On 2/16/07, Tom R. [email protected] 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.
Assuming a real-only signal, the AD9862 has a dual 14-bit DAC which
gives +/-8191 values. This probably doesn’t mean anything since it
goes through interpolation, but the whole normalization of this stuff
would seem to get quite hairy after a while.
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?
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.
You just use the one symbol for a preamble of sorts? Is that reliable
enough to not false acquire enough to get data through?
Is that what you’re looking for?
Very much so. Thank you very much!