power levels in areas of poor reception or during busy periods, most of the time a CDMA channel is operating at a very low power level. This gives CDMA an advantage over AMPS and GSM in terms of power consumption and battery recharge interval. One way to prolong battery life and to reduce radio interference as well is to shut down the transmitter when the user is not speaking. Normal human speech has short pauses between words and long pauses while the user is listening rather than speak- ing. The air interfaces have protocols that allow for discontinuous transmission (DTx). 4 The receiver receives the modulated signal coming from the base station. It also has an amplifier, but one that only needs enough power to produce an internal signal rather than a transmitted radio wave. Even that amplifier uses some battery power, and the receiver is always monitoring the paging channel while the mobile telephone is pow- ered on. Thus GSM and CDMA proscribe a time schedule for paging so that the mobile telephone can conserve more power using discontinuous reception (DRx). The transmitter and receiver share enough technology and components that it makes sense to form them into a single unit, a transceiver. AMPS and CDMA require the 1We have seen the SEND and END keys marked YES and NO on some portable user terminals. Telephones do this because speakers get nervous when they cannot hear their own voices. The round-trip delay of a telephone call, particularly a long-distance call, is long enough that it would be disconcerting to have the speaker's voice come back through the two-way link. The long- distance telephone network has echo-cancelers specifically to eliminate these round-trip echoes. Fax and modem signals do not have these pauses, and they present their own capacity engi- neering issues as a result. 4Tx is the radio engineering abbreviation for transmit. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any u