MATLAB Examples

Reduce ISI Using Raised Cosine Filtering

Employ raised cosine filtering to reduce inter-symbol interference (ISI) that results from a nonlinear amplifier.

Create a 16-QAM modulator having an average power of 1.

qamModulator = comm.RectangularQAMModulator('ModulationOrder',16, ...
    'NormalizationMethod','Average power','AveragePower',1);

Create square root raised cosine filter objects.

txfilter = comm.RaisedCosineTransmitFilter;
rxfilter = comm.RaisedCosineReceiveFilter;

Create a Memoryless Nonlinearity System object to introduce nonlinear behavior to the modulated signal. Using name-value pairs, set the Method property to Saleh model to emulate a high power amplifier.

hpa = comm.MemorylessNonlinearity('Method','Saleh model', ...
    'InputScaling',-10,'OutputScaling',0);

Generate random integers and apply 16-QAM modulation.

x = randi([0 15],1000,1);
modSig = qamModulator(x);

Plot the eye diagram of the modulated signal. At time 0, there are three distinct "eyes" for 16-QAM modulation.

eyediagram(modSig,2)

Amplify the modulated signal using hpa.

txSigNoFilt = hpa(modSig);

Plot the eye diagram of the amplified signal without RRC filtering. At time 0, there are multiple eyes. This is a result of inter-symbol interference from the nonlinear amplifier.

eyediagram(txSigNoFilt,2)

Filter the modulated signal using the RRC transmit filter.

filteredSig = txfilter(modSig);

Release hpa and amplify the filtered signal. The release function is needed because the input signal dimensions change due to filter interpolation.

release(hpa)
txSig = hpa(filteredSig);

Filter txSig using the RRC matched receive filter.

rxSig = rxfilter(txSig);

Plot the eye digram of the signal after the application of the receive filter. There are once again three distinct eyes as the matched RRC filters mitigate ISI.

eyediagram(rxSig,2)