Developers Brainstorm About Smart Antennas

The worldwide surge in the deployment of fixed wireless networks as pushed development of smart antenna technology into high gear, raising hopes that serious interference and capacity problems associated with over-the-air applications will soon be overcome.

A hint of what's in store can be found in and around the New Jersey town of Crawford Hill, where engineers from Lucent Technologies' Bell Laboratories are putting cutting-edge antenna technologies to the test in a wide range of networking conditions at various frequency tiers now being used for fixed wireless applications. Using military communications trailers with mobile transmitter towers that extend skyward by 180 feet, members of Bell Labs' Wireless Research Laboratory can operate three base stations in virtually any networking configuration as they work out the finer details of a new integrated approach to using various types of smart antenna technologies.

"We've come a long way in figuring out how to integrate and apply our innovations in real-world networking situations," says Rich Howard, director of the wireless lab at Lucent. Now, he adds, it's a question of when market demand will drive systems integrators to begin making use of the technology.

"If you believe there's going to be a widespread market push for wireless data, wireless operators will have no choice but to use every trick in the book to improve bandwidth efficiency," Howard says. "What we're working on is the biggest trick of all."

It's A BLAST

Based on what they've accomplished so far, Lucent's wireless lab group is confident the advanced antenna techniques could be quickly put to use in network systems to achieve tenfold to twentyfold in-creases in capacity over a given wireless link and to overcome the interference problems that will intensify as use of wireless spectrum for delivering services in competition with wireline networks accelerates.

These techniques include BLAST, an innovation announced late last year that involves the use of multiple transmit and receive antennas to exploit the multipath nature of wireless communications. BLAST technology combines the use of multiple antennas with "steerable" antenna beam technology to deliver more robust wireless communications for geographical areas ranging from urban centers to suburban fringes, where base station operating range is a bigger issue than density.

BLAST uses Lucent-developed algorithms to assign specific signals to specific transmission paths in the multipath dissemination of a radio wave at a given frequency, thereby allowing reuse of the frequency many times over for delivery of different messages to and from different users. In contrast, Lucent's multibeam antenna system serves multiple customers over the same frequency by moving a wide beam from customer to customer in quick, millisecond hops that are timed to coincide with the time slots assigned to each customer in a Time Division Multiplexing (TDM) configuration.

While BLAST is ideal for an urban environment, where densely packed buildings create the reflective patterns needed for multipath communications, the steerable beam technology is good for increasing the information-carrying capacity in suburban areas, Howard notes. By using multiple antenna arrays at the transmitter and receiver, Lucent has found it can combine the two techniques to maximize the benefits of each, depending on where a base station is located, he adds.

"The hardware is identical - multiple antennas with multiple radios," Howard says. The key, he explains, is to activate the hardware via algorithms that match the local situation. Digital signal processors are still too expensive to allow for multiple antennas at the receiver end, Howard says, but, within the next year or so, the cost of DSPs could fall far enough to make feasible commercial applications of the integrated advanced antenna system.

Faith In Angel

Nobody is more interested in deriving higher data-carrying capacity from limited spectrum than AT&T Wireless. Com-pany executives have spoken often about the long-promised Project Angel as a service that will deliver high-speed data and voice services over cellular, personal communications service (PCS) and other spectrum as an alternative to wireline access. But despite repeated mention of Project Angel's potential, officials continue to refuse to go into technical details or to describe ongoing tests as they attempt to drive the system to market.

Nonetheless, it is clear that smart antenna technology plays an important role in Project Angel, as evidenced in tests the company has conducted with various vendors. One of those companies, Radix Technologies, is pursuing a combination of smart antenna and other techniques on the theory that companies with limited spectrum availability like AT&T Wireless are going to need at least a tenfold increase in bandwidth efficiency over what they're seeing today.

"One issue is the rate at which information can be transmitted," says Bob Kelsch, vice president of wireless products at Radix. "The second issue is, if you can create a link that delivers information at the desired speed, how many of those links can you supply within a given amount of spectrum?"

Kelsch believes the market for fixed wireless data services will require delivery at speeds of 1.5 megabits per second, or 10 times the rate envisioned for initial applications of next-generation data delivery systems over mobile links. "We're creating a whole new air link technology with multiple elements, including adaptive antenna arrays, to meet these requirements," Kelsch says, but he declines to say when his company will have product available or to discuss any role Radix might play in bringing Project Angel to market.

Matching Spectrum

Smart antennas factor well into the portable data world because they enhance the capacity of networks that operate at lower frequencies, such as 2 gigahertz to 5 GHz. These lower frequencies provide the ubiquitous coverage essential for portable services - something that is harder to do at the very high-frequency allocations used by Local Multipoint Distribution Service and by LMDS-like services at 24 GHz and 38 GHz used by broadband wireless operators like Advanced Radio Telecom, Teligent and WinStar Communications. Such services typically offer hundreds of megahertz worth of bandwidth to work with but have to be engineered in perfect line-of-sight matchups between transmitters and receivers.

