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Re: [Discuss-gnuradio] Question about LFTX and RFTX


From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] Question about LFTX and RFTX
Date: Tue, 30 Oct 2007 10:45:57 +0000
User-agent: Thunderbird 2.0.0.6 (Windows/20070728)

Ismail Mohamed wrote:
Hi again,

What is the best way to do symbol timing recovery at baseband (i.e. using the LFRX)?

I've tried a simple method based on power detection, and it's not too bad. I'm using a pwr_squelch block and the first non-zero sample that comes in is considered as the first sample of my symbol. This method is ok, but doesn't solve my problem totally due to the large phase offset when the sampling is totally out of phase.

The second option I had was to use a fractional_interpolator block to make up for the slight sampling frequency offset between Tx and Rx. I've measured the frequency offset at about 14ppm in my setup, so I don't think the fractional_interpolator block will help that much as 16bit float accuracy would not be enough.


I had another idea of oversampling on the receiver but I haven't tried it out fully yet.

Anyway, I would really be appreciative if anyone could point me in the right direction to get over this problem.

Many thanks,
Ismail Mohamed
There's a Mueller and Muller timing recovery block that should handle this for you. There's both a complex and floating point version.
gr_clock_recovery_mm_cc
gr_clock_recovery_mm_ff

Tom






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