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Re: [Discuss-gnuradio] in-band signaling & dependent packets (i.e., ACK

From: George Nychis
Subject: Re: [Discuss-gnuradio] in-band signaling & dependent packets (i.e., ACK generation)
Date: Tue, 04 Dec 2007 20:43:21 -0500
User-agent: Thunderbird (X11/20071022)

You can't go lower than the PHY layer.  There's a reason it's the
lowest on the stack.

Yeah, stupid comment by me ;)

You can use a matched filter for this, but a generalized matched
filter uses multipliers.  If you limited yourself to GMSK, PSK, or QAM
you can get away with sign manipulations.  OFDM would require
different processing in general.
If some mechanism could be built in the FPGA that was highly
reconfigurable, that would be fine... but it seems to be boiling down to
different techniques per-modulation.

Not really an option for the USRP as it currently is due to the lack
of hardware multipliers.  Maybe more of an option for the USRP2.

So there is no way of getting a generalized matched filter on the USRP? Is there anything that can be done to get around the hardware multipliers? If there is absolutely no way, limiting to GMSK, PSK, and QAM is not that bad. What makes OFDM need different processing? I'm trying to read up on matched filters now.

On a side note, you should not feel bad about making something that
considers a trade-off in the realm of software defined radio.  You are
giving up (slight) PHY layer flexibility for much improved latency.
We're still using an antenna.  We're still using a super heterodyne
receiver.  Some things just can't be software and are limitations -
working around them is fine.

I agree, it's not even that this functionality is truly needed. The host can still be used to decode and generate ACKs. It's merely an optional mechanism and certainly better than nothing.

- George

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