wlanVHTEqualize

Equalize demodulated VHT field symbols

Since R2025a

Syntax

[eqSym,csi] = wlanVHTEqualize(sym,chEst,noiseEst,cfg,field)

[eqSym,csi] = wlanVHTEqualize(___,userIdx)

Description

[eqSym,csi] = wlanVHTEqualize(sym,chEst,noiseEst,cfg,field) equalizes the demodulated very high-throughput (VHT) field symbols sym from the specified field. The function uses the channel estimate chEst, the noise estimate noiseEst, and the parameters specified in cfg.

[eqSym,csi] = wlanVHTEqualize(___,userIdx) specifies the user index in combination with the previous syntax. This syntax applies only when field is "VHT-SIG-B" or "VHT-Data", the STBC property of the cfg input is false, and the NumSpaceTimeStreams property of the cfg input is a vector.

Examples

Equalize L-SIG Field

Create a VHT configuration object with default parameters. Generate a time-domain waveform for the configuration.

cfg = wlanVHTConfig;
tx = wlanWaveformGenerator([1;0;0;1],cfg);

Pass the waveform through an AWGN channel with a signal-to-noise ratio of 15 dB.

rx = awgn(tx,15);

Get indices for the L-LTF. Use the indices to isolate the part of the received waveform that corresponds to the L-LTF.

indLLTF = wlanFieldIndices(cfg,"L-LTF");
rxLLTF = rx(indLLTF(1):indLLTF(2),:);

Demodulate the L-LTF. Use the demodulated symbols to estimate the channel and noise at the L-LTF.

demodLLTF = wlanVHTDemodulate(rxLLTF,"L-LTF",cfg);
chEst = wlanLLTFChannelEstimate(demodLLTF,cfg);
noiseEst = wlanLLTFNoiseEstimate(demodLLTF);

Isolate the part of the received waveform that corresponds to the L-SIG field.

field = "L-SIG";
indLSIG = wlanFieldIndices(cfg,field);
rxLSIG = rx(indLSIG(1):indLSIG(2),:);

Demodulate the L-SIG field.

sym = wlanVHTDemodulate(rxLSIG,field,cfg);

Equalize the L-SIG field using the channel and noise estimates you obtained at the L-LTF.

[eqSym,csi] = wlanVHTEqualize(sym,chEst,noiseEst,cfg,field);

Equalize VHT-Data Field

Create a VHT configuration object with four users, four transmit antennas, and one space-time stream per user.

cfg = wlanVHTConfig(NumUsers=4,UserPositions=[0 1 2 3], ...
    NumTransmitAntennas=4,NumSpaceTimeStreams=[1 1 1 1],GroupID=1);

Create a four-element cell array of PSDU bits. Each element of the array contains the bits for one user.

psdu = cell(1,cfg.NumUsers);
for i = 1:cfg.NumUsers
    psdu{i} = randi([0 1],cfg.PSDULength(i)*8,1);
end

Generate time-domain waveforms for the VHT-LTF and VHT-Data field of the configuration.

txVHTLTF = wlanVHTLTF(cfg);
txData = wlanVHTData(psdu,cfg);

Pass both waveforms through an AWGN channel with a signal-to-noise ratio of 5 dB, returning the noise variance.

[rxVHTLTF,noiseEst] = awgn(txVHTLTF,5);
rxData = awgn(txData,5);

Demodulate the VHT-LTF. Use the demodulated symbols to estimate the channel at the VHT-LTF.

demodVHTLTF = wlanVHTDemodulate(rxVHTLTF,"VHT-LTF",cfg);
chEst = wlanVHTLTFChannelEstimate(demodVHTLTF,cfg);

Demodulate the VHT-Data field.

field = "VHT-Data";
sym = wlanVHTDemodulate(rxData,field,cfg);

Equalize the demodulated symbols for the first user.

userIdx = 1;
[eqSym,csi] = wlanVHTEqualize(sym,chEst,noiseEst,cfg,field,userIdx);

Input Arguments

sym — Demodulated VHT field symbols

3-D array

Demodulated VHT field symbols, specified as a 3-D array. The size of the array is N_SC-by-N_SYM-by-N_R, where N_SC is the number of subcarriers, N_SYM is the number of OFDM symbols, and N_R is the number of receive antennas.

Data Types: double | single
Complex Number Support: Yes

chEst — Channel estimate

matrix | 3-D array

Channel estimate, specified as a matrix or 3-D array. The size of this input must be:

• N_SC-by-1-by-N_R if the field input is "L-SIG" or "VHT-SIG-A".

• N_SC-by-N_STS-by-N_R if the field input is "VHT-SIG-B" or "VHT-Data". If the STBC property of the cfg input is 1 (true), N_STS must be even.

N_STS is the number of space-time streams specified in the cfg input. If the NumSpaceTimeStreams property is a scalar, N_STS is equal to it. If the NumSpaceTimeStreams property is a vector, N_STS is the sum of its entries.

