FreeBSD TDMA: Legalizing 440MHz 802.11 modems

[Bart Kus]

Well I'll be damned, looks like 5MHz mode on Mikrotik is just slower 
signalling!  All 52-subcarriers are indeed there, although a little hard 
to see:



If you count peaks left/right of center, you'll get 26.  2*26=52, so 
that's every sub accounted for.

Can you point me at your 5/10MHz docs?  And which analog filter you're 
referring to?

--Bart




On 8/27/2014 4:47 PM, Adrian Chadd wrote:
> On 27 August 2014 16:38, Bart Kus <me at bartk.us> wrote:
>> I'm guessing the chip generates its own internal clocks from an external
>> reference.  Can that PLL be slowed down ahead of timers overflowing?
>> Hopefully the PCI clock is generated independently.
> Yeah, that's what you're slowing down - you program the PLL.
>
> The PCI clock is generated separately.
>
>> Also, I think Mikrotik implements narrower bands by dropping subcarriers
>> instead of slowing down their symbol rates.  I'll try to get a good spectrum
>> picture of their 5MHz mode tonight. Keeping the subcarrier symbol rates
>> relatively higher might offset some analog droops, at the cost of OFDM skirt
>> sharpness.
> Hey cool, if they're doing that then I should likely go digging for
> the PHY documentation for the AR5414 and find out.
>
>> Also, a slight correction.  I think you meant the subs are 312.5kHz wide,
>> which would result in a 200kHz emission having 3.125kHz wide subs.  Or,
>> perhaps more realistically, running at 1/128th the speed, 2.44140625kHz
>> wide.  How can you not love a number like that? :)
> Someone else can do better math, I'm busy doing non-wifi at the moment. :)
>
> But, it really is 20MHz / 64 carriers == each subcarrier width.
>
>> Does the project have a map of all these clocks + timers which might need
>> tweaking for spectrum reduction?  I know you can't cite original Atheros
>> docs, but perhaps there's been derivative documentation works created?
> Not at the moment I'm afraid. I haven't really dug into that level of
> detail. I documented what's needed for 5 and 10MHz modes.
>
> However - there's an analog filter that I don't know if it's
> programmable or not. It's used for both transmit and receive
> filtering. I know on these chips it can do 5/10/20/40 but I don't know
> if it's an arbitrary width.
>
>
> -a