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

From: rear1019
Subject: Re: [Discuss-gnuradio] Determining distance from OFDM signals
Date: Sat, 8 Jun 2019 08:08:46 +0200

On Wed, 05 Jun 2019 at 16:20:12 -0700, Andrew Wolfram wrote:
> 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.

You cannot use the output of the equalizer block to get information on
channel phase (or amplitude). That’s the point of equalization. A
received subcarrier in a OFDM system is given by Y = H·X + N where H is
the channel transfer function at the subcarrier, X is the sent symbol
and N is noise (in frequency domain, i.e. after the FFT; note some
assumption regarding synchronisation must hold, see [1]). The equalizer
returns hat{X} = Y/hat{Y} where hat{·} denotes estimation. Note that
the serializer block is a downstream block of the equalizer in the file
you mention.

> 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?

This might be related to the number of used pilots and their position.
Do you use 86 pilots? How do you define their position? It’s rather odd
that the position of carriers without phase information is not

[1] https://ieeexplore.ieee.org/document/803501

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