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Re: [Discuss-gnuradio] Determining distance from OFDM signals

From: Andrew Wolfram
Subject: Re: [Discuss-gnuradio] Determining distance from OFDM signals
Date: Fri, 7 Jun 2019 09:10:19 -0700

Thanks for your responses! I'm trying to implement what's already been done with USRPs in this paper (https://ieeexplore.ieee.org/document/8555795), which is based on this paper (https://ieeexplore.ieee.org/document/5505942). My apologies if you can't access the full versions of these. In the first paper with the USRP's they're using an external clock to perfectly frequency sync the two devices, but otherwise they are taking relative distance measurements. That is, after frequency and time synchronization, they are accounting for the different delays in each device by only measuring changes in distance rather than absolute distance. Assuming I had the same setup as in the paper, does my original question make more sense?

On Fri, Jun 7, 2019 at 5:20 AM Müller, Marcus (CEL) <address@hidden> wrote:
Well, you can go ahead and at least to a degree enforce a known
relative phase between transmitter and receiver, but yeah, without
extensive external synchronization effort (e.g. GPSDOs – hi there, u-
blox :) ), the receiver phase is random relative to the transmitter's
phase. So, the distance can't be recovered from a one-way transmission.

Best regards,

On Fri, 2019-06-07 at 10:21 +0000, Jonas Manthey wrote:
> Hi Andrew,

> What do you mean by “information from the carrier data”? I’m no OFDM expert, but my intuition tells me that in a zero-IF architecture (which I assume your USRP has) any carrier phase information is lost. There’s some results when googling for “OFDM ranging” maybe that helps.

> Cheers,
> Jonas

> From: Discuss-gnuradio [mailto:discuss-gnuradio-bounces+jonas.manthey=address@hidden] On Behalf Of Andrew Wolfram
> Sent: Donnerstag, 6. Juni 2019 01:20
> To: address@hidden
> Subject: [Discuss-gnuradio] Determining distance from OFDM signals

> Hi,

> I'm trying to alter the file ofdm_txrx.py (https://github.com/gnuradio/gnuradio/blob/master/gr-digital/python/digital/ofdm_txrx.py) to get phase information from the carrier data so I can calculate an approximate distance between two USRP devices. Ideally I want to grab data from one of the blocks in the rx pipeline in the above python file after the frequency/timing corrections have been applied. I tried using the data after the serializer block, but it appears that this has no phase information. I tried with the equalizer block output but I'm not sure how to interpret its output data. For my particular payload with 300 occupied carriers and an fft size of 512 only the first 214 items of the equalizer output vector have any phase information, so I'm somewhat confused. Any ideas?

> Thanks,
> Andrew
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