Abstract:
The present invention provides a base station device capable of improving the throughput of a whole network by minimizing interference between respective base stations. In a communication system including a plurality of base stations, which transmit a signal by using any of a plurality of different ABS patterns which are sub frame ABS patterns represented by the combination of a transmission sub frame for transmitting a signal and a non-transmission sub frame for suspending the transmission of the signal, a determining unit ( 104 ), on the basis of the presence or absence of the interference between an HeNB ( 100 ) and other base stations except for the HeNB ( 100 ), determines the ABS pattern used by the HeNB ( 100 ) from among the plurality of ABS patterns, and a transmitting unit ( 105 ) transmits a signal to a terminal connected to the HeNB ( 100 ); in accordance with the determined ABS pattern.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a base station apparatus and a transmission method. 
       BACKGROUND ART 
       [0002]    In recent years, small cell base station apparatuses each used for forming a cell having a communication area smaller than a conventional cell have been developed for the purpose of eliminating the dead zones of mobile phones or improving the cell average throughput or cell edge throughput. Examples of such small cell base station apparatuses include “Pico eNB” and “Home eNB” (hereinafter, these base station apparatuses are collectively called “HeNB” for the sake of convenience). HeNBs are deployed for covering only restricted small areas such as homes or offices. Accordingly, HeNBs form a small cell as compared with the existing macro base station apparatuses, which form a cell having a large communication area (“Macro base station apparatus (Macro eNB) (hereinafter, referred to as “MeNB”)). For this reason, HeNBs are less likely to involve congestion caused by traffic concentration and can be expected to achieve high throughput. 
         [0003]    However, the users of HeNBs can change the installation locations of HeNBs (i.e., HeNBs installed in homes) to any location, so that it is difficult for telecommunication carriers to manage the operational states of HeNBs. In addition, since HeNBs use the same frequency band as an MeNB (Macro base station), interference between the MeNB and HeNBs is a problem. 
         [0004]      FIG. 1  is a diagram provided for describing interference between an HeNB and an MeNB, In  FIG. 1 , one HeNB is installed in the communication area of the McNB, and a mobile station communicating with the MeNB (Macro User Equipment (hereinafter, abbreviated as “MUE”)) is located in the communication area ((i.e., located within the cell)) of the MeNB. In addition, another mobile station communicating with the HeNB (Home User Equipment (hereinafter, abbreviated as “HUE”)) is located in the communication area ((i.e., located within the cell)) of the HeNB. 
         [0005]    If the distance between the McNB and HeNB is relatively short in  FIG. 1 , the HUE receives not only a downlink signal from the HeNB, which is a desired wave (solid line), but also a downlink signal from the MeNB, which is an interference wave (broken line). In this case, the reception quality of the HUE is degraded, which results in a decrease in throughput. Likewise, when the MUE illustrated in  FIG. 1  moves closer to the communication area of the HeNB, the MUE receives not only a downlink signal (desired wave) from the McNB but also a downlink signal (interference wave) from the HeNB. In this case, the MUE is interfered by the signal from the HeNB, and the reception quality of the MUE is degraded, which results in a decrease in throughput. 
         [0006]    As a solution to this problem, a method called “Almost Blank Subframe” (ABS) disclosed in Non-Patent Literature (hereinafter, abbreviated as “NPL”) 1 has been discussed, for example. In the ABS disclosed in NPL 1, the MeNB periodically stops downlink transmission. In  FIG. 2 , the MeNB sets a non-transmission subframe at every fourth subframe to stop downlink transmission. Thus, the interfered base station (victim, which is the HcNB in  FIG. 2 ) is no longer interfered in a subframe where the interfering base station (aggressor, which is the MeNB in  FIG. 2 ) stops transmission (in non-transmission subframes illustrated in  FIG. 2 ). As a result, the throughput of a UE located in the cell provided by the interfered base station is improved. 
       CITATION LIST 
     Non-Patent Literature 
     NPL 1. 
