Abstract:
When a base station of a mobile communications system attempts to reduce its cell radius for power saving, frequency re-use enhancement and the like, a risk is that an ongoing communication between a mobile station served by the base station may be disconnected, which is avoided by: the base station transmitting information about a new cell radius to mobile stations in the current cell; the mobile stations responding to the base station with a positive or a negative reply determined from the information received from the base station; the base station suspending or revising the attempted cell radius reduction according to the responses from the mobile stations; and further by the mobile stations being capable of performing hand-over more efficiently than ever owing to abilities to detect final cell radii of adjacent base stations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/237,697, filed Sep. 10, 2002, which is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2001-274214, filed Sep. 10, 2001, the entire contents of each are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention generally relates to a control method of cell formation and a mobile communications system therewith, and specifically relates to the cell formation method and the mobile communications system therewith, wherein one or more mobile stations are served by a base station that is capable of changing and securing a service area of the mobile stations by changing the cell formation while communications are going on.  
         [0004]     The present invention further relates to the base station and mobile station that can communicate according to the cell formation control method.  
         [0005]     2. Description of the Related Art  
         [0006]     An area where a base station can provide a communication service, i.e., a cell, depends on the quality of a control signal received by a mobile station, the control signal being for connecting the mobile station with the base station. When the base station serves a cell, a conventional method has been that transmission electric power of the control signal is predetermined, and the predetermined value is used. In recent years, an autonomous cell formation control method has been proposed, wherein a base station checks cell-formation of adjacent cells, and adjusts its cell formation such that areas not covered by the adjacent cells are efficiently covered. The autonomous cell formation control method mediates traffic congestion by controlling the cell formation according to a congestion state of the adjacent cells, and can raise frequency use efficiency. If the technology of such autonomous cell formation control is applied to a mobile communications system, a base station will be capable of changing cell formation, i.e., increasing and decreasing the radius of the cell, while communications are continuing.  
         [0007]     When the cell radius is expanded, an increased number of base stations will become available to a mobile station that is in communication with a first base station. The communication connection to the first base station can be switched to a second base station, if the second base station is carrying a lower amount of traffic, such that a higher through put may be obtained.  
         [0008]     On the other hand, when the cell radius of the first base station with which the mobile station is connected is reduced, it is necessary for the mobile station to switch the ongoing communication connection to another base station before the service of the first base station to the mobile station becomes unavailable.  
         [0009]     However, according to the autonomous cell formation control of a conventional base station, a relief is not provided to the mobile station that would be made outside the service area of the base station before the base station actually reduces its service area.  
         [0010]     For example, as shown in  FIG. 16 , when a base station  3  reduces its service area from an area indicated by a dotted line to an area indicated by a solid line, although a mobile station  3  served by the base station is thereby put outside the reduced service area of the base station  3 , the mobile station  3  can be switched to the base station  2  by hand-over, such that the communication can continue. However, when a base station  1  reduces its service area from an area indicated by a dotted line to an area indicated by a solid line, there is no adjacent base station to perform hand-over for a mobile station  1  that is being served by the base station  1 . Consequently, when the base station  1  reduces its service area, the mobile station  1  will be put outside the reduced service area, and the communication will be disconnected.  
         [0011]     Thus, the conventional problem is that a normal communication of a mobile station with the first base station cannot be continued due to disconnection, a packet loss and the like, when the first base station reduces its cell radius, i.e., service area.  
         [0012]     Even if there is the second base station available for the service to be continued in an adjacent area, hand-over has to be performed from the first base station to the second base station. Further, there is a possibility that the second base station may later reduce its service area. In this case, further hand-over is required from the second base station to a third base station, or back to the first base station, as the case may be.  
       SUMMARY OF THE INVENTION  
       [0013]     It is a general object of the present invention to provide a method, a system using the method, and a base station and a mobile station that operate in the system, which substantially obviate one or more of the problems caused by the limitations and disadvantages of the related art.  
         [0014]     Features and advantages of the present invention will be set forth in the description that follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by the method, the system using the method, and the base station and the mobile station that operate in the system, which are particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.  
         [0015]     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides the method, the system, the base station and the mobile station that enable the system to function as summarized below.  
