TDD then and now
To understand TDD's appeal in 2010, it's worth looking at why TDD didn't catch on during the 00s despite being commercially available. Part of the reason the industry favored FDD was to do with technology, explains Dr Ray Owen, head of pre-sales for the Mobile Devices and Home division of Motorola Asia and general director of Motorola Vietnam.
"FDD separates the uplink and downlink, which is much simpler from an engineering standpoint than TDD where both links are combined," he says. "That also helps in terms of battery life. Also, W-CDMA was really designed as a voice network that supported data and video, and voice and video perform better with symmetrical links. TDD is great for pure-play data, but 3G operators weren't focused on that model then."
Another issue, he adds, was that with everyone focused on FDD, TDD was only likely to be deployed if dual-mode FDD/TDD devices were available. "It's not impossible to put FDD and TDD in the same handset, but it is very difficult from an engineering point of view," says Owen.
By contrast, today chipset makers like Qualcomm and ST-Ericsson have FDD/TDD chipsets on their roadmaps, with China Mobile pushing for a dual-mode strategy as part of its TD-LTE plans.
Related to that is the close development of both FDD and TDD-based LTE on the network side, says Isaac Liang, international TD system marketing director for ZTE.
"FDD and TDD [LTE] are like two brothers, like twins - they are very similar in terms of coverage, throughput, spectral efficiency, and so on," Liang says. "Many vendors using the same platform for their FDD and TDD LTE products, and chipset vendors in their roadmap will have dual-mode FDD/TDD wireless chipsets. These are incentives for operators."
Another factor is the rapid rise of dongles and IP-savvy smartphones like the iPhone, which have driven mobile data traffic to such levels that operators in many markets are hungry for new spectrum - and they aren't necessarily going to wait for paired LTE spectrum to be freed up, says Tom Gruba, Motorola's senior director of wireless product marketing.
"Between iPhones, netbooks and flat-rate pricing, operators are finding they don't have enough spectrum, which is one reason they want 2.6-GHz LTE, but they'll also use all available spectrum they can get," he says. "They're seeing all this TDD spectrum being used in China, the US and in southeast Asia with 2.3 GHz. So there's all this opportunity for spectrum, and China has broken that inertia to get things moving."
Julien Grivolas, principal analyst for networks and technology at Ovum, agrees, noting the example of Softbank acquiring Willcom primarily for its spectrum. "They're not sure what they'll do with it, but what they want is the spectrum. You will not say 'I'll only take this kind of spectrum', you'll take what you can."