BER problem while transmitting 64 OFDM symbols

Hi,

I am trying to implement OFDM system using 64 point FFT and the following equation. I am trying to transmit 64 OFDM symbols, each containing 64 qpsk symbols.

Y = HS + W;

where,

Y = 64x64 matrix of received signal
H = 64 x 64 matrix of channel coefficient
W = 64x64 matrix of noise

I have done the following steps in beginning,

1) I multiplied tx-block with Fourier Matrix (64 x 64).
2) on Rx side I just try to recover the transmitted bits, BER is constant for all SNR values i.e 0:1:10 dB even in the absence of channel and noise.

But when I done the same exercise by transmitting a single OFDM symbol, multiplied it with channel coefficient and add noise, I got fine BER.

Following is code:-

nFFT = 64; % fft size
bits_symbol = 2; % Bits per qpsk symbols
nSym = 10^2; %10^4; % number of symbols
bits_number = nFFT*bits_symbol*nSym; % Total number of bits to be generated

EbN0dB = 0:1:10; % Bit to Noise Ratio
SNR = EbN0dB + 10*log10(bits_symbol);

h = [1 1 1];

% Generate N-Point FFT matrix

N = nFFT;
for K_ind = 1:nFFT
for L_ind = 1:nFFT
F(K_ind,L_ind)=1/sqrt(N)*exp(-1j*((2*pi)/N)*(K_ind-1)*(L_ind-1));
end
end

for ii = 1:length(EbN0dB)

snr = 10.^(SNR (ii)/10); % SNR/link spectral efficiency(bits/s/Hz) = Eb/ N0 --> SNR = 2 Eb/N0 -> 10log10(SNR) = 2 Eb/N0 -> snr = 10^((2Eb/N0)/10)=10^(SNR/10)

% Transmitter

uncoded_bits = rand(1,bits_number)>0.5; % random 1's and 0's
s_mod_serial = qpsk_mod(uncoded_bits); % QPSK modulation
sn = reshape(s_mod_serial,nSym,nFFT);

no_Run = round(length(sn)/nFFT)-1;

err_bit = 0;

for kkk = 1:no_Run

tx_symbols_block = sn(((kkk-1)*nFFT+1:kkk*nFFT),(1:nFFT)); % 64 QPSK symbols for transmission
uncoded_bits0 = uncoded_bits(1,(kkk-1)*nFFT*bits_symbol*nFFT+1:kkk*nFFT*bits_symbol*nFFT); % 128 bits for transmissio in a single Run

% Serial to Parallel conversion

XF = tx_symbols_block.';

% Imposed onto orthogonal sub-carriers by means of IFFT

xt =F'*XF;
r = xt;
% Receiver

% FFT operation. conversion from time Domain Frequency Domain
yF = F*r;
yF = reshape(yF,1,nFFT*nFFT);

% Demodulation of qpsk symbols

decoded_bits = qpsk_dem(yF);

% Error bits calculation

err_bit = err_bit + sum(uncoded_bits0~=decoded_bits);

end %end of no_Run

% Bit Error Rate Calculation

BER(ii)= err_bit/length(uncoded_bits);
end

simBer=BER;


I would appreciate help or comment for the said problem.

Thanks,

Regards,
usman