         [0016]     The base station, serving a cell, is equipped with a facility to change formation of the cell, typically, the radius of the cell within which the service of the base station is available, and a facility to provide information about a contemplated cell radius change to one or more mobile stations in the cell. Each mobile station is capable of transmitting a request not to change the cell radius, and also capable of searching for another base station to which a hand-over is to take place, if the mobile station determines that the mobile station will be placed outside the cell if the cell radius change is actually performed by the base station. Then, the base station, upon receiving the request, is capable of suspending the contemplated cell radius change.  
         [0017]     An extension to the above is that the mobile station is capable of calculating the distance between the base station and the mobile station, using signal strength of a signal from the base station, and other information, and the mobile station transmits information about the distance to  
         [0018]     the base station, such that the base station can adjust the cell radius enough to cover the mobile station concerned.  
         [0019]     As an extension to the above, the base station is equipped with a facility to change the formation of the cell, i.e., the cell radius, gradually in stages until the contemplated cell radius change is finally reached, and a facility to inform the mobile stations about a next stage cell radius one by one at a predetermined interval. The mobile station is capable of transmitting the request not to change the cell radius when a cell radius change of any of the stages that is contemplated will place the mobile station outside the cell. This contributes to reducing processing load of the cell, because the number of control signals from the mobile stations to the base station in a given period of time is reduced, with transmission of the control signals being distributed over the stages.  
         [0020]     A further extension to the above is that the base station is capable of transmitting information both about the next stage cell radius and about the cell radius in the final stage. In this case, the mobile station is informed of the final cell radius from the beginning, and is enabled to attempt a hand-over to a second base station if the final cell radius will not cover the mobile station, even if a cell radius of any prior stage may cover the mobile station. This alleviates frequency and traffic of the hand-over. Further, in this case, even if the second base station is in the process of a cell radius change, the mobile station can select a base station that will provide the most favorable communication conditions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is a figure showing an example of a structure of a mobile communications system to which a cell formation control method of an embodiment of the present invention is applied;  
         [0022]      FIG. 2  is a figure showing a structure of a base station of the present invention;  
         [0023]      FIG. 3  is a figure showing a structure of a mobile station of the present invention;  
         [0024]      FIG. 4  is a figure showing an outline of a first embodiment of a cell radius changing process;  
         [0025]      FIG. 5  is a flowchart showing steps of a process performed by a base station of the first embodiment;  
         [0026]      FIG. 6  is a flowchart showing steps of a process performed by a mobile station of the first embodiment;  
         [0027]      FIG. 7  is a figure showing an outline of a second embodiment of the cell radius change process;  
         [0028]      FIG. 8  is a flowchart showing steps of a process performed by a base station of the second embodiment;  
         [0029]      FIG. 9  is a flowchart showing a continuation of the steps of the process performed by the base station of the second embodiment (continuation of  FIG. 8 );  
         [0030]      FIG. 10  is a flowchart showing steps of a process performed by a mobile station of the second embodiment;  
         [0031]      FIG. 11  is a figure showing an outline of a third embodiment of the cell radius change process;  
         [0032]      FIG. 12  is a flowchart showing steps of a process performed by the base station of the third embodiment;  
         [0033]      FIG. 13  is a flowchart showing a continuation of the steps of the process performed by the base station of the third embodiment (continuation of  FIG. 12 );  
         [0034]      FIG. 14  is a flowchart showing steps of a process performed by the mobile station of the third embodiment;  
         [0035]      FIG. 15  is a flowchart showing a continuation of the steps of the process performed by the mobile station of the third embodiment (continuation of  FIG. 14 ); and  
         [0036]      FIG. 16  is a figure explaining a conventional technology. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]     Hereafter, embodiments of the present invention are explained based on attached figures. A mobile communications system to which the cell formation control methods of the embodiments of the present invention are applied is structured as shown in  FIG. 1 .  
         [0038]     In  FIG. 1 , the mobile communications system is, for example, a PDC (Personal Digital Cellular) system, and includes a base station and two or more mobile stations. In this example, a mobile station A  100  is communicating with a base station A  200 , and a mobile station B  110  and a mobile station C  120  are communicating with a base station B  210 , and each base station is capable of changing cell formation. The cell formation is changed according to a cell formation change of an adjacent base station, a congestion situation, and the like. For example, the base station A  200  is capable of changing the cell formation from A 1  to A 2 . The base station B 210  is also capable of changing its cell formation. A change of the cell formation can be realized by changing transmission electric power of a perch channel (a control channel used in order that a mobile station may synchronize with a base station), thereby a cell radius, i.e., the radius of a service area, is changed. Therefore, a change of transmission electric power is equivalent to a cell formation change. Further, the base station may use a sector antenna. In that case, a change of the transmission electric power of any of one, more than one, and all sectors is regarded as a cell formation change.  
