802.11ax Waveform Generation
This example shows how to parameterize and generate different IEEE® 802.11ax™ (Wi-Fi 6) high efficiency (HE) format packets.
Introduction
IEEE Std 802.11ax™-2021 [1] specifies four high efficiency (HE) packet formats:
Single-user
Extended-range single-user
Multi-user
Trigger-based
This example shows you how to generate packets for these different formats and demonstrates some of the key features of the standard [1].
HE Single-User Format
An HE single-user (SU) packet is a full-band transmission to a single user. To configure transmit parameters for the HE SU format, use a wlanHESUConfig object. You can configure the wlanHESUConfig object to operate in extended-range mode. To enable or disable this mode, set the ExtendedRange property to true or false. In this example you create a configuration for an HE SU transmission and configure transmission properties.
cfgSU = wlanHESUConfig; cfgSU.ExtendedRange = false; % Do not use extended-range format cfgSU.ChannelBandwidth = "CBW20"; % Channel bandwidth cfgSU.APEPLength = 1000; % Payload length in bytes cfgSU.MCS = 0; % Modulation and coding scheme cfgSU.ChannelCoding = "LDPC"; % Channel coding cfgSU.NumSpaceTimeStreams = 1; % Number of space-time streams cfgSU.NumTransmitAntennas = 1; % Number of transmit antennas
You can generate a single-user packet with the waveform generator, wlanWaveformGenerator. The getPSDULength object function returns the required PSDU length given the transmission configuration. The example creates a random PSDU of this length for the transmission.
psdu = randi([0 1],getPSDULength(cfgSU)*8,1,"int8"); % Random PSDU txSUWaveform = wlanWaveformGenerator(psdu,cfgSU); % Create packet
HE Extended-Range Single-User Format
An extended-range single-user packet has the same fields as the standard single-user format, but the powers of some fields are boosted, and some fields are repeated to improve performance at low signal-to-noise ratios (SNRs). You can configure an extended-range packet using a wlanHESUConfig object with ChannelBandwidth set to "CBW20" and ExtendedRange set to true. An extended-range packet can transmit only in the upper 106-tone resource unit (RU) within the 20 MHz channel, or over the entire bandwidth. You can choose one of these options by setting the Upper106ToneRU property:
cfgExtSU = cfgSU; cfgExtSU.ExtendedRange = true; % Enable extended-range format cfgExtSU.Upper106ToneRU = true; % Use only upper 106-tone RU % Generate a packet psdu = randi([0 1],getPSDULength(cfgExtSU)*8,1,'int8'); % Random PSDU txExtSUWaveform = wlanWaveformGenerator(psdu,cfgExtSU); % Create packet
Display the spectrum and spectrogram of the generated signal. In the spectrogram the packet headers use the available bandwidth, but the data portion occupies only the upper half of the channel.
fs = wlanSampleRate(cfgExtSU); % Get baseband sample rate ofdmInfo = wlanHEOFDMInfo("HE-Data",cfgExtSU); fftsize = ofdmInfo.FFTLength; % Use the data field fft size rbw = fs/fftsize; % Resoluton bandwidth spectrumScope = spectrumAnalyzer(SampleRate=fs, ... RBWSource="property", ... RBW=rbw, ... AveragingMethod="exponential", ... ForgettingFactor=0.25, ... YLimits=[-50,20], ... Title="HE Extended-Range SU with Active Upper 106-Tone RU"); spectrumScope.ViewType = "Spectrum and Spectrogram"; spectrumScope.TimeSpanSource = "Property"; spectrumScope.TimeSpan = length(txExtSUWaveform)/fs; spectrumScope(txExtSUWaveform)
Plot the power of the L-STF and L-LTF fields. The extended-range transmission is boosted by 3 dB.
figure; ind = wlanFieldIndices(cfgExtSU); t = (0:(double(ind.LLTF(2))-1))/fs*1e6; plot(t,20*log10(movmean(abs(txSUWaveform(1:ind.LLTF(2))),20)),"-b") hold on; plot(t,20*log10(movmean(abs(txExtSUWaveform(1:ind.LLTF(2))),20)),"-r") grid on; title("Power of L-STF and L-LTF (1 us Moving Average)"); xlabel("Time (us)"); ylabel("Power (dBW)"); legend("HE SU","HE Extended-Range SU",Location="SouthWest");
HE Multi-User Format - OFDMA
You can configure the HE multi-user (HE MU) format for an OFDMA transmission, an MU-MIMO transmission, or a combination of the two. This flexibility enables an HE MU packet to transmit to a single user over the whole band, multiple users over different parts of the band (OFDMA), or multiple users over the same part of the band (MU-MIMO).
