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[Discuss-gnuradio] OFDM update

From: Tom Rondeau
Subject: [Discuss-gnuradio] OFDM update
Date: Sat, 04 Aug 2007 16:30:35 +0100
User-agent: Thunderbird (X11/20070604)

Matt and I have had a bit of a chance to work out some kinks in the OFDM
world. My latest merge should allow people to operate over the air ok.
The problem turns out to be false correlations when using the BasicRX
boards because with the low gain, the noise only toggles the last 1 or 2
LSBs, which correlate nicely and caused the assertions down the chain.
The new version avoids that issue as well as provides better default
parameters to run over the air.

I recommend testing this on an RFX board and not a BasicRX/TX. Also,
adjust the --tx-amplitude option on the command line for performance.
It's set low still (200) as opposed to 1. This should be fine for a
couple of feet. Watch going too high with the amplitude because of
clipping. I think 4000 was about the max we saw with an RFX400, but
between 1000 and 2000 should work nicely for in-house tests.

I also set the default packet size to 400, down from 1450 bytes. We have
not implemented frequency/phase tracking throughout the packet, just on
the preambles, so rotation throughout the packet can cause high packet
loss with long packets.

I think this current version provides a usable base for people to play
with, though it's by no means perfect. However, I've got to bury myself
in my dissertation, so I won't doing any more work on it for a couple of


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