RE: [Discuss-gnuradio] OFDM PHY (802.11) ?

[Tom Rondeau]

> -----Original Message-----
> From: Brian Padalino [mailto:address@hidden
> Sent: Thursday, February 15, 2007 3:15 PM
> To: Tom Rondeau
> Cc: gnuradio mailing list
> Subject: Re: [Discuss-gnuradio] OFDM PHY (802.11) ?
> 
> On an semi-related note, which of the RF front end cards will work
> with the OFDM waveform?  From my understanding, it has a pretty high
> peak-to-average ratio which might cause some problems with some
> transmitters.

An excellent question (which is academic speak for 'I don't have an
answer').

Hopefully we will know next week. You are right in the high peak-to-average
power, but we have to work a few more issues out before we will know how it
works over the air and through different daughterboards.
 
> How are the floating point numbers calculated on the PC going to be
> normalized into proper scale fixed-point numbers the FPGA works with?

Our biggest concern will probably be getting the signal to work on the RFX
boards, which (I think) all have the same output amplifier stage. What we do
with all of the modulators is scale it from +1 to -1 (even my QAM modulators
are normalized amplitudes), then digitally amplify it to +-16384. When you
look at the output power of the transmitter, at full power it only starts to
go non-linear and you can start to see the third-order product (Matt's done
a really good job with these guys). At around +-15000, I don't really see
anything, so that's about the arbitrary max I use.

With the OFDM work, we still send modulate the maximum value to be 1 (for
example, we send a "known symbol" at the start of a frame of all 1's), then
we will digitally amplify it, so there shouldn't be a difference, and we
shouldn't see any problems with the high peak-to-average ratio. I say
shouldn't because I'm an experimentalist and don't like to make concrete
claims until I see it.

Is that what you're looking for?

Tom


> This is very interesting to me, so I guess I am just a little curious
> how this is going to be done.
> 
> Thanks,
> Brian