Re: N310 phase sync issue with antenna array

[Johannes Demel]

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!