OV- erage most necessarily includes propagation for the several environments, propagation impairments, methods to mitigate those impairments, access techniques, bandwidth limi- tations, and ways around this problem. It will cover several mobile radio standards and compare a number of existing and planned systems. The chapter objective is to provide an appreciation of mobile/personal communications. Space limitations force us to confine the discussion to what might loosely be called "land mobile systems." 18.2 BASIC CONCEPTS OF CELLULAR RADIO Cellular radio systems connect a mobile terminal to another user, usually through the PSTN. The "other user" most commonly is a telephone subscriber of the PSTN. However, 18.2 BASIC CONCEPTS OF CELLULAR RADIO the other user may be another mobile terminal. Most of the connectivity is extending "plain old telephone service" (POTS) to mobile users. Data and facsimile services are in various stages of implementation. Some of the terms used in this section have a strictly North American flavor. Figure 18.1 illustrates a conceptual layout of a cellular radio system. The heart of the system for a specific serving area is the MTSO (mobile telephone switching office). The MTSO is connected by a trunk group to a nearby telephone exchange providing an interface to and connectivity with the PSTN. The area to be served by a cellular geographic serving area (CGSA) is divided into small geographic cells, which ideally are hexagonal. 1 Cells are initially laid out with centers spaced about 4-8 m (6.4-12.8 km) apart. The basic system components are the cell sites, the MTSO, and mobile units. These mobile units may be hand-held or vehicle- mounted terminals. Each cell has a radio facility housed in a building or shelter. The facility's radio equip- ment can connect and control any mobile unit within the cell's responsible geographic area. Radio transmitters located at the cell site have a maximum effective radiated power (ERP) of 100 W. 2 Combiners are used to connect multiple transmitters to 