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Re: phase difference between Tx/Rx1 Tx/Rx2


From: Marcus D. Leech
Subject: Re: phase difference between Tx/Rx1 Tx/Rx2
Date: Tue, 13 Oct 2020 12:42:32 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 10/12/2020 11:32 PM, Xiang Ma wrote:
Think about this.

The signal transmitted at USRP TX is 
image.png
Then after transmission, received at USRP RX is 
image.png

Here, ignore the Doppler shift, and just consider the phase change due to the transmission distance,
I know due to the wavelength, the phase received might be mod 2*pi, however, how can we get the received below in USRP?
image.png
Should we use analog signals to get the phase offset? or After ADC?

Thx

Let us for a moment think about how radios actually work, rather than how mathematical models of radios work.

You construct some signal at baseband--for simplicity a sinusoid.  That sinusoid is sampled at some rate, typically a rate that is a fraction
  of the clock rate of the hardware.   The basebanded signal is passed through a special interpolating filter called a DUC, before being
  presented to the DAC at the master clock rate.  Once it leaves the DAC it passes through an analog band-limiting filter, and then to
  a mixer.  The mixer has an LO at some phase that is random with respect to the baseband signal.  Once it leaves the mixer, it is amplified
  and then presented to the antenna port, where it heads down a cable or is propagated via an antenna--the new signal coming out of the
  antenna or down a cable will have a phase that is a combination of all of those stages in processing your baseband signal.

Now, the signal arrives at the RX, and the logical inverse of all of that happens.  Now, in the best of all possible worlds, the RX side is
  a complete and perfect "mirror" of the TX side.  The LOs are perfectly synchronized (so you can eliminate them from the phase
  estimates), as are the ADC and DAC clocks, and all the digital transformations on the signal.  Now, depending on the type of
  USRP hardware you have, you *MAY* be able to synchronize the TX and RX LOs.   There are very few applications "out in the world"
  that assume/require that the TX and RX frequency-conversion machinery be perfectly in-phase and coherent.  Radar is one of them.
  But 99% of telecom type applications out there *necessarily* MUST assume that the two "ends" of a conversation are not
  mutually coherent with respect to all of the frequency-translation machinery.

Now, there are comms applications that require that the TX machinery be mutually-coherent with itself--because of MIMO, and this applies
  to the RX as well.  But what it DOES NOT require is that the TX and RX be mutually coherent with each other, because the TX and RX
  will be separate pieces of hardware, potentially separated by a considerable distance, operated by different parties, and usually
  different equipment.

So, in order to proceed further, it would be good to know, in a high-level way, what it is you're ACTUALLY trying to achieve rather than
 focussing on these small details.  Because it's very easy to spend a long time on these details and with a "bigger picture" it may be
  easier and more expeditious to help you...





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