Simulating OFDM systems

The popular LabVIEW simulation package from National Instruments can be used to develop

virtual instruments (VI’s) that implement OFDM in a graphical user interface. See also [1].

Other communication system building blocks and multicarrier modulation tools are available in

the National Instruments Modulation Toolkit.

OFDM functions can also be developed in Matlab.6 Here, we provide Matlab code for a

baseband OFDM transmitter and receiver for QPSK symbols. These functions can be modified

to transmit and receive M-QAM symbols or passband—complex baseband—signals.

function x=OFDMTx(N, bits, num, nu)

%==========================================================

% x=OFDMTx( N, bits, num, nu,zero_tones)

%

% APSK transmitter for OFDM

%

% N is the FFT size

% bits is a {1,-1} stream of length (N/2-1)*2*num (baseband, zero DC)

% num is the number of OFDM symbols to produce

% nu is the cyclic prefix length

%==========================================================

if length(bits) ~= (N/2-1)*2*num

error('bits vector of improper length -- Aborting');

end x=[];

real_index = -1; %initial values

imag_index = 0;

for a=1:num

real_index = max(real_index)+2:2:max(real_index)+N-1;

imag_index = max(imag_index)+2:2:max(imag_index)+N-1;

X = (bits(real_index) + j*bits(imag_index))/2;

X=[0 X 0]; % zero nyquist and DC

x_hold=sqrt(N)*ifft([X,conj(fliplr(X(2:length(X)-1)))]); %Baseband so symmeteic

x_hold=[x_hold(length(x_hold)-nu+1:length(x_hold)),x_hold]; %Add CP

x=[x,x_hold];

end

x=real(x);

function bits_out = OFDMRx(N,y,h,num_symbols,nu)

%=========================================================

% bits_out = OFDMRx(M,N,y,num_symbols,nu,zero_tones)

%

% N is the FFT size

% y is received channel output for all symbols (excess delay spread removed)

% h is the (estimated) channel impulse response

% num_symbols is the # of symbols (each of N*sqrt(M) bits and N+nu samples)

% nu is the cyclic prefix length

%=========================================================

if length(y) ~= (N + nu)*num_symbols

error('received vector of improper length -- Aborting');

end bits_out=[]; bits_out_cur = zeros(1,N-2);

for a=1:num_symbols

y_cur = y((a-1)*(N+nu)+1+nu:a*(N+nu)); % Get current OFDM symbol, strip CP

X_hat = 1/sqrt(N)*fft(y_cur)./fft(h,N); %FEQ

X_hat = X_hat(1:N/2);

real_index = 1:2:N-2-1; % Don’t inc. X_hat (1) because is DC, zeroed

imag_index = 2:2:N-2;

bits_out_cur(real_index) = sign(real(X_hat(2:length(X_hat))));

bits_out_cur(imag_index) = sign(imag(X_hat(2:length(X_hat))));

bits_out=[bits_out,bits_out_cur];

end