Mobile communication system and mobile station performing diversity handover

#Recently, code division multiple access (CDMA) technology has been proposed as one of promising radio transmission methods for implementing multimedia communications in the next generation mobile communication network systems. A CDMA system carries out diversity handover (DHO) that establishes multiple communication links (branches) between a mobile station and a plurality of base stations while the mobile station is moving near a boundary of a radio area, and communicates by performing signal combining (selection combining) on the multiple communication links. In the diversity condition, it is possible to increase a radio capacity by controlling such that the mobile station and the base stations can communicate at minimum transmission power (transmission power control). Furthermore, it is possible in the diversity condition to eliminate instantaneous chopping during handover, which can occur in the conventional time division multiple access (TDMA).

#Establishing a new branch in the DHO is specifically referred to as “addition DHO” in the DHO, and deleting a communication branch not contributing to the communications in the diversity condition is specifically referred to as “deletion DHO” in the DHO. To carry out the addition DHO and deletion DHO, the mobile station normally detects a candidate of the addition DHO (called “addition DHO candidate” or “addition branch candidate”), and a candidate of the deletion DHO (called “deletion DHO candidate” or “deletion branch candidate”). Detecting an addition or deletion branch candidate by monitoring a radio condition (transmission loss, for example) of a perch in the current sector or peripheral sectors, the mobile station notifies the network side of the detected candidate. The network side performs the DHO (addition DHO or deletion DHO) based on the notified candidate. The term “network side” usually refers to a switching center or a control center of base stations, it is possible to provide this function to the base stations.

#The mobile station communicates with the base stations using individual radio links (radio branches) associated with the base stations. A switching center comprises cable links (cable branches) connecting it with the base stations that are communicating with the mobile station, and carries out, with a handover (HO) processor or a diversity handover trunk (DHT), the selection combining of the signals sent from the base stations. The fundamental operation of the diversity handover branch is disclosed in Japanese Patent Application Laid-open No. 9-508773 (1997), and network configurations and control methods of the diversity handover trunks are disclosed in Japanese Patent Application No. 8-348900 (1996).

#To further clarify the above and other advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

#FIG. 1 is a schematic diagram illustrating an embodiment of a diversity handover branch control method in accordance with the present invention;

#FIG. 2 is a diagram illustrating images of branch connection and release on a mobile station side and a network side in respective DHO operations;

#The invention will now be described with reference to the accompanying drawings.

#FIG. 1 is a schematic diagram illustrating an embodiment of a diversity handover branch control method in accordance with the present invention.

#In FIG. 1, a mobile station (MS) 20 is a device such as a mobile phone and a mobile information terminal, which has radio communication functions, and base stations (BS's) 10, 14 and 16 are radio stations that communicate with the mobile station 20. The mobile station communicates with the base stations 10, 14 and 16 at first. Mobile services switching center (MSC) 30, 32 or 34, which is connected with one of the base stations 10, 12, 14, 16 and 18 through a cable, has base station control functions that perform radio control of the base stations 10 and others, and a paging control function that provides the mobile station 20 and others with communication services through the base stations 10 and others. It will be needless to say that the mobile switching center 30, 32 or 34 can be connected with the base station 10, 12, 14, 16 or 18 through fixed radio channels instead of the cable. One mobile switching center can handle a plurality of base stations. Although the base station 10 is connected double through cable channels to both the mobile services switching centers 30 and 32 in FIG. 1, this is for the purpose of load distribution or switching between operating and standby systems, and each base station is connected to one of the mobile services switching centers during a single communication term. When the mobile station 20 moves to the area of the mobile station 22, and acquires quality communication condition with the base stations 12 and 18, the base stations 12 and 18 each become an addition branch candidate. In this case, if the radio condition between the mobile station 20 and the base stations 10 and 14 deteriorates, the base stations 10 and 14 each become a deletion branch candidate. Thus, because the addition branch candidate is present, it does not necessarily follow that the deletion branch candidate is present. The mobile services switching centers 30, 32 and 34 are interconnected through a trunk circuit network.