Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-09-01
2002-12-17
Nguyen, Lee (Department: 2683)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S331000, C370S332000, C455S336000
Reexamination Certificate
active
06496492
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a cellular communication system, and more particularly to a hand-off control method and hand-off control system for performing hand-off by giving priorities to calls.
2. Description of the Related Art
Subscribers to mobile communication systems have steadily increased in recent years, and an increase in capacity for subscribers is consequently in great demand. Reducing cell radius has been considered as one way of increasing the subscriber capacity in a mobile communication system. Reducing the cell radius, however, brings about an increase in the number of base stations in a particular area and can increase the number of channels used by subscribers.
FIG. 1
a
illustrates a hand-off control method in a typical mobile communication system, and
FIG. 1
b
shows a case in which the cell radius is reduced from the case shown in
FIG. 1
a.
A case is described as shown in
FIG. 1
a
in which mobile station
110
, which is present within cell
130
a
of base station
120
a
and receiving service from base station
120
a
, moves toward cell
130
b
of base station
120
b.
As mobile station
110
moves away from base station
120
a
, the intensity of signals from base station
120
a
received at mobile station
110
gradually diminishes. In this case, the intensity of received signals refers to the level of power received from the base station.
The intensity of signals from base station
120
a
received at mobile station
110
is periodically measured. When the measured received signal intensity falls below a predetermined threshold, a hand-off request is transmitted from mobile station
110
to base station
120
a
, and mobile station
110
is accordingly allowed to also receive service from base station
120
b.
The hand-off request sent from mobile station
110
to base station
120
a
is notified to base station
120
b
from base station
120
a
by way of a network (not shown), whereupon mobile station
110
can receive service from both base stations
120
a
and
120
b.
Then, as mobile station
116
moves farther away from base station
120
a
, the service from base station
120
a
is halted and mobile station
110
receives service only from base station
120
b.
The region in which mobile station
110
can receive service from both base stations
120
a
and
120
b
is area
140
, which is the region in which cell
130
a
and cell
130
b
overlap.
The above-described hand-off operation is carried out with greater frequency as the cell radius is reduced as shown in
FIG. 1
b
. Hand-off traffic thus increases as the cell radius is reduced, making forced disconnection of communication more likely to occur.
To prevent such forced disconnection, a method in which queues are maintained for requests from mobile stations requiring hand-off is disclosed in, for example, “Traffic model and performance analysis for cellular mobile radio telephone systems with prioritized and nonprioritized handoff procedures ” by D. Hong and S. S. Rappaport (IEEE Transactions on Vehicular Technology, Vol. VT-35, 1986) and “Performance analysis of mobile cellular radio systems with priority reservation hand-off procedures ” by Q. A. Zeng, K. Mukumoto, and A. Fukuda (IEEE Proceedings, Vehicular Technology Conference-94, Vol. 3, 1994).
The former paper discloses a technique in which several channels dedicated to hand-off are always secured among the set channels. These secured channels are not used for new calls, and the rate of lost calls during hand-off can thus can be reduced.
The latter paper proposes adding a buffer for new call use to the technique disclosed in the former paper to provide a further reduction in the loss probability of new calls without appreciably raising the rate of disconnection of hand-off calls.
In addition, Japanese Patent Laid-open No. 264656/95 discloses a technique in which priority is assigned to a hand-off process request from a mobile station based on a calculation equation that takes into consideration the speed or the direction of movement of a moving mobile station, the hand-off process then being carried out in accordance with the priority.
Mobile stations that are moving between cells also move at various speeds, the speed of movement of mobile stations differing, for example, for a mobile station that moves by automobile and a mobile station that moves at walking speed. The permissible time interval from the request for hand-off to the completion of the hand-off process differs when the speeds of movement differ as described above, and merely carrying out the hand-off process sequentially in the order in which hand-off requests were received entails the danger of forced disconnection of a communication in mobile stations moving at high speed due to the lengthy interval from the time of hand-off request to the completion of the hand-off process.
A mobile station that is moving at high speed also passes through many cells in a prescribed time interval, and the number of requested hand-offs during a communication, as well as the possibility of forced disconnection of the communication, is therefore increased. Mobile stations that are moving between cells also have various paths of movement, with some mobile stations only moving away from the base station from which they are currently receiving service, and others maintaining a uniform distance from the base station.
The permissible time interval from the request for hand-off up to the completion of the hand-off process differs according to the various different paths of movement described above. Therefore, if the hand-off procedure is carried out merely sequentially in response to the requests from mobile stations requiring hand-off, there is the possibility that processing will be too late, resulting in the forced disconnection of communication in the case that the hand-off procedure is not carried out immediately upon request for hand-off for a mobile station that is moving away from the base station from which it is currently receiving service.
In addition, the method disclosed in Japanese Patent Laid-open No. 264656/95 suffers from the problems that calculation for giving priorities to calls is complex and, because priority is given to each call, processing is carried out frequently whenever hand-off is requested, make the processing complicated.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a hand-off control system and hand-off control method that can reduce the rate of occurrence of forced disconnection of communication by taking into consideration the permissible time interval from the request for hand-off by a mobile station until completion of the hand-off process.
To achieve the above-described object, the hand-off control system of this invention comprises: measuring means, calculating means, queue storing means, request processing means, waiting rank determining means; queue reallocating means, and queue control means.
The present invention thus constituted operates as follows: The intensity of signals received from a base station at a mobile station is periodically measured by the measuring means, and the amount of relative change for each measurement interval in the intensity of received signals measured by the measuring means is calculated by the calculating means. When hand-off is requested, the request processing means allocates the call of the mobile station that has requested hand-off to an appropriate queue of queues to which priorities are given in advance according to the amount of relative change based on the amount of relative change in the received signal intensity in the mobile station. The waiting rank determining means determines the waiting rank of a call of the mobile station within the queue in which the call of the mobile station has been allocated based on the intensity of received signals measured by the measuring means for every measurement interval of the intensity of received signals. When the amounts of relative change are updated, the queuing reallocating means pe
NEC Corporation
Nguyen Lee
Sughrue & Mion, PLLC
Tran Congvan
LandOfFree
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