In 2003, Nortel Networks rolled out of its labs a cellular radio that managed to shove 20 Mb/s of capacity into a single 5 MHz PCS channel. In July 2004, Motorola Labs packed 300 Mb/s into a 20 MHz channel and went one step further: It took its show on the road, driving its prototype receiver up and down Chicago's highways, demonstrating throughputs exceeding 20 Mb/s while breaking freeway speed limits. And in December 2004, Siemens unleashed from its Munich labs its own wireless wonder, packing 1 Gb/s of capacity into a swathe of spectrum 100 MHz wide.

All of these trials had one thing in common: They were based on orthogonal frequency division multiplexing, or OFDM, technology and variations of new multiple input/multiple output (MIMO) smart antenna technology. OFDM is nothing new — its core multiplexing principles have been applied to everything from satellite broadcast to ADSL. Over the last decade, the technology has played a critical role in wireless, forming the basis of the IEEE 802.11a standard and, most recently, the critical multiplexing scheme for the WiMAX Forum's next generation of wireless broadband specifications.

But OFDM might have an even bigger role to play as vendors and the standards bodies they occupy warm up to the idea of OFDM as a mobile cellular technology. Vendor after vendor is developing new prototype gear based on OFDM and submitting proposals to the GSM-based 3G Partnership Project and its CDMA-oriented counterpart, the 3GPP2. At the heart of those proposals is the goal of making OFDM the basis for 4G.

“We're very high on OFDM,” said Paul Sergeant, product marketing director for Motorola's alternative network solutions group. “Though I wouldn't say it will be the only physical layer candidate for the next-generation cellular networks, right now there are not a lot of competing technologies beyond OFDM.”

OFDM's supporters believe the technology will be the key to cutting down on the multipath distortion inherent in single-carrier cellular networks, leading toward greater spectral efficiency and ultimately broadband speeds unthinkable on today's networks. As the technology's name implies, OFDM splits a single-carrier signal into multiple signals, dividing the transmitted data among them. In a single-carrier system, a data call is sent on a single signal stream from transmitter to receiver. But the individual signals in that transmission don't make a beeline for the receiver. Radio waves bounce off the ground and off buildings or bend around obstacles. The result is that two bits of data sent simultaneously from a transmitter don't arrive at the receiver at the same time. This mixing of bits results in distortion and the degradation of the original message.

Today's wireless technologies easily sort out this distortion with equalization technologies, but as capacity increases over any given network, more data is being packed into the same amount of spectral bandwidth, compounding the effects of multipath distortion.

OFDM's answer to this problem is to divide one extremely “fast” signal into numerous “slow” signals, each spaced apart at precise frequencies. While each of those individual subcarriers is subject to the same multipath distortion faced by a single-carrier transmission, the data is traveling slowly enough that the effects of the distortion become negligible. The numerous slow transmissions are then all collected at the receiver and recombined to form one high-speed transmission.

Until recently, OFDM has not been economically feasible for mobile wireless networks, namely because of the “orthogonal” part of its acronym. The orthogonal aspect of the technology is the complex spacing between each data-carrying frequency. That spacing is constantly changing throughout a transmission, requiring a series of complex calculations for the most minute re-tuning. Making the chips with that kind of processing power for commercial handsets and laptop cards just wasn't feasible, but today, silicon technologies have finally caught up.

In November, the 3GPP decided to create an official OFDM project to explore implementing the technology into future specifications for cellular technology. Though no decision was made on where OFDM would fit into the GSM evolution ladder, some vendors are saying it may fall right behind the already finalized specifications for high-speed downlink packet access (HSDPA) and high-speed uplink packet access (HSUPA). Though vendors have said they have begun submitting proposals to 3GPP2, the standards body hasn't said anything publicly about OFDM as a candidate for the next evolution of CDMA 2000 networks. But vendors said it is only a matter of time before the 3GPP2 begins weighing the technology.

Dan Tell, a fellow on Motorola's technical staff, said that the trend in broadband data is toward wider channels. The spread spectrum CDMA technology on which current 3G networks are based won't be able to cut it when faced with the demands of greater bandwidth.

“Below 5 MHz, CDMA still has legs,” Tell said. “But if you're going to wider channels, you have to go beyond spread spectrum technologies. You can have 10 MHz channels over OFDM but not over CDMA.”

So far, CDMA stalwart Qualcomm has made OFDM the basis for its CDMA MediaFLO broadcast technology (see story page 30), and other CDMA-centric vendors have begun developing the technology for ultrawideband and WiMAX products. But 3GPP2 adopting OFDM as a future cellular technology is definitely not a given. According to some vendors, CDMA still has a lot of overhead clearance for increasing speed and capacity, making any consideration of OFDM premature.

“At this point, there's been nothing to convince us that OFDM-based technologies are superior to the technology we have today,” said Sanjeev Verma, vice president of marketing for Airvana, a vendor specializing in 3G CDMA 2000 technologies. “We haven't seen any compelling enough advantage in OFDM for us to consider switching from CDMA. In fact, the two technologies appear to be performing roughly the same.”

