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Power amplifier nonlinearity impairment in MATLAB

This section of MATLAB source code covers power amplifier nonlinearity impairment and its effect on constellation diagram using matlab code. PA nonlinearity incorporates AM-AM conversion and AM-PM conversion.

Part A and PART C of the matlab code is same as mentioned on AWGN page.

Source code-PART B

PA_backoff_dB = 2; % Power Amplifier Back off w.r.t. 1dB compression point
% < 0 --> average power above P1dB
% > 0 --> average power below P1dB
in=mapper_out_ori;
in=in';
out=TX_PA(in,PA_backoff_dB);
figure;plot(real(out),imag(out),'r+');title('constellation with PA nonlinearities');

TX_PA.m

function out = TX_PA(in, Backoff_dB);
[M N] = size(in);
for k=1:N
% Parameters of nonlinear model
alpha1 = 1;
alpha3 = -(1-10^(-1/20))*4/3; % This is an inversion of the formula
% giving the input 1 dB compression point
% RMS amplitude of signal
Arms = sqrt(var(in(:,k)));
% Brings signal to back-off dB below 1dB compression point
in(:,k) = in(:,k)*10^(-Backoff_dB/20)/Arms;
% Computes saturation values
IAsat = 2/3*sqrt(alpha1/abs(alpha3));
OAsat = 4/9*alpha1*sqrt(alpha1/abs(alpha3));
% For the lowpass equivalent, multiply alpha3 by 3/4 !
out(:,k) = alpha1*in(:,k) + 0.75*alpha3*in(:,k).*abs(in(:,k)).^2;
% Clipping to ensure monotonicity
ind = find(abs(in(:,k))>IAsat);
out(ind,k) = OAsat*sign(in(ind,k));
% Brings signal energy back to its original value
YRMS = sqrt(var(out(:,k)));
out(:,k) = out(:,k)/YRMS*Arms;
end

Reference

The code is taken from book by name digital front end compensation for emerging wireless systems
By authors Francois Horlin and Andre Bourdoux