another, is the normal form of handoff. Hard handoff was the only kind of handoff available in the AMPS days, but in CDMA, hard handoff is used only when soft handoff will not work. Therefore, we will discuss soft handoff first and then hard handoff. 11.4.5 Soft handoff Soft handoff (see Sec. 8.7) requires close coordination between or among cells whose only connection is through the MSC or the MSC network. 13 The MSC receives multiple copies of speech data from multiple base stations for the same call. For each speech data frame coming from the user terminal, the MSC analyzes all the copies it receives and selects the version with best speech quality. Each base station receiving a signal from a particular user terminal sends power- control signals to the user terminal and also forwards the frames it receives to the MSC. This performs two important functions: It allows the user terminal to adjust to the lowest power level acceptable to any receiver, reducing interference for all receivers. It allows for higher call quality because the MSC can choose the best voice signal frame by frame. The soft handoff process allows the system to manage user terminals as both sources of calls and as sources of interference. Any time a call is being served by two or more base stations, it is in the soft handoff state. Soft handoff is a state of a call rather than a transition from one state to another. The serving base station receives radio signal information from the user terminal and decides when a call should go into the soft handoff state. The call remains in soft hand- off as long as the radio conditions warrant. When a base station's signal for the call be- comes too weak, that base station is dropped. If a call should start being served by base station A and then be in the soft handoff state, served by both base station A and base station B, and then move so that it is out 13 Most soft handoffs are between two cells, but some are among three cells, a subtle point for the English grammarians reading this book. Downloaded 