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wlanVHTLTFDemodulate

Demodulate VHT-LTF waveform

Description

example

sym = wlanVHTLTFDemodulate(rx,cfg) returns a demodulated VHT-LTF[1] waveform y by demodulating time-domain input signal rx for very high throughput (VHT) transmission parameters cfg.

example

sym = wlanVHTLTFDemodulate(rx,cbw,numSTS) specifies channel bandwidth cbw and number of space-time streams numSTS.

example

sym = wlanVHTLTFDemodulate(___,symOffset) specifies the OFDM symbol offset as a fraction of the cyclic prefix length.

Examples

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Create a VHT format configuration object.

vht = wlanVHTConfig;

Generate a VHT-LTF signal.

txVHTLTF = wlanVHTLTF(vht);

Add white noise to the signal.

rxVHTLTF = awgn(txVHTLTF,1);

Demodulate the received signal.

y = wlanVHTLTFDemodulate(rxVHTLTF,vht);

Specify a VHT format configuration object and generate a VHT-LTF.

vht = wlanVHTConfig;
txltf = wlanVHTLTF(vht);

Multiply the transmitted VHT-LTF by 0.1 + 0.1i . Pass the signal through an AWGN channel.

rxltfNoNoise = txltf * complex(0.1,0.1);
rxltf = awgn(rxltfNoNoise,20,'measured');

Demodulated the received VHT-LTF with a symbol offset of 0.5.

dltf = wlanVHTLTFDemodulate(rxltf,vht,0.5);

Estimate the channel using the demodulated VHT-LTF. Plot the result.

chEst = wlanVHTLTFChannelEstimate(dltf,vht);
scatterplot(chEst)

Figure Scatter Plot contains an axes object. The axes object with title Scatter plot contains an object of type line. This object represents Channel 1.

The estimate is very close to the previously introduced 0.1+0.1i multiplier.

Generate a VHT waveform. Extract and demodulate the VHT long training field (VHT-LTF) to estimate the channel coefficients. Recover the data field by using the channel estimate and use this field to determine the number of bit errors.

Configure a VHT-format configuration object with two paths.

vht = wlanVHTConfig('NumTransmitAntennas',2,'NumSpaceTimeStreams',2);

Generate a random PSDU and create the corresponding VHT waveform.

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

Pass the signal through a TGac 2x2 MIMO channel.

tgacChan = wlanTGacChannel('NumTransmitAntennas',2,'NumReceiveAntennas',2, ...
    'LargeScaleFadingEffect','Pathloss and shadowing');
rxSigNoNoise = tgacChan(txSig);

Add AWGN to the received signal. Set the noise variance for the case in which the receiver has a 9-dB noise figure.

nVar = 10^((-228.6+10*log10(290)+10*log10(80e6)+9)/10);
awgnChan = comm.AWGNChannel('NoiseMethod','Variance','Variance',nVar);
rxSig = awgnChan(rxSigNoNoise);

Determine the indices for the VHT-LTF and extract the field from the received signal.

indVHT = wlanFieldIndices(vht,'VHT-LTF');
rxLTF = rxSig(indVHT(1):indVHT(2),:);

Demodulate the VHT-LTF and estimate the channel coefficients.

dLTF = wlanVHTLTFDemodulate(rxLTF,vht);
chEst = wlanVHTLTFChannelEstimate(dLTF,vht);

Extract the VHT-Data field and recover the information bits.

indData = wlanFieldIndices(vht,'VHT-Data');
rxData = rxSig(indData(1):indData(2),:);
rxPSDU = wlanVHTDataRecover(rxData,chEst,nVar,vht);

Determine the number of bit errors.

numErrs = biterr(txPSDU,rxPSDU)
numErrs = 0

Recover bits from the VHT-Data field of a VHT multi-user transmission recovered from a fading MU-MIMO channel by using channel estimation on the VHT-LTF.

This example can return high bit error rates because the transmission does not include precoding to mitigate the interference between space-time streams. However, the example shows a typical VHT signal recovery workflow and appropriate syntax use for the functions involved.

