Abstract:
A hand-off control method is disclosed that can reduce the rate of occurrence of forced disconnection of communication. The intensity of signals received from a base station is periodically measured in a mobile station, the amount of relative change in received signal intensity is calculated for every measurement interval, and the measurement results and calculation results are notified to the base station and the base station of the hand-off destination. If the mobile station requests hand-off, the call of the mobile station is allocated in the hand-off destination base station to an appropriate queue among a plurality of queues that are given an order of priority, based on the amount of relative change for the call. The call is reallocated to a queue that corresponds to the new amount of relative change whenever the amount of relative change is updated. In addition, the waiting rank of the call within the queue is determined based on the received signal intensity at the mobile station. When free channels are available in the destination cell of the moving mobile station, calls allocated to queues are assigned to free channels in an order that is based on the priority of the queues and the waiting rank within the queue.

Description:
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 periodically reallocates calls stored in each queue, for every measurement interval of the received signal intensity, such that the calls are stored in queues according to the amount of relative change in received signal intensity. The queue control means, on the other hand, monitors whether or not there are free channels within the destination cell of a moving mobile station. If there are free channels in the cell, the queue control means assigns calls allocated to the queue to free channels in an order that is based on the priority of the queue and the waiting rank within that queue. 
     Hand-off is thus performed in: an order that is based on the intensity of received signals and amount of relative change in the intensity of received signals at a mobile station. As a result, a mobile station for which the permissible time interval from the request for hand-off to the completion of the hand-off process is short, i.e., a mobile station that exhibits a large amount of relative change in the intensity of received signals, is processed before a mobile station for which the permissible time interval from the request for hand-off until the completion of the hand-off process is long, i.e., a mobile station that exhibits a small amount of relative change. In addition, the hand-off process for a mobile station for which the received signal intensity is weak and which is liable to be disconnected is also given higher priority. 
     In the hand-off control system of this invention, when the received signal intensity of a particular hand-off request call is updated, the waiting rank determining means updates the waiting rank of calls stored in each of the queues only in cases in which the received signal intensity is greater than a value obtained by adding a predetermined first hysteresis value (H 1 ) to an upper threshold of the current waiting rank of that hand-off request call or smaller than a value obtained by subtracting the first hysteresis value (H 1 ) from the lower threshold of the current waiting rank of the hand-off request call. 
     In the hand-off control system of this invention, when the amount of relative change of a particular hand-off request call is updated, the queue reallocating means reallocates calls stored in each of the queues only in cases in which the amount of relative change is greater than a value obtained by adding a predetermined second hysteresis value (H 2 ) to the upper threshold of the class of the queue in which the hand-off request call is currently allocated or less than a value obtained by subtracting the second hysteresis value (H 2 ) from the lower threshold of the class of queue in which the hand-off request call is currently allocated. 
     Thus, the use of a hysteresis characteristic in the judgment when waiting rank is determined by the waiting rank determining means and when queues are reallocated by the queue reallocating means, decreases the number of updating procedures, thereby cutting down the amount of control processing. 
