802.11ac Multiuser MIMO Precoding with WINNER II Channel Model

This example uses:

This example shows the transmit and receive processing for a 802.11ac™ multiuser downlink transmission over a WINNER II fading channel. You must download and install the WINNER II Channel Model for Communications Toolbox™ Add-On to run this example. Only one WINNER II channel System object™ is needed to set up the channels from one access point to all users.

Introduction

802.11ac supports downlink (access-point to station) multiuser transmissions for up to four users and up to eight transmit antennas to increase the aggregate throughput of the link [ 1 ]. Based on a scheduled transmission time for a user, the scheduler looks for other smaller packets ready for transmission to other users. If available, it schedules these users over the same interval, which reduces the overall time taken for multiple transmissions.

This simultaneous transmission comes at a higher complexity because successful reception of the individual user's payloads requires precoding, also known as transmit-end beamforming. Precoding assumes that channel state information (CSI) is known at the transmitter. A sounding packet, as described in the 802.11ac Transmit Beamforming (WLAN Toolbox) example, is used to determine the CSI for each user in a multiuser transmission. Each of the users feed back their individual CSI to the beamformer. The beamformer uses the CSI from all users to set the precoding (spatial mapping) matrix for subsequent data transmission.

This example uses a channel inversion technique for a three-user transmission with a different number of spatial streams allocated per user and different rate parameters per user. The system can be characterized by the figure below.

The example generates the multiuser transmit waveform, passes it through a multiuser WINNER II channel and decodes the received signal for each user to calculate the bits in error. Prior to the data transmission, the example uses a null-data packet (NDP) transmission to sound the different channel for each user and determines the precoding matrix under the assumption of perfect feedback.

Check for Support Package Installation

Check if the 'WINNER II Channel Model for Communications Toolbox' support package is installed.

commSupportPackageCheck('CST_WINNER2');

Simulation Parameters and Configuration

For 802.11ac, a maximum of eight spatial streams is allowed. An 8x8 MIMO configuration for three users is used in this example, where the first user has three streams, second has one, and the third has four streams allocated to it. Different rate parameters and payload sizes for each user are specified as vector parameters for the transmission configuration.

s = rng(10);                           % Set RNG seed for repeatability

% Transmission parameters
chanBW      = 'CBW80';               % Channel bandwidth
numUsers    = 3;                     % Number of users
numSTSVec   = [3 1 4];               % Number of streams per user
userPos     = [0 1 2];               % User positions
mcsVec      = [4 6 8];               % MCS per user: 16QAM, 64QAM, 256QAM
apepVec     = [520 192 856];         % Payload per user, in bytes
chCodingVec = {'BCC','LDPC','LDPC'}; % Channel coding per user

% Precoding and equalization parameters
precodingType = 'ZF';                % Precoding type; ZF or MMSE
snr           = 47;                  % SNR in dB
eqMethod      = 'ZF';                % Equalization method

% Create the multiuser VHT format configuration object
numTx = sum(numSTSVec);
cfgVHTMU = wlanVHTConfig('ChannelBandwidth',chanBW, ...
    'NumUsers',numUsers, ...
    'NumTransmitAntennas',numTx, ...
    'GroupID',2, ...
    'NumSpaceTimeStreams',numSTSVec,...
    'UserPositions',userPos, ...
    'MCS',mcsVec, ...
    'APEPLength',apepVec, ...
    'ChannelCoding',chCodingVec);

The number of transmit antennas is set to be the sum total of all the used space-time streams. This implies no space-time block coding (STBC) or spatial expansion is employed for the transmission.

Sounding (NDP) Configuration

For precoding, channel sounding is first used to determine the channel experienced by the users (receivers). This channel state information is sent back to the transmitter, for it to be used for subsequent data transmission. It is assumed that the channel varies slowly over the two transmissions. For multiuser transmissions, the same NDP (Null Data Packet) is transmitted to each of the scheduled users [ 2 ].

% VHT sounding (NDP) configuration, for same number of streams
cfgVHTNDP = wlanVHTConfig('ChannelBandwidth',chanBW, ...
    'NumUsers',1, ...
    'NumTransmitAntennas',numTx, ...
    'GroupID',0, ...
    'NumSpaceTimeStreams',sum(numSTSVec),...
    'MCS',0, ...
    'APEPLength',0);

The number of streams specified is the sum total of all space-time streams used. This allows the complete channel to be sounded.

