Huawei stuffs LTE into TV bands

4G without the expensive spectrum

While the world's network operators pay billions for 4G radio frequencies, Huawei is calmly testing LTE in the licence-free white space spectrum, though squeezing it into the required mask will be tough.

Huawei will be testing the technique, which uses time division duplex (TDD) LTE in the television-broadcast bands not being used locally. That could, in theory, deliver 4G telephony without having to spend any money on radio spectrum, but making it happen will be a technical challenge of epic proportions.

The problem is that the FCC (which oversees white space use in the USA) is mandating extremely square radio signals, something which doesn't suit LTE at all. Competitors have been dismissive of attempts to get LTE, or Wi-Fi, working within those limits, but with so much at stake Huawei reckons it is worth a punt.

Outside the USA the rules aren't quite so strict, but still push hard against what's possible. The problem comes from the intersection between digital logic and the messy reality of physics – we happily talk about a radio signal being broadcast at 854MHz, and being 8MHz wide thus extending to 862MHz, but the reality is that radio signals don't kick in cleanly at 854 and drop off instantly at 862. They are more bell-shaped, and tail off into their neighbours' space, as well as only hitting peak power in the middle of the band.

The FCC requires equipment operating in the white space bands to be particularly square, to ensure maximum occupancy, and that makes the band more suited to some technologies than others.

But LTE radios are going to be very cheap, thanks to worldwide adoption of the standard, and the standard is flexible enough to use regionally-different bands (UK television channels are 8MHz wide, while the US uses 6MHz bands). TDD uses the same frequency for sending and receiving (it flips back and forth) and is expected to be used heavily in China, so Huawei knows the technology well.

Such a solution will still require base stations to check with an online database for locally-free channels, and transmission power is limited to avoid interference with broadcast TV. It is also not guaranteed: one has to play nicely with the other unlicenced users. So established network operators are unlikely to switch into white space any time soon.

But if LTE can be proven to work in white space, it opens up a whole lot of potential, enough to justify six months of testing, after which we'll see if Huawei really can run a 4G network without paying for 4G spectrum. ®

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