     In an embodiment of the present invention, the amount of relative change in the intensity of received signals, when a mobile station for which the intensity of received signals was P 0  at a time to moves and the intensity of received signals is P 1  at time t 1 , is defined as: 
     
       
         ( P   1   −P   0 )/( P   1   +P   0 ), ( P   1   −P   0 )/ P   1 , ( P   1   −P   0 )/ P   0  or ( P   1   −P   0 )/( t   1   −t   0 ) 
       
     
     The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is an explanatory view of a hand-off control system in a typical mobile communication system; 
     FIG. 1 b  shows a case in which the cell radius has been reduced from the size shown in FIG. 1 a;    
     FIG. 2 shows a hand-off control system according to a first embodiment of the present invention; 
     FIG. 3 a  is an explanatory view showing the calculation method of the intensity of received signals in a case in which two mobile stations have moved from positions having the same received signal intensities; 
     FIG. 3 b  is an explanatory view showing the calculation method of received signal intensity: in a case in which two mobile stations have moved from positions having different received signal intensities to positions for which the received signal intensities have reached the hand-off threshold value; 
     FIG. 4 is an explanatory view showing the determination method of priority in the mobile station hand-off process of the hand-off control system shown in FIG. 2; 
     FIG. 5 is a block diagram showing an example of the construction of a mobile station in the hand-off control system shown in FIG. 2; 
     FIG. 6 is a block diagram showing an example of the construction of a base station in the hand-off control system shown in FIG. 2; 
     FIG. 7 a  is a flow chart illustrating procedures in hand-off request processor  28 , and FIG. 7 b  is a flow chart illustrating procedures in waiting rank determination unit  34 ; 
     FIG. 8 is a flow chart illustrating procedures in queue reallocation unit  35 ; 
     FIG. 9 is a flow chart illustrating procedures in queue control unit  32 ; 
     FIG. 10 is an explanatory view showing the control according to the path of movement of a mobile station in the hand-off control system shown in FIG. 2; 
     FIG. 11 shows the change in the amount of relative change in the hand-off control system according to the first embodiment of the invention; 
     FIG. 12 is a flow chart illustrating procedures in waiting rank determination unit  34  in the hand-off control system according to the second embodiment of the present invention; 
     FIG. 13 is a flow chart illustrating procedures in queue reallocation unit  35  in the hand-off control system according to the second embodiment of the present invention; and 
     FIG. 14 shows the change in the amount of change in the hand-off control system according to the second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     As shown in FIG. 2, in the hand-off control method according to a first embodiment of the present invention, mobile station  10  is located within cell  30   a  of base station  20   a , receiving service from base station  20   a , and moving toward cell  30   b  of base station  20   b.    
     The intensity of signals received from base station  20   a  is periodically measured at mobile station  10 . Base station  20   a  is informed of the measured intensity of received signals and the amount of relative change in received signal intensity for each measurement interval. 
     Base station  20   b  within cell  30   b , which is adjacent to cell  30   a  of base station  20   a , is also informed of the received signal intensity and the amount of relative change in received signal intensity from base station  20   a  by way of a network (not shown). 
     At base stations  20   a  and  20   b , not only are the received signal intensity and the amount of relative change in received signal intensity notified from mobile station  10  stored, but queues are produced in advance in which priority is given based on the amount of relative change in the received signal intensity. Upon transmission of a request for hand-off from mobile station  10 , mobile station  10  is allocated at base stations  20   a  and  20   b  to an appropriate queue based on the amount of relative change in received signal intensity of mobile station  10  and, in addition, a waiting rank within the queue to which mobile station  10  is:assigned is determined based on the received signal intensity of mobile station  10 . Hand-off control is subsequently performed based on the priority. 
     Explanation will next be presented with reference to FIGS. 3 a  and  3   b  regarding the method of calculating the amount of relative change in received signal intensity and the method of determining priority in this embodiment. 
     FIG. 3 a  and FIG. 3 b  are figures for explaining the method of calculating the received signal intensity in the hand-off control system shown in FIG. 2, FIG. 3 a  showing a case in which two mobile stations move from positions having the same received signal intensity, and FIG. 3 b  showing a case in which two mobile stations move from positions having different received signal intensities to positions in which the received signal intensities reach the hand-off threshold value. In FIG. 3 a  and FIG. 3 b , the horizontal axis designates time, and the vertical axis designates received signal intensity. 
     In FIG. 3 a , it is assumed that a mobile station having a received signal intensity of P 0  at time t 0  moves such that the received signal intensity becomes P L1  at time t 1 . In this case, the amount of relative change is defined as: 
     
       
         ( P   L1   −P   0 )/( P   L1   +P   0 ) 
       
     
     In addition, a mobile station having a received signal intensity of P 0  at time to moves such that the received signal intensity becomes P H1  at time t 1 . The amount of relative change in this case is defined as: 
     
       
         ( P   H1   −P   0 )/( P   H1   +P   0 ) 
       
     
     In this case, the absolute change of the received field signal intensity of the mobile station for which the received signal intensity becomes P L1  at time t 1  is ΔP L1 . In addition, the absolute change in received field signal intensity of the mobile station for which the received signal intensity becomes P H1  at time t 1  is ΔP H1 , which is greater than the absolute change ΔP L1  in received field signal intensity of the mobile station for which the received signal intensity becomes P L1  at time t 1 . That is, the mobile station at which the received signal intensity becomes P H1  at time t 1  is moving away from the base station more rapidly than the mobile station at which the received signal intensity becomes P L1  at time t 1 . 
