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.
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.
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.