A Better Antenna for Cellular Networks
When you talk on a cellular phone, you’re sharing radio frequencies with everyone else who’s using one within a three-kilometer radius of the nearest base station. As everyone knows, this sharing doesn’t always work perfectly – network congestion can lead to static, dropped calls, and slow data downloads.
But what if you didn’t have to share a cell-phone signal? What if the nearest base station could aim a radio beam directly at your phone as you moved around, rather than spewing signals in all directions? In that scenario, you could expect clearer voice calls and speedier delivery of digital information such as Web pages or video. And by sending out multiple beams, your cellular carrier could deliver enhanced signals to other customers, too.
This approach to increasing the capacity of cellular networks is called “adaptive beamforming.” And engineers at Nokia are quickly bringing it closer to commercial use. Although the Finnish telecommunications giant is best known for its phones, it’s also a major supplier of networking and transmission equipment to mobile operators. In the sub-sub-basement of the Nokia Research Center in Helsinki, Finland, where their equipment is quarantined from the clamor of cellular signals, researchers are building and testing a prototype beamforming base station antenna that could triple the capacity of the newest generation of cellular networks.
[Click here to view images of the antenna, its environs, and its keepers.]
Those new networks aren’t overloaded yet. But that’s no cause for complacency. “The 3G systems, such as wideband CDMA, are just beginning to be deployed around the world, so the networks are by no means congested at the moment,” says Hannu Kauppinen, senior research manager for radio technologies at Nokia Research Center. “But we anticipate that in the future, operators will have a need for capacity increases. That is why we are investigating this feature.”
Whereas a traditional cell-phone tower works like a lawn sprinkler, radiating in a circle, a beamforming antenna works like a hose. “The basic idea is that in a crowded area you want to give the maximum signal to the appropriate person, rather than wasting the energy by spreading it out over a broader volume,” explains Greg Hindman, president and cofounder of Torrance, CA-based Nearfield Systems, which builds testing and measurement systems for manufacturers of radio equipment. “A lot of our customers are working on this.”
New ways to support more callers are needed because cellular-phone networks employ a finite resource: the radio spectrum. The original technique for serving multiple wireless users in a populated space, pioneered more than 40 years ago, was to divide the space into cells, each served by a separate base station. But since cells were large and might contain many customers, that wasn’t enough. Signals had to be divided up using different radio frequencies, or channels.
In the United States, however, the spectrum allocated by the government for first-generation, analog cellular networks was enough to support only 56 channels per cell –the 57th caller in any given cell was out of luck. So frequencies had to be divided up further.
In Time Division Multiple Access (TDMA) digital networks each burst of information on a particular frequency is split into three time slots, each a few milliseconds long. These slots are assigned to three different phones, each of which can piece together the data from its time slot into a continuous conversation. The result is that three phones at a time can use the same frequency, tripling the capacity of each cell, to roughly 168 channels. TDMA is the basic technique behind protocols such as the Global System for Mobile Communications, or GSM, used by major companies such as China Mobile, T-Mobile, the Cingular division of the new AT&T, and Personal Communications Services, or PCS, used by Sprint.
An alternative technique is to abandon channels altogether and instead spread multiple conversations in small pieces across the entire cellular spectrum. In this method, known as Code Division Multiple Access (CDMA), all phones in a particular cell listen to the same range of frequencies and receive the same raw data, but each piece of data is prefaced by a digital code unique to one customer’s phone. Only that phone can pick out and reassemble the pieces that constitute the user’s conversation. CDMA is the preferred wireless protocol of Verizon Wireless in the United States, Orange in Europe, and NTT DoCoMo in Japan.
The third-generation (or “3G”) version of CDMA is called Wideband CDMA, referring to its greater capacity to carry data such as music and live moving images. In ideal circumstances, WCDMA networks can send data at near-DSL speeds: 384 kilobits per second to moving users and 2 megabits per second to stationary users, compared with about 50 kilobits per second for second-generation networks. This standard has already been adopted by NTT DoCoMo and other carriers, and Nokia has invested heavily in the protocol, building the necessary phones, base-station equipment, computer systems, and software.
As Nokia gears up now to handle anticipated congestion on WCDMA networks, its researchers have come full circle: they’ve returned to the idea of dividing cellular signals spatially. Just as first-generation cellular technology divided space into cells, beamforming divides cells into slices, each served by a different beam. (Beamforming technology can be applied to any type of digital cellular network, not just CDMA-based ones.)
While beamforming itself isn’t a novel idea, it’s never been successfully applied to cellular telephony. “It’s basically old military technology,” says Kauppinen. “Some radars have been functioning with this principle for a very long time. But only in the last few years have we had an understanding of how beamforming would actually function in cellular networks.”
The beamforming antenna being tested in the Helsinki laboratory is actually eight antennas in one. It’s fashioned out of copper strips each about eight centimeters across, welded together into a surface covering about one square meter. The device cleverly modulates the overlapping radio waves from the eight antennas to steer signals in specific directions. (More antennas could be used, but the computations required to steer the signals increase drastically as more antennas are added.)
Imagine dropping two stones simultaneously into a still pond. At some spots, the peaks of the spreading ripples will coincide, creating higher peaks. At other spots, the peaks of one ripple will cancel out the troughs of the other, leaving calm water. Furthermore, dropping the stones at slightly different times will change the locations where the peaks coincide. By computing the time intervals exactly, you could, in theory, cause the highest peaks to line up in a specific direction.
That is how Nokia’s beamforming antenna works. A case behind the copper sheet contains the sophisticated amplifiers and digital signal-processing circuits needed to steer as many as eight separate beams in different directions. In practice, there would likely be many callers within the arc of each beam, so standard code-division techniques would be used within each beam to serve multiple callers, theoretically increasing overall network capacity by a factor of eight. However, because of complicating factors, such as geography and interference among beams, using eight beams wouldn’t actually increase network capacity eight times. “In simulations of semi-urban and urban environments, we found that [the beamforming antenna] increased capacity by a factor of two to three,” Kauppinen says.
Nokia thinks that’s enough of an improvement to interest mobile operators. And there’s another reason for the technology’s appeal: unlike other kinds of antenna arrays, a beamforming antenna doesn’t need multiple thick, heavy, and expensive copper cables to connect to amplifying equipment on the ground. Instead, all of the necessary equipment is inside the antenna itself.
“If you have to have four cables, each maybe one inch thick, going up to an antenna array, that’s a practical obstacle, and it is the chief reason for the reluctance of operators to install antenna arrays,” says Thomas Höhne, a researcher in Kauppinen’s lab. “Now that the amplifier is integrated into the antenna, it means we can run a thin optical fiber up to the antenna. And the power amplifier doesn’t need to be extra-strong, because we are adding the signals of the antennas together.”
Kauppinen says the prototype’s electronics are working well. In a few weeks, the team will test the beamforming antenna in the company’s underground anechoic chamber. Then they’ll take it to the roof and see how it performs in Helsinki’s brisk air. “We want to show that our simulations are true, and to gather practical experience,” says Kauppinen.
It’s unclear when beamforming antennas might be available for commercial use. “It’s a proof-of-concept” project, Kauppinen emphasizes – designed to convince the company’s business units that the technology can be developed into a viable product.
Even if Nokia goes ahead, it won’t be alone. According to Hindman of Nearfield Systems, many companies, including quite a few in China, South Korea, and Taiwan, are buying equipment to test beamforming. The technology seems likely to become another one of the tricks that mobile operators are employing to deliver on the promise of high-quality broadband wireless service.
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