QPSK Receiver with ADALM-PLUTO Radio

Introduction

The comm.SDRRxPluto System object receives the QPSK signal impaired by over the air transmission. This example provides a reference design of a practical QPSK receiver that decodes the impaired QPSK signal by addressing the channel impairments. The QPSK receiver includes correlation based coarse frequency compensator, phase locked loop (PLL) based fine frequency compensation, timing recovery with fixed rate sampling and bit stuffing or stripping, frame synchronization, and phase ambiguity resolution.

This example serves two main purposes:

Initialize Receiver Parameters

The plutoradioqpskreceiver_init script initializes the simulation parameters and generates the structure prmQPSKReceiver .

% Receiver parameter structure
prmQPSKReceiver = plutoradioqpskreceiver_init;
% Specify Radio ID
prmQPSKReceiver.Address = 'usb:0'

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
                   DesiredPower: 2
                AveragingLength: 50
                   MaxPowerGain: 60
         MaximumFrequencyOffset: 6000
     PhaseRecoveryLoopBandwidth: 0.0100
     PhaseRecoveryDampingFactor: 1
    TimingRecoveryLoopBandwidth: 0.0100
    TimingRecoveryDampingFactor: 1
        TimingErrorDetectorGain: 5.4000
     PreambleDetectionThreshold: 0.8000
                    MessageBits: [11200×1 double]
                        BerMask: [7700×1 double]
           PlutoCenterFrequency: 915000000
                      PlutoGain: 30
        PlutoFrontEndSampleRate: 400000
               PlutoFrameLength: 11226
                 PlutoFrameTime: 0.0281
                       StopTime: 10
                        Address: 'usb:0'

Code Architecture

The function runPlutoradioQPSKReceiver uses two System objects, QPSKReceiver and comm.SDRRxPluto, to implement the QPSK receiver.

ADALM-PLUTO Receiver

The comm.SDRRxPluto System object communicates with the ADALM-PLUTO radio connected to the host computer. This component returns the QPSK signal received.

QPSK Receiver

The QPSKReceiver demodulates and retrieves the original transmitted message. The QPSKReceiver has six subcomponents, modeled using System objects.

Automatic gain control: The automatic gain control (AGC) subcomponent sets the output power to a particular level to ensure that the equivalent gains of the phase and timing error detectors are 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.

Coarse frequency compensation: The coarse frequency compensator subcomponent 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 or converge. Hence, the coarse frequency offset is estimated using a comm.CoarseFrequencyCompensator System object and an averaging formula. The comm.PhaseFrequencyOffset performs the compensation.

Timing recovery: Performs timing recovery with closed-loop scalar processing to counteract the channel-induced delays, using a comm.SymbolSynchronizer System object. The comm.SymbolSynchronizer object implements a PLL to correct the symbol timing error in the received signal. For this example, you select the rotationally-invariant Gardner timing error detector, allowing timing recovery to take place before fine frequency compensation. The input to the comm.SymbolSynchronizer object is a fixed-length frame of samples and the output is a frame of symbols whose length can vary due to bit stuffing and stripping, depending on actual channel delays.

Fine frequency compensation: The fine frequency compensation subcomponent performs closed-loop scalar processing and compensates for the frequency offset accurately using a comm.CarrierSynchronizer System object. The comm.CarrierSynchronizer object implements a PLL to track the residual frequency offset and the phase offset in the input signal.

Frame synchronization: The frame synchronization sub component performs frame synchronization and converts the variable length symbol inputs into fixed-length outputs using a FrameSynchronizer System object. The FrameSynchronizer object has a secondary boolean scalar output that indicates the validity of the first frame output.

Data decoder: The data decoder subcomponent 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 ADALM-PLUTO Radio in Simulink.

Receive QPSK Signal and Calculate BER

Run the example to start receiving the QPSK signal. The QPSK receiver demodulates and calculates the bit error rate (BER) of the received signal.

printReceivedData = false;    % true if the received data is to be printed

BER = runPlutoradioQPSKReceiver(prmQPSKReceiver, printReceivedData);

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));

## Establishing connection to hardware. This process can take several seconds.
Error rate is = 0.000000.
Number of detected errors = 0.
Total number of compared samples = 2741200.

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.

If the message is not properly decoded by the receiver system, you can vary the gain of the source signals in the ADALM-PLUTO Transmitter and ADALM-PLUTO Receiver System objects by changing the SimParams.PlutoGain value in the transmitter initialization file and in the receiver initialization file.

Also, a large relative frequency offset between the transmit and receive radios can prevent the receiver functions from properly decoding the message. If that happens, you can determine the offset by running the models in Frequency Offset Calibration with ADALM-PLUTO Radio in Simulink example then applying that offset to the center frequency of the comm.SDRRxPluto System object.

Supporting Functions

This example uses the following functions:

References

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