[Discuss-gnuradio] Channel Model block - Epsilon effect on band edges

[Murphy, John]

Questions about the Channel Model block from gr::channels::channel_model

Noiose Voltage and Frequency Offset work as expected.
I've been using a Seed of 0 and no multipath so Taps of 1+0i. Have not
seen any issue there although one does have to be careful to not use
Taps of just 1 for whatever reason - you have to fully specify the
complex number or weird stuff will happen. The default is 1+1j and I
guess you use that and just shift stuff 45 degrees which will likely
be un-noticed, I have not tried that.

Anyhow, my questions relate to the Epsilon that lets you simulate
sample rate error at baseband, which you can use for a controlled
symbol timing error.

I read the code and figured out that 1 is perfect timing. Values less
than 1 will add samples (input rate over output rate equals Epsilon),
which will make my sync late and I see this. Also I see the opposite
for values greater than 1. That seems as expected.

What I have encountered, did not expect and have some trouble with, is
the way non-unity Epsilon affects the frequency response of the band
edges in a time-varying fashion. With Epsilon just slightly offset
from unity I can see this causing the edges of my signal to droop and
then come back at a rate proportional to the offset I am using. This
gets to be problematic when I have a pilot tone near the band edge
which is having not only its amplitude but its phase shifted.

Anyone try to use this before? Really all I need is just something to
stick extra samples in (or remove them) at some rate. I suppose I
could loop thru the USRP at slightly different Tx and Rx sample rates.

Thanks,
John