ScienTECH ST2115, Руководство пользователя

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ST2115
Scientech Technologies Pvt. Ltd. 18
Soft handoff:
Soft handoff (or soft handover) is an innovation in mobility. It refers to the technique
of adding additional base stations (in IS-95 as many as 5) to a connection to be certain
that the next base is ready as you move through the terrain. However, it can also be
used to move a call from one base station that is approaching congestion to another
with better capacity. As a result, signal quality and handoff robustness is improved
compared to TDMA systems.
In TDMA and analog systems, each cell transmits on its own frequency, different
from those of its neighbouring cells. If a mobile device reaches the edge of the cell
currently serving its call, it is told to break its radio link and quickly tune to the
frequency of one of the neighbouring cells where the call has been moved by the
network due to the mobile's movement. If the mobile is unable to tune to the new
frequency in time the call is dropped.
In CDMA, a set of neighbouring cells all use the same frequency for transmission and
distinguish cells (or base stations) by means of a number called the "PN offset", a
time offset from the beginning of the well-known pseudo-random noise sequence that
is used to spread the signal from the base station. Because all of the cells are on the
same frequency, listening to different base stations is now an exercise in digital signal
processing based on offsets from the PN sequence, not RF transmission and reception
based on separate frequencies.
As the CDMA phone roams through the network, it detects the PN offsets of the
neighbouring cells and reports the strength of each signal back to the reference cell of
the call (usually the strongest cell). If the signal from a neighbouring cell is strong
enough, the mobile will be directed to "add a leg" to its call and start transmitting and
receiving to and from the new cell in addition to the cell (or cells) already hosting the
call. Likewise, if a cell's signal becomes too weak the mobile is directed to drop that
leg. In this way, the mobile can move from cell to cell and add and drop legs as
necessary in order to keep the call up without ever dropping the link.
It should be noted that this "soft handoff" does not happen via CDMA from cell tower
to cell tower. A group of cell sites are linked up with wire and the call is synced over
wire, over TDM, ATM, or even IP.
When there are frequency boundaries between different carriers or sub-networks, a
CDMA phone behaves in the same way as TDMA or analog and performs a hard
handoff in which it breaks the existing connection and tries to pick up on the new
frequency where it left off.
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