Data Types: double | single
Complex Number Support: Yes

noiseEst — Noise estimate

nonnegative real scalar

Noise estimate, specified as a nonnegative real scalar. This input determines how the function equalizes the input symbols. For more information, see Equalization Methods.

Data Types: double | single

cfg — Format configuration

wlanVHTConfig object

Format configuration, specified as a wlanVHTConfig object.

field — Field to equalize

"L-SIG" | "VHT-SIG-A" | "VHT-SIG-B" | "VHT-Data"

Field to equalize, specified as one of these values:

• "L-SIG" — Equalize the legacy signal (L-SIG) field.

• "VHT-SIG-A" — Equalize the VHT signal A (VHT-SIG-A) field.

• "VHT-SIG-B" — Equalize the VHT signal B (VHT-SIG-B) field.

• "VHT-Data" — Equalize the VHT-Data field.

Data Types: char | string

userIdx — User index

positive integer

User index, specified as a positive integer.

Note

This input is required only when field is "VHT-SIG-B" or "VHT-Data", the STBC property of the cfg input is 0 (false), and the NumSpaceTimeStreams property of the cfg input is a vector. If you specify this input in any other situation, the function ignores it.

Output Arguments

eqSym — Equalized symbols

matrix | 3-D array

Equalized symbols, returned as a matrix or 3-D array. The size of this output depends on the value of the field input:

• If field is "L-SIG" or "VHT-SIG-A", eqSym has size N_SC-by-N_SYM, where N_SC is the number of data, pilot, or combined data and pilot subcarriers in a 20 MHz channel bandwidth and N_SYM is the number of OFDM symbols.

• If field is "VHT-SIG-B" or "VHT-Data", eqSym has size N_SC-by-N_SYM-by-N_SS. N_SC is the number of subcarriers in the sym input. N_SYM is the number of OFDM symbols. N_SS is the number of spatial streams for the user of interest. Its value varies as follows:

  • When field is "VHT-Data" and the STBC property of the cfg input is 1 (true), N_SS is half of N_STS.

  • When the previous conditions are not both satisfied and the NumSpaceTimeStreams property of the cfg input is a scalar, N_SS is equal to N_STS.

  • Otherwise, N_SS is equal to the vector entry in the NumSpaceTimeStreams property that corresponds to the user of interest as specified by userIdx.

Data Types: double | single
Complex Number Support: Yes

csi — Channel state information

real-valued matrix

Channel state information, returned as a real-valued matrix. The size of the matrix depends on the field input:

• If field is "L-SIG" or "VHT-SIG-A", csi has size N_SC-by-1, where N_SC has the same value as in eqSym.

• If field is "VHT-SIG-B" or "VHT-Data", csi has size N_SC-by-N_SS, where N_SS has the same value as in eqSym.

Data Types: double | single

More About

L-SIG

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 uses BPSK modulation with rate 1/2 binary convolutional coding (BCC).

The L-SIG consists of 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, 160, and 320	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 formats, 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 formats, specify the transmission time as described in Sections 19.3.9.3.5 and 10.27.4 of IEEE Std 802.11-2020.

  • For VHT formats, specify the transmission time as described in Section 21.3.8.2.4 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.

Note

Signaling fields added for HT (wlanHTSIG) and VHT (wlanVHTSIGA, wlanVHTSIGB) formats provide data rate and configuration information for those formats.

• For the HT-mixed format, Section 19.3.9.4.3 of IEEE Std 802.11-2020 describes HT-SIG bit settings.

• For the VHT format, Sections 21.3.8.3.3 and 21.3.8.3.6 of IEEE Std 802.11-2020 describe bit settings for the VHT-SIG-A and VHT-SIG-B fields, respectively.

VHT-SIG-A

The very high throughput signal A (VHT-SIG-A) field contains information required to interpret VHT format packets. Similar to the non-HT signal (L-SIG) field for the non-HT OFDM format, this field stores the actual rate value, channel coding, guard interval, MIMO scheme, and other configuration details for the VHT format packet. Unlike the HT-SIG field, this field does not store the packet length information. Packet length information is derived from L-SIG and is captured in the VHT-SIG-B field for the VHT format.

For a detailed description of the VHT-SIG-A field, see Section 21.3.8.3.3 of IEEE Std 802.11-2016. The VHT-SIG-A field consists of two symbols: VHT-SIG-A1 and VHT-SIG-A2. These symbols are located between the L-SIG and the VHT-STF portion of the VHT format PPDU.

The VHT-SIG-A field includes the following components. The bit field structures for VHT-SIG-A1 and VHT-SIG-A2 vary for single-user or multi-user transmissions.

• BW — A two-bit field that indicates 0 for 20 MHz, 1 for 40 MHz, 2 for 80 MHz, or 3 for 160 MHz.

• STBC — A bit that indicates the presence of space-time block coding.

• Group ID — A six-bit field that indicates the group and user position assigned to a STA.

• N_STS — A three-bit field for a single user or 4 three-bit fields for a multi-user scenario that indicates the number of space-time streams per user.

