FDD/TDD: WiMAX and LTE Crossing Paths? - WiMAX.com

FDD/TDD: WiMAX and LTE Crossing Paths?
WiMAX.com
Monica Paolini

Oct. 15, 2009

At the ITU show in Geneva last week, there was a lot of talk on IMT-advanced technologies as expected. At this point, both WiMAX and LTE are obvious candidates for inclusion.

The timing is still uncertain, but inclusion in IMT-advanced seems to be uncontroversial for both technologies. What is interesting, however, is that the issue around duplexing is somewhat breaking down - with WiMAX using TDD (time division duplex) and LTE using FDD (frequency division duplex).

The WiMAX camp is pushing to have an FDD version of WiMAX as an IMT-2000 and as an IMT-Advanced technology (WiMAX TDD is already an IMT-2000 technology). Support for FDD is included in the IEEE standard-so nothing new from a standards perspective. Is there a market for FDD WiMAX though? Not much to date and this is mostly because TDD is almost universally accepted by WiMAX operators as the best option for the inherently asymmetric data traffic they have to transport.

Besides, there is currently no beamforming solution available for FDD wireless interfaces (either WiMAX or LTE-and this constitutes an advantage for WiMAX, although one that is seldom acknowledged). So the only reason to use FDD WiMAX is linked to regulatory requirements. Some WiMAX operators may have to use FDD in their allocated spectrum, but to date it does not seem that there are enough of them to justify FDD WiMAX 16e product development.

On the LTE side, there is a growing interest for a TDD version of LTE, mostly driven by China Mobile, but relevant to other mobile operators too, which often have TDD spectrum and do not know what to do with it. TD-LTE is currently being trialed in China by multiple vendors. Chipsets are being developed by Qualcomm, Ericsson, Huawei, Sequans, Altair. TD-LTE will support MIMO, but beamforming as well and will, like FDD LTE, use channel sizes up to 20 MHz.

A femtocell prototype from Nokia was also on display in Geneva in the China Mobile's booth. Combined (TDD and FDD) chipsets in subscriber devices will make it easier to roam across TDD and FDD networks. With China Mobile's commitment, TD-LTE has gained the needed momentum, but it still uncertain where and how TD-LTE will be deployed in other countries.

At this stage, TD-LTE appears to have better prospects than FDD WiMAX, but it is interesting to see that while it is not possible at this stage for WiMAX and LTE to converge into a single standard, they seem to increasingly cross paths and move in the same direction.