Configure a VHT transmission with a channel bandwidth of 160 MHz, two users, and four transmit antennas. Assign one space-time stream to the first user and three space-time streams to the second user.

cbw = 'CBW160';
numSTS = [1 3];
cfgVHT = wlanVHTConfig('ChannelBandwidth',cbw,'NumUsers',2, ...
    'NumTransmitAntennas',4,'NumSpaceTimeStreams',numSTS);

Generate a payload of bits for each user. This payload must be in the a 1-by-N cell array, where N is the number of users.

psduLength = 8*cfgVHT.PSDULength;
numUsers = cfgVHT.NumUsers;
bits = cell(1,2);
for nu = 1:numUsers
    bits{nu} = randi([0 1],psduLength(nu),1);
end

Generate VHT-LTF and VHT-Data field signals.

txLTF  = wlanVHTLTF(cfgVHT); 
txDataSym = wlanVHTData(bits,cfgVHT);

Pass the VHT-Data field signal for the first user through a 4x1 channel because this signal consists of a single space-time stream. Pass the VHT-Data field for the second user data through a 4x3 channel because this signal consists of three space-time streams. Apply AWGN to each signal, assuming an SNR of 15 dB.

snr = 15; 
H{1} = complex(randn(4,1),randn(4,1))/sqrt(2);
H{2} = complex(randn(4,3),randn(4,3))/sqrt(2);
number = zeros(2,1);
ratio = zeros(2,1);
for userIdx = 1:numUsers
    rxDataSym = awgn(txDataSym*H{userIdx},snr,'measured');

Apply the same channel processing to the VHT-LTF for each user.

    rxLTF = awgn(txLTF*H{userIdx},snr,'measured');

Calculate the received signal power for each user and estimate the noise variance.

    powerDB = 10*log10(var(rxDataSym));
    noiseVarEst = mean(10.^(0.1*(powerDB-snr)));

Estimate the channel characteristics by using the VHT-LTF.

    demod = wlanVHTLTFDemodulate(rxLTF,cbw,numSTS);
    chEst = wlanVHTLTFChannelEstimate(demod,cbw,numSTS);

Recover the bits from the received VHT-Data field for each user and determine the bit error rate by comparing the recovered bits with the original payload bits.

    dataBits = wlanVHTDataRecover(rxDataSym,chEst,noiseVarEst,cfgVHT,userIdx);
    [number(userIdx),ratio(userIdx)] = biterr(bits{userIdx},dataBits);
    disp(number(userIdx))
    disp(ratio(userIdx))
end
        4269
    0.5082
        2444
    0.0968

Input Arguments

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Received time-domain signal, specified as a complex-valued matrix of size Ns-by-Nr.

  • Ns is the number of time-domain samples. If Ns is not an integer multiple of the OFDM symbol length, Ls, for the specified field,then the function ignores the remaining mod(Ns,Ls) symbols.

  • Nr is the number of receive antennas.

Data Types: double
Complex Number Support: Yes

VHT format configuration, specified as a wlanVHTConfig object.

Channel bandwidth, specified as 'CBW20', 'CBW40', 'CBW80', or 'CBW160'. If the transmission has multiple users, the same channel bandwidth is applied to all users.

Data Types: char | string

Number of space-time streams in the transmission, specified as a scalar or vector.

  • For a single user, the number of space-time streams is a scalar integer from 1 to 8.

  • For multiple users, the number of space-time streams is a 1-by-NUsers vector of integers from 1 to 4, where the vector length, NUsers, is an integer from 1 to 4.

Example: [1 3 2] indicates that one space-time stream is assigned to user 1, three space-time streams are assigned to user 2, and two space-time streams are assigned to user 3.

Note

The sum of the space-time stream vector elements must not exceed eight.

Data Types: double

OFDM symbol sampling offset, as a fraction of the cyclic prefix length, specified as a scalar in the interval [0, 1].

The value that you specify indicates the start location for OFDM demodulation relative to the beginning of the cyclic prefix.

Example: 0.45

Data Types: double

Output Arguments

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Demodulated frequency-domain signal, returned as a complex-valued array of size Nsc-by-Nsym-by-Nr.

  • Nsc is the number of active occupied subcarriers in the demodulated field.

  • Nsym is the number of OFDM symbols.

  • Nr is the number of receive antennas.

Data Types: double
Complex Number Support: Yes

More About

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VHT-LTF

The very high throughput long training field (VHT-LTF) is located between the VHT-STF and VHT-SIG-B portion of the VHT packet.

It is used for MIMO channel estimation and pilot subcarrier tracking. The VHT-LTF includes one VHT long training symbol for each spatial stream indicated by the selected MCS. Each symbol is 4 μs long. A maximum of eight symbols are permitted in the VHT-LTF.

For a detailed description of the VHT-LTF, see section 21.3.8.3.5 of IEEE® Std 802.11™-2016.

References

[1] IEEE Std 802.11ac™-2013 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 4: Enhancements for Very High Throughput for Operation in Bands below 6 GHz.

[2] IEEE Std 802.11™-2012 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.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

Introduced in R2015b

[1] IEEE Std 802.11ac™-2013 Adapted and reprinted with permission from IEEE. Copyright IEEE 2013. All rights reserved.