         [0039]     In the present invention, when the base station A  200 , for example, carries out a cell formation change, i.e., a service area (=cell) change, e.g., from A 1  to A 2 , it is carried out such that the mobile station A  100  does not come outside of the service area. According to the present invention, the base station performs the cell formation change based on a response from the mobile station that informs the base station whether or not the mobile station will be placed outside of the service area, thereby the base station can adjust the cell formation change such that the mobile station can remain inside the service area. Consequently, disconnection of communication of the mobile station, due to the cell formation change, can be prevented.  
         [0040]     The base station is structured as shown in  FIG. 2 , for example.  
         [0041]      FIG. 2  is a block diagram of the base station of the present invention.  
         [0042]     The base station includes a receiving antenna  11 , a receiving unit  12 , a control signal detection unit  13 , a control information processing unit  14 , a cell formation processing unit  15 , an information signal processing unit  16 , a transmitting unit  17 , and a transmitting antenna  18 .  
         [0043]     In  FIG. 2 , an arrow shows a flow of a signal.  
         [0044]     The receiving unit  12  receives a signal transmitted from the mobile station through the receiving antenna  11 , and provides the received signal to the control signal detection unit  13  of the following stage. In the control signal detection unit  13 , a change stop signal that is the control signal transmitted by the mobile station is detected from the signal received by the receiving unit  12 , and information contained in the signal is provided to the control information processing unit  14  of the following stage. The control information processing unit  14  extracts and temporarily stores the information provided by the control signal detection unit  13 . Here, since the change stop signal may be transmitted from two or more mobile stations, the information is stored with a mobile station identifier corresponding to each mobile station. Then, the control information processing unit  14  provides the cell formation processing unit  15  with the information including a value of the cell radius (L MAX ) that is to be used in the cell formation change. Details about L MAX  will be described later. The cell formation processing unit  15  provides the information signal processing unit  16  with a new cell radius R 1  that the cell formation change is to take. Further, the information signal processing unit  16  provides the new cell radius R 1  (changed cell radius) to the transmitting unit  17 . However, at this juncture, in the case that L MAX  is provided by the control information processing unit  14 , R 1  is set with the value of L MAX , and then, R 1  is provided to the transmitting unit  17  through the information signal processing unit  16 . The information signal processing unit  16  updates contents of an information signal, and provides it to the transmitting unit  17 . The transmitting unit  17  transmits the updated information signal to all the mobile stations in the cell. Further, the transmitting unit  17  is equipped with a function, such as transmission power control, to change the cell radius to R 1 .  
         [0045]     On the other hand, if no change stop signals from the mobile stations are detected by the control signal detection unit  13 , the cell formation processing unit  15  provides R 1 , with no regard to L MAX , to the transmitting unit  17 , and cell formation change is carried out in the transmitting unit  17 .  
         [0046]     The mobile station is structured as shown in  FIG. 3 , for example.  
         [0047]      FIG. 3  is a block diagram of the mobile station of the present invention  
         [0048]     The mobile station includes a receiving antenna  21 , a receiving unit  22 , a connectable base station detection unit  23 , a distance calculation unit  24 , an information signal detection unit  25 , a cell radius comparison unit  26 , a hand-over processing unit  27 , a control signal processing unit  28 , a transmitting unit  29 , and a transmitting antenna  30 . In  FIG. 3 , an arrow shows a flow of a signal.  
         [0049]     The receiving unit  22  receives a signal transmitted from the base station through the receiving antenna  21 , and an information signal contained in the received signal is provided to the information signal detection unit  25  and the connectable base station detection unit  23 . In the connectable base station detection unit  23 , information required to determine a hand-over destination, such as the number of base stations that can communicate with the mobile station, a base station identifier, a receiving level, and the like, is extracted from the information signal provided by the receiving unit  22 , and the information is provided to the hand-over processing unit  27 . The information in the hand-over processing unit  27  contains information about hand-over destination base station candidates when the cell radius comparison unit  26  requires a hand-over control.  