One of the leading operating companies seeking a way around these problems is Metricom, which recently drew capital infusions of $300 million each from MCI WorldCom and Vulcan Ventures, the investment company controlled by Microsoft cofounder Paul Allen. Metricom is delivering its 28.8-kilobit-per-second Ricochet wireless data service in three markets over a 26-kilohertz slice of spectrum in the unlicensed band at 902 MHz, with plans to roll out 128-Kbps service in 46 cities nationwide by the middle of next year using unlicensed and licensed spectrum at the 2.3-GHz tier.

"Our ultimate goal is to allow mo-bile professionals to take their full desktop functionality anywhere they need to work," Metricom Chief Executive Timothy Dreisbach says. "This means accessing data and applications on corporate LANs [local area networks] and the Internet anywhere in the same fashion as sitting in an office."

In a recent study, International Data Corp. predicted that by 2001 the market for Metricom's services in territories served by the company will consist of 7.3 million professionals who carry cellular/PCS phones, laptops and other mobile computing devices, growing to 13 million by 2005.

"Critical success factors in this segment of the mobile data market are the ability to provide cost-effective, reliable mobile Internet and LAN access throughout the major metropolitan areas," says Iain Gillott, vice president for worldwide consumer and small business telecommunications at IDC.

Metricom has one prospective technology partner: ArrayComm, which has built its business on a proprietary active array antenna technology now being deployed in Brazil, Japan, Malaysia and the Philippines. ArrayComm would like to team up with Metricom to help carriers expand their potential capacity with use of smart antenna technology, says Martin Cooper, ArrayComm's chairman and CEO.

The MCI WorldCom-Metricom tie-in, which will result in a 38 percent ownership stake for MCI WorldCom once the deal closes, coincides with MCI WorldCom's acquisition of wireless spectrum at the Multichannel Multipoint Distribution Service (MMDS) frequencies, where, again, limited spectrum comes into play as a barrier that can be addressed by smart antenna technology. Here the goal is to complement the Metricom service with a fixed wireless data service targeted to business users.

Recycled Cycles

While MMDS operators have 200 MHz to work with at the 2.5-GHz tier, they must spread that spectrum among users across a much larger serving area than is the case with the small areas served by base stations deployed by PCS, cellular and unlicensed providers such as Metricom. MCI WorldCom and Sprint, which also acquired major MMDS holdings earlier this year, have both indicated they intend to "cellularize" their MMDS territories by putting in multiple transmitters, thereby supporting reuse of the 200 MHz of frequency several times per area.

But this is easier said than done, given the significant interference problems that can occur in a multiple-transmitter environment where signals can travel 30 or more miles from each transmitter site. And even though propagation distances are only one-tenth that distance at LMDS frequencies, a dense clustering of LMDS transmitters poses interference issues as well.

One of the companies applying smart antenna technology to the higher-frequency fixed broadband interference issues is Raytheon, which is tapping into technology developed through its E-Systems division for mobile networks along with creating innovations specific to fixed wireless networks. "We've been working on problems associated with frequency reuse since 1992, and we think we've come up with some interesting ways to enhance performance with regard to both sectorizing at the hub and optimizing CPE [customer premise equipment] to get more bits per hertz," says Dave Lacinski, manager at Raytheon's terrestrial broadband business unit.

One of the attributes of the technology that Raytheon is focused on is support for very high-dynamic-range systems, where networks can operate in a very linear mode in fairly dense environments or less so where there's less potential for interference, Lacinski says. "Some things we're looking at provide very high-frequency selectivity and agility in the recovery of signals, which is something we can see coming as a requirement for all these wireless categories," he adds.

While Lacinski is circumspect about the techniques under development, his allusion to E-Systems provides one important clue, since that unit was one of the first to deliver adaptive array antennas for cellular networks. While such antennas are intended for use in mobile networks, where they pick up the path delivering the strongest signal and eliminate all the others on the same frequency, the basic technique also can be used with steerable beams to provide a highly focused, interference-free connection along with reuse of frequency in the TDM described by Lucent's Howard.

Lacinski, declining to comment on such capabilities, mentions that another technique, known as circular polarization, is coming into play as a way to minimize interference. Circular polarization uses the shape of the antenna to differentiate between signals on the same frequency. Lacinski adds that Raytheon is looking at supplying flat-panel as opposed to parabolic horn antennas in areas where customers want them.

The microwave communications division of Harris is also responding to market demands for flat-panel antennas, which don't use microprocessors - as is the case with "smart" antennas - but do provide significant protection against unwanted interference. Harris, which is working with Wavtrace to integrate their products into platforms suited for MMDS and other microwave and fixed wireless bands, is exploring the use of flat panels in point-to-multipoint applications, says David Giguere, senior manager of business development at the microwave division.

"Flat panels operate at much lower radiated power on the side lobes than the typical parabolic antennas do," Giguere says. This means less interference is generated by users over the return paths than might be the case using parabolic antennas, he explains.

But when it comes to resolving the more complex interference issues regarding densely clustered transmitters operating over high-frequency tiers, the wireless industry is going to have to turn to smart antennas, Giguere says, noting that Harris is working on such technology for point-to-multipoint millimeter wave applications. "Sectorization is a brute force approach to reusing frequencies that becomes far less effective as you deploy more transmitters," he says.