% Generate the null data packet, with no data
txNDPSig = wlanWaveformGenerator([],cfgVHTNDP);
NPDSigLen = size(txNDPSig, 1);

WINNER II Channel for Indoor Office (A1) Scenario

In this example, one comm.WINNER2Channel System object™ in the WINNER II Channel Model for Communications Toolbox™ is set up to simulate the three channels to different users. The indoor office (A1) non-line-of-sight (NLOS) scenario is configured for each user. With a fixed power delay profile, each user experiences a 16-path fading channel with the largest delay of 175 us. Each user is also assigned a low mobility as appropriate for 802.11ac.

The access point employs a uniform circular array (UCA) with a radius of 20cm. Each user employs a uniform linear array (ULA) with 5cm spacing between elements. It is also assumed that each user's number of receive antennas is equal to the number of space-time streams allocated to them.

% Set up layout parameters for WINNER II channel
AA = winner2.AntennaArray('UCA',numTx,0.2);
for i = 1:numUsers
    AA(i+1) = winner2.AntennaArray('ULA',numSTSVec(i),0.05);
end
STAIdx   = 2:(numUsers+1);
APIdx   = {1};
rndSeed = 12;
cfgLayout = winner2.layoutparset(STAIdx,APIdx,numUsers,AA,[],rndSeed);
cfgLayout.Pairing = [ones(1,numUsers);2:(numUsers+1)]; % One access point to all users
cfgLayout.ScenarioVector = ones(1,numUsers);           % A1 scenario for all links
cfgLayout.PropagConditionVector = zeros(1,numUsers);  % NLOS
for i = 1:numUsers % Randomly set velocity for each user
    v = rand(3,1) - 0.5;
    cfgLayout.Stations(i+1).Velocity = v/norm(v,'fro');
end

% Set up model parameters for WINNER II channel
cfgModel = winner2.wimparset;
cfgModel.FixedPdpUsed       = 'yes';
cfgModel.FixedAnglesUsed    = 'yes';
cfgModel.IntraClusterDsUsed = 'no';
cfgModel.RandomSeed         = 111;    % Repeatability

% The maximum velocity for the 3 users is 1m/s. Set up the SampleDensity
% field to ensure that the sample rate matches the channel bandwidth.
maxMSVelocity = max(cell2mat(cellfun(@(x) norm(x,'fro'), ...
    {cfgLayout.Stations.Velocity},'UniformOutput',false)));
cfgModel.UniformTimeSampling = 'yes';
cfgModel.SampleDensity = round(physconst('LightSpeed')/ ...
    cfgModel.CenterFrequency/2/(maxMSVelocity/wlanSampleRate(cfgVHTMU)));

% Create the WINNER II channel System object
WINNERChan = comm.WINNER2Channel(cfgModel,cfgLayout);

% Call the info method to check some derived channel parameters
chanInfo = info(WINNERChan)

chanInfo = 

  struct with fields:

               NumLinks: 3
          NumBSElements: [8 8 8]
          NumMSElements: [3 1 4]
               NumPaths: [16 16 16]
             SampleRate: [8.0000e+07 8.0000e+07 8.0000e+07]
     ChannelFilterDelay: [7 7 7]
    NumSamplesProcessed: 0

The channel filtering delay for each user is stored to account for its compensation at the receiver. In practice, symbol timing estimation would be used. At transmitter, an extra ten all-zero samples are appended to account for channel filter delay.

chanDelay   = chanInfo.ChannelFilterDelay;
numPadZeros = 10;

% Set ModelConfig.NumTimeSamples to match the length of the input signal to
% avoid warning
WINNERChan.ModelConfig.NumTimeSamples = NPDSigLen + numPadZeros;

% Sound the WINNER II channel for all users
chanOutNDP = WINNERChan([txNDPSig;zeros(numPadZeros,numTx)]);

% Add AWGN
rxNDPSig = cellfun(@awgn,chanOutNDP, ...
    num2cell(snr*ones(numUsers,1)),'UniformOutput',false);

Channel State Information Feedback

Each user estimates its own channel using the received NDP signal and computes the channel state information that it can send back to the transmitter. This example uses the singular value decomposition of the channel seen by each user to compute the CSI feedback.

mat = cell(numUsers,1);
for uIdx = 1:numUsers
    % Compute the feedback matrix based on received signal per user
    mat{uIdx} = vhtCSIFeedback(rxNDPSig{uIdx}(chanDelay(uIdx)+1:end,:), ...
        cfgVHTNDP,uIdx,numSTSVec);
end

Assuming perfect feedback, with no compression or quantization loss of the CSI, the transmitter computes the steering matrix for the data transmission using either Zero-Forcing or Minimum-Mean-Square-Error (MMSE) based precoding techniques. Both methods attempt to cancel out the intra-stream interference for the user of interest and interference due to other users. The MMSE-based approach avoids the noise enhancement inherent in the zero-forcing technique. As a result, it performs better at low SNRs.