         [0050]     In the information signal detection unit  25 , the information about the cell radius is extracted from the information signal received from the receiving unit  22 . The information here includes, for example, the received electric power P r  of the signal, the new cell radius R 1  if the cell formation change actually takes place, and the present base station transmission electric power P s , and the like. The value of the cell radius after change R 1  is provided to the cell radius comparison unit  26 , and other values, such as a value of P s , which are needed for calculating a distance (distance between the base station and the mobile station) are provided to the distance calculation unit  24 .  
         [0051]     The cell radius comparison unit  26  checks whether or not the mobile station will remain within the service area of the base station by comparing the new cell radius after change R 1  with the distance between the mobile station and the base station. If it is determined that the mobile station will become outside the service area if the cell radius is  10  actually changed, a hand-over direction is provided to the hand-over processing unit  27  or a change stop direction is provided to the control signal processing unit  28 , depending on whether another base station is available for hand-over. The control signal processing unit  28  generates either of the change stop signal or the control signal relative to hand-over, and the generated signal is transmitted from the transmitting unit  29 .  
         [0052]     Next, an outline of the first embodiment of cell radius change processing of the present invention is described with reference to  FIG. 4 .  
         [0053]     Section (a) of  FIG. 4  shows a mobile communications system referenced to explain the outline of the first embodiment. In this figure, mobile stations A and B are present in the service area (R 0 =initial cell radius) that a base station forms, and each mobile station can communicate with the base station. The base station is to change the cell radius from the initial radius R 0  to a new cell radius R 1 .  
         [0054]     In  FIG. 4 , the distance between the base station and the mobile station A is expressed as L A , and the distance between the base station and the mobile station B is expressed as L B .  
         [0055]     Section (b) of  FIG. 4  shows an outline of the cell radius change processing that the base station performs when only one mobile station A is present in the cell radius R 0  of the base station in the mobile communications system of the section (a) of  FIG. 4 . Here, the horizontal axis of this graph expresses the time lapse starting when the base station proposes a new cell radius, and the vertical axis expresses the distance (cell radius) from the base station concerned.  
         [0056]     In the section (b) of  FIG. 4 , at the time of t=0, the base station starts proposing to change the cell formation and provides information about the change to all the mobile stations in the cell as a part of the information signal. During a period between t=0 and t=T, the information signal is transmitted to the mobile stations, and the cell radius is made small at t=T. The cell radius after change is expressed by R 1 . Here, it is assumed that the position of the mobile station A is not changed during the period. As the section (a) of  FIG. 4  shows, the mobile station A is within the service area before and after the cell radius change. In this case, no special processing is necessary as to the mobile station A.  
         [0057]     Section, (c) of  FIG. 4  shows an outline of the cell radius change processing where only the mobile station B is present in the cell radius R 0  formed by the base station in the mobile communications system as shown by the section (a) of  FIG. 4 .  
         [0058]     When the cell radius is made small as shown in the section (b) of  FIG. 4 , the mobile station B becomes outside of the service area after t=T at which time the cell radius is made small. However, in the present invention, the mobile station B is warned in advance of the cell radius change, that is, the mobile station B becoming outside the service area if the cell radius is actually made small according to the information signal received during the period between t=0 and t=T. Consequently, the mobile station B can attempt to hand-over the existing communication connection to another base station. The hand-over is performed if another base station is available, avoiding a disconnection. If the other base station is not available, the mobile station B transmits a change stop signal to the base station, such that the base station suspends the cell radius change, or revises the new cell radius such that the mobile station B can continuously be served.  
         [0059]     Next, details of the processing procedure of the first embodiment are explained.  
         [0060]      FIG. 5  is a flowchart that shows the processing procedure of the base station, and  FIG. 6  is a flowchart that shows the processing procedure of the mobile station.  
         [0061]     First, the processing procedure of the base station is explained, referring to  FIG. 5 .  
         [0062]     When the base station decides to make a cell radius change, the information signal processing unit  16  generates an information signal that includes information contents such as the new cell radius R 1  and the present transmission electric power P s . The information signal is periodically and repeatedly transmitted (S 1 ) to the mobile stations in the cell, a timer is started, and a response from the mobile station (S 2 ) is waited for. The time length from the transmission start time of the information signal to scheduled time of the cell radius change implementation is set at T as shown in  FIG. 4 . When the predetermined time elapses, i.e., timeout, (YES at S 2 ), the control signal detection unit  13  checks whether a change stop signal has been received from one or more of the mobile stations (S 3 ). If the change stop signal has been received from one or more mobile stations (YES at S 3 ), a value L i  that indicates the distance between a mobile station and the base station is extracted from the change stop signal, and the value is provided to the control information processing unit  14  (S 4 ).  
