Handoff/Handover

Handoff (also known as handover) is the ability of a subscriber to maintain
a call while moving within the network. The term handoff is typically used with AMPS, IS-136, and IS-95, while handover is used in GSM. The two
terms are synonymous.
Handoff usually means that a subscriber travels from one cell to another
while engaged in a call, and that call is maintained during the transition
(ideally without the subscriber noticing any change). In general, handoff
means that the subscriber is transitioned from one radio channel (and/or
timeslot) to another. Depending on the two cells in question, the handoff can
be between two sectors on the same base station, between two BSCs,
between two MSCs belonging to the same operator, or even between two
networks. (Note that inter-network handoff is not supported in some systems,
often mainly for billing reasons.)
It is also possible to handoff a call between two channels in the same cell.
This could occur when a given channel in a cell is experiencing interference
that is affecting the communication quality. In such a case, the subscriber
would be moved to another frequency that is subject to less interference. A
handoff scenario is depicted in Figure 1-9.
How does the system determine that a handoff needs to occur? Basically,
two main approaches are used. In first-generation technologies, a handoff is generally controlled by the network. The network measures the signal
strength from a mobile as received at the serving cell. If it begins to fall
below a certain threshold, then nearby cells are requested to perform signal
strength measurements. If a nearby cell records a better signal strength,
then it is highly likely that the subscriber has moved to the coverage of that
cell. The new cell is instructed by the BSC or MSC (typically just the MSC,
since first-generation systems do not have BSCs) to allocate a channel for
the subscriber. Once that allocation is performed, the network instructs the
mobile to swap to the new channel. This is known as a network-controlled
handoff, because the network determines when and how a handoff is to
occur.
In more recent technologies, a technique known as mobile assisted handover
(MAHO) is the most common. In the approach, the network provides
the mobile with a list of base station frequencies (those of nearby base stations).
The mobile makes periodic measurements of the signals received
from those base stations (as well as the serving base station), including signal
strength and signal quality (usually determined from bit error rates),
and it sends the corresponding measurement reports to the network. The
network analyzes the reports and makes a determination of if and how a
handoff should occur. Assuming that a handoff is required, then the network
reserves a channel on the new cell and sends an instruction to the
mobile to move to that channel, which it does.