On 01/02/2021 02:44 AM, Ofer Saferman via USRP-users wrote:
Hello,
I must say that I am a bit frustrated.
When I purchased multiple E310 units it was with the hope I will be
able to synchronize them mainly based on:
https://kb.ettus.com/Synchronization_and_MIMO_Capability_with_USRP_Devices,
it shows the E310 as MIMO capable and having a reference clock input.
Of course, now I know that not to be true. At least the reference
clock part. It seems that the MIMO part might be incorrect as well.
If I hack the clock and patch an external clock connector to be used
instead of the internal TCXO will I be able, using existing UHD
software and firmware, to synchronize multiple E310 units?
I will be able to provide a common clock to multiple units and I will
be able to provide a 1-PPS signal through the SYNC pins of multiple
units that is generated from the same common clock.
I need these E310 to transmit and have their base-band samples
perfectly time aligned.
How is multi-chip synchronization of the AD9361 devices being done by
UHD? By using the SYNC pin and setting
usrp->set_time_source("external")? Is it enough? Do I need to handle
it on my own? How?
I figured the internal TCXO frequency of E310 is 40 MHz. Can I provide
an external clock of different frequency? Maybe 10 MHz? Or does it
require a major change in software or firmware?
Does anybody have a different idea how to get E310 units synchronized
at sample level without voiding the warranty and hacking an external
clock?
Please help.
Regards,
Ofer Saferman
The E310 was never designed for multi-unit MIMO, although it does 2x2
MIMO just fine, and there are plenty of folks using it for that,
including stacks for various wireless protocols. The AD9361
inherently supports 2x2 MIMO due to the shared LO architecture between
the two TX and two RX channels.
The AD9361 SYNC_IN pin does get tied to the Zynq FPGA, but the UHD
code-base doesn't use it. I'll note that for most telecom-type
MIMO implementations, zero-phase-offset is NOT a requirement, just
ongoing phase coherence.
Similarly the B2xx series does 2x2 MIMO just fine, but it was never
designed for multi-unit MIMO, although due to the clocking architecture,
the B2xx could likely (no guarantees) be made to do a multi-unit MIMO
implementation.
The SYNC input connector on the E310 is designed to:
(A) Provide a higher longer-term quality than the on-board clock.
(B) Allow for TDM synchronization on system deployments where that is
important.
Due to the high residual phase-noise of the DPLL implementation on the
SYNC input on the E310, it cannot be used to produce a working
multi-unit MIMO (or any other kind of phase-coherent) application.
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