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Recover feedback status from HE TB feedback NDP



status = wlanHETBNDPFeedbackStatus(rxSym,cfg) recovers the feedback status of an HE trigger-based (HE TB) feedback null data packet (NDP) transmission parameterized by cfg. The function recovers the feedback status by using rxSym, the demodulated high-efficiency long training field (HE-LTF) of the transmission. The function estimates the feedback status by using the algorithm defined in [1].

The recovered feedback status indicates the value of the bit used for tone modulation in each tone set specified by the RUToneSetIndex property of the cfg input.

For more information about the HE TB feedback NDP, see section 27.3.17 of [2].


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Configure an uplink HE TB feedback NDP transmission with four stations (STAs), a channel bandwidth of 20 MHz, and a signal-to-noise ratio (SNR) of 20 dB.

numSTA = 4;
cbw = 'CBW20';
snr = 20;
cfgSTA = cell(1,numSTA);

Specify the resource unit (RU) tone set index, starting space-time stream, and feedback status for all STAs.

ruToneSetIndex = repmat([1 2],1,round(numSTA/2));
startingSTS = repmat([1 2],1,round(numSTA/2));
feedbackStatus = repmat([1 0],1,round(numSTA/2));

Create a valid HE TB feedback NDP configuration.

cfg = wlanHETBConfig;
cfg = getNDPFeedbackConfiguration(cfg);

Configure the channel for transmission, assuming no variation across STAs.

tgax = wlanTGaxChannel('ChannelBandwidth',cbw, ...
    'TransmissionDirection','Uplink', ...
chanInfo = info(tgax);
awgn = comm.AWGNChannel('NoiseMethod','Signal to noise ratio (SNR)', ...

Configure STAs and generate an HE TB feedback NDP waveform.

rx = 0;
for idx = 1:numSTA
    % Configure STAs
    cfg.RUToneSetIndex = ruToneSetIndex(idx);
    cfg.StartingSpaceTimeStream = startingSTS(idx);
    cfg.FeedbackStatus = feedbackStatus(idx);
    cfgSTA{idx} = cfg;
    % Generate transmit waveform
    waveform = wlanWaveformGenerator([],cfg);
    % Pass waveform through TGax channel
    rx = rx + tgax([waveform; zeros(15,size(waveform,2))]);

Pass the waveform through the AWGN channel, accounting for the noise energy in nulls to ensure the SNR is defined per active and complementary subcarrier.

field = 'HE-LTF';
ofdmInfo = wlanHEOFDMInfo(field,cbw,cfg.GuardInterval);
awgn.SNR = snr - 10*log10(ofdmInfo.FFTLength/12);
rx = awgn(rx);

Get the field indices and extract the HE-LTF.

ind = wlanFieldIndices(cfgSTA{1});
offset = chanInfo.ChannelFilterDelay;
heltf = rx(offset+(ind.HELTF(1):ind.HELTF(2)),:);

Demodulate the HE-LTF.

rxSym = wlanHEDemodulate(heltf,field,cbw,cfg.GuardInterval,cfg.HELTFType);

Recover the feedback status for the STAs.

status = zeros(1,numSTA);
for n = 1:numSTA
    status(n) = wlanHETBNDPFeedbackStatus(rxSym,cfgSTA{n});

Compare the transmitted and received feedback status for the STAs.


Input Arguments

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Demodulated HE-LTF of the received HE TB feedback NDP, specified as a complex-valued array of size Nst-by-2-by-Nr.

  • Nst is the number of subcarriers.

    • For transmissions that include active and complementary subcarriers, Nst must be 12.

    • For all other transmissions, Nst must be 242, 484, 996, or 1992.

  • Nr is the number of receive antennas.

Data Types: double
Complex Number Support: Yes

HE TB feedback NDP transmission configuration, specified as a wlanHETBConfig object.

Output Arguments

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Feedback status, returned as one of these values.

  • 1 — Transmission detected on the first tone set

  • 0 — Transmission detected on the second tone set

  • -1 — Transmission not detected on either tone set

This output indicates the value of the bit used for tone modulation in each tone set specified by the RUToneSetIndex property of the cfg input. The feedback status and RU tone set index determine the HE-LTF subcarrier mapping in accordance with Table 27-30 of [2].

Data Types: double


[1] Montreuil, Leo et al. NDP Short Feedback Design. IEEE® 802.11™-17/0044r4 (May 2017).

[2] IEEE P802.11ax™/D4.1. “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. Amendment 1: Enhancements for High Efficiency WLAN.” Draft Standard for Information technology — Telecommunications and information exchange between systems. Local and metropolitan area networks — Specific requirements.

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