[srslte-users] How is subframe-level timing established and adhered to for over-the-air (OTA) eNB transmissions?

[Vikas Jain]

Dear Community,

I have two USRP B210 boards, each connected to a Linux host over USB. One
Linux host is running the example program lib/examples/pdsch_enodeb.c and
the other linux host is running the lib/examples/pdsch_ue.c. The UE side is
able to successfully decode PSS/SSS/PBCH and subsequently decode all the
PDSCH subframe transmissions.

On the eNB side, I am running the pdsch_enodeb program with MCS =1 and PRB
= 50, which translates to sampling rate of 11.52 MHz, which means 11520 I/Q
samples per subframe (1ms).  The  pdsch_enode program invokes
srslte_rf_send_multi() API to send the I/Q samples, which does not have any
notion of time.  The I/Q samples for every subframe are simply transmitted
asynchronously over USB one after the other. Please see the attached eNB
USB wireshark trace (collected from one of the runs) showing I/Q sample
transmission for two consecutive subframes. I/Q samples for first subframe
is sent in a span of ~590us and the second subframe's I/Q samples are sent
immediately after it in a span of ~700 us; similarly, other subframes
follow right after this and so on.

Following are the questions:

(1) How does USRP determine the set of USB I/Q packets that make up one
subframe? From the Wireshark trace, I couldn't see any fields in the CHDR
header that would indicate the group of USB I/Q packets which constitute
one subframe?
(2) I presume USRP internally has a notion of OTA subframe boundary time,
which is strictly maintained. Is this correct?
(3) How is the OTA subframe number determined in USRP for a group of USB
I/Q packets (for one subframe) received from Linux host?
(4) How is it ensured that the USRP is not overwhelmed or starved for USB
I/Q packets being received from Linux host? From the Wireshark trace, it
seems that the Linux host application just sends the I/Q packets one after
the other, without any regards to USRP aspects.
(5) Does Linux host know the OTA subframe number in which the USB I/Q
packets (sent over USB link) were actually transmitted over the air? I
presume such association would be needed to correlate e.g. HARQ ACK for the
PDSCH transmissions etc.

Will appreciate any help in understanding the above. If there is any
document or material on these aspects that can help understand how subframe
timing is managed between USRP and Linux host, it will be very helpful.


Thanks.
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