       [0000]    
       
         R1-105779 “Way Forward on time-domain extension of Rel 8/9 backhaul-based ICIC” (RAN 1) 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    If transmission is stopped by using ABSs as illustrated in NPL 1, the throughput unfavorably decreases in the interfering base station (MeNB) due to non-transmission. In particular, as illustrated in NPL 1 (FIG. 2), if the MeNB having a wide coverage stops transmission, a large number of UEs (MUEs) are affected by the decrease in the throughput due to the non-transmission, which leads to a decrease in the throughput of the whole network. 
         [0009]    It is an object of the present invention to provide a base station apparatus and a transmission method each of which makes it possible to improve the throughput of the whole network while limiting interference between base stations (MeNB and HeN B) to the minimum level. 
       Solution to Problem 
       [0010]    A base station apparatus according to a first aspect of the present invention in a communication system includes plurality of base station apparatuses each being configured to transmit a signal using one of a plurality of different subframe configuration patterns, each of the plurality of different subframe configuration patterns being represented by a combination of a transmission subframe that transmits signal and a non-transmission subframe that stops transmission of a signal, the base station apparatus including: a determining section configured to determine a subframe configuration pattern to be used in the base station apparatus of the determining section from among the plurality of subframe configuration patterns on a basis of the presence or absence of interference between the base station apparatus thereof and another one or more of the base station apparatuses; and a transmitting section configured to transmit a signal to a terminal apparatus connected to the base station apparatus of the transmitting section, in accordance with the determined subframe configuration pattern. 
         [0011]    A transmission method according to a second aspect of the present invention in a communication system includes plurality of base station apparatuses each configured to transmit a signal using one of a plurality of different subframe configuration patterns, each of the plurality of different subframe configuration patterns being represented by a combination of a transmission subframe that transmits a signal and a non-transmission subframe that stops transmission of a signal, the method including: determining a subframe configuration pattern to be used in a specific one of the base station apparatuses from among the plurality of subframe configuration patterns on a basis of the presence or absence of interference between the specific one of the base station apparatuses and another one or more of the base station apparatuses; and transmitting a signal to a terminal apparatus connected to the specific one of the base station apparatuses, in accordance with the determined subframe configuration pattern. 
       Advantageous Effects of Invention 
       [0012]    According to the present invention, is possible to improve the throughput of the whole network while limiting interference between base stations (MeNB and HeNB) to the minimum level. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a diagram illustrating a configuration of a network system including an MeNB and an HeNB; 
           [0014]      FIG. 2  is a diagram provided for describing ABS; 
           [0015]      FIG. 3  is a diagram illustrating a configuration of a network system according to Embodiment 1 of the present invention; 
           [0016]      FIG. 4  is a block diagram illustrating a configuration of a small cell base station according to Embodiment 1 of the present invention; 
           [0017]      FIG. 5  is a diagram illustrating ABS patterns according to Embodiment 1 of the present invention; 
           [0018]      FIG. 6  is a diagram illustrating an ABS white list according to Embodiment 1 of the present invention; 
           [0019]      FIG. 7  is a flowchart illustrating processing performed by a small cell base station according to Embodiment 1 of the present invention; and 
           [0020]      FIG. 8  is a diagram illustrating an example of how ABS configurations according to Embodiment 1 of the present invention are set. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       Embodiment 1 
       [0022]      FIG. 3  illustrates a configuration example of a network system (communication system) according to Embodiment 1. As illustrated in  FIG. 3 , two HeNBs including HeNB  1  (cell ID-9711) and HeNB  2  (cell ID-11094) are installed in the communication area (cell range) of MeNB  1  (cell ID-2169). In addition, an assumption is made that HeNB  1  is in operation and HeNB  2  is not in operation (in power-off state) in  FIG. 3 . The term “cell ID” used herein refers to a number assigned to a specific base station. In addition, as illustrated in  FIG. 3 , MUEs  11  to  13  are located in the communication area of MeNB  1 , while HUE  11  is located in the communication area of HeNB  1 , and HUE  21  is located in the communication area of HeNB  2 . Each of the base stations (MeNB and HeNBs) located in the network system illustrated in  FIG. 3  transmits signals using any of a plurality of different ABS patterns. The term “ABS pattern” used herein refers to a subframe configuration pattern represented by a combination of transmission subframes that transmit a signal and non-transmission subframes that stop transmission of a signal. In  FIG. 3 , an operation and maintenance center (OMC) is connected to each of MeNB  2  and MeNB  3  (not illustrated) in addition to MeNB  1 , HeNB  1  and HeNB  2 , and is a management apparatus having a function to manage the setting information on the ABS patterns of the respective base station apparatuses (hereinafter, referred to as “ABS configuration”). 
