Channel estimation using HT-LTF
Estimate SISO Channel Using HT-LTF
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.
The channel estimate matches the complex channel multiplier.
Estimate MIMO Channel Using HT-LTF
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
demodSig — Demodulated HT-LTF signal
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.
Complex Number Support: Yes
cfg — Configuration information
Configuration information, specified as a
span — Filter span
positive odd integer
Filter span of the frequency smoothing filter, specified as a positive odd
integer and expressed as a number of subcarriers. Frequency smoothing is
applied only when
span is specified and greater than
one. See Frequency Smoothing.
chEst — Channel estimate
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.
Complex Number Support: Yes
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 126.96.36.199.6 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.
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.
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.
Frequency smoothing can improve channel estimation by averaging out noise.
Frequency smoothing is recommended only for cases in which a single transmit antenna is used. Frequency smoothing consists of applying a moving-average filter that spans multiple adjacent subcarriers. Channel conditions dictate whether frequency smoothing is beneficial.
If adjacent subcarriers are highly correlated, frequency smoothing results in significant noise reduction.
In a highly frequency-selective channel, smoothing can degrade the quality of the channel estimate.
 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.
 Perahia, E., and R. Stacey. Next Generation Wireless LANs: 802.11n and 802.11ac . 2nd Edition, United Kingdom: Cambridge University Press, 2013.
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1 IEEE Std 802.11-2012 Adapted and reprinted with permission from IEEE. Copyright IEEE 2012. All rights reserved.