# Channel Model block - Epsilon effect on band edges **URL:** \ **Category:** GNU Radio\ **Created:** [June 17, 2015, 5:59pm UTC](https://www.ruby-forum.com/t/channel-model-block-epsilon-effect-on-band-edges/242350 "2015-06-17T17:59:55Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\ **Post date:** [June 17, 2015, 5:59pm UTC](https://www.ruby-forum.com/t/channel-model-block-epsilon-effect-on-band-edges/242350/1 "2015-06-17T17:59:55Z")
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 ---
**Author:** ![MurphyS\_John](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ce7236/32.png) [@MurphyS\_John](https://www.ruby-forum.com/u/MurphyS_John)\ **Post date:** [June 17, 2015, 6:30pm UTC](https://www.ruby-forum.com/t/channel-model-block-epsilon-effect-on-band-edges/242350/2 "2015-06-17T18:30:18Z")
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: > Anyhow, my questions relate to the Epsilon that lets you simulate > edges in a time-varying fashion. With Epsilon just slightly offset > John > > * * * > > Discuss-gnuradio mailing list > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) [1] Links: ---
**Author:** ![MurphyS\_John](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ce7236/32.png) [@MurphyS\_John](https://www.ruby-forum.com/u/MurphyS_John)\ **Post date:** [June 17, 2015, 6:57pm UTC](https://www.ruby-forum.com/t/channel-model-block-epsilon-effect-on-band-edges/242350/3 "2015-06-17T18:57:45Z")
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 M. removed\_email\_address@domain.invalid ---
**Author:** ![MurphyS\_John](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ce7236/32.png) [@MurphyS\_John](https://www.ruby-forum.com/u/MurphyS_John)\ **Post date:** [June 17, 2015, 7:07pm UTC](https://www.ruby-forum.com/t/channel-model-block-epsilon-effect-on-band-edges/242350/4 "2015-06-17T19:07:41Z")
On Wed, Jun 17, 2015 at 12:56 PM, Murphy, John [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > Timing change is done by adding or removing samples, which is a form of > resampling. All resamplers must also be filters to handle the signal > changes correctly. What you’re seeing here is the effect of the filter > in > the fractional\_resampler. How much are you off by 1? It doesn’t need to be that much. Remember that normally our clocks are accurate to within parts per million. Also, any signal you have, you really need to make sure that the receiver can handle this kind of thing. And you should always be suspect of the band edges since most hardware will likely distort that as well to some degree. Tom ---
**Author:** ![MurphyS\_John](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ce7236/32.png) [@MurphyS\_John](https://www.ruby-forum.com/u/MurphyS_John)\ **Post date:** [June 17, 2015, 8:32pm UTC](https://www.ruby-forum.com/t/channel-model-block-epsilon-effect-on-band-edges/242350/5 "2015-06-17T20:32:23Z")
On Wed, Jun 17, 2015 at 2:14 PM, Murphy, John [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > is probably easier just to vernier the sample rate on one end). > > John M. > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) Yep, that’s the expected behavior of that fractional\_resampler block. One thing to do to see this easily is just connect a noise source → fractional\_resampler → freq\_sink (and turn on averaging in the freq display) and that shows you the filter shape, which is about 20 dB lower at the edges with a fairly slow roll-off. Instead of the fractional\_resampler with an interpolation ratio of r, plug in a pfb\_arb\_resampler (in Python use the filter.pfb.arb\_resampler) with a ratio of 1.0/r. This one builds a better half-band filter to reduce those edge effects. If that’s still not good enough for you, you can design your own filter for the Taps parameter to control that better. Tom ---
**Author:** ![MurphyS\_John](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ce7236/32.png) [@MurphyS\_John](https://www.ruby-forum.com/u/MurphyS_John)\ **Post date:** [June 18, 2015, 7:21pm UTC](https://www.ruby-forum.com/t/channel-model-block-epsilon-effect-on-band-edges/242350/6 "2015-06-18T19:21:12Z")
On further thought, not all resamplers need filters. In fact this may be about the ideal case where a resampler does not need a filter. If you are resampling a signal that does extend fully to the band edge, and your resample ratio is close enough to unity to avoid aliasing at the target new rate, do you still need to filter? And with a Epsilon on the order of 1+/-1e-7 or so the change in bandwidth from sample rate change is so small you can not even see it on a spectrum with human vision. Am I outside the box or up the creek without a paddle? John M. removed\_email\_address@domain.invalid ---
**Author:** ![MurphyS\_John](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ce7236/32.png) [@MurphyS\_John](https://www.ruby-forum.com/u/MurphyS_John)\ **Post date:** [June 17, 2015, 8:16pm UTC](https://www.ruby-forum.com/t/channel-model-block-epsilon-effect-on-band-edges/242350/7 "2015-06-17T20:16:02Z")
I uploaded a pair of images to show. [http://imgur.com/a/qAtEX#1](http://imgur.com/a/qAtEX#1) This is with an Epsilon of 1-1e-7. The signal droops at the edges as shown in the images then returns to normal, cyclically, at a rate that depends on the offset of Epsilon from unity. With these settings the cyclical rate is about 30 seconds, keeping in mind I have this throttled down to 320k fsamp without changing the displays (to work on this I bypassed the radio link temporarily - it does the same thing when used thru the radio although in that case it is probably easier just to vernier the sample rate on one end). John M. removed\_email\_address@domain.invalid ---
**Author:** ![MurphyS\_John](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ce7236/32.png) [@MurphyS\_John](https://www.ruby-forum.com/u/MurphyS_John)\ **Post date:** [June 18, 2015, 7:32pm UTC](https://www.ruby-forum.com/t/channel-model-block-epsilon-effect-on-band-edges/242350/8 "2015-06-18T19:32:18Z")
eh, nevermind, hit send too quickly on a long day (decimators are probably a special case, but rational resamplers when they interpolate are going to generate aliases that fold back in band when decimated, so yeah never mind) John M. removed\_email\_address@domain.invalid