Abstract:
A base station serving terminals, the base station including: a memory, and a processor coupled to the memory and configured to: select a terminal among the served terminals based on reception powers or reception qualities at the served terminals, when a number of the served terminals exceeds a threshold, determine an amount of change in transmission parameter so that a reception quality at the selected terminal from another base station satisfies a certain condition in case where a handover of the selected terminal to the other base station is performed, the other base station being selected based on the reception powers or the reception qualities, perform the handover of the selected terminal to the other base station, and change the transmission parameter by the determined amount of change.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-119283 filed on Jun. 5, 2013, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to a base station and a handover control method. 
       BACKGROUND 
       [0003]    In recent years, base stations called femto base stations have been known. Examples of femto base stations include base stations that form femto cells covering smaller ranges than those of the macrocells of macro base stations and those of the picocells of pico base stations. Femto base stations are used, for example, to provide services for users in environments where it is difficult for radio waves to reach, such as in basements or buildings. 
         [0004]    When femto base stations are used, they are in some cases installed at indoor spots in shopping malls and stations where a large number of general users are present. In such cases, a large number of mobile stations (may be referred to as user equipment(s) (UE(s)) or terminal(s)) may concentrate temporarily in the femto cell of a certain femto base station, thereby sharply increasing the load on that femto base station. 
         [0005]    However, from the viewpoint of apparatus size and communication load, the number of mobile stations that are connectable to the femto cell of each femto base station (namely, the number of mobile stations that can be served or managed by the femto cell of each femto base station) is smaller than that of macro base stations and pico base stations. Consequently, when a large number of mobile stations concentrate temporarily in the femto cell of a certain femto base station, it is difficult for each mobile station to connect to the femto base station. 
         [0006]    To address this difficulty, there is a known connection system that connects the mobile stations to the femto cell in the order that connection requests (may be referred to as attach request(s)) from the mobile stations (that have not be served or managed by the femto cell) arrive at the femto base station. In the connection system, the femto base station monitors the number of mobile stations that are currently connected to the femto cell thereof and rejects a connection request from a new mobile station when the number of connected mobile stations reaches a pre-defined threshold. This arrangement guarantees communication of mobile stations that are currently connected to a certain base station, when a large number of mobile stations concentrate in the cell of the base station. 
         [0007]    Related art is disclosed in Japanese Laid-open Patent Publication No. 2011-182009 and Japanese Laid-open Patent Publication No. 2010-283440. 
       SUMMARY 
       [0008]    According to an aspect of the invention, a base station serving terminals, the base station includes a memory, and a processor coupled to the memory and configured to: select a terminal among the served terminals based on reception powers or reception qualities at the served terminals, when a number of the served terminals exceeds a threshold, determine an amount of change in transmission parameter so that a reception quality at the selected terminal from another base station satisfies a certain condition in case where a handover of the selected terminal to the other base station is performed, the other base station being selected based on the reception powers or the reception qualities, perform the handover of the selected terminal to the other base station, and change the transmission parameter by the determined amount of change. 
         [0009]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0010]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a diagram illustrating the overall configuration of a wireless communication system including a femto base station according to a first embodiment; 
           [0012]      FIG. 2  is a block diagram illustrating the configuration of the femto base station according to the first embodiment; 
           [0013]      FIG. 3  is a diagram illustrating a change in the coverage of the femto cell  1  when transmission power is reduced; 
           [0014]      FIG. 4  is a schematic graph illustrating processing for recovering the transmission power; 
           [0015]      FIGS. 5A to 5C  are diagrams illustrating a flow of processing in which the femto base station according to the first embodiment connects an unconnected mobile station to the femto cell and then makes a mobile station selected as an HO candidate to execute an HO process; 
           [0016]      FIG. 6  is a flowchart illustrating a procedure of HO control processing performed by the femto base station according to the first embodiment; 
           [0017]      FIG. 7  is a sequence diagram illustrating an example of the overall flow of HO-candidate selection processing, transmission-power control processing, and HO command processing performed by the femto base station according to the first embodiment; 
           [0018]      FIG. 8  is a flowchart illustrating a procedure of the HO-candidate selection processing performed by the femto base station according to the first embodiment; 
           [0019]      FIG. 9  is a flowchart illustrating a procedure of the transmission-power control processing performed by the femto base station according to the first embodiment; 
           [0020]      FIG. 10  is a flowchart illustrating a procedure of the transmission-power recovery processing performed by the femto base station according to the first embodiment; 
           [0021]      FIG. 11  is a block diagram illustrating the configuration of a femto base station according to a second embodiment; 
           [0022]      FIG. 12  is a flowchart illustrating a procedure of HO-candidate selection processing performed by the femto base station according to the second embodiment; 
           [0023]      FIG. 13  is a block diagram illustrating the configuration of a femto base station according to a third embodiment; 
           [0024]      FIG. 14  is a flowchart illustrating a procedure of HO-candidate selection processing performed by the femto base station according to the third embodiment; 
           [0025]      FIG. 15  is a block diagram illustrating the configuration of a femto base station according to a fourth embodiment; 
           [0026]      FIG. 16  is a schematic graph illustrating processing for recovering the transmission power; 
           [0027]      FIG. 17  is a flowchart illustrating a procedure of transmission-power recovery processing performed by the femto base station according to the fourth embodiment; 
           [0028]      FIG. 18  is a block diagram illustrating the configuration of the femto base station according to the fifth embodiment; 
           [0029]      FIGS. 19A to 19C  are diagrams illustrating a flow of processing in which the femto base station according to the fifth embodiment makes a mobile station selected as an HO candidate to execute an HO process and then makes an unconnected mobile station to connect to the femto cell; 
           [0030]      FIG. 20  is a flowchart illustrating a procedure of HO control processing performed by the femto base station according to the fifth embodiment; 
           [0031]      FIG. 21  is a diagram illustrating an example of the hardware configuration of the femto base station; 
           [0032]      FIG. 22  is a diagram illustrating a problem in related art; and 
           [0033]      FIG. 23  is a diagram illustrating a problem in the related art. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]    However, in the known connection system in which mobile stations are connected to the femto cell of a femto base station in the order that connection requests from the mobile stations arrive at the femto base station, no consideration has been given to appropriately switching the connection destinations of the mobile stations to another cell when a large number of mobile stations concentrate in the cell of a certain base station. 
         [0035]    That is, in the known connection system, when the number of mobile stations (hereinafter referred to as “existing mobile stations”) that are currently connected to the femto cell of the femto base station reaches a threshold, a connection request from a new mobile station is rejected. Thus, the new mobile station is not connected to the femto cell. 
         [0036]    Such a problem in the related art will now be described with reference to  FIGS. 22 and 23 .  FIGS. 22 and 23  are diagrams illustrating problems in the related art. For example, assume a situation in which, as illustrated in  FIG. 22 , an existing mobile station UE#1 is currently connected to a femto cell  1  of a femto base station HeNB#1, a mobile station UE#2 has not been connected to the femto cell  1 , and a threshold for the number of mobile stations that are connectable to the femto cell  1  is “1”. In the example illustrated in  FIG. 22 , it is also assumed that reception power for signals transmitted from the femto cell  1  to the new mobile station UE#2 is larger than reception power for signals transmitted from the femto cell  1  to the existing mobile station UE#1. In this case, since the number of mobile stations that are currently connected to the femto cell  1  has reached the threshold “1”, the femto base station HeNB#1 rejects a connection request from the new mobile station UE#2 whose reception power is larger than that of the existing mobile station UE#1. Consequently, the new mobile station UE#2 is not connected to the femto cell  1 . 
         [0037]    Also, for example, assume a situation in which, as illustrated in  FIG. 23 , the existing mobile station UE#1 is currently connected to the femto cell  1  of the femto base station HeNB#1, the mobile station UE#2 has not been connected to the femto cell  1 , and the threshold for the number of mobile stations that are connectable to the femto cell  1  is “1”. In the example in  FIG. 23 , it is also assumed that the existing mobile station UE#1 is located at a cell border between the femto cell  1  and another femto cell  2  of a femto base station HeNB#2 that is adjacent to the femto base station HeNB#1. In this case, some of the signals transmitted from the other femto cell  2  of the femto base station HeNB#2 arrive at the existing mobile station UE#1. It is therefore expected that the existing mobile station UE#1 can also obtain, from the femto cell  2 , reception power that is equivalent to the reception power from the femto cell  1 . However, the number of mobile stations that are currently connected to the femto cell  1  has already reached the threshold “1”. Thus, the femto base station HeNB#1 rejects a connection request from the new mobile station UE#2 whose reception power for signals transmitted from the femto cell  1  is expected to be sufficiently large. Consequently, the new mobile station UE#2 is not connected to the femto cell  1 . 
         [0038]    In addition, in the example in  FIG. 23 , there may also be a situation in which the mobile station UE#2 that is not connected to the femto cell  1  is connected to the other femto cell  2 . However, in this situation, there is a possibility that the femto cell  1  gives interference to the mobile station UE#2 in downlink or the femto cell  1  receives interference from the mobile station UE#2 in uplink. 