The best OFDM projections show that the technology will have twice the spectral efficiencies of today's CDMA-based 3G technologies, a gap that CDMA vendors could easily overcome as they revise standards. But other OFDM's proponents claim that the technology's benefits aren't in overall capacity, at least not initially. OFDM will act as the foundation to build faster and more efficient technologies in the future. OFDM's multiple carriers allow them multiple antenna and multiple path technologies such as MIMO, which can exponentially increase an OFDM network's speed but can't be deployed over a CDMA network, said John Hoadley, Nortel vice president of next-generation access.

“Inherently, OFDM itself doesn't offer huge speed increases or capacity increases over time,” Hoadley said. “It's where we take OFDM that we'll see the advantages.”

Most vendors believe there won't be a commercial OFDM cellular network before 2009 or 2010, though some have optimistically set their sights for 2008, putting it right in the time frame that carriers will be deploying HSUPA or one of the further revisions of CDMA 1X EV-DO or EV-DV. But Flarion Technologies asks, why wait?

The broadband wireless vendor has optimized its proprietary Flash-OFDM gear for cellular networks and already has two major trials of its technology running. T-Mobile has augmented its GSM network with Flash-OFDM in the Netherlands, and Nextel has launched trials in North Carolina (though its pending merger with Sprint may kill that project). Flarion's system boasts downlink speeds of 1.5 Mb/s per sector per carrier with an uplink of 350 kb/s and sub-50 millisecond latencies. Flash-OFDM's main drawback is its a proprietary access technology that can't easily interoperate with other technologies, but according to Ronny Haraldsvik, Flarion's vice president of global communications, standards are becoming less and less of a concern for carriers.

“It's no longer about standards,” Haraldsvik said. “It's about differentiating yourself and your network. Networks are already becoming standardized under IP, so carriers now have the freedom to pick and choose the best access technology.”

Haraldsvik said carriers wouldn't wait years for standards approval if alternate proprietary OFDM technologies were available today. While a Flash-OFDM handset or data card may not be able to roam on a 3G network or future OFDM-based 4G networks, the costs of integrating disparate networks are falling rapidly. Carriers no longer have to custom build ever aspect of their networks around a particular technology as IP makes its way to the base station, and the cost of dual-mode devices will become negligible compared with the revenue boost for launching unique broadband data services, Haraldsvik said.

“I have no doubt OFDM will make its way into the standards. It's not a matter of if, but a matter of when,” Haraldsvik said. “But OFDM already has a lot of momentum. This train isn't stopping to wait for the standards bodies.”

While 3GPP and the 3GPP2 may be years away from incorporating OFDM into their 4G specifications, one standards body already has a jump on the technology. The WiMAX Forum has made OFDM the central technology in its standardization efforts of 802.16 technologies, and its first truly mobile broadband specification based on 802.16e will be finalized this year. As WiMAX's traditionally data-centric technology becomes more mobile and cellular's mobile-centric technology becomes more data-hungry, the two sectors will begin to overlap.

And if both technologies are based on OFDM, the differences between the technologies may decrease even further. According to WiMAX Forum board member Mo Shakouri, who is assistant vice president of business development for broadband wireless vendor Alvarion, the WiMAX and cellular technologies will co-exist in their separate sectors for the next five to 10 years — but after that point, the networks will eventually start mixing, destroying the distinction between a broadband wireless and mobile cellular network. The question is, if the cellular standards group adopts OFDM, will they essentially create their own version of WiMAX for their networks?

Shakouri thinks not. WiMAX, by that point, will achieve a penetration among data users on par with the way Wi-Fi has infected laptop users today, and customers will push their cellular carriers to incorporate the technology on their networks, Shakouri said. That said, the forum must ensure that its networks interface with those of the 3GPP and 3GPP2, but with most of the cellular network vendors already participating in the WiMAX Forum, that shouldn't be a problem. If WiMAX can keep its technological edge, it could be the standards body that pushes OFDM into the 4G network.

“That's the key,” Shakouri said. “We have to be better than the next-generation network technology.”

VENDORS WORKING ON OFDM CELLULAR TECHNOLOGIES

Flarion Technologies

After Lucent Technologies' Bell Labs devised Flash-OFDM, it launched Flarion to develop and commercially market the technology. Flarion now has trials running with two major cellular carriers: T-Mobile and Nextel.

Motorola

In July, Motorola was the first major cellular vendor to take its OFDM/MIMO trials out of the lab, demonstrating speeds of 20 Mb/s and latencies of 25 milliseconds while traveling at 62 mph over Chicago's freeways.

Nortel Networks

One of the earliest proponents of OFDM, Nortel has been testing the technology since 2000. In February 2003, Nortel demonstrated OFDM/MIMO peak data rates of 20 Mb/s over a standard 5 MHz PCS band.

Qualcomm

Qualcomm has incorporated OFDM into its multicasting technologies: 1X EV-DO Platinum and FLO. The EV-DO evolution is intended for carriers wanting to broadcast digital media over their current networks and spectrum, while FLO is a specialty technology intended for establishing dedicated digital broadcast networks.

Siemens

Siemens has been trialing OFDM and MIMO in its labs and in December announced achieving a transmission speed of 1 Gb/s over a 100 MHz swathe of spectrum.

WiLAN

WiLAN has been optimizing its Wideband OFDM (W-OFDM) for mobile networks with hopes of applying it as an eventual 4G cellular technology. While it has demonstrated that its Libra fixed-wireless product can be used in a mobile environment, in October, it released Mobilis, a W-OFDM system specifically tailored for high-speed mobile networks.