         [0063]     Otherwise, that is, if it is determined that no change stop signals have been received from the mobile stations (NO at S 3 ), the process ends, and returns to the first processing step.  
         [0064]     The control information processing unit  14  determines the largest value L MAX  of the distance values L i  and the largest value L MAX  is provided to the cell formation processing unit  15  (S 5 ). The cell formation processing unit  15  sets the value L MAX  as the value of the new cell radius R 1  such that the mobile station that gives the largest value can remain within the service area of the base station (S 6 ). Then, the process ends, and returns to the first processing step, and repeats the above-mentioned procedure.  
         [0065]     Next, the processing procedure of the mobile station is described with reference to  FIG. 6 . When the receiving unit  22  of the mobile station receives the information signal from the base station with which it is communicating, the received signal is provided to the information signal detection unit  25 . The information signal detection unit  25  extracts data is such as the received electric power Pr, the new cell radius R 1  after change, and the present transmission electric power P s  of the base station from the received signal (S 11 ), and provides the data to the distance calculation unit  24 . The distance calculation unit  24  calculates the distance L i  between the mobile station and the base station concerned, applying the received electric power P r  and P s  received from the information signal detection unit  25  to a formula (Formula 1) that gives a relationship between the signal electric power and the distance (S 12 ). 
 
 P   r   =A×P   s ×(1/ L   i ) 4 , where   (Formula 1) 
 
         [0066]     A is a coefficient indicative of the propagation path situation between the base station and the mobile station.  
         [0067]     Although the present embodiment employs Formula 1, for example, for calculating the distance between the mobile station and the base station, another formula of the relationship between the signal electric power and the distance suitable for the propagation path characteristics of the system can be used, and the present invention is not limited to using Formula 1.  
         [0068]     When the distance calculation unit  24  calculates L i , the calculation result is provided to the cell radius comparison unit  26 . The cell radius comparison unit  26  compares L i  with R 1  (S 13 ). If it is determined that R 1  is larger than L i  (NO at S 13 ), the process ends because it means that the mobile station will remain within the service area after the cell radius change. Otherwise, if it is determined that R 1  is smaller than L i  (YES at S 13 ), the process moves to the next step (S 14 ) where it is determined whether a hand-over to another base station is possible (S 14 ). The determination (S 14 ) is performed as follows.  
         [0069]     The hand-over processing unit  27  receives information items sent one by one from the connectable base station detection unit  23 , and determines the availability of a hand-over. The result of the determination (availability of base stations for hand-over) is periodically provided to the cell radius comparison unit  26 . The cell radius comparison unit  26  determines whether a hand-over to another base station is possible using the information about the availability of connectable base stations provided by the hand-over processing unit  27 .  
         [0070]     When the cell radius comparison unit  26  performs determining as mentioned above (S 14 ) and determines that a hand-over to another base station is possible (NO at S 14 ), it directs the control signal processing unit  28  to generate a control signal that is needed by the mobile station for carrying out hand-over. Then, the control signal is provided to the mobile station such that the mobile station carries out the hand-over to another base station, and continues communication.  
         [0071]     Otherwise, that is, when it is determined that the hand-over to another base station is impossible (YES at S 14 ), the transmitting unit  29  of the mobile station concerned transmits the change stop signal that contains L i  to the base station (S 16 ). According to this embodiment, a mobile station is capable of preventing the base station from reducing the cell radius any smaller than L i  by transmitting information that contains the value of L i  to the base station.  
         [0072]     Next, an outline of the second embodiment of the cell radius change processing is described with reference to  FIG. 7 , wherein a base station changes cell formation based on the cell formation control method of the present invention.  