         [0039]    In the situations illustrated in  FIGS. 22 and 23 , a scheme can be conceived from the viewpoint of load balancing that is to autonomously perform processing for switching the connection destination of the existing mobile station from the femto cell to the other cell. Processing for switching the connection destination of an existing mobile station to another cell is called a handover (HO) process. However, when the HO process is merely autonomously performed, there is a possibility that interference between signals transmitted from an HO-source femto cell and signals transmitted from another HO-destination cell increases. 
         [0040]    The technology disclosed herein has been conceived in view of the foregoing, and an object of the technology is to provide a base station and a handover control method that autonomously perform an HO process when a large number of mobile stations concentrate in the cell of a particular base station and that are capable of reducing signal interference due to a forcibly executed HO process caused by control of the number of connections. 
         [0041]    Embodiments of a base station and a handover control method disclosed herein will be described below in detail with reference to the accompanying drawings. The embodiments are not intended to limit the technology disclosed herein. In the following embodiments, a description will be given of a case in which an example of a base station is a femto base station that forms a femto cell covering a smaller range than that of the macrocell of a macro base station and that of the picocell of a pico base station. The technology disclosed herein, however, is not limited to this example, and the following embodiments are also applicable to macro base stations and pico base stations. 
       First Embodiment 
       [0042]      FIG. 1  is a diagram illustrating the overall configuration of a wireless communication system including a femto base station according to a first embodiment. The wireless communication system illustrated in  FIG. 1  includes mobile stations  20   a  and  20   b , a femto base station  10 - 1 , and another base station  10 - 2 . 
         [0043]    As illustrated in  FIG. 1 , the mobile stations  20   a  and  20   b  are currently connected to a femto cell  1  of the femto base station  10 - 1 . Of the mobile stations  20   a  and  20   b , the mobile station  20   a  is located at a cell border between the femto cell  1  and a cell  2  (hereinafter referred to as an “adjacent cell  2 ”, as appropriate) of the other base station  10 - 2  that is adjacent to the femto base station  10 - 1 . Consequently, some of the signals transmitted from the adjacent cell  2  arrive at the mobile station  20   a  as interference signals. 
         [0044]    In this case, the number of mobile stations that are connectable to the femto cell  1  is limited by a predetermined threshold. In the example illustrated in  FIG. 1 , a threshold indicating the number of mobile stations that are connectable to the femto cell  1  is assumed to be “1”. Thus, in the present embodiment, a process for switching any of the connection destinations of the mobile stations  20   a  and  20   b  from the femto cell  1  to the adjacent cell  2  (this process is hereinafter referred to as a “handover (HO) process”) is autonomously performed in order to reduce the number of mobile stations that are currently connected to the femto cell  1 . Autonomously performing an HO process means, for example, performing an HO process under a situation in which a femto base station does not have a special interface (such as an X2 interface) for performing direct exchange with an adjacent femto base station and also is not under the control of a controller that centrally controls a plurality of femto base stations. Now, a description will be given of an example of a procedure of an HO control method for autonomously performing an HO process. 
         [0045]    For example, the femto base station  10 - 1  monitors the number of connected mobile stations, which are mobile stations currently connected to the femto cell  1  of the femto base station  10 - 1 . In the example illustrated in  FIG. 1 , since two mobile stations, namely, the mobile stations  20   a  and  20   b , are currently connected to the femto cell  1 , the femto base station  10 - 1  detects that the number of connected mobile stations is “2”. 
         [0046]    When the number of connected mobile stations exceeds the threshold indicating the number of mobile stations that are connectable to the femto cell  1 , the femto base station  10 - 1  selects, from the connected mobile stations, an HO candidate that is made to execute an HO process for switching the connection destination from the femto cell  1  to the adjacent cell  2 . In the example illustrated in  FIG. 1 , since the number of connected mobile stations, “2”, exceeds the threshold “1”, the femto base station  10 - 1  selects an HO candidate from the mobile stations  20   a  and  20   b , which are connected mobile stations. In this case, it is assumed that the mobile station  20   a  located at the cell border between the femto cell  1  and the adjacent cell  2  is selected, as an HO candidate, from the mobile stations  20   a  and  20   b , which are connected mobile stations. The reason why the mobile station  20   a  is selected as the HO candidate is that the mobile station  20   a  can receive signals, transmitted from the adjacent cell  2 , at a sufficient level. 
         [0047]    The femto base station  10 - 1  controls (namely changes or adjusts) a parameter related to transmission signals transmitted from the femto cell  1  so that, of the transmission signals, signals that arrive at a mobile station selected as an HO candidate decrease. In the example illustrated in  FIG. 1 , the mobile station  20   a  located at the cell border between the femto cell  1  and the adjacent cell  2  is selected as the HO candidate. Thus, the femto base station  10 - 1  controls the parameter related to transmission signals transmitted from the femto cell  1  so that, of the transmission signals, signals that arrive at the mobile station  20   a , which is an HO candidate, decrease. In the present embodiment, the parameter related to transmission signals is assumed to be transmission power used for transmitting the transmission signals. The femto base station  10 - 1  performs, for example, control for reducing the transmission power so that, of the transmission signals transmitted from the femto cell  1 , signals that arrive at the mobile station  20   a , which is the HO candidate, decrease. The parameter related to transmission signals may also be, in addition to the transmission power, a parameter for changing a beam pattern or directivity of transmission signals radiated from an antenna. Examples of the beam pattern include the orientation (tilt) and a gain of the antenna that radiates the transmission signals into space. The parameter related to transmission signals may correspond to a transmission parameter for changing a cell coverage of the femto base station  10 - 1 . 
         [0048]    The femto base station  10 - 1  instructs the mobile station selected as the HO candidate to execute an HO process. In the example illustrated in  FIG. 1 , the mobile station  20   a  located at the cell border between the femto cell  1  and the adjacent cell  2  has been selected as the HO candidate. Thus, the femto base station  10 - 1  instructs the mobile station  20   a  to execute an HO process. As a result, the mobile station  20   a  executes the HO process to switch the connection destination from the femto cell  1  to the adjacent cell  2 . Thus, the number of mobile stations that are currently corrected to the femto cell  1  is changed from “2” to “1”. 
         [0049]    As described above, when a large number of mobile stations concentrate in the femto cell  1  of the femto base station  10 - 1  in the present embodiment, the femto base station  10 - 1  selects an HO candidate from the large number of mobile stations and controls the parameter related to transmission signals so that, of the transmission signals, signals that arrive at the HO candidate decrease. Thus, the connection destination of the mobile station selected as the HO candidate can be appropriately switched to the adjacent cell  2 , which is an HO destination, and it is also possible to reduce signal interference between the femto cell  1 , which is an HO source, and the adjacent cell  2 , which is an HO destination. Consequently, at the mobile station that has perform HO, a reception quality from the adjacent cell (HO destination) satisfies a certain condition. As a result, according to the present embodiment, when a large number of mobile stations concentrate in the femto cell  1  of the femto base station  10 - 1 , the femto base station  10 - 1  autonomously performs an HO process, thereby making it possible to reduce signal interference due to the HO process. 
         [0050]    Next, the configuration of the femto base station  10 - 1  illustrated in  FIG. 1  will be described with reference to  FIG. 2 .  FIG. 2  is a block diagram illustrating the configuration of the femto base station  10 - 1  according to the first embodiment. As illustrated in  FIG. 2 , the femto base station  10 - 1  has a wireless communication unit  11 , a number-of-connected-mobile-stations monitoring unit  12 , an HO-candidate selecting unit  13 , a transmission-power control unit  14 , an HO command unit  15 , and a call connection unit  16 . The wireless communication unit  11  is implemented by, for example, an analog circuit, such as a radio frequency (RF) circuit. The number-of-connected-mobile-stations monitoring unit  12 , the HO-candidate selecting unit  13 , the transmission-power control unit  14 , the HO command unit  15 , and the call connection unit  16  are implemented by, for example, a central processing unit (CPU) or a digital signal processor (DSP). 
         [0051]    The wireless communication unit  11  transmits/receives, via an antenna  11   a , various signals and various types of data transmitted from mobile stations located in the femto cell  1 . Signals radiated from the wireless communication unit  11  into space via the antenna  11   a  serve as the transmission signals transmitted from the femto cell  1 . 
         [0052]    For example, upon receiving, via the antenna  11   a , radio signals transmitted from a connected mobile station, which is a mobile station currently connected to the femto cell  1 , the wireless communication unit  11  outputs the received radio signals to the call connection unit  16 . 
         [0053]    Also, for example, upon receiving, via the antenna  11   a , a connection request transmitted from an unconnected mobile station, which is a mobile station located in the femto cell  1  but is not connected to the femto cell  1 , the wireless communication unit  11  outputs the received connection request to the call connection unit  16 . 
         [0054]    Also, for example, for each connected mobile station, the wireless communication unit  11  receives, via the antenna  11   a , measurement values including, for example, reception power and a reception quality for transmission signals transmitted from the femto cell  1  and outputs the measurement values, received for each connected mobile station, to the HO-candidate selecting unit  13 . The reception power for transmission signals transmitted from the femto cell  1  refers to reception power when a connected mobile station receives transmission signals transmitted from the femto cell  1 . The reception quality for transmission signals transmitted from the femto cell  1  refers to a reception quality when a connected mobile station receives transmission signals transmitted from the femto cell  1 . 
