Power control

Date: 2015-10-07; view: 438.

In-call handover

At all times during a call (whether L-M or M-L) the base station currently serving the mobile monitors the signal (strength and/or quality) from the mobile. If the signal falls below a predesignated threshold, the network will command neighbouring base stations to measure the signal from the mobile (Figure 47.3(a)). If another base station is receiving the mobile with a stronger signal than the current base station, a signalling message is sent to the mobile on the voice channel from the current base station commanding the mobile to a new voice channel, namely a free voice channel from those allo­cated to the neighbouring cell. The mobile changes frequency (and thereby the serving base station) and simultaneously the network connects the call to the new base station (Figure 47.3(b)).

The measuring process and new cell selection may take several seconds, but the user will only be aware of a brief break in trans­mission as the mobile tunes to the new voice channel.

Since the size of a cell may be anything from one kilometre to tens of kilometres across, it is not necessary for a mobile to transmit on full power at all times in order to maintain a satisfactory signal level at the base station's receiver. Most cellular standards therefore incorporate mobile power control, the base station commanding the mobile to transmit at one of a number of power levels. As the mobile moves closer to or further from the base station, further commands are issued to keep the received signal level to prescribed limits. By reducing the average mobile power level, co-channel interference is reduced, improving overall system quality.