ansmission. Roaming is a term used for a mobile unit that travels such distances that the route covers more than one cellular organization or company. The cellular industry is moving toward technical and tariffing standardization SO that a cellular unit can operate anywhere in the United States, Canada, and Mexico. 18.3 RADIO PROPAGATION IN THE MOBILE ENVIRONMENT 18.3.1 The Propagation Problem Line-of-sight microwave and satellite communications covered in Chapter 9 dealt with fixed systems. Such systems are optimized. They are built up and away from obstacles. Sites are selected for best propagation. This is not SO with mobile systems. Motion and a third dimension are additional variables. The end-user terminal often is in motion; or the user is temporarily fixed, but that point can be anywhere within a serving area of interest. Whereas before we dealt with point-to-point, here we deal with point-to-multipoint. One goal in line-of-sight microwave design is to stretch the distance as much as possible between repeaters by using high towers. In this chapter there are some overriding circumstances where we try to limit coverage extension by reducing tower heights, what we briefly introduced in Section 18.2. Even more importantly, coverage is area coverage, where shadowing is frequently encountered Examples are valleys, along streets with high 18.3 RADIO PROPAGATION IN THE MOBILE ENVIRONMENT buildings on either side, verdure such as trees and inside buildings, to name a few typical situations. There are two notable results. Transmission loss increases notably and such an environment is rich with multipath scenarios. Paths can be highly dispersive, as much as 10 usec of delay spread (Ref. 2). If a user is in motion, Doppler shift can be expected. The radio-frequency bands of interest are UHF, especially around 800 and 900 MHz, and 1700-2000 MHz. In certain parts of the world, there is usage in the 400-MHz band. 18.3.2 Propagation Models We concentrate on cellular operation. There is a fixed station (FS) and mobile sta