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Re: N310 phase sync issue with antenna array

From: Johannes Demel
Subject: Re: N310 phase sync issue with antenna array
Date: Wed, 30 Jun 2021 17:31:23 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1

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.


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!

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