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Re: [Discuss-gnuradio] Large number of overflows...

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Large number of overflows...
Date: Wed, 21 Apr 2010 23:44:53 -0700
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4

On 04/11/2010 09:22 PM, Ian Holland wrote:
Hi All

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?

These overflows indicate one of two things:

- that your flowgraph is too slow to execute in real time on your computer

- You haven't enabled realtime scheduling.


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