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function subr = demultiplex(r, alpha, puncture); 
% Copyright 1998, Yufei Wu 
% MPRG lab, Virginia Tech. 
% for academic use only 
 
% At receiver end, serial to paralle demultiplex to get the code word of each 
% encoder 
% alpha: interleaver mapping  
% puncture = 0: use puncturing to increase rate to 1/2; 
% puncture = 1; unpunctured, rate 1/3; 
 
% Frame size, which includes info. bits and tail bits 
L_total = length(r)/(2+puncture); 
 
% Extract the parity bits for both decoders 
if puncture == 1        % unpunctured 
  for i = 1:L_total   
      x_sys(i) = r(3*(i-1)+1); 
      for j = 1:2 
          subr(j,2*i) = r(3*(i-1)+1+j); 
      end 
   end 
else            % unpunctured 
   for i = 1:L_total 
       x_sys(i) = r(2*(i-1)+1); 
       for j = 1:2 
          subr(j,2*i) = 0; 
       end    
       if rem(i,2)>0 
          subr(1,2*i) = r(2*i); 
       else 
          subr(2,2*i) = r(2*i); 
       end       
   end 
end        
 
% Extract the systematic bits for both decoders 
for j = 1:L_total 
% For decoder one 
   subr(1,2*(j-1)+1) = x_sys(j); 
% For decoder two: interleave the systematic bits   
   subr(2,2*(j-1)+1) = x_sys(alpha(j)); 
end