© 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 to CDMA Capacity and Quality Principles cdma2000 and wideband CDMA (W-CDMA) have rapid feedback for power control in both directions. The design tradeoff in CDMA power control is that we can have more precise power control only by spending more power on power-control messages. When the required rate of data transfer is relatively low, as is the case with cdmaOne and cdma2000 voice calls, the capacity devoted to extra power control costs more than the advantage that it provides. However, with the higher data rates offered on 3G systems, cdma2000 and W-CDMA are designed to have high-speed power control in both forward and reverse directions. The more data required on the channel, the more the tradeoff favors having more signal-control information. Signal-control information includes both forward power control and reverse pilot going from the user terminal to the base station. The smaller voice channel does not justify the extra overhead. Let us work it out by an example. Suppose that we are considering whether to add an 800-bit-per-second power-control stream to the 9600-bit-per-second cdmaOne speech channel. To have this power-control stream be as accurate as the speech chan- nel means sending 10,400 bits per second instead of 9600 bits per second with the same accuracy, a cost of 8.3 percent in power, 0.35 dB. If the benefit of that 800-bit-per- second power-control channel is a 0.30-dB reduction in power variation, then it is not worth the extra overhead. Everything becomes capacity in CDMA: The increase in power-control power costs 0.35 dB of capacity, and the decrease in power variation gains 0.30 dB in capacity. A losing proposition, but not by much. The same decision for a 64,000-bit-per-second data channel comes out the other way, however. The same 800 bits per second gains the same 0.30 dB, but now our total data requirement goes from 64,000 to 64,800 bits per seco