 into smaller cells, each with its own base station, as shown in Figure 18.3. With smaller cells lower transmitter power and antennas with less height are used, thus permitting greater frequency reuse. These subdivided cells can be split still further for even greater frequency reuse. However, there is a practical limit to cell splitting, often with cells with a 1-m (1.6-km) radius. Radio system design for cellular operation differs from that used for LOS microwave operation. For one thing, mobility enters the picture. Path characteristics are constantly changing. Mobile units experience multipath scattering, reflection, and/or diffraction by 18.2 BASIC CONCEPTS OF CELLULAR RADIO Figure 18.2 Cell separation with four different sets of frequencies. Figure 18.3 Staged growth by cell splitting (subdividing). obstructions and buildings in the vicinity. There is shadowing, often very severe. The resulting received signal under these conditions varies randomly as the sum of many individual waves with changing amplitude, phase, and direction of arrival. The statistical autocorrelation distance is on the order of one-half wavelength (Ref. 1). Space diversity at the base station tends to mitigate these impairments. In Figure 18.1, the MTSO is connected to each of its cell sites by a voice trunk for each of the radio channels at the site. Also, two data links (AMPS design) connect the MTSO to each cell site. These data links transmit information for processing calls and for controlling mobile units. In addition to its "traffic" radio equipment, each cell site has installed signaling equipment, monitoring equipment, and a setup radio to establish calls. CELLULAR AND PCS RADIO SYSTEMS When a mobile unit becomes operational, it automatically selects the setup channel with the highest signal level. It then monitors that setup channel for incoming calls destined for it. When an incoming call is sensed, the mobile terminal in question again samples signal levels of all appropriate setup channels SO it can respond through the cel