Abstract:
When a femto cell base station detects an intense uplink interference, the femto cell base station autonomously extends the femto cell so as to raise a probability that a mobile terminal around the original femto cell may be connected to the femto cell base station. If the mobile terminal connects to the femto cell base station, the uplink interference is reduced and total throughput is improved. Additionally, if a resultant effect of interference reduction is low, the femto cell base station returns the mobile terminal, which is connected to the femto cell base station due to the extension of the femto cell, to the original connection destination. If an evaluative criterion meets a predetermined condition, the femto cell base station restores the extended femto cell to the original size. These actions prevent degradation of performance caused by concentration of the connections of mobile terminals to the femto cell base station.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
       [0001]    Japan Priority Application 2010-259435, filed Nov. 19, 2010 including the specification, drawings, claims and abstract, is incorporated herein by reference in its entirety. This application is a Continuation of U.S. application Ser. No. 13/283,128, filed Oct. 27, 2011, incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a wireless communication system and a method for wireless communication, or more particularly, to a cellular wireless communication system including small-scale base stations (for example, femto cell base stations) and a method for a cellular wireless communication. 
         [0004]    2. Description of the Related Art 
         [0005]    In cellular wireless communication systems for cellular portable phones or the like, a macro cell method in which one base station covers a wide area has been employed in the past. The macro cell method features its adaptability to high-speed movement due to a low handover frequency and its capability to expand an area using a small number of base stations. However, the macro cell method finds it hard to cope with a dead zone created indoors or locally. 
         [0006]    In contrast, an approach to elimination of the dead zone by installing a base station, which provides a cell smaller than a macro cell, in a place where radio waves from a macro cell base station are hard to reach has been conceived. 
         [0007]    In particular, a femto cell method in which a small-scale base station capable of covering, for example, one house is installed in each household is attracting attention these days. In a so-called third-generation cellular wireless communication system employing the Wideband Code Division Multiple Access (W-CDMA) or Evolution Data Optimized (EV-DO), provision of commercial services according to the femto cell method is beginning. 
         [0008]    In addition, for the Long Term Evolution (LTE) that is the standard succeeding W-CDMA, specifications for the femto cell method are formulated. 
         [0009]    In the femto cell method, a small-scale base station (femto cell base station) that covers a radio-wave propagation range nearly identical to the range covered by a wireless LAN is installed in each household or building, and the femto cell base station is connected onto a cellular communication network over the Internet. For the purpose of eliminating the dead zone, the femto cell base station may be used in such a manner that a femto cell locally overlaps a macro cell. A frequency band employed by the femto cell base station is identical to or overlaps a frequency band employed by a macro cell base station. 
       SUMMARY OF THE INVENTION 
       [0010]    Assuming that cells provided by two base stations which share the same frequencies overlap, an uplink signal to be transmitted from a mobile terminal connected to one of the base stations becomes an interference signal to an uplink signal to be received by the other base station. 
         [0011]    Now, assume that a mobile terminal connected to a macro cell base station is located in the vicinity of a femto cell that overlaps a macro cell concerned. In a cellular wireless communication system, as a mobile terminal recedes from a base station that is a connection destination, the mobile terminal transmits an uplink signal with a larger power. This is intended to ensure receiving quality required by the base station. Therefore, the mobile terminal transmits the uplink signal to the macro cell base station, which is separated from the mobile terminal by a long distance, with the large power. Further, since a distance from the mobile terminal to a femto cell base station is short, the uplink signal transmitted from the mobile terminal reaches the femto cell base station without a large decay. Therefore, the uplink signal sent from the mobile terminal becomes a very intense interference signal to the femto cell base station. As a result, presumably, the receiving quality of the uplink signal at the femto cell base station may be markedly degraded, and an uplink throughput in the femto cell may be decreased. 
         [0012]    As a solution for the foregoing problem, Japanese Unexamined Patent Application Publication No. 2010-171885 has disclosed a method in which a macro cell base station posts information on radio resources, which the macro cell base station has allocated to subordinate mobile terminals, to a femto cell base station, and the femto cell base station in turn allocates radio resources, which the macro cell base station has not allocated, to mobile terminals. Accordingly, a situation in which an uplink signal sent from a mobile terminal connected to the macro cell base station becomes an interference signal to the femto cell base station can be avoided. 
         [0013]    However, as described in Japanese Unexamined Patent Application Publication No. 2010-171885, according to the method of exchanging information between base stations, an information exchange frequency increases along with an increase in the number of femto cell base stations located in a macro cell. Eventually, a load on a network increases. 
