Wireless local loop technology falls into four general categories: analog cellular, digital cellular, proprietary fixed wireless, and cordless telecommunications.
First-generation wireless local loops are based on analog cellular technologies, which have a solid track record of performance, support a relatively wide coverage area, and provide economies of scale for infrastructure and handsets. The key drawback with analog cellular is that it is optimized for mobility rather than local loop service; since user bit rates are low, wireline voice quality is elusive.
Digital cellular offers greater capacity and better voice quality than analog cellular, but it too is geared toward mobile applications. Coverage areas are usually smaller than with analog and there is a profusion of industry standards. Even when there is general agreement on standards—as there is with GSM, for example—the frequencies over which it operates may differ from country to country.
Proprietary systems—such Nortel's Proximity I Series—are designed from the start as alternatives to the copper-based local loop. They operate at higher frequencies, usually 3.5 GHz or above, where the spectrum is less crowded. They provide high-quality voice at 32 Kbps via Adaptive Differential Pulse Code Modulation (ADPCM)—versus 8 Kbps or 13 Kbps used in most WLL systems—and support voice-band data modems and high-speed fax transmission at up to 28.8 Kbps. These systems are interoperable with the PSTN and are aimed at new operators in competitive markets, where the challenger must be able to match the advanced service offerings of the wired incumbent to have any chance of success. The key disadvantage with these proprietary approaches is that they do not usually support mobility.
Some WLL systems make use of the standardized cordless telecommunications systems—including CT2, DECT, PACS, and PHS. While all are well suited for deployment in dense urban areas, and offer higher quality voice at 32 Kbps and data services up to 28.8 Kbps, each has its strong points. CT2 (Cordless Telephone Two) makes a good pair-gain system for countries with an existing, but insufficient, feeder network infrastructure. DECT (Digital Enhanced Cordless Telecommunications) is a proven technology widely used in Europe in wireless PBX implementations, while PHS (Personal Handyphone System) has been successful in Japan and is headed toward becoming a pan-Asian standard.
PHS offers high-quality, low-cost mobile telephone services using a fully digital system operating in the 1.9-GHz spectrum. Originally developed by NTT, the Japanese telecommunications giant, PHS is based on GSM technology. In addition to personal communications, PHS is being used in wireless PBX and wireless local loop applications.
PACS (Personal Access Communications System) is based on Bellcore's Wireless Access Communications System (WACS) and on Japan's PHS. Operating in the 1.9-GHz licensed PCS band, PACS provides an approach to PCS that is fully compatible with the local exchange telephone network and interoperable with existing cellular systems. PACS supports mobility better than the other standards—at vehicular speeds at over 65 miles per hour—and it also can be used for pedestrian venues, commuting routes, and indoor wireless. Although some vendors are now getting behind PACS in the United States, they face an uphill battle for market acceptance, if only because they got off to a late start—the major service providers having already committed to their WLL strategies.
All of these technologies work well enough and the costs are very attractive. In fact, WLL technology is now far more economical than copper-based local loops. Wireline local loops now cost anywhere from $1000 to $2000 per subscriber to provision, depending on the distance of the subscriber to the central office switch, while wireless local loops are down to about $500 per subscriber, regardless of the distance of the subscriber to the local switch.
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