FreeBSD TDMA: Legalizing 440MHz 802.11 modems

[Adrian Chadd]

On 27 August 2014 16:09, Bart Kus <me at bartk.us> wrote:
> Is the underclocking affecting the digital domain only?  Or would there be
> some analog frequency response curves that would start falling off too?  If
> it's a digital-only underclock I don't see why there would be any
> degradation (aside from the obvious speed decrease).  Is this easily
> testable somehow, with a single clock variable?

It isn't a single clock variable. The chip is a huge state machine and
lots of timers, so all of the timing related things that are counts
that reflect time (eg number of ticks for 802.11 things to occur) need
to be recalculated.

> Yes, the subcarriers would get really narrow, but the sampling time would
> increase proportionately, so the FFT resolution would stay the same.  I
> wonder if we'd be exceeding the hold time of the S&H circuit(s)...  I don't
> really know anything about these chips, just making wild ass guesses based
> on generic modem architecture.  :)

Well, there's an FFT going on, and you still will have 52 subcarriers,
so divide your 200KHz up into 64 bins (52 subcarriers + guard bits +
sync tones.)

normally for 20MHz they're 31.25KHz wide.

For 200KHz that's 0.3125KHz wide, or 312.5Hz wide. There's not a lot
of gap between each carrier and we aren't over-sampling. I don't think
the chip was ever really designed for that.

Also yes, there's AGC and other bits that have likely only been
characterised for a max hold time of hm, 12mS? Whatever the 1MHz CCK *
maximum packet length is.


-a