How can i transmit a modulated signal through a AWGN channel which adds a zero mean gaussian real random number with variance s^2

M = 4;
data=randi([0, M-1], 10000,1);
modData = pammod(data,M);
scatterplot(modData)
title('4-PAM');
snr=[0 2 4 6 8 10 12 15];
y=awgn(modData,snr);
scatterplot(y)

[John BG]

Simulink already has a block for AWGN channels where you can change the setting to your values of interest.

[Vidya Viswanathan]

You could consider using "comm.AWGNChannel" System object. This provides you with a lot of options including one to specify the noise variance. Refer to the following documentation link for the same:

http://www.mathworks.com/help/comm/ref/comm.awgnchannel-class.html?s_tid=answers_rc2

As pointed out by John, the same is available in Simulink as well. The following link describes the block corresponding to AWGN channel: http://www.mathworks.com/help/comm/ref/awgnchannel.html