         [0014]    In order to solve the problem of an uplink interference, which is applied to a femto cell base station by a mobile terminal connected to a macro cell base station, without exchanging information among base stations, connecting the mobile terminal, which acts as a source of interference signal, to the femto cell base station is thought to be effective. When the mobile terminal is connected to the femto cell base station, the uplink signal sent from the mobile terminal does not any longer become an interference signal to the femto cell base station. In contrast, the uplink signal from the mobile terminal becomes an interference signal to the macro cell base station to which the mobile terminal is previously connected. However, since a distance from the mobile terminal to the femto cell base station is short, the transmission power for the uplink signal gets diminished and the interference with the macro cell base station gets weakened. As a result, as the whole of a system including the macro cell and femto cell, the throughput can be expected to improve. 
         [0015]    However, adoption of the foregoing technique is confronted with two problems that should be solved. 
         [0016]    The first problem is how a femto cell base station should control handover, which is a process of connecting a mobile terminal to another base station. Handover is initiated by a mobile terminal itself that is an object or by a base station that is a connection destination from which handover is made. Therefore, the femto cell base station that is a handover destination cannot directly instruct the mobile terminal to make handover to the femto cell base station itself. 
         [0017]    The second problem is how to avoid concentration of connections to a femto cell base station. When numerous mobile terminals are connected to the femto cell base station owing to the foregoing technique, a load or a consumption of resources on the femto cell base station increases. Eventually, there arises a possibility that a throughput in a femto cell may decrease. 
         [0018]    Accordingly, an object of the present invention is to, when an uplink interference power at a base station such as a femto cell base station is high, raise a probability that a mobile terminal, which is suspected of being a source of interference signal and located on the perimeter of a cell such as a femto cell, may make handover to the femto cell base station, to thus autonomously reduce an uplink interference to be applied to the base station such as the femto cell base station, and to eventually improve a throughput of an entire system. 
         [0019]    Another object of the present invention is to suppress an increase in a load on a base station such as a femto cell base station deriving from connection of a mobile terminal to the base station such as the femto cell base station. 
         [0020]    Still another object of the present invention is to prevent a mobile terminal, which is not a source of serious interference signal to a base station such as a femto cell base station, from repeating handover from an original connection destination to a cell such as a femto cell or vice versa, by autonomously restoring the radius of the cell such as the femto cell. 
         [0021]    In order to solve the above problems, when a femto cell base station relating to the present invention detects an intense uplink interference, the femto cell base station autonomously extends the radius of a femto cell formed by the femto cell base station, and thus raises a probability that a mobile terminal located on the perimeter of the original femto cell may be connected to the femto cell base station itself. 
         [0022]    Further, a femto cell base station relating to the present invention evaluates an effect of interference reduction deriving from connection of a mobile terminal, which is located on the perimeter of a femto cell, to the femto cell base station itself as a result of the foregoing action. If the effect is low, the mobile terminal connected to the femto cell base station is returned to a base station that is an original connection destination. 
         [0023]    Further, for example, if an evaluative criterion such as a magnitude of interference reduction or the number of connected mobile terminals meets a predetermined condition as a result of extension of the radius of a femto cell, a femto cell base station relating to the present invention restores the extended radius of the femto cell to the original size. 
         [0024]    According to a first solution of the present invention, there is provided a wireless communication system including a first base station that forms a first cell and a second base station that forms a second cell which contains the first cell of the first base station or overlaps the first cell. 
         [0025]    Herein, the first base station estimates a first radio-wave interference in an uplink direction, which the first base station receives from a group of mobile terminals which wirelessly communicates with another base station or other plural base stations. When the first radio-wave interference is equal to or larger than a predetermined threshold, the first base station increases a transmission power so as to thus extend the radius of the first cell. Thus, out of the group of mobile terminals, one mobile terminal or plural mobile terminals that exist in the expanded first cell and are connected to the second base station are disconnected from the second base station and connected to the first base station. 
         [0026]    According to a second solution of the present invention, there is provided a method for wireless communication to be implemented in a wireless communication system including a first base station that forms a first cell and a second base station that forms a second cell which contains the first cell of the first base station or overlaps the first cell. 
         [0027]    Herein, the first base station estimates a first radio-wave interference in an uplink direction which the first base station receives from a group of mobile terminals which wirelessly communicates with another base station or other plural base stations. When the first radio-wave interference is equal to or larger than a predetermined threshold, the first base station increases a transmission power so as to extend the radius of the first cell. Thus, out of the group of mobile terminals, one mobile terminal or plural mobile terminals that exist in the expanded first cell and are connected to the second base station are disconnected from the second base station and connected to the first base station. 