• Partial AID — An identifier that combines the association ID and the BSSID.

• TXOP_PS_NOT_ALLOWED — An indicator bit that shows whether client devices are allowed to enter dose state. This bit is set to false when the VHT-SIG-A structure is populated, indicating that the client device is allowed to enter dose state.

• Short GI — A bit that indicates use of the 400 ns guard interval.

• Short GI NSYM Disambiguation — A bit that indicates if an extra symbol is required when the short GI is used.

• SU/MU[0] Coding — A bit field that indicates if convolutional or LDPC coding is used for a single user or for user MU[0] in a multi-user scenario.

• LDPC Extra OFDM Symbol — A bit that indicates if an extra OFDM symbol is required to transmit the data field.

• MCS — A four-bit field.

  • For a single-user scenario, it indicates the modulation and coding scheme used.

  • For a multi-user scenario, it indicates the use of convolutional or LDPC coding and the MCS setting is conveyed in the VHT-SIG-B field.

• Beamformed — An indicator bit set to 1 when a beamforming matrix is applied to the transmission.

• CRC — An eight-bit field used to detect errors in the VHT-SIG-A transmission.

• Tail — A six-bit field used to terminate the convolutional code.

VHT-SIG-B

Receivers use the very high throughput signal B (VHT-SIG-B) field in multi-user scenarios to set up the data rate and to fine-tune MIMO reception. The field uses MCS 0 and consists of a single OFDM symbol.

The VHT-SIG-B field is located between the VHT-LTF and the data portion of the VHT format PPDU.

The VHT-SIG-B field contains the actual rate and A-MPDU length value per user. For a detailed description of the VHT-SIG-B field, see Section 21.3.8.3.6 of IEEE Std 802.11-2016. The number of bits in the VHT-SIG-B field varies with the channel bandwidth. The assignment of the bits depends on whether there is a single user or multiple users. For single-user configurations, the same information is available in the L-SIG field, but the VHT-SIG-B field is included for continuity purposes.

Field	VHT MU PPDU Allocation (bits)			VHT SU PPDU Allocation (bits)			Description
	20 MHz	40 MHz	80 MHz, 160 MHz	20 MHz	40 MHz	80 MHz, 160 MHz
VHT-SIG-B	B0–15 (16)	B0–16 (17)	B0–18 (19)	B0–16 (17)	B0–18 (19)	B0–20 (21)	This variable-length field indicates the size of the data payload in four-byte units. The length of the field depends on the channel bandwidth.
VHT-MCS	B16–19 (4)	B17–20 (4)	B19–22 (4)	N/A	N/A	N/A	This four-bit field is included for multi-user scenarios only.
Reserved	N/A	N/A	N/A	B17–19 (3)	B19–20 (2)	B21–22 (2)	All ones
Tail	B20–25 (6)	B21–26 (6)	B23–28 (6)	B20–25 (6)	B21–26 (6)	B23–28 (6)	This six bit field contains all zeros to terminate the convolutional code.
Total number of bits	26	27	29	26	27	29	N/A
Bit field repetition	1	2	4	1	2	4	For 160 MHz, the 80 MHz channel repeats twice.

For a null data packet (NDP), the VHT-SIG-B bits are set according to Table 21-15 of IEEE Std 802.11-2016.

VHT-Data Field

The VHT-Data field carries one or more frames from the medium access control (MAC) layer. This field follows the VHT-SIG-B field in a VHT PPDU.

For a detailed description of the VHT-Data field, see section 21.3.10 of IEEE Std 802.11-2020. The VHT Data field consists of four subfields.

• Service field — Contains a seven-bit scrambler initialization state, one bit reserved for future considerations, and eight bits for the VHT-SIG-B cyclic redundancy check (CRC) field

• PSDU — Variable-length field containing a PLCP service data unit

• PHY Pad — Variable number of bits passed to the transmitter to create a complete OFDM symbol

• Tail — Bits required to terminate a convolutional code (not required when the transmission uses LDPC channel coding)

Algorithms

Equalization Methods

wlanVHTEqualize uses one of two equalization methods: zero forcing (ZF) or minimum mean square error (MMSE).

ZF equalization compensates for the effects of channel distortion by assuming the channel to be noiseless and by designing a filter using the inverse of the channel matrix. If the channel matrix is invertible and the channel is truly noiseless, the output of the equalizer is identical to the transmitted signal. However, if the channel is noisy, this method performs poorly because the filter exaggerates the effects of the noise.

MMSE equalization minimizes the mean squared error (MSE) between the original transmitted signal and the equalizer output. This method performs better for noisy channels because, unlike ZF, it takes the effects of noise into account. However, it is more computationally complex than ZF. Therefore, ZF is better suited to noiseless channels than MMSE.

Because of these facts, wlanVHTEqualize uses ZF when the noiseEst input is 0 and MMSE for all other values.

References

[1] 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.

Extended Capabilities

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

Version History

Introduced in R2025a