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Soft Handoff and Power Control in IS-95 CDMA

Soft handoff is different from the traditional hard-handoff process. With hard handoff, a definite decision is made on whether to hand off or not. The handoff is initiated and executed without the user attempting to have simultaneous traffic channel communications with the two base stations. With soft handoff, a conditional decision is made on whether to hand off. Depending on the changes in pilot signal strength from the two or more base stations involved, a hard decision will eventually be made to communicate with only one. This normally happens after it is evident that the signal from one base station is considerably stronger than those from the others. In the interim period, the user has simultaneous traffic channel communication with all candidate base stations. It is desirable to implement soft handoff in power-controlled CDMA systems because implementing hard handoff is potentially difficult in such systems. A system with power control attempts to dynamically adjust transmitter power while in operation. Power control is closely related to soft handoff. IS-95 uses both power control and soft handoff as an interferencereduction mechanism. Power control is the main tool used in IS-95 to combat the near-far problem. It is theoretically unnecessary to have power control if one can successfully implement a more intelligent receiver than that used in IS-95, which is the subject of the field of multiuser detection (MUD), a feature being proposed for the 3G CDMA systems. Power control is necessary in order for a CDMA system to achieve a reasonable level of performance in practice. The use of power control in the CDMA system necessitates the use of soft handoff when the original and new channels occupy the same frequency band. For power control to work properly, the mobile must attempt to be linked at all times to the base station from which it receives the strongest signal. If this does not happen, a positive power control feedback loop could inadvertently occur, causing system problems. Soft handoff can guarantee that the mobile is indeed linked at all times to the base station from which it receives the strongest signal, whereas hard handoff cannot guarantee this.