Recovering PSK from FM demodulated data

Thread Starter


Joined Feb 28, 2013
Hi, I'm quite new to the field of communications and I've been recording satellite data for demodulation for kicks. The system I'm looking at, Orbcomm, modulates data in the form of Symmetric Differential Phase Shift Keying (SDPSK). However, there is some software that takes in a Narrow FM (NFM) demodulated version of the data. The link below shows someone using SDR to stream NFM demodulated audio to the software which then demodulates the digital data.
I've been puzzling over how this demodulation works. My understanding is that PM modulated data can be recovered from the FM demodulated data by integration. What I have tried in MATLAB is the following:
1. Resample the audio data to some multiple of the baud rate. Call it X1
2. Integrate the data. Call it X2
3. For various time offsets (i.e. 0, 1, ... over-sampling rate - 1), downsample X2 to the baud rate
3. Plot the differential of X2 (i.e. X2[2] - X2[1], - X2[3] - X2[2], ...). Call it X3.
4. Plot X3 with time

I expect that since SDPSK requires that the angle difference between each symbol is either +-90 deg, I would see two distinct clusters (thick lines of dots) corresponding to these two angles. I have done this but the results which I got from the Youtube audio seems rather poor. In addition, the audio that I recorded from my own radio reception but I can't really see a distinction of the the two states, even though the software seems to be able to demodulate quite well. Does anyone have a suggestion for a better method of demodulation?