MIMO ON THE MARCH

Before the initial round of Fixed WiMAX network gear and customer premises equipment was certified in January, that milestone was seen as an extremely important phase in the development of a healthy WiMAX market. For a market chasing so much hype, Fixed WiMAX certification in reality may well have struck a note of significant progress, but it also seemed like a perfunctory step, the opening act before the headliner took the stage.

The headliner, in this case, is Mobile WiMAX. This commercial realization of the IEEE's 802.16e standard has been eagerly awaited by many vendors and service providers, to a large degree because it will bring mobility — limited portability at first, but eventually full mobility with seamless hand-off at vehicular speeds — to the evolving wireless broadband technology.

However, that's not the only reason that vendors and service providers are hungry for Mobile WiMAX. Chris Knudsen, chief technology officer for Intel's service provider group, said in a recent interview, “Mobile WiMAX will deliver a much more robust usage experience. The benefits of a wireless broadband experience can be had through Fixed WiMAX, but those benefits will be much richer with Mobile WiMAX.”

Using words like “robust” and “rich” to describe Mobile WiMAX might make it sound like a good cup of coffee, and in some ways, that analogy may be appropriate. Thanks to smart antenna technologies, such as multiple input/multiple output (MIMO) and beam forming, Mobile WiMAX may well deliver a shot of undiluted caffeine to the broadband wireless market.

“802.16e is the real WiMAX,” said Tom Flak, vice president of product strategy for SOMA Networks. “It's the one with the hooks for these smart antenna technologies.”

Perhaps it's the funny-sounding acronym, but MIMO (pronounced “mime-oh,” as often as it is “meem-oh”) has caught the telecom industry's attention of late — even though it's a technology based on a concept that has been waiting in the wings for broad commercial implementation for two decades. Based on the concept of multi-antenna arrays deployed at both transmitter and receiver locations, MIMO takes advantage of a traditional weakness of wireless technology by harnessing the once-unusable multi-path propagation that occurs in a single antenna transmission and using it to create additional transmission channels within the same frequency. When companies talk about MIMO in the modern sense, what they usually mean is a system with two antenna arrays at the transmission location and two antenna arrays inside the receiver, whether that is a modem, handset or other device. This translates to a doubling in bandwidth capacity and more efficient use of a single frequency.

Bell Labs researcher Jack Winters first filed a patent on multiple antenna arrays in the mid-1980s, and research continued through the 1990s until Gerard Foschini and M.J. Gans of Bell Labs wrote a paper in 1996 that refined the concept a bit and clearly stated how “multi-element arrays” could be used to deliver increased capacity in wireless local area network (LAN) deployments. Though the days of commercial Wi-Fi hot spots in coffee houses were years away, the paper caused a sensation in the wireless technology research community, and many other researchers contributed important findings and theories. Greg Raleigh, eventual co-founder of Airgo Networks, popularized the concept of MIMO in an orthogonal frequency division multiplexing (OFDM) architecture, which fueled a road map for the 802.11n wireless LAN standard. That standard was just ratified in January and will propel Wi-Fi past the 100 Mb/s bandwidth mark in the next year or two.

Meanwhile, MIMO has made its way from the LAN into the wide area network (WAN). The technology was specified in the 802.16e standard approved last December, and the WiMAX Forum has made it part of the certification profile for Mobile WiMAX.

“MIMO was specified in the 802.16e standard, but it's even more important what the WiMAX Forum has done — any Mobile WiMAX equipment will invariably include MIMO,” said Lars Johnsson, vice president of business development for Beceem Communications.

However, MIMO actually is classified as “implementation-optional” for Mobile WiMAX, according to Frank Van Heeswyk, chief scientist for software systems at SOMA Networks.

Byron Young, vice president of marketing and product management for Adaptix, added, “When vendors really will be expected to support MIMO isn't that clear. The second wave of Mobile WiMAX certification in the second half of next year will probably be the first that has testing and certification of MIMO-based systems.”

MIMO may have taken a while to go mainstream because it was viewed as complex and expensive to integrate into user devices such as handsets, vendor sources said. “You have to have two antennas on each side to take advantage of it,” said Johnsson, meaning that antennas on a mobile handset would be inches or less apart.

“One of the downsides is the effect of the cost of CPE,” said SOMA's Flak. “MIMO takes more space, and it takes more power.” While theoretically doubling capacity, actual increases may be less, and also may vary depending on how much bandwidth service providers are willing to allocate.