For an OFDMA transmission, the channel bandwidth is divided into resource units (RUs). An RU is a group of subcarriers assigned to one or more users. An RU is defined by a size (the number of subcarriers) and an index. The RU index specifies the location of the RU within the channel. For example, in an 80 MHz transmission there are four possible 242-tone RUs, one in each 20 MHz subchannel. RU# 242-1 (size 242, index 1) is the RU occupying the lowest absolute frequency within the 80 MHz, and RU# 242-4 (size 242, index 4) is the RU occupying the highest absolute frequency. The draft standard defines possible sizes and locations of RUs in Section 27.3.2.2 of [1]. The assignment of RUs in a transmission is defined by the allocation index. The allocation index is defined in Table 27-26 of [1]. For each 20 MHz subchannel, an 8-bit index describes the number and size of RUs, and the number of users transmitted on each RU. The allocation index also determines which content channel is used to signal a user in the HE-SIG-B field. The heRUAllocationTable function returns a table that shows the allocation indices in Table 27-26 and the corresponding RU assignments. The example shows the first 10 allocations in the table. For each allocation index, the table shows the 8-bit allocation index, the number of users, number of RUs, RU indices, RU sizes, and number of users per RU. The table also contains a note about allocations that are reserved or serve a special purpose. You can view the full allocation table in the Appendix.
allocationTable = heRUAllocationTable;
disp('First 10 entries in the allocation table: ')
disp(allocationTable(1:10,:));
First 10 entries in the allocation table:
Allocation BitAllocation NumUsers NumRUs RUIndices RUSizes NumUsersPerRU Note
__________ _____________ ________ ______ _____________________ ______________________________ _____________________ ____
0 "00000000" 9 9 {[1 2 3 4 5 6 7 8 9]} {[26 26 26 26 26 26 26 26 26]} {[1 1 1 1 1 1 1 1 1]} ""
1 "00000001" 8 8 {[ 1 2 3 4 5 6 7 4]} {[ 26 26 26 26 26 26 26 52]} {[ 1 1 1 1 1 1 1 1]} ""
2 "00000010" 8 8 {[ 1 2 3 4 5 3 8 9]} {[ 26 26 26 26 26 52 26 26]} {[ 1 1 1 1 1 1 1 1]} ""
3 "00000011" 7 7 {[ 1 2 3 4 5 3 4]} {[ 26 26 26 26 26 52 52]} {[ 1 1 1 1 1 1 1]} ""
4 "00000100" 8 8 {[ 1 2 2 5 6 7 8 9]} {[ 26 26 52 26 26 26 26 26]} {[ 1 1 1 1 1 1 1 1]} ""
5 "00000101" 7 7 {[ 1 2 2 5 6 7 4]} {[ 26 26 52 26 26 26 52]} {[ 1 1 1 1 1 1 1]} ""
6 "00000110" 7 7 {[ 1 2 2 5 3 8 9]} {[ 26 26 52 26 52 26 26]} {[ 1 1 1 1 1 1 1]} ""
7 "00000111" 6 6 {[ 1 2 2 5 3 4]} {[ 26 26 52 26 52 52]} {[ 1 1 1 1 1 1]} ""
8 "00001000" 8 8 {[ 1 3 4 5 6 7 8 9]} {[ 52 26 26 26 26 26 26 26]} {[ 1 1 1 1 1 1 1 1]} ""
9 "00001001" 7 7 {[ 1 3 4 5 6 7 4]} {[ 52 26 26 26 26 26 52]} {[ 1 1 1 1 1 1 1]} ""
Use a wlanHEMUConfig object to configure the transmission of an HE MU packet. When you create a wlanHEMUConfig object, you must specify the allocation index for each 20 MHz subchannel. For each subchannel, you must specify an integer between 0 and 223. These integers correspond to the 8-bit numbers in Table 27-26 of [1].
