Alamouti Coding Based MIMO Transmitter using USRP Radio

This example demonstrates the implementation of a 2x2 Multiple Input Multiple Output (MIMO) transmitter based on Alamouti based Space-Time Block Coding (STBC) using a USRP™ radio. The MIMO transmitter combines a training signal with a QPSK-modulated payload and then transmits over the air. The Alamouti Coding Based MIMO Receiver using USRP Radio example receives and demodulates the signal transmitted by this example

Required Hardware and Software

To run this example, you require one of the following hardware:

• A B210 radio or two bundled N210 radios and Communications Toolbox Support Package for USRP Radio, required when using the radio as the transmitter. For information on mapping an NI™ USRP device to an Ettus Research 200-series USRP device, see Supported Hardware and Required Software.

• A 300-Series USRP radio (X3xx or N3xx) and Wireless Testbench Support Package for NI USRP Radios. For information on mapping an NI USRP device to an Ettus Research 300-series USRP device, see Supported Radio Devices (Wireless Testbench).

• A MIMO cable for use with two N210 radios.

The example requires two MATLAB™ sessions, one for the transmitter and one for the receiver. You run the Alamouti Coding Based MIMO Receiver using USRP Radio example in another MATLAB session to receive the transmitted signal.

Configure USRP Radio Parameters

Define the USRP radio parameters including the platform type, radio address, gain, center frequency, and sample rate. If you use two N200 or N210 radios, specify radioAddress for two radios separated by a comma. Example: "192.168.10.2,192.168.10.3"

platform        = "B210"; % Select radio for Transmission
radioAddress    = "30F59A1"; % Serial number/IP Address of USRP radio.
gain            = 45; % Set radio gain
centerFrequency = 3200000000; % Set RF center frequency
sampleRate      = 1000000; % Set sample rate
stopTime        = 100; % Set stop time

Transmitter Design

Initialize Transmitter Parameters

The sdruOSTBCTransmitterInit function initializes the USRP radio System object and returns the initialization parameters. Initialize the system objects required for the transmitter, including the radio object, scrambler, Alamouti encoder, and raised cosine transmit filter:

[radio, initParams] = sdruOSTBCTransmitterInit(platform, radioAddress,...
    gain, centerFrequency, sampleRate);

% Create a scrambler object to scramble the input bits
scrambler = comm.Scrambler(initParams.ScramblerBase, initParams.ScramblerPolynomial, ...
    initParams.ScramblerInitialConditions);

% Create Alamouti encoder object to perform Alamouti encoding on input
% symbols
alamoutiEnc = comm.OSTBCEncoder( ...
    'NumTransmitAntennas', initParams.NTx);

% Create Raised cosine transmit filter object to interpolate input signal
% using raised cosine FIR filter
txFilter = comm.RaisedCosineTransmitFilter('RolloffFactor' , initParams.RolloffFactor ,...
     'OutputSamplesPerSymbol', initParams.UpSamplingFactor);

Construct Training Signal and Payload

Generate a training signal based on Gold sequences, scramble the message bits, modulate the data, and encode it using the Alamouti space-time Block Encoder

trainingSignal = sdruOSTBCInitGoldSeq; % Based on Gold Sequences

% Scramble the message
scrambledBits = scrambler(initParams.MessageBits);

% Modulate data
modOut = pskmod(scrambledBits, initParams.ModulationOrder, pi/4, ...
    InputType="bit", OutputDataType="double");

% Alamouti Space-Time Block Encoder
payload = alamoutiEnc(modOut);

Transmit Signal

Prepend the training signal to the payload. Pulse shape the data and transmit it using the configured USRP radio. Repeat this process until the specified stop time is reached.

% Prepend training signal to the payload
data = [trainingSignal; payload; zeros(10,2)];

% Pulse shape data
txSig = txFilter(data);

currentTime = 0;
underrun = uint32(0)

underrun = uint32

0

USRPFrameTime = length(txSig(:,1))/sampleRate;

% Transmit signal over the air
while currentTime < stopTime
   
    % Data transmission
    tunderrun = radio(txSig);
    underrun = underrun + tunderrun;

    if tunderrun
        underrun
    end

    % Update simulation time
    currentTime=currentTime+USRPFrameTime;
end

Finally, release the system objects to free up resources

release(radio);
release(scrambler);
release(alamoutiEnc);
release(txFilter);

Helper Functions

The following scripts are used in this example.

• sdruOSTBCInitGoldSeq.m

• sdruOSTBCTransmitterInit.m