hange the speech coding rate. When the trans- mit power is getting close to its maximum, the AMR bit rate is reduced. The coding scheme is algebraic code excited linear prediction (ACELP), and the mul- tirate version is referred to as MR-ACELP. The bits are arranged according to their subjective importance into three classes, A, B, and C, where class A is the most sensi- tive. The forward error correction is designed to provide the most protection for class A bits. AMR has voice activity detection (VAD) for discontinuous transmission (DTx) to ex- tend battery life and to reduce CDMA interference. The AMR transmitter sends a si- lence descriptor (SID) frame at regular intervals SO that the receiver can present com- fort noise during the silent intervals. The bit rate is controlled by the wireless network, so lower bit rates can be used dur- ing high system loads. The lower bit rates are also used during periods of poor radio connection to extend cell coverage. The AMR system allows a smooth tradeoff between system capacity and coverage on one side and speech quality on the other. 10.3.3 W-CDMA radio bandwidth The carrier spacing of W-CDMA is 5 MHz. 12 There is some debate about the need for guard bands in W-CDMA. In the standard, the proposal is to use complex filters so that guard bands are not needed, and 5-MHz bands can be 5 MHz apart, center to center. However, the CDG (which promotes cdma2000) has doubts about the feasibility of this approach. The CDG claims that it remains to be seen if a system can be specified and developed that will meet power emission requirements of the U.S. FCC. The CDG also says that in many nations there may be difficulty placing 5-MHz W-CDMA adjacent to other wireless services without adding guard bands. The potential for unmanageable problems is higher if the adjacent service is not CDMA because that service will not be able to manage interference as well as a CDMA service would. To date, all wireless ser- vices of different types have been separated by guard bands. If operated in 