I am very much a rookie, so apologies for the terrible knowledge or misconceptions. I'm looking at this mostly in the context of GPS (or sending a signal with a very low SNR ratio).
I understand BPSK for the most part. Shifting phase by 180 degrees which is essentially just flipping it (atleast in my case).
I am just trying to understand how to do CDMA demodulation. The modulation makes enough sense, essentially just encoding each bit into the spread spectrum code.
But I don't understand how to demodulate it, and all the sources I've been looking at are very vague about it (sources would be greatly appreciated!)
Pretty much what I've gathered is this:
First correlate (multiply and then integrate) the recieved signal with the PRN code. And then apply BPSK to that result.
And my understanding of it is that the PRN code is going to cause the signal to flip at the appropriate intervals. If you essentially just...apply it again, then it's going to undo the effect it had in the first place. And leave you with the BPSK signal.
The other thing I'm thinking it might be is that rather than directly correlating with the PRN, you correlate with a wave copy. This copied wave would just be the carrier wave following the PRN code. And then any correlations should perfectly line up.
But I couldn't get either of these to work very well in the presence of noise!
What I'm curious about doesn't even necessarily have to work in the presence of other transmitted signals overlaid on top. Just the one signal is good right now. I guess I'm just not understanding how GPS can work so well in the presence of noise.
Thanks so much.
