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wlanVHTLTFChannelEstimate

Channel estimation using VHT-LTF

Syntax

chEst = wlanVHTLTFChannelEstimate(demodSig,cfg)
chEst = wlanVHTLTFChannelEstimate(demodSig,cbw,numSTS)
chEst = wlanVHTLTFChannelEstimate(___,span)

Description

example

chEst = wlanVHTLTFChannelEstimate(demodSig,cfg) returns the channel estimate, using the demodulated VHT-LTF[1] signal, demodSig, given the parameters specified in wlanVHTConfig object cfg.

example

chEst = wlanVHTLTFChannelEstimate(demodSig,cbw,numSTS) returns the channel estimate for the specified channel bandwidth, cbw, and the number of space-time streams, numSTS.

example

chEst = wlanVHTLTFChannelEstimate(___,span) specifies the span of a moving-average filter used to perform frequency smoothing.

Examples

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Display the channel estimate of the data and pilot subcarriers for a VHT format channel using its long training field.

Create a VHT format configuration object. Generate a VHT-LTF based on cfg.

cfg = wlanVHTConfig;
txSig = wlanVHTLTF(cfg);

Multiply the transmitted VHT-LTF signal by 0.3 - 0.15i and pass it through an AWGN channel having a 30 dB signal-to-noise ratio. Demodulate the received signal.

rxSig = awgn(txSig*(0.3-0.15i),30);
demodSig = wlanVHTLTFDemodulate(rxSig,cfg);

Estimate the channel response using the demodulated VHT-LTF signal.

est = wlanVHTLTFChannelEstimate(demodSig,cfg);

Plot the channel estimate.

scatterplot(est)
grid

The channel estimate matches the complex channel multiplier.

Estimate and display the channel coefficients of a 4x2 MIMO channel using the VHT-LTF.

Create a VHT format configuration object for a channel having four spatial streams and four transmit antennas. Transmit a complete VHT waveform.

cfg = wlanVHTConfig('NumTransmitAntennas',4, ...
    'NumSpaceTimeStreams',4,'MCS',5);
txWaveform = wlanWaveformGenerator([1;0;0;1;1;0],cfg);

Set the sampling rate, and then pass the transmitted waveform through a 4x2 TGac channel.

fs = 80e6;
tgacChan = wlanTGacChannel('SampleRate',fs, ...
    'NumTransmitAntennas',4,'NumReceiveAntennas',2);
rxWaveform = tgacChan(txWaveform);

Determine the VHT-LTF field indices and demodulate the VHT-LTF from the received waveform.

indVHTLTF = wlanFieldIndices(cfg,'VHT-LTF');
ltfDemodSig = wlanVHTLTFDemodulate(rxWaveform(indVHTLTF(1):indVHTLTF(2),:), cfg);

Generate the channel estimate by using the demodulated VHT-LTF signal. Specify a smoothing filter span of five subcarriers.

est = wlanVHTLTFChannelEstimate(ltfDemodSig,cfg,5);

Plot the magnitude response of the first space-time stream for both receive antennas. Due to the random nature of the fading channel, your results may vary.

plot(abs(est(:,1,1)))
hold on
plot(abs(est(:,1,2)))
xlabel('Subcarrier')
ylabel('Magnitude')
legend('Rx Antenna 1','Rx Antenna 2')

Recover VHT-Data field bits for a multiuser transmission using channel estimation on a VHT-LTF field over a quasi-static fading channel.

