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);