how to generate Gaussian noisse AWGN?

[mariuszprus]

AWGN chanel

Hi I am from Poland and I need your helpi must generate Gaussian noisse AWGN
I implement AWGN channel in trellis code modulation.
i have coder and modulation 8PSK to this signal i must add gaussian noise and depend this noise SNR ( we must give SNR)

supose we have SNR=10log(Psignal/Pnoise) and how can i generate noise
??
Psignal=1
we have one more SNR 10log(Asignal/Anoise)^2 =10log(1/Asignal)

and i dont know what is No/2 in **broken link removed**

My code

double noise(Random ^generator)
		{
			
		if(amp==0)
		{
			return 0;
		}
		else
		{	
		    double x=0,y=0;
			for(int i=0;i<12;i++)
			{
				y=generator->NextDouble();
				x=x+y;
			}
			x=x-6;
			return (x*amp);
		}

where amp i is the amplitude of signal noise Anoise

amp=sqrt(pow(10.0,(-SNR/10.0)));

or No/2 and we must divide amp/2 ??
please help me!!!;( and soory for the error