[LINK] Ubiquiti´s totally disruptive 24Ghz unlicensed "AirFiber" looks awesome

[Jeremy Visser]

On 9/01/2013 09:16, Fernando Cassia wrote:
> On Tue, Jan 8, 2013 at 7:03 PM, Tom Worthington  wrote:
>> But how many of these units could be serviced from one central tower,
>> before mutual interference degrades the signal?
>
> It's designed for point-to-point, not point-to-multipoint.

I think you could be misinterpreting the G.P. (alternatively, I could be 
:-) ). It is definitely possible to have multiple point-to-point links 
pointing to the same tower. It requires a radio unit per PTP link, but 
hey, that's not unheard of.

After all, you lose a lot with omni-directional or sector antennas. If 
your point-to-point radios are very directional, and you have 
appropriate shielding, you can actually get away with reusing spectrum.

I don't think airFiber has this particular feature (not this generation, 
anyway), but some Ubiquiti Rocket M gear has GPS synchronisation that 
lets you reuse spectrum in the same location. [0] The base stations use 
a very precise GPS-based TDMA to coordinate this.

The airFiber equipment itself only gives you a choice of two channels 
(24.1 GHz and 24.2 GHz, each 100 MHz wide). [1] I haven't looked into 
whether one, both, or none of these are allowed by the ACMA.

That opens you up for a fair bit of interference if they get popular in 
your area. Ubiquiti don't make any point-to-multipoint 24 GHz gear 
(thankfully!) but who knows what else is out there.

The only way around interference is going licensed. And even that's not 
enough in some cases. (We get a noise floor of -80 dBm on our 3.6 GHz 
allocations due to some other nearby allocations on the same frequency 
that the ACMA incorrectly calculated should only reach -100 dBm. When we 
pointed this out, the response was basically "sorry, our bad, but suck it").

--
[0] http://dl.ubnt.com/datasheets/rocketmgps/Rocket_M_GPS_Datasheet.pdf
[1] http://dl.ubnt.com/datasheets/airfiber/airFiber_DS.pdf