Movement from cell to cell and handover

The use of multiple cells means that, if the distributed transceivers are mobile and moving from place to place, they also have to change from cell to cell. The mechanism for this differs depending on the type of network and the circumstances of the change. For example, if there is an ongoing continuous communication and we don't want to interrupt it, then great care must be taken to avoid interruption. In this case there must be clear coordination between the base station and the mobile station. Typically such systems use some kind of multiple access independently in each cell, so an early stage of such a handover (handoff) is to reserve a new channel for the mobile station on the new base station which will serve it. The mobile then moves from the channel on its current base station to the new channel and from that point on communication takes place. The exact details of the mobile system's move from one base station to the other varies considerably from system to system. For example, in all GSM handovers and W-CDMA inter-frequency handovers the mobile station will measure the channel it is meant to start using before moving over. Once the channel is confirmed okay, the network will command the mobile station to move to the new channel and at the same time start bi-directional communication there, meaning there is no break in communication. In CDMA2000 and W-CDMA same-frequency handovers, both channels will actually be in use at the same time (this is called a soft handover or soft handoff). In IS-95 inter-frequency handovers and older analog systems such as NMT it will typically be impossible to measure the target channel directly whilst communicating. In this case other techniques have to be used such as pilot beacons in IS-95. This means that there is almost always a brief break in the communication whilst searching for the new channel followed by the risk of an unexpected return to the old channel.