wlanHTSIGRecover

R2026b

Recover HT-SIG information bits

Syntax

recBits = wlanHTSIGRecover(rxSig,chanEst,nVar,cbw)

recBits = wlanHTSIGRecover(rxSig,chanEst,nVar,cbw,Name,Value)

[recBits,failCRC] = wlanHTSIGRecover(___)

[recBits,failCRC,eqSym] = wlanHTSIGRecover(___)

[recBits,failCRC,eqSym,cpe] = wlanHTSIGRecover(___)

Description

recBits = wlanHTSIGRecover(rxSig,chanEst,nVar,cbw) returns the recovered information bits from the HT-SIG field and performs a CRC check. Inputs include the channel estimate data chanEst, noise variance estimate nVar, and channel bandwidth cbw.

recBits = wlanHTSIGRecover(rxSig,chanEst,nVar,cbw,Name,Value) specifies algorithm parameters by using one or more name-value pair arguments. For example, PilotPhaseTracking="None" disables pilot phase tracking.

[recBits,failCRC] = wlanHTSIGRecover(___) returns the result of the CRC check, failCRC, using any of the arguments from the previous syntaxes.

[recBits,failCRC,eqSym] = wlanHTSIGRecover(___) returns the equalized symbols, eqSym.

[recBits,failCRC,eqSym,cpe] = wlanHTSIGRecover(___) returns the common phase error, cpe.

Examples

Recover HT-SIG Information Bits in Perfect Channel

Create a wlanHTConfig object having a channel bandwidth of 40 MHz. Use the object to create an HT-SIG field.

cfg = wlanHTConfig(ChannelBandwidth="CBW40");
[txSig,txBits] = wlanHTSIG(cfg);

Because a perfect channel is assumed, specify the channel estimate as a column vector of ones and the noise variance estimate as zero.

chanEst = ones(104,1);
nVar = 0;

Recover the HT-SIG information bits. Verify that the received information bits are identical to the transmitted bits.

rxBits = wlanHTSIGRecover(txSig,chanEst,nVar,"CBW40");
numerr = biterr(txBits,rxBits)

numerr = 
0

Recover HT-SIG Bits Using Zero-Forcing Equalizer

Configure an HT transmission with a channel bandwidth of 40 MHz by creating a wlanHTConfig object. Generate the corresponding HT-SIG field.

cfg = wlanHTConfig(ChannelBandwidth="CBW40");
[txSig,txBits] = wlanHTSIG(cfg);

Pass the transmitted HT-SIG waveform through an additive white Gaussian noise (AWGN) channel.

snrdB = 15; % SNR (dB)
% Convert the SNR per active subcarrier to total SNR to account for noise energy in null subcarriers
ofdmInfo = wlanHTOFDMInfo("HT-LTF",cfg);
snrValue = convertSNR(snrdB,"snrsc","snr",FFTLength=ofdmInfo.FFTLength,NumActiveSubcarriers=ofdmInfo.NumTones);
nVar = 10^((-snrValue-30)/10); % Noise variance (W) for 0 dBm (-30 dBW) signal power
rxSig = awgn(txSig,snrValue);

Recover the HT-SIG field assuming a perfect channel and a noise variance estimate, specifying zero-force equalization. Verify that the received information has no bit errors.

recBits = wlanHTSIGRecover(rxSig,ones(104,1),nVar,"CBW40",EqualizationMethod="ZF");
biterr(txBits,recBits)

ans = 
0

Recover HT-SIG in 2x2 MIMO Channel

Recover HT-SIG in a 2x2 MIMO channel with AWGN. Confirm that the CRC check passes.

Configure a 2x2 MIMO TGn channel.

chanBW = "CBW20";
cfg = wlanHTConfig( ...
    ChannelBandwidth=chanBW, ...
    NumTransmitAntennas=2, ...
    NumSpaceTimeStreams=2);

Generate L-LTF and HT-SIG waveforms.

txLLTF  = wlanLLTF(cfg);
txHTSIG = wlanHTSIG(cfg);

Set the sample rate to correspond to the channel bandwidth. Create a TGn 2x2 MIMO channel without large scale fading effects.

fsamp = 20e6;
tgnChan = wlanTGnChannel(SampleRate=fsamp, ...
    LargeScaleFadingEffect="None", ...
    NumTransmitAntennas=2, ...
    NumReceiveAntennas=2);

Pass the L-LTF and HT-SIG waveforms through a TGn channel with white noise.

rxLLTF = awgn(tgnChan(txLLTF),20);
rxHTSIG = awgn(tgnChan(txHTSIG),20);

Demodulate the L-LTF signal. Generate a channel estimate by using the demodulated L-LTF.

demodLLTF = wlanLLTFDemodulate(rxLLTF,chanBW,1);
chanEst = wlanLLTFChannelEstimate(demodLLTF,chanBW);

Recover the information bits, the CRC failure status, and the equalized symbols from the received HT-SIG field.

[recHTSIGBits,failCRC,eqSym] = wlanHTSIGRecover(rxHTSIG, ...
    chanEst,0.01,chanBW);

Verify that HT-SIG passed a CRC check by examining the status of failCRC.

failCRC

failCRC = logical
   0

Because failCRC is 0, HT-SIG passed the CRC check.

