A mobile communication system is provided with a handover function such that a call is not interrupted even when a user is talking while moving. The third generation mobile telephone system is called W-CDMA (Wideband Code Division Multiple Access) system. With this system, a mobile unit is capable of communicating with a plurality of base stations (called “cells”) simultaneously if the base stations use the same frequency. Accordingly, in the case of a call while moving, the mobile unit maintains communication with a first base station with which the mobile unit is currently communicating while starting communicating with a second base station, and thereafter, communication with the first base station is interrupted in the case where the communication level with respect to the first base station is lowered. Such handover is called soft handover (SHO) or diversity handover (DHO). The state in which a mobile unit is communicating with a plurality of base stations is called a soft handover state or diversity handover state.
The technique related to a conventional mobile communication system having the handover function is disclosed in the following document 1.
Patent Document 1
Japanese Patent Application Laid-Open No. 2002-112311
However, the conventional mobile communication system has a problem in that its handover function is insufficient.