function [y,t,theta] = OFDM(signalPresent,SNR,fs,theta)

% DVBT() generates four OFDM signals over a single symbol period of 1.25 ms (cyclic frequency
% included).   The OFDM signals have baseband carriers between 592 kHz and 7408 kHz inclusive
% in increments of 1 kHz.  Carrier frequencies of the signals range from 48 MHz to 96 MHz
% in increments of 16 MHz.

% Inputs:
% SNR: desired SNR (in dB)
% fs: sampling frequency (in Hz)
% theta: phase of cosine modulator (in radians)
% signalPresent: if signalPresent is 0, QPSK symbols are 0; otherwise, QPSK symbols are randomized

% Outputs:
% y: the resultant OFDM signal with AWGN noise producing the desired SNR
% t: times corresponding to the signal
% theta: the phase of the cosine modulator

if(nargin() <= 3)
	theta = unifrnd(0,2*pi,1);
end
if(nargin() <= 2)
	fs = 500*10^6;
end
if (nargin() <= 1)
	SNR = 0;
end
if (nargin() == 0)
	signalPresent = 1;
end

n = 4;						% number of OFDM signals
nss = 6817;					% number of symbols per signal per symbol time

fsc = 10^3;					% frequency separation of subcarriers

fc = 60*10^6;				% OFDM signal carrier frequency
fbb = (592:592+nss-1)*fsc;	% subcarrier frequencies at baseband


fbw = 8*10^6;				% nominal bandwidth

cp = 0.25;					% cyclic prefix

% generate random QPSK symbols
symbols = (2*binornd(1,.5,[nss,1])-1 + 1i*(2*binornd(1,.5,[nss,1])-1))/sqrt(2);
	
% define the DFT of the four signals
S_baseband = [zeros(fbb(1)/fsc,1);symbols;zeros(fs/fsc-fbb(1)/fsc-length(symbols),1)]*fs/fsc;

% move the baseband signals into the time domain
s_baseband = ifft(S_baseband);

% modulate the baseband signal
t = (0:1/fs:1/fsc-1/fs)';
s = cos(2*pi*fc*t+theta).*s_baseband;

% add a cyclic prefix
s = [s((length(s)*3/4+1):length(s));s];

% update t in consideration of cyclic prefix
t = [t;max(t)+t(1:length(t)/4)];

% apply noise to the received signal
y = awgn(s,SNR,'measured');

% if signal isn't supposed to be present, keep the noise
if(signalPresent==0)
	y = y-s;
end

endfunction