You can specify the allocation index as a decimal or as an 8-bit binary sequence. This example creates a 20 MHz HE MU configuration with 8-bit allocation index 10000000. This is equivalent to the decimal allocation index 128. This configuration specifies three RUs, each with one user.
allocationIndex = "10000000"; % 3 RUs, 1 user per RU cfgMU = wlanHEMUConfig(allocationIndex);
The showAllocation method visualizes the occupied RUs and subcarriers for the configuration you specify. The colored blocks show the occupied subcarriers in the pre-HE and HE portions of the packet. White indicates unoccupied subcarriers. The pre-HE portion illustrates the occupied subcarriers in the fields preceding the HE-STF. The HE portion illustrates the occupied subcarriers in the HE-STF, HE-LTF and HE-Data fields and therefore shows the RU allocation. You can display information about an RU by clicking on it. The RU number corresponds to the i th RU element of the cfgMU.RU property. RUs are labeled with their size and index. The RU index is the i th possible RU of the corresponding RU size within the channel bandwidth. For example, Index 2 is the second possible 106-tone RU within the 20 MHz channel bandwidth. The user number corresponds to the i th User element of the cfgMU.User property, and the user field in HE-SIG-B. The middle RU (RU #2) is split across the DC subcarriers.
showAllocation(cfgMU);
axAlloc = gca; % Get axis handle for subsequent plotting
The ruInfo object function returns details about the RUs in the configuration. In this case there are three users and three RUs.
allocInfo = ruInfo(cfgMU);
disp("Allocation info:")
disp(allocInfo)
Allocation info:
NumUsers: 3
NumRUs: 3
RUIndices: [1 5 2]
RUSizes: [106 26 106]
NumUsersPerRU: [1 1 1]
NumSpaceTimeStreamsPerRU: [1 1 1]
PowerBoostFactorPerRU: [1 1 1]
RUNumbers: [1 2 3]
RUAssigned: [1 1 1]
The properties of cfgMU describe the transmission configuration. The cfgMU.RU and cfgMU.User properties are cell arrays. Each element of the cell arrays contains an object that configures an RU or a user. When you create the cfgMU object, the software configures the elements of cfgMU.RU and cfgMU.User to create your desired number of RUs and users. Each element of cfgMU.RU is a wlanHEMURU object that describes the configuration of an RU. Similarly, each element of cfgMU.User is a wlanHEMUUser object that describes the configuration of a user. You can see this object hierarchy here:
This example specifies three RUs by setting the allocation index to 128. Therefore, cfgMU.RU is a cell array with three elements. After you create an object, you can configure each RU to create your desired transmission configuration by setting the properties of the appropriate RU object. For example, you can configure the spatial mapping and power boost factor for each RU. Once you create an object, the Size and Index properties of each RU are fixed, and therefore are read-only properties. Similarly, the UserNumbers property is read-only and indicates which user is transmitted on the RU. For this configuration the first RU has size 106, index 1.
disp("First RU configuration:")
disp(cfgMU.RU{1})
First RU configuration:
wlanHEMURU with properties:
PowerBoostFactor: 1
SpatialMapping: 'Direct'
Read-only properties:
Size: 106
Index: 1
UserNumbers: 1
In this example, the allocation index specifies three users in the transmission, so cfgMU.User contains three elements. You can configure the transmission properties of users by modifying individual user objects. For example, you can modify the MCS, APEP length and channel coding scheme. The read-only RUNumber property indicates which RU is used to transmit this user.
disp("First user configuration:")
disp(cfgMU.User{1})
First user configuration:
wlanHEMUUser with properties:
APEPLength: 100
MCS: 0
NumSpaceTimeStreams: 1
DCM: 0
ChannelCoding: 'LDPC'
STAID: 0
NominalPacketPadding: 0
PostFECPaddingSource: 'mt19937ar with seed'
PostFECPaddingSeed: 1
Read-only properties:
RUNumber: 1
The allocation index determines the number of users per RU and the mapping of users to RUs. The UserNumbers property of an RU object indicates which users (elements of the cfgMU.User cell array) are transmitted on that RU. Similarly, the RUNumber property of each User object indicates which RU (element of the cfgMU.RU cell array) is used to transmit the user:
This enables you to access the properties of an RU associated with a user easily:
ruNum = cfgMU.User{2}.RUNumber; % Get the RU number associated with user 2
disp(cfgMU.RU{ruNum}.SpatialMapping); % Display the spatial mapping
Direct
When an RU serves multiple users in an MU-MIMO configuration, the UserNumbers property can index multiple users:
Once you create the cfgMU object, you can set the transmission parameters as shown here:
% Configure RU 1 and user 1 cfgMU.RU{1}.SpatialMapping = 'Direct'; cfgMU.User{1}.APEPLength = 1e3; cfgMU.User{1}.MCS = 2; cfgMU.User{1}.NumSpaceTimeStreams = 4; cfgMU.User{1}.ChannelCoding = 'LDPC'; % Configure RU 2 and user 2 cfgMU.RU{2}.SpatialMapping = 'Fourier'; cfgMU.User{2}.APEPLength = 500; cfgMU.User{2}.MCS = 3; cfgMU.User{2}.NumSpaceTimeStreams = 2; cfgMU.User{2}.ChannelCoding = 'LDPC'; % Configure RU 3 and user 3 cfgMU.RU{3}.SpatialMapping = 'Fourier'; cfgMU.User{3}.APEPLength = 100; cfgMU.User{3}.MCS = 4; cfgMU.User{3}.DCM = true; cfgMU.User{3}.NumSpaceTimeStreams = 1; cfgMU.User{3}.ChannelCoding = 'BCC';
Some transmission parameters are common to all users in the HE MU transmission.
