Re: N310 phase sync issue with antenna array

Hi Larry,

Which carrier frequency do you use for your tests? Something like 2.4GHz?
do the 4 receive channels have a stable phase between them? i.e. if you
don't move the transmit antenna or your RX array, the phases do not change?

I assume you have 1 TX antenna and 4 RX antennas. I would expect that
your N310 receives this signal with a stable phase and time aligned.

However, your wireless channel will introduce a phase shift (you
compensated it with a delay, if I understand correctly). This is
expected and introduced by your wireless channel.
In your wired test case you fixed the "different distances" issue by
using same-length cables.

> What I believe should be the expected results of this setup is that
each AD9371 should receive synchronous signals aligned in phase, with a
random 180 degree offset between each AD9371 transceiver (please correct
me if this assumption if wrong).

Your assumption is probably wrong. You can only expect a fixed phase
relation between your RX antennas. Your RX signal will not have the same
phase on all RX antennas. And there might still be a 180 degree ambiguity.

In case you aim for a SIMO configuration, you need to do channel
estimation for every RX stream.

Cheers
Johannes


On 29.06.21 22:51, Larry wrote:
> Hello everyone,
>
> I am having an issue with achieving a phase-synchronous RF configuration
> using an N310 with an Octoclock and a linear antenna array. What I
> believe should be the expected results of this setup is that each AD9371
> should receive synchronous signals aligned in phase, with a random 180
> degree offset between each AD9371 transceiver (please correct me if this
> assumption if wrong). Here is a summary of my setup, issues, and outcomes:
>
> 1)  Hardware/software setup: Using an N310 running HG image at 1-gigabit
> network connection on UHD version 3.15 on Ubuntu 18.04. This is
> supported by an Octoclock-G serving as the 10MHz reference and PPS
> source for the N310. Equal length cables are used between all channels
> of the N310, to facilitate better phase synchronization. My GnuRadio
> flowgraph consists of a USRP source into a simple squelch, feed forward
> AGC, frequency xlating fir filter, and then converted from complex to
> real going into a QT time sink.
>
> 2) Testing & results: I am attempting to receive a bursty signal using a
> four element linear dipole antenna array, with the elements spaced
> slightly under lambda/2 distance apart. Two main tests have been
> performed; one with the N310 directly wired to another SDR that is
> injecting a generated sine wave into the N310, and another test over the
> air using a radio transmitter.
>
> i. Testing with a wired connection results in the correct expected
> results - phase-aligned signals, with the channel pairs on each AD9371
> transceiver offset by presumably + or - 180 degrees. I can then align
> using simple delays to achieve phase alignment between all channels.
> This works with 2, 3, or 4 channels used.
>
> ii. Testing over the air results in very unsynchronized signals among
> all four channels. These results tend to be repeatable and consistent in
> their behavior, but the channels all are received both wildly out of
> phase (even channels on the same AD9371 transceiver), and even
> (depending on location of the transmitter relative to the antenna array)
> inverted in amplitude relative to other channels (particularly
> interesting was that the imaginary component of one channel would match
> the inverse of a different channel's real component). This test has been
> performed at ranges exceeding 75~ feet, and as near as 5 feet away. The
> results are similar in either situation. It is also worth noting that
> varying the transmitter's location parallel to the antenna array
> (finding a 'sweet spot', so to speak) resulted in at most 2, possibly 3
> of the channels to align properly in phase without calibrating using
> delays (at least one channel would always stay wildly different).
> Testing over the air using fewer than 4 channels yields marginally
> improved, but overall similarly poor results.
>
> I have tried using an external LO source for the N310 as well as
> operating the Octoclock with and without GPS functionality enabled. I
> have varied the sample rates, distances, and testing environments as
> well as changing cables and splitters to try to rule out any hardware
> component errors. These seem to have no real impact on the strange
> results I get with the over the air RF configuration. Any help to sanity
> check or troubleshoot my issues would be greatly appreciated. Thank you!
>
>