Dear all,

I've a problem demodulating in QPSK.

My transmitter is a vector signal analyzer (VSG) and a labVIEW programme is used
to capture the data. Matlab is used to despread the data using PN sequence of
1024 bits, after which the data is being demodulated.

Below is a portion of the code using BPSK,

% Read off Gain Factor and offset

Gain_factor = fread(fid, 1, 'double');

Offset = fread(fid, 1, 'double');

% take out the unscaled data

Rx = fread(fid, [1, length], 'int16');

r = 0.9; % the rolloff factor, r.

[num,den] = rcosine(Fsym,Fsamp,'sqrt',r,2);

SQRCfilter = num;

PN_filter = filter(num, 1, PN);

w = 2*pi*(IF_freq/Fsamp );

n = 0 : (length -1); % time index in point number

% Demodulation in BPSK

sI = (Gain_factor.*Rx+Offset).*cos(w*n + 0.0);

sQ = (Gain_factor.*Rx+Offset).*sin(w*n + 0.0);

My trial and error QPSK code,

% Demodulation in QPSK

sI=(Gain_factor.*Rx+Offset).*sqrt(2)*cos(w*n);

sQ=(Gain_factor.*Rx+Offset).*(-sqrt(2)*sin(w*n));

Please advise. Thank you.

Regards,

Yingmu