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

[Murphy, John]

Any signal must be correct just because it is different?
I am talking about pretty severe amplitude effects (several 10's of dB
droop, accompanied by phase shift beyond what the timing error does)
near the band edges.
I don't think I should expect that from timing change. If I connected
another radio and ran it at a small delta sample rate, it would not
lok like that.


John Murphy
address@hidden


On Wed, Jun 17, 2015 at 12:29 PM,  <address@hidden> wrote:
> Shouldn't this be an expected effect?
>
> if signal.isactuallysampledat() != signal.thewaywepretenditssampled()
>
> Then should you expect phase and frequency effects?
>
>
>
>
>
>
>
> On 2015-06-17 11:59, Murphy, John wrote:
>
> 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
>
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio