Method of handover control in mobile radio communication system

Abstract: In the method of handover control according to the present invention, when the mobile station recognizes that it has moved to an overlapping area, the mobile station notifies this fact to the network side and in response the network side sets up only a communication channel between the handover target base station and the exchange station for controlling that handover target base station, without setting up a radio channel between the handover target base station and the mobile station, while continuing the communication by not releasing the communication channel between the currently located base station and the exchange station and the radio channel between the currently located base station and the mobile station, so that it becomes possible to reduce a transmission control load for transmission between the base station and the exchange station as well as a control load for controlling the exchange station itself.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a method of handover control in a mobile radio communication system in which a service area for supporting a communication with respect to a mobile station is formed by a plurality of radio zones at which a plurality of base stations are distributedly provided respectively, for controlling a handover in the case where a mobile station moves from a currently located radio zone to a neighbouring radio zone and a handover in the case of switching a type of channel to be used for the communication with respect to the mobile station. DESCRIPTION OF THE BACKGROUND ART In WO 95/18513, there is described a cellular mobile radio system with improved handover, where it is proposed to keep as short as possible interrupt times due to a handover of a connection from one radio cell to another radio cell, that different stations have a coupling device for inserting an incoming signal from a switching center into a signal going to the switching center. The conventional handover control in the mobile radio communication is realized in such a manner that, when a request for switching a radio channel occurs, this request is notified from a mobile station to a handover target base station or a currently located base station, via the currently located base station and an exchange station for controlling the currently located base station. However, this conventional handover control is associated with a problem of increasing a transmission control load for transmission between the base station and the exchange station, a control load for controlling the exchange station itself, and a handover control delay. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a method of handover control in a mobile radio communication which is capable of reducing a transmission control load for transmission between the base station and the exchange station, a control load for controlling the exchange station itself, and a handover control delay. According to one aspect of the present invention there is provided a method of handover control having the features of claim 1. Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of a mobile radio communication system for realizing a handover control method according to one embodiment of the present invention, in a state before a mobile station enters an overlapping area. Fig. 2 is a schematic diagram of a mobile radio communication system for realizing a handover control method according to one embodiment of the present invention, in a state in which a mobile station just enters an overlapping area. Fig. 3 is a schematic diagram of a mobile radio communication system for realizing a handover control method according to one embodiment of the present invention, in a state in which a request for switching a radio channel occurs at a mobile station. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Fig. 1 to Fig. 3, one embodiment of a handover control method according to the present invention will be described. Fig. 1 to Fig. 3 show a mobile radio communication system which realizes the handover control method of this embodiment. First, in Fig. 1, a mobile station 1 is currently located in a radio zone 5 under a currently located base station 3 and communicating with the currently located base station 3 through a radio channel 15, while the currently located base station 3 is connected with an exchange station 7 through a communication channel 17. In this state, the mobile station 1 has not yet entered into an overlapping area 13 between the radio zone 5 and a neighboring radio zone 11 under a handover target base station 9. When the mobile station 1 moves from a state shown in Fig. 1 into the overlapping area 13 as shown in Fig. 2, the mobile station recognizes this state and notifies this fact to a network side. At the network side, upon receiving this notification, a communication channel 19 alone is set up between the handover target base station 9 and the exchange station 7 for controlling the handover target base station 9. Also, in this case, the handover target base station 9 acquires a logical number unique within the handover target base station 9 for the purpose of identifying the mobile station 1 at the handover target base station 9, and notifies this logical number to the mobile station 1 via the exchange station 7 and the currently located base station 3. Then, when the mobile station 1 moves more deeply into the overlapping area 13 as shown in Fig. 3 and a request for switching a radio channel occurs at the mobile station 1, this request is directly notified from the mobile station 1 to the handover target base station 9 by sending the logical number so as to set up a radio channel 21 between the mobile station 1 and the handover target base station 9, while the radio channel 15 between the mobile station 1 and the currently located base station 3 is released. Here, in order to establish a relationship of the radio channel 21 between the mobile station 1 and the handover target base station 9 with the communication channel 19 between the handover target base station 9 and the exchange station 7, the mobile station 1 notifies the logical number to the handover target base station 9. Upon receiving this notification, the handover target base station 9 connects the radio channel 21 and the communication channel 19 according to the logical number as managed therein. According to this handover control, it is possible to switch the radio channel without sending signals through the currently located base station 3 and the exchange station 7, so that it is possible to reduce the channel control load within the network as well as the handover control delay. It is to be noted here that this handover control method is generally applicable to the case in which a request for switching the radio channel to that of a base station that is constituting the overlapping area occurs at the mobile station while the mobile station is located in the overlapping area. As described, according to the handover control method of this embodiment, when the mobile station recognizes that it has moved to an overlapping area, the mobile station notifies this fact to the network side and in response the network side sets up only a communication channel between the handover target base station and the exchange station for controlling that handover target base station, without setting up a radio channel between the handover target base station and the mobile station, while continuing the communication by not releasing the communication channel between the currently located base station and the exchange station and the radio channel between the currently located base station and the mobile station, so that it becomes possible to reduce a transmission control load for transmission between the base station and the exchange station as well as a control load for controlling the exchange station itself. In addition, according to the handover control method of this embodiment, when a request for switching the radio channel to that of a base station that is constituting the overlapping area occurs at the mobile station while the mobile station is located in the overlapping area, the mobile station directly notifies this request to a handover target base station, without utilizing a transmission via the currently located base station, so that it becomes to reduce a transmission control load for transmission between the base station and the exchange station, a control load for controlling the exchange station itself, and a handover control delay.

