With mobile analog and TDMA cellular communications, as with WLANs, there is
a scheme for transferring a moving user from one cell to an adjacent cell, when
the signal in the current cell becomes too weak. This process is known as a
handoff. The actual procedure used for a handoff varies, but is similar among
the different cellular technologies. At any moment, each mobile device is
located in one specific cell or cell sector and under the control of the base
station of that cell, as shown in Figure
. When a device
leaves a cell, the base station notices the signal fading away and asks all the
surrounding base stations how much power they are getting from the signal. The
base station then transfers ownership to the cell receiving the strongest
signal. The device is informed of the new base station, and if a call is in
progress, it is instructed to switch to a new channel or frequency. This
handoff process takes about 300 msec. Actual channel assignment is done by the
MTSO, since the base stations are really just radio relays. The devices
involved in this handoff are said to be roaming.
The handoff to an
adjacent cell occurs on a channel of a different frequency to reduce the
possibility of interference. This type of handoff is called a hard handoff and
can take place between different cell sites or different sectors of one cell
site.
In mobile CDMA cellular communications, the moving user can be
connected to multiple cells at one time, adding and dropping connections as
required as the mobile moves about the coverage area of the system. Each cell
is typically operating on the same channel but using different spread spectrum
codes, and handoffs between cells on the same channel are called soft handoffs.
Handoffs between different sectors of a cell are called softer handoffs. If a
CDMA handoff involves different channels or bands, it is a hard handoff. In a
soft or softer handoff process, there is no time delay as the new connection is
added before the old one is lost.
The handoff to an adjacent cell occurs
on a channel of a different frequency to reduce the possibility of
interference.