Re: [Discuss-gnuradio] Issue with MMSE interp. filter (broken?)

[Tom Rondeau]

On Tue, May 31, 2011 at 4:37 PM, Colby Boyer <address@hidden> wrote:

Related. It seems that this delay is not taking in account for the MM clock recovery block? It some cases, this delay will not matter (2 or 4 samples per symbol), but for other rates it will.

--Colby

That's an excellent point. Any chance for a patch? :)

Tom

 

On Tue, May 31, 2011 at 1:29 PM, Tom Rondeau <address@hidden> wrote:

On Tue, May 31, 2011 at 4:25 PM, Colby Boyer <address@hidden> wrote:

On Tue, May 31, 2011 at 5:59 AM, Tom Rondeau <address@hidden> wrote:

On Tue, May 31, 2011 at 2:13 AM, Colby Boyer <address@hidden> wrote:

Hi All,

Recently I've been using the MMSE interp filter. I found that when I shift a signal by a fractional amount of 0 (or anything really), the signal goes way off! I would expect SOME difference, but not this much...

Some example output.

Mu:0 In:(-1.67869,0.480381) Out:(0.0418351,-0.16734)

Mu:0 In:(-1.23772,-0.104519) Out:(0.16003,-0.115883)

Mu:0 In:(-1.7598,-0.0618457) Out:(0.0986395,-0.33428)

On the imgur links are two Re-Im scatter plots of a bpsk signal, one with the fractional shift of 0 and the other with a fractional shift of 0.1 The points with 'x' are the resampled points and points with 'o' is the original signal. As you can see, the mmse fractional interp more or less destroys the signal! Unless I'm using it wrong!?

Any comments?

imgur: http://imgur.com/a/w98SX top picture is 0 delay, bottom is 0.1 delay.

Thanks!
Colby

Colby,

This block has been around for years without any changes, and I and others have been successfully using it in various projects, so my guess is that you have some misconception about what it's doing or what the parameters are.

You can see how it's used to simulate a timing offset in gnuradio-core/src/lib/hier/gr_channel_model.cc where it's used inside of the gr_fractional_interpolator_cc block. We also use it in the gr_clock_recovery_mm_XX and gr_mpsk_receiver_cc.

Tom

Hi Tom,

I looked at the issue again and I was not taking in account the group delay of the interp filter (its 5 taps into the future or 3 in the past), so I was sampling a few taps out of phase! After a time shift, it passes the sanity check of '0.0' delay. Thanks for the response!

--Colby

Excellent, glad that worked out. I was pretty sure it works, but you never know... good to have extra confirmation.

Tom