Page 176 - From GMS to LTE

162 From GSM to LTE-Advanced Pro and 5G

Soft Handover
With this kind of handover, a voice call is not interrupted at any time during the proce-
dure. On the basis of signal quality measurements of the current and neighboring cells,
the RNC can decide to set the mobile device into soft handover state. All data from and
to the mobile device will then be sent and received not only over a single cell but also
over two or even more cells simultaneously. All cells that are part of the communication
are put into the so‐called Active Set of the connection. If a radio connection of a cell in
the Active Set deteriorates, it is removed from the connection. Thus it is ensured that
despite the cell change, the mobile device never loses contact with the network. The
Active Set can contain up to six cells at the same time, although in operational networks
no more than two or three cells are used at a time. Figure 3.25 shows a soft handover
situation with three cells.
The soft handover procedure has a number of advantages over the hard handover
described before. As no interruption of the user data traffic occurs during the soft
handover procedure the overall connection quality increases. As the soft handover pro-
cedure can be initiated while the signal quality of the current cell is still acceptable the
possibility of a sudden loss of the connection is reduced.
Furthermore, the transmission power and hence the energy consumption of the
mobile device can be reduced in some situations as shown in Figure 3.26. In this sce-
nario, the subscriber first roams into an area in which it has good coverage by cell 1. As
the subscriber moves, there are times when buildings or other obstacles are in the way
of the optimal transmission path to cell 1. As a consequence, the mobile device needs to
increase its transmission power. If the mobile device is in soft handover state, however,
cell 2 still receives a good signal from the mobile device and can thus compensate for the
deterioration of the transmission path to cell 1. As a consequence, the mobile device is
not instructed to increase the transmission power. This does not mean, however, that
the connection to cell 1 is released immediately, as the network speculates on an
improvement of the signal conditions.
As the radio path to cell 1 is not released, the RNC receives the subscriber’s data
frames from both cell 1 and cell 2 and can decide, on the basis of the signal quality

User moves through
the coverage areas of
several Node-Bs.
Network activates
the soft handover
mode. Iub

Iu(cs), Iu(ps)
RNC

Data is received by the RNC but
is discarded as the same frame
is received from another Node-B
with a better signal quality rating.
Figure 3.25 Connections to a mobile device during a soft handover procedure with three cells.