         [0073]     In  FIG. 7 , the horizontal axis expresses the time lapse from the time when the base station decides to change the cell radius, and the vertical axis expresses the distance (cell radius) from the base station, like  FIG. 4 . Here, in the base station, R 0  represents the present cell radius, and R 1  represents the new cell radius after the change that the base station proposes to perform. In the second embodiment of the present invention, the cell radius change from R 0  to R 1  is performed in N steps, without changing all at once. For example, the cell radius change is carried out in three steps (N=3) in this embodiment, namely, the cell radius is to be reduced from R 0  to R t =R 0 −R L , R t =R 0 −2R L , and then to R t =R 0 −3R L =R 1 , as shown in this graph. Specifically, the following R t  values of stage change cell radius are transmitted during each specified period. Namely, between t=0 and t=T, R t =R 0 −R L , 
        between t=T and t=2T, R t =R 0 −2R L , and,     between t=2T and t=3T, R t =R 0 −3R L =R 1  are 
 
 provided to the mobile station as the new cell radius values to which a cell formation change is proposed. If the mobile station is located at a distance L B , the radius changes proposed during t=0 and t=2T do not affect the mobile station, and no further action is necessary. However, the radius change proposed during t=2T and t=3T will make the mobile station become outside the service area. Therefore, the mobile station transmits the change stop signal to the base station such that the cell radius change that makes the mobile station outside the area is stopped. 
       
 
         [0076]     Next, details of the processing procedure of the second embodiment are explained.  
         [0077]      FIGS. 8 and 9  are flowcharts showing the processing procedure of the base station, and  FIG. 10  is a flowchart showing the processing procedure of the mobile station.  
         [0078]     First, the processing procedure of the base station is explained, referring to  FIG. 8  and  FIG. 9 .  
         [0079]     When a cell radius change is decided upon by the base station, a radius decrement interval R L  is obtained by dividing the difference between the new cell radius after change R 1  and the present cell radius R 0  by the number of times N of change as indicated by the following formula (Formula 2) (S 21 ). 
 
 R   L =( R   0   −R   1 ) /N    (Formula 2) 
 
         [0080]     Next, a radius R t  that the base station changes to in steps is obtained by the following formula (Formula 3) (S 22 ). 
 
 R   t   =R   0   −R   L    (Formula 3) 
 
         [0081]     The above processes are performed by the cell formation processing unit  15 . The resulting Rt is provided to the information signal processing unit  16 . The information signal processing unit  16  generates an information signal including data of R t  and the present transmission electric power PS, transmits the information signal to the mobile station in the cell periodically and repeatedly through the transmitting unit  17 , operates a timer, and waits for responses from the mobile stations (S 23 ). The time length from the transmission start time of the information signal to the scheduled implementation time of the cell radius change is set at T as shown in  FIG. 7 . After waiting for the response for the predetermined time, i.e., after a time out (YES at S 24 ), the base station determines whether a change stop signal has been received from one or more mobile stations (S 25 ). If a change stop signal has been received from one or more mobile stations (YES at S 25 ), transmission of the contents of the proposed cell change is stopped, the value of the cell radius setting value set up in the cell formation processing unit  15  is deleted, and the cell radius change processing is stopped such that the mobile station that transmitted the change stop signal can remain in the service area. That is, if the new cell radius R 1  is set equal to R t  (S 26 ), the cell radius is made the smallest possible while keeping the mobile station within the service area.  
         [0082]     On the other hand, if it is determined after the timeout (YES at S 24 ) that no change stop signals have been received from any of the mobile stations (NO at S 25 ), the process reached the final stage and is completed if R t =R 1  (YES at S 27 ). If R t  is not equal to R 1  (NO at S 27 ), R t  is set at R t −RL, and provided to the transmitting unit (S 28 ), and the process returns to the first step.  
         [0083]     Next, the processing procedure of the mobile station is described in reference to  FIG. 10 . The information signal from the base station with which communications are ongoing is received by the receiving unit  22  of the mobile station, and provided to the information signal detection unit  25  that extracts data such as the received electric power Pr, the cell radius Rt after change and the present transmission electric power PS of the base station included in the information signal (S 31 ), and sends the data out to the distance calculation unit  24 . The distance calculation unit  24  calculates the distance Li between the mobile station and the base station by applying the received electric power Pr and PS to the above-mentioned Formula 1 (S 32 ).  
         [0084]     In addition, although the present embodiment employs Formula 1, for example, for calculating the distance between the mobile station and the base station, another formula for the relationship between the signal electric power and the distance suitable for the propagation path characteristics of the system can be used, and the present invention is not limited to using Formula 1.  
