On 1/30/08, Dev Ramudit <[EMAIL PROTECTED]> wrote: > Not sure if you've had a chance to take a look at this yet. I've been > trying to figure out what's going on, but even with Martin's advice I > still havent had much luck. Could you tell me what the slip rate is on > the sliding correlator? Whenever you have some free time.
Martin's description of the issue is very accurate. Due to frequency offset, the impulse response peaks do not come at the proper intervals. What is worse, is that the algorithm in the FPGA is using an n-squared algorithm to calculate the impulse response, so it is extremely sensitive to frequency offset. You can adjust the frequency manually on either side to accomodate, but simple drift will soon cause the clocks to diverge. You may also be able to post-process the impulse response records by estimating the frequency offset using an FFT and then resampling/interpolating them to the proper time base. Unfortunately, the only true way to correct this is with a PLL on the receive side, inside the FPGA, after the despreading occurs. Fortunately, I am developing one of these as part of another project. This is a direct sequence spread spectrum transceiver, and is really a superset of and replacement for gr-sounder. With the proper frequency and PN-code phase locked loops, you will be able to get channel impulse responses with much greater accuracy than the open-loop gr-sounder code does today. I normally wouldn't have "announced" this until there was working code people could try out. -- Johnathan Corgan Corgan Enterprises LLC http://corganenterprises.com/ _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
