Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-05-07
2003-03-04
Urban, Edward F. (Department: 2685)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S440000, C455S512000, C455S513000, C455S226200, C370S332000
Reexamination Certificate
active
06529733
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of handoff control and a handoff control system in a cellular communication system and, more particularly, to a method of handoff control and a handoff control system which perform handoff processing by assigning a priority to each request for handoff from a mobile terminal.
2. Description of the Related Art
In recent years, the number of subscribers in mobile communication systems is unceasingly on the increase, and hence an increase in subscriber capacity is required. Reducing each cell radius in the cellular communication system is proposed as one of solutions for increasing the subscriber capacity. Reducing the cell radius means that a total number of base stations in the service area of the cellular communication system increases. As a consequence, the number of channels available for subscribers can be increased.
On the other hand, reducing the cell radius causes a mobile terminal to increase a chance of handoff operation. FIG.
11
(A) is a schematic view for explaining a handoff operation in a general cellular communication system; and FIG.
11
(B) is a view showing a case wherein the cell radius is reduced.
In FIG.
11
(A), assume that the mobile terminal
110
is locating in the cell
130
a
of the base station
120
a
and is communicating with the base station
120
a
, and the mobile terminal
110
is moving into the cell
130
b
of the base station
120
b
. As the mobile terminal
110
is moving away from the base station
120
a
, the reception signal intensity from the base station
120
a
gradually decreases at the mobile terminal
110
. In this case, the reception signal intensity indicates the magnitude of signal power received from the base station.
The reception signal intensity from the base station
120
a
is periodically measured by the mobile terminal
110
. When the measured reception signal intensity becomes equal to or less than a predetermined threshold level, the mobile terminal
110
sends a handoff request to the base station
120
a
for continuing the present communication of a call by changing the base station from the base station
120
a
to the base station
120
b
. With this operation, the mobile terminal
110
is set in a state in which it can also receive service from the base station
120
b.
The handoff request sent from the mobile terminal
110
to the base station
120
a
is notified from the base station
120
a
to the base station
120
b
through a network apparatus such as a base station control or mobile switching center equipment (not shown). Thereafter, the mobile terminal
110
can communicate with both the base stations
120
a
and
120
b
. When the mobile terminal
110
is further moving away from the base station
120
a
, the communication with the base station
120
a
is disconnected, and the mobile terminal
110
only communicate with the base station
120
b.
An area where the mobile terminal
110
can communicate with both the base stations
120
a
and
102
b
is an overlap area
140
where the cells
130
a
and
130
b
overlap.
When the cell radius is reduced as shown in FIG.
11
(B), the above handoff operation is frequently performed. For this reason, as the cell radius decreases, the handoff traffic increases, and forced termination of communication tends to occur due to lack of communication channels.
A method of avoiding such a situation by preparing a queue for handoff requests by each of mobile terminals is disclosed in, for example, D. Hong et al., “Traffic model and performance analysis for cellular mobile radio telephone systems with prioritized and nonprioritized handoff procedures”, IEEE Trans. Veh. Technol., vol. VT-35, August 1986 (reference 1) and Q. A. Zeng et al., “Performance analysis of mobile cellular radio system with priority reservation handoff procedures”, IEEE Proc. VTC-94, vol. 3, June 1994 (reference 2).
According to reference 1, of all the set channels, some number of channels are always reserved as handoff channels, and the dedicated handoff channels are only used for the handoff operation, and not used for ordinary new calls. With this arrangement, the call loss probability during handoff operation due to lack of communication channels is reduced. According to reference 2, a buffer for new calls is provided in addition to the technique disclosed in reference 1 to decrease the call loss probability for new calls without increasing the blocking probability and probability of forced termination of handoff calls much.
In addition, Japanese Patent Laid-Open No. 7-264656 (reference 3) discloses a technique of assigning priority to each handoff process on the basis of a calculation result considering the moving speed and direction of each mobile terminal, and performing the handoff processes in accordance with the priority.
Each mobile terminal is moving across cells at various speeds. For example, the moving speed of a mobile terminal that is moving on a car differs from that of a mobile terminal carried by a walking user. As the moving speed of each mobile terminal differs in this manner, the time allowed between the instant at which a handoff request is generated and the instant at which the handoff process is completed changes. When handoff processes are performed only in the order of accepting handoff requests, forced termination of communication may occur at a certain mobile terminal that is moving at a high speed if between the instant at which a handoff request is generated and the instant at which the handoff process is completed is prolonged.
In addition, a mobile terminal that moves at a high speed generates handoff requests many times during one communication because it passes many cells in a predetermined period of time, and hence the probability of forced termination of communication tends to increase. Furthermore, mobile terminals move across cells through various routes; some mobile terminals move away from the base station from which service is provided, and some mobile terminals move while keeping distances to the base station constant.
When mobile terminals move through different routes as described above, the time allowed between the instant at which a handoff request is generated and the instant at which the handoff process is completed changes as well. When handoff processes are simply performed in the order of handoff requests, a delay in performing a handoff process may cause forced termination of communication at a mobile terminal which moves away rapidly from the base station currently under communication.
In the technique disclosed in reference 3, since complicated arithmetic operation is required to assign priorities, and a priority is assigned to each call, processing for a handoff request is frequently performed and complicated.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a handoff control system and method which can decrease the probability of forced termination of communication by considering the time allowed between the instant at which a handoff request is generated and the instant at which the handoff process is completed.
In order to achieve the above object, according to the present invention, a handoff control system for performing a handoff processing for a mobile terminal, that moves across cells of a plurality of base stations while performing mobile communication, comprises the following mobile terminal and a base station.
A mobile terminal which periodically measures a reception signal intensity of a radio signal used for a current communication, calculates a relative change value of reception signal intensity at measurement time intervals, and
periodically reports the measured reception signal intensity and the calculated relative change value of reception signal intensity; and
a base station which periodically receives and stores the reception signal intensity and the relative change value of reception signal intensity respectively reported from the mobile terminal, and performs a handoff processing by giving a higher priority to
Davis Temica M.
Foley & Lardner
NEC Corporation
Urban Edward F.
LandOfFree
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