         [0085]     When the distance calculation unit  24  has calculated L i  as mentioned above, the calculation result is provided to the cell radius comparison unit  26 . The cell radius comparison unit  26  compares R t  with L i  (S 33 ). If the comparison determines that R t  is larger than L i  (NO at S 33 ), which indicates that the mobile station concerned will remain within the service area after the cell radius is changed, the process ends and returns to the first processing step.  
         [0086]     Otherwise, if it is determined that R t  is smaller than L i  (YES at S 33 ), the process moves to the next step, where it is determined whether a hand-over to another base stations is possible (S 34 ). This determination (S 34 ) is performed as follows.  
         [0087]     The hand-over processing unit  27  receives the information items sent one by one from the connectable base station detection unit  23 , and determines the availability of a connectable base station for hand-over from the received information. The result of the determination (the information about the availability of a connectable base station that is acquired as a determination result in this case) is periodically provided to the cell radius comparison unit  26 . The cell radius comparison unit  26  determines whether the hand-over to another base station is possible using the information about the availability of a connectable base station provided by the hand-over processing unit  27 .  
         [0088]     The cell radius comparison unit  26  directs the control signal processing unit  28  to generate the control signal that is needed for the mobile station to carry out the hand-over, when it is determined that the hand-over to another base station is possible by the above-mentioned check (S 34 ) (NO at S 34 ). Using the control signal, the mobile station performs the hand-over to another base station, and continues communications.  
         [0089]     Otherwise, when it is determined that the hand-over to another base station is impossible by the above-mentioned check (YES at S 34 ), the mobile station concerned transmits a change stop signal to the base station (S 35 ). Thereby, the mobile station can stop cell radius change processing of the base station, and can avoid disconnection of ongoing communications.  
         [0090]     Next, an outline of the third embodiment of the cell radius change processing is described with reference to  FIG. 11 , wherein a base station changes cell formation based on the cell formation control method of the present invention.  
         [0091]     As for  FIG. 11 , like  FIG. 7 , the horizontal axis expresses the time lapse from the time when the base station decides to make a cell radius change, and the vertical axis expresses the distance (cell radius) from the base station concerned. Here, in the base station cell, R 0  represents the present cell radius, and R 1  represents the new cell radius after change. In the third embodiment of the present invention, the cell radius change from R 0  to R 1  is divided in N stages. For example, the cell radius change is carried out in three steps (N=3) in this embodiment, namely, the cell radius is to be reduced from R 0  to R t =R 0 −R L , R t =R 0 −2R L , and then to R t =R 0 −3R L =R 1 , as shown in this figure. Specifically, the following R t  values of stage change cell radius are transmitted during each specified period. Namely, 
        between t=0 and t=T, R t =R 0 −R L ,     between t=T and t=2T, R t =R 0 −2R L , and,     between t=2T and t=3T, R t =R 0 −3R L =R 1  are 
 
 provided to the mobile station as the new cell radius to which the cell formation is proposed to be changed. 
       
 
         [0095]     In the third embodiment, information about the final cell radius R 1  is added to the information signal in each stage, which is in addition to the second embodiment.  
         [0096]     In this example, if the distance between the mobile station and the base station is L B , the cell radius change proposed during t=0−2T is acceptable, and no special process is needed. However, in the third embodiment, the final cell radius R 1  that the base station will ultimately propose is also provided from the beginning of t=0 to the mobile station. Therefore, the mobile station can start attempting to find another base station, a second base station, for hand-over at an earlier stage. Further, even when the second base station is in the process of changing its cell radius, the mobile station can determine whether the final cell radius R 1  of the second base station will contain the mobile station. In this manner, the number of times of hand-over can be decreased. Further, even when there is no candidate base station to which the distance from the mobile station is shorter than the final cell radius of the base station, it is possible to choose a base station for hand-over, with which communication can be maintained for the longest possible time, based on R 1  and the stage change cell radius R t . Consequently, since the number of times of hand-over can be minimized, and transmission and reception of control signals, such as a change stop signal, can be minimized, wireless resources can be used efficiently.  
         [0097]     Next, the detailed processing procedure of the third embodiment is explained.  
         [0098]      FIG. 12  and  FIG. 13  are flowcharts showing the processing procedure of the base station, and  FIG. 14  and  FIG. 15  are flowcharts showing the processing procedure of the mobile station. First, the processing procedure of the base station is explained, referring to  FIG. 12  and  FIG. 13 .  
