FreeBSD TDMA: Legalizing 440MHz 802.11 modems

[Adrian Chadd]

Yup, that's what I thought.

I'll go dig it up.

The analog filter is something on the front end, just before the
transmit/receive RF side. It acts as a filter to ensure (a) decent
rejection of adjacent channel signals being received and (b) the transmit
spectral mask is being met.


-a



On 27 August 2014 21:00, Bart Kus <me at bartk.us> wrote:

>  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> <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
>
>
>