% Configure common parameters for all users
cfgMU.NumTransmitAntennas = 4;
cfgMU.SIGBMCS = 2;
To generate the HE MU waveform, first create a random PSDU for each user. Since the PSDU lengths differ, use a cell array to store the PSDU for each users. The getPSDULength object function returns a vector with the required PSDU per user given the configuration. Use the wlanWaveformGenerator function to create a packet.
psduLength = getPSDULength(cfgMU); psdu = cell(1,allocInfo.NumUsers); for i = 1:allocInfo.NumUsers psdu{i} = randi([0 1],psduLength(i)*8,1,"int8"); % Generate random PSDU end % Create MU packet txMUWaveform = wlanWaveformGenerator(psdu,cfgMU);
To configure an OFDMA transmission with a channel bandwidth greater than 20 MHz, you must specify an allocation index for each 20 MHz subchannel. For example, to configure an 80 MHz OFDMA transmission, you must specify four allocation indices. This example configures four 242-tone RUs. The allocation index 192 specifies one 242-tone RU with a single user in a 20 MHz subchannel. Therefore the vector of allocation indices [192 192 192 192] specifies four of these RUs, over an 80 MHz channel bandwidth:
% Display allocation index 192 properties in the table (the 193rd row) disp('Allocation #192 table entry:') disp(allocationTable(193,:)) % Create 80 MHz MU configuration, with four 242-tone RUs cfgMU80MHz = wlanHEMUConfig([192 192 192 192]);
Allocation #192 table entry:
Allocation BitAllocation NumUsers NumRUs RUIndices RUSizes NumUsersPerRU Note
__________ _____________ ________ ______ _________ _______ _____________ ____
192 "11000000" 1 1 {[1]} {[242]} {[1]} ""
When you specify multiple 20 MHz subchannels, the object sets the ChannelBandwidth property to the appropriate value. For this configuration the property is "CBW80" as four 20 MHz subchannels are specified. This is also shown in the allocation plot.
disp("Channel bandwidth for HE MU allocation:")
disp(cfgMU80MHz.ChannelBandwidth)
showAllocation(cfgMU80MHz,axAlloc)
Channel bandwidth for HE MU allocation: CBW80
HE Multi-User Format - MU-MIMO
An HE MU packet can also transmit an RU to multiple users using MU-MIMO. For a full band MU-MIMO allocation, the allocation indices between 192 and 199 configure a full-band 20 MHz allocation (242-tone RU). The index in this range determines the number of users. You can view the details in the allocation table. The NumUsers column in the table grows with index but NumRUs is always 1. You can also view the allocation table in the Appendix.
disp("Allocation #192-199 table entries:") disp(allocationTable(193:200,:)) % Indices 192-199 (rows 193 to 200)
Allocation #192-199 table entries:
Allocation BitAllocation NumUsers NumRUs RUIndices RUSizes NumUsersPerRU Note
__________ _____________ ________ ______ _________ _______ _____________ ____
192 "11000000" 1 1 {[1]} {[242]} {[1]} ""
193 "11000001" 2 1 {[1]} {[242]} {[2]} ""
194 "11000010" 3 1 {[1]} {[242]} {[3]} ""
195 "11000011" 4 1 {[1]} {[242]} {[4]} ""
196 "11000100" 5 1 {[1]} {[242]} {[5]} ""
197 "11000101" 6 1 {[1]} {[242]} {[6]} ""
198 "11000110" 7 1 {[1]} {[242]} {[7]} ""
199 "11000111" 8 1 {[1]} {[242]} {[8]} ""
The allocation index 193 transmits a 20 MHz 242-tone RU to two users. In this example, you create a transmission with a random spatial mapping matrix that maps a single space-time stream for each user onto two transmit antennas.
