Soft Handoffs

Soft handoffs are an integral part of CDMA. The determination of which
pilots will be used in the soft handoff process has a direct impact on the
quality of the voice call or packet-data session as well as the capacity for the
system. Therefore setting the soft handoff parameters is a key element in
the system design for CDMA2000.
The parameters associated with soft handoffs involve the determination
of which pilots are in the active, candidate, neighbor, and remaining sets.
The list of neighbor pilots is sent to the subscriber unit when it acquires the
cell site or is assigned a traffic channel.
A brief description of each type of pilot is the same as that used for IS-95
systems and discussed in Chapter 3; however, it is repeated here for clarity.
The active set is the set of pilots associated with the forward traffic channels
assigned to the subscriber unit. The active set can contain more than
one pilot because a total of three carriers, each with its own pilot, could be
involved in a soft handoff process.
The candidate set are the pilots that the subscriber unit has reported are
of sufficient signal strength to be used. The subscriber unit also promotes
the neighbor set and remaining set pilots that meet the criteria to the candidate
set.
The neighbor set is a list of the pilots that are not currently on the active
or candidate pilot list. The neighbor set is identified by the base station via
the neighbor list and neighbor list update messages.
The remaining set is the set of all possible pilots in the system that can
be possibly used by the subscriber unit. However, the remaining set pilots
that the subscriber unit looks for must be a multiple of the Pilot_Inc.
An example of the interaction between active, candidate, neighbor, and
remaining sets is shown in Figure 3-30 and the associated description that
accompanies the figure.
Several issues need to be addressed regarding soft handoffs with 1xRTT
whether it is a 1x, 1xEV-DO, or 1xEV-DV configuration. The issues that need
to be factored in are the different radio configurations between all the base
stations involved with the soft handoff process. More specifically, the radio configurations
involved must be the same. In addition, radio resources must be
available for use by the mobile during soft handoff with all involved base stations.
The resources available could possibly involve excluding the subscriber
unit soft handoff with a target cell due to the lack of resources available.
If the mobile downgrades from one RC, say RC3 to RC2, it cannot
upgrade back to RC3 when resources become available.
An equally important issue is that a 2G mobile having RC1 and RC2
capability can be involved with numerous soft handoffs thereby taking
resources away from possible 2.5G/3G mobile use.
In addition, when the mobile negotiates a new service option, it can be
any one of the available RCs.