Moving to the front of the pack
When it comes to wireless transmission technology, the 1960s are fashionable again.
Orthogonal frequency division multiplexing (OFDM) technology, developed more than 40 years ago and first used in broadcast television stations in Europe, has emerged as the de facto standard in the broadband wireless world. And it has crept into standards-setting bodies governing mobile wide area networks that currently are dominated by CDMA technology.
“OFDM is not a technology that was invented yesterday, but the complexity of the implementation has been a challenge,” said Bruce Gustafson, director of marketing for carrier networks for Nortel Networks, which is investing a significant amount of resources in WiMAX and next-generation technologies based on OFDM Access, or OFDMA. “It has come into its own the last couple of years.”
Now OFDM and its derivatives are everywhere. Engineers have concluded that OFDMA can fulfill the ever-growing desire for faster data transmission speeds in a spectrally efficient manner. Meanwhile, OFDM already is transforming fixed wireless standards, moving 802.11-based technologies from practical speeds of 11 Mb/s to once-futuristic transmission speeds of 200 Mb/s and beyond in 802.11n. Furthermore, OFDM is used by Qualcomm’s MediaFLO technology to transmit mobile television services to handsets.
OFDMA also is the basis for the emerging and much-hyped broadband technology known as WiMAX, in both fixed and mobile standards. Sprint Nextel gave the technology a major boost with its plans to deploy Mobile WiMAX (802.16e) some time in 2008 in the 2.5 GHZ band. Beyond 802.16-based WiMAX products, the wireless broadband market consists of FLASH-OFDM, a proprietary technology developed by Qualcomm’s Flarion division.
Perhaps the most important nod for OFDMA comes from the world’s two 3G standards-setting bodies, which are making OFDMA the basis of the wideband-CDMA (W-CDMA) world’s 4G standard, called long-term evolution (LTE) and CDMA 1xEV-DO Rev. C, otherwise known as ultra mobile broadband (UMB).
Peter Jarich, analyst with Current Analysis, predicted in a recent research report that OFDMA-based technologies would overtake CDMA-based 3G in the next decade as the “gold standard” for mobile broadband. “It might be WiMAX. It might be LTE or even EV-DO Rev. C.,” Jarich said. “Whatever form it comes in, OFDMA is well understood as the follow-on to today’s 3G.”
Continually increasing data transmission speeds is the name of the game, and that’s where OFDMA comes in. It promises high spectral efficiency, resiliency to radio-frequency interference and high data speeds because a radio signal is split into several narrowband channels at different frequencies.
Though often mistaken for one another, OFDM and OFDMA actually are two different variants of the same technology. Both divide one extremely fast signal into numerous slow signals, each spaced apart at precise frequencies. The advantage of each is that those individual slow signals, or subcarriers, aren’t subject to the same intensity of multipath distortion faced by a single-carrier transmission because the data is traveling slowly enough that the effects of the distortion become negligible. The numerous subcarriers are then collected at the receiver and recombined to form one high-speed transmission.
The difference between OFDM and OFDMA is that the latter has the ability to dynamically assign a subset of those subcarriers to individual users, attuning the technology to the particular demands of mobility. Thus, OFDM technologies occupy nomadic, fixed and one-way transmission standards, ranging from TV transmission and Wi-Fi to Fixed WiMAX and newer multicast wireless systems such as Qualcomm’s MediaFLO. OFDMA, however, adds true mobility to the mix, forming the backbone of Mobile WiMAX, LTE and UMB. Furthermore, scalable OFDMA allows for an increase in range of channel bandwidths from 1.25 MHz up to 20 MHz.
A major validation for OFDMA-based technology will come from commercial deployments of Mobile WiMAX technology. The biggest coup for the technology has come from Sprint Nextel, which is spending an estimated $2.5 billion to $3 billion, according to the carrier, on a platform that could be leveraged by an estimated 100 million people by 2008.
Full mobility is still a question mark for many OFDMA deployments, however. Korea Telecom and SK Telecom last summer launched WiBro, a technology based on the WiMAX standard, in limited parts of Seoul. So far the network only supports limited mobility and lower-speed applications. WiBro is considered an early version of Mobile WiMAX and does not include key features such as advanced antenna systems, including multiple input/multiple output (MIMO) systems. MIMO and beam forming can significantly increase range and increase data speeds for most end-users, while also lowering investment costs for operators (see story on page 42).
WiBro base stations are designed to offer an aggregate data throughput of 30 Mb/s to 50 Mb/s, but Allen Nogee, analyst with In-Stat, said that once users move faster than 20 mph, the network experiences latency problems. “Many standards work great standing still but when they go faster is when problems arise,” Nogee said.
Andy Fuertes, analyst with Visant Strategies, agreed it’s dangerous to declare OFDMA as the panacea for better capacity and higher data speeds.
“The jury is still out on which implementations can take advantage of OFDMA’s advantages. Certain implementations of OFDM can actually cause self-interference,” he said. “It all depends on the implementation.”
Qualcomm’s FLASH-OFDM technology is one OFDMA-based technology that has proved effective in a highly mobile environment. This wideband spread-spectrum technology divides spectrum into a number of equally spaced tones or frequencies, which ensures there is no interference between users on the same cell. The IP-based technology is spectrum-agnostic, can operate in interference-riddled spectrum and is able to transmit data at peak rates of 3 Mb/s, with average throughputs of 1.5 Mb/s. Several major operators trialed the technology but were concerned about its proprietary nature.
Qualcomm acquired the technology when it bought Flarion in 2005 but has yet to announce any plans for the technology or whether it will enter the WiMAX market using it.
Meanwhile, Nortel concedes that WiMAX won’t be perfect coming out of the starting gate, but said its network simulations have shown that fully Mobile WiMAX works. “This will not hit the ground without hurdles,” said Richard Lowe, president of Nortel’s mobility and converged core networks group. “It will get put in the ground and it won’t be nationwide because that is not the business model a lot of people are using. It will get shaken out.”
Another question mark for WiMAX is its actual cost, both in terms of capital and operational expenditures. Though the WiMAX community long has touted WiMAX as being “one-tenth of the cost” of 3G networks, some analysts say that although WiMAX may offer better spectral efficiency, it consumes more battery power and requires more base stations to achieve coverage and penetrate walls. In fact, analysts at Pyramid Research have said they don’t believe WiMAX will be cheaper than existing 3G networks on the capital expenditure front.
Ron Resnick, president and chairman of the WiMAX Forum, debunks such notions. He said every operator would use different assumptions on the design and buildout of the network. “It’s like you can stack the deck any way you like,” he said.
For instance, Sprint Nextel makes some impressive claims about the planned cost of its WiMAX networks, which are in part based on leveraging existing network assets and on the use of a 20 MHz channel, which the WiMAX Forum doesn’t support yet.
But one major advantage that could positively affect costs is WiMAX’s ability to achieve network handoff in various modes, Resnick said. Since most WiMAX operators won’t introduce voice-over-IP services initially, operators have the option of using a hard handoff network architecture.
“That is what is scaring other players to death,” Resnick said. “WiMAX’s hard-handoff model needs fewer base stations and there is less burden on the system. You can space the base stations further apart so it actually costs less for the base station. For voice … you have to have soft handoff and more base stations.”
THE ADVANTAGES OF OFDM
- Immunity to delay spread and multipath
- Resistance to frequency selective fading
- Simple equalization
- Efficient bandwidth
THE DISADVANTAGES OF OFDM
- Synchronization
- Need FFT units at transmitter, receiver
- Sensitive to carrier frequency offset
- High peak to average power ratio
Source: CCU Wireless Access Tech Lab
