A version of LTE that could give consumers more mobile bandwidth for downloading content or apps is moving from the margins to the mainstream at Mobile World Congress this week.
TD (Time-Division) LTE, which uses a single block of radio frequencies instead of the paired blocks used in typical FDD (Frequency-Division Duplexing) cellular networks, has shown up in many places at the world's annual mobile gathering. Numerous carriers and vendors are building the technology into their gear and demonstrating uses for it, in a departure from the scant attention given TD-LTE a few years ago.
The big prize that shines over all this activity is the prospect of China Mobile's planned national deployment of TD-LTE, which is still waiting on the Chinese government's spectrum allocation but is already gathering steam with trial services in six cities. Yet carriers elsewhere are also using or planning to use TD-LTE, including Softbank in Japan, Sprint Nextel and Clearwire in the U.S., and operators in Brazil, Russia, India, Sweden, Saudi Arabia, and other countries. TD spectrum blocks are being set aside in yet other countries, including in a recent auction in the U.K.
"There's a lot of momentum behind it, and it's not all China," said Ovum analyst Daryl Schoolar. Still, with more than 600 million subscribers, China Mobile is big enough to make TD-LTE attractive to network vendors, chip designers and device makers for a long time, he said. "The volume opportunity is going to keep everyone interested."
At its booth at MWC, China Mobile showed off dozens of chips and devices designed for its planned network. They included smartphones from LG Electronics, Samsung Electronics, Huawei Technologies, ZTE and Quanta; USB dongles and personal hotspots from most of those vendors, and tablets from Huawei and Quanta. The display of chips included ones from big names such as Marvell Technology Group and Qualcomm. All those devices can be used with FDD as well as TD, along with backward compatibility with 2G and 3G networks.
Alcatel-Lucent has already developed a TD-LTE small cell, through subsidiary Alcatel Shanghai Bell, that will be used to add capacity to China Mobile's network in busy areas. Mindspeed Technologies, which supplied the silicon for it, showed off the cell at MWC.
Also at the show, Nokia Siemens Networks demonstrated a patented algorithm for balancing subscriber loads among LTE cells, including between TD and FDD equipment.
Advocates of TD-LTE say flexibility is its main advantage. Most LTE networks so far have been built with FDD (Frequency-Division Duplexing) technology, which uses two separate and equal-sized spectrum blocks, one for upstream and one for downstream traffic. Because TD-LTE uses just one large block, the frequencies within that block can be divided up in any way that makes sense for the way subscribers will use it.
That means a TD-LTE service could look more like home broadband, with a relatively thin pipe for sending email messages and URLs and a fatter one for downloading the pages that come with those URLs, as well as video, music, images and other content from the Internet.
China Mobile promotes this feature as one of the main things that will make its network better. The carrier could divide its spectrum differently in various areas depending on how the network might be used there, said Lei Cao, a China Mobile representative in the company's MWC booth.
Some said TD-LTE saves carriers money and is just a better way to use spectrum.
"This is hotly debated, but the TD-LTE advocates will tell you that it can be deployed in cheaper unpaired spectrum and is more efficient when the downlink/uplink is asymmetric," Tolaga Research analyst Phil Marshall said in an email interview. Dedicating the same amount of spectrum to uplinks as to downlinks leaves a lot of uplink spectrum unused, he said.
The biggest reason FDD is still used is tradition, according to Marshall. When cell phones were used mostly for voice, upstream and downstream traffic was equal.
"Most of the cellular spectrum is allocated in FDD and systems are deployed this way," Marshall said. "The advocates of FDD will tell you that you get better performance consistency with FDD and it is easier to implement -- particularly when coordinatedA with other FDD systems."
Without the need for pairing, it's also easier to cobble together various frequencies. In January, China Mobile and ZTE said they had demonstrated combining two separate TD-LTE spectrum blocks into one virtual block and assigned 75 percent of the whole to downstream traffic.
It's not especially challenging to implement TD-LTE, Schoolar said. Nor is it hard to hand off subscribers from those networks to LTE FDD systems, according to China Mobile and others. Despite the dominance of FDD, most existing LTE base stations can be set up for TD use with a software upgrade or a new line card, Schoolar said. Sprint plans to mix FDD and TD networks by using the Clearwire TD-LTE network for extra capacity in busy areas, shifting users from one to the other as needed.
China Mobile Hong Kong has already launched a combined TD and FDD network. It puts subscribers on TD-LTE where it's available, then shifts them onto FDD where possible, and puts them onto GSM when necessary. All these transitions are transparent to users, Lei said.
The pre-commercial network in mainland China is growing rapidly despite the fact that China Mobile can't offer commercial service yet. There are about 20,000 base stations there today and will be 200,000 in 100 cities by the end of this year, Lei said. And China Mobile is not expected to be the only Chinese carrier to deploy TD-LTE.
That bodes well for a high-volume market that should make TD-LTE devices cheap and plentiful in other parts of the world, with the help of big silicon vendors, analysts said. "It really depends on guys like Qualcomm to make it happen," Marshall said.