# Multi-User Transmit Beamforming with USRP® Hardware

This example uses beamforming techniques to send two different payloads to two receivers simultaneously in the same frequency band. Channel estimates from the receivers are used continuously to update the transmitted beams.

The transmitter uses multiple USRP® radios as a composite radio with 4 channels. Two X300/X310 radios or four N-series radios are required. All radios for the transmitter must be connected to the same PPS and 10 MHz clock generator via cables of equal lengths.

Besides the transmitter radios, the host computer must be connected to two more USRP® radios, one for each receiver. The receiver radios must also be connected to the same 10 MHz clock generator.

To run this example, run MultiUserBeamformingExample, and then run helperMUBeamformRx1 and helperMUBeamformRx2 in two separate MATLAB sessions on the same host computer.

## Contents

## Configure radios

Find attached radios and allocate radios for transmitter and receivers.

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

Checking radio connections... transmitter = comm.SDRuTransmitter with properties: Platform: 'X310' IPAddress: '192.168.20.3,192.168.20.2' ChannelMapping: [1 2 3 4] CenterFrequency: 900000000 LocalOscillatorOffset: 0 Gain: 8 PPSSource: 'External' ClockSource: 'External' MasterClockRate: 200000000 InterpolationFactor: 500 TransportDataType: 'int16' EnableBurstMode: false

## Construct training signals and payloads

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;

## Initialize variables and temporary files for channel feedback

% 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');

## Main loop

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);

Sending two different payloads to two receivers simultaneously in the same frequency band ... Channel estimates from the receivers are used continuously to update the transmitted beams. Please run helperMUBeamformRx1 and helperMUBeamformRx2 in two separate MATLAB sessions on this computer.

## Appendix

This example uses the following helper functions:

## Copyright Notice

Universal Software Radio Peripheral® and USRP® are trademarks of National Instruments Corp.