     In a case in which a mobile station having a received signal intensity of P 0  at time t 0  moves such that the received signal intensity becomes P L1  at time t 1 , the aforementioned amount of relative change can be defined as (P L1 −P 0 )/P L1  or (P L1 −P 0 )/P 0 . Alternatively, in a case in which a mobile station having a received signal intensity of P 0  at time to moves such that the received signal intensity becomes P H1  at time t 1 , the amount of relative change can be defined as (P H1 −P 0 )/P H1  or (P H1 −P 0 )/P 0 . 
     In FIG. 3 b , it is assumed that a mobile station having a received signal intensity of P L2  at time to moves such that the received signal intensity at time t 1  becomes the hand-off threshold value  P   1 . The amount of relative change in this case is defined as (P 1 −P L2 )/(t 1 −t 0 ). 
     A mobile station having a received signal intensity of P H2  at time to moves such that the received signal intensity becomes the hand-off threshold value P 1  at time t 1 . The amount of relative change in this case can be defined as (P 1 −P H2 )/(t 1 −t 0 ). 
     In this case, the absolute change in received field signal intensity of the mobile station having a received signal intensity of P L2  at time to is ΔP L2 . The absolute change in received field signal intensity of the mobile station having a received signal intensity of P H2  at time t 0  is ΔP H2 , which is greater than the absolute change ΔP L2  in received field signal intensity of the mobile station having a received signal intensity of P L2  at time t 0 . That is, the mobile station having a received signal intensity of P H2  at time t 0  is moving away from the base station more rapidly than the mobile station having a received signal intensity of P L2  at time t 0 . 
     As shown in FIG. 4, the priority of a mobile station in this embodiment is determined based on the amount of relative change in received signal intensity of that mobile station. The amount of relative change in received signal intensity is grouped into four classes by amount, and the priority of each mobile station is determined according to the class of the amount of relative change in the received signal intensity for that mobile station. As an example, if mobile station  10   a  fits into class  2 , mobile station  10   b  fits into class  3 , and mobile station  10   c  fits into class  4 , class  4  has the highest priority and class  1  has the lowest priority. 
     Explanation will next be presented regarding the details of the constructions of a mobile station and base station in this embodiment. 
     As shown in FIG. 5, mobile station  10  in this embodiment comprises: antenna  11  for transmitting and receiving radiowaves, transmission/reception: amplifier  12 , radio unit  13 , baseband signal processor  14 , relative change calculator  15 , mobile station interface unit  16 , and control unit  17 . 
     Transmission/reception amplifier  12  not only amplifies received RF signals received by way of antenna  11  and transmitted RF signals transmitted by way of antenna  11 , but also demultiplexes received RF signals and transmitted RF signals. 
     Radio unit  13  quasi-synchronously and detects received RF signals amplified by transmission/reception amplifier  12  for conversion to digital signals, converts signals to be transmitted by way of antenna  11  to analog signals, and converts the signals to transmission RF signals through orthogonal modulation. 
     Baseband signal processor  14  performs baseband signal processing such as the demodulation, synchronization, error-correcting decoding, and data demultiplexing of received signals converted to digital signals by radio unit  13  as well as the error-correcting encoding, framing, and data modulation of signals to be transmitted by way of antenna  11 . Baseband signal processor  14  includes measuring means for periodically measuring the intensity of received signals from base station  20   a.    
     Relative change calculator  15  is a calculating means for calculating the amount of relative change by measurement interval in the intensity of received signals measured at baseband processor  14 . 
     Mobile station interface unit  16  has a speech CODEC and data adapter function, and interfaces with a handset connected to the outside or with an outside data mobile station (not shown). 
     Control unit  17  includes a radio control function to transmit and receive control signals and controls the operations of transmission/reception amplifier  12 , radio unit  13 , baseband signal processor  14 , relative change calculator  15  and mobile station interface unit  16 . 
     When mobile station  10  transmits signals to base station  20   a , signals received by way of mobile station interface unit  16  undergo baseband signal processing in baseband signal processor  14 , and then are converted to analog signals in radio unit  13 . 