         [0055]    In addition, for example, the wireless communication unit  11  receives, from the HO command unit  15 , a command signal for instructing a mobile station selected as an HO candidate to execute an HO process and transmits, via the antenna  11   a , the received command signal to the mobile station selected as the HO candidate. 
         [0056]    The number-of-connected-mobile-stations monitoring unit  12  receives, from the call connection unit  16 , an input indicating the number of connected mobile stations, N UE , which is a detection result. The number-of-connected-mobile-stations monitoring unit  12  uses the detection result to monitor the number of connected mobile stations, N UE . The number-of-connected-mobile-stations monitoring unit  12  outputs the number of connected mobile stations, N UE , to the HO-candidate selecting unit  13  as a monitoring result. 
         [0057]    The HO-candidate selecting unit  13  receives the monitoring result input from the number-of-connected-mobile-stations monitoring unit  12 . The HO-candidate selecting unit  13  receives, from the call connection unit  16 , a notification indicating the timing at which an unconnected mobile station was connected to the femto cell  1 . After the call connection unit  16  connects the unconnected mobile station to the femto cell  1 , the HO-candidate selecting unit  13  determines whether or not the number of connected mobile stations, N UE /exceeds a threshold N max  indicating the number of mobile stations that are connectable to the femto cell  1 , based on the monitoring result. It is assumed that the threshold N max  in the present embodiment has been set to a value that is an upper limit of the number of mobile stations that are connectable to the femto cell  1  and that is smaller than a maximum number of connections, M, indicating an upper limit pre-specified as an apparatus specification of the femto base station  10 - 1 . When the number of connected mobile stations, N UE , exceeds the threshold N max , the HO-candidate selecting unit  13  selects an HO candidate from the connected mobile stations. 
         [0058]    Now, a description will be given of details of the HO-candidate selection processing performed by the HO-candidate selecting unit  13 . When the number of connected mobile stations, N UE , exceeds the threshold N max , the HO-candidate selecting unit  13  transmits, to each of the connected mobile stations via the wireless communication unit  11 , a measurement command indicating that measure measurement values including reception power, a reception quality, and so on are to be measured and to be transmitted to the femto base station  10 - 1 . The HO-candidate selecting unit  13  obtains, via the wireless communication unit  11 , the measurement values that the connected mobile stations have reported in response to the measurement command. The measurement values reported from each of the connected mobile stations include, for example, reception power for transmission signals transmitted from the femto cell  1  and the femto cell  2  and a reception quality for transmission signals transmitted from the femto cell  1 . The HO-candidate selecting unit  13  selects, as an HO candidate, a mobile station that is included in the connected mobile stations and whose reception power or reception quality for the transmission signals transmitted from the femto cell  1  is the lowest (namely the worst). In the present embodiment, the HO-candidate selecting unit  13  selects, as an HO candidate, a mobile station whose reception power for the transmission signals transmitted from the femto cell  1  is the lowest. Upon completing the HO candidate selection, the HO-candidate selecting unit  13  outputs a selection completion notification and information of the HO candidate to the transmission-power control unit  14  and the HO command unit  15 . 
         [0059]    The transmission-power control unit  14  receives the selection completion notification and the HO-candidate information from the HO-candidate selecting unit  13 . Upon being triggered by the selection completion notification, the transmission-power control unit  14  performs control for reducing the transmission power used for transmitting transmission signals transmitted from the femto cell  1  (hereinafter referred to as “transmission power”, as appropriate) so that, of the transmission signals, signals that arrive at the mobile station selected as the HO candidate decrease. For example, the transmission-power control unit  14  performs control for reducing the transmission power, by adjusting the gain of an amplifier built into the wireless communication unit  11  or by changing average amplitude of digital signal. 
         [0060]    Now, a description will be given of details of the transmission-power control processing performed by the transmission-power control unit  14 . The transmission-power control unit  14  estimates a reception quality for signals transmitted from the adjacent cell  2  to a mobile station selected as an HO candidate, when transmission signals transmitted from the femto cell  1  are assumed to be reduced in the transmission power by a power control amount Δ to be calculated. The reception quality for signals transmitted from the adjacent cell  2  to a mobile station selected as an HO candidate refers to a reception quality when a mobile station selected as an HO candidate is connected to the adjacent cell  2 . In the present embodiment, a signal-to-interference-plus-noise power ratio (SINR) is used as an example of the reception quality. When transmission signals transmitted from the femto cell  1  are assumed to be reduced in the transmission power by the power control amount Δ to be calculated, an SINR D     —     TPC  for signals transmitted from the adjacent cell  2  to the mobile station selected as the HO candidate is estimated using equation (1) below. 
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         [0061]    where P S  denotes reception power [dB] for transmission signals transmitted from an HO-source cell (the femto cell  1 ), 
         [0062]    P D  denotes reception power [dB] for signals transmitted from an HO-destination cell (the adjacent cell  2 ), and 
         [0063]    I denotes interference power [dB], including noise power, from cells other than the adjacent cell. 
         [0064]    Subsequently, the transmission-power control unit  14  calculates the power control amount Δ with which the estimated reception quality (the SINR D     —     TPC  in the present embodiment) is made to match a target value. When the target value of the SINR D     —     TPC  is indicated by SINR target     —     D , the power control amount Δ is calculated using equation (2) below. 
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                   2 
                   ) 
                 
               
             
           
         
       
     
         [0065]    Subsequently, the transmission-power control unit  14  performs control for reducing the transmission power, based on the calculated power control amount Δ. For example, by adjusting the gain of the amplifier built into the wireless communication unit  11  or by changing average amplitude of digital signal, the transmission-power control unit  14  reduces the transmission power by the power control amount Δ. 
         [0066]      FIG. 3  is a diagram illustrating a change in the coverage of the femto cell  1  when the transmission power is reduced.  FIG. 3  illustrates an example of a state in which the mobile station  20   a  located at the cell border between the femto cell  1  and the adjacent cell  2  is selected as an HO candidate and the mobile station  20   a  switches the connection destination thereof from the femto cell  1  to the adjacent cell  2 . When the transmission-power control unit  14  reduces the transmission power for transmission signals transmitted from the femto cell  1 , the coverage of the femto cell  1  is reduced, as indicated by an arrow in  FIG. 3 . In other words, of the transmission signals transmitted from the femto cell  1 , signals that arrive at the mobile station  20   a  selected as the HO candidate decrease. 
         [0067]    When performing the control for reducing the transmission power, the transmission-power control unit  14  may also specify an upper limit Δmax of the power control amount Δ in order to maintain the communication quality of a mobile station that is not selected as the HO candidate, that is, a mobile station that stays in the femto cell  1 . Now, a description will be given of a scheme for specifying the upper limit Δmax of the power control amount Δ. The transmission-power control unit  14  estimates a reception quality for transmission signals transmitted from the femto cell  1  to a mobile station that is not selected as the HO candidate, when transmission signals are assumed to be transmitted from the femto cell  1  by using the transmission power reduced by the power control amount Δ to be calculated. The transmission-power control unit  14  then specifies the upper limit Δmax of the power control amount Δ so that the estimated reception quality does not fall below a predetermined lower-limit value. In this case, the lower-limit value of the SINRs of the mobile stations that stay in the femto cell  1  is indicated by SINR target     —     S , and the SINR of a mobile station whose SINR is the lowest among the SINRs of the mobile stations that stay in the femto cell  1  is indicated by SINR S     —     min . In this case, the power control amount Δ and the upper limit Δmax of the power control amount Δ have a relationship given by equations (3) below. 
         [0000]      Δ max =SINR S     —     min −SINR target     —     S  
 
         [0000]      Δ=max{min{Δ,Δ max },0}  (3)
 
         [0068]    When a predetermined time has passed after starting to execute the control for reducing the transmission power, the transmission-power control unit  14  recovers (or restore) the reduced transmission power to a value before executing the control. However, if the transmission power recovers instantaneously to the value before executing the control, the level of interference between the transmission signals transmitted from the femto cell  1  and the signals transmitted from the adjacent cell  2  increases sharply. Accordingly, the transmission-power control unit  14  in the present embodiment recovers, in a stepwise manner, the reduced transmission power to the initial value before executing the control. More specifically, when a predetermined time T Shrink  has passed from time 0 at which the transmission power was reduced by the power control amount Δ, the transmission-power control unit  14  increases the transmission power in a stepwise manner in increments of α recov  at a period T recov , with an initial value TxPow0 of the transmission power being the upper limit, as illustrated in  FIG. 4 .  FIG. 4  is a schematic graph illustrating the processing for recovering the transmission power. In  FIG. 4 , the horizontal axis indicates time t, and the vertical axis indicates transmission power TxPow. The transmission power TxPow is represented by equation (4) below. 