         [0023]      FIG. 4  is a block diagram illustrating a configuration of an HeNB (e.g., HeNB  1  and HeNB  2  illustrated in  FIG. 3 ). HeNB  100  illustrated in  FIG. 4  collects neighboring base station (MeNB and HeNB) information (such as cell IDs) as soon as turned on. HeNB  100  may collect this information periodically (e.g., daily). 
         [0024]    Specifically, receiving section  101  in HeNB  100  illustrated in  FIG. 4  receives downlink signals (downlink radio signals) from the neighboring base stations (MeNB and HeNB) as soon as HeNB  100  is turned on. The signals to be received herein include a reference signal, a synchronization signal, an ABS configuration, and the like, transmitted from other base stations, for example. The ABS configuration is transmitted on a broadcast channel (is broadcasted), for example. Receiving section  101  outputs the received downlink signals to search section  103 . 
         [0025]    Turning on HeNB  100  triggers control section  102  to instruct search section  103  to search for a base station located around HeNB  100 . Control section  102  instructs search section  103  to measure a reference signal received quality (RSRQ), for example. 
         [0026]    Search section  103  searches for a base station located around HeNB  100  and detects an ABS configuration (i.e., ABS pattern) used in the base station located around HeNB  100  in accordance with the instruction from control section  102  (i.e., turning on HeNB  100  triggers the instruction). For example, search section  103  searches for other base stations located around HeNB  100  (MeNB and HeNB) on the basis of the downlink signals (synchronization signals) received by receiving section  101 . Moreover, search section  103  identifies the base station found by the search as a base station that causes interference with HeNB  100 . Furthermore, search section  103  monitors a broadcast channel from the other base station found by the search and acquires an ABS configuration indicating the ABS pattern used in the base station found by the search. Moreover, search section  103  measures an RSRQ by using the reference signal from the other base station found by the search. The lower the RSRQ measurement value of the other base station is, the smaller the amount of interference between the other base station and HeNB  100  is. Search section  103  outputs the acquired ABS configuration to determining section  104 . Note that, processing to search for a neighboring base station in search section  103  will be described in detail, hereinafter. 
         [0027]    Determining section  104  determines the ABS configuration to be used in HeNB  100 , on the basis of the ABS configuration acquired by search section  103 . For example, determining section  104  determines any of ABS configurations different from the ABS configurations collected by search section  103 , as the ABS configuration (ABS pattern) to be used in HeNB  100 . Specifically, determining section  104  determines, as the ABS configuration (ABS pattern) to be used in HeNB  100 , an ABS configuration other than the ABS configurations used in neighboring base stations of HeNB  100  (apparatuses which cause interference with HeNB  100 ). Determining section  104  outputs the determined ABS configuration (ABS pattern) to transmitting section  105 . The processing to determine the ABS configuration in determining section  104  will be described in detail, hereinafter. 
         [0028]    HeNB  100  applies the ABS configuration determined by determining section  104  to a UE (HUE) connected to HeNB  100 . Accordingly, transmitting section  105  transmits signals to the terminal (HUE) connected to HeNB  100 , in accordance with the ABS configuration (ABS pattern) determined by determining section  104 . In addition, transmitting section  105  reports the ABS configuration (ABS configuration used in HeNB  100 ) received from determining section  104  to the OMC. 
