Re: [Discuss-gnuradio] Synchronous MIMO TX on X310

[Marcus Müller]

Hi Dmitry,

sorry, I've just dug your mail out of my spam folder; please don't think ill of no one following up on you!

There's basically no other way than experimentally figuring out how large your ε must be so that GNU Radio can deliver samples in time.

You did set both devices to the same device time using some common trigger (either a PPS pulse or a GPSDO)?

Best regards,
Marcus

On 22.07.2015 14:09, Dmitry Kramarev wrote:

Hello,

I am trying to implement a MIMO bursty transmission on USRP X310 with 2 CBX daughterboards using GNU Radio. In order to make both antennas to start transmission synchronously I set up the time spec in the USRP Sink code usrp_sink_impl.cc:

_metadata.time_spec = get_time_now() + ::uhd::time_spec_t(epsilon);

where epsilon is 1ms fixed delay. This fixed delay is a problem, because of the GNU Radio time jitter, i.e. in one case epsilon is too small and the burst is late (LLLL), in another case epsilon is too large and the USRP just wastes time for waiting.

I want to minimize the waiting time, and make the USRP to start transmission simultaneously as soon as the TX chains of both antennas have some data to transmit. One option is making changes in the X310 FPGA image. My question however is, can I minimize the waiting time just by changing the GNU Radio code or settings?

Thank you in advance,

Dmitry.

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