f base stations, which connect to mobile users. According to the reference, PCS base stations connect directly to the PSTN. However, a number of PCS strategies have a hierarchy similar to cellular where an MSC (mobile switching center) provides the connectivity to the PSTN. Cellular radio systems operate in the 800- and 900-MHz band; in the United States narrowband PCS operates in the 900-MHz band, and wideband PCS operates in the 1850- to 1975-MHz band. Other PCS operations are specialized, such as the wireless PABX, wireless LAN (WLAN), and wireless local loop (WLL). By WLL we mean a transmission method that will operate in lieu of, supplement, or complement the telephone subscriber loop based on a wire pair. 18.1.1 Background The earliest radio techniques served a mobile community, namely, ocean vessels. This was followed by vehicular mobile including aircraft. Prior to World War II, there were police and ambulance dispatching systems followed by growth in the airline industry. However, not until Bell Telephone Laboratories published the famous issue of the Bell Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045-8 Copyright © 2005 by Roger L. Freeman CELLULAR AND PCS RADIO SYSTEMS System Technical Journal devoted entirely to a new system called AMPS (advanced mobile phone system) did the cellular idea take hold. It tends to remain a major player in the cellular radio scene in the United States and in some Latin American countries. It uses FM radio allocating 30 kHz per voice channel. AMPS set the scene for explosive cellular radio growth and usage. What set AMPS apart from previous mobile radio systems is that it was designed to interface with the PSTN. It was based on an organized scheme of adjoining cells and had a unique capability of handoff when a vehicle moves through one cell to another; or when another cell receives a higher signal level from the vehicle, it will then take over the call. Vehicles can roam from one service area to another with appropriate handoffs. 