nd. This is a 1.25 percent increase, which is a 0.05-dB change in total power. Another way to think of this is that spending 800 bits per second for signal improvement becomes a better deal when the signal be- ing improved is bigger. Both cmda2000 and W-CDMA offer high-speed forward power control because they support higher data rates than cdmaOne. This is similar to the decision for a reverse pilot. The extra overhead of a pilot is traded off against the signal improvement of coherent demodulation. In cdmaOne, the small reverse signal does not justify a reverse pilot, and the cdmaOne base station re- ceiver uses a noncoherent demodulation scheme. Both cdma2000 and W-CDMA main- tain a pilot signal in the reverse direction, in different ways, allowing the system to use a coherent demodulation scheme on the higher data signal with its higher rate. 30.5 Speech Coding CDMA, like the Global System for Mobility (GSM), supports a low-bit-rate speech cod- ing system. The air interface is a naturally scarce resource, and its bit-rate capacity is defined by bandwidth and signal-to-noise (S/N) ratio. Therefore, the designers of voice channels for wireless telephone systems want to make every bit count. We start with low-bit-rate speech coder and reduce the rate further when we can. The cdmaOne and cdma2000 systems use Qualcomm code-excited linear prediction (QCELP), whereas W-CDMA uses adaptive multirate (AMR) speech coding. cdmaOne and cdma2000 have two different speech systems, a mandatory 8000-bit- per-second speech coder and an optional 13,000-bit-per-second coder that offers fewer calls with higher speech quality. The bit rate is reduced when the speech does not require the full rate. While GSM turns off the speech data stream when the speaker is idle, the CDMA air interface 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. Capacity Issues Specific 