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 coordinated 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.
