nships. challenge to design cost-effective radio transmitters and receivers in the microwave band, and it becomes more difficult as frequencies get higher. 1.3 Multiple Access In the very first days of radio, it sufficed to get a signal from here to there over the ra- dio airwaves. We can imagine the listeners' excitement the first time they heard a live voice carried across the ocean on a radio wave and received for their ears. We also can imagine the desire to carry more than one radio signal. While radio link users can wait their turns in an ordered sequence, radio is only really useful when many users can use it simultaneously. The ability to send more than one signal at the same time is called multiple access. 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. Radio Engineering Concepts Key Radio Concepts Radio has been used for both broadcast and two-way communication. In broadcast, a single signal is meant for a large community of receivers, whereas we typically picture two-way radio as having two individual stations communicating with each other. The usual picture of broadcast is a commercial radio station, but there are private broadcast channels of distribution. Pagers are a form of broadcast radio; a single source sends data over the airwaves for a large community of receivers. The two-way walkie-talkie has evolved into sophisticated communications systems used in aviation, trucking, railroads, police, and the wireless telephone systems of to- day. Some systems often have one broadcast direction, a dispatcher talking to all the taxicabs or an air traffic controller talking to all the airplanes, with individuals reply- ing on a common frequency with no privacy. Wireless telephone systems require another level of sophistication because they manage separate two-way communication links in the same system. Unlike airline pi- lots, wireles