# BER for 4QAM with AWGN Channel

Started by February 13, 2008
hi everyone,

I am trying to simulate a OFDM system with AWGN channel, 4QAM symbol in matlab.
Theoritically, the curve for BER should be the same as for BPSK

BER = Q(sqrt(2*Eb/No));

But I have a problem: When I normalized the symbols 1/sqrt(2)(1+j) (For example), that doesn't work. But when I leave out the factor 1/sqrt(r), it works perfectly. Anybody knows the answer? Please help.

Thank you very much

Kenny
My guess is that your noise power is off by the same factor of 1/sqrt(2).
Check how you generate complex noise and that each of the real and imaginary
part should have noise variance half of the total noise power.

Nandan

On Feb 13, 2008 6:10 AM, loc truong wrote:

> hi everyone,
>
> I am trying to simulate a OFDM system with AWGN channel, 4QAM symbol in
> matlab.
> Theoritically, the curve for BER should be the same as for BPSK
>
> BER = Q(sqrt(2*Eb/No));
>
> But I have a problem: When I normalized the symbols 1/sqrt(2)(1+j) (For
> example), that doesn't work. But when I leave out the factor 1/sqrt(r), it
>
> Thank you very much
>
> Kenny
>
hello,

the white noise I did as below

sigma = sqrt(1/(2*Eb_No_linear));

AWGN_Noise = sigma.*(randn(1,N)+j*randn(1,N));

but when I simulate for Rayleigh Fading channel , with the same Noise, I need to scale the 4QAM symbols with factor 1/sqrt(2), then it works.

thanks,

Kenny
I think what it means is that your average channel power (averaged over the

Nandan

On Thu, Feb 14, 2008 at 4:46 AM, loc truong wrote:

>
> but when I simulate for Rayleigh Fading channel , with the same Noise, I
> need to scale the 4QAM symbols with factor 1/sqrt(2), then it works.
>
> thanks,
>
> Kenny
>
Your answer is right, I need to normalize the energie of the Rayleigh Channel to 1. Thanks for all the answers.

Kenny