Rise of the Smart Antenna

A few years ago, the future looked bright for smart antennas. Visionaries spoke of fixed antennas that could follow mobile users around electronically, creating what might be called personal communications tractor beams. Smart antennas promised to increase range, reduce interference and boost network capacity.

But this smart idea has taken longer than expected. Even proponents agree smart antennas still are too complex and costly for general use. They only have managed to escape the laboratory by focusing on niche applications. Widespread acceptance and use of smart antennas is just a matter of time.

A couple of vendors have achieved limited commercial success. ArrayComm is marketing its adaptive array antennas for use over Personal Handyphone System (PHS) networks in Asia and Latin America. Metawave is selling its beam-switching antennas for use over AMPS and CDMA networks and plans to enter the GSM market later this year. And researchers at Virginia Tech's Mobile and Portable Radio Research Group (MPRG) continue to explore ways to make smart antennas as common as dumb aluminum.

ADAPTIVE ARRAYSArrayComm has remained true to the original vision: antennas that create -- and maintain in real time -- unique beam patterns for each user. But it doesn't involve custom, directional antennas. Instead, ArrayComm's IntelliCell solution uses four to 12 off-the-shelf, omnidirectional antennas per base station. In essence, a radio beam is shaped for each user by carefully distributing that user's signal among multiple antennas. The "smarts" to do that are located not in the antennas themselves, but alongside the radio transceivers.

Advanced adaptive arrays that work on both the uplink and downlink are easiest to implement with air interfaces that transmit and receive on the same frequency. Why? Because information collected from the uplink signal can be applied immediately to the downlink signal. For air interfaces that transmit and receive on separate frequencies, a feedback loop (requiring special handset support) would be needed to optimize the downlink.

Thus, ArrayComm has focused its efforts on one of the few technologies in commercial use that uses time division duplexing (TDD) to transmit and receive on a single frequency: the PHS. ArrayComm also is looking at developing smart antennas for other TDD technologies, some of which employ variants of CDMA.

ArrayComm's PHS-focus may seem surprising. PHS is used almost exclusively in Asia. And it's been widely reported that the total number of PHS subscribers is declining. ArrayComm believes PHS has gotten a bad rap: The decline in total subscribers is due to one operator's (NTT of Japan) faltering PHS business. A competing Japanese operator, DDI, has built a robust and profitable PHS business.

ArrayComm hastens to point out PHS' unique strengths. Besides offering a low-cost solution, PHS employs 32kb/s transmission to achieve landline audio quality. ArrayComm also points out PHS carriers soon will deliver 64kb/s data services and already are supporting 32kb/s data. True, PHS did suffer early on from limited capacity and range, but both parameters have been i mproved greatly through the use of smart antenna systems.

ArrayComm claims it has proved smart-antenna systems actually can save carriers money. Agreed, smart antennas are quite expensive when added to existing base- station designs as an afterthought. The cost plummets, however, when manufacturers design base stations with integrated smart antennas from the ground up.

SWITCHED-BEAM SYSTEMSMetawave, in contrast, uses a simpler solution: switched-beam antenna systems. Rather than custom-shaping signal beams for each user, Metawave's SpotLight system intelligently switches between 12 directional antennas -- each with a fixed, 30-degree beam. In essence, Metawave creates 12-sector cells -- not only increasing capacity, but also enabling more cost-effective use of linear power amplifiers (LPAs).

The cost of an LPA increases disproportionately with power. Thus, if you can reduce LPA power by a factor of four, you probably will reduce cost by an even greater factor. To convert conventional 3-sector cells into 12-sector cells, you need four times as many LPAs, but with just one-quarter as much power apiece.

A big advantage to the intelligent, switched-beam antenna system is that it can be applied readily to air interfaces employing separate transmit and receive frequencies such as AMPS, cdmaOne and GSM. In fact, Metawave has found cellular carriers upgrading from AMPS to cdmaOne to be its best prospects.

In order to bring cdmaOne on-line, a cellular carrier has to clear some of its existing AMPS spectrum. But that means the network must serve the same number of AMPS users in less AMPS spectrum -- at least until enough AMPS subscribers can be switched to cdmaOne. One potential solution is to add more cells in capacity "hot spots." But one of the main reasons for selecting cdmaOne was to avoid such cell splitting. Metawave's SpotLight smart-antenna system can be deployed selectively to add capacity only where it's needed.

Cellular operators also don't want to bring additional 1.25MHz cdmaOne carriers on-line until it's absolutely necessary. Again, a smart-antenna system can be used to add capacity only where it's needed.

Metawave's SpotLight system has been adapted to AMPS and cdmaOne cellular base stations made by Motorola and Lucent. But Metawave has chosen not to adapt its technology to PCS base stations, explaining that capacity is not yet an issue for PCS operators. Plus, switched-beam antenna systems do more to enhance the downlink, while PCS operators need more help with their uplinks.

Although ArrayComm and Metawave have found their respective market niches, Professor Jeff Reed of Virginia Tech's MPRG believes smart antennas will come into wider use as the industry begins to offer high-speed data services. If everything else remains the same, an increase in speed will cause a decrease in range. As operators roll out 3G services, they will find providing the same coverage with their existing cell layout much harder.

HANDSET SMARTSMPRG also is working on smarter antennas for handsets. Although no handset manufacturer is about to incorporate 12 antennas in a portable package, there are advantages to adding a second antenna -- and ways of concealing it. Two antennas, separated by a fraction of a wavelength, can provide sufficient diversity to combat multipath fading. Diversity antennas for handsets may prove particularly valuable for operation inside buildings, where smaller differences in signal-path length are encountered routinely. Based on what's been learned about signal propagation in urban settings, Reed believes there may be merit to equipping handsets with cross-polarized antennas; there certainly seems to be enough processing power in today's handsets.

The spread of smart antennas will be spurred by the increase in knowledge about mobile radio propagation, the development of new base stations with integrated smart-antenna technology and the emergence of handsets with multiple antennas. But the key is to remember that a smart antenna is really a system.

Dumb aluminum is not going away; it's just going to be used more intelligently.