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 call and the capacity of the system.
Therefore, setting the soft handoff parameters is a key element in the system
design for CDMA.
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 set follows:
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 is made up of the pilots that the subscriber unit has
reported are of a 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 lists. The Neighbor set is identified by the base station via
the Neighbor list and Neighbor list update messages.
The Remaining set consists of all possible pilots in the system that can
possibly be used by the subscriber unit. However, the remaining set pilots
that the subscriber unit looks for must be a multiple of the Pilot_Inc.
Figure 3-29 shows an example of a soft handoff region, which is an area
between cells A and B. Naturally, as the subscriber unit travels farther away
from cell A, cell B increases in signal strength for the pilot. When the pilot
from cell B reaches a certain threshold, it is added to the active pilot list.
The process of how a pilot channel moves from a neighbor to a candidate,
to active, and then back to neighbor is best depicted in Figure 3-30.
Here are the steps that a pilot channel takes:

1. Pilot exceeds T_ADD and the subscriber unit sends a Pilot Strength
Measurement Message (PSMM) and a transfer pilot to the candidate
set.
2. The base station sends an extended handoff direction message.
3. The subscriber unit transfers the pilot to active set and acknowledges
this with a handoff completion message.
4. The pilot strength drops below T_DROP and the subscriber unit begins
the handoff drop time.
5. The pilot strength goes above T_DROP prior to the handoff drop time
expiring and the T_DROP sequences topping.
6. The pilot strength drops below T_DROP and the subscriber unit begins
the handoff drop timer.
7. The handoff drop timer expires and the subscriber unit sends a PSMM.
8. The base station sends an extended handoff direction message.
9. The subscriber unit transfers the pilot from the Active set to the
Neighbor set and acknowledges this with a handoff completion
message.

To help augment the previous description, Figure 3-31 highlights how
T_Comp is factored into the decision matrix for adding and removing pilots
from the Neighbor, Candidate, and Active sets.