Page 66 - From GMS to LTE

52 From GSM to LTE-Advanced Pro and 5G

which can change at any time because of the mobility of the user. If the user roams into
the area of a new cell, it may need to inform the network of this change. To reduce the
signaling load in the radio network, several cells are grouped into a location area. The
network informs the mobile device via the BCCH of a cell not only of the cell ID but also
of the LAC that the new cell belongs to. The mobile device thus only has to report its
new location if the new cell belongs to a new location area. Grouping several cells into
location areas not only reduces the signaling load in the network but also the power
consumption of the mobile. A disadvantage of this method is that the network operator
is only aware of the current location area of the subscriber but not of the exact cell.
Therefore, the network has to search for the mobile device in all cells of a location area
for an incoming call or SMS. This procedure is called paging. The size of a location area
can be set by the operator depending on its particular needs. In operational networks,
several dozen cells are usually grouped into a location area (Figure 1.41).
Figure 1.39 shows how a location area update procedure is performed. While idle, the
mobile measures the signal strengths of the serving cell and of the neighboring cells.
Neighboring cells can be found because their transmission frequency is announced on
the broadcast channel (BCCH) of the serving cell. Typical values that a signal is received
with are −100 dBm, which indicates that it is very far away from the base station, and
−60 dBm, which indicates that it is very close to the base station. This value is also
referred to as the received signal strength indication (RSSI). Once the signal of a neigh-
boring cell becomes stronger than the signal of the current cell by a value that can be set
by the network operator, the mobile reselects the new cell and reads the BCCH. If the
LAC that is broadcast is different from that of the previous cell a location update proce-
dure is started. After a signaling connection has been established, the mobile device
sends a Location Update Request message to the MSC, which is transparently forwarded

Location area 2708

Location area 5578

Normal cell change,
no location area update
necessary Location area has changed,
location area update procedure
started
Figure 1.41 Cells in different location areas.