% Configure 2 users in a 20 MHz channel cfgMUMIMO = wlanHEMUConfig(193); % Set the transmission properties of each user cfgMUMIMO.User{1}.APEPLength = 100; % Bytes cfgMUMIMO.User{1}.MCS = 2; cfgMUMIMO.User{1}.ChannelCoding = "LDPC"; cfgMUMIMO.User{1}.NumSpaceTimeStreams = 1; cfgMUMIMO.User{2}.APEPLength = 1000; % Bytes cfgMUMIMO.User{2}.MCS = 6; cfgMUMIMO.User{2}.ChannelCoding = "LDPC"; cfgMUMIMO.User{2}.NumSpaceTimeStreams = 1; % Get the number of occupied subcarriers in the RU ruIndex = 1; % Get the info for the first (and only) RU ofdmInfo = wlanHEOFDMInfo("HE-Data",cfgMUMIMO,ruIndex); numST = ofdmInfo.NumTones; % Number of occupied subcarriers % Set the number of transmit antennas and generate a random spatial mapping % matrix numTx = 2; allocInfo = ruInfo(cfgMUMIMO); numSTS = allocInfo.NumSpaceTimeStreamsPerRU(ruIndex); cfgMUMIMO.NumTransmitAntennas = numTx; cfgMUMIMO.RU{ruIndex}.SpatialMapping = "Custom"; cfgMUMIMO.RU{ruIndex}.SpatialMappingMatrix = rand(numST,numSTS,numTx); % Create packet with a repeated bit sequence as the PSDU txMUMIMOWaveform = wlanWaveformGenerator([1 0 1 0],cfgMUMIMO);
You can create a full band MU-MIMO transmission with a channel bandwidth greater than 20 MHz by specifying a single RU allocation index in the range 200-223 when you create a wlanHEMUConfig object. These indices specify that the transmission uses HE-SIG-B compression.
The allocation indices between 200 and 207 configure a full-band MU-MIMO 40 MHz allocation (484-tone RU). The index in this range determines how many users are configured. You can view the details in the allocation table. The NumUsers column in the table grows with index but NumRUs is always 1.
disp("Allocation #200-207 table entries:") disp(allocationTable(201:208,:)) % Indices 200-207 (rows 201 to 208)
Allocation #200-207 table entries:
Allocation BitAllocation NumUsers NumRUs RUIndices RUSizes NumUsersPerRU Note
__________ _____________ ________ ______ _________ _______ _____________ ____
200 "11001000" 1 1 {[1]} {[484]} {[1]} ""
201 "11001001" 2 1 {[1]} {[484]} {[2]} ""
202 "11001010" 3 1 {[1]} {[484]} {[3]} ""
203 "11001011" 4 1 {[1]} {[484]} {[4]} ""
204 "11001100" 5 1 {[1]} {[484]} {[5]} ""
205 "11001101" 6 1 {[1]} {[484]} {[6]} ""
206 "11001110" 7 1 {[1]} {[484]} {[7]} ""
207 "11001111" 8 1 {[1]} {[484]} {[8]} ""
Similarly, the allocation indices between 208 and 215 configure a full-band MU-MIMO 80 MHz allocation (996-tone RU), and the allocation indices between 216 and 223 configure a full-band MU-MIMO 160 MHz allocation (2x996-tone RU).
For example, the allocation index 203 specifies a 484-tone RU with 4 users:
cfg484MU = wlanHEMUConfig(203); showAllocation(cfg484MU,axAlloc)
HE Multi-User Format - OFDMA with RU Sizes Greater Than 242 Subcarriers
For an HE MU transmission with a channel bandwidth greater than 20 MHz, two HE-SIG-B content channels signal user configurations. These content channels are duplicated over each 40 MHz subchannel for larger channel bandwidths, as described in Section 27.3.11.8.5 of [1]. When you specify an RU size greater than 242 as part of an OFDMA configuration, you can signal the users assigned to the RU on either of the two HE-SIG-B content channels. The allocation index you specify when you create a wlanHEMUConfig object controls which content channel each user is signaled on. The allocation table in the Appendix shows the relevant allocation indices.
