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Reduce ISI Using Raised Cosine Filtering

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

Initialize a simulation variable for modulation order.

M = 16; % Modulation order

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 M-1],1000,1);
modSig = qammod(x,M,'UnitAveragePower',true);

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

eyediagram(modSig,2)

Figure Eye Diagram contains 2 axes. Axes 1 with title Eye Diagram for In-Phase Signal contains an object of type line. This object represents In-phase. Axes 2 with title Eye Diagram for Quadrature Signal contains an object of type line. This object represents Quadrature.

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)

Figure Eye Diagram contains 2 axes. Axes 1 with title Eye Diagram for In-Phase Signal contains an object of type line. This object represents In-phase. Axes 2 with title Eye Diagram for Quadrature Signal contains an object of type line. This object represents Quadrature.

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 diagram 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)

Figure Eye Diagram contains 2 axes. Axes 1 with title Eye Diagram for In-Phase Signal contains an object of type line. This object represents In-phase. Axes 2 with title Eye Diagram for Quadrature Signal contains an object of type line. This object represents Quadrature.