Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-04-02
2002-11-26
Bost, Dwayne (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S435100, C455S560000
Reexamination Certificate
active
06487409
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a handoff control system in a cellular communication system and, more particularly, to a handoff control system and method which perform handoff processing by assigning priorities.
Recently, the number of subscribers in mobile communication systems is unceasingly on the increase, and hence an increase in subscriber capacity is required. To increase the subscriber capacity of a mobile communication system, a technique of reducing the cell radius has been studied. If the cell radius is reduced, the number of base stations in the service area increases. As a consequence, the number of channels used by subscribers can be increased.
Assume that a terminal
110
is present in a cell
130
a
of a base station
120
a
and is receiving service from the base station
120
a
, and the terminal
110
moves into a cell
130
b
of a base station
120
b
, as shown in FIG.
9
A. As the terminal
110
moves away from the base station
120
a
, the reception signal strength from the base station
120
a
gradually decreases in the terminal
110
. In this case, the reception signal strength indicates the magnitude of power received from the base station.
The reception signal strength from the base station
120
a
is periodically measured in the terminal
110
. When the measured reception signal strength becomes equal to or less than a predetermined threshold, the terminal
110
sends a handoff request to the base station
120
a
. With this operation, the 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 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 (not shown). Thereafter, the terminal
110
can receive service from both the base stations
120
a
and
120
b
. When the terminal
110
further moves away from the base station
120
a
, the service from the base station
120
a
is stopped, and the terminal
110
receives service from only the base station
120
b.
An area where the terminal
110
can receive service from both the base stations
120
a
and
102
b
is an area
140
where the cells
130
a
and
130
b
overlap.
When the cell radius is reduced as shown in
FIG. 9B
, the above handoff operation is frequently performed. For this reason, as the cell radius decreases, the handoff rate increases, and forced termination of communication tends to occur.
A method of avoiding such a situation by preparing a queue for requests from terminals that demand handoffs 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 ensured as handoff channels, and the handoff channels are not used for new calls. With this setting, the loss probability during handoff operation is reduced.
According to reference 2, a buffer for new calls is used in addition to the technique disclosed in reference 1 to decrease the loss probability of 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 priorities to handoff processes on the basis of a mathematical expression set in consideration of the moving speeds and directions of terminals, and performing the handoff processes in accordance with the priorities.
Terminals move across cells at various speeds. For example, the moving speed of a terminal that moves on a car differs from that of a terminal carried by a walking user. When the moving speeds of terminals differ 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. For this reason, when handoff processes are performed in the order of handoff requests, forced termination of communication may occur at a terminal that is moving at a high speed if the time 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 terminal that moves at a high speed generates handoff requests many times during one communication operation because it passes many cells in a predetermined period of time, and hence the frequency of forced termination of communication tends to increase.
Furthermore, terminals move across cells through various routes; some terminals move away from the base stations from which service is received, and some terminals move while keeping distances to the base stations constant.
When 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 to be simply performed in the order of handoff requests, a delay in performing a handoff process upon generation of a handoff request may cause forced termination of communication at a terminal that only moves away from the base station from which service is currently received because of a process delay.
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 terminal generates a handoff request and the instant at which the handoff process is completed.
In order to achieve the above object, according to the present invention, there is provided a handoff control system for performing handoff processing for a terminal that moves across cells of a plurality of base stations while performing speech communication, comprising measuring means for periodically measuring a reception signal strength in the terminal, calculation means for calculating the relative change amount of reception signal strength at measurement time intervals on the basis of the measurement result output from the measuring means, queue storage means storing, in advance, a plurality of queues to which priories based on relative change amounts of reception signal strength are assigned, request processing means for, when the terminal generates a handoff request, distributing a call from the terminal to one of the queues on the basis of the relative change amount of reception signal strength in the terminal, and queue control means for, when an available channel is present in an adjacent cell to which the terminal, which generated the request, has moved, assigning the call, distributed into the queue according to the order based on the priority, to the available channel.
REFERENCES:
patent: 5465389 (1995-11-01), Agrawal et al.
patent: 5535425 (1996-07-01), Watanbe
patent: 5640676 (1997-06-01), Garncarz et al.
patent: 5790954 (1998-08-01), Tayloe et al.
patent: 669 775 (1995-08-01), None
patent: 2-44929 (1990-02-01), None
patent: 7-240959 (1995-09-01), None
patent: 7-264656 (1995-10-01), None
patent: WO 93/12588 (1993-06-01), None
Yi Bing Lin et al., PCS Channel Assignment Strategies for Hand-off and Initial Access, IEEE Personal Communications, 1(1994) 3rd Quarter, No. 3, New York, NY, U.S., 1994.*
Hong , D., et al., “Traffic Model and Performance Analysis for Cellular Mobile Radio Telephone Systems with Prioritized and Nonprioritized Handoff Procedures,” IEEE Transactions on on Vehicular Technology, v
Bost Dwayne
Contee Joy K.
Foley & Lardner
NEC Corporation
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