function [p]=cm_sm41(snr_in_dB)
% [p]=cm_sm41(snr_in_dB)
% CM_SM41 finds the probability of error for the given
% value of snr_in_dB, SNR in dB.
N=10000;
d=1; % min. distance between symbols
Eav=10*d^2; % energy per symbol
snr=10^(snr_in_dB/10); % SNR per bit (given)
sgma=sqrt(Eav/(8*snr)); % noise variance
M=16;
% Generation of the data source follows.
for i=1:N,
temp=rand; % a uniform R.V. between 0 and 1
dsource(i)=1+floor(M*temp); % a number between 1 and 16, uniform
end;
% Mapping to the signal constellation follows.
mapping=[-3*d 3*d;
-d 3*d;
d 3*d;
3*d 3*d;
-3*d d;
-d d;
d d;
3*d d;
-3*d -d;
-d -d;
d -d;
3*d -d;
-3*d -3*d;
-d -3*d;
d -3*d;
3*d -3*d];
for i=1:N,
qam_sig(i,:)=mapping(dsource(i),:);
end;
% received signal
for i=1:N,
[n(1) n(2)]=gngauss(sgma);
r(i,:)=qam_sig(i,:)+n;
end;
% detection and error probability calculation
numoferr=0;
for i=1:N,
% Metric computation follows.
for j=1:M,
metrics(j)=(r(i,1)-mapping(j,1))^2+(r(i,2)-mapping(j,2))^2;
end;
[min_metric decis] = min(metrics);
if (decis~=dsource(i)),
numoferr=numoferr+1;
end;
end;
p=numoferr/(N);
