The WiMax Fixed-to-Mobile Evolution

In December 2005, Wavesat announced that its Chief Scientist, Dr. Jonathan Labs, was named Chair of the WiMax Forum’s new Evolutionary Task Group, created to develop technical specifications for the evolution of WiMax standards from fixed (802.16-2004) to mobile (802.16e). The key challenge for the group lies in ensuring a smooth transition between the two standards.

What makes the transition between fixed and mobile particularly challenging, Labs says, is the fact that the WiMax Forum is supporting two 802.16 technologies – OFDM 256 and scalable OFDMA. “What WiMax has adopted for fixed is the OFDM 256 mode – and for mobility, they’re pushing the scalable OFDMA mode,” he says. “And they are incompatible technologies, as they exist today.”

According to Labs, each technology has particular strengths for particular applications. “The difference between the two is how they deal with the channel characteristics,” he says. “In a mobile environment, your channels are going to be rapidly varying, and they’ve designed the PHY around that fact. And the more mobile you get, the more challenging the channel is to deal with.”

A Portable Compromise

The group’s first task, Labs says, is to define the profiles that enable portable operation – with the aim of being able to certify portable equipment relatively soon, by the end of 2006. “The real challenge is the timetable that we’re working under, and being able to provide all the documentation and test procedures and test cases that are needed to certify that products are built according to the standard,” he says.

With service providers soon to be deploying fixed WiMax equipment, Labs says the idea is to give them a clear migration path from fixed to portable, and then to mobile. “So we are using the OFDM 256, and adding the features that will allow at least portable operation,” he says. “Portable is mobile, but maybe not as fast – it’s a limited mobility.”

With fully mobile WiMax not expected until 2007 or 2008, Labs says the portable solution is intended to build upon the fixed standard and provide limited mobility in the interim, while requiring only a basic update to systems that are already deployed. “We are trying to make it as simple as possible in terms of the enhancements that are needed,” he says. “The hope is that it would be mostly a software upgrade.”

Looking Ahead

The real driving force behind this effort, Labs says, is competition. “WiMax has the 3G market to compete against,” he says. “They’re trying to make their foothold into the broadband wireless mobile market, in terms of being able to have things rolling out, showing that there is this upgrade path as we move on to providing mobile WiMax products.”

Due to the lack of compatibility between fixed and mobile WiMax, though, Labs anticipates that dual mode chips will eventually have to be used to allow full interoperability between the two. “Unless there are changes introduced in the standards to allow them to be more compatible, it would have to be some sort of dual mode,” he says.

Labs says the Evolutionary Task Group has been meeting unofficially since June, which means a lot has already been accomplished – but there’s still an enormous amount left to do. “We’ve gotten through the first round of profiles, so we’re getting into the stage now that we will have to start putting together the test cases and test procedures for validating equipment conforming to this technology,” he says.

A related white paper entitled “Can OFDM Enhancement Drive WiMax Mobility Forward?” is available for download from Wavesat’s Web site.