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QPSK Receiver with USRP® Hardware

This example shows how to use the Universal Software Radio Peripheral® (USRP®) device using SDRu (Software Defined Radio USRP®) System objects to implement a QPSK receiver. The receiver addresses practical issues in wireless communications, such as carrier frequency and phase offset, timing offset and frame synchronization. This system receives the signal sent by the QPSK Transmitter with USRP® Hardware example. The receiver demodulates the received symbols and prints a simple message to the MATLAB® command line.

Please refer to the Setup and Configuration section of Documentation for USRP® Radio for details on configuring your host computer to work with the SDRu Receiver System object.

Implementations

This example describes the MATLAB implementation of a QPSK receiver with USRP® Hardware. There is another implementation of this example that uses Simulink®.

MATLAB script using System objects: sdruQPSKReceiver.m.

Simulink implementation using blocks: sdruqpskrx.mdl.

You can also explore a simulation only QPSK Transmitter and Receiver example without SDR hardware that models a general wireless communication system using an AWGN channel and simulated channel impairments at QPSK Transmitter and Receiver.

Introduction

This example has the following motivation:

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

  • To illustrate the use of key Communications Toolbox™ System objects for QPSK system design, including coarse and fine carrier frequency compensation, timing recovery with bit stuffing and stripping, frame synchronization, carrier phase ambiguity resolution, and message decoding.

In this example, the SDRuReceiver System object receives data corrupted by the transmission over the air and outputs complex baseband signals which are processed by the QPSK Receiver System object. This example provides a reference design of a practical digital receiver that can cope with wireless channel impairments. The receiver includes correlation-based coarse frequency compensation, PLL-based fine frequency compensation, timing recovery with fixed-rate resampling and bit stuffing/skipping, frame synchronization, and phase ambiguity resolution.

Discover Radio

Discover radio(s) connected to your computer. This example uses the first USRP® radio found using the findsdru function. Check if the radio is available and record the radio type. If no available radios are found, the example uses a default configuration for the system.

connectedRadios = findsdru;
if strncmp(connectedRadios(1).Status, 'Success', 7)
  platform = connectedRadios(1).Platform;
  switch connectedRadios(1).Platform
    case {'B200','B210'}
      address = connectedRadios(1).SerialNum;
    case {'N200/N210/USRP2','X300','X310','N300','N310','N320/N321'}
      address = connectedRadios(1).IPAddress;
  end
else
  address = '192.168.10.2';
  platform = 'N200/N210/USRP2';
end
Checking radio connections...

Initialization

The sdruqpskreceiver_init.m script initializes the simulation parameters and generates the structure prmQPSKReceiver.

printReceivedData = true;    % true if the received data is to be printed
compileIt         = false;   % true if code is to be compiled for accelerated execution
useCodegen        = false;   % true to run the latest generated code (mex file) instead of MATLAB code

% Receiver parameter structure
prmQPSKReceiver = sdruqpskreceiver_init(platform, useCodegen)
prmQPSKReceiver.Platform = platform;
prmQPSKReceiver.Address = address;
prmQPSKReceiver = 

  struct with fields:

                           Rsym: 200000
                ModulationOrder: 4
                  Interpolation: 2
                     Decimation: 1
                           Tsym: 5.0000e-06
                             Fs: 400000
                     BarkerCode: [1 1 1 1 1 -1 -1 1 1 -1 1 -1 1]
                   BarkerLength: 13
                   HeaderLength: 26
                        Message: 'Hello world'
                  MessageLength: 16
                NumberOfMessage: 100
                  PayloadLength: 11200
                      FrameSize: 5613
                      FrameTime: 0.0281
                  RolloffFactor: 0.5000
                  ScramblerBase: 2
            ScramblerPolynomial: [1 1 1 0 1]
     ScramblerInitialConditions: [0 0 0 0]
         RaisedCosineFilterSpan: 10
...

To transmit successfully, ensure that the specified center frequency of the SDRu Receiver is within the acceptable range of your USRP® 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 runSDRuQPSKReceiver implements the QPSK receiver using two System objects, QPSKReceiver and comm.SDRuReceiver.

SDRu Receiver

This example communicates with the USRP® board using the SDRu receiver System object. The parameter structure prmQPSKReceiver sets the CenterFrequency, Gain, and InterpolationFactor etc.

