Channel estimation using pre-HE fields
returns the channel estimate at the legacy signal field (L-SIG) using the demodulated L-SIG
chEst = wlanPreHEChannelEstimate(
demodSym, and the channel estimate at the legacy long
training field (L-LTF) ,
chEstLLTF. The channel bandwidth is specified
cbw. For more information about these fields, see L-SIG and L-LTF.
Channel Estimation Using Pre-HE Fields
Calculate and plot the channel estimate of an HE SU format channel by using pre-HE fields.
Create an HE SU format configuration object and extract its channel bandwidth. Generate a time-domain waveform for an 802.11ax packet.
cfg = wlanHESUConfig; cbw = cfg.ChannelBandwidth; txSig = wlanWaveformGenerator([1;0;0;1],cfg);
Multiply the transmitted signal by 0.1 + 0.4i and pass it through an AWGN channel with a signal-to-noise ratio of 30 dB.
rxSig = awgn((0.1 + 0.4i)*txSig,30);
Extract the field indices of the HE SU configuration object, demodulate the L-LTF, and perform L-LTF channel estimation.
ind = wlanFieldIndices(cfg); demodSigLLTF = wlanHEDemodulate(rxSig(ind.LLTF(1):ind.LLTF(2),:),"L-LTF",cfg); chEstLLTF = wlanLLTFChannelEstimate(demodSigLLTF,cbw);
Demodulate the L-SIG and RL-SIG fields.
demodSigLSIG = wlanHEDemodulate(rxSig(ind.LSIG(1):ind.RLSIG(2),:),"L-SIG",cfg);
Using a frequency smoothing span of 3, perform the full channel estimation.
est = wlanPreHEChannelEstimate(demodSigLSIG,chEstLLTF,cbw,3);
Plot the channel estimate.
demodSym — Demodulated L-SIG field symbol
Demodulated L-SIG field symbol, specified as an NST-by-NSYM-by-NR array. NST is the number of occupied subcarriers. NSYM is the number of OFDM symbols in the L-SIG and repeated legacy signal (RL-SIG) fields. NR is the number of receive antennas.
Complex Number Support: Yes
chEstLLTF — L-LTF channel estimate
Channel estimate at the legacy long training field, specified as an NST-by-1-by-NR array. NST is the number of occupied subcarriers and NR is the number of receive antennas.
Complex Number Support: Yes
cbw — Channel bandwidth
Channel bandwidth, specified as one of these values.
"CBW20"– Channel bandwidth of 20 MHz.
"CBW40"– Channel bandwidth of 40 MHz.
"CBW80"– Channel bandwidth of 80 MHz.
"CBW160"– Channel bandwidth of 160 MHz.
"CBW320"– Channel bandwidth of 320 MHz.
span — Filter span
positive odd integer
Span of the frequency smoothing filter, specified as a positive odd integer and
expressed as a number of subcarriers. The function applies frequency smoothing only when
span is greater than one. For more information on when to specify
this input, see Frequency Smoothing.
chEst — Channel estimate
Channel estimate at all data and pilot subcarriers, returned as an NST-by-1-by-NR array. NST is the number of occupied subcarriers and NR is the number of receive antennas. The output includes the channel estimates for the extra four subcarriers per each 20 MHz subchannel in the L-SIG field.
The L-SIG is the third field of the 802.11™ OFDM PLCP legacy preamble. This field is a component of EHT, HE, VHT, HT, and non-HT PPDUs. It consists of 24 bits that contain rate, length, and parity information. The L-SIG field is transmitted using BPSK modulation with rate 1/2 binary convolutional coding (BCC).
The L-SIG is one OFDM symbol with a duration that varies with channel bandwidth.
|Channel Bandwidth (MHz)||Subcarrier Frequency Spacing, ΔF (kHz)||Fast Fourier Transform (FFT) Period (TFFT = 1 / ΔF)||Guard Interval (GI) Duration (TGI = TFFT / 4)||L-SIG Duration (TSIGNAL = TGI + TFFT)|
|20, 40, 80, and 160||312.5||3.2 μs||0.8 μs||4 μs|
|10||156.25||6.4 μs||1.6 μs||8 μs|
|5||78.125||12.8 μs||3.2 μs||16 μs|
The L-SIG contains packet information for the received configuration.
Bits 0 through 3 specify the data rate (modulation and coding rate) for the non-HT format.
Rate (Bits 0–3) Modulation
Coding Rate (R)
Data Rate (Mb/s) 20 MHz Channel Bandwidth 10 MHz Channel Bandwidth 5 MHz Channel Bandwidth 1101 BPSK 1/2 6 3 1.5 1111 BPSK 3/4 9 4.5 2.25 0101 QPSK 1/2 12 6 3 0111 QPSK 3/4 18 9 4.5 1001 16-QAM 1/2 24 12 6 1011 16-QAM 3/4 36 18 9 0001 64-QAM 2/3 48 24 12 0011 64-QAM 3/4 54 27 13.5
For HT and VHT formats, the L-SIG rate bits are set to
'1 1 0 1'. Data rate information for HT and VHT formats is signaled in format-specific signaling fields.
Bit 4 is reserved for future use.
Bits 5 through 16:
For non-HT, specify the data length (amount of data transmitted in octets) as described in table 17-1 and section 10.27.4 IEEE® Std 802.11-2020.
For HT-mixed, specify the transmission time as described in sections 126.96.36.199.5 and 10.27.4 of IEEE Std 802.11-2020.
For VHT, specify the transmission time as described in section 188.8.131.52.4 of IEEE Std 802.11-2020.
Bit 17 has the even parity of bits 0 through 16.
Bits 18 through 23 contain all zeros for the signal tail bits.
For the HT-mixed format, section 184.108.40.206.3 of IEEE Std 802.11-2020 describes HT-SIG bit settings.
For the VHT format, sections 220.127.116.11.3 and 18.104.22.168.6 of IEEE Std 802.11-2020 describe bit settings for the VHT-SIG-A and VHT-SIG-B fields, respectively.
The RL-SIG is a repeat of the L-SIG, used to distinguish an HE or EHT PPDU from a non-HT, HT, and VHT PPDU.
The L-LTF is the second field in the 802.11 OFDM PLCP legacy preamble. The L-LTF is a component of HE, VHT, HT, and non-HT PPDUs.
Channel estimation, fine frequency offset estimation, and fine symbol timing offset estimation rely on the L-LTF.
The L-LTF is composed 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|
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 802.11-2020 (Revision of IEEE Std 802.11-2016). “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.” IEEE Standard for Information Technology — Telecommunications and Information Exchange between Systems— Local and Metropolitan Area Networks — Specific Requirements.
 IEEE Std 802.11ax™-2021 (Amendment to IEEE Std 802.11-2020). “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. Amendment 1: Enhancements for High Efficiency WLAN.” IEEE Standard for Information technology — Telecommunications and information exchange between systems. Local and metropolitan area networks — Specific requirements.
 IEEE P802.11be™/D1.5. “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. Amendment 8: Enhancements for Extremely High Throughput (EHT).” Draft Standard for Information Technology — Telecommunications and Information Exchange between Systems — Local and Metropolitan Area Networks — Specific Requirements.
C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.
Introduced in R2022b