Page 51 - From GMS to LTE

Global System for Mobile Communications (GSM) 37

Mobile device BTS (old) BSC BTS (new)

Existing connection,
BSC makes handover decision
Chan act. (TCH)

Chan act. ack.
Handover command
FACCH: Handover (ARFCN, TS) MS
command
(ARFCN, TS)

Handover access
(MS changes cell) Handover access
Handover access
Handover access
FACCH: SABM
Establish ind.
FACCH: UA

BSC redirects connection to TCH in new cell
Chan. rel.
Chan rel. ack.
Handover performed (to MSC)
Figure 1.30 Message flow during a handover procedure.

To reduce interference, the BSC is also in charge of controlling the transmission
power for every air interface connection. For the mobile device, an active power control
has the advantage that the transmission power can be reduced under favorable recep-
tion conditions. Transmission power is controlled using the signal quality measurements
of the BTS for the connection. If the mobile device’s transmission power has to be
increased or decreased, the BSC sends a Power Control message to the BTS. The BTS
in turn forwards the message to the mobile device and repeats the message on the
SACCH in every frame. In practice, it can be observed that power control and adapta-
tion is performed every 1–2 seconds. During call establishment, the mobile device
always uses the highest allowed power output level, which is then reduced or increased
again by the network step by step. Table 1.6 gives an overview of the mobile device
power levels. A distinction is made for the 900 MHz versus the 1800 MHz band. While
mobile devices operating on the 900 MHz band are allowed to use up to 2 W, connec-
tions on the 1800 MHz band are limited to 1 W. For stationary devices or car phones
with external antennas, power values of up to 8 W are allowed. The power values in the
table represent the power output when the transmitter is active in the assigned timeslot.
As the mobile device only sends on one of the eight timeslots of a frame, the average
power output of the mobile device is only one‐eighth of this value. The average power
output of a mobile device that sends with a power output of 2 W is thus only 250 mW.
The BSC is also able to control the power output of the base station. This is done by
evaluating the signal measurements of the mobile devices in the current cell. It is impor-
tant to note that power control can only be performed for downlink carriers that do not