Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-11-19
2001-09-25
Vo, Nguyen T. (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S525000, C455S439000, C455S453000
Reexamination Certificate
active
06295451
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a mobile communication system of a cellular scheme such as a automobile telephone system or a portable telephone system, and a base station (BS) and base station controller (BSC) which are used for the mobile communication system.
It is generally known that a radio wave emitted from a given radio station is attenuated in proportion to the power of the distance from the radio station. For this reason, according to the cellular scheme, many base stations are distributed to form cover areas called cells, and each base station takes charge of the mobile stations in each cover area.
In such a cellular scheme, when a mobile station moves between cells during communication, the base station taking charge of the mobile station changes. In this mobile communication system, handoff processing is performed to change the base station that connects the radio path to the mobile station.
In general, handoff processing is performed to connect a radio path between the mobile station and a base station with a higher reception power on the basis of the reception power of a transmission signal received by the mobile station from each base station. More specifically, in a mobile communication system using a CDMA (Code Division Multiple Access) scheme, each base station transmits a pilot channel. Each mobile station measures the reception electric field strength of each pilot channel that can be received, and notifies the base station that connects the radio path to the mobile station of the measurement results. The base station compares the respective pilot reception powers (the reception electric field strengths of the respective pilot signals) received from the mobile station of which the base station is taking charge. If there is another base station that has a pilot reception power higher than the reception electric field strength of the pilot channel transmitted by the above base station, handoff processing is executed to shift the charge of the mobile station to this other base station.
FIG. 1
shows the distribution of pilot reception power associated with adjacent base stations A and B which transmit pilot channels with the same transmission power.
As is obvious from this graph, the pilot reception power of the base station A becomes equal to that of the base station B at a point C as a middle point between the base stations A and B. The pilot reception power of the base station A becomes larger than that of the base station B at any point closer to the base station A than the point C. The pilot reception power of the base station B become larger than that of the base station A at any point closer to the base station B than the point C. Consequently, the handoff execution boundary between the base stations A and B is the point C, i.e., the middle point between the base stations A and B.
When, therefore, base stations are arranged as in
FIG. 2
, if each base station has its communicative area within the circle indicated by the dashed line in
FIG. 2
, the cell formed by each base station falls within the hexagonal range indicated by the solid line in FIG.
2
.
Since the transmission power of each base station is almost fixed, each cell is fixed to the state shown in FIG.
2
. For this reason, even if the traffic in a specific cell is larger than that in an adjacent cell, when a mobile station that has been located in the adjacent cell moves closer to the base station that manages the specific cell, handoff to the base station in the specific cell must be performed. As a result, traffic may concentrate on the specific cell.
For example, a method of locally forming a small cell (micro-cell) at a portion, in a cell, in which the traffic is heavy may be used as a method of preventing traffic concentration. In this case, however, many facilities must be added to the base station, and the coexistence of cells (macro-cells) and micro-cells results in complex, cumbersome control.
As described above, according to the prior art, since the size of each cell is fixed, traffic concentration on a specific cell cannot be avoided.
If micro-cells are formed to prevent such traffic concentration, the base station facilities must be increased, and complex, cumbersome control is required.
BRIEF SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a mobile communication system which can distribute traffic without increasing base station facilities and requiring complicated and cumbersome control, thereby providing stable communication services, and a base station and base station controller which can implement the mobile communication system.
According to one aspect of the present invention, there is provided a mobile communication system in which a plurality of base stations are distributed such that a communication range of each base station overlaps communication ranges of other adjacent base stations, and handoff control for a mobile station communicating through one of the plurality of base stations is performed on the basis of reception powers in the mobile station which are related to predetermined signals output from the respective base stations, the system comprising: means for determining a base station that should take charge of the mobile station, for which the handoff control is to be performed, on the basis of reception powers in the mobile station which are related to predetermined signals output from the respective base stations and traffic amounts in the respective base stations; and means for performing handoff control to allow the mobile station to perform communication through the determined base station.
In the above system, the base station determination means preferably selects a base station on the basis of priority values obtained by adding predetermined correction values corresponding to the traffic amounts in the respective base stations to reception power values, in the mobile station for which handoff control is to be performed, which are related to the predetermined signals output from the respective base stations. In this case, the base station determination means may adopt, as a priority value, a reception power value in the mobile station which is related to a predetermined signal output from a base station whose traffic amount is smaller than a predetermined value.
The above system may further comprise correction value generating means for generating a correction value that changes with a delay upon a change in traffic in each base station, and the base station determination means may use the correction value generated by the correction value generating means to select a base station.
The above system may further comprise correction value generating means having a conversion table for converting a traffic amount in each base station into a predetermined correction value, and the base station determination means may use the correction value generated by the correction value generating means to select a base station.
In the above system, the handoff control means may be prepared in each of the plurality of base stations and perform handoff control for a mobile station communicating through the base station in which the handoff control means is provided. In this case, the above system may further comprise a control station for collectively controlling the plurality of base stations, the control station comprising traffic amount notification means for notifying each of the plurality of base stations of traffic amounts in the remaining base stations. The traffic amount notification means may notify the number of available traffic channels in the remaining base stations.
According to another aspect of the present invention, there is provided a base station used in a mobile communication system in which a plurality of base stations are distributed such that a communication range of each base station overlaps communication ranges of other adjacent base stations, and handoff control for a mobile station communicating through one of the plurality of base stations is performed on the basis
Banner & Witcoff , Ltd.
Kabushiki Kaisha Toshiba
Lee John J.
Vo Nguyen T.
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