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All Hail OFDMA!

OFDMA (orthogonal frequency division multiple access) is emerging as the clear choice for the next-generation radio interface in mobile wireless networks, said Mick Reeve, group technology officer at BT Group plc (NYSE: BT; London: BTA), in his keynote at Light Reading’s Future of Telecom Europe event in London yesterday.

Reeve said that “almost everyone agrees” OFDMA will be the next major evolution in mobile wireless technology, but he noted there are unanswered questions about how, when, and by whom it will be implemented.

“Mobile guys are saying it’s just an enhancement to 3G, and WiMax guys are saying it’s WiMax,” said Reeve. “We’ve seen agreement on technology, but there’s a lack of clarity on standards and spectrum.”

OFDM works by dividing a given spectrum block into a number of sub-channels, each of which is individually modulated and then transmitted orthogonally to minimize interference with one another.

The result is faster, more robust radio links than can be achieved with alternative modulation/multiplexing schemes. It is often claimed, for example, that OFDM is more efficient, in terms of bits per Hertz, than CDMA and WCDMA (wideband code division multiple access) technology used in 3G systems, and/or delivers better performance in non-line-of-sight urban environments due to better resistance to multipath interference.

For these reasons OFDM has been adopted as the radio interface for 802.11g and 802.11a (but not 802.11b), for digital TV broadcast systems such as Digital Video Broadcast (DVB), and for Fixed WiMax systems based on the 802.16-2004 standards. Now the adoption of OFDM in more demanding cellular systems is starting to have an impact on the mobile infrastructure market.

Specifically, the demands of mobility and two-way communications are pushing the industry towards a variant of OFDM called OFDMA , which can assign a subset of sub-carriers to individual users. This is the radio interface being adopted by Mobile WiMax, via the upcoming 802.16e specifications, and being implemented in the Korean WiBro equipment.

For non-line-of-sight applications, OFDMA is now considered superior to the 256-carrier OFDM specified by the Fixed WiMax 802.16-2004 standards to the extent that some in the industry (mainly those not buried under a mountain of time and money sunk into Fixed WiMax) believe that fixed wireless broadband access will, in fact, become just an application of Mobile WiMax, due to its superior physical layer and anticipated higher unit volumes.

The OFDM physical layer is also being developed by mobile system providers currently outside the WiMax sphere. At this point, terminology gets seriously confusing and the stakes get even higher.

Qualcomm Inc.’s (Nasdaq: QCOM) recent $600 million acquisition of Flarion Technologies for its “FLASH-OFDM” technology was widely seen as public recognition by Qualcomm that its CDMA hegemony is under long-term threat from newer OFDMA systems. Interestingly, in its announcement of the acquisition, Qualcomm employed OFDMA terminology, even though Flarion had not previously used the term publicly.

Meanwhile OFDMA pioneers, such as Adaptix Inc., are working to differentiate themselves by presenting their technology as OFDMA/TDD -- which works by assigning sub channels to individual users dynamically in time. (See Adaptix, NY3G Team Up ).

There is also a widespread expectation that OFDMA will be adopted by the 3rd Generation Partnership Project (3GPP) through its Long Term Evolution (3G LTE) working group, also referred to as “Super 3G.” The difference here is that it’s likely to be several years until anything much happens. This is because the world’s 3G systems providers and operators have more pressing issues to deal with, including implementation of HSPA (High Speed Packet Access) and EV-DO Rev A radio interface upgrades.

— Gabriel Brown, Chief Analyst, Unstrung Insider

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