TD-LTE: Ready for Liftoff
The news was good for stakeholders in TD-LTE: China Mobile, the world’s largest wireless operator by subscriber base, said at Mobile World Congress last month that it would begin large-scale trials of the technology.
With an operator the size of China Mobile set to deploy 200,000 TD-LTE base stations between now and 2013 - more than are believed to be used in the average U.S. network - the technology seemed well-positioned to getting the scale it needed to become a global success.
But less than two weeks later, reports emerged that the Chinese government would likely delay granting TD-LTE licenses for two to three years. It’s not official yet – the government hasn’t put out a formal policy statement – but it’s definitely not a positive sign for China Mobile’s mobile broadband plans.
So with its most high-profile backer sent back to the bench, does that mean TD-LTE is out of the running? Not by a long shot.
The technology is being adopted worldwide by operators with unpaired spectrum looking to get in on the growing LTE ecosystem. The overlap between the time-division and frequency-division flavors is about 90 percent, making them nearly indistinguishable.
Clearwire is set to deploy its first 5,000 TD-LTE base stations by mid-2013 for its overlay network; DirecTV subsidiary Sky Brazil lit up its service late last year; and Bharti Airtel and Reliance Industries Limited plan to light up their respective networks this year in the high-growth, critically important Indian market. The standard has also made inroads in a number of other markets including Australia, Japan, Malaysia, Sweden and the Middle East.
“What’s really important here is the operators with unpaired spectrum can now enjoy some of the economy of scale advantages that come from FDD technologies,” Yankee Group analyst Ken Rehbehn says.
Even with China Mobile’s fate uncertain, TD-LTE is gaining steam. Operators whose unpaired spectrum formerly limited them to WiMAX have found a way to latch on to LTE’s rapid rise.
So set aside the bad news about China Mobile and focus on the positive: TD-LTE is ready for takeoff, and 2012 could be the year that it happens.
The Clearwire Factor
Clearwire has long been at the forefront of TD-LTE development, even if it will be far from the first to deploy when its network goes live next year.
It founded the Global TD-LTE Initiative with China Mobile, Vodafone, Softbank and Bharti Airel last year and also began working with China Mobile on devices, interoperability testing and roaming.
"If you look at volumes of scale, I'd rather hitch my bandwagon on these guys and have a globally harmonized spectrum solution than not," says Clearwire Chief Technology Officer John Saw, citing GTI estimates that TD-LTE will cover 2 billion people by 2014. "The scale is tremendous."
China Mobile’s trial run at the Shanghai Expo and its partnership with Clearwire helped legitimize TD-LTE, bringing on board top-tier vendors like Qualcomm and Alcatel-Lucent.
"What we have shown is TDD can scale. It will be as formidable as FDD in terms of LTE services," Saw says. "We’re giving a level of comfort to carriers around the world - it's not to be underestimated and the ecosystem is growing by leaps and bounds."
But for all of Clearwire’s work to establish commonalities in the ecosystem, its network architecture will be very different from the majority of other operators’ deployments.
Instead of the broad, even coverage that typifies most networks, Clearwire is using its limited financial resources to deploy TD-LTE on only its most high-traffic cell sites. About half of its sites carry a disproportionate 80-90 percent of traffic, CFO Hope Cochran said at a recent investor conference.
The result: Clearwire’s new network will function as the LTE equivalent of Wi-Fi offload, alleviating congestion where it’s needed most for wholesale customers Sprint and Cricket Communications. The network will also be LTE-Advanced-ready, capable of supporting carrier aggregation technology once it’s standardized.
Carrier aggregation will take LTE where it’s never gone before, Saw says. The technique combines different spectrum bands into a single, wider channel, increasing the network’s capacity.
"Other carriers look at carrier aggregation as a way to stop the spectrum crunch... Clearwire is looking at carrier aggregation to set a pace that no one will be able to follow - I'm talking about combining a 20 MHz channel with another 20 MHz channel to create a 40 MHz pipe," he says. "That is essentially stringing a virtual fiber with you wherever you go at a reasonable cost."