     Transmission/reception amplifier  12  then amplifies the signals converted to analog signals in radio unit  13 , and transmits the amplified signals to base station  20   a  by way of antenna  11 . 
     In the case of receiving signals transmitted from base station  20   a , on the other hand, the signals received by way of antenna  11  are first amplified in transmission/reception amplifier  12 . The amplified signals are quasi-synchronously, detected and converted to digital signals in radio unit  13 , following which the signals converted to digital signals in radio unit  13  undergo baseband processing in baseband signal processor  14  and are then outputted by way of mobile station interface unit  16 . 
     In addition, the intensity of received signals from base station  20   a  is periodically measured in baseband signal processor  14 , and the amount of relative change in the received signal intensity measured; in baseband signal processor  14  is calculated for each measurement interval in relative change calculator  15 . 
     The received signal intensity measured in baseband signal processor  14  and the amount of relative change in received signal intensity calculated in relative change calculator  15  are simultaneously notified to base station  20   a  at a prescribed period. 
     FIG. 6 is a block diagram showing an example of the construction of base stations  20   a  and  20   b  in the hand-off control system shown in FIG.  2 . 
     As shown in FIG. 6, each of base stations  20   a  and  20   b  in this embodiment comprises antenna  21  for receiving and transmitting radiowaves, transmission/reception amplifier  22 , radio unit  23 , baseband signal processor  24 , relative change table  25 , received signal intensity table  33 , wire transmission line interface unit  26 , queue unit  31 , hand-off request processor  28 , switch  29 , waiting rank determination unit  34 ; queue control unit  32 , and control unit  27 . 
     Transmission/reception amplifier  22  amplifies received RF signals received by way of antenna  21  and transmitted RF signals to be transmitted by way of antenna  21 , and demultiplexes received RF signals and transmitted RF signals. 
     Radio unit  23  not only quasi-synchronously and detects received RF signals amplified by transmission/reception amplifier  22  and converts the signals to digital signals, but also converts signals to be transmitted by way of antenna  21  to analog signals and converts the signals to transmitted RF signals by modulation. 
     Baseband signal processor  24  carries out baseband signal processing such as demodulation, synchronization, error-correcting decoding, and data demultiplexing of received signals that have been converted to digital signals by radio unit  23 , as well as the error-correcting encoding, framing, and data modulation of transmitted signals to be transmitted by way of antenna  21 . 
     Relative change table  25  is a first storing means for storing, of the signals-processed at baseband signal processor  24 , the amount of relative change in received signal intensity that is notified from mobile station  10 . 
     Received signal intensity table  33  is a second storing means for storing, of the signals processed at baseband signal processor  24 , the received signal intensity notified from mobile station  10 . 
     Wire transmission line interface  26  interfaces with host device  50  connected to the outside. 
     Queue unit  31  stores queues  31 - 1 - 31 -n that are given priority based on the amount of relative change in received signal intensity. 
     When a mobile station requests hand-off, hand-off request processor  28  allocates the call of the mobile station to a queue  31 - 1 - 31 -n in queue unit  31  based on the amount of relative change in received signal intensity of the mobile station that is stored in relative change table  25 . 
     Switch  29  performs switching of processes of queues  31 - 1 - 31 -n in queue unit  31  based on judgment in hand-off request processor  28 . 
     Waiting rank determination unit  34  determines the waiting rank of calls in each of queues  31 - 1 - 31 -n based on the received signal intensity of that mobile station stored in received signal intensity table  33 . 
     If amounts of relative change stored in relative change table  25  are updated, queue reallocation unit  35  periodically reallocates hand-off request calls stored in each of queues  31 - 1 - 31 -n such that the calls are stored in queues that accord with the amounts of relative change of received signal intensities for each mobile station stored in relative change table  25 . 
     Queue control unit  32  monitors whether or not there are free channels in a cell, and if free channels are present monitors whether or not there are hand-off request calls within queues  31 - 1 - 31 -n. If hand-off request calls are present, queue control unit  32  assigns hand-off request calls to free channels based on the priority of queues  31 - 1 - 31 -n and the waiting rank of the calls in each of queues  31 - 1 - 31 -n. 
     Control unit  27  controls the operations of transmission/reception amplifier  22 , radio unit  23 , baseband signal processor  24 , wire transmission line interface  26 , and queue control unit  32 , and transmits and receives control signals with a host device  50 , to manage, sets, and disconnects radio lines. 
