CODEWITHSUNDEEP
gnuradio
VikingTrav
VikingTrav

Reputation: 5

Incorporate the spreading code into the matched filter input to a PFB clock sync

I have a BPSK modulator/demodulator working that I have added a few blocks around to effectively have a DSSS system working. However, when I try to add a second user (or spreading code) I can only lock on to one of the signals, I am assuming because I am using the standard root raised cosine filter in the PFB clock sync block, and it has no direct knowledge of the spreading code used.

My question is if there is a way to somehow incorporate the spreading code into the root raised cosine filter, or maybe incorporate it some other way into the PFB clock sync block so that I can perform symbol timing recovery on the correct set of symbols?

The RRC I am using now is:

firdes.root_raised_cosine(nfilts,nfilts,1.0,0.35,11*sps*nfilts)

where nfilts = 32 and sps = 2.

Upvotes: 0

Views: 229

Answers (1)

eeKnud
eeKnud

Reputation: 106

I am sorry I am not directly answering your question, but first we need to understand where the RRC is applied. If you are using the Constellation Modulator (CM) block to generate the BPSK and then spreading, the RRC is being applied before the spreading; i.e., it's performed by the CM. If this is true, then I think it may be just luck that it worked for one spreading code.

On the other hand, if you apply an RRC post-spreading, then the PFB Clock Sync should not care. I suggest changing sps to 4 and then looking at the time domain signal post-spreading. Do you see RRC-shaped symbols?

Upvotes: 1

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