovered HE-SIG-A Bits

Update the wlanHERecoveryConfig object after interpreting the recovered HE-SIG-A bits.

cfgRx = interpretHESIGABits(cfgRx,sigaBits);
ind = wlanFieldIndices(cfgRx); % Update field indices

Display the common transmission configuration obtained from the HE-SIG-A field for an HE MU packet. The properties indicated by -1 are unknown or undefined. You can update the unknown user-related properties after successfully decoding the HE-SIG-B field.

disp(cfgRx)
  wlanHERecoveryConfig with properties:

                    PacketFormat: 'HE-MU'
                ChannelBandwidth: 'CBW20'
                      LSIGLength: 383
                 SIGBCompression: 0
                         SIGBMCS: 0
                         SIGBDCM: 0
          NumSIGBSymbolsSignaled: 5
                            STBC: 0
                 LDPCExtraSymbol: 0
             PreFECPaddingFactor: 4
                  PEDisambiguity: 0
                   GuardInterval: 3.2000
                       HELTFType: 4
                 NumHELTFSymbols: 2
                UplinkIndication: 0
                        BSSColor: 0
                    SpatialReuse: 0
                    TXOPDuration: 127
                     HighDoppler: 0
                 AllocationIndex: -1
       NumUsersPerContentChannel: -1
         RUTotalSpaceTimeStreams: -1
                          RUSize: -1
                         RUIndex: -1
                           STAID: -1
                             MCS: -1
                             DCM: -1
                   ChannelCoding: 'Unknown'
                     Beamforming: -1
             NumSpaceTimeStreams: -1
    SpaceTimeStreamStartingIndex: -1

HE-SIG-B Decoding

For an HE MU packet the HE-SIG-B field stores this information:

  • The RU allocation information for a non-compressed SIGB waveform is stored in the HE-SIG-B common field [1 Table. 27-24]. For a compressed SIGB waveform the RU allocation information is determined by the recovered HE-SIG-A bits.

  • The HE-SIG-B user field stores user transmission parameters for both SIGB compressed and non-compressed waveforms [1 Table. 27-27, 27-28].

For a non-compressed SIGB waveform, update the number of HE-SIG-B symbols in the wlanHERecoveryConfig object. The example updates the symbols only if the number of HE-SIG-B symbols indicated in the HE-SIG-A field is 15 and all content channels pass the CRC. The example does not update the number of HE-SIG-B symbols indicated in the HE-SIG-A field if any HE-SIG-B content channel fails the CRC.

To decode the HE-SIG-B field, you require an estimate of the channel and noise power obtained from the L-LTF.

if strcmp(pktFormat,'HE-MU')
    fprintf('Decoding HE-SIG-B...\n');
    if ~cfgRx.SIGBCompression
        fprintf(' Decoding HE-SIG-B common field...\n');
        s = getSIGBLength(cfgRx);
        % Get common field symbols. The start of HE-SIG-B field is known
        rxSym = rx(pktOffset+(double(ind.HESIGA(2))+(1:s.NumSIGBCommonFieldSamples)),:);
        % Decode HE-SIG-B common field
        [status,cfgRx] = heSIGBCommonFieldDecode(rxSym,preHEChanEst,noiseVar,cfgRx);

        % CRC on HE-SIG-B content channels
        if strcmp(status,'Success')
            fprintf('  HE-SIG-B (common field) CRC pass\n');
        elseif strcmp(status,'ContentChannel1CRCFail')
            fprintf('  ** HE-SIG-B CRC fail for content channel-1\n **');
        elseif strcmp(status,'ContentChannel2CRCFail')
            fprintf('  ** HE-SIG-B CRC fail for content channel-2\n **');
        elseif any(strcmp(status,{'UnknownNumUsersContentChannel1','UnknownNumUsersContentChannel2'}))
            error('  ** Unknown packet length, discard packet\n **');
        else
            % Discard the packet if all HE-SIG-B content channels fail
            error('  ** HE-SIG-B CRC fail **');
        end
        % Update field indices as the number of HE-SIG-B symbols are
        % updated
        ind = wlanFieldIndices(cfgRx);
    end

    % Get complete HE-SIG-B field samples
    rxSIGB = rx(pktOffset+(ind.HESIGB(1):ind.HESIGB(2)),:);
    fprintf(' Decoding HE-SIG-B user field... \n');
    % Decode HE-SIG-B user field
    [failCRC,cfgUsers] = heSIGBUserFieldDecode(rxSIGB,preHEChanEst,noiseVar,cfgRx);

    % CRC on HE-SIG-B users
    if ~all(failCRC)
        fprintf('  HE-SIG-B (user field) CRC pass\n\n');
        numUsers = numel(cfgUsers);
    elseif all(failCRC)
        % Discard the packet if all users fail the CRC
        error('  ** HE-SIG-B CRC fail for all users **');
    else
        fprintf('  ** HE-SIG-B CRC fail for at least one user\n **');
        % Only process users with valid CRC
        numUsers = numel(cfgUsers);
    end

else % HE SU, HE EXT SU
    cfgUsers = {cfgRx};
    numUsers = 1;
end
Decoding HE-SIG-B...

 Decoding HE-SIG-B common field...

  HE-SIG-B (common field) CRC pass

 Decoding HE-SIG-B user field... 

