Code

Constant complexity rectangular QAM demodulator

Vashishth November 14, 2010 Coded in C for the TI C67x

This code can demodulate an input to a constellation of size 'm'. The number of steps to perform the demodulation is independent of 'm'.

% N is the size of the constellation.
% v is the input data.
% For details email me.
void function_qam()
{

co=0;
 m=0;

for(q=0; q<N; q++)

        {

 a=v[m];// a holds the real part
 b=v[m+1];// b holds the imaginary part

 d=a-onern[max];  %onern stores the negative real axis points and onein stores the negative imag axis points. eg for 256 QAM, each will store 0 to -8 as an array.
 l=d;         //l stores the distance
 s=(d-l);
 if(l<max-1) % max is the maximum value that the 1-d coordinate can take.
       {

if(d>0){
 if(s<=.5)
       {
 hi=max-l;
 flagtwo=1;
       }
 else
 {
 hi=max-l-1;
 flagtwo=1;
 }
      }
	  else
	  {
	  hi=max;
	  flagtwo=1;
	  }
        
      }

 if(l==max-1)
 {
 hi=1;
 flagtwo=1;
 }
 if(l==max)//
 {
 flagone=1;
 hi=1;
 } 

 if(l>max)
        {
		if(d<2*max)
		{

 if(s<=.5)
 {
 hi=l-max;
 flagone=1;
 }
 else
 {
 hi=l-max+1;
 flagone=1;
 }
        }
		else
		{
		hi=max;
		flagone=1;
		}
       
          
        }

 de=b-onein[max];
 le=de;         //l stores the distance
 se=(de-le);

 if(le<max-1)
      {

	  if(de>0)
	  {
 if(se<=.5)
 {
 hm=max-le;
 flagfour=1;
 }
 else
 {
 hm=max-le-1;
 flagfour=1;
 }
      
      }
	  else
	  {
	  hm=max;
	  flagfour=1;
      }
      
      
      }

 if(le==max-1)
 {
 hm=1;
 flagfour=1;
 }
 if(le==max)
 {
 flagthree=1;
 hm=1;
 }

 if(le>max)
     {

if(de<2*max){

 if(se<=.5)
 {
 hm=le-max;
 flagthree=1;
 }
 else
 {
 hm=le-max+1;
 flagthree=1;
 }
    }
	else
	{
	hm=max;
	flagthree=1;
	}
    
       
    
    }

 if(flagone==1)
 {
  vv[m]=hi;  // now v[m] holds one x coordinate of the qam mapping and v[m+1] holds the y coordinate
 }
 
 else
 {
 vv[m]= 100-hi;
 }

 
 if(flagthree==1)
  {
 vv[m+1]=hm;  
  }
		else
		{
 vv[m+1]= 100-hm;
        }

flagone=0;
flagtwo=0;
flagthree=0;
flagfour=0;
        r=0;
        
        
          
           output[co]= valuetable[ vv[m] ][ vv[m+1] ] ;
       
       
          
          
                        
      m = m+ 2;
	  
	  co = co+ 1;
                
                
                 }

  for(t=0;t<(N/2);t++)
  {
   p[t] = (output[t]);
 
  }

 

}

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