/* Simulate AWGN channel * Copyright 1994 Phil Karn, KA9Q */#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;}