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[Discuss-gnuradio] Packet chopping

From: Peter Vizi
Subject: [Discuss-gnuradio] Packet chopping
Date: Tue, 10 Mar 2009 16:13:35 -0400


We are implementing a packet-based system with gnuradio, and we
experience that the packets are not sent in one flush from the USRP.

In our transmitter we are maintaining a data path and a control path, so
we exactly know where is the beginning of a packet and where is the end.
We use this method to pad the outgoing complex waveform to the multiple
of 128 samples, as it is required by the USRP (in a particular setup our
packet is 468 samples long, so it is padded to 512 samples).

Image [1] shows the abs of the complex waveform that our transmitter
block produces, this is fed into the USRP sink (a copy of the ofdm
benchmark's USRP sink). Image [2] shows abs from the received signal
when the transmitter was operating. You can discover the preamble in the
received signal and similar things to the rest of the packet, but this
is not continuous. Interestingly there seems to be some parts from the
previous packet before the preamble.

We tried to set the fusb_block_size of the USRP sink to 8, so this
buffer will hold a whole packet, but this did not help either.

Any ideas would be greatly appreciated.

Best, Peter.

[1] http://nd.edu/~pvizi/transmitted.png
[2] http://nd.edu/~pvizi/onair.png

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