Hello community,
I am working on demodulating 1030 MHz Mode S traffic. As this signal uses DBPSK demodulation I’m using the deprecated GNU radio block ‘DPSK Demod’. However, there are a few parameters within this block that I can’t figure out how to set:
- Excess bandwidth: I’ve read that in general the more excess bandwidth supplied the better you can expect your synchronization algorithm to perform. This is [0,1], and when I set it
to 1 it’s noise resilience appreciably increases.
- FLL Bandwidth (assuming this is the same as filter lock-in bandwidth): This and the two subsequent values default to 6.28/100. I believe the higher this bandwidth is the faster the
phase locked loop can adjust the output of the frequency. This leads me to believe I want this as high as possible, however I don’t know where 6.28 and 100 come from.
- Phase Loop Bandwidth: I know that lower values lead to reduced levels of phase noise and refence spurs at the expense of longer lock times and less phase margin. I’m assuming I want
the least phase noise possible, however I don’t know where 6.28 and 100 come from.
- Timing Bandwidth: A dsp exchange question mentions that optimum loop bandwidth is usually somewhere between R/100 and R/20, where R is the symbol clock rate being recovered. My symbol
rate is 2 due to it being BPSK, is this the same as symbol clock rate? Where do the 100 or 20 denominators come from? Should this value mirror the values of FLL and Phase Loop bandwidths?
My end goal is being able to identify the Mode S message preambles within the demodulated bitstream. Any help is appreciated, thank you!
Adam Gorski
Virginia Tech Applied Research Corporation (VT-ARC)
Lead Communications Engineer
410-818-3188