How is it possible that I receive a packet from the same USRP that transmitted it?

GNU Radio
1

Hello all,

Currently I am testing a modified version of tunnel.py, but I am seeing
some
strange results. My modifications were essentially a re-factoring to
prepare
for future additions, however somehow I was able to receive a
transmitted
packet on the same USRP that transmitted it.

The USRP transmit/receive paths are set-up exactly the same as in the
original tunnel.py, the only difference is that I abstracted the tun/tap
layer a bit (in order to improve the MAC layer functionality). I did not
change the cs_mac loop and I am only using a single USRP with the
modified
tunnel to flood PING requests. I am using only a single antenna on a WBX
d’board.

I know that I am receiving a transmitted packet because in
phy_rx_callback()
I am using scapy to dump the packet and inspect its contents. This
doesn’t
happen consistently - I am flooding out PING requests but only a
relative
few are “looped back” (for lack of a better word).

Now, can anyone explain how this could possibly happen?

Thanks,
Tom

2

On 06/01/2011 03:41 PM, Thomas Arcuri wrote:

I did not change the cs_mac loop and I am only using a single USRP

Thanks,
Tom

Put an antenna on the RX port, and you’ll get much more consistent
results–that is, you’ll receive ALL your packets. Even without an
antenna,
there’s enough TX energy coupled into the RX port to receive data,
but not reliably. The receiver doesn’t shut-down during transmit.
If you want that behaviour, you have to arrange for it yourself, but
muting the RX side when you transmit, or similar.

–
Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium