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QPSK Transmitter with SDR Hardware

This model shows how to use SDR hardware with MATLAB® and SDR System objects to implement a QPSK transmitter. The SDR device in this system will keep transmitting indexed "Hello world" messages at its specified center frequency. You can demodulate the transmitted message using the QPSK Receiver with SDR HardwareQPSK Receiver with SDR Hardware example with an additional SDR device.

Please refer to documentationdocumentation for details on configuring your host computer to work with the SDR Transmitter System object.

Implementations

This example describes the MATLAB® implementation of a QPSK transmitter with SDR hardware. There is another implementation of this example that uses Simulink®.

MATLAB script using System objects: sdrfQPSKTransmitter.msdrfQPSKTransmitter.m.

Simulink implementation using blocks: sdrfqpsktxsdrfqpsktx.

You can also explore a non-radio QPSK Transmitter and Receiver example that models a general wireless communication system using an AWGN channel and simulated channel impairments at commQPSKTransmitterReceiver.mcommQPSKTransmitterReceiver.m.

Introduction

This example has the following motivation:

  • To implement a real QPSK-based transmission-reception environment in MATLAB using SDR System objects.

  • To illustrate the use of key Communications System Toolbox™ System objects for QPSK system design.

In this example, the transmitter generates a message using ASCII characters, converts the characters to bits, and prepends a Barker code for receiver frame synchronization. This data is then modulated using QPSK and filtered with a square root raised cosine filter. The filtered QPSK symbols can be transmitted over the air using the SDR transmitter System object and the SDR hardware.

Initialization

The sdrfqpsktransmitter_init.msdrfqpsktransmitter_init.m script initializes the simulation parameters and generates the structure prmQPSKTransmitter.

prmQPSKTransmitter = sdrfqpsktransmitter_init % Transmitter parameter structure
compileIt  = false; % true if code is to be compiled for accelerated execution
useCodegen = false; % true to run the latest generated mex file

To achieve a successful transmission, ensure that the specified center frequency of the SDR Transmitter is within the acceptable range of your SDR daughterboard.

Also, by using the compileIt and useCodegen flags, you can interact with the code to explore different execution options. Set the MATLAB variable compileIt to true in order to generate C code; this can be accomplished by using the codegen command provided by the MATLAB Coder™ product. The codegen command compiles MATLAB® functions to a C-based static or dynamic library, executable, or MEX file, producing code for accelerated execution. The generated executable runs several times faster than the original MATLAB code. Set useCodegen to true to run the executable generated by codegen instead of the MATLAB code.

Code Architecture

The function runSDRFQPSKTransmitter implements the QPSK transmitter using two System objects, QPSKTransmitter and the SDR transmitter.

QPSK Transmitter

The transmitter includes the Bit Generation, QPSK Modulator and Raised Cosine Transmit Filter objects. The Bit Generation object generates the data frames. Each frame contains 200 bits. The first 26 bits are header bits, a 13-bit Barker code that has been oversampled by two. The Barker code is sent on both in-phase and quadrature components of the QPSK modulated symbols. This is achieved by repeating the Barker code bits twice before modulating them with the QPSK modulator.

The remaining bits are the payload. The first 105 bits of the payload correspond to the ASCII representation of 'Hello world ###', where '###' is an incrementing sequence of '001', '002', '003',..., '100'. The remaining payload bits are random bits. The payload is scrambled to guarantee a balanced distribution of zeros and ones for the timing recovery operation in the receiver object. The scrambled bits are modulated by the QPSK Modulator (with Gray mapping). The Raised Cosine Transmit Filter upsamples the modulated symbols by four, and has a roll-off factor of 0.5. The output rate of the Raised Cosine Filter is set to be 200e3 samples per second.

SDR Transmitter

The host computer communicates with the SDR radio using the SDR transmitter System object. You can supply the IP address of the SDR radio as an argument when you construct the object. The IP address can be any address within the same subnetwork as the host computer, and should match the IP address of the radio connected to the host. The CenterFrequency and InterpolationFactor arguments are set by the parameter variable prmQPSKTransmitter.

Execution

Before running the script, first turn on the SDR radio and connect it to the computer. As already mentioned, you can check the correct data transmission by running the QPSK Receiver with SDR HardwareQPSK Receiver with SDR Hardware example while running the transmitter script.

if compileIt
    codegen('runSDRFQPSKTransmitter', '-args', {coder.Constant(prmQPSKTransmitter)}); %#ok<UNRCH>
end
if useCodegen
   clear runSDRFQPSKTransmitter_mex %#ok<UNRCH>
   runSDRFQPSKTransmitter_mex(prmQPSKTransmitter);
else
   runSDRFQPSKTransmitter(prmQPSKTransmitter);
end

The gain behavior of different SDR daughter boards varies considerably. Thus, the gain setting in the receiver defined in this example may not be well suited for your daughter boards. If the message is not properly decoded by the receiver object, you can vary the gain of the source signals in the SDR Receiver System objects by changing the SimParams.RadioGain value in the transmitter initialization filetransmitter initialization file and in the receiver initialization filereceiver initialization file.

Also, a large relative frequency offset between the transmit and receive SDR radios can prevent the receiver functions from properly decoding the message. If that happens, you can determine the offset by sending a tone at a known frequency from the transmitter to the receiver, then measuring the offset between the transmitted and received frequency, then applying that offset to the center frequency of the SDR Receiver System object.

Appendix

This example uses the following script and helper functions:

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