[Discuss-gnuradio] Lock onto QPSK signal

[Henry Barton]

Hi, does anyone know of any good tutorials that explain how to lock onto the clock of a QPSK signal and/or correlate? I know this is all very simple in GNUradio, but I hope someone knows of a website or, preferably, a video series that will explain this.

 

If I have 10 different QPSK users, spreaded and on the same frequency, that would make CDMA. Assuming no manufacturing tolerance or frequency drift, all the clocks on the transmitters would be perfectly the same. What I’m trying to understand is:

 

1.       No two separate transmitters can ever be perfectly in sync. Even if the clocks were 100% accurate, User 1 might start his transmitter at 10:00:00 AM while User 2 starts his at 10:00:00.250 AM. The signals would be a little out of sync with each other. How would I pick one out in DSP?

2.       I’ve heard of “clock recovery”, which implies to me that you “look” at a signal to find its clock, but surely you must have at least a very close idea of the desired clock before you can begin, right? If you had 3 different CDMA signals  but different chip rates, they could probably coexist nicely, but how would you pick out the faster signal or the slower one? (see picture)

 

Please advise if my current understanding is erroneous. Thank you in advance.

 

Sent from Mail for Windows 10

 

cdma_different.png
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