     As for the priority of queues  31 - 1 - 31 -n within queue unit  31 , queue  31 - 1  has the highest priority and queue  31 -n has the lowest priority. In addition, waiting rank within each of queues  31 - 1 - 31 -n is controlled such that mobile stations are arranged in order starting from the mobile station having the weakest received signal intensity. Further, the received signal intensities stored in received signal intensity table  33  are updated with every notification of received signal intensity from mobile station  10 , and waiting rank determination unit  34  determines the waiting rank of calls in each of queues  31 - 1 - 31 -n each time the received signal intensities stored in received signal intensity table  33  are updated. 
     Operation of the hand-off control system that is configured will next be described with reference to FIGS. 7 a  to  9 . 
     Explanation is first presented with reference to FIG. 7 a  regarding the operations in hand-off request processor  28 . 
     The intensity of signals received from base station  20   a  is periodically measured at mobile station  10 , and the measured received signal intensity and the amount of relative change in received signal intensity for each measurement interval are notified to base station  20   a.    
     The received signal intensity of mobile station  10  that has been notified to base station  20   a  is stored in received signal intensity table  33  in base station  20   a , the amount of relative change in received signal intensity is stored in relative change table  25  in base station  20   a , and the received signal intensity stored in received signal intensity table  33  is updated each time notification comes from mobile station  10 . 
     The received signal intensity and the amount of relative change in received signal intensity of mobile station  10  are notified from base station  20   a  to base station  20   b  by way of wire transmission line interface  26  and a network, and the received signal intensity of mobile station  10  is stored in received signal intensity table  33  within base station  20   b  and the amount of relative change in received signal intensity of mobile station  10  is stored in relative change table  25  within base station  20   b.    
     The amounts of relative change in received signal intensity of mobile stations are grouped into a plurality of classes depending on their values; and queues  31 - 1 - 31 -n, each of which are given a priority, are prepared in advance and stored in queue unit  31 . Of queues  31 - 1 - 31 -n, queue  31 - 1  is the class having the greatest amount of relative change in received signal intensity and has the highest priority, and queue  31 -n is the class having the smallest amount of relative change in received signal intensity and has the lowest priority. 
     The intensity of signals received from base station  20   a  at mobile station  10  gradually diminishes as mobile station  10  moves away from base station  20   a . The intensity of signals received from base station  20   a  is periodically measured at mobile station  10 , and when the measured received signal intensity falls below a predetermined hand-off threshold value, hand-off request is transmitted from mobile station  10  to base station  20   a . The hand-off request transmitted to base station  20   a  from mobile station  10  is further notified from base station  20   a  to base station  20   b  by way of wire transmission line interface  26  and network. 
     In Step S 1 , if a request for hand-off processing is directed from mobile station  10  to base station  20   a , this hand-off request is transmitted to base station  20   b . In Step S 2 , hand-off request processor  28  in base station  20   b  extracts from relative change table  25  the amount of relative change in received signal intensity of mobile station  10  that has requested hand-off. 
     In Step S 3 , hand-off request processor  28  allocates the call of mobile station  10  that has requested hand-off to the queue in queue unit  31  that is of the class corresponding to the amount of relative change in received signal intensity extracted in Step S 2 . 
     Explanation will next be presented with reference to FIG. 7 b  regarding the operation of waiting rank determination unit  34 . 
     In Step S 4 , waiting rank determination unit  34  extracts the received signal intensity of mobile station  10  that has requested hand-off processing from received signal intensity table  33 . In Step S 5 , the waiting rank of the call of mobile station  10  within the queue to which the call of mobile station  10  that has requested hand-off is assigned is determined under the control of waiting rank determination unit  34  based on the received signal intensity extracted in Step S 4 . The waiting ranks of calls in each of queues  31 - 1 - 31 -n are updated each time the received signal intensities stored in received signal intensity table  33  are updated. 
     As shown in FIG. 8, in Step S 6 , queue reallocation unit  35  periodically extracts the amounts of relative change of the hand-off request calls stored in each of queues  31 - 1 - 31 -n from relative change table  25 . Then, when a hand-off request call stored in a particular queue no longer belongs to the class corresponding to that queue due to updating of the amount of relative change, queue reallocation unit  35  reallocates the hand-off request call to another queue that corresponds to that class in Step S 7 . 