Clearwire’s goal is to make its pipe as fat as possible. Sprint – its first TD-LTE wholesale customer – has a somewhat different goal.
Bob Azzi, senior vice president of networks at Sprint, says it plans to use that pipe to give its subscribers a "consistent experience," not unexpected boosts in speed. The added capacity will be used to even out speeds during times of congestion, when data rates would normally slow.
"When we combine Clearwire's network with ours at heavy traffic sites, theoretically customers would be at a faster peak speed, but we're offloading that traffic at times of congestion in our base network, so the customer experience is going to look very consistent in terms of the average rates," Azzi says.
The first Sprint smartphones to run on both its network and Clearwire’s TD-LTE service are expected to hit shelves some time next year.
Vendor Perspective: The Attitude Shift
Sequans was asked by China Mobile back in 2009 to develop chips for its TD-LTE trial network at the Shanghai Expo. As Sequans marketing executive Craig Miller puts it, "What are you going to do, say no to the world's largest operator?"
The company had a long history with WiMAX, which is also a TDD technology, and had sample TD-LTE chipsets available for China Mobile in a matter of months. China Mobile's TD-LTE evangelizing "legitimized" the use of unpaired spectrum for time-division-duplex-based wireless service, Miller says.
"Most of the traditional providers of cellular technology have been what I call 'FDD bigots' - they've been very anti-TDD for years, and for good reason," Miller says.
It's easy to understand why providers with a long history in voice-centric network design favor FDD. Early wireless networks were built for voice, so frequency-division-duplex was ideal because it is symmetric and allows signals to be sent and received at the same time - a good fit when you're talking about as much as you're listening.
But that model no longer applied as the industry moved to more data-centric services. Time-division duplexing allows providers to adjust the uplink/downlink ratio, lending itself more flexibly to download-heavy data traffic.
The unpaired spectrum that was once a forgotten set of bands is now being put to use for TD-LTE, helping to change longstanding attitudes of time-division duplexing.
"Now I think people see a lot of value in having TDD spectrum for some of these high-capacity, high-speed networks a lot of countries will need," Miller says. As an added benefit, the spectrum is abundant in many markets - Clearwire has about 160 MHz of 2.5 GHz spectrum across the country’s top 100 markets.
Bill Clifford, vice president of Alcatel-Lucent's LTE product line, agrees with Miller's characterization of the industry's change in attitude. "There has been industry bias toward the usefulness of TDD spectrum, at least in the old days," Clifford says. "With this adoption of TD-LTE, I definitely don’t see that bias any more."
Like Sequans, Alcatel-Lucent was a participant in the Shanghai Expo demo. Years after the influential trial, Clifford is bullish on the standard's prospects.
"There's a significant amount of spectrum that is TDD only. Spectrum, as we all know, is a highly valuable item and it's not going to sit fallow for long," he says. "WiMAX is fading away, and the natural technology to take over is TD-LTE. It's a boom, it's happening, it's for real."
Qualcomm technical marketing director Rasmus Hellberg agrees – but thinks it might be time to stop thinking separately about TD-LTE and FDD-LTE.
“It’s not two ecosystems, it’s one ecosystem – it just has two flavors,” he says. Qualcomm has taken that to heart, incorporating both technologies in its LTE chips.
Maravedis estimates that 31 operators had committed to launching TD-LTE by the end of last year. The firm predicts that by 2016, about one-quarter of the world’s estimated 469 million LTE subscribers will use the time-division version of the technology.
So with China Mobile and its 650 million wireless subscribers on board, could TD-LTE eventually outpace FDD-LTE? Rehbehn isn’t ready to go that far, calling the standard a “sleeping giant.” If the technology maintains its current trajectory, that giant just might wake up.