         [0029]    Next, the processing performed in HeNB  100  will be described in detail. 
         [0030]    Base stations such as an MeNB and an HeNB (including HeNB  100 ) each have an ABS configuration table as illustrated in  FIG. 5 , for example. This ABS configuration table illustrated in  FIG. 5  shows the association between ABS configurations (0 to 7) and ABS patterns C ABS  (m) (m=0 to 39). In this table, “m” represents the counter which is incremented every subframe. More specifically, the ABS patterns C ABS  (in) illustrated in  FIG. 5  define downlink transmission/non-transmission for 40 subframes. In  FIG. 5 , a subframe that transmits a downlink signal (transmission subframe) is represented by “0” while a subframe where transmission of a downlink signal is stopped (non-transmission subframe) is represented by “1.” For example, a downlink signal is transmitted in all subframes when the ABS configuration=0, while transmission of a downlink signal is stopped at every eighth subframe when the ABS configuration=1 in  FIG. 5 . 
         [0031]    In addition, each of the base stations such as an MeNB and an HeNB (including HeNB  100 ) includes an ABS white list indicating ABS configurations usable by the base station, as illustrated in  FIG. 6 , for example. In the ABS white list illustrated in  FIG. 6 , when the parameter indicating whether or not the corresponding ABS configuration is usable indicates “1,” this means that the ABS configuration is usable, and when the parameter indicates “0,” this means that the corresponding ABS configuration is not usable. For example, ABS configurations=0 and 4 are not usable and ABS configurations=1, 2, 3, 5, 6, and 7 are usable in  FIG. 6 . 
         [0032]      FIG. 7  illustrates a flowchart illustrating the flow of processing to search for a neighboring base station and the processing to determine an ABS configuration in HeNB  100  according to Embodiment 1. In the following description, the processing in HeNB  2  (not in operation) in  FIG. 3  will be described. 
         [0033]    In  FIG. 7 , search section  103  of HeNB  2  initializes the ABS white list (see, e.g.,  FIG. 6 ) in step (hereinafter, simply referred to as “ST”)  101 . More specifically, search section  103  initializes all the usable/unusable states of the ABS configurations illustrated in the ABS white list to all usable (“1”). 
         [0034]    In ST  102 , search section  103  initializes each parameter. Specifically, search section  103  sets, as the measurement target cell ID (T PCID ), the smallest cell ID (PCID MIN ) within a range of cell IDs (PCID MIN  to PCID MAX ) which is set as a blind detection target. In addition, search section  103  sets, as the minimum RSRQ buffer (P MIN ) for storing the minimum RSRQ, the maximum RSRQ (P MAX ) measureable by HeNB  2 . In addition, search section  103  sets “0” as ABS configuration (C MIN ) of the base station having the minimum RSRQ. For example, the following values are set: PCID MIN =0, PCID MAX =65535 and P MIN −24 (dBm) 
         [0035]    In ST  103 , search section  103  determines whether or not the measurement target cell ID (T PCID ) has exceeded PCID MAX . 
         [0036]    In ST  104 , search section  103  generates replicas of the synchronization signals if measurement target cell ID (T PCID ) has not exceeded PCID MAX  (ST  103 : No). There are two types of synchronization signals, which are the primary synchronization signal (PSS) and the secondary synchronization signal (SSS). 
         [0037]    Next, in ST  105 , search section  103  performs cell search using the replicas of the synchronization signals (PSS and SSS) generated in ST  104 . Specifically, search section  103  performs a correlation operation between the received signal and the replicas of the synchronization signals (PSS and SSS). If the correlation value between the received signal and replicas of the synchronization signals is equal to or greater than a previously set threshold, search section  103  determines that the base station of the measurement target cell ID (T PCID ) is located around the base station (HeNB  2 ) of search section  103  (i.e., as successful cell search). Meanwhile, if the correlation value between the received signal and the replicas of the synchronization signals is less than the threshold, search section  103  determines that the base station of the measurement target cell ID (T PCID ) is not located around the base station (HeNB  2 ) of search section  103  (i.e., as cell search failure). If the cell search is successful (ST 105 : Yes), the processing proceeds to the process of ST  106 , and if the cell search is not successful (ST 105 : No), the processing proceeds to the process of ST  111 . 