         [0028]    According to the present invention, when an uplink interference power at a base station such as a femto cell base station is high, a probability that a mobile terminal suspected of being a source of interference signal and located on the perimeter of a cell such as a femto cell may make handover to the femto cell base station gets higher. Therefore, an uplink interference to be applied to the base station such as the femto cell base station is autonomously reduced, and a throughput of an entire system is improved. 
         [0029]    In addition, among mobile terminals that have made handover to a base station such as a femto cell base station and are located on the perimeter of a cell such as a femto cell, a mobile terminal that is not a source of serious interference signal to the base station such as the femto cell base station is returned to a base station that is an original connection destination. Further, by autonomously restoring the radius of the cell such as the femto cell, a situation in which numerous mobile terminals are connected to the base station such as the femto cell base station can be avoided. Accordingly, an increase in a load on the base station such as the femto cell base station deriving from connection of a mobile terminal to the base station such as the femto cell base station can be suppressed. 
         [0030]    Further, by autonomously restoring the radius of the cell such as the femto cell, a mobile terminal that is not a source of serious interference signal to a base station such as a femto cell base station can be prevented from repeating handover from an original connection destination to the cell such as the femto cell or vice versa. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    The present invention will become fully understood from the detailed description given hereinafter and the accompanying drawings, wherein: 
           [0032]      FIG. 1  is a diagram showing the configuration of a cellular wireless communication system in accordance with a first embodiment of the present invention including femto cell base stations; 
           [0033]      FIG. 2  is a diagram showing the internal configuration of a femto cell base station employed in the first embodiment of the present invention; 
           [0034]      FIG. 3  is a diagram showing the internal configuration of a macro cell base station; 
           [0035]      FIG. 4  is a diagram showing the internal configuration of a mobile terminal; 
           [0036]      FIG. 5A  and  FIG. 5B  are diagrams showing examples of arrangement of mobile terminals connected to a macro cell base station and a femto cell base station; 
           [0037]      FIG. 6A  is a sequence diagram showing a handover procedure to be activated with an increase in a downlink transmission power by the femto cell base station employed in the first embodiment of the present invention; 
           [0038]      FIG. 6B  is a sequence diagram showing a handover procedure to be activated with an increase in a downlink transmission power by a femto cell base station employed in a second or third embodiment of the present invention; 
           [0039]      FIG. 7  is a sequence diagram showing a detailed procedure of the handover from a macro cell base station to the femto cell base station included in the procedure mentioned in  FIG. 6A  or  FIG. 6B ; 
           [0040]      FIG. 8  is a flowchart of processing of deciding whether an effect of interference reduction is exerted in the femto cell base station employed in the first embodiment of the present invention; 
           [0041]      FIG. 9  is a flowchart of processing of deciding whether an effect of interference reduction is exerted in the femto cell base station employed in the second embodiment of the present invention; 
           [0042]      FIG. 10  is a flowchart of processing of deciding whether an effect of interference reduction is exerted in the femto cell base station employed in the third embodiment of the present invention; 
           [0043]      FIG. 11  is a sequence diagram showing a detailed procedure of handover from the femto cell base station to the macro cell base station included in the procedure mentioned in  FIG. 6 ; 
           [0044]      FIG. 12  is a flowchart of processing of deciding whether it is necessary to perform an action of restoring a downlink transmission power of the femto cell base station, which is employed in the first embodiment of the present invention, to an original value; 
           [0045]      FIG. 13  is a flowchart of processing of deciding whether it is necessary to perform an action of restoring a downlink transmission power of the femto cell base station, which is employed in the second embodiment of the present invention, to an original value; 
           [0046]      FIG. 14  is a flowchart of processing of deciding whether it is necessary to perform an action of restoring a downlink transmission power of the femto cell base station, which is employed in the third embodiment of the present invention, to an original value; and 
           [0047]      FIG. 15  is a flowchart of processing of starting an interference control action in the femto cell base station employed in the first embodiment of the present invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     1. First Embodiment 
       [0048]    Referring to the drawings, a first embodiment of the present invention will be described below. 
         [0049]      FIG. 1  is a diagram showing an example of the configuration of a cellular wireless communication system in accordance with the first embodiment of the present invention including femto cell base stations. 
