Handoffs

Several types of handoffs are available with CDMA. The types of handoffs
involve soft, softer, and hard. The difference between the types is dependent
upon what is trying to be accomplished.
Several user-adjustable parameters help the handoff process take place.
The parameters that need to be determined involve the values to add or
remove a pilot channel from the active list and the search window sizes.
Several values determine when to add or remove a pilot from consideration.
In addition, the size of the search window cannot be too small, nor can it be
too large.
As mentioned previously, the handoff process for CDMA can take on several
variants. Each of the handoff scenarios is a result of the particular system
configuration and where the subscriber unit is in the network.
The hand-off process begins when a mobile detects a pilot signal that is
significantly stronger than any of the forward traffic channels assigned to
it. When the mobile detects the stronger pilot channel, the following
sequence should take place. The subscriber unit sends a pilot strength measurement
message to the base station, instructing it to initiate the handoff
process. The cell site then sends a handoff direction message to the mobile
unit, directing it to perform the handoff. Upon the execution of the handoff
direction message, the mobile unit sends a handoff completion message on
the new reverse traffic channel.

In CDMA, a soft handoff involves a inter-cell handoff and is a makebefore-
break connection. The connection between the subscriber unit and
the cell site is maintained by several cell sites during the process. A soft
handoff can only occur when the old and new cell sites are operating on the
same CDMA frequency channel.

The advantage of the soft handoff is path diversity for the forward and
reverse traffic channels. Diversity on the reverse traffic channel results in
less power being required by the mobile unit, reducing the overall interference,
which increases the traffic-handling capacity.
The CDMA softer handoff is an intracell handoff occurring between the
sectors of a cell site and is a make-before-break type. The softer handoff
occurs only at the serving cell site.

The hard handoff process is meant to enable a subscriber unit to handoff
from a CDMA call to an analog call. The process is functionally a break-before-make and is implemented in areas where CDMA service is no
longer available for the subscriber to utilize while on a current call.The continuity
of the radio link is not maintained during the hard handoff.
A hard handoff can also occur between two distinct CDMA channels that
are operating on different frequencies.