WiFi in White Spaces

January 17, 2012, By Sanjeev

Imagine this, Wi-Fi signals being sliced up into bundles small enough to fit between the broadcast TV channels, and running them in White Spaces. This one is Microsoft’s idea, called WiFi Narrow Channel. The Microsoft team wants to slice up the signal into chunks, instead of just squeezing the existing wireless networking protocol into a smaller space, and then feed those chunks through separate channels.

WiFi in White Spaces

This proposal means reusing lots of WiFi technology, which currently runs in unlicensed bands at 2.4GHz and 5GHz, and can cope with the interference-heavy environment of White Space.

The frequencies are not being used locally to transmit TV channels. Also, WiFi 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, 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.

Microsoft is not the only one in the US who to provide a record of such channels. The four major contenders are: 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 Microsoft’s Wi-Fi NC.

Spectrum Bridge will launch 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, there is no restriction on what can be filled.

Hurdles are not few. Apart from slimming them down, FCC has imposed strict out-of-band limits on White Space transmissions. That means the standard signals doesn’t suit LTE or Wi-Fi as it standards.

Microsoft has thought of course.  There is 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.

The Microsoft team will be presenting their technology at Networked Systems Design and Implementation in April. Let’s wait for what’s in store.

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