         [0000]        TxPow =min{ TxPow+α   recov   ,TxPow 0}  (4)
 
         [0069]    Now, a description will be given while referring back to  FIG. 2 . The HO command unit  15  receives the selection completion notification and the HO-candidate information from the HO-candidate selecting unit  13 . The HO command unit  15  instructs the mobile station selected as the HO candidate to execute the HO process. More specifically, the HO command unit  15  transmits, to the base station  10 - 2  that forms the adjacent cell  2 , an HO request for requesting that the mobile station selected as the HO candidate be connected to the adjacent cell  2 . Thereafter, upon receiving an Ack response, which is an acknowledgement response for acknowledging the HO request, from the base station  10 - 2 , the HO command unit  15  transmits, via the wireless communication unit  11 , an command signal for instructing the mobile station selected as the HO candidate to execute an HO process. 
         [0070]    When a connection request is received from an unconnected mobile station via the wireless communication unit  11 , the call connection unit  16  connects the unconnected mobile station to the femto cell  1  so that the unconnected mobile station becomes a connected mobile station. The call connection unit  16  then notifies the HO-candidate selecting unit  13  of the timing at which the unconnected mobile station was connected to the femto cell  1  to become a connected mobile station. 
         [0071]    Using radio signals input from the wireless communication unit  11 , the call connection unit  16  detects the number of mobile stations connected to the femto cell  1 , that is, the number of connected mobile stations, N UE . The call connection unit  16  then outputs the number of connected mobile stations, N UE , to the number-of-connected-mobile-stations monitoring unit  12  as a detection result. 
         [0072]    Next, a flow of processing in which the femto base station  10 - 1  according to the present embodiment connects an unconnected mobile station to the femto cell  1  and then makes a mobile station selected as an HO candidate to execute an HO process will be described with reference to  FIGS. 5A to 5C .  FIGS. 5A to 5C  are diagrams illustrating a flow of processing in which the femto base station  10 - 1  according to the first embodiment connects an unconnected mobile station to the femto cell  1  and then makes a mobile station selected as an HO candidate to execute an HO process. In the example illustrated in  FIGS. 5A to 5C , it is assumed that the threshold N max  indicating the number of mobile stations that are connectable to the femto cell  1  is “1” and the threshold N max  is set to a smaller value than the maximum number of connections, M. It is also assumed that the mobile station  20   a  is currently connected to the femto cell  1 . 
         [0073]    Upon receiving a connection request R1 (see  FIG. 5A ) transmitted from the mobile station  20   b , which is an unconnected mobile station that is located in the femto cell  1  but is not connected to the femto cell  1 , the femto base station  10 - 1  connects the mobile station  20   b  to the femto cell  1  (see  FIG. 5B ). 
         [0074]    After the mobile station  20   b  is connected to the femto cell  1 , the femto base station  10 - 1  monitors the number of connected mobile stations and determines whether or not the monitored number of connected mobile stations, N UE , exceeds the threshold N max =“1”. In  FIG. 5B , since the number of connected mobile stations, N UE =“2”, exceeds the threshold N max =“1”, the femto base station  10 - 1  selects an HO candidate from the mobile stations  20   a  and  20   b , which are connected mobile stations. In this case, it is assumed that the mobile station  20   a  located at the cell border between the femto cell  1  and the adjacent cell  2  is selected, as an HO candidate, from the mobile stations  20   a  and  20   b , which are connected mobile stations. The femto base station  10 - 1  then performs control for reducing the transmission power so that, of transmission signals transmitted from the femto cell  1 , signals that arrive at the mobile station  20   a  selected as the HO candidate decrease. 
         [0075]    Subsequently, the femto base station  10 - 1  transmits, to the mobile station  20   a  selected as the HO candidate, an command signal D1 for issuing an command for executing an HO process, to thereby cause the mobile station  20   a  to execute the HO process for switching the connection destination thereof from the femto cell  1  to the adjacent cell  2  (see  FIG. 5C ). 
         [0076]    Next, a procedure of the HO control processing performed by the femto base station  10 - 1  according to the present embodiment will be described with reference to  FIG. 6 .  FIG. 6  is a flowchart illustrating a procedure of the HO control processing performed by the femto base station  10 - 1  according to the first embodiment. 
         [0077]    As illustrated in  FIG. 6 , if a connection request from an unconnected mobile station is not received (NO in step S 101 ), the call connection unit  16  in the femto base station  10 - 1  returns the process to step S 101 . If a connection request from an unconnected mobile station is received (YES in step S 101 ), the call connection unit  16  determines whether or not the number of connected mobile stations, N UE , is smaller than the maximum number of connections, M (step S 102 ). If the number of connected mobile stations, N UE , is larger than or equal to the maximum number of connections, M (NO in step S 102 ), the call connection unit  16  advances the process to step S 105 . 
         [0078]    On the other hand, if the number of connected mobile stations, N UE , is smaller than the maximum number of connections, M (YES in step S 102 ), the call connection unit  16  connects the unconnected mobile station to the femto cell  1  (step S 103 ). 
         [0079]    Thereafter, based on a monitoring result obtained by the number-of-connected-mobile-stations monitoring unit  12 , the HO-candidate selecting unit  13  determines whether or not the number of connected mobile stations, N UE , exceeds the threshold N max  indicating the number of mobile stations that are connectable to the femto cell  1  (step S 104 ). If the number of connected mobile stations, N UE , does not exceed the threshold N max  (NO in step S 104 ), the HO-candidate selecting unit  13  ends the processing. 
         [0080]    On the other hand, if the number of connected mobile stations, N UE , exceeds the threshold N max  (YES in step S 104 ), the HO-candidate selecting unit  13  advances the process to step S 105  for HO-candidate selection processing, transmission-power control processing, and HO command processing. 
         [0081]    Next, an example of an overall flow of the HO-candidate selection processing, the transmission-power control processing, and the HO command processing in step S 105  illustrated in  FIG. 6  will be described with reference to  FIG. 7 .  FIG. 7  is a sequence diagram illustrating an example of the overall flow of the HO-candidate selection processing, the transmission-power control processing, and the HO command processing performed by the femto base station  10 - 1  according to the first embodiment. 
         [0082]    As illustrated in  FIG. 7 , the HO-candidate selecting unit  13  in the femto base station  10 - 1  executes the HO-candidate selection processing (step S 113 ). 
         [0083]    Now, a procedure of the HO-candidate selection processing in step S 113  illustrated in  FIG. 7  will be described with reference to  FIG. 8 .  FIG. 8  is a flowchart illustrating the procedure of the HO-candidate selection processing performed by the femto base station  10 - 1  according to the first embodiment. 
         [0084]    As illustrated in  FIG. 8 , the HO-candidate selecting unit  13  in the femto base station  10 - 1  obtains measurement values that the mobile stations  20   a  and  20   b , which are connected mobile stations, have reported in response to the measurement command (step S 121 ). The measurement values reported from each of the connected mobile stations include, for example, reception power for transmission signals transmitted from the femto cell  1  and a reception quality for transmission signals transmitted from the femto cell  1 . The HO-candidate selecting unit  13  holds the obtained measurement values for each connected mobile station. 
         [0085]    The HO-candidate selecting unit  13  selects, as an HO candidate, a mobile station that is included in the mobile stations  20   a  and  20   b , which are connected mobile stations, and whose reception power or reception quality for the transmission signals transmitted from the femto cell  1  is minimized (step S 122 ). In the present embodiment, it is assumed that the HO-candidate selecting unit  13  selects, as an HO candidate, the mobile station  20   a  whose reception power for the transmission signals transmitted from the femto cell  1  is minimized. Upon completing the HO-candidate selection, the HO-candidate selecting unit  13  outputs a selection completion notification and information of the HO candidate to the transmission-power control unit  14  and the HO command unit  15 . 
         [0086]    Now, a description will be given while referring back to  FIG. 7 . Upon receiving the selection completion notification and the HO-candidate information from the HO-candidate selecting unit  13 , the HO command unit  15  transmits, to the base station  10 - 2  that forms the adjacent cell  2 , an HO request for requesting that the mobile station  20   a  selected as the HO candidate be connected to the adjacent cell  2  (step S 114 ). Upon receiving the HO request, the base station  10 - 2  transmits, to the femto base station  10 - 1 , an Ack response, which is an acknowledgement response for acknowledging the HO request (step S 115 ). 
         [0087]    If the Ack response is received from the base station  10 - 2 , the HO command unit  15  transmits, to the mobile station  20   a  selected as the HO candidate, an command signal for issuing an command for executing an HO process (step S 116 ). Thereafter, the transmission-power control unit  14  executes the transmission-power control processing (step S 117 ). 
         [0088]    Now, a procedure of the transmission-power control processing illustrated in  FIG. 7  will be described with reference to  FIG. 9 .  FIG. 9  is a flowchart illustrating the procedure of the transmission-power control processing performed by the femto base station  10 - 1  according to the first embodiment. 
         [0089]    As illustrated in  FIG. 9 , upon being triggered by the selection completion notification from the HO-candidate selecting unit  13 , the transmission-power control unit  14  in the femto base station  10 - 1  calculates a power control amount Δ (step S 131 ). For example, the transmission-power control unit  14  estimates an SINR D     —     TPC  corresponding to signals transmitted from the adjacent cell  2  to the mobile station  20   a  that is the HO candidate, when transmission signals are assumed to be transmitted from the femto cell  1  by using the transmission power reduced by a power control amount Δ to be calculated. The transmission-power control unit  14  then calculates the power control amount Δ with which the estimated SINR D     —     TPC  is made to match the target value SINR target     —     D . 
