The TIA/EIA Interim Standard, Mobile Station – Base Station Compatibility Standard of Dual- Mode Wideband Spread Spectrum Cellular System (TIA/EIA/IS-95), states that a CDMA base station shall support three types of handoff processes.
CDMA to CDMA Hard Handoff
A CDMA to CDMA hard handoff is a handoff in which the base station directs the mobile station to transition between disjoint sets of base stations, different frequency assignments, or different frame offsets.
CDMA to Analog Hard Handoff
A CDMA to Analog hard handoff is a handoff in which the base station directs the mobile station from a Forward TCH to an analog voice channel.
A soft handoff is a handoff in which a new base station commences communications with the subscriber station without interrupting the communications from the old base station. The base station can direct the subscriber station to perform a soft handoff only when all Forward Traffic Channels assigned to the subscriber station have identical frequency assignments. While soft handoff is being performed, more than 1 TCH shall be assigned to the subscriber.
Subscribers in the overlapping region are power controlled by both sectors during softer-handoffs and their signals are coherently combined. The threshold for activation of this procedure is a system control parameter. Softer handoff mitigates both path loss differences due to different shadowing and fades. In the activated region, both sector antennas are engaged and received along opposite slopes of their patterns to help differentiate multipath components.
Overhead Erlang Capacity for Soft Handoff
The soft handoff factor is used to determine the overhead Erlangs to support different kinds of soft handoffs. The factor is likely to vary from 1.3 to 2.0. It should be noted that the soft handoff factor (SHOF) defined here is a linear scaling factor of the actual usable Erlangs but not the number of traffic channels.
Soft Handoff Factor = 1*(1-a-b) + 2*a + 3*b
2-way soft Handoff fraction, a = Average two-way software handoff duration per access/hold time
3-way soft Handoff fraction, b = Average three-way soft handoff duration per access /