For example, consider the following 80 MHz configuration that serves seven users:
One 484-tone RU (RU #1) with four users (users #1-4)
One 242-tone RU (RU #2) with one user (user #5)
Two 106-tone RUs (RU #3 and #4), each with one user (users #6 and #7)
To configure an 80 MHz OFDMA transmission, you need four allocation indices, one for each 20 MHz subchannel. To configure the above scenario, use allocation indices of the form:
[X Y 192 96]
XandYconfigure the 484-tone RU, with users #1-4. The possible values ofXandYare discussed below.192configures a 242-tone RU with one user, user #5.96signals two 106-tone RUs, each with one user, users #6 and #7.
The selection of X and Y configures the correct number of users in the 484-tone RU, and determines which HE-SIG-B content channel signals each user. A 484-tone RU spans two 20 MHz subchannels, so you must specify two allocation indices. In the 484-tone RU, you can signal the four users on the two HE-SIG-B content channels in the combinations shown in Table 1.
An allocation index in the range 200-207 specifies 1-8 users on a 484-tone RU. To signal no users on a content channel, you can use the allocation indices 114 or 115, for a 484-tone or 996-tone RU respectively. Therefore, you can define the combinations in Table 1 using two allocation indices as shown in Table 2. The two allocation indices in each row of Table 2 are X and Y respectively.
For example, to configure Combination E use the following 80 MHz allocation indices:
[114 203 192 96]
114and203configure the 484-tone RU, with users #1-4.192configures a 242-tone RU with one user, user #5.96signals two 106-tone RUs, each with one user, users #6 and #7.
cfg484OFDMA = wlanHEMUConfig([114 203 192 96]); showAllocation(cfg484OFDMA,axAlloc);
To view the HE-SIG-B allocation signaling, use the hePlotHESIGBAllocationMapping function. This shows the user fields signaled on each HE-SIG-B content channel, as well as the RUs and users in the wlanHEMUConfig object that each user field signals. In this case the users on RUs #1, 3 and 4 are all signaled on content channel 2, and the user of RU #2 is signaled on content channel 1. The second content channel signals six users, while the first content channel signals only one user. Therefore, the first content channel is padded up to the length of the second for transmission. In the diagram, the RU allocation information is provided in the form index-size, e.g. RU8-106 is the 8th 106-tone RU.
figure;
hePlotHESIGBAllocationMapping(cfg484OFDMA);
axSIGB = gca; % Get axis handle for subsequent plotting
To balance the user field signaling in HE-SIG-B, you can use Combination B when you specify the allocation index for the 484-tone RU. This results in two users being signaled on each content channel of HE-SIG-B, meaning a better balance of user fields, and potentially fewer HE-SIG-B symbols in the transmission.
cfg484OFDMABalanced = wlanHEMUConfig([201 201 96 192]); hePlotHESIGBAllocationMapping(cfg484OFDMABalanced,axSIGB);
HE Multi-User Format - Central 26-Tone RU
In an 80 MHz transmission, when you do not use a full-band RU, you can choose whether the central 26-tone RU is active. Activate or deactivate the central 26-tone RU using a name-value argument when you create a wlanHEMUConfig object.
% Create a configuration with no central 26-tone RU cfgNoCentral = wlanHEMUConfig([192 192 192 192],LowerCenter26ToneRU=false); showAllocation(cfgNoCentral,axAlloc); % Create a configuration with a central 26-tone RU cfgCentral = wlanHEMUConfig([192 192 192 192],LowerCenter26ToneRU=true); showAllocation(cfgCentral,axAlloc);
Similarly, for a 160 MHz transmission, you can choose whether the central 26-tone RU in each 80 MHz segment is active. Activate or deactivate each central 26-tone RU using name-value arguments when you create a wlanHEMUConfig object. This example creates only the upper central 26-tone RU. The allocation index [200 114 114 200 200 114 114 200] specifies four 242-tone RUs, each with one user.
cfgCentral160MHz = wlanHEMUConfig([200 114 114 200 200 114 114 200],UpperCenter26ToneRU=true); disp(cfgCentral160MHz)
wlanHEMUConfig with properties:
RU: {1×5 cell}
User: {1×5 cell}
PrimarySubchannel: 1
NumTransmitAntennas: 1
STBC: 0
GuardInterval: 3.2000
HELTFType: 4
SIGBMCS: 0
SIGBDCM: 0
UplinkIndication: 0
BSSColor: 0
SpatialReuse: 0
TXOPDuration: 127
HighDoppler: 0
Read-only properties:
ChannelBandwidth: 'CBW160'
AllocationIndex: [200 114 114 200 200 114 114 200]
LowerCenter26ToneRU: 0
UpperCenter26ToneRU: 1
HE Multi-User Format - Preamble Puncturing
In an 80 MHz or 160 MHz transmission, you can puncture 20 MHz subchannels to allow a legacy system to operate in the punctured channel. This method is also described as channel bonding. You can use the 20 MHz subchannel allocation index 113 to null a 20 MHz subchannel. Use the showAllocation method to view the punctured 20 MHz subchannel.
