[USRP-users] Issues with dual full duplex on x310

Michael Carosino m.carosino at gmail.com
Tue Jul 25 17:18:24 EDT 2017


I'm using gnuradio 3.7.12 and rfnoc-devel commit 1908672.

I'm trying to use both daughter boards on the x310 each at full duplex
(that is, I will have 2 independent tx paths and 2 independent rx paths).
So far I have accomplished this by using the UHD: USRP Sink/Source blocks
and setting them to 2 channels, selecting the appropriate antennas, and
specifying a subdev spec of A:0 B:0. The flowgraph works perfectly sending
and receiving 2 sets of data simultaneously.

However, I need to have the same capability when using the RFNoC blocks.
I've attempted to use 2 RFNoC radio blocks, one set to Tx, 2 channels, and
the other set to Rx, 2 channels, however the block's parameters have only a
radio select of A or B unlike the usrp source's subdev spec, so this setup
does not appear to work no matter what configurations I try. Am I mistaken
about what the 2 channels refers to here?

I did finally get an RFNoC setup that works by using 4 RFNoC radio blocks
(2 for transmit with radio select A, B respectively, and 2 for receive with
radio select A, B respectively). However, with this setup I get tons of
underflow "U" in the console. Curiously this happens even when using the
DMAFIFO as ettus recommends so I'm not sure what's going on. My flowgraph
is attached showing this setup.

(A quick note on why I want to use the rfnoc blocks, apparently the uhd
usrp sink block now automatically includes a DMAFIFO in the tx chain to
deal with the underflow issue caused by tcp flow control, however for my
application the depth/size of this fifo is much too large and causing
massive delays, I've found that by reducing the depth I can minimize my
delay and I'll only get underflows on flowgraph startup which I assume is
due to the tcp slow start phase before backoff occurs. All this to say, if
there is a way to adjust the dma fifo depth from the usrp sink blocks I'd
gladly use that instead).

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