of user terminals. 3.1 Mobile and Portable Telephones In the early days of Advanced Mobile Phone Service (AMPS), mobile telephones in ve- hicles were distinct from portable telephones, which were light and convenient enough to be carried by hand. Now the two technologies have merged, and a mobile telephone in a car is simply a physical holder for the subscriber's hand-held portable telephone with an external power supply drawing on the car's electrical system. These days, cel- lular engineers refer to a user terminal that functions as a telephone as the mobile or the mobile telephone, and we will do so here. Subscribers commonly refer to them as cell phones. Subscribers, however, have adopted the term portable telephone for house- hold cordless telephones that do not use cellular technology at all. The mobile telephone is a significant component of the cellular system, and its tech- nology is changing constantly. Mobile telephones are becoming smaller, cheaper, and more common every year. Even though each phone is a low-cost item, the sheer num- ber of phones means that the cost of user terminals is a major component of the cost of cellular systems. The engineering design of the radio link is a crucial element that gov- erns the capacity of any cellular system. Yet the size and power of the battery are the key issues of the engineering design of the portable telephone itself. 3.1.1 Components and engineering Figure 3.1 illustrates the components of a portable telephone. The external power source charges the battery and usually receives its power from a household or car elec- trical system. Customers are likely to own more than one battery charger for each cell 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 User Terminal Key Radio Concepts Antenna Speaker Receiver Decoder Transceiver CODEC Transmitter External Power Battery Coder Amplifi