         [0050]    In the present system, a macro cell base station  101  forms a macro cell  102 , and one or more mobile terminals  103  exist in the macro cell  102 . The macro cell base station  101  and mobile terminals  103  are connected to one another over wireless links. 
         [0051]    A femto cell base station  111  installed in the macro cell  102  forms a femto cell  112 , and one or more mobile terminals  113  exist in the femto cell  112 . The femto cell base station  111  and mobile terminals  113  are connected to one another over wireless links. 
         [0052]    The macro cell base station  101  and femto cell base station  111  are connected to a core network  121  over wired links. 
         [0053]      FIG. 2  shows an example of the internal configuration of the femto cell base station  111 . The femto cell base station  111  includes a memory  201 , a CPU  202 , a radio interface  203 , a logic circuit  204 , and a network interface  205 . 
         [0054]    The memory  201  preserves a measurement result of received RS (Reference Signal) power  210 . 
         [0055]    The CPU  202  includes a transmitted data processing unit  211 , a received data processing unit  212 , a handover processing unit  213 , a handover decision unit  214 , a transmission power adjustment unit  215 , a transmission power reversion decision unit  216 , an interference mitigation judgment unit  217 , an interference control decision unit  218 , and an interference measurement unit  219 . 
         [0056]    The transmitted data processing unit  211  converts data, which should be transmitted to a mobile terminal, into a radio signal to be transmitted to the mobile terminal over a wireless link. 
         [0057]    The received data processing unit  212  extracts data, which a base station should receive, from a radio signal that comes from the mobile terminal over the wireless link. 
         [0058]    The handover processing unit  213  performs transmitting/receiving processing on a message to be transferred between base stations at the time of handover, or establishes or releases a wireless link to a mobile terminal. 
         [0059]    The handover decision unit  214  uses a report on a result of measurement of a downlink RS power, which is received from a mobile terminal, to decide a cell that is an appropriate connection destination of the mobile terminal. 
         [0060]    The transmission power adjustment unit  215  increases or decreases a downlink transmission power of the femto cell base station  111 . 
         [0061]    When the femto cell base station  111  tentatively increases a downlink transmission power, the transmission power reversion decision unit  216  decides whether the downlink transmission power should be decreased to a value which the downlink transmission power takes on before being increased. 
         [0062]    The interference mitigation judgment unit  217  decides whether an uplink interference power received by the femto cell base station  111  has decreased. 
         [0063]    The interference control decision unit  218  decides whether the femto cell base station  111  should perform an interference control action. 
         [0064]    The interference measurement unit  219  measures the uplink interference power to be received by the femto cell base station  111 . 
         [0065]    The radio interface  203  is an interface through which the femto cell base station  111  communicates with a mobile terminal over a wireless link. 
         [0066]    The logic circuit  204  appends an error-correcting code to data that is transmitted to a mobile terminal, or decodes the data having the error-correcting code appended thereto received from a mobile terminal. 
         [0067]    The network interface  205  is an interface through which the femto cell base station  111  communicates with the core network system or another base station over a wired link. 
         [0068]      FIG. 3  shows an example of the internal configuration of the macro cell base station  101 . The macro cell base station  101  includes a memory  301 , a CPU  302 , a radio interface  303 , a logic circuit  304 , and a network interface  305 . 
         [0069]    The memory  301  preserves a measurement result of received RS power  310 . 
         [0070]    The CPU  302  includes a transmitted data processing unit  311 , a received data processing unit  312 , a handover processing unit  313 , and a handover decision unit  314 . Actions to be performed by these units are identical to those performed by the transmitted data processing unit  211 , received data processing unit  212 , handover processing unit  213 , and handover decision unit  214  respectively included in the femto cell base station  111 . 
         [0071]    The radio interface  303  is an interface through which the macro cell base station  101  communicates with a mobile terminal over a wireless link. 
         [0072]    The logic circuit  304  appends an error-correcting code to data that is transmitted to a mobile terminal, or decodes the data having the error-correcting code appended thereto received from a mobile terminal. 
         [0073]    The network interface  305  is an interface through which the macro cell base station  101  communicates with the core network system or another base station over a wired link. 
         [0074]      FIG. 4  shows an example of the internal configuration of the mobile terminal  103  or  113 . The mobile terminal  103  or  113  includes a memory  401 , a CPU  402 , a radio interface  403 , and a logic circuit  404 . 
         [0075]    The memory  401  preserves a measurement result of received RS power  410 . 
         [0076]    The CPU  402  includes a transmitted data processing unit  411 , a received data processing unit  412 , an RS measurement report generating unit  413 , and an RS power measurement unit  414 . 
