Capacity boost

Scientists at Lucent Technologies' Bell Labs in Murray Hill, N.J., recently showed how wireless operators could increase reliability, capacity and transmission speeds in their networks by boosting radio-signal capacity.

Bell Labs found that radio signals use only one-third of the possible capacity today.

“The extra capacity is available because there are six distinguishable electric and magnetic states of polarization at a given point, rather than [the] two that have been used,” said Partha Mitra, a Bell Labs researcher.

Historically, scientists have looked at radio waves as having only two points of polarization, Mitra said, using the example of a stone dropped into a pond. The ensuing ripple effect goes not only up and out but also down. But the third point of polarization has yet to be used.

Similarly, the electric and magnetic spectrums, though close in nature, are not identical, providing another three points of polarization, Mitra said. Because today's systems use only two of those points in the electric spectrum, the other four could increase capacity as much as six times, giving wireless operators several different options, the Lucent scientists said.

But to implement the technology, vendors would have to alter their existing hardware or develop new hardware altogether.

For example, some of today's antennas use double polarization, enabling them to have a redundant signal so the user doesn't lose the transmission if one signal fails. Using multiple antennas in the same location could help provide even more redundancy if used in conjunction with the six-signal capability, the Lucent scientists said.

One of the benefits of this technology is that it boosts transmission speeds, but it's not likely to be implemented until third-generation technologies take hold. “Any use of these findings will probably be with 3G technology; no one is spending money anymore on 2G technology,” said Alan Haase, CEO of SkyCross, a Melbourne, Fla.-based manufacturer of antennas for hand-held devices. SkyCross' own Meander Line Antenna technology is the basis for the company's multi-frequency antennas.

With 2D antennas already in production, the development of 3D antennas, which could take advantage of the availability of additional signals, is a logical next step for SkyCross, Haase said.

By providing more potential signals, the discovery also could mean wireless companies could support more subscribers on the same frequency and provide better coverage in urban areas where the volume of wireless users sometimes makes it hard or impossible to get a signal, Haase said. The additional points of polarization can take advantage of the signals that bounce off buildings and typically cause interference and other problems, Mitra said.

The scientists used three antennas and proved that multiple antennas in the same location can detect the different polarization states created by scattering. Historically, multiple antennas have been effective only when separated, Mitra said.

The company already is building next-generation base stations with multiple antennas in mind. This developing technology, which Lucent calls BLAST (Bell Labs Layered Space-Time) is built on the concept that it is possible to have several transmissions occupy the same frequency band. Each transmission uses its own transmitting antenna.

On the receiving end, multiple antennas are used, along with innovative signal processing, to separate the mutually interfering transmissions from each other. Thus, the capacity of a given frequency band increases proportionally to the number of antennas.

The BLAST prototype, built to test this theory, uses eight transmit and 12 receive antennas. During its first weeks of operation, it achieved unprecedented wireless capacities of at least 10 times the capacity of today's fixed wireless loop systems.