The CDMA Principle Key Radio Concepts signal quality for power control, it sends power-up and power-down messages that both A and B obey. In the reverse direction, we have two receivers picking up the same message and co- ordinating their signals. When the two receivers are two sectors at the same base sta- tion, the rake filter is used to combine them optimally. When the two receivers A and B are at different base stations, one of them remains the primary base station, while the others are called secondary base stations. The signals from A and B are the same except, perhaps, for power control. When A and B send conflicting power-control messages, this means that the signal is good enough for one cell and not good enough for the other cell. The idea of soft handoff is to maintain just enough signal strength to keep one base station happy, so the mobile tele- phone powers down when it gets conflicting power-control messages. When one of the two signals is no longer helping the call, that signal is dropped. If the signal being dropped is the primary base station, then a secondary base station be- comes the primary base station for the call. This concept of a shared radio channel is possible because CDMA uses the same ra- dio carrier in the same frequency range for all cell sectors. When A and B are two separate base stations, we have a soft handoff until one of them is dropped. When A and B are two sectors of the same cell, we have a softer hand- off, as shown in Fig. 8.8. Softer Handoff Handoff Figure 8.8 Soft and softer handoff. 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. The CDMA Principle The CDMA Principle The CDMA channel allows more than two signals in the soft and softer handoff process. cdmaOne supports up to six simultaneous signals. 13 Thus we have three-way soft handoffs, four-way soft handoffs, and various kinds of soft-softer