         [0077]    The transmitted data processing unit  411  converts data, which should be transmitted to a base station, into a radio signal to be transmitted to the base station over a wireless link. 
         [0078]    The received data processing unit  412  extracts data, which a mobile terminal should receive, from the radio signal that comes from the base station over a wireless link. 
         [0079]    The RS measurement report generating unit  413  produces a message with which a measured RS power is reported to a base station that is a connection destination. 
         [0080]    The RS power measurement unit  414  measures an RS power in a connection-destination cell or neighbor cells. 
         [0081]    The radio interface  403  is an interface through which the mobile terminal  103  or  113  communicates with the macro cell base station  101  or femto cell base station  111  over a wireless link. 
         [0082]    The logic circuit  404  appends an error-correcting code to data that is transmitted to or received from a base station, or decodes the data having the error-correcting code appended thereto. 
         [0083]    Next, an uplink interference control action of the femto cell base station  111  will be described below. 
         [0084]    As shown in  FIG. 5A , a mobile terminal  104  and a mobile terminal  105  shall exist in the macro cell  102 , and the mobile terminal  105  shall be located near the border of the femto cell  112 . 
         [0085]      FIG. 6A  shows a sequence for uplink interference control by the femto cell base station  111 . Incidentally, actions at respective steps will be detailed later. 
         [0086]    The mobile terminals  104  and  105  each establish a connection to the macro cell base station  101  (step  1001 ). 
         [0087]    In the femto cell base station  111 , the interference measurement unit  219  measures an uplink interference power at certain timing (step  1002 ). As a result, if the interference power exceeds a predetermined threshold, the femto cell base station  111  activates an interference control action (step  1003 ), and handover from the macro cell  102  to the femto cell  112  is executed (step  1004 ). 
         [0088]    Subsequently to step  1004 , the interference measurement unit  219  in the femto cell base station  111  measures an uplink interference power again (step  1004 ). Thereafter, the femto cell base station  111  uses a result of step  1005  to evaluate an effect of interference reduction, and decides whether the mobile terminal should be indwelled in the femto cell  112  (step  1010 ). 
         [0089]    If the femto cell base station  111  decides as a result of step  1010  that the mobile terminal is connected to the macro cell, handover from the femto cell  112  to the macro cell  102  is executed (step  1011 ). 
         [0090]    Thereafter, if the femto cell base station  111  decides at step  1004  that it is unnecessary to transmit a downlink signal with a tentatively increased power, the downlink transmission power is decreased to a transmission power attained prior to step  1004  (step  1012 ). 
         [0091]      FIG. 15  is a flowchart of step  1003 . 
         [0092]    The interference control decision unit  218  decides whether the value of an interference power A measured at step  1002  is equal to or larger than a predetermined threshold “g” (step  2601 ). 
         [0093]    The value of an interference power can be measured, for example, as mentioned below. Specifically, each base station has a period during which transmissions from all mobile terminals connected to the own base station is stopped. A power measured during the period is a power of radio waves received from one or plural mobile terminals that are not connected to the own base station. Therefore, the base station can measure the power as an interference power. 
         [0094]    If it is found as a result of step  2601  that the interference power A is equal to or larger than the threshold “g”, the interference control decision unit  218  decides starting of interference control (step  2602 ). 
         [0095]    In contrast, if the interference power A falls below the threshold “g”, the interference control decision unit  218  terminates its action without doing anything. 
         [0096]      FIG. 7  shows a detailed sequence of step  1004 . 
         [0097]    The femto cell base station  111  activates an interference control action at step  1003 , and increases a downlink transmission power (step  1101 ). Accordingly, as shown in  FIG. 5B , the radius of the femto cell  112  is extended. 
         [0098]    Thereafter, the femto cell base station  111  and macro cell base station  101  each transmit a downlink RS (steps  1102  and  1103 ). Incidentally, the downlink RS is regularly transmitted from each of the base stations. A mobile terminal creates a report on a result of measurement of the power of the downlink RS received from each of the base stations (step  1104 ), and transmits the report to the macro cell base station  101  (step  1105 ). 
         [0099]    The macro cell base station  101  decides an appropriate connection destination of the mobile terminal on the basis of the contents of the report on the result of measurement of the downlink RS power received at step  1105  (step  1106 ). At this time, since the transmission power for a downlink RS transmitted from the femto cell base station  111  has been tentatively increased due to step  1101 , and the radius of the femto cell  112  has been extended, a probability that the femto cell  112  may be selected at step  1106  gets higher. 
