orrect, but it is not the industry standard. GSM engineers call a particular frequency band a channel, whereas we are calling it a carrier. In GSM language, a single radio channel is time-divided to carry multiple voice channels. We chose to use the term carrier so that we would not have two different meanings for the term channel. 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. TDMA Wireless Telephony (GSM) TDMA Wireless Telephony (GSM) frequencies where more spectrum is available, so these systems can have more GSM carriers and channels, and there is more frequency separation between the forward and reverse directions. The GSM channel has 30 us between its time slots, which works out to 4.5 km for the round trip at the speed of light. If a mobile telephone is further from the base station than 4.5 km, then GSM allows for it to be resynchronized using a feature called dy- namic time alignment up to 237 us, about 40 km. Any range further than this requires the call to use extended dynamic time alignment, which takes two time slots for the call instead of just one. There is not much GSM traffic operating further than 40 km (25 miles) from the cell tower. Pauses in speech can be matched by pauses in transmission using DTx. DTx is the radio buzzword for discontinuous transmission. (Tx is the abbreviation for transmit.) The speech coder uses voice-activity detection (VAD) to determine when to transmit and when to keep quiet. The GSM receiver turns silent frames into comfort noise so that the listener does not hear the clipped muting so common in less sophisticated voice-activated systems such as those often found on inexpensive tape recorders. Rayleigh fading is less severe in a 200-kHz GSM carrier than it is in a single 30-kHz AMPS channel because Rayleigh fades are concentrated in a particular place at a par- ticular frequency. Even with its b