wlanVHTDemodulate

OFDM-demodulate received time-domain VHT waveform

Since R2025a

Syntax

sym = wlanVHTDemodulate(rx,field,cfg)

sym = wlanVHTDemodulate(___,Name=Value)

Description

sym = wlanVHTDemodulate(rx,field,cfg) OFDM-demodulates the received time-domain very high-throughput (VHT) waveform rx.

sym = wlanVHTDemodulate(___,Name=Value) specifies additional options using one or more name-value arguments in combination with the previous syntax.

Examples

Demodulate VHT-LTF

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 20 dB.

snr = 20;
rx = awgn(tx,snr);

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

field = "VHT-LTF";
ind = wlanFieldIndices(cfg,field);
rx = rx(ind(1):ind(2),:);

Demodulate the VHT-LTF.

sym = wlanVHTDemodulate(rx,field,cfg);

Demodulate VHT-SIG-A with Oversampling

Create a VHT configuration object with a channel bandwidth of 160 MHz.

cfg = wlanVHTConfig(ChannelBandwidth="CBW160");

Generate a time-domain waveform for the configuration, specifying an oversampling factor of 2.

osf = 2;
tx = wlanWaveformGenerator([1;0;0;1],cfg,OversamplingFactor=osf);

Pass the waveform through an AWGN channel with a signal-to-noise ratio of 10 dB.

snr = 10;
rx = awgn(tx,snr);

Get indices for the VHT-SIG-A field, specifying the oversampling factor. Use the indices to isolate the part of the received waveform that corresponds to the VHT-SIG-A field.

field = "VHT-SIG-A";
ind = wlanFieldIndices(cfg,field,OversamplingFactor=osf);
rx = rx(ind(1):ind(2),:);

Demodulate the VHT-SIG-A field, specifying the oversampling factor.

sym = wlanVHTDemodulate(rx,field,cfg,OversamplingFactor=osf);

Input Arguments

rx — Received time-domain signal

complex-valued matrix

Received time-domain signal, specified as a complex-valued matrix of size N_s-by-N_r.

• N_s is the number of time-domain samples. If N_s is not an integer multiple of the number of OFDM symbols, L_s, in the specified field, then the function ignores the remaining mod(Ns,Ls) symbols.

• N_r is the number of receive antennas.

Data Types: double | single
Complex Number Support: Yes

field — Field to be demodulated

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

Field to be demodulated, specified as one of these values:

• "L-LTF" — Demodulate the legacy long training field (L-LTF).

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

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

• "VHT-LTF" — Demodulate the VHT long training field (VHT-LTF).

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

• "VHT-Data" — Demodulate the VHT-Data Field.

Data Types: char | string

cfg — PHY format configuration

wlanVHTConfig object

Physical layer (PHY) format configuration, specified as a wlanVHTConfig object.

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: OversamplingFactor=2

OversamplingFactor — Oversampling factor

1 (default) | scalar greater than 1

Oversampling factor, specified as a scalar greater than or equal to 1. The oversampled cyclic prefix length must be an integer number of samples. For more information, see FFT-Based Oversampling.

OFDMSymbolOffset — OFDM symbol sampling offset

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

OFDM symbol sampling offset, specified as a scalar in the interval [0, 1]. The scalar represents the sampling offset as a fraction of the cyclic prefix length.

The value that you specify indicates the start location for OFDM demodulation relative to the beginning of the cyclic prefix.

Example: 0.45

Data Types: double | single

Output Arguments

sym — Demodulated frequency-domain signal

complex-valued array

Demodulated frequency-domain signal, returned as a complex-valued array of size N_sc-by-N_sym-by-N_r.

• N_sc is the number of active occupied subcarriers in the demodulated field.

• N_sym is the number of OFDM symbols.

• N_r is the number of receive antennas.

Data Types: double
Complex Number Support: Yes

More About

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.

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

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-LTF

The very high throughput long training field (VHT-LTF) is between the VHT-STF and VHT-SIG-B portion of the VHT packet.

Receivers use this field for MIMO channel estimation and pilot subcarrier tracking. The VHT-LTF includes one VHT long training symbol for each spatial stream indicated by the selected modulation and coding scheme (MCS). Each symbol is 4 μs long. The VHT-LTF consists of eight symbols at most.

For a detailed description of the VHT-LTF, see Section 21.3.8.3.5 of IEEE Std 802.11-2016.

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

FFT-Based Oversampling

An oversampled signal is a signal sampled at a frequency that is higher than the Nyquist rate. WLAN signals maximize occupied bandwidth by using small guardbands, which can pose problems for anti-imaging and anti-aliasing filters. Oversampling increases the guardband width relative to the total signal bandwidth, which increases the number of samples in the signal.

This function performs oversampling by using a larger IFFT and zero pad when generating an OFDM waveform. This diagram shows the oversampling process for an OFDM waveform with N_FFT subcarriers made up of N_g guardband subcarriers on either side of N_st occupied bandwidth subcarriers.

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