% Pack the per user CSI into a matrix
numST = length(mat{1});         % Number of subcarriers
steeringMatrix = zeros(numST,sum(numSTSVec),sum(numSTSVec));
%   Nst-by-Nt-by-Nsts
for uIdx = 1:numUsers
    stsIdx = sum(numSTSVec(1:uIdx-1))+(1:numSTSVec(uIdx));
    steeringMatrix(:,:,stsIdx) = mat{uIdx};     % Nst-by-Nt-by-Nsts
end

% Zero-forcing or MMSE precoding solution
if strcmp(precodingType, 'ZF')
    delta = 0; % Zero-forcing
else
    delta = (numTx/(10^(snr/10))) * eye(numTx); % MMSE
end
for i = 1:numST
    % Channel inversion precoding
    h = squeeze(steeringMatrix(i,:,:));
    steeringMatrix(i,:,:) = h/(h'*h + delta);
end

% Set the spatial mapping based on the steering matrix
cfgVHTMU.SpatialMapping = 'Custom';
cfgVHTMU.SpatialMappingMatrix = permute(steeringMatrix,[1 3 2]);

Data Transmission

Random bits are used as the payload for the individual users. A cell array is used to hold the data bits for each user, txDataBits. For a multiuser transmission the individual user payloads are padded such that the transmission duration is the same for all users. This padding process is described in Section 9.12.6 of [ 1 ]. In this example for simplicity the payload is padded with zeros to create a PSDU for each user.

% Create data sequences, one for each user
txDataBits = cell(numUsers,1);
psduDataBits = cell(numUsers,1);
for uIdx = 1:numUsers
    % Generate payload for each user
    txDataBits{uIdx} = randi([0 1],cfgVHTMU.APEPLength(uIdx)*8,1,'int8');

    % Pad payload with zeros to form a PSDU
    psduDataBits{uIdx} = [txDataBits{uIdx}; ...
        zeros((cfgVHTMU.PSDULength(uIdx)-cfgVHTMU.APEPLength(uIdx))*8,1,'int8')];
end

Using the format configuration, cfgVHTMU, with the steering matrix, to generate the multiuser VHT waveform.

txSig = wlanWaveformGenerator(psduDataBits,cfgVHTMU);

The WINNER II channel object does not allow the input signal size to change once locked, so we have to call the release method before passing the waveform through it. In addition, as we restart the channel, we want it to re-process the NDP before the waveform so as to accurately mimic the channel continuity. Only the waveform portion of the channel's output is extracted for the subsequent processing of each user.

release(WINNERChan);

% Set ModelConfig.NumTimeSamples to match the total length of NDP plus
% waveform and padded zeros
WINNERChan.ModelConfig.NumTimeSamples = ...
    WINNERChan.ModelConfig.NumTimeSamples + length(txSig) + numPadZeros;

% Transmit through the WINNER II channel for all users, with 10 all-zero
% samples appended to account for channel filter delay
chanOut = WINNERChan([txNDPSig; zeros(numPadZeros,numTx); ...
    txSig; zeros(numPadZeros,numTx)]);

% Extract the waveform output for each user
chanOut = cellfun(@(x) x(NPDSigLen+numPadZeros+1:end,:),chanOut,'UniformOutput',false);

% Add AWGN
rxSig = cellfun(@awgn,chanOut, ...
    num2cell(snr*ones(numUsers,1)),'UniformOutput',false);

Data Recovery per User

The receive signals for each user are processed individually. The example assumes that there are no front-end impairments and that the transmit configuration is known by the receiver for simplicity.

A user number specifies the user of interest being decoded for the transmission. This is also used to index into the vector properties of the configuration object that are user-specific.

