Re: how to calculate noise vector from Eb/No to plot a EbNo/BER graph (noise generation)

On 27 juil, 19:01, Phil <philguilleme...@xxxxxxxxxxxxxxxxxxx> wrote:
Anna wrote:
On Jul 27, 4:40 pm, Anna <petitmou...@xxxxxxxxx> wrote:
On Jul 27, 4:00 pm, John Sampson <jo...@xxxxxxxxxxxx> wrote:

Anna wrote:
Hi,
I'm doing a final project in OFDM simulation and I would like to plot
a EbNo/BER graph. For a range of a specified EbNo, I I have to
generate a noise added to the signal. I have no idea how I could
generate a noise vector from this Eb/No.

supose that my signal is x = randint(1,N)
some people generate noise vector by 1/sqrt*(randn(N)+jrandn(N))
do I have to multiply this with a constealltion number? or do I have
to multiply this with any OFDM process too???

Thank you very much for your reply. I'm a bit stressful right now
because I don't have a lot of time to finish it.

Thank you very much
best regards,
Anna

Here's an approach. Suppose your noiseless OFDM signal is 'z', the bit
rate is 'Rb', sample rate is 'Fs', and target Eb/No is 'EbNo':

z = z./std(z);
n = (randn(length(z),1) + j*randn(length(z),1))/sqrt(2);
snr = 10.^(EbNo/10) * Rb / Fs;
n = n./sqrt(snr);
z = z + n;

John

I still have one little question (I'm sorry, i know so little thing in
DSP) , why do we derive n by sqrt(snr) ,but not simply snr?

Hi Anna,

The snr is a ratio of powers. Since n can be considered as a voltage,
we need the sqrt(snr) to have the proper ratio between the normalized
z vector and the n vector.

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Ahhh I see. thank you very much Phil. have a nice day :-)
Anna

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