As in a cellular network, a wireless local loop consists of cells that cover a specific region. Each cell contains a Base Transceiver Subsystem (BTS). The subscriber is normally in an office or home where a fixed subscriber station is installed to communicate wirelessly with the nearest BTS. The subscriber station appears and functions much like a wireline phone. The information received by the antennas of the BTS is processed and is then sent through T1/E1 land lines to the Base Station Controller (BSC) for connection to the Public Switched Telephone Network (PSTN).
Each cell can be further divided into sectors to achieve greater capacity. In the case of WLL, up to nine sectors can be accommodated, dramatically increasing system capacity. Various antenna configurations can be arranged to serve sectors with varying sizes in line with traffic demands. In doing so, coverage is adjusted for unevenly distributed subscriber populations surrounding the BTS. The BTS is strategically placed where it can cover the largest geographic area, forcing issues such as terrain to be taken into account during the planning stage. In the case of shopping centers, office buildings, or apartment buildings, coverage can be enhanced by using a remote antenna or alternative indoor products.
The BSC manages the signals arriving from different BTSs and connects the wireless network to the local central office switch for access to the PSTN. The BSC and the central office switch communicate through interconnecting T1/E1 links.
The base stations in a WLL system are deployed as needed to provide the necessary geographic coverage, with each base station connected back to the telephone network, typically either by wire or microwave links. In this way, a WLL system resembles a mobile cellular system: each base station supports a cell or several sectors of coverage, servicing subscribers within the coverage area and providing the link back to the PSTN. The extent of the coverage area is determined by the transmit power and the frequencies at which the base station and subscriber terminal radios operate, by the associated local propagation characteristics as a function of local geography and terrain, and by the radiation patterns of the base station and subscriber terminal antennas. In WLL systems that do not support user mobility, some reductions in cost can be obtained by optimizing the base station design and its site coverage patterns to best serve the known fixed subscriber locations.
The number of base stations that must be deployed depends on the anticipated traffic to be supported, on the capacity of the system, on the availability of base station sites, on the range of coverage provided by the system and by local propagation characteristics, and on the bandwidth that is available for use by the WLL network. Different systems offer different degrees of spectral efficiency in terms of users supportable per unit bandwidth, providing an advantage to high capacity systems in regions with high subscriber densities or with severely constrained access to bandwidth. In general, though, the greater the available bandwidth, the greater the capacity of the deployed network.
As noted, subscribers to a WLL system are linked via radio to a network of radio base stations which, in turn, are tied by a backhaul network to the PSTN. The WLL system's interface to the telephone network is then supported either by its own switch or through direct connection to the local exchange.
WLL systems are available that incorporate their own switch or that only connect to one or more specific switches. In part, this approach to WLL system architecture has reflected the difficulty of supporting direct connection to the wide variety of switches globally deployed. This is changing, however, with the adoption of V5.2 standard interfaces. It may also reflect a given WLL system's heritage as an adaptation of cellular technology designed for the support of mobile services. Because of the history of mobile services as competitive independent networks distinct from wireline service providers, cellular systems have been developed for use with specific mobile switches.
Direct connection, either through analog or digital interfaces, to the central office switch can allow the use of existing but underutilized switching resources. Such capacity left idle while waiting for full deployment of the local loop represents an inefficient and costly use of resources that effectively increases the switch's contribution to the overall cost per subscriber.
Analog two- or four-wire interfaces are necessary for copper line local loops, but they represent a cumbersome and relatively expensive interface when used with WLL technology. Digital interfaces, on the other hand, can be more convenient and less expensive, but compatibility between a specific switch and WLL system cannot always be ensured.
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