         [0100]    In the example shown in  FIG. 5B , both the mobile terminals  104  and  105  exist in the femto cell  112  whose radius has been extended. Therefore, the mobile terminals each report a result of measurement, which signifies that a receiving power of a downlink RS in the femto cell  112  is high, to the macro cell base station. The macro cell base station  101  decides based on the report of the result of measurement, which is received at step  1106 , that the mobile terminals  104  and  105  should be handed over to the femto cell  112 . 
         [0101]    If handover to the femto cell  112  is activated at step  1106 , the macro cell base station  101  transmits a handover request to the femto cell base station  111  (step  1107 ). If the femto cell base station  111  can accept handover, the femto cell base station  111  transmits a handover acknowledgement to the macro cell base station  101  (step  1108 ). 
         [0102]    When the macro cell base station  101  receives the handover acceptance response from the femto cell base station  111 , the macro cell base station  101  instructs the mobile terminal to make handover to the femto cell  112  (step  1109 ). 
         [0103]    When the mobile terminal is instructed to make handover to the femto cell  112 , the mobile terminal establishes a connection to the femto cell base station  111  (step  1110 ). 
         [0104]      FIG. 8  is a flowchart describing step  1010 . 
         [0105]    A mobile terminal that is handed over to the femto cell base station  111  because the radius of the femto cell is extended is no longer a source of interference signal to the femto cell base station  111 . Therefore, an interference power is expected to decrease accordingly. In this processing flow, at step  2001 , the degree of the decrease is recognized, and only a mobile terminal that offers a large degree of decrease remains connected to the femto cell base station  111 , but a mobile terminal that offers a small degree of decrease is handed over to the macro cell base station  101 . 
         [0106]    The interference mitigation judgment unit  217  decides whether an interference power B measured by the interference measurement unit  219  at step  1004  has decreased by a predetermined threshold “a” or more with respect to the interference power A measured by the interference measurement unit  219  at step  1002  (step  2001 ), and passes the result of the decision to the handover decision unit  214 . 
         [0107]    If it is found as a result of step  2001  that the interference power B has decreased by the threshold “a” or more with respect to the interference power A, the handover decision unit  214  decides to keep the target mobile terminal connected to the femto cell base station  111  (step  2002 ). 
         [0108]    In contrast, if the interference power B has not decreased by the threshold “a” or more with respect to the interference power A, the handover decision unit  214  decides to the target mobile terminal handover to the macro cell base station  101  (step  2003 ). 
         [0109]    The processing mentioned in  FIG. 8  is executed successively to every completion of step  1004 . 
         [0110]    In the example shown in  FIG. 5B , the mobile terminal  105  is located quite close to the femto cell  112  whose radius has not been extended. Therefore, before the mobile terminal  105  is handed over to the femto cell  112 , an uplink signal sent from the mobile terminal  105  becomes an intense interference signal to the femto cell base station. Therefore, as for the mobile terminal  105 , the interference power B largely decreases with respect to the interference power A. As a result of step  2001 , the mobile terminal  105  is left connected to the femto cell base station  111 . 
         [0111]    In contrast, the mobile terminal  104  is not located very close to the femto cell  112  whose radius has not been extended. Therefore, before the mobile terminal  104  is handed over to the femto cell  112 , an uplink signal from the mobile terminal  104  is not an intense interference signal to the femto cell base station. Therefore, as for the mobile terminal  104 , the interference power B does not decrease very much with respect to the interference power A. As a result of step  2001 , the mobile terminal  104  is handed over to the macro cell base station  101 . 
         [0112]      FIG. 11  shows a detailed sequence of step  1011 . 
         [0113]    When handover to the macro cell  102  is activated at step  1301 , the femto cell base station  111  transmits a handover request to the macro cell base station  101  (step  1302 ). 
         [0114]    If the macro cell base station  101  can accept handover, the macro cell base station  101  transmits a handover acceptance response to the femto cell base station  111  (step  1303 ). 
         [0115]    When the femto cell base station  111  receives the handover acceptance response from the macro cell base station  101 , the femto cell base station  111  instructs a mobile terminal to make handover to the macro cell  102  (step  1304 ). 
         [0116]    When the mobile terminal is instructed to make handover to the macro cell  102 , the mobile terminal establishes a connection to the macro cell base station  101  (step  1305 ). 
         [0117]      FIG. 12  shows a processing flow of step  1012 . 
