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Microsoft carves up Wi-Fi into White Spaces

So many options, so little room to work in

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Researchers from Microsoft have proposed running Wi-Fi in White Spaces, by slicing up the Wi-Fi signal into bundles small enough to fit between the broadcast TV channels.

The proposal, termed Wi-Fi Narrow Channel (Wi-Fi NC), means reusing lots of Wi-Fi technology. But unlike those who want to just squeeze the existing wireless networking protocol into a smaller space, the Microsoft team wants to slice up the signal into chunks, and then feed those chunks through separate channels.

Wi-Fi currently runs in unlicensed bands at 2.4GHz and 5GHz, and can therefore cope with the interference-heavy environment of White Space - the frequencies that aren't being used locally to transmit TV channels. However Wi-Fi uses bands that are at least 20MHz wide, while White Space allocations come in 8MHz or 6MHz chunks.

The variation in White Space is down to the size of a TV transmission slot. In the UK our PAL transmissions were 8MHz wide, while the US squeezed their NTSC transmissions into 6MHz bands. The transition to digital uses slots of the same size for convenience for both PAL and NTSC, though a frequency slot can hold multiple channels.

In the USA Microsoft is one of the dozen or so companies that intends to provide a record of such channels. Spectrum Bridge will be launching the first such database later this month, and White Space devices will be required to check a list of locally available channels before transmitting. But once a device has the frequencies it's free to fill them with whatever it wishes.

So far we have four major contenders for that role: Weightless, a M2M protocol being pushed by Cambridge-based Neul; the IEEE's 802.22 wide-area star-topology network; LTE, the telephony standard which Huawei is co-opting into white spaces; and now Microsoft's Wi-Fi NC.

Getting things working in White Space isn't just a matter of slimming them down to fit. The FCC (probably to be followed by Ofcom) has imposed strict out-of-band limits on White Space transmissions – so signals have to be incredibly square (ramping up to full power, and then down again very steeply rather than falling off in the traditional bell shape). That doesn't suit LTE or Wi-Fi as it stands.

But the Microsoft researchers, who'll be presenting their technology at Networked Systems Design and Implementation in April, aren't just squeezing the Wi-Fi signal; they're imposing an additional step in cutting up the Wi-Fi transmission into sections which can then be transmitted in multiple White Space slots. The idea is named "compound radio" and could achieve good Wi-Fi speeds over much longer ranges, while also maintaining the flexibility to scale back and, critically, gaining access to all the higher layers of Wi-Fi functionality that already exist.

There's no need for a single standard to dominate completely in White Spaces. Just as 2.4GHz is full of Bluetooth, baby listeners and car keys so White Spaces can be full of all sorts of electronics using different radio standards. We'll likely see all these standards implemented in White Space equipment for a few years until things boil down to two or three popular standards, no doubt followed by impassioned debate from those who believe in the superiority of a particular protocol and/or have the patents to cover it. ®

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