Create a VHT configuration object having a 160 MHz channel bandwidth, 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];
vht = wlanVHTConfig('ChannelBandwidth',cbw,'NumUsers',2, ...
    'NumTransmitAntennas',4,'NumSpaceTimeStreams',numSTS);

Because there are two users, the PSDU length is a 1-by-2 row vector.

psduLen = vht.PSDULength
psduLen =

        1050        3156

Generate multiuser input data. This data must be in the form of a 1-by- N cell array, where N is the number of users.

txDataBits{1} = randi([0 1],8*vht.PSDULength(1),1);
txDataBits{2} = randi([0 1],8*vht.PSDULength(2),1);

Generate VHT-LTF and VHT-Data field signals.

txVHTLTF  = wlanVHTLTF(vht);
txVHTData = wlanVHTData(txDataBits,vht);

Pass the data field for the first user through a 4x1 channel because it consists of a single space-time stream. Pass the second user's data through a 4x3 channel because it consists of three space-time streams. Apply white Gaussian noise to each user signal.

snr = 15;
H1 = 1/sqrt(2)*complex(randn(4,1),randn(4,1));
H2 = 1/sqrt(2)*complex(randn(4,3),randn(4,3));

rxVHTData1 = awgn(txVHTData*H1,snr,'measured');
rxVHTData2 = awgn(txVHTData*H2,snr,'measured');

Repeat the process for the VHT-LTF fields.

rxVHTLTF1  = awgn(txVHTLTF*H1,snr,'measured');
rxVHTLTF2  = awgn(txVHTLTF*H2,snr,'measured');

Calculate the received signal power for both users and use it to estimate the noise variance.

powerDB1 = 10*log10(var(rxVHTData1));
noiseVarEst1 = mean(10.^(0.1*(powerDB1-snr)));

powerDB2 = 10*log10(var(rxVHTData2));
noiseVarEst2 = mean(10.^(0.1*(powerDB2-snr)));

Estimate the channel characteristics using the VHT-LTF fields.

demodVHTLTF1 = wlanVHTLTFDemodulate(rxVHTLTF1,cbw,numSTS);
chanEst1 = wlanVHTLTFChannelEstimate(demodVHTLTF1,cbw,numSTS);

demodVHTLTF2 = wlanVHTLTFDemodulate(rxVHTLTF2,cbw,numSTS);
chanEst2 = wlanVHTLTFChannelEstimate(demodVHTLTF2,cbw,numSTS);

Recover VHT-Data field bits for the first user and compare against the original payload bits.

rxDataBits1 = wlanVHTDataRecover(rxVHTData1,chanEst1,noiseVarEst1,vht,1);
[~,ber1] = biterr(txDataBits{1},rxDataBits1)
ber1 =

    0.4983

Determine the number of bit errors for the second user.

rxDataBits2 = wlanVHTDataRecover(rxVHTData2,chanEst2,noiseVarEst2,vht,2);
[~,ber2] = biterr(txDataBits{2},rxDataBits2)
ber2 =

    0.0972

The bit error rates are quite high because there is no precoding to mitigate the interference between streams. This is especially evident for the user 1 receiver because it receives energy from the three streams intended for user 2. The example is intended to show the workflow and proper syntaxes for the LTF demodulate, channel estimation, and data recovery functions.

Input Arguments

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Demodulated VHT-LTF signal, specified as an NST-by-NSYM-by-NR array. NST is the number of occupied subcarriers, NSYM is the number of VHT-LTF OFDM symbols, and NR is the number of receive antennas.

Data Types: double
Complex Number Support: Yes

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

Filter span of the frequency smoothing filter, specified as an odd integer. The span is expressed as a number of subcarriers.

Note

If adjacent subcarriers are highly correlated, frequency smoothing results in significant noise reduction. However, in a highly frequency-selective channel, smoothing can degrade the quality of the channel estimate.

Data Types: double

Output Arguments

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Channel estimate between all combinations of space-time streams and receive antennas, returned as an NST-by-NSTS,total-by-NR array. NST is the number of occupied subcarriers. NSTS,total is the total number of space-time streams for all users. For the single-user case, NSTS,total = NSTS. NR is the number of receive antennas. The channel estimate includes coefficients for both the data and pilot subcarriers.

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.

The VHT-LTF is defined in IEEE® Std 802.11ac™-2013, Section 22.3.8.3.5.

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.

[3] Perahia, E., and R. Stacey. Next Generation Wireless LANs: 802.11n and 802.11ac. 2nd Edition, United Kingdom: Cambridge University Press, 2013.

Extended Capabilities

Introduced in R2015b


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

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