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.