         [0090]    The transmission-power control unit  14  performs control for reducing the transmission power by the calculated power control amount Δ (step S 132 ). 
         [0091]    Now, a description will be given while referring back to  FIG. 7 . After the transmission-power control unit  14  completes the transmission power control, the mobile station  20   a  selected as the HO candidate executes an HO process to switch the connection destination from the femto cell  1  to the adjacent cell  2  (step S 118 ). 
         [0092]    Although a case in which the process in step S 117  is executed after the process in step S 116  has been described in the example illustrated in  FIG. 7 , the process in step S 116  may also be executed after the process in step S 117 . 
         [0093]    Next, a procedure of the transmission-power recovery processing performed by the femto base station  10 - 1  according to the present embodiment will be described with reference to  FIG. 10 .  FIG. 10  is a flowchart illustrating the procedure of the transmission-power recovery processing performed by the femto base station according to the first embodiment. 
         [0094]    As illustrated in  FIG. 10 , if the transmission power has not been reduced (NO in step S 141 ), the transmission-power control unit  14  in the femto base station  10 - 1  returns the process to step S 141 . On the other hand, if the transmission power has been reduced (YES in step S 141 ), the transmission-power control unit  14  initializes a timer t and sets a predetermined time T Shrink  for timer expiration time T (step S 142 ). 
         [0095]    The transmission-power control unit  14  determines whether or not the timer t is greater than or equal to the timer expiration time T (step S 143 ). If the timer t is smaller than the timer expiration time T (NO in step S 143 ), the transmission-power control unit  14  increments the timer t (step S 144 ) and returns the process to step S 143 . 
         [0096]    On the other hand, if the timer t is greater than or equal to the timer expiration time T (YES in step S 143 ), the transmission-power control unit  14  increases the transmission power by α recov  (step S 145 ). Thereafter, the transmission-power control unit  14  initializes the timer t and sets the period T recov  for the timer expiration time T (step S 146 ). 
         [0097]    The transmission-power control unit  14  determines whether or not the transmission power TxPow is larger than or equal to the initial value TxPow0 (step S 147 ). If the transmission power TxPow is smaller than the initial value TxPow0 (NO in step S 147 ), the transmission-power control unit  14  returns the process to step S 143 . On the other hand, if the transmission power TxPow is larger than or equal to the initial value TxPow0 (YES in step S 147 ), the transmission-power control unit  14  ends the processing. 
         [0098]    As described above, when a large number of mobile stations concentrate in the femto cell  1  of the femto base station  10 - 1  in the present embodiment, the femto base station  10 - 1  selects an HO candidate from the large number of mobile stations, and controls a parameter related to transmission signals, based on the reception quality when the HO candidate is connected to the adjacent cell  2 . Thus, the connection destination of the mobile station selected as the HO candidate can be appropriately switched to the adjacent cell  2 , which is an HO destination, and it is also possible to reduce the level of signal interference from the femto cell  1 , which is an HO source, to the adjacent cell  2 , which is the HO destination. As a result, according to the present embodiment, when a large number of mobile stations concentrate in the femto cell  1  of the femto base station  10 - 1 , it is possible to autonomously perform an HO process and it is also possible to reduce the signal interference due to a forcibly executed HO process caused by the control of the number of connections. 
         [0099]    In addition, the femto base station  10 - 1  in the present embodiment selects, as an HO candidate, a mobile station that is included in the connected mobile stations and whose reception power or reception quality for transmission signals transmitted from the femto cell  1  is minimized. Thus, according to the present embodiment, a mobile station whose reception power or reception quality for transmission signals transmitted from the femto cell  1 , which is an HO source, is favorable is allowed to stay in the femto cell  1 , which is an HO source, with high priority. 
         [0100]    Also, the femto base station  10 - 1  in the present embodiment executes control for reducing the transmission power used for transmitting transmission signals, the transmission power being a parameter related to the transmission signals. Thus, according to the present embodiment, it is possible to easily reduce, of transmission signals transmitted from the femto cell  1 , signals that arrive at the mobile station  20   a  selected as the HO candidate, thus making it possible efficiently reduce the coverage of the femto cell  1 . 
         [0101]    Also, the femto base station  10 - 1  in the present embodiment estimates a reception quality for signals transmitted from the adjacent cell  2  to the mobile station  20   a  selected as the HO candidate, when transmission signals are assumed to be transmitted from the femto cell  1  by using transmission power reduced by the power control amount to be calculated. The femto base station  10 - 1  then calculates a power control amount with which the estimated reception quality is made to match a target value, and performs control for reducing the transmission power, based on the calculated power control amount. Thus, according to the present embodiment, it is possible to reduce signal interference between the femto cell  1 , which is an HO source, and the adjacent cell  2 , which is an HO destination, while maintaining, at the target value, the reception quality for signals transmitted from the adjacent cell  2  to a mobile station selected as an HO candidate. 
         [0102]    The femto base station  10 - 1  in the present embodiment estimates a reception quality for transmission signals transmitted from the femto cell  1  to a mobile station that is not the HO candidate, when transmission signals are assumed to be transmitted from the femto cell  1  by using the transmission power reduced by the power control amount to be calculated. The femto base station  10 - 1  specifies the upper limit of the power control amount so that the estimated reception quality does not fall below the predetermined lower-limit value. Thus, according to the present embodiment, it is possible to appropriately maintain the communication quality of a mobile station that is not selected as the HO candidate, that is, a mobile station that stays in the femto cell  1 . 
         [0103]    Also, when a predetermined time has passed after starting to execute the control for reducing the transmission power, the femto base station  10 - 1  in the present embodiment recovers the reduced transmission power to the value before executing the control. Thus, according to the present embodiment, when the predetermined time has passed after starting to execute the control for reducing the transmission power, the size of the coverage of the femto cell  1 , the size being reduced as a result of the reduction of the transmission power, can be recovered to the initial size. 
         [0104]    Also, the femto base station  10 - 1  in the present embodiment transmits, to the base station  10 - 2  that forms the adjacent cell  2 , an HO request for requesting that the mobile station selected as the HO candidate be connected to the adjacent cell  2 . Upon receiving, from the base station  10 - 2 , an acknowledgement response for acknowledging the HO request, the femto base station  10 - 1  instructs the mobile station selected as the HO candidate to execute an HO process. Thus, according to the present embodiment, under the initiative of the femto base station  10 - 1 , which is an HO source, it is possible to cause a mobile station, which is an HO candidate, to appropriately execute an HO process. 
         [0105]    Also, after an unconnected mobile station that has transmitted a connection request is connected to the femto cell  1  to become a connected mobile station, the femto base station  10 - 1  in the present embodiment determines whether or not the number of connected mobile stations exceeds the threshold N max . When the number of connected mobile stations exceeds the threshold N max , the femto base station  10 - 1  causes the mobile station selected as the HO candidate to execute an HO process. Thus, according to the present embodiment, after an unconnected mobile station is connected to the femto cell  1 , the connection destination of an HO candidate selected connected mobile stations can be switched from the femto cell  1  to the adjacent cell  2 . 
       Second Embodiment 
       [0106]    A description in the first embodiment has been given of an example in which a mobile station that is included in mobile stations currently connected to the femto cell  1  and whose reception power or reception quality for transmission signals transmitted from the femto cell  1  is minimized (namely the worst) is selected as an HO candidate. A mobile station that is included in mobile stations currently connected to the femto cell  1  and whose reception power or reception quality for signals transmitted from the adjacent cell is maximized may also be selected as an HO candidate. In a second embodiment, a description will be given of an example in which a mobile station that is included in mobile stations currently connected to the femto cell  1  and whose reception power or reception quality for signals transmitted from the adjacent cell is maximized (namely the best) is selected as an HO candidate. 
         [0107]      FIG. 11  is a block diagram illustrating the configuration of a femto base station according to the second embodiment. In  FIG. 11 , elements that are the same as or similar to those in  FIG. 2  are denoted by the same reference numerals, and descriptions thereof are not given hereinafter. As illustrated in  FIG. 11 , a femto base station  20 - 1  according to the present embodiment has an HO-candidate selecting unit  23 , instead of the HO-candidate selecting unit  13  illustrated in  FIG. 2 . 
         [0108]    The HO-candidate selecting unit  23  receives a monitoring result input from the number-of-connected-mobile-stations monitoring unit  12 . The HO-candidate selecting unit  23  receives, from the call connection unit  16 , a notification indicating the timing at which an unconnected mobile station was connected to the femto cell  1 . After the call connection unit  16  connects the unconnected mobile station to the femto cell  1 , the HO-candidate selecting unit  23  determines whether or not the number of connected mobile stations, N UE , has exceeded the threshold N max  indicating the number of mobile stations that are connectable to the femto cell  1 , based on a monitoring result. It is assumed that the threshold N max  in the present embodiment has been set to a value that is an upper limit of the number of mobile stations that are connectable to the femto cell  1  and that is smaller than a maximum number of connections, M, indicating an upper limit pre-specified as an apparatus specification of the femto base station  20 - 1 . When the number of connected mobile stations has exceeded the threshold N max , the HO-candidate selecting unit  23  selects an HO candidate from the connected mobile stations. 