QPSK Receiver

This component regenerates the original transmitted message. It is divided into five subcomponents, modeled using System objects. Each subcomponent is modeled by other subcomponents using System objects.

1) Automatic Gain Control: Sets its output power to a level ensuring that the equivalent gains of the phase and timing error detectors keep constant over time. The AGC is placed before the Raised Cosine Receive Filter so that the signal amplitude can be measured with an oversampling factor of two. This process improves the accuracy of the estimate.

2) Coarse frequency compensation: Uses a correlation-based algorithm to roughly estimate the frequency offset and then compensate for it. The estimated coarse frequency offset is averaged so that fine frequency compensation is allowed to lock/converge. Hence, the coarse frequency offset is estimated using a comm.CoarseFrequencyCompensator System object and an averaging formula; the compensation is performed using a comm.PhaseFrequencyOffset System object.

3) Timing recovery: Performs timing recovery with closed-loop scalar processing to overcome the effects of delay introduced by the channel, using a comm.SymbolSynchronizer System object. The object implements a PLL to correct the symbol timing error in the received signal. The rotationally-invariant Gardner timing error detector is chosen for the object in this example; thus, timing recovery can precede fine frequency compensation. The input to the object is a fixed-length frame of samples. The output of the object is a frame of symbols whose length can vary due to bit stuffing and stripping, depending on actual channel delays.

4) Fine frequency compensation: Performs closed-loop scalar processing and compensates for the frequency offset accurately, using a comm.CarrierSynchronizer System object. The object implements a phase-locked loop (PLL) to track the residual frequency offset and the phase offset in the input signal.

5) Preamble Detection: Detects the location of the known Barker code in the input using a comm.PreambleDetector System object. The object implements a cross-correlation based algorithm to detect a known sequence of symbols in the input.

6) Frame Synchronization: Performs frame synchronization and, also, converts the variable-length symbol inputs into fixed-length outputs, using a FrameSynchronizer System object. The object has a secondary output that is a boolean scalar indicating if the first frame output is valid.

7) Data decoder: Performs phase ambiguity resolution and demodulation. Also, the data decoder compares the regenerated message with the transmitted one and calculates the BER.

For more information about the system components, refer to the QPSK Receiver with USRP® Hardware example using Simulink.

Execution and Results

Before running the script, first turn on the USRP® and connect it to the computer. To ensure data reception, first start the QPSK Transmitter with USRP® Hardware example.

if compileIt
    codegen('runSDRuQPSKReceiver', '-args', {coder.Constant(prmQPSKReceiver), coder.Constant(printReceivedData)});
end
if useCodegen
   clear runSDRuQPSKReceiver_mex %#ok<UNRCH>
   BER = runSDRuQPSKReceiver_mex(prmQPSKReceiver, printReceivedData);
else
   BER = runSDRuQPSKReceiver(prmQPSKReceiver, printReceivedData);
end

fprintf('Error rate is = %f.\n',BER(1));
fprintf('Number of detected errors = %d.\n',BER(2));
fprintf('Total number of compared samples = %d.\n',BER(3));

When you run the simulations, the received messages are decoded and printed out in the MATLAB command window while the simulation is running. BER information is also shown at the end of the script execution. The calculation of the BER value includes the first received frames, when some of the adaptive components in the QPSK receiver still have not converged. During this period, the BER is quite high. Once the transient period is over, the receiver is able to estimate the transmitted frame and the BER dramatically improves. In this example, to guarantee a reasonable execution time of the system in simulation mode, the simulation duration is fairly short. As such, the overall BER results are significantly affected by the high BER values at the beginning of the simulation. To increase the simulation duration and obtain lower BER values, you can change the SimParams.StopTime variable in the receiver initialization file.

Also, the gain behavior of different USRP® daughter boards varies considerably. Thus, the gain setting in the transmitter and receiver defined in this example may not be well-suited for your daughter boards. If the message is not properly decoded by the receiver system, you can vary the gain of the source signals in the SDRu Transmitter and SDRu Receiver System objects by changing the SimParams.USRPGain value in the transmitter initialization file and in the receiver initialization file.

Finally, a large relative frequency offset between the transmit and receive USRP® 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 SDRu Receiver System object.

Appendix

This example uses the following script and helper functions:

References

1. Rice, Michael. Digital Communications - A Discrete-Time Approach. 1st ed. New York, NY: Prentice Hall, 2008.

Copyright Notice

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