     Meanwhile, queue control unit  32  first checks whether or not free channels are present within cell  30   b  in Step S 11 . If the result of the check in Step S 11  is “Yes”, queue control unit  32  checks whether or not hand-off request calls are present within queues  31 - 1 - 31 -n in queue unit  31  in Step S 12 . If the result of the check in Step S 12  is “Yes”, the hand-off request calls existing within queues  31 - 1 - 31 -n are assigned to free channels within cell  30   b  in an order that is based on the priority and the waiting rank within queues  31 - 1 - 31 -n in Step S 13 . 
     As for the priority of queues  31 - 1 - 31 -n, queue  31 - 1  has the highest priority and queue  31 -n has the lowest priority, and hand-off request calls are therefore sequentially processed with hand-off request calls in queue  31 - 1  being processed first, followed by hand-off request calls in queue  31 - 2 , hand-off request calls in queue  31 - 3 , and so on down to hand-off request calls in queue  31 -n. If a plurality of calls are present within the same queue, the calls are sequentially processed in an order based on the waiting rank determined by waiting rank determination unit  34 . If there are a plurality of calls within the same queue that have the same waiting rank, the calls are processed in the order in which they were allocated to that queue. 
     In the event that the received signal intensity and the amount of relative change in received signal intensity stop being notified from mobile station  10  and calls for which mobile station  10  has requested hand-off remain in queues  31 - 1 - 31 -n, the remaining calls are discarded from the queues without being processed in queue control unit  32 , and hand-off processing is carried out for calls given the next waiting rank. 
     Further, in the event mobile station  10  is unable to hand off within the hand-off area, processing in queue control unit  32  of calls for which hand-off has been requested by mobile station  10  is not carried out, and hand-off processing is carried out for calls given the next waiting rank. 
     Normal processing is carried but when new calls originate. 
     In this embodiment, the amount of relative change in received signal intensity at mobile station  10  is calculated by relative change calculator  15  within mobile station  10  and is notified together with the received signal intensity at mobile station  10  to base stations  20   a  and  20   b . However, a means for calculating the amount of relative change in received signal intensity at mobile station  10  for each measurement interval based on the received signal intensity notified from mobile station  10  may be provided within base stations  20   a  and  20   b  or the exchange which is host device  50  of base stations  20   a  and  20   b , whereby only the received signal intensity need be notified to base stations  20   a  and  20   b  from mobile station  10 , and the amount of relative change in the received signal intensity at mobile station  10  for each measurement interval is calculated by the base stations or the exchange. 
     Hand-off processing is carried out in an order based on the speed of movement of the mobile station in the explanation of this embodiment. However, since priority in this invention is determined based on the amount of relative change in the received signal intensity of the mobile station, the path of movement of the mobile station may be taken into consideration and hand-off processing carried out in an order based on the speed away from the base station. 
     Assume that mobile station  10   a  moves linearly across cell  30  of base station  20 , while mobile station  10   b  moves during a prescribed interval within cell  20  at a substantially uniform distance from base station  20 , as shown in FIG.  10 . In this example, mobile station  10   a  and mobile station  10   b  have the same speed of movement. 
     Although the received signal intensity of mobile station  10   a  changes, the received signal intensity of mobile station  10   b  remains substantially uniform during the prescribed interval. The amount of relative change in received signal intensity for mobile station  10   a  is thus greater, and in the event that hand-off processing is requested in mobile stations  10   a  and  10   b , the hand-off request call for mobile station  10   a  will be given higher priority than the hand-off request call for mobile station  10   b  and accordingly will be processed first. 
     Second Embodiment 
     Explanation will next be presented regarding hand-off control system according to the second embodiment of the invention. In the first embodiment, when the amount of relative change in received signal intensity stored in relative change table  25  is updated, queue reallocation unit  35  reallocated hand-off request calls in queues  31 - 1 - 31 -n of the queues according to the updated amounts of relative change. When the received signal intensities stored in received signal intensity table  33  is updated, waiting rank determination unit  34  updates waiting ranks within queues  31 - 1 - 31 -n. 
