Hello I have made a program in matlab that can demodulate an OQPSK-modulated signal with noise (additive white gaussian noise). The carrier is phase-shifted with an arbitrary constant between 0 and 2*pi. Here is what I do to demodulate the signal: When the signal is received at time t=0 I know the following will happen for the first T seconds: 1) If there is no or little noise and the carrier has not been phase-shifted the I-phase should go from 0 to 1 while the Q-phase remains 0. 2) According to IEEE 802.15.4 (zigbee standard) the first symbol must be the symbol (I,Q)=(-1,-1). If s(t) is the signal received I lowpassfilter s(t)*cos(2*pi*f*t+p(t)) and s(t)*sin(2*pi*f*t+p(t)) where p(t)=0 to begin with and f is the carrier frequency. By doing this I now have the (possibly rotated) coordinates (I,Q). I trace the received (I,Q) from time t=0 to time t=T and calculate a trend-line of where (I,Q) is going from t=0 to t=T. This trend-line is actually a vector and the vector is one of the basis-vectors in the signal-space. So I calculate the angle between this basisvector and the vector (1,0). As I expect a symbol (-1,-1) I know that the angle has to be 180 degrees. So if it is not the calculated angle directly tells me how much the signal has been phase-shifted (rotated). So I just derotate the signal by adjusting p(t) such that the first symbol received will be (-1,-1). After that I just compare the received (I,Q) with the expected (I,Q) and any difference results in an adjustment of p(t). Is this right? Thanks. Jakob

# Implementation of OQPSK-demodulator in matlab

Started by ●May 9, 2004

Reply by ●May 9, 20042004-05-09