         [0118]    The interference mitigation judgment unit  217  decides whether the value of the interference power B measured by the interference measurement unit  219  at step  1004  falls below a predetermined threshold “d” (step  2301 ), and passes the result of the decision to the transmission power reversion decision unit  216 . 
         [0119]    If it is found as a result of step  2301  that the interference power B falls below the threshold “d”, the transmission power reversion decision unit  216  decides to decrease a downlink transmission power to the value attained prior to step  1004  (step  2302 ), and instructs the transmission power adjustment unit  215  to decrease the downlink transmission power. 
         [0120]    In contrast, if the interference power B is equal to or larger than the threshold “d”, the transmission power reversion decision unit  216  decides to keep the downlink transmission power at the value increased at step  1004  (step  2303 ). 
         [0121]    The femto cell base station  111  may execute the series of pieces of processing mentioned in  FIG. 12  every after executing step  1004  or may comprehensively execute the series of pieces of processing after executing step  1004  for plural mobile terminals. 
       2. Second Embodiment 
       [0122]    Now, a second embodiment of the present invention will be described below. 
         [0123]    The present embodiment is different from the first embodiment in actions to be performed at step  1006  and subsequent steps. In particular, actions for handover necessity decision (step  1010 ′) and for power decrease (step  1012 ′) are different from those of steps  1010  and  1012  in the first embodiment. In addition, since the action for handover necessity decision (step  1010 ′) is different, an uplink interference control action sequence is different. 
         [0124]      FIG. 6B  shows an uplink interference control sequence to be followed by the femto cell base station  111 . 
         [0125]    Actions of steps  1001  to  1004  are identical to those in the first embodiment mentioned in  FIG. 6A . 
         [0126]    The femto cell base station  111  and macro cell base station  101  each regularly transmit a downlink RS. Therefore, an opportunity to transmit the downlink RS from each base station succeeds to step  1004  (steps  1006  and  1007 ). A mobile terminal creates a report on a result of measurement of the power of the downlink RS received from each of the base stations (step  1008 ), and transmits the report to the femto cell base station  111  (step  1009 ). 
         [0127]    On receipt of the report on the result of measurement of the power of the downlink RS from the mobile terminal at step  1009 , the femto cell base station  111  decides at step  1010 ′ whether the mobile terminal should be indwelled in the femto cell  112 . 
         [0128]    Actions to be performed at step  1011  and subsequent steps are identical to those in the first embodiment mentioned in  FIG. 6A . If it is decided as a result of step  1010 ′ that the femto cell base station  111  connects the mobile terminal to a macro cell, handover from the femto cell  112  to the macro cell  102  is executed (step  1011 ). 
         [0129]    Thereafter, if the femto cell base station  111  decides that it is unnecessary to transmit a downlink signal with a power tentatively increased at step  1004 , the femto cell base station  111  decreases the downlink transmission power to the value attained prior to step  1004  (step  1012 ′). 
         [0130]      FIG. 9  is a flowchart of step  1010 ′ in the second embodiment of the present invention. 
         [0131]    The interference mitigation judgment unit  217  decides whether a downlink RS receiving power which is notified by a mobile terminal at step  1009  and is relevant to the femto cell base station  111  is equal to or larger than a predetermined threshold “b” (step  2101 ), and passes the result of the decision to the handover decision unit  214 . 
         [0132]    If it is found as a result of step  2101  that the downlink RS receiving power relevant to the femto cell base station  111  is larger than the threshold “b”, the handover decision unit  214  decides to keep the mobile terminal connected to the femto cell base station  111  (step  2102 ). 
         [0133]    In contrast, if the downlink RS receiving power relevant to the femto cell base station  111  is smaller than the threshold “b”, the handover decision unit  214  decides to the target mobile terminal handover to the macro cell base station  101  (step  2103 ). 
         [0134]    Incidentally, the femto cell base station  111  may execute the series of pieces of processing mentioned in  FIG. 9  every after executing step  104  or may comprehensively execute the series of pieces of processing after executing step  104  for plural mobile terminals. 
         [0135]      FIG. 13  is a flowchart of step  1012 ′ in the second embodiment of the present invention. 
         [0136]    The transmission power reversion decision unit  216  decides whether the number of mobile terminals handed over to the femto cell  112  at step  1004  is larger than a predetermined threshold “e” (step  2401 ). 
         [0137]    If it is found as a result of step  2401  that the number of mobile terminals handed over to the femto cell  112  is larger than the threshold “e”, the transmission power reversion decision unit  216  decides to decrease the downlink transmission power so as to decrease it to the value attained prior to step  1004  (step  2402 ), and instructs the transmission power adjustment unit  215  to decrease the downlink transmission power. 
