1. Field of the Invention
The present invention relates to a cellular mobile communication system, and in particular, to a method for performing a hard hand-off between service areas of base stations.
2. Description of the Related Art
FIG. 1 is a diagram illustrating a construction of a conventional cellular mobile communication system. Referring to FIG. 1, a plurality of base transceiver subsystems (BTSs) 20, 30, 40 connect mobile communication terminal 10 within the corresponding cells 21, 31, 41 to radio channels, and perform communications. A base station controller (BSC) 50 controls the plurality of BTSs 20, 30, 40. A mobile switching center (MSC) 60 connects the BSC 50 to other subscriber systems. The mobile communication system constructed above provides a soft or hard hand-off function so that the mobile communication terminal 10 can maintain a call while moving between the cells.
In a mobile communication system operating under code division multiple access (CDMA) technology, a single frequency channel can be basically classified into a plurality of code channels. Therefore, all the base stations have at least a single common frequency assignment (a common FA). However, where there is a need to increase capacity due to an increase of users, new frequency assignments may be added to several base stations. For instance, downtown areas of cities having quite a number of subscribers require many more frequency assignments, while suburbs require less number of frequency assignments. Here, a hard hand-off occurs when the mobile communication terminal is moving from a source station to a destination station, and when the destination station is unable to provide services with the frequency assignment currently engaged in communication with the mobile terminal.
The service area covered by a particular frequency assignment unavailable to adjacent stations is defined to be a border cell. Data on the border cell is stored in the BSC 50. The mobile communication terminal entering into the border cell searches a target frequency assignment (a target FA) under any conditions so as to be ready to perform the hard hand-off. At this stage, the mobile communication terminal searches the common FA of the base station it is currently engaged in communication with as well as of all the adjacent stations. Normally, the signal of the common FA of the base station that the mobile communication terminal is currently engaged in communication with is detected to be the most intensive.
FIG. 2 is a diagram illustrating an operation of a hard hand-off according to the conventional technology. The source station and the destination station are divided into three sectors α, β, γ, respectively. The sectors in the source station can provide services with 1FA and 2FA, while the sectors in the destination station can provide services with 1FA only. If the α sector is the only sector among all the sectors in the source station adjacent to the destination station, the area of services provided with the 2FA of the α sector is set to be a border cell. Data on such border cell is pre-stored in the BSC.
Referring to FIG. 2, if the mobile communication terminal, currently engaged in communication in the β sector of the source station with the 2FA, enters the overlapping area between the α sector, which is a boundary cell, and the β sector, the mobile communication terminal connects the communication with both the 2FA in the α sector and the 2FA in the β sector due to a soft hand-off, and starts searching the target FA from all the available common FAs. This is because the α sector is a boundary cell. As the mobile communication terminal moves toward the destination station, the signal from the α sector becomes weak. Then, the target FA should be determined.
Under normal circumstances, however, the characteristic of the radio frequency signal is unstable in the boundary cell area due to interference between frequencies. Therefore, the 1FA, which is the common FA of the source station rather than that of the destination station, can be determined as the target FA. In that case, the mobile communication terminal drops both the 2FA of the α sector and the 2FA of the β sector to perform a hard hand-off to the 1FA of the α sector. In other words, the mobile communication terminal performs a hard hand-off in the overlapping area between the α sector and the β sector as well as in the boundary cell. Moreover, if the mobile communication terminal enters deeper into the destination station, the hand-off to the destination station needs to be performed once more.
In the method for performing a hard hand-off under the conventional technology as described above, even the overlapping area between the boundary cells and adjacent cells thereof are recognized as boundary cells. As a result, an unnecessary hard hand-off operation is performed because the common FAs of all the base stations, including the base station currently engaged in communication, have been searched. As is well known, the hard hand-off poses a problem of lowering the quality of communication because the call is re-connected once it is dropped.