I am trying a modified example of the digital-bert routines, for
communication between 2 USRP2s, and notice that I am getting a very
large number of overflows (SSSS…) even with decimation rate at the
receiver of 20, and 4 samples per symbol (sometimes even with 20
samples/symbol). If I don’t get overflows (as has occurred when I used
20 for decimation as well as 20 for samples/symbol in one instance), I
am able to capture the demodulated bits as 111111111111111111111111…,
as expected for the example. However, with overruns, which seem to occur
more for lower samples per symbol and/or lower decimation values, I get
a large number of bit errors.
My receiver flowgraph is of the form:
USRP2 Source --> RRC Filter --> Costas Loop --> Mueller and Muller Synch
–> Complex to Real --> Binary Slicer --> Descrambler --> File Sink.
The transmitter flowgraph uses the same blocks as per
digital-bert/transmit_path.py, but with a USRP2 sink.
I am transmitting over-the-air, and clocks are not synchronised between
Tx and Rx.
I have a gigabit Ethernet link, and 2 x 2 GHz CPUs in my PC, which is
running Ubuntu 9.10.
Can anyone suggest why I am getting so many over-runs, and how I could
get around this problem?