% Null second lowest 20 MHz subchannel in a 160 MHz configuration cfgNull = wlanHEMUConfig([192 113 114 200 208 115 115 115]); % Plot the allocation showAllocation(cfgNull,axAlloc);
You can also view punctured 20 MHz with the generated waveform and the spectrumAnalyzer object:
% Set the transmission properties of each user in all RUs cfgNull.User{1}.APEPLength = 100; cfgNull.User{1}.MCS = 2; cfgNull.User{1}.ChannelCoding = "LDPC"; cfgNull.User{1}.NumSpaceTimeStreams = 1; cfgNull.User{2}.APEPLength = 1000; cfgNull.User{2}.MCS = 6; cfgNull.User{2}.ChannelCoding = "LDPC"; cfgNull.User{2}.NumSpaceTimeStreams = 1; cfgNull.User{3}.APEPLength = 100; cfgNull.User{3}.MCS = 1; cfgNull.User{3}.ChannelCoding = "LDPC"; cfgNull.User{3}.NumSpaceTimeStreams = 1; % Create packet txNullWaveform = wlanWaveformGenerator([1 0 1 0],cfgNull); % Visualize signal spectrum fs = wlanSampleRate(cfgNull); ofdmInfo = wlanHEOFDMInfo("HE-Data",cfgNull,1); fftsize = ofdmInfo.FFTLength; spectrumScope = spectrumAnalyzer(SampleRate=fs, ... AveragingMethod="exponential", ... ForgettingFactor=0.99, ... RBWSource="property", ... RBW=fs/fftsize, ... Title="160 MHz HE MU Transmission with Punctured 20 MHz Channel"); spectrumScope(txNullWaveform);
Trigger-Based MU Format
The HE trigger-based (TB) format enables OFDMA or MU-MIMO transmission in the uplink. Each station (STA) simultaneously transmits a TB packet when the access point (AP) triggers it. The AP entirely controls a TB transmission. The AP provides all the parameters required for the transmission in a trigger frame, which it transmits to all STAs participating in the TB transmission. This example configures a TB transmission in response to a trigger frame for three users in an OFDMA/MU-MIMO system. Three STAS transmit simultaneously to an AP.
Use the 20 MHz allocation 97. This setting specifies two RUs, one of which serves two users in MU-MIMO.
disp("Allocation #97 table entry:") disp(allocationTable(98,:)) % Index 97 (row 98)
Allocation #97 table entry:
Allocation BitAllocation NumUsers NumRUs RUIndices RUSizes NumUsersPerRU Note
__________ _____________ ________ ______ _________ ___________ _____________ ____
97 "01100001" 3 2 {[1 2]} {[106 106]} {[1 2]} ""
Obtain the allocation information by creating an MU configuration with a wlanHEMUConfig object.
% Generate an OFDMA allocation
cfgMU = wlanHEMUConfig(97);
allocationInfo = ruInfo(cfgMU);
In a TB transmission several parameters are the same for all users in the transmission. Some of these are specified here:
% These parameters are the same for all users in the OFDMA system trgMethod = "TriggerFrame"; % Method used to trigger an HE TB PPDU channelBandwidth = cfgMU.ChannelBandwidth; % Bandwidth of OFDMA system lsigLength = 142; % L-SIG length preFECPaddingFactor = 2; % Pre-FEC padding factor ldpcExtraSymbol = false; % LDPC extra symbol numHELTFSymbols = 2; % Number of HE-LTF symbols
Configure a TB transmission for a single user within the system with a wlanHETBConfig object. Create a cell array of three objects to describe the transmission of the three users.
