[SOLVED] how to add noise iinto QPSK OFDM code --urgent

[ReComm]

dear friends,

using the help of regular search , i modified the next code, but kindly i don't know how to add noise to it. kindly advice
1. how to generate noise
2. plot BER vs EbNo

code
====
% QPSK OFDM
%===========
close all;
clear;
clc;

%Initilization

nBitPerSym = 52; % number of bits per OFDM symbol
nSym = 10^4; % number of symbols

nFFT = 64; % fft size
nDSC = 52; % number of data subcarriers


% Input Data Generation
data = rand(1,nBitPerSym*nSym*2) > 0.5; % random 1's and 0's
oddData = data(1:2:end);
evenData = data(2:2:end);

qpskModulated = sqrt(1/2)*(1i*(2*oddData-1)+(2*evenData-1)); %QPSK Mapping
% and normalization of energy to 1

qpskModulated_Reshape = reshape(qpskModulated,nBitPerSym,nSym).'; % grouping into multiple symbols

% Taking FFT
xt = (nFFT/sqrt(nDSC))*ifft(fftshift(qpskModulated_Reshape.')).';
% Appending cylic prefix
xt = [xt,[41:52]) xt];
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Noise Insertion


yt=xt;

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Receiver
%==========
yt = yt,[13:64]); % removing cyclic prefix
% converting to frequency domain
yF = (sqrt(nDSC)/nFFT)*fftshift(fft(yt.')).';

%--------------Demodulation-----------------------------
%Threshold Detector
TX = reshape(yF.',nBitPerSym*nSym,1).';
detected_real = real(TX)>=0;
detected_img = imag(TX)>=0;

estimatedBits=reshape([detected_img;detected_real],1,[]);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%-------------------------------------------------------
nErr = size(find([data - estimatedBits]),2); % couting the number of errors
Err_QPSK_OFDM = nErr/(nSym*nBitPerSym*2);




wait your reply,

 

[ReComm]

is the group receive my posts? Kindly help.

 

[David83]

Define:

SNRdB=[0:30];%Define a range of SNR in dB
SNRLin=10.^(SNRdB./10);%Convert SNR to linear scale
.
.
.
for ii=1:length(SNRLin)%For each SNR

n=(1/sqrt(2))*(randn(1,1)+1i*randn(1,1));%generate a Guassian noise with zero mean and unit variance
.
.
.
yt=xt+(1/sqrt(SNRLin))*n;%add the noise to the transmitted signal.

.
.
.
Err(ii)=...%Compute the number of errors for each SNR
end
.
.
semilogy(SNRdB,Err_QPSK_OFDM)

 

[ReComm]

Thanks David83 for your reply,

But kindly why you use 1/sqrt(2) in n generation? and is this value changed by modulation scheme?
Also why you multiply n by (1/sqrt(SNRLin)) in adding noise to transmitted signal?

Thanks and wait your reply,

 

[David83]

for 1/sqrt(2): because randn in MATLAB generates a Gaussian random variable with zero mean and unit variance. The term randn()+1i*randn() will be zero mean and variance of 2. But (1/sqrt(2))*(randn(1,1)+1i*randn(1,1)) will be zero mean and unit variance, which most convenient.

For (1/sqrt(SNRLin)): because in the received signal yt=xt+(1/sqrt(SNRLin))*n the average SNR will be: 1/(1/SNRLin)=SNRLin. In other words, we add a noise term to make the average SNR equals SNRLin. I know it sounds confusing, but think a little about it.