would run a base station for many hours in the event of a power failure. 4.6 Microcells, Picocells, and Repeaters We often expand coverage of a cell by using a higher base station tower, but sometimes there is a particular region that is hard to reach. Other times there is a particular com- munity of interest, a lot of wireless telephone demand in a small space. We can deal with these issues locally using microcells, picocells, and repeaters. Amicrocell is a cell smaller than 500 m in radius. This is a full base station connected to the wireless telephone system just as any other cell. Because of its small coverage area, a microcell can use smaller and simpler radio equipment. Microcells are sold to wireless service providers as a package, a single piece of equipment, sometimes with a built-in microwave link for its connection to the MSC. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. The Base Station The Base Station When the service area is very small, perhaps a single building, then we use even smaller units called picocells. When the community of wireless subscribers is not large enough to justify a micro- cell but hard to reach with the usual base-station towers, we use repeater technology to extend coverage. The obvious examples are buildings, subways, and tunnels, but even a deep valley can be a coverage problem. As the name suggests, a repeater receives the signal from a sector of a base station and retransmits the signal into a hard-to-reach area such as a tunnel. Repeaters do re- quire an external power source, and they have two sets of receiving and transmitting antennas. However, they do not have any direct link to the MSC or PSTN, the way base stations, microcells, and picocells do. They rely on their air interface to a base station to carry the signal back to the PSTN. The repeater can be a major source of multipath interferen