/* Simulate AWGN channel * Copyright 1994 Phil Karn, KA9Q * May be used under the terms of the GNU Public License */#include <math.h>#define	OFFSET	128double normal_rand(double mean, double std_dev);/* Turn binary symbols into individual 8-bit channel symbols * with specified noise and gain */voidmodnoise(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 */doublenormal_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;}