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Re: [Discuss-gnuradio] gr-sounder tx and rx results

From: Johnathan Corgan
Subject: Re: [Discuss-gnuradio] gr-sounder tx and rx results
Date: Wed, 30 Jan 2008 14:27:17 -0800

On 1/30/08, Dev Ramudit <address@hidden> 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

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