Still, Johnsson said that MIMO has benefited from its long journey into the WAN and the tremendous amount of research that has gone into the technology, and the result is a much-improved and more cost-effective technology.

“The algorithms have gotten better,” he said. “People can do more elegant number-crunching now, and that improves the signal-processing capability, which ultimately means cheaper devices.”

In addition to increasing the data rate, the ability to craft two usable channels in the same spectrum allows service providers to get more value out of the frequencies they own, which is particularly important as the WiMAX market's spectrum future remains in flux in many countries. Also, as WiMAX service providers increasingly look to offer voice-over-IP service and video service in addition to Internet access and other data applications, they can consider keeping the more sensitive applications like voice and video on a separate channel from data.

Initially, WiMAX systems using MIMO will be of the 2×2 (two antennas at the base station, two in the user device) variety, and that is the only approach mentioned in the Mobile WiMAX profile, but the bandwidth can be increased by continuing to add more antennas. Some companies have discussed the deployment of 4×4 MIMO solutions, but the cost and complexity challenges resurface as you up the antenna ante.

“The first wave of Mobile WiMAX gear that gets certified will be 2×2 MIMO,” Intel's Knudsen said. “There is no specific plan to do more than that, but as other silicon gets developed, more sophisticated multi-antenna systems will continue to come to market and get certified.”

While the WiMAX community awaits its MIMO experience, the technology also is being positioned as part of other mobile broadband genres. Not long after publishing his influential MIMO paper, Bell Labs' Foschini developed a MIMO-based technology for Lucent Technologies called Bell Labs Layered Space-Time (BLAST). The “space-time” referred to the fact that MIMO-based BLAST brought a spatial enhancement to the time, or frequency, domain wireless transmission. The technology was highly touted during 1998 as Lucent claimed demonstrated bandwidth increases of at least 10 to 20 times and more with experimental systems that used far more than just the two antennas being talked about in current systems. Lucent eventually announced plans to incorporate BLAST into its Flexent base station architecture.

However, a Lucent spokesman said last week that the company still has not commercialized BLAST because it hasn't yet seen the demand for vastly higher mobile bandwidth among its customers. The spokesman said Lucent is continuing to study the technology and monitor market demand.

Meanwhile, MIMO certainly looks to have some kind of future in the mobile broadband road map. Looking beyond today's 3G system, MIMO and OFDM were incorporated into the 3GPP/3GPP2 Long-Term Evolution (LTE) standards document in December 2005. Nortel Networks also recently demonstrated its high-speed OFDM packet access (HSOPA) technology, which employs MIMO, and has been an aggressive proponent of MIMO.

Doug Wolff, vice president and general manager of Nortel's CDMA group, recently said, “We're very keen on MIMO and OFDM being in the LTE and part of [CDMA 1X] EV-DO Rev. C. We think you'll see this as a commercial technology in 2008 or maybe 2009.”

In improving overall capacity, MIMO delivers on one part of the broadband wireless promise. The technology is sometimes misconstrued as providing network coverage enhancements, but it actually requires signal strength as a foundation for being able to provide its capacity enhancements. “MIMO has nothing to do with coverage, and it requires a strong signal, so you can't really do it at the cell's edge,” said Beceem's Johnsson.

But, Mobile WiMAX won't be lacking in the coverage department either because another long-heralded smart antenna technology called beam-forming (or adaptive antenna system, or smart beam-forming, by some) can be used to strengthen weak antenna signals. Beam-forming, as the name implies, enables antenna transmissions to be dynamically shaped to avoid the interference that weakens them.

“Beam-forming makes a weak signal stronger, but with MIMO, it's the rich getting richer,” Johnsson said.

However, some companies — including SOMA Networks, according to Flak — favor beam-forming over MIMO.

For that reason, the technologies might be used in different situations depending on the pre-existing coverage conditions, but also could be used in the same market. For instance, if there's a lack of coverage in a particular area, beam-forming might be used to rectify that situation, while in another zone, MIMO will enhance capacity for users because the signal already is strong enough. Beam-forming also could be used in advance of MIMO to create the right signal conditions for the latter technology to be employed.

“Beam-forming is something you might see first in an early network rollout, when network operators are always more fixated on coverage,” Johnsson said. “In the second stage of network life, it's all about capacity, and that means MIMO.”