 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 The User Terminal features. Mobile telephones use preorigination dialing; that is, the number is entered first, and then the call is initiated when the SEND key is pressed. 1 Preorigination dial- ing was conceived when the market was vehicular. The driver could enter two or three digits at a time, with breaks to look up and check traffic. Several other advantages have been possible due to this design. Battery use is lower because the transmitter is not activated during dialing. In addition, since keystroke management is local to the telephone, the keypad can have a backspace or erase function and also can be used for a number of other alphanumeric data entry activities, such as maintaining a speed-dial list. The microphone picks up the user's voice and sends it through the modulator to the radio transmitter during a call. The sound of the voice is also sent to the speaker so that the subscriber can hear his or her own voice. This deliberate recreation of the speaker's voice on a telephone call is called sidetone. The microphone receive voice commands, such as voice-activated speed dialing. The modulator turns the electrical audio signal from the microphone into a radio sig nal, analog FM modulation for AMPS and digital modulation for Global System for Mo- bility (GSM) or code division multiple access (CDMA), as we discussed in Sec. 1.10. The transmitter amplifies the modulated signal and sends it to the antenna, where it goes over the reverse radio link to the receiver in the base station. This amplifier is the critical power component that limits call-duration battery life. It has to produce a radio signal with enough effective radiated power (ERP) to meet the signal-to-noise (S/N) demands of the base-station receiver and the air interface. While CDMA can de- mand high peak 