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Re: [Discuss-gnuradio] OFDM PHY (802.11) ?
From: |
Eric Blossom |
Subject: |
Re: [Discuss-gnuradio] OFDM PHY (802.11) ? |
Date: |
Sun, 18 Feb 2007 11:21:45 -0800 |
User-agent: |
Mutt/1.5.9i |
On Fri, Feb 16, 2007 at 09:56:39AM -0500, Brian Padalino wrote:
> On 2/16/07, Tom Rondeau <address@hidden> wrote:
> >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.
>
> Ah, so you really want to go full scale with this - neat. I am
> worried about what type of transients you could see if you do some big
> impulse signal with the interpolation that happens when transmitting.
> I suppose your 1.0 normalized signal would really be the number of
> active frequency bins since you could possibly hit a peak at each one
> of those frequencies? Does that sound right?
>
> On the scale values - a complex signal going down to the USRP is
> 8-bits real / 8-bits imaginary if I remember correctly. That should
> give +/-127 for those values.
We almost always use 16-bit I & Q across the USB.
> For a fixed-point model, maybe it would be worth while to choose a Q12
> or Q15 number to be the standard normalized value - therefore any
> bit-slicing done in the FPGA can be reliably done without the fear of
> a loss of data?
Of course you can build an arbitrary width data path in the FPGA...
Eric