why
are four frequency sets necessary in
Cellular mobile Otelephony?
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Cellular Telephone Systems
Cellular telephone systems, also referred to as Personal Communication Systems (PCS), are extremely popular and lucrative worldwide: these systems have sparked much of the optimism about the future of wireless networks. Cellular telephone systems are designed to provide two-way voice communication at vehicle speeds with regional or national coverage. Cellular systems were initially designed for mobile terminals inside vehicles with antennas mounted on the vehicle roof. Today these systems have evolved to support lightweight handheld mobile terminals operating inside and outside buildings at both pedestrian and vehicle speeds.
The basic feature of the cellular system is frequency reuse, which exploits path loss to reuse the same frequency spectrum at spatially separated locations. Specifically, the coverage area of a cellular system is divided into nonoverlapping cells where some set of channels is assigned to each cell.
Cellular telephone systems are now evolving to smaller cells with base stations close to street level or inside buildings transmitting at much lower power. These smaller cells are called microcells or picocells, depending on their size. This evolution is driven by two factors: the need for higher capacity in areas with high user density, and the reduced size and cost of a base station. A cell of any size can support roughly the same number of users if the system is scaled accordingly. Thus, for a given coverage area, a system with many microcells has a higher number of users per unit area than a system with just a few macrocells. Small cells also have better propagation conditions since the lower base stations have reduced shadowing and multipath.