         [0109]    Now, a description will be given of details of the HO-candidate selection processing performed by the HO-candidate selecting unit  23 . When the number of connected mobile stations exceeds the threshold N max , the HO-candidate selecting unit  23  transmits, to each of the connected mobile stations via the wireless communication unit  11 , a measurement command indicating that measure measurement values including reception power, a reception quality, and so on are to be measured and to be transmitted to the femto base station  20 - 1 . The HO-candidate selecting unit  23  obtains, via the wireless communication unit  11 , the measurement values that the connected mobile stations have reported in response to the measurement command. The measurement values reported by each of the connected mobile stations include, for example, reception power P S  for transmission signals transmitted from the femto cell  1 , a reception quality SINRs for transmission signals transmitted from the femto cell  1 , and reception power P D  for signals transmitted from the adjacent cell  2 . Based on the reception power P S , the reception quality SINR S , and the reception power P D , the HO-candidate selecting unit  23  calculates a reception quality SINR D  for the signals transmitted from the adjacent cell  2 . The reception quality for the signals transmitted from the adjacent cell  2  refers to a reception quality when the connected mobile station is connected to the adjacent cell  2 . The reception quality SINR D  for the signals transmitted from the adjacent cell  2  is calculated using equation (5). 
         [0000]    
       
         
           
             
               
                 
                   
                       
                   
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         [0110]    I in equation (5) represents interference power including noise power and is calculated using equation (6) below. 
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         [0111]    Subsequently, the HO-candidate selecting unit  23  selects, as an HO candidate, a mobile station that is included in the connected mobile stations and whose reception power P D  or reception quality SINR D  for the signals transmitted from the adjacent cell  2  is the highest. According to the present embodiment, the HO-candidate selecting unit  23  selects, as an HO candidate, a mobile station whose reception quality SINR D  is the highest. Upon completing the HO-candidate selection, the HO-candidate selecting unit  23  outputs a selection completion notification and information of the HO candidate to the transmission-power control unit  14  and the HO command unit  15 . 
         [0112]    Next, a procedure of the HO-candidate selection processing performed by the femto base station  20 - 1  according to the present embodiment will be described with reference to  FIG. 12 .  FIG. 12  is a flowchart illustrating the procedure of the HO-candidate selection processing performed by the femto base station according to the second embodiment. The procedure of the HO-candidate selection processing illustrated in the flowchart in  FIG. 12  corresponds to the procedure of the HO-candidate selection processing in step S 113  illustrated in  FIG. 7 . 
         [0113]    As illustrated in  FIG. 12 , the HO-candidate selecting unit  23  in the femto base station  20 - 1  obtains measurement values that the mobile stations  20   a  and  20   b , which are connected mobile stations, have reported in response to the measurement command (step S 151 ). The measurement values reported by each of the connected mobile stations include, for example, the reception power P S  for transmission signals transmitted from the femto cell  1 , the reception quality SINR S  for transmission signals transmitted from the femto cell  1 , and the reception power P D  for signals transmitted from the adjacent cell  2 . The HO-candidate selecting unit  23  holds the obtained measurement values for each connected mobile station. 
         [0114]    Based on the reception power P S , the reception quality SINR S , and the reception power P D , the HO-candidate selecting unit  23  calculates a reception quality SINR D  when each connected mobile station is connected to the adjacent cell  2  (step S 152 ). For example, the HO-candidate selecting unit  23  calculates the reception quality SINR D  for each connected mobile station by using equation (5). 
         [0115]    The HO-candidate selecting unit  23  selects, as an HO candidate, a mobile station that is included in the connected mobile stations and whose reception power P D  or reception quality SINR D  for the signals transmitted from the adjacent cell  2  is the highest (step S 153 ). In the present embodiment, the HO-candidate selecting unit  23  selects, as an HO candidate, a mobile station whose reception quality SINR D  is the highest. Upon completing the HO-candidate selection, the HO-candidate selecting unit  23  outputs a selection completion notification and information of the HO candidate to the transmission-power control unit  14  and the HO command unit  15 . 
         [0116]    As described above, the femto base station  20 - 1  in the present embodiment selects, as an HO candidate, a mobile station that is included in the connected mobile stations and whose reception power or reception quality for signals transmitted from the adjacent cell  2  is the highest. Thus, according to the present embodiment, the connection destination of a mobile station whose reception power or reception quality for signals transmitted from the adjacent cell  2  is favorable can be switched to the adjacent cell  2  with high priority. As a result, it is possible to efficiently reduce signal interference due to the HO process. 
       Third Embodiment 
       [0117]    The description in the second embodiment has been given of an example in which a mobile station that is included in mobile stations currently connected to the femto cell  1  and whose reception power or reception quality for signals transmitted from the adjacent cell  2  is the highest is selected as an HO candidate. However, when the reception power or the reception quality of the mobile station selected as the HO candidate, the reception power or the reception quality being used for signals transmitted from the adjacent cell  2 , does not satisfy a predetermined value, a mobile station whose reception power or reception quality for transmission signals transmitted from the femto cell  1  may also be newly selected (or reselected) as the HO candidate. Accordingly, in a third embodiment, a description will be given of an example in which, when the reception power or the reception quality of the mobile station selected as the HO candidate, the reception power or the reception quality being used for signals transmitted from the adjacent cell  2 , does not satisfy a predetermined value, a mobile station whose reception power or reception quality for transmission signals transmitted from the femto cell  1  is the lowest is newly selection as the HO candidate. 
         [0118]      FIG. 13  is a block diagram illustrating the configuration of a femto base station according to the third embodiment. In  FIG. 13 , elements that are the same as or similar to those in  FIG. 11  are denoted by the same reference numerals, and descriptions thereof are not given hereinafter. As illustrated in  FIG. 13 , a femto base station  30 - 1  according to the present embodiment has an HO-candidate selecting unit  33 , instead of the HO-candidate selecting unit  23  illustrated in  FIG. 11 . 
         [0119]    Similarly to the HO-candidate selecting unit  23  illustrated in  FIG. 11 , the HO-candidate selecting unit  33  selects, as an HO candidate, a mobile station that is included in connected mobile stations and whose reception power P D  or reception quality SINR D  for signals transmitted from the adjacent cell  2  is the highest. 
         [0120]    In addition, the HO-candidate selecting unit  33  determines whether or not the reception power P D  or the reception quality SINR D  of the mobile station selected as the HO candidate satisfies a predetermined value. When the reception power P D  or the reception quality SINR D  does not satisfy the predetermined value, the HO-candidate selecting unit  33  newly selects, as the HO candidate, a mobile station that is included in the connected mobile stations and whose reception power or reception quality for transmission signals transmitted from the femto cell  1  is the lowest. Upon completing the HO-candidate selection, the HO-candidate selecting unit  33  outputs a selection completion notification and information of the HO candidate to the transmission-power control unit  14  and the HO command unit  15 . 
         [0121]    Next, a procedure of the HO-candidate selection processing performed by the femto base station  30 - 1  according to the present embodiment will be described with reference to  FIG. 14 .  FIG. 14  is a flowchart illustrating the procedure of the HO-candidate selection processing performed by the femto base station  30 - 1  according to the third embodiment. The procedure of the HO-candidate selection processing illustrated in the flowchart  FIG. 14  corresponds to the procedure of the HO-candidate selection processing in step S 113  illustrated in  FIG. 7 . 
         [0122]    As illustrated in  FIG. 14 , the HO-candidate selecting unit  33  in the femto base station  30 - 1  obtains measurement values that the mobile stations  20   a  and  20   b , which are connected mobile station, have reported in response to the measurement command (step S 161 ). The measurement values reported by each of the connected mobile stations include, for example, the reception power P S  for transmission signals transmitted from the femto cell  1 , the reception quality SINR S  for transmission signals transmitted from the femto cell  1 , and the reception power P D  for signals transmitted from the adjacent cell  2 . The HO-candidate selecting unit  33  holds the obtained measurement values for each connected mobile station. 
         [0123]    Based on the reception power P S , the reception quality SINR S , and the reception power P D , the HO-candidate selecting unit  33  calculates a reception quality SINR D  for the signals transmitted from the adjacent cell  2  (step S 162 ). For example, the HO-candidate selecting unit  33  calculates the reception quality SINR D  for each connected mobile station by using equation (5) noted above. 
         [0124]    The HO-candidate selecting unit  33  selects, as an HO candidate, a mobile station that is included in the connected mobile stations and whose reception power P D  or reception quality SINR D  for the signals transmitted from the adjacent cell  2  is maximized (step S 163 ). In the present embodiment, the HO-candidate selecting unit  33  selects, as an HO candidate, a mobile station whose reception quality SINR D  is maximized. 
         [0125]    The HO-candidate selecting unit  33  determines whether or not the reception power P D  or the reception quality SINR D  of the mobile station selected as the HO candidate satisfies a predetermined value (step S 164 ). If the reception power P D  or the reception quality SINR D  of the mobile station selected as the HO candidate satisfies the predetermined value (NO in step S 164 ), the HO-candidate selecting unit  33  ends the processing without newly selecting an HO candidate. 