% Create a trigger configuration for each user
numUsers = allocationInfo.NumUsers;
cfgTriggerUser = repmat({wlanHETBConfig},1,numUsers);
Set the non-default system-wide properties for each user.
for userIdx = 1:numUsers cfgTriggerUser{userIdx}.TriggerMethod = trgMethod; cfgTriggerUser{userIdx}.ChannelBandwidth = channelBandwidth; cfgTriggerUser{userIdx}.LSIGLength = lsigLength; cfgTriggerUser{userIdx}.PreFECPaddingFactor = preFECPaddingFactor; cfgTriggerUser{userIdx}.LDPCExtraSymbol = ldpcExtraSymbol; cfgTriggerUser{userIdx}.NumHELTFSymbols = numHELTFSymbols; end
Next, set the per-user properties. When multiple users are transmitting in the same RU in an MU-MIMO configuration, each user must transmit on different space-time stream indices. To ensure that different space-time streams are used, you must set the StartingSpaceTimeStream and NumSpaceTimeStreamSteams properties for each user. In this example user 1 and 2 are in an MU-MIMO configuration, therefore StartingSpaceTimeStream for user two is set to 2, because user one is configured to transmit 1 space-time stream with StartingSpaceTimeStream = 1.
% These parameters are for the first user - RU#1 MU-MIMO user 1 cfgTriggerUser{1}.RUSize = allocationInfo.RUSizes(1); cfgTriggerUser{1}.RUIndex = allocationInfo.RUIndices(1); cfgTriggerUser{1}.MCS = 4; % Modulation and coding scheme cfgTriggerUser{1}.NumSpaceTimeStreams = 1; % Number of space-time streams cfgTriggerUser{1}.NumTransmitAntennas = 1; % Number of transmit antennas cfgTriggerUser{1}.StartingSpaceTimeStream = 1; % The starting index of the space-time streams cfgTriggerUser{1}.ChannelCoding = "LDPC"; % Channel coding % These parameters are for the second user - RU#1 MU-MIMO user 2 cfgTriggerUser{2}.RUSize = allocationInfo.RUSizes(1); cfgTriggerUser{2}.RUIndex = allocationInfo.RUIndices(1); cfgTriggerUser{2}.MCS = 3; % Modulation and coding scheme cfgTriggerUser{2}.NumSpaceTimeStreams = 1; % Number of space-time streams cfgTriggerUser{2}.StartingSpaceTimeStream = 2; % The starting index of the space-time streams cfgTriggerUser{2}.NumTransmitAntennas = 1; % Number of transmit antennas cfgTriggerUser{2}.ChannelCoding = "LDPC"; % Channel coding % These parameters are for the third user - RU#2 cfgTriggerUser{3}.RUSize = allocationInfo.RUSizes(2); cfgTriggerUser{3}.RUIndex = allocationInfo.RUIndices(2); cfgTriggerUser{3}.MCS = 4; % Modulation and coding scheme cfgTriggerUser{3}.NumSpaceTimeStreams = 2; % Number of space-time streams cfgTriggerUser{3}.StartingSpaceTimeStream = 1; % The starting index of the space-time streams cfgTriggerUser{3}.NumTransmitAntennas = 2; % Number of transmit antennas cfgTriggerUser{3}.ChannelCoding = "BCC"; % Channel coding
A packet containing random data is now transmitted by each user with the wlanWaveformGenerator function. Store the waveform each user transmits for analysis.
trigInd = wlanFieldIndices(cfgTriggerUser{1}); % Get the indices of each field
txTrigStore = zeros(trigInd.HEData(2),numUsers);
for userIdx = 1:numUsers
% Generate waveform for a user
cfgTrigger = cfgTriggerUser{userIdx};
txPSDU = randi([0 1],getPSDULength(cfgTrigger)*8,1);
txTrig = wlanWaveformGenerator(txPSDU,cfgTrigger);
% Store the transmitted STA waveform for analysis
txTrigStore(:,userIdx) = sum(txTrig,2);
end
The spectrum of the transmitted waveform from each STA shows the different portions of the spectrum used, and the overlap in the MU-MIMO RU.
fs = wlanSampleRate(cfgTriggerUser{1});
ofdmInfo = wlanHEOFDMInfo("HE-Data",cfgTriggerUser{1});
spectrumScope = spectrumAnalyzer(SampleRate=fs, ...
Method="welch", ...
AveragingMethod="exponential", ...
ForgettingFactor=0, ...
ChannelNames={"RU#1 User 1","RU#1 User 2","RU#2"},...
ShowLegend=true,Title="Transmitted HE TB Waveform per User");
spectrumScope(txTrigStore);
Appendix
The RU allocation table for allocations <= 20 MHz is shown below, with annotated descriptions.
The RU allocation and HE-SIG-B user signaling for allocations > 20 MHz is shown in the table below, with annotated descriptions.
Selected Bibliography
IEEE Std 802.11ax™-2021. 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 - Amendment 1: Enhancements for High-Efficiency WLAN.
See Also
wlanHESUConfig | wlanHEMUConfig | wlanHETBConfig
