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Nature’s Speed Limit

This new, hybrid vision remains contrarian in the wireless industry, largely because mobile broadband evangelists like Chapman believe in “build it and they will come.” Their companies are proposing a jumble of competing protocols to move us beyond today’s second-generation, or 2G, digital cell-phone networks. They plan to roll out so-called 2.5G systems that combine voice and data this year in Japan and Europe, and in 2002 in the United States. Japan claims it will soon follow with the third generation, or 3G. Yet nobody seems certain when 3G, the standard that will support true broadband applications, will actually be implemented.

Working against its rapid appearance is a fundamental law governing data communications that was laid down way back in telecom’s primordial era: 1948. That year, Claude E. Shannon of Bell Labs stated that the maximum amount of data that can be transmitted through any channel is limited by the available bandwidth (the amount of radio-frequency spectrum it occupies) and by its signal-to-noise ratio (the signal to be communicated versus interference).

Both limits are strikes against mobile-wireless communications. A wireless channel can only use the portion of the spectrum approved for it by the International Telecommunication Union and licensed by one of its 189 member states. The licensing fees are appalling: carriers spent more than $46 billion for the 3G spectrum in Germany alone. At those prices, a carrier must maximize the payback of its channels by packing as much data as possible into as narrow a frequency band as possible-a practice that runs counter to the principle of filling a broad band with data-intensive multimedia streams, which is, technologically, the optimum strategy. To resolve the conflict, carriers must devise technology that can send signals faster in tight bands.

To make matters worse, the medium through which the signals flow-the earth’s surface atmosphere-is a very noisy place these days. Cell-phone signals careen off buildings, hillsides and each other, creating interference and decay. To improve fidelity, manufacturers must boost the signal power or reduce the noise. But they can’t increase power because the Federal Communications Commission and its European and Asian counterparts restrict the electromagnetic radiation cell towers and handsets can emit. Besides, raising a handset’s power level kills its batteries.

No surprise, then, that engineers focus on reducing noise. This game began in earnest in the mid-1990s, when digital cell phones started to replace analog versions, increasing voice clarity dramatically. Even though telecom companies had to spend billions of dollars to add digital transceivers to their cell towers, the upgrade quickly paid for itself, because it also allowed the providers to cram many more simultaneous voice calls into the same slice of expensive bandwidth, with less interference.

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