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Re: [Discuss-gnuradio] diminishing returns with increasing frequency off

From: Bastian Bloessl
Subject: Re: [Discuss-gnuradio] diminishing returns with increasing frequency offset
Date: Mon, 21 Mar 2016 03:42:22 +0100


On 19 Mar 2016, at 12:08, tom x <address@hidden> wrote:

I tried doubling both, the sample rate to 8MHz and Omega to 4, but still no progress. The setup is simply the 802.15.4 PHY block connected to a USRP source, listening for an over the air transmission. Can you give some insight on how you picked the other MM Clock Recovery params? Is there any other dependence in the PHY block? The papers and guides I looked at explain what the MM params are but don't give much intuition for choosing them.

I didn’t set them and am not an expert, but I think there is no justification for this particular parameter set, i.e., there is no easy formula for calculating ideal parameters. I guess someone started with an educated guess and made simulations to find parameters that worked reasonably well.

Anyhow, I don’t think that the parameters of the MM block are the problem (once you adapted omega of course). As far as I see
- the gain parameters are independent from the sample rate as the error/feedback is calculated once per symbol
- mu is the initial phase and doesn’t matter
- omega relative limit could be cut in half (when using 8MHz). IIRC, it’s the maximum frequency deviation normalised with the sample rate.

What you could try is to adapt alpha of the single pole IIR, to get the same averaging with the increased sample rate.

Additionally, based on posts in this thread: https://lists.gnu.org/archive/html/discuss-gnuradio/2015-07/msg00585.html ,
 I thought it would be better to replace the MM CR block with a polyphase clock sync block, as "M&M block isn't great in fading environments", and I would like to receive possibly weak signals,

When this transceiver was implemented, the polyphase clock sync block was not available yet. You are right, it should improve performance. If you switch to that block and see performance improvements, it would be great if you made a pull request.

 Again, works for 4 MHz sample rate and 2 samples per symbol, but not 8 MHz rate and 4 samples per signal. What do you think?

I think there is no reason why that shouldn’t work. It would be helpful if you could upload your flow graph, maybe there is a mistake somewhere (the picture you posted did not show the USRP and xlating filters, for example).


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