Transmit and Recover L-SIG, VHT-SIG-A, and VHT-SIG-B in Fading Channel

Transmit a VHT waveform through a noisy MIMO channel. Extract the L-SIG, VHT-SIG-A, and VHT-SIG-B fields and verify that they were correctly recovered.

Set the parameters used throughout the example.

cbw = 'CBW40';                     % Channel bandwidth
fs = 40e6;                         % Sample rate (Hz)
ntx = 2;                           % Number of transmit antennas
nsts = 2;                          % Number of space-time streams
nrx = 3;                           % Number of receive antennas

Create a VHT configuration object that supports a 2x2 MIMO transmission and has an APEP length of 2000.

vht = wlanVHTConfig('ChannelBandwidth',cbw,'APEPLength',2000, ...
    'NumTransmitAntennas',ntx,'NumSpaceTimeStreams',nsts, ...
    'SpatialMapping','Direct','STBC',false);

Generate a VHT waveform containing a random PSDU.

txPSDU = randi([0 1],vht.PSDULength*8,1);
txPPDU = wlanWaveformGenerator(txPSDU,vht);

Create a 2x2 TGac channel and an AWGN channel with an SNR=10 dB.

tgacChan = wlanTGacChannel('SampleRate',fs,'ChannelBandwidth',cbw, ...
    'NumTransmitAntennas',ntx,'NumReceiveAntennas',nrx, ...
    'LargeScaleFadingEffect','Pathloss and shadowing', ...
    'DelayProfile','Model-C');

chNoise = comm.AWGNChannel('NoiseMethod','Signal to noise ratio (SNR)',...
    'SNR',10);

Pass the VHT waveforms through a 2x2 TGac channel and add the AWGN channel noise.

rxPPDU = chNoise(tgacChan(txPPDU));

Add additional white noise corresponding to a receiver with a 9 dB noise figure. The noise variance is equal to k*T*B*F, where k is Boltzmann's constant, T is the ambient temperature, B is the channel bandwidth (sample rate), and F is the receiver noise figure.

nVar = 10^((-228.6+10*log10(290) + 10*log10(fs) + 9 )/10);
rxNoise = comm.AWGNChannel('NoiseMethod','Variance','Variance',nVar);

rxPPDU = rxNoise(rxPPDU);

Find the start and stop indices for all component fields of the PPDU.

ind = wlanFieldIndices(vht)

ind = struct with fields:
       LSTF: [1 320]
       LLTF: [321 640]
       LSIG: [641 800]
    VHTSIGA: [801 1120]
     VHTSTF: [1121 1280]
     VHTLTF: [1281 1600]
    VHTSIGB: [1601 1760]
    VHTData: [1761 25600]

The preamble is contained in the first 1760 symbols. Plot the preamble.

plot(abs(rxPPDU(1:1760)))

Extract the L-LTF from the received PPDU using the start and stop indices determined by the wlanFieldIndices function. Demodulate the L-LTF and estimate the channel coefficients.

rxLLTF = rxPPDU(ind.LLTF(1):ind.LLTF(2),:);
demodLLTF = wlanLLTFDemodulate(rxLLTF,vht);
chEstLLTF = wlanLLTFChannelEstimate(demodLLTF,vht);

Extract the L-SIG field from the received PPDU and recover its information bits.

rxLSIG = rxPPDU(ind.LSIG(1):ind.LSIG(2),:);
infoLSIG = wlanLSIGRecover(rxLSIG,chEstLLTF,nVar,cbw);

Inspect the L-SIG rate information and confirm that the sequence [1 1 0 1] is received. This sequence corresponds to a 6 MHz data rate, which is used for all VHT transmissions.

rate = infoLSIG(1:4)'

rate = 1x4 int8 row vector

   0   1   1   1

Extract the VHT-SIG-A and confirm that the CRC check passed.

rxVHTSIGA = rxPPDU(ind.VHTSIGA(1):ind.VHTSIGA(2),:);
[infoVHTSIGA,failCRC] = wlanVHTSIGARecover(rxVHTSIGA, ...
    chEstLLTF,nVar,cbw);
failCRC

failCRC = logical
   1

Extract and demodulate the VHT-LTF. Use the demodulated signal to estimate the channel coefficients needed to recover the VHT-SIG-B field.

rxVHTLTF = rxPPDU(ind.VHTLTF(1):ind.VHTLTF(2),:);
demodVHTLTF = wlanVHTLTFDemodulate(rxVHTLTF,vht);
chEstVHTLTF = wlanVHTLTFChannelEstimate(demodVHTLTF,vht);

Extract and recover the VHT-SIG-B.

rxVHTSIGB = rxPPDU(ind.VHTSIGB(1):ind.VHTSIGB(2),:);
infoVHTSIGB = wlanVHTSIGBRecover(rxVHTSIGB,chEstVHTLTF,nVar,cbw);

Verify that the APEP length, contained in the first 19 bits of the VHT-SIG-B, corresponds to the specified length of 2000 bits.

pktLbits = infoVHTSIGB(1:19);
pktLen = bit2int(double(pktLbits),19,false)*4

pktLen = 
1676920