         [0099]     When the base station decides to make a cell radius change, a radius decrement interval R L  of the cell radius is obtained by dividing the difference between the present cell radius R 0  and the final cell radius after change R 1  by the number of times of change N (refer to Formula 2) (S 41 ). Next, the base station obtains Rt from the difference between R 0  and R L , using Formula 3 (S 42 ), R t  being the stage change cell radius that the base station uses when gradually changing the cell radius. The processes are performed by the cell formation processing unit  15 . The information signal processing unit  16  generates information containing data such as R 1 , R t , P s , R L , and T, and transmits the information to mobile stations in its cell periodically and repeatedly through the transmitting unit  17 , operates a timer, and waits for responses from the mobile stations (S 43 ). The time length from the time when transmission of the information signal is started to the scheduled time of cell radius change implementation is set at T as shown in  FIG. 11 . After waiting for the responses for the predetermined time, that is, after a timeout (YES at S 44 ), it is determined whether a change stop signal is received from one or more mobile stations (S 45 ). If it is determined that a change stop signal is received from one or more mobile stations (YES at S 45 ), transmission of the information of the proposed cell change is stopped, the value of the cell radius set in the cell formation processing unit  15  is deleted, and the cell radius change processing is stopped, such that the mobile station that transmitted the change stop signal can remain in the service area. By replacing R 1  with R t  (S 46 ), the cell radius is set at the smallest possible while keeping the mobile station in question within the service area.  
         [0100]     Otherwise, if it is determined after the timeout (YES at S 44 ) that no change stop signals are received from any of the mobile stations (NO at S 45 ), the process reaches the final step and is completed if R t =R 1  (YES at S 47 ). If R t  is not equal to R 1  (NO at S 47 ), R t  is replaced with R t −R L , which is provided to the transmitting unit  17  (S 48 ), and the process returns to the first step.  
         [0101]     Next, the processing procedure of the mobile station is explained with reference to  FIG. 14  and  FIG. 15 .  
         [0102]     The processing procedure of the mobile station in the third embodiment is similar to the processing procedure of the second embodiment, as shown in this figure. However, the mobile station in the third embodiment is capable of selecting: a base station that can continue connection longer than others as the hand-over destination. That is, the mobile station can receive the information signal from a candidate base station, and calculate remaining time until the cell radius of the candidate base station reaches R 1 , using R t , R 1 , R L , and T contained in the information signal. In this manner, the mobile station can attempt to hand-over to the candidate base station before the mobile station becomes outside the service area of the candidate base station.  
         [0103]     Further, the mobile station is capable of calculating the distance to the candidate base station from P s  and the receiving signal level of the information signal from the candidate base station. By comparing the calculation result with R 1 , the mobile station can determine whether the mobile station will remain within the service area after the cell radius change. Using this information, the mobile station can select a base station to communicate with such that the number of times of hand-over is minimized.  
         [0104]     Although the embodiments described above are based on the base station autonomously controlling the cell formation, the present invention is applicable to a structure wherein a central station that governs a plurality of base stations controls the cell formation. In such a structure, the cell formation control is based on information acquired from the base stations governed by the central station. The central station may be, for example, a wireless circuit control station.  
         [0105]     In the embodiments, the cell formation control function of the cell formation processing unit  15  of the base station includes a cell formation change stop means, a cell formation change means, and a cell radius gradual change means, and the information function of the information signal processing unit  16  includes a first cell information providing means and a second cell information providing means.  
         [0106]     Further, the comparison function of the cell radius comparison unit  26  of the mobile station represents a preparatory outside-the-cell detection means, the signal transmitting function of the transmitting unit  29  represents a detection result providing means and a distance information providing means, the distance calculation function of the distance calculation unit  24  represents a distance calculation means, and the hand-over processing function of the hand-over processing unit  27  represents a first hand-over means and a second hand-over means.  
         [0107]     As mentioned above, the cell formation control method and the mobile communications system of the present invention can prevent a disconnection of communication that is due to a reduced cell radius excluding a mobile station in communication from occurring by the mobile station being capable of informing the base station whether a planned cell radius reduction is acceptable, and by the base station reducing the cell radius enough to cover the mobile station based on the information from the mobile station.  
         [0108]     The present invention further realizes a base station that can change cell formation according to the above cell formation control method.  
         [0109]     The present invention further realizes a mobile station that can maintain communication without being disconnected, according to the above cell formation control method.  
         [0110]     Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention.