Re: [Discuss-gnuradio] Split-function implementation of 802.11g OFDM PHY and MAC on USRP2

[Colby Boyer]

On Wed, Aug 17, 2011 at 10:50 AM, Nick Foster <address@hidden> wrote:

On Wed, 2011-08-17 at 21:47 +0400, Alexander Chemeris wrote:
> On Wed, Aug 17, 2011 at 21:15, Nick Foster <address@hidden> wrote:
> > On Wed, 2011-08-17 at 18:00 +0400, Alexander Chemeris wrote:
> >> Hi Nick,
> >>
> >> On Sat, Aug 13, 2011 at 02:41, Nick Foster <address@hidden> wrote:
> >> > On Fri, 2011-08-12 at 23:26 +0100, Nemanja Trecakov wrote:
> >> >> 1) Did anybody use this external SRAM
> >> >> on USRP2 before? According to the FAQ page the usual FPGA build is
> >> >> not using it.
> >> >> (http://gnuradio.org/redmine/projects/gnuradio/wiki/USRP2GenFAQ)
> >> >> However, my experience with these pages is that many of them are
> >> >> outdated. I also found the folder called extramfifo in
> >> >> uhd/fpga/usrp2 folder for which I believe has something to do with
> >> >> external RAM.True?
> >> >
> >> > The external SRAM is currently used for buffering TX packets.
> >>
> >> Is it true for USRP N and latest git too?
> >>
> >
> > yes
> >
>
> By Tx do you mean Ethernet->RF aka RF Tx, or RF->Ethernet aka Eth Tx?

RF TX. It's there as a buffer so you can use timestamped TX packets to
say "send this data at exactly time X".

--n

>
>



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Is the FPGA even big enough to support a full 802.11G receiver (so many multipliers)? Unless there is a onboard microprocessor/DSP for you to use, trying to run everything in hardware will get very complicated very fast. Granted the MAC is offloaded to the host computer, but handling all the PHY functions is daunting. 

There is preamble search, then symbol/freq synchronization + whatever filtering is needed to accomplish this. After that, there is OFDM equalization from the pilot symbols. Also viterbi decoders for the FEC. 

--Colby