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Clearwire and China Mobile link up for 4G time-div trials

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As part of the Global TD-LTE Initiative US-based Clearwire and China Mobile are joining forces to test time-divisioned networks in both countries, though we'll likely see the technology in the UK too.

That's because Ofcom, the UK regulator, has earmarked 50MHz of spectrum for the TD variant of 4G telephony, despite the fact that the regulator is supposed to be technology neutral. TD-LTE has a solid technical specification but it still needs lots of interoperability testing if it's going to achieve the economies of scale accorded to its frequency-divisioned contemporary.

When cellular telephony was designed it made sense for the sending and receiving channels to be on separate frequencies, that way the users don't have to shout "over" at the end of every exchange and can even interrupt each other's diatribes with their own interjections.

But giving every user a separate frequency is overly generous, so GSM divides up the band into time slots and expects each user to share a pair of frequencies with seven others nearby. Each user gets an equal share, one time slot per voice call, though a GPRS handset may make use of empty slots for data communications.

Even with CDMA (Code Divisioned Multiple Access) allowing users to share the same frequency at the same time (the base station differentiating between signals with an identifying code) users still transmit on one frequency, and receive on another, ensuring the two don't interfere with each other and keeping things simple.

But using frequency division does require the network operator to own two separate bands, so the regulators regularly licence spectrum in paired blocks suited to frequency divisioning and even refer to companies as owning, for example, 2x25MHz rather than just 50MHz of bandwidth.

WiMAX was the notable exception to the use of frequency-divisioned standards, instead requiring both ends to switch from receive to transmit in perfect harmony. That allows a single frequency so be used for a duplex connection, as well as providing greater flexibility for asynchronous connections (as the time slots for send and receive don't have to be the same size).

WiMAX is all but dead now, but the bands it was using have no convenient pair, which is why Clearwire (who previously deployed WiMAX around the USA) is so keen to explore TD-LTE.

In the UK our 4G auctions are almost entirely paired, but there's a 50MHz chunk at 2.67GHz which lacks a pair and is widely expected to be used for TD-LTE, though the regulator remains officially neutral about technology choices and the licences won't specify what the airwaves are to be filled with.

Who will deploy TD-LTE in the UK we don’t know: the existing operators are more likely to prefer the frequency divisioning they're familiar with, but if China Mobile and Clearwire can demonstrate the technical capabilities of the TD-LTE standard it might increase interest in technology, and the bands needed to support it. ®

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