         [0138]    In contrast, if the number of mobile terminals handed over to the femto cell  112  is equal to or smaller than the threshold “e”, the transmission power reversion decision unit  216  decides to keep the downlink transmission power at the value increased at step  1004  (step  2403 ). 
         [0139]    Incidentally, the femto cell base station  111  may execute the series of pieces of processing mentioned in  FIG. 13  every after executing step  1004  or may comprehensively execute the series of pieces of processing after executing step  104  for plural mobile terminals. 
       3. Third Embodiment 
       [0140]    A third embodiment of the present invention will be described below. In the present embodiment, a sequence for uplink interference control to be followed by the femto cell base station  111  is, as mentioned in  FIG. 6B , identical to that in the second embodiment, but actions at steps  1010 ′ and  1012 ′ are different from those in the second embodiment. 
         [0141]      FIG. 10  is a flowchart of step  1010 ′ in the third embodiment of the present invention. 
         [0142]    The interference mitigation judgment unit  217  decides whether a difference between a downlink RS receiving power that is relevant to the femto cell base station  111  and notified by a mobile terminal at step  1009  in  FIG. 6B , and a downlink RS receiving power relevant to the macro cell base station  101  is equal to or larger than a predetermined threshold “c” (step  2201 ), and passes the result of the decision to the handover decision unit  214 . 
         [0143]    If it is found as a result of step  2201  that the difference between the downlink RS receiving power relevant to the femto cell base station  111  and the downlink RS receiving power relevant to the macro cell base station  101  is larger than the threshold “c”, the handover decision unit  214  decides to keep the target mobile terminal connected to the femto cell base station  111  (step  2202 ). 
         [0144]    In contrast, if the difference between the downlink RS receiving power relevant to the femto cell base station  111  and the downlink RS receiving power relevant to the macro cell base station  101  is smaller than the threshold “c”, the handover decision unit  214  decides to make the target mobile terminal handover to the macro cell base station  101  (step  2203 ). 
         [0145]    Incidentally, the femto cell base station  111  may execute the series of pieces of processing, which is mentioned in  FIG. 10 , every after executing step  1004  or may comprehensively execute the series of pieces of processing after executing step  1004  for plural mobile terminals. 
         [0146]      FIG. 14  is a flowchart of step  1012 ′ in the third embodiment of the present invention. 
         [0147]    The transmission power reversion decision unit  216  decides whether an elapsed time having elapsed since a transmission power of the femto cell base station  111  is increased at step  1101  in  FIG. 6B  is equal to or larger than a predetermined threshold “f” (step  2501 ). 
         [0148]    If it is found as a result of step  2501  that the elapsed time is equal to or larger than the threshold “f”, the transmission power reversion decision unit  216  decides to decrease a downlink transmission power to the value attained prior to step  1004  (step  2502 ), and instructs the transmission power adjustment unit  215  to decrease the downlink transmission power. 
         [0149]    In contrast, if the elapsed time falls below the threshold “f”, the transmission power reversion decision unit  216  decides to keep the downlink transmission power at the value increased at step  1004  (step  2503 ). 
         [0150]    Incidentally, the femto cell base station  111  may execute the series of pieces of processing, which is mentioned in  FIG. 24 , every after executing step  1004 , or may comprehensively execute the series of pieces of processing after executing step  1004  for plural mobile terminals. 
       4. Modification 
       [0151]    In relation to the three aforesaid embodiments, three methods mentioned in  FIG. 8 ,  FIG. 9 , and  FIG. 10  are introduced as handover necessity decision (step  1010  or step  1010 ′), and three methods mentioned in  FIG. 12 ,  FIG. 13 , and  FIG. 14  are introduced as power decrease (step  1012  or step  1012 ′). An example of a combination of a handover necessity decision method (step  1010  or step  1010 ′) and a power decrease method (step  1012  or step  1012 ′) has been cited in relation to each of the embodiments. However, the present invention is not limited to the combinations but permits selection of appropriate methods. As for power decrease (step  1012  or step  1012 ′), two or more of the methods mentioned in  FIG. 12 ,  FIG. 13 , and  FIG. 14  may be combined. 
         [0152]    Handover especially between a femto cell base station and a macro cell base station has been described so far. However, the present invention is not limited to the handover between the femto cell base station and macro cell base station, but can be applied to handover between femto cell base stations, handover between macro cell base stations, handover between other base stations, or handover between different types of base stations or between base stations of the same type.