Hi all,
I'm observing a multi-tap channel on a wire even after using an octo-clock to sync the two usrps (ref/pps). I've also tried using timed commands, but that didn't help either. Any ideas on why its still happening? Is there such a things as a fractional sampling offset? That is what a lab-mate suggests is the cause of the issue, but I can't find any sources on it. Thank you, Arjun ________________________________ From: Edwin Li <lbyhp...@gmail.com> Sent: Friday, September 1, 2017 5:52:28 PM To: Bakshi, Arjun; usrp-users@lists.ettus.com Subject: Re: [USRP-users] Initial phase offset at TX/ DC offset and LTE Hi AB, Let me make a guess here. Because of the clock is not perfectly synchronized, there is non-linearity. Therefore the channel estimator gives multiple coefficient. Try connecting 2 USRP with the same reference clock and pulse per second signal and see if there are still multipath. Regards, Edwin Bakshi, Arjun <bakshi...@buckeyemail.osu.edu<mailto:bakshi...@buckeyemail.osu.edu>>于2017年8月31日周四 下午3:07写道： Correction: The thing I mentioned about all taps being phase shifted by the same amount isn't always observed. I tried it again with the same USRP, (and 2 USRPs with a MIMO) and I still get different channels every time I start the flow graph. Identifying multiple taps: the channel estimate I get has >1 coefficient that is significantly greater than a noise like value. Figure attached. Shows only abs(chan). Thank you, AB ________________________________ From: Edwin Li <lbyhp...@gmail.com<mailto:lbyhp...@gmail.com>> Sent: Wednesday, August 30, 2017 11:49:30 PM To: Bakshi, Arjun; usrp-users@lists.ettus.com<mailto:usrp-users@lists.ettus.com> Subject: Re: [USRP-users] Initial phase offset at TX/ DC offset and LTE Hi AB, I suppose you are using 2 USRPs, right? Each USRP has its own clock. These clocks have different phases. That's why you see a phase offset. As for the multi-tap channel, how do you identify it? Not sure how LTE eliminates the phase offset. But in my communication program, I use preamble for phase estimation, then use costas loop to eliminate the offset. Hope it helps you. Regards, Edwin Bakshi, Arjun via USRP-users <usrp-users@lists.ettus.com<mailto:usrp-users@lists.ettus.com>>于2017年8月30日周三 下午9:09写道： Hello everyone, I have a setup that records the channel between a tx and rx while they are connected by a wire. However I've noticed that while the channel is constant over one run, when I restart the code/flow graph I get a different (but still constant) channel. Can anyone shed some light on why this is happening? Also, I'm observing a multi-tap channel even on a wired connection. That seems suspicious to me. I've however verified that my estimation code is correct based on simulations. I'm using an N210 with a WBX board. Lastly, does LTE do anything to force the same offset or eliminate random offsets at the transmitting node? I searched the srsLTE github repo and found some variable tx_corr_dc_phase/gain, that suggests a fix, but I'd like some more inputs on this. github link: https://github.com/srsLTE/srsLTE/blob/590fff9c69214b4858de2e987410f0d24ad4275c/srsue/ue.conf.example Thank you, AB _______________________________________________ USRP-users mailing list USRP-users@lists.ettus.com<mailto:USRP-users@lists.ettus.com> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
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