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

[Ismail Mohamed]

> With the LFTX/RX boards, DC does indeed remain DC at the other end.
> However, the crystals in each USRP still govern the sampling rate, and
> each can be up to 50 PPM off (new USRP hardware has 20 PPM spec'd
> crystals now.)
>
> So the same issues with respect to symbol timing recovery exist.
>
> In practice I've never encountered a USRP that had more than a few PPM
> crystal offset--but you can only rely on the spec range, especially in
> variable environments.
>
>   
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