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.”