Problem with ber for ofdm system in matlab

hello , i need help for my matlab program for calculating ber in ofdm..actually in my program i am got getting right curve for simulated data so plezz help me to knw where the problem is ..my progam is written below ...... plezz help me to get this done.

M=2;
N = 4;
G=N/4;
nsym = 2;
EbN0dB = [0:35];
EsN0dB = EbN0dB + 10*log10((N)/(N+G));
for ii = 1:length(EbN0dB)
data=randsrc(1,N*nsym,[0:M-1]);
psk_data=pskmod(data,M);
xF = reshape(psk_data,N,nsym).';
xt1 =ifft(xF.').';
xt = [xt1,[N-G+1:N]) xt1];
yt1 = sqrt(N+G/N)*xt;
yt = yt1,[G+1:N+G]);
yF = fft(yt.').';
yMod = reshape(yF.',N*nsym,1).';
psk_dataout=fft(yMod);
nErr(ii) = size(find(psk_dataout -data),2);
end
simBer = nErr/(nsym*N);
theoryBer = (1/2)*erfc(sqrt(10.^(EbN0dB/10)));
close all; figure
semilogy(EbN0dB,theoryBer,'bs-','LineWidth',2);
hold on
semilogy(EbN0dB,simBer,'mx-','LineWidth',2);
axis([0 10 10^-5 1])
grid on

Hi

Can you explain the signaifiance of variable "G" & "nsym=2". What it excatly represent and why you have used.