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Re: [Discuss-gnuradio] USRP Receiver config problem

From: satarkar
Subject: Re: [Discuss-gnuradio] USRP Receiver config problem
Date: Sun, 5 Apr 2009 04:16:42 -0400 (EDT)
User-agent: SquirrelMail/1.4.16

> On Thu, Mar 26, 2009 at 01:24:37PM -0400, address@hidden wrote:
>> Hi,
>> I am trying to run ALOHA MAC on gnu radio. The modulation scheme used is
>> GMSK. The Tx and Rx chains are as follows:
>> Tx Chain:
>> src -> framer -> crc enoder -> fec encoder -> preamble -> nrz ->
>> interpolator -> Gauus filter -> FM modulator -> gain -> usrp_tx
>> Rx Chain
>> usrp_rx -> filter -> demod -> integrator filter -> correlator -> fec
>> decoder -> crc decoder -> extract frame
>> For some reason, I am unable to receive the frame when I run my python
>> scripts. I tested USRP boards by passing a simple sine-wave and it is
>> received correctly. I checked the output of usrp_tx on oscilloscope and
>> it
>> does display the frame going through. To test, whether there was any
>> problem in the signalblocks I replaced usrp_tx by a filesink and dumped
>> the output of Tx chain in a file. I replaced the usrp_rx by this file
>> and
>> the frame was correctly received. I think this means that there is
>> problem
>> of configuration with usrp_tx and usrp_rx. Following are the
>> configuration
>> scripts:
> How are you doing symbol timing recovery on the receiver?
> Generating logging data using gr.file_sink's at varous points in the graph
> is
> often helpful in seeing what's going on.
> Eric

The correlator block uses a 64-bit preamble for synchronization(The block
is same as the gr_simple_correlator.cc). I made it print out the detected
SYNC word whenever it reports successful detection and the printed word is
correct. So I guess the synchronization is happening correctly.

I tried generating the logging data at various points as you suggested.
The output of usrp_rx, when plotted, clearly displayed the bursts of
incoming frames. The output of the filter block has the amplitude same as
the output of the gain block in Tx (this was expected according to the
documentation I got with this code). I tried using "grep" with the output
of the convolution encoder in Tx and input of convolution decoder in Rx,
but it did produce any match.

I am really at a loss about how to proceed now. Any help would be welcome.


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