sim.c

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/* Simulate AWGN channel
 * Copyright 1994 Phil Karn, KA9Q
 * May be used under the terms of the GNU Public License
 */
#include <math.h>

#define	OFFSET	128

double normal_rand(double mean, double std_dev);

/* Turn binary symbols into individual 8-bit channel symbols
 * with specified noise and gain
 */
void
modnoise(
unsigned char *symbols,		/* Input and Output symbols, 8 bits each */
unsigned int nsyms,		/* Symbol count */
double amp,			/* Signal amplitude */
double noise			/* Noise amplitude */
){
  double s;

  while(nsyms-- != 0){
    s = normal_rand(*symbols ? 1.0 : -1.0,noise);
    s *= amp;
    s += OFFSET;
    if(s > 255)
      s = 255;	/* Clip to 8-bit range */
    if(s < 0)
      s = 0;
    *symbols++ = floor(s+0.5);	/* Round to int */
  }
}

#define	MAX_RANDOM	0x7fffffff

/* Generate gaussian random double with specified mean and std_dev */
double
normal_rand(double mean, double std_dev)
{
  double fac,rsq,v1,v2;
  static double gset;
  static int iset;
  
  if(iset){
    /* Already got one */
    iset = 0;
    return mean + std_dev*gset;
  }
  /* Generate two evenly distributed numbers between -1 and +1
   * that are inside the unit circle
   */
  do {
    v1 = 2.0 * (double)random() / MAX_RANDOM - 1;
    v2 = 2.0 * (double)random() / MAX_RANDOM - 1;
    rsq = v1*v1 + v2*v2;
  } while(rsq >= 1.0 || rsq == 0.0);
  fac = sqrt(-2.0*log(rsq)/rsq);
  gset = v1*fac;
  iset++;
  return mean + std_dev*v2*fac;
}