FreeBSD TDMA: Legalizing 440MHz 802.11 modems

[Bart Kus]

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?

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.  :)

--Bart


On 08/27/2014 03:53 PM, Adrian Chadd wrote:
> 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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