wlanHTLTFChannelEstimate

Channel estimation using HT-LTF

Description

example

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

example

chEst = wlanHTLTFChannelEstimate(demodSig,cfg,span) returns the channel estimate and specifies the span of a moving-average filter used to perform frequency smoothing.

Examples

collapse all

Estimate and plot the channel coefficients of an HT-mixed format channel by using the high throughput long training field.

Create an HT format configuration object. Generate the corresponding HT-LTF based on the object.

cfg = wlanHTConfig;
txSig = wlanHTLTF(cfg);

Multiply the transmitted HT-LTF signal by 0.2 + 0.1i and pass it through an AWGN channel. Demodulate the received signal.

rxSig = awgn(txSig*(0.2+0.1i),30);
demodSig = wlanHTLTFDemodulate(rxSig,cfg);

Estimate the channel response using the demodulated HT-LTF.

est = wlanHTLTFChannelEstimate(demodSig,cfg);

Plot the channel estimate.

scatterplot(est)
grid

The channel estimate matches the complex channel multiplier.

Estimate the channel coefficients of a 2x2 MIMO channel by using the high throughput long training field. Recover the HT-data field and determine the number of bit errors.

Create an HT-mixed format configuration object for a channel having two spatial streams and four transmit antennas. Transmit a complete HT waveform.

cfg = wlanHTConfig('NumTransmitAntennas',2, ...
    'NumSpaceTimeStreams',2,'MCS',11);
txPSDU = randi([0 1],8*cfg.PSDULength,1);
txWaveform = wlanWaveformGenerator(txPSDU,cfg);

Pass the transmitted waveform through a 2x2 TGn channel.

tgnChan = wlanTGnChannel('SampleRate',20e6, ...
    'NumTransmitAntennas',2, ...
    'NumReceiveAntennas',2, ...
    'LargeScaleFadingEffect','Pathloss and shadowing');
rxWaveformNoNoise = tgnChan(txWaveform);

Create an AWGN channel with noise power, nVar, corresponding to a receiver having a 9 dB noise figure. The noise power is equal to kTBF, where k is Boltzmann's constant, T is the ambient noise temperature (290K), B is the bandwidth (20 MHz), and F is the noise figure (9 dB).

nVar = 10^((-228.6 + 10*log10(290) + 10*log10(20e6) + 9)/10);
awgnChan = comm.AWGNChannel('NoiseMethod','Variance', ...
    'Variance',nVar);

Pass the signal through the AWGN channel.

rxWaveform = awgnChan(rxWaveformNoNoise);

Determine the indices for the HT-LTF. Extract the HT-LTF from the received waveform. Demodulate the HT-LTF.

indLTF  = wlanFieldIndices(cfg,'HT-LTF');
rxLTF = rxWaveform(indLTF(1):indLTF(2),:);
ltfDemodSig = wlanHTLTFDemodulate(rxLTF,cfg);

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

chEst = wlanHTLTFChannelEstimate(ltfDemodSig,cfg,3);

Extract the HT-data field from the received waveform.

indData = wlanFieldIndices(cfg,'HT-Data');
rxDataField = rxWaveform(indData(1):indData(2),:);

Recover the data and verify that there no bit errors occurred.

rxPSDU = wlanHTDataRecover(rxDataField,chEst,nVar,cfg);

numErrs = biterr(txPSDU,rxPSDU)
numErrs = 0

Input Arguments

collapse all

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

Data Types: double
Complex Number Support: Yes

Configuration information, specified as a wlanHTConfig object.

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 will result in significant noise reduction. However, in a highly frequency selective channel, smoothing may degrade the quality of the channel estimate.

Data Types: double

Output Arguments

collapse all

Channel estimate between all combinations of space-time streams and receive antennas, returned as an NST-by-(NSTS+NESS)-by-NR array. NST is the number of occupied subcarriers, NSTS is the number of space-time streams. NESS is the number of extension spatial streams. NR is the number of receive antennas. Data and pilot subcarriers are included in the channel estimate.

Data Types: double
Complex Number Support: Yes

More About

collapse all

HT-LTF

The high throughput long training field (HT-LTF) is located between the HT-STF and data field of an HT-mixed packet.

As described in Section 19.3.9.4.6 of IEEE® Std 802.11™-2016, the receiver can use the HT-LTF to estimate the MIMO channel between the set of QAM mapper outputs (or, if STBC is applied, the STBC encoder outputs) and the receive chains. The HT-LTF portion has one or two parts. The first part consists of one, two, or four HT-LTFs that are necessary for demodulation of the HT-Data portion of the PPDU. These HT-LTFs are referred to as HT-DLTFs. The optional second part consists of zero, one, two, or four HT-LTFs that can be used to sound extra spatial dimensions of the MIMO channel not utilized by the HT-Data portion of the PPDU. These HT-LTFs are referred to as HT-ELTFs. Each HT long training symbol is 4 μs. The number of space-time streams and the number of extension streams determines the number of HT-LTF symbols transmitted.

Tables 19-12, 19-13 and 90-14 from IEEE Std 802.11-2012 are reproduced here.

NSTS DeterminationNHTDLTF DeterminationNHTELTF Determination

Table 19-12 defines the number of space-time streams (NSTS) based on the number of spatial streams (NSS) from the MCS and the STBC field.

Table 19-13 defines the number of HT-DLTFs required for the NSTS.

Table 19-14 defines the number of HT-ELTFs required for the number of extension spatial streams (NESS). NESS is defined in HT-SIG2.

NSS from MCSSTBC fieldNSTS
101
112
202
213
224
303
314
404

NSTSNHTDLTF
11
22
34
44

NESSNHTELTF
00
11
22
34

Additional constraints include:

  • NHTLTF = NHTDLTF + NHTELTF ≤ 5.

  • NSTS + NESS ≤ 4.

    • When NSTS = 3, NESS cannot exceed one.

    • If NESS = 1 when NSTS = 3 then NHTLTF = 5.

References

[1] 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.

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

Extended Capabilities

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

Introduced in R2015b


[1] IEEE Std 802.11-2012 Adapted and reprinted with permission from IEEE. Copyright IEEE 2012. All rights reserved.