     In the event that the amount of relative change fluctuates in the vicinity of a threshold level for classifying different queues, however, reallocation by queue reallocation unit  35  is effected at high frequency, putting a heavy processing on control unit  27 . 
     If the amount of relative change fluctuates, for example, as shown in FIG. 11, queue reallocation unit  35  performs reallocation at times t 1 , t 2 , t 3 , t 4 , and t 5 . 
     Similarly, fluctuation in received signal intensity in the vicinity of the threshold level for updating waiting rank within a queue causes waiting rank determination unit  34  to update waiting rank at high frequency, again placing a heavy processing load on control unit  27 . 
     The hand-off control system of this embodiment is intended for reducing the processing load in the foregoing cases, and therefore differs from the first embodiment in the processing operations during updating in waiting rank determination unit  34  and queue reallocation unit  35 . Other processing operations and construction of this embodiment are equivalent to those of the first embodiment. 
     Operation in the hand-off control system according to this embodiment will next be described with reference to FIG.  12  and FIG.  13 . 
     Explanation is first presented regarding the operation of waiting rank determination unit  34  using FIG.  12 . 
     In Step S 21 , waiting rank determination unit  34  extracts from received signal intensity table  33  the received signal intensity of mobile station  10  that has requested hand-off processing. 
     Waiting rank determination unit  34  then determines whether or not the received signal intensity extracted from received signal intensity table  33  in Step S 22  is greater than a value obtained by adding a predetermined hysteresis value (H 1 ) to the upper threshold value of the current waiting rank of the hand-off request call or lower than a value obtained by subtracting a predetermined hysteresis value (H 1 ) from the lower threshold value of the current waiting rank of the hand-off request call. 
     If the result of the determination in Step S 22  is “Yes”, waiting rank determination unit  34  determines the waiting rank of the call of mobile station  10  within the queue to which the hand-off request call of mobile station  10  has been allocated in Step S 23  based on the received signal intensity extracted in Step S 21 . 
     The upper threshold in Step S 22  is the value of the received signal intensity of a hand-off request call that is set one rank ahead of the waiting rank of the hand-off request call for which processing is being carried out, and the lower threshold is the value of the received signal intensity of the hand-off request call that is set one rank after the hand-off request call being processed. 
     The operation of queue reallocation unit  35  will next be described using FIG.  13 . 
     Queue reallocation unit  35  periodically extracts from relative change table  25  the amounts of relative change of hand-off request calls stored in each of queues  31 - 1 - 31 -n in Step S 6 . 
     In Step S 30 , queue reallocation unit  35  determines whether or not the amount of relative change extracted from relative change table  25  is greater than a value obtained by adding a predetermined hysteresis value (H 2 ) to the upper threshold value of the class of the queue to which the hand-off request call is currently assigned or less than a value obtained by subtracting the predetermined hysteresis value (H 2 ) from the lower threshold value of the class of the queue to which the hand-off request call is currently assigned. 
     If the result of the determination in Step S 30  is “Yes”, and if a hand-off request call stored in a particular queue no longer belongs to the class corresponding to that queue as a result of updating of the amount of relative change, queue reallocation unit  35  reallocates the hand-off request call to another queue that corresponds to that class in Step S 37 . 
     In the case in which a hand-off request call corresponds, for example, to class  2  in FIG. 4, the, upper threshold value in the shown of Step S 30  is a level at the boundary between class  2  and class  3 , and the lower threshold value is a level at the boundary between class  1  and class  2 . 
     If the amount of relative change fluctuates as shown in FIG. 14, reallocation by queue reallocation unit  35  is performed just once at time t 6 . This demonstrates that the number of updating is reduced from the case shown in FIG. 11, in which reallocation is carried out five times. 
     In this embodiment, the use of a hysteresis characteristic in the judgment when determining waiting rank by waiting rank determination unit  34  and when reallocating to queues by queue reallocation unit  35  enables the number of updating processes to be reduced, thereby cutting the amount of control processing. 
     Although the present embodiment has been described regarding a case in which a hysteresis characteristic is used in both when determining waiting rank by waiting rank determination unit  34  and reallocating to queues by queue reallocation unit  35 , it is understood that the present invention is not limited to this form, and the hysteresis characteristic may be used in only one of the processes. 
     While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.