Visualize the scatter plot of the equalized symbols, eqSym.

scatterplot(eqSym(:))

Input Arguments

rxSig — Received HT-SIG field

matrix

Received HT-SIG field, specified as an N_S-by-N_R matrix. N_S is the number of samples and increases with channel bandwidth.

Channel Bandwidth	NS
"CBW20"	160
"CBW40"	320

N_R is the number of receive antennas.

Data Types: double | single
Complex Number Support: Yes

chanEst — Channel estimate

vector | 3-D array

Channel estimate, specified as an N_ST-by-1-by-N_R array. N_ST is the number of occupied subcarriers and increases with channel bandwidth.

Channel Bandwidth	NST
"CBW20"	52
"CBW40"	104

N_R is the number of receive antennas.

The channel estimate is based on the L-LTF.

Data Types: double | single

nVar — Noise variance estimate

nonnegative scalar

Noise variance estimate, specified as a nonnegative scalar.

Data Types: double | single

cbw — Channel bandwidth

"CBW20" | "CBW40"

Channel bandwidth in MHz, specified as "CBW20" or "CBW40".

Data Types: char | string

Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Example: PilotPhaseTracking="None" disables pilot phase tracking.

OFDMSymbolOffset — OFDM symbol sampling offset

0.75 (default) | scalar in the interval [0, 1]

OFDM symbol sampling offset represented as a fraction of the cyclic prefix (CP) length, specified as the name-value argument consisting of OFDMSymbolOffset and a scalar in the interval [0, 1]. The value you specify indicates the start location for OFDM demodulation relative to the beginning of the CP. The value 0 represents the start of the CP, and the value 1 represents the end of the CP.

Data Types: double

EqualizationMethod — Equalization method

"MMSE" (default) | "ZF"

Equalization method, specified as one of these values.

• "MMSE" — The receiver uses a minimum mean-square error equalizer.

• "ZF" — The receiver uses a zero-forcing equalizer.

When the received signal has multiple receive antennas, the function exploits receiver diversity during equalization. When the number of transmitted space-time streams is one and you specify this argument as "ZF", the function performs maximal-ratio combining.

Data Types: char | string

PilotPhaseTracking — Pilot phase tracking

"PreEQ" (default) | "None"

Pilot phase tracking, specified as one of these values.

• "PreEQ" — Enable pilot phase tracking, which the function performs before any equalization operation.

• "None" — Disable pilot phase tracking.

Data Types: char | string

Output Arguments

recBits — Recovered HT-SIG information

vector

Recovered HT-SIG information bits, returned as a 48-element column vector. The number of elements corresponds to the length of the HT-SIG field.

failCRC — CRC failure status

true | false

CRC failure status, returned as a logical scalar. If HT-SIG fails the CRC check, failCRC is true.

eqSym — Equalized symbols

matrix

Equalized symbols, returned as a 48-by-2 matrix corresponding to 48 data subcarriers and 2 OFDM symbols.

Data Types: double | single

cpe — Common phase error

column vector

Common phase error in radians, returned as a 2-by-1 column vector.

Data Types: double | single

More About

HT-SIG

The high throughput signal (HT-SIG) field is located between the L-SIG field and HT-STF and is part of the HT-mixed format preamble. It consists of two symbols, HT-SIG₁ and HT-SIG₂. This diagram¹ shows the HT-SIG field in the HT-mixed preamble.

HT-SIG carries information used to decode the HT packet, including the MCS, packet length, FEC coding type, guard interval, number of extension spatial streams, and whether there is payload aggregation. The HT-SIG symbols are also used for auto-detection between HT-mixed format and legacy OFDM packets. This diagram² shows the information that the bits in the HT-SIG symbols carry.

For a detailed description of the HT-SIG field, see Section 19.3.9.4.3 of IEEE^® Std 802.11™-2020.

L-LTF

The L-LTF is the second field in the 802.11 OFDM PLCP legacy preamble. The L-LTF is a component of EHT, HE, VHT, HT, and non-HT PPDUs. This diagram³ shows the L-LTF in the legacy preamble.

Channel estimation, fine frequency offset estimation, and fine symbol timing offset estimation rely on the L-LTF.

The L-LTF consists of a cyclic prefix (CP) followed by two identical long training symbols (C1 and C2). The CP consists of the second half of the long training symbol.

The L-LTF duration varies with channel bandwidth.

Channel Bandwidth (MHz)	Subcarrier Frequency Spacing ΔF (kHz)	Fast Fourier Transform (FFT) Period (TFFT = 1 / ΔF)	Cyclic Prefix or Training Symbol Guard Interval (GI2) Duration (TGI2 = TFFT / 2)	L-LTF Duration (TLONG = TGI2 + 2 × TFFT)
20, 40, 80, 160, and 320	312.5	3.2 μs	1.6 μs	8 μs
10	156.25	6.4 μs	3.2 μs	16 μs
5	78.125	12.8 μs	6.4 μs	32 μs

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.

Extended Capabilities

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

Version History

Introduced in R2015b

R2024a: Single precision support

This function supports single-precision values for its numeric input arguments.