Claims: A method of handover control in a mobile radio communication system in which a service area for supporting a communication with respect to a mobile station (1) is formed by a plurality of radio zones (5, 11) at which a plurality of base stations (3, 9) are distributedly provided, respectively, for controlling a handover in which a mobile station (1) moves from a currently located radio zone (5) to a neighbouring radio zone (11), the method comprising the steps of: (a) recognizing at the mobile station (1) a moving from a non-overlapping area of the currently located radio zone (5) and the neighbouring radio zone (11) to an overlapping area (13) of the currently located radio zone (5) and the neighbouring radio zone (11) within the currently located radio zone (5), and notifying the moving from the mobile station (1) to a network side; and (b) setting up only a new communication channel (19) between a handover target base station (9) that constitutes the overlapping area and an exchange station (7) for controlling the handover target base station (9), while continuing the communication by continuing connections of an original communication channel (17) between a currently located base station (3) and the exchange station (7) and an original radio channel (15) between the currently located base station (3) and the mobile station (1), at the network side upon receiving a notification of the moving from the mobile station (1); characterized by (c) notifying a request for switching a radio channel from the original radio channel (15) to a new radio channel (21) from the handover target base station, directly from the mobile station (1) to the handover target base station (9), when the request occurs at the mobile station (1) while the mobile station (1) is located in the overlapping area (13). The method of claim 1, further comprising the step of: (d) setting up the new radio channel (21) and connecting the new radio channel (21) with the new communication channel (19) while releasing the original communication channel (17), a the network side upon receiving the request from the mobile station (1). The method of claim 2, further comprising the step of: (e) notifying a logical number for uniquely identifying the mobile station (1) within the handover target base station (9), from the handover target base station (9) to the mobile station (1) via the currently located base station (3) and the exchange station (7) upon receiving the notification of the moving from the mobile station (1); wherein the step (c) notifies the request by using the logical number and the step (d) connects the new radio channel (21) with the new communication channel (19) according to the logical number as notified by the step (c).

Inventors: HAMAJIMA TAKUYA, SATO TAKAAKI, YAMAGATA KATSUHIKO, HAMAJIMA, TAKUYA, SATO, TAKAAKI, YAMAGATA, KATSUHIKO
Applicants: NTT DOCOMO INC, NTT DOCOMO INC.
Application year: 1998
Main classification: H04Q7
IPC codes: H04B7, H04W36