  HE-SIG-B (user field) CRC pass

HE-Data Decoding

Use the updated wlanHERecoveryConfig object for each user to recover the PSDU bits for each user in the HE-Data field.

cfgDataRec = trackingRecoveryConfig;
fprintf('Decoding HE-Data...\n');
Decoding HE-Data...

for iu = 1:numUsers
    % Get recovery configuration object for each user
    user = cfgUsers{iu};
    if strcmp(pktFormat,'HE-MU')
        fprintf(' Decoding User:%d, STAID:%d, RUSize:%d\n',iu,user.STAID,user.RUSize);
    else
        fprintf(' Decoding RUSize:%d\n',user.RUSize);
    end

    heInfo = wlanHEOFDMInfo('HE-Data',chanBW,user.GuardInterval,[user.RUSize user.RUIndex]);

    % HE-LTF demodulation and channel estimation
    rxHELTF = rx(pktOffset+(ind.HELTF(1):ind.HELTF(2)),:);
    heltfDemod = wlanHEDemodulate(rxHELTF,'HE-LTF',chanBW,user.GuardInterval,user.HELTFType,[user.RUSize user.RUIndex]);
    [chanEst,pilotEst] = wlanHELTFChannelEstimate(heltfDemod,user);

    % Number of expected data OFDM symbols
    symLen = heInfo.FFTLength+heInfo.CPLength;
    numOFDMSym = double((ind.HEData(2)-ind.HEData(1)+1))/symLen;

    % HE-Data demodulation with pilot phase and time tracking
    % Account for extra samples when extracting data field from the packet
    % for sample rate offset tracking. Extra samples may be required if the
    % receiver clock is significantly faster than the transmitter.
    maxSRO = 120; % Parts per million
    Ne = ceil(numRxSamples*maxSRO*1e-6); % Number of extra samples
    Ne = min(Ne,rxWaveLen-numRxSamples); % Limited to length of waveform
    numRxSamplesProcess = numRxSamples+Ne;
    rxData = rx(pktOffset+(ind.HEData(1):numRxSamplesProcess),:);
    if user.RUSize==26
        % Force phase only tracking for 26-tone RU as algorithm susceptible to noise
        cfgDataRec.PilotTimeTracking = false;
        cfgDataRec.PilotPhaseTracking = phaseTracking;
    else
        cfgDataRec.PilotTimeTracking = timeTracking;
        cfgDataRec.PilotPhaseTracking = phaseTracking;
    end
    demodSym = helperTrackingOFDMDemodulate(rxData,chanEst,numOFDMSym,user,cfgDataRec);

    % Estimate noise power in HE fields
    demodPilotSym = demodSym(heInfo.PilotIndices,:,:);
    nVarEst = wlanHEDataNoiseEstimate(demodPilotSym,pilotEst,user);

    % Equalize
    [eqSym,csi] = wlanHEEqualize(demodSym,chanEst,nVarEst,user,'HE-Data');

    % Discard pilot subcarriers
    eqSymUser = eqSym(heInfo.DataIndices,:,:);
    csiData = csi(heInfo.DataIndices,:);

    % Demap and decode bits
    rxPSDU = wlanHEDataBitRecover(eqSymUser,nVarEst,csiData,user,'LDPCDecodingMethod','norm-min-sum');

    % Deaggregate the A-MPDU
    [mpduList,~,status] = wlanAMPDUDeaggregate(rxPSDU,wlanHESUConfig);
    if strcmp(status,'Success')
        fprintf('  A-MPDU deaggregation successful \n');
    else
        fprintf('  A-MPDU deaggregation unsuccessful \n');
    end

    % Decode the list of MPDUs and check the FCS for each MPDU
    for i = 1:numel(mpduList)
        [~,~,status] = wlanMPDUDecode(mpduList{i},wlanHESUConfig,'DataFormat','octets');
        if strcmp(status,'Success')
            fprintf('  FCS pass for MPDU:%d\n',i);
        else
            fprintf('  FCS fail for MPDU:%d\n',i);
        end
    end

    % Plot equalized constellation of the recovered HE data symbols for all
    % spatial streams per user
    helperPlotEQConstellation(eqSymUser,user,ConstellationDiagram,iu,numUsers);

    % Measure EVM of HE-Data symbols
    release(EVM);
    EVM.ReferenceConstellation = wlanReferenceSymbols(user);
    rmsEVM = EVM(eqSymUser(:));
    fprintf('  HE-Data EVM:%2.2fdB\n\n',20*log10(rmsEVM/100));

    % Plot EVM per symbol of the recovered HE data symbols
    helperPlotEVMPerSymbol(eqSymUser,user,EVMPerSymbol,iu,numUsers);

    % Plot EVM per subcarrier of the recovered HE data symbols
    helperPlotEVMPerSubcarrier(eqSymUser,user,EVMPerSubcarrier,iu,numUsers);
end
 Decoding User:1, STAID:1, RUSize:52

  A-MPDU deaggregation successful 

  FCS pass for MPDU:1

  HE-Data EVM:-28.89dB

 Decoding User:2, STAID:2, RUSize:52

  A-MPDU deaggregation successful 

  FCS pass for MPDU:1

  HE-Data EVM:-39.59dB

 Decoding User:3, STAID:3, RUSize:106

  A-MPDU deaggregation successful 

  FCS pass for MPDU:1

  HE-Data EVM:-28.07dB

 Decoding User:4, STAID:4, RUSize:106

  A-MPDU deaggregation successful 

  FCS pass for MPDU:1

  HE-Data EVM:-30.89dB

Selected Bibliography

  1. IEEE Std 802.11ax™-2021. IEEE Standard for Information Technology - Telecommunications and Information Exchange between Systems - Local and Metropolitan Area Networks - Specific Requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications - Amendment 1: Enhancements for High-Efficiency WLAN.

  2. IEEE Std 802.11™-2020 Standard for Information Technology - Telecommunications and Information Exchange between Systems - Local and Metropolitan Area Networks - Specific Requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.