         [0126]    On the other hand, if the reception power P D  or the reception quality SINR D  does not satisfy the predetermined value, the HO-candidate selecting unit  33  performs the following process (YES in step S 164 ). That is, the HO-candidate selecting unit  33  newly selects, as the HO candidate, a mobile station whose reception power P S  or reception quality SINR S  for the transmission signals transmitted from the femto cell  1  is minimized (step S 165 ). In the present embodiment, the HO-candidate selecting unit  33  newly selects, as the HO candidate, a mobile station whose reception power P S  is minimized. Upon completing the HO-candidate selection, the HO-candidate selecting unit  33  outputs a selection completion notification and information of the HO candidate to the transmission-power control unit  14  and the HO command unit  15 . 
         [0127]    As described above, the femto base station  30 - 1  in the present embodiment selects, as an HO candidate, a mobile station that is included in connected mobile stations and whose reception power or reception quality for signals transmitted from the adjacent cell  2  is maximized. When the reception power or the reception quality of the mobile station selected as the HO candidate, the reception power or the reception quality being used for signals transmitted from the adjacent cell  2 , does not satisfy the predetermined value, the femto base station  30 - 1  newly selects, as the HO candidate, a mobile station whose reception power or reception quality for transmission signals transmitted from the femto cell  1  is minimized. Hence, according to the present embodiment, when the reception power or the reception quality of the mobile station selected as the HO candidate, the reception power or the reception quality being used for signals transmitted from the adjacent cell  2 , does not satisfy the predetermined value, a mobile station whose reception power or reception quality for transmission signals transmitted from a cell, which is an HO source, is the worst can be newly selected as the HO candidate. 
       Fourth Embodiment 
       [0128]    In the first embodiment, a description has been give of an example in which, when a predetermined time has passed after starting to execute the control for reducing the transmission power, the reduced transmission power is recovered to the value before executing the control. However, in the period in which the control for reducing the transmission power is executed, the reduced transmission power may also be recovered to the value before executing the control, when the number of connected mobile stations becomes smaller than or equal to the threshold N max . Accordingly, a description in a fourth embodiment will be given of an example in which, when the number of connected mobile stations becomes smaller than or equal to the threshold N max  in the period in which the control for reducing the transmission power is executed, the reduced transmission power is recovered to the value before executing the control. 
         [0129]      FIG. 15  is a block diagram illustrating the configuration of a femto base station according to the fourth embodiment. In  FIG. 15 , elements that are the same as or similar to those in  FIG. 2  are denoted by the same reference numerals, and descriptions thereof are not given hereinafter. As illustrated in  FIG. 15 , a femto base station  40 - 1  according to the present embodiment has a transmission-power control unit  44 , instead of the transmission-power control unit  14  illustrated in  FIG. 2 . 
         [0130]    The transmission-power control unit  44  receives a selection completion notification and HO-candidate information from the HO-candidate selecting unit  13 . Similarly to the transmission-power control unit  14  illustrated in  FIG. 2 , upon being triggered by the selection completion notification, the transmission-power control unit  44  performs control for reducing the transmission power so that, of transmission signals transmitted from the femto cell  1 , signals that arrive at the mobile station selected as the HO candidate decrease. For example, the transmission-power control unit  44  performs control for reducing the transmission power by adjusting the gain of an amplifier built into the wireless communication unit  11  or by changing average amplitude of digital signal. 
         [0131]    In addition, the transmission-power control unit  44  receives a monitoring result input from the number-of-connected-mobile-stations monitoring unit  12 . In the period in which the control for reducing the transmission power is executed, based on the monitoring result, the transmission-power control unit  44  determines whether or not the number of connected mobile stations, N UE , becomes smaller than or equal to the threshold N max  indicating the number of mobile stations that are connectable to the femto cell  1 . When the number of connected mobile stations, N UE , becomes smaller than the threshold N max , the transmission-power control unit  44  recovers the reduced transmission power to the value before executing the control. 
         [0132]    However, if the transmission power recovers instantaneously to the value before executing the control, the level of interference between the transmission signals transmitted from the femto cell  1  and the signals transmitted from the adjacent cell  2  increases sharply. Accordingly, the transmission-power control unit  44  in the present embodiment recovers, in a stepwise manner, the reduced transmission power to the initial value before executing the control. More specifically, when the number of connected mobile stations, N UE , becomes smaller than the threshold N max , the transmission-power control unit  44  increases the transmission power in a stepwise manner in increments of α recov  at a period T recov , with an initial value TxPow0 of the transmission power being the upper limit, as illustrated in  FIG. 16 .  FIG. 16  is a schematic graph illustrating the processing for recovering the transmission power. In  FIG. 16 , the horizontal axis indicates time t, and the vertical axis indicates the transmission power TxPow. The transmission power TxPow is represented by equation (4) noted above. 
         [0133]    Next, a procedure of the transmission-power recovery processing performed by the femto base station  40 - 1  according to the present embodiment will be described with reference to  FIG. 17 .  FIG. 17  is a flowchart illustrating the procedure of the transmission-power recovery processing performed by the femto base station according to the fourth embodiment. 
         [0134]    As illustrated in  FIG. 17 , if the transmission power has not been reduced (NO in step S 171 ), the transmission-power control unit  44  in the femto base station  40 - 1  returns the process to step S 171 . On the other hand, if the transmission power has been reduced (YES in step S 171 ), the transmission-power control unit  44  determines whether or not the number of connected mobile stations, N UE , becomes smaller than or equal to the threshold N max  indicating the number of mobile stations that are connectable to the femto cell  1 , based on a monitoring result (step S 172 ). 
         [0135]    If the number of connected mobile stations, N UE , exceeds the threshold N max  (NO in step S 172 ), the transmission-power control unit  44  returns the process to step S 172 . 
         [0136]    On the other hand, if the number of connected mobile stations, N UE , becomes smaller than or equal to the threshold N max  (YES in step S 172 ), the transmission-power control unit  44  initializes the timer t and sets “0” for the timer expiration time T (step S 173 ). 
         [0137]    The transmission-power control unit  44  determines whether or not the timer t is greater than or equal to the timer expiration time T (step S 174 ). If the timer t is smaller than the timer expiration time T (NO in step S 174 ), the transmission-power control unit  44  increments the timer t (step S 175 ) and returns the process to step S 174 . 
         [0138]    On the other hand, if the timer t becomes larger than or equal to the timer expiration time T (YES in step S 174 ), the transmission-power control unit  44  increases the transmission power by α recov  (step S 176 ). Thereafter, the transmission-power control unit  44  initializes the timer t and sets a period T recov  for the timer expiration time T (step S 177 ). 
         [0139]    The transmission-power control unit  44  determines whether or not the transmission power TxPow is larger than or equal to the initial value TxPow0 (step S 178 ). If the transmission power TxPow is smaller than the initial value TxPow0 (NO in step S 178 ), the transmission-power control unit  44  returns the process to step S 174 . On the other hand, if the transmission power TxPow is larger than or equal to the initial value TxPow0, the transmission-power control unit  44  ends the processing (YES in step S 178 ). 
         [0140]    As described above, when the number of connected mobile stations becomes smaller than or equal to the threshold N max  in the period in which the control for reducing the transmission power is executed, the femto base station  40 - 1  in the present embodiment recovers the reduced transmission power to the value before executing the control. Thus, according to the present embodiment, when the number of connected mobile stations becomes smaller than or equal to the threshold N max  in the period in which the control for reducing the transmission power is executed, the size of the coverage of the femto cell  1 , the size being reduced as a result of the reduction of the transmission power, can be recovered to the initial size. 
       Fifth Embodiment 
       [0141]    The description in the first embodiment has been given of an example in which the threshold N max  indicating the number of mobile stations that are connectable to the femto cell  1  is set to a value that is the upper limit of the number of mobile stations that are connectable to the femto cell  1  and that is smaller than the maximum number of connections, M, indicating the upper limit pre-specified as an apparatus specification of the femto base station  10 - 1 . However, the threshold N max  may also be set to the same value as the maximum number of connections, M. In such a case, the femto base station may also be adapted to determine whether or not the number of connected mobile stations exceeds the threshold N max , before an unconnected mobile station that has transmitted a connection request is connected to the femto cell  1  to become a connected mobile station. In this case, it is assumed that a case in which the number of connected mobile stations exceeds the threshold N max  includes a case in which the number of connected mobile stations becomes larger than or equal to the threshold N max . Accordingly, a description in a fifth embodiment will be given of an example in which, before an unconnected mobile station that has transmitted a connection request is connected to the femto cell  1  to become a connected mobile station, a determination is made as to whether or not the number of connected mobile stations exceeds the threshold N max . 
         [0142]      FIG. 18  is a block diagram illustrating the configuration of a femto base station according to the fifth embodiment. In  FIG. 18 , elements that are the same as or similar to those in  FIG. 2  are denoted by the same reference numerals, and descriptions thereof are not given hereinafter. As illustrated in  FIG. 18 , a femto base station  50 - 1  according to the present embodiment has an HO-candidate selecting unit  53 , an HO command unit  55 , and a call connection unit  56 , instead of the HO-candidate selecting unit  13 , the HO command unit  15 , and the call connection unit  16  illustrated in  FIG. 2 . 
