Radio Propagation In The Mobile Environment 1031 The Propagation Problem

LOS microwave and satellite communications covered in Chapters 2 and 3 dealt with fixed systems. Such systems were and 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 LOS microwave design was 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 10.2. Even more important, coverage is area coverage where shadowing is frequently encountered. Examples are valleys, along city streets with high buildings on either side, in verdure such as trees, and inside buildings, to name just a few situations. Such an environment is rich with multipath scenarios. Paths can be highly dispersive, as much as 10 fs of delay spread (Ref. 3). Due to a user's motion, Doppler shift can be expected.

The radio-frequency bands of interest are UHF, especially around 800 and 900 MHz and 1700-2000 MHz. Some 400-MHz examples will also be covered.

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