optimize UL test to support higher system bandwidth

[tingjunchen]

Here's what I have tested on USRP X310 SDRs with a server:

• For 1x1 SISO sounding with one "U" in the frame schedule, Sounder support up to up to 50 MHz bandwidth. When I further increase the bandwidth to 100 MHz, the BS side quickly gets this error:

53:702207 INFOR: Sync BS host and FPGA timestamp for thread 0
54:229086 INFOR: Start BS main recv loop in thread 0
54:683848 INFOR: Saving HD5F: 2000 frames saved on CPU 1
55:142102 INFOR: Saving HD5F: 4000 frames saved on CPU 1
55:377018 ERROR: thread 0 buffer full

It seems like we will need to either increase the buffer size or to use more thread for the txrx streaming.

• For 2x2 MIMO sounding with one "U" in the frame schedule, Sounder cannot even support 5 MHz bandwidth on USRP SDRs. I think this may be caused by the fact that the client keeps reading UL symbols from a file, and that the file I/O and txrx streaming are not fully paralyzed/optimized. For example, I am getting a lot of late packets from the USRP indicating that the host is not able to catch up with the USRP timestamp. Could we optimize the client code for UL tests so that the file I/O and txrx streaming can happen more effectively in the MIMO mode?

Please also note that for 2x2 MIMO sounding without "U" in the frame schedule, the sounder works perfectly fine for 50 MHz bandwidth, that's why I suspect that the large performance drop comes from the use of the UL .bin file...

At least now the UL mode fully works on USRP devices and in the SISO mode it works for up to 50 MHz bandwidth. This is great and thanks everyone for your help on this thus far!