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

Input and output constellation diagram

Reference

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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