% Get field indices from configuration, assumed known at receiver
ind = wlanFieldIndices(cfgVHTMU);

% Single-user receivers recover payload bits
rxDataBits = cell(numUsers,1);
scaler = zeros(numUsers,1);
spAxes = gobjects(sum(numSTSVec),1);
hfig = figure('Name','Per-stream equalized symbol constellation');
for uIdx = 1:numUsers
    rxNSig = rxSig{uIdx}(chanDelay(uIdx)+1:end, :);

    % User space-time streams
    stsU = numSTSVec(uIdx);

    % Estimate noise power in VHT fields
    lltf = rxNSig(ind.LLTF(1):ind.LLTF(2),:);
    demodLLTF = wlanLLTFDemodulate(lltf,chanBW);
    nVar = helperNoiseEstimate(demodLLTF,chanBW,sum(numSTSVec));

    % Perform channel estimation based on VHT-LTF
    rxVHTLTF  = rxNSig(ind.VHTLTF(1):ind.VHTLTF(2),:);
    demodVHTLTF = wlanVHTLTFDemodulate(rxVHTLTF,chanBW,numSTSVec);
    chanEst = wlanVHTLTFChannelEstimate(demodVHTLTF,chanBW,numSTSVec);

    % Recover information bits in VHT Data field
    rxVHTData = rxNSig(ind.VHTData(1):ind.VHTData(2),:);
    [rxDataBits{uIdx},~,eqsym] = wlanVHTDataRecover(rxVHTData, ...
        chanEst,nVar,cfgVHTMU,uIdx, ...
        'EqualizationMethod',eqMethod,'PilotPhaseTracking','None', ...
        'LDPCDecodingMethod','layered-bp','MaximumLDPCIterationCount',6);

    % Plot equalized symbols for all streams per user
    scaler(uIdx) = ceil(max(abs([real(eqsym(:)); imag(eqsym(:))])));
    for i = 1:stsU
        subplot(numUsers,max(numSTSVec),(uIdx-1)*max(numSTSVec)+i);
        plot(reshape(eqsym(:,:,i),[],1),'.');
        axis square
        spAxes(sum([0 numSTSVec(1:(uIdx-1))])+i) = gca; % Store axes handle
        title(['User ' num2str(uIdx) ', Stream ' num2str(i)]);
        grid on;
    end
end

% Scale axes for all subplots and scale figure
for i = 1:numel(spAxes)
    xlim(spAxes(i),[-max(scaler) max(scaler)]);
    ylim(spAxes(i),[-max(scaler) max(scaler)]);
end
pos = get(hfig,'Position');
set(hfig,'Position',[pos(1)*0.7 pos(2)*0.7 1.3*pos(3) 1.3*pos(4)]);

Per-stream equalized symbol constellation plots validate the simulation parameters and convey the effectiveness of the technique. Note the discernible 16QAM, 64QAM and QPSK constellations per user as specified on the transmit end. Also observe the EVM degradation over the different streams for an individual user. This is a representative characteristic of the channel inversion technique.

The recovered data bits are compared with the transmitted payload bits to determine the bit error rate.

% Compare recovered bits against per-user APEPLength information bits
ber = inf(1, numUsers);
for uIdx = 1:numUsers
    idx = (1:cfgVHTMU.APEPLength(uIdx)*8).';
    [~,ber(uIdx)] = biterr(txDataBits{uIdx}(idx),rxDataBits{uIdx}(idx));
    disp(['Bit Error Rate for User ' num2str(uIdx) ': ' num2str(ber(uIdx))]);
end

rng(s); % Restore RNG state

Bit Error Rate for User 1: 0
Bit Error Rate for User 2: 0
Bit Error Rate for User 3: 0.00043808

The small number of bit errors, within noise variance, indicate successful data decoding for all streams for each user, despite the variation in EVMs seen in individual streams.

Conclusion and Further Exploration

The example shows how to use the WINNER II fading channel System object to model a multiuser VHT transmission in 802.11ac. Further exploration includes modifications to the transmission parameters, antenna arrays, channel scenarios, LOS vs. NLOS propagations, path loss modeling and shadowing modeling.

There is another version of this example in the WLAN Toolbox™, which uses three independent TGac fading channels for three users: 802.11ac Multi-User MIMO Precoding (WLAN Toolbox).

Appendix

This example uses the following helper functions from WLAN Toolbox:

Selected Bibliography

IEEE® Std 802.11ac-2013 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 4: Enhancements for Very High Throughput for Operation in Bands below 6 GHz.
Perahia, E., R. Stacey, "Next Generation Wireless LANS: 802.11n and 802.11ac", Cambridge University Press, 2013.
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.
IST WINNER II, "WINNER II Channel Models", D1.1.2, Sep. 2007.
Breit, G., H. Sampath, S. Vermani, et al., "TGac Channel Model Addendum", Version 12. IEEE 802.11-09/0308r12, March 2010.