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

[Eric Blossom]

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