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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
figure;plot(real(out),imag(out),'r+');title('constellation with PA nonlinearities');


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;

Input and output constellation diagram

constellation input
power amplifier nonlinearity effect on constellation


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

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