FreeBSD TDMA: Legalizing 440MHz 802.11 modems

[Adrian Chadd]

Hi!

So, I'm not sure if we can underclock it _that_ far. The 5/10MHz
channels are implemented by underclocking various parts of the chip,
which results in everything being some fraction of 20MHz (or 2x
clocking it, resulting in 40MHz.) Bringing it all the way down to
200KHz is a pretty tall ask.

I know people have gone down to 2.5MHz on these chips, but I don't
know of anything lower than that. The OFDM subcarriers start being way
too narrow and I doubt you'd end up with anything sensible.

It's likely that you'd want to create a custom DSP based solution to
do 200KHz side stuff at that symbol rate.



-a


On 27 August 2014 15:34, Bart Kus <me at bartk.us> wrote:
> Hello,
>
> I'm wondering if you can tell me if it's possible to modify the FBSD TDMA
> code to make this card:
>
> http://www.doodlelabs.com/products/radio-transceivers/sub-ghz-range/420-450-mhz-dl435-30/
>
> legal to use in its intended spectrum.  By default the card violates two
> Part 97 rules:
>
> 1) Emissions are limited to 200kHz bandwidth
> 2) Symbol rate is limited to 56kSym/s
>
> Is it possible to slow down the subcarrier symbol rate in that Atheros chip?
> Is it also possible to then space the subcarriers tighter together to
> respect the 200kHz emission bandwidth limit? They'd have to come closer
> together anyway to uphold OFDM subcarrier orthogonality.
>
> Thanks for any clues,
>
> --Bart
>
> PS: I'm asking in the interest of the HamWAN.org project.  We're trying to
> find a more mobile solution, which penetrates through forests and some
> buildings.  440MHz seems to fit the bill, but hardware is really hard to
> come by.
>
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