Multi-User Transmit Beamforming with USRP™ Hardware

radioConfig = helperMUBeamformAllocateRadios;

% Save radio configuration to a file for helperMUBeamformRx1 and helperMUBeamformRx2
save(fullfile(tempdir,'helperMUBeamformRadioConfig.mat'),'radioConfig');

transmitter = comm.SDRuTransmitter('Platform',radioConfig.txPlatform,...
                                   'IPAddress',radioConfig.txIPAddrs);
transmitter.MasterClockRate = radioConfig.txMasterClockRate;
transmitter.InterpolationFactor = radioConfig.txInterpolationfactor;
transmitter.ChannelMapping = [1 2 3 4];
transmitter.CenterFrequency = 900e6;
transmitter.Gain = 8;
transmitter.ClockSource = 'External'; % Synchronize all 4 channels in frequency
transmitter.PPSSource = 'External';   % Synchronize all 4 channels in time

% Radio settings
transmitter

trainingSig = helperMUBeamformInitGoldSeq; % Based on Gold Sequences                    

% Construct payload 1:
% 64 symbols with IFFT length of 256
% Each symbol uses 8 subcarriers
% Subcarrier 5 uses 64-QAM. Each point of 64-QAM is used once.
% Other subcarriers use QPSK
modOut1 = [zeros(4,64);
           qammod(randperm(64)-1,64,'UnitAveragePower',true);   % 64-QAM
           qammod(randi([0 3],7,64),4,'UnitAveragePower',true); % QPSK
           zeros(256-4-8,64)];
payload1 = reshape(ifft(modOut1),[],1);
% Scale time-domain signal appropriately
payload1 = payload1/max(real(payload1))*0.5;

% Construct payload 2:
% 64 symbols with IFFT length of 256
% Each symbol uses 8 subcarriers
% Subcarrier 5 uses 16-QAM. Each point of 16-QAM is used 4 times.
% Other subcarriers use QPSK
modOut2 = [zeros(4,64);
           qammod([(randperm(16)-1) (randperm(16)-1) ...
                   (randperm(16)-1) (randperm(16)-1)], ...
                   16,'UnitAveragePower',true);                 % 16-QAM
           qammod(randi([0 3],7,64),4,'UnitAveragePower',true); % QPSK
           zeros(256-4-8,64)];
payload2 = reshape(ifft(modOut2),[],1);
% Scale time-domain signal appropriately
payload2 = payload2/max(real(payload2))*0.5;

% Have no knowledge of the channel yet
% Generate channel estimate randomly
lastFeedback1 = rand(1,4) + 1j*rand(1,4);
fid1 = fopen(fullfile(tempdir,'helperMUBeamformfeedback1.bin'),'wb');
fwrite(fid1,[real(lastFeedback1) imag(lastFeedback1)],'double');
fclose(fid1);

% Have no knowledge of the channel yet
% Generate channel estimate randomly
lastFeedback2 = rand(1,4) + 1j*rand(1,4);
fid2 = fopen(fullfile(tempdir,'helperMUBeamformfeedback2.bin'),'wb');
fwrite(fid2,[real(lastFeedback2) imag(lastFeedback2)],'double');
fclose(fid2);

% Open files for reading only
fid1 = fopen(fullfile(tempdir,'helperMUBeamformfeedback1.bin'),'rb');
fid2 = fopen(fullfile(tempdir,'helperMUBeamformfeedback2.bin'),'rb');

disp('Sending two different payloads to two receivers simultaneously in the same frequency band ...');

disp('Channel estimates from the receivers are used continuously to update the transmitted beams.');

disp('Please run helperMUBeamformRx1 and helperMUBeamformRx2 in two separate MATLAB sessions on this computer.');

for i = 1:30000
    fseek(fid1,0,'bof'); % Read from the beginning of the file
    % The channel between 4 TX antennas and 1 RX antenna is modeled
    % by 4 complex gains. This approximation works because the
    % signal has very narrow bandwidth (400k samples per second).
    channelEst1 = fread(fid1,8,'double');
    if length(channelEst1) == 8
        % File content has expected length
        channelEst1 = (channelEst1(1:4) + 1j*channelEst1(5:8)).';
        lastFeedback1 = channelEst1;
    else
        % Use last feedback
        channelEst1 = lastFeedback1;
    end

    fseek(fid2,0,'bof'); % Read from the beginning of the file
    % The channel between 4 TX antennas and 1 RX antenna is modeled
    % by 4 complex gains. This approximation works because the
    % signal has very narrow bandwidth (400k samples per second).
    channelEst2 = fread(fid2,8,'double');
    if length(channelEst2) == 8
        % File content has expected length
        channelEst2 = (channelEst2(1:4) + 1j*channelEst2(5:8)).';
        lastFeedback2 = channelEst2;
    else
        % Use last feedback
        channelEst2 = lastFeedback2;
    end

    % For payload 1, create a spectral null at receiver 2 by
    % inverting the channel response and applying phase offsets of
    % 0, pi/2, pi, and 3*pi/2 to achieve destructive interference.
    % Hence, payload 1 will be suppressed for receiver 2.
    beamWeight1 = 1./channelEst2;
    beamWeight1 = beamWeight1/max(abs(beamWeight1)); % Normalize the gain
    beamWeight1 = beamWeight1 .* [1 1j -1 -1j];

    % Rotate payload 1 so that receiver 1 does not need to
    % correct the phase of payload 1
    phaseCorrection1 = beamWeight1 * channelEst1.';
    phaseCorrection1 = phaseCorrection1/abs(phaseCorrection1);
    beamWeight1 = beamWeight1 ./ phaseCorrection1;

    % For payload 2, create a spectral null at receiver 1 by
    % inverting the channel response and applying phase offsets of
    % 0, pi/2, pi, and 3*pi/2 to achieve destructive interference
    % Hence, payload 2 will be suppressed for receiver 1.
    beamWeight2 = 1./channelEst1;
    beamWeight2 = beamWeight2/max(abs(beamWeight2));
    beamWeight2 = beamWeight2 .* [1 1j -1 -1j];

    % Rotate payload 2 so that receiver 2 does not need to
    % correct the phase of payload 2
    phaseCorrection2 = beamWeight2 * channelEst2.';
    phaseCorrection2 = phaseCorrection2/abs(phaseCorrection2);
    beamWeight2 = beamWeight2 ./ phaseCorrection2;

    % Beamforming for payload
    payload = payload1*beamWeight1 + payload2*beamWeight2;

    % Send signals to the radios
    txSig = [trainingSig; zeros(400,4); payload; zeros(100,4)] * 0.2;
    transmitter(txSig);
end

release(transmitter);