mobile to switch to a designated voice channel, namely one of the threshold, the network will command neighbouring base stations to channels allocated to that cell. The mobile retunes to the channel measure the signal from the mobile (Figure 47.3(a)). If another base indicated and the network proceeds to set up the call to the desired station is receiving the mobile with a stronger signal than the current Mobile signal measured Mobile commanded and new BS selected to new channel Path of mobile Figure 47.3 In-call handover Radio planning 47/5 base station, a signalling message is sent to the mobile on the voice of planning a cellular radio system is to design the cell repeat pattern channel from the current base station commanding the mobile to a and frequency allocation in order to maximise the capacity of the new voice channel, namely a free voice channel from those allo- system whilst controlling co-channel interference to within accept- cated to the neighbouring cell. The mobile changes frequency (and able limits. thereby the serving base station) and simultaneously the network connects the call to the new base station (Figure 47.3(b)). 47.3.1 Cell repeat patterns The measuring process and new cell selection may take several seconds, but the user will only be aware of a brief break in trans- The cell plan has to be chosen such that the number of channel sets mission as the mobile tunes to the new voice channel. (N) fit together in a regular fashion without gaps or overlaps. Only certain values of N achieve this, and typical arrangements of interest 47.2.6 Power control to cellular radio are N = 4, 7 and 12 as shown in Figure 47.4. The value of N has a major effect on the capacity of the cellular system. 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 As the number of channels sets is decreased, the number of full power at all times in order to maintain a satisfactory signal level channels per cell increases, hence the sys