         [0143]    When a connection request from an unconnected mobile station is received via the wireless communication unit  11 , the call connection unit  56  outputs a notification indicating that the connection request is received to the HO-candidate selecting unit  53 , without connecting the unconnected mobile station to the femto cell  1  so that the unconnected mobile station becomes a connected mobile station. The HO command unit  55  receives, from the call connection unit  56 , a notification indicating the timing at which an HO process is to be executed. After the HO command unit  55  issues the command for executing the HO process, the call connection unit  56  connects the unconnected mobile station to the femto cell  1  so that the unconnected mobile station becomes a connected mobile station. 
         [0144]    The HO-candidate selecting unit  53  receives a monitoring result input from the number-of-connected-mobile-stations monitoring unit  12 . The HO-candidate selecting unit  53  receives, from the call connection unit  56 , a notification indicating that the connection request was received. Upon receiving, from the call connection unit  56 , the notification indicating that the connection request was received, the HO-candidate selecting unit  53  performs the following processing before the call connection unit  56  connects the unconnected mobile station to the femto cell  1  so that the unconnected mobile station becomes a connected mobile station. That is, based on the monitoring result, the HO-candidate selecting unit  53  determines whether or not the number of connected mobile stations, N UE , exceeds the threshold N max  indicating the number of mobile stations that are connectable to the femto cell  1 . It is assumed the threshold N max  in the present embodiment has been set to a value that is the upper limit of the number of mobile stations that are connectable to the femto cell  1  and that is the same as the maximum number of connections, M, indicating the upper limit pre-specified as an apparatus specification of the femto base station  50 - 1 . When the number of connected mobile stations, N UE , exceeds the threshold N max , the HO-candidate selecting unit  53  selects an HO candidate from the connected mobile stations. Details of the HO-candidate selection processing performed by the HO-candidate selecting unit  53  are analogous to the details of the HO-candidate selection processing performed by the HO-candidate selecting unit  13  illustrated in  FIG. 2 . 
         [0145]    The HO command unit  55  receives a selection completion notification and HO-candidate information from the HO-candidate selecting unit  53 . The HO command unit  55  instructs the mobile station selected as the HO candidate to execute an HO process. In addition, the HO command unit  55  notifies the call connection unit  56  of the timing at which the HO process is to be executed. 
         [0146]    Next, a flow of processing in which the femto base station  50 - 1  according to the present embodiment makes a mobile station selected as an HO candidate to execute an HO process and then makes an unconnected mobile station to connect to the femto cell  1  will be described with reference to FIGS.  19 A to  19 C.  FIGS. 19A to 19C  are diagrams illustrating a flow of processing in which the femto base station according to the fifth embodiment makes a mobile station selected as an HO candidate to execute an HO process and then makes an unconnected mobile station to connect to the femto cell. In the example illustrated in  FIGS. 19A to 19C , it is assumed that the threshold N max  indicating the number of mobile stations that are connectable to the femto cell  1  is “2” and the threshold N max  is set to the same value as the maximum number of connections, M. It is also assumed that mobile stations  20   a  and  20   b  are currently connected to the femto cell  1 . 
         [0147]    The femto base station  50 - 1  receives a connection request R2 (see  FIG. 19A ) transmitted from a mobile station  20   c , which is an unconnected mobile station that is located in the femto cell  1  but is not connected to the femto cell  1 . The femto base station  50 - 1  then monitors the number of connected mobile stations and determines whether or not the monitored number of connected mobile stations, N UE , exceeds the threshold N max =“2”. In  FIG. 19A , since the number of connected mobile stations, N UE , =“2” has reached the threshold N max =“2”, the femto base station  50 - 1  selects an HO candidate from the mobile stations  20   a  and  20   b , which are connected mobile stations. In this case, it is assumed that the mobile station  20   a  located at the cell border between the femto cell  1  and the adjacent cell  2  is selected, as an HO candidate, from the mobile stations  20   a  and  20   b , which are connected mobile stations. The femto base station  50 - 1  then performs control for reducing the transmission power so that, of transmission signals transmitted from the femto cell  1 , signals that arrive at the mobile station  20   a  selected as the HO candidate decrease. 
         [0148]    Subsequently, the femto base station  50 - 1  transmits, to the mobile station  20   a  selected as the HO candidate, an command signal D2 for issuing an command for executing an HO process, to thereby cause the mobile station  20   a  to perform the HO process for switching the connection destination thereof from the femto cell  1  to the adjacent cell  2  (see  FIG. 19B ). 
         [0149]    After issuing the command for executing the HO process to the mobile station  20   a , the femto base station  50 - 1  connects the mobile station  20   c , which is an unconnected mobile station, to the femto cell  1  so that the mobile station  20   c  becomes a connected mobile station (see  FIG. 19C ). 
         [0150]    Next, a procedure of HO control processing performed by the femto base station  50 - 1  according to the present embodiment will be described with reference to  FIG. 20 .  FIG. 20  is a flowchart illustrating the procedure of the HO control processing performed by the femto base station  50 - 1  according to the fifth embodiment. 
         [0151]    As illustrated in  FIG. 20 , if a connection request from an unconnected mobile station is not received (NO in step S 181 ), the call connection unit  56  in the femto base station  50 - 1  returns the process to step S 181 . If a connection request from an unconnected mobile station is received (YES in step S 181 ), the call connection unit  56  outputs a notification indicating that the connection request is received to the HO-candidate selecting unit  53 , without connecting the unconnected mobile station to the femto cell  1  so that the unconnected mobile station becomes a connected mobile station. 
         [0152]    The HO-candidate selecting unit  53  receives the monitoring result input from the number-of-connected-mobile-stations monitoring unit  12 . The HO-candidate selecting unit  53  receives, from the call connection unit  56 , a notification indicating that the connection request was received. Upon receiving, from the call connection unit  56 , the notification indicating that the connection request was received, the HO-candidate selecting unit  53  determines whether or not the number of connected mobile stations, N UE , exceeds the threshold N max  indicating the number of mobile stations that are connectable to the femto cell  1 , based on the monitoring result (step S 182 ). If the number of connected mobile stations, N UE , does not exceed the threshold N max  (NO in step S 182 ), the HO-candidate selecting unit  53  advances the process to step S 184 . 
         [0153]    On the other hand, if the number of connected mobile stations, N UE , exceeds the threshold N max  (YES in step S 182 ), the HO-candidate selecting unit  53  advances the process to the HO-candidate selection processing, the transmission-power control processing, and the HO command processing (step S 183 ). The HO-candidate selection processing, the transmission-power control processing, and the HO command processing illustrated in step S 183  respectively correspond to the HO-candidate selection processing, the transmission-power control processing, and the HO command processing in step S 105  illustrated in  FIG. 6 . 
         [0154]    If step S 183  is finished or if the number of connected mobile stations, N UE , does not exceed the threshold N max  (NO in step S 182 ), the call connection unit  56  connects the unconnected mobile station to the femto cell  1  so that the unconnected mobile station becomes a connected mobile station (step S 184 ). 
         [0155]    As described above, the femto base station  50 - 1  according to the present embodiment determines whether or not the number of connected mobile stations exceeds the threshold N max , before an unconnected mobile station that has transmitted a connection request is connected to the femto cell  1 . When the number of connected mobile stations exceeds the threshold N max , the femto base station  50 - 1  causes the mobile station selected as the HO candidate to execute an HO process. According to the present embodiment, after the connection destination of an HO candidate selected from connected mobile stations is switched from the femto cell  1  to the adjacent cell  2 , an unconnected mobile station can be connected to the femto cell  1 . 
         [0156]    (Hardware Configuration) 
         [0157]    The femto base station in each embodiment described above may be realized by, for example, a hardware configuration as illustrated in  FIG. 21 .  FIG. 21  is a diagram illustrating an example of the hardware configuration of the femto base station. 
         [0158]    As illustrated in  FIG. 21 , the femto base station has a processor  101 , a memory  102 , and a radio frequency (RF) circuit  103  as hardware elements. The RF circuit  103  has an antenna. The memory  102  is implemented by, for example, a random access memory (RAM), a read only memory (ROM), or a flash memory. The wireless communication unit  11  is implemented by, for example, an analog circuit, such as the RF circuit  103 . The number-of-connected-mobile-stations monitoring unit  12 , the HO-candidate selecting unit  13 , the transmission-power control unit  14 , the HO command unit  15 , and the call connection unit  16  are realized by, for example, an integrated circuit, such as the processor  101 . 
         [0159]    The various types of processing described above in each embodiment can be realized by executing a prepared program with a computer. In this case, programs corresponding to the processing executed by the number-of-connected-mobile-stations monitoring unit  12 , the HO-candidate selecting unit  13 , the transmission-power control unit  14 , the HO command unit  15 , and the call connection unit  16  are recorded to the memory  102 , and the programs are read out to the processor  101  and function as processes. 
         [0160]    By the way, in this application, for example, “connected to” is able to be replaced with “coupled to”. Moreover, for example, when an element is referred to as being “connected to” or “coupled to” another element, it can be not only directly but also indirectly connected or coupled to the other element (namely, intervening elements may be present). So do “connecting to”, “coupling to”, “connection to”, “coupling to” and so on. 
         [0161]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.