         [0038]    In ST  106 , search section  103  monitors a broadcast channel from the base station of the measurement target cell ID (T PCID ) and collects the ABS configuration (C PCID ) used in the base station. 
         [0039]    In ST  107 , search section  103  updates the ABS white list by setting the usable/unusable state corresponding to the ABS configuration (C PCID ) collected in ST  106  to be unusable (‘0’). 
         [0040]    In ST  108 , search section  103  monitors a downlink reference signal from the base station of the measurement target cell ID (T PCID ) and measures an RSRQ (P RSRQ ). 
         [0041]    In ST  109 , search section  103  compares the RSRQ (P RSRQ ) measured in ST  108  with P MIN . When P RSRQ  is smaller than P MIN  (ST  109 : Yes), search section  103  updates P MIN  to P RSRQ , and updates C MIN  to C PCID  in step ST  110 . Repeating the processes of ST  108  to ST  110  on the measurement target base stations identifies the ABS configuration (C MIN ) used in the base station having the smallest RSRQ among the measurement target base stations (i.e., base station which causes the smallest amount of interference with the base station of search section  103 ). Meanwhile, when P RSRQ  is equal to or greater than P MIN  (ST  109 : No), the processing proceeds to the process of ST  111 . 
         [0042]    In ST  111 , search section  103  updates the measurement target base station by incrementing T PCID  indicating the measurement target cell ID and returns to the process of ST  103 . Search section  103  repeats the processes of ST  103  to ST  111  until T PCID  becomes equal to PCID MAX  (until Yes in ST  103 ). 
         [0043]    Upon completion of the processing to search for a neighboring base station in search section  103  (ST  103 : Yes), determining section  104  determines the ABS configuration of HeNB  2 . Specifically, in ST  112 , determining section  104  determines whether or not there is an ABS configuration usable in the base station (HeNB  2 ) of determining section  104  with reference to the ABS white list updated in ST  107  (i.e., determines whether or not all ABS configurations are unusable). 
         [0044]    If there is a usable ABS configuration (ST  112 : No), determining section  104  randomly selects an ABS configuration to be used in the base station (HeNB  2 ) of determining section  104  from among the usable ABS configurations in the ABS white list (ABS configurations=1,2,3,5,6, and 7 in the case of  FIG. 6 ) in ST  113 . Stated differently, determining section  104  excludes the unusable ABS configurations in the ABS white list (ABS configurations=0, and 4 in the case of  FIG. 6 ) the ABS configuration used in a neighboring base station from the ABS configuration targets (selection targets) to be used in the base station (HeNB  2 ). Note that, the method of determining an ABS configuration to be used in the base station (HeNB  2 ) is not limited to the method of randomly determining an ABS configuration, and it is also possible to select the ABS configuration having the smallest number from among usable ABS configurations, for example. In  FIG. 5 , a smaller ABS configuration number is associated with a smaller number of non-transmission subframes. Thus, selecting the ABS configuration of the smallest number makes it possible to reduce the number of non-transmission subframes where transmission of a downlink signal is stopped, as much as possible. 
         [0045]    When there is no usable ABS configuration (ST  112 : Yes), determining section  104  sets, in ST  114 , as the ABS configuration to be used in the base station (HeNB  2 ) of determining section  104 , the ABS configuration (C MIN ) of the base station corresponding to the minimum RSRQ set in ST  110 . Specifically, when all of the plurality of ABS configurations (ABS patterns) are used by a plurality of other base stations, determining section  104  determines, as the ABS configuration (ABS pattern) to be used in the base station of determining section  104 , the ABS configuration (ABS pattern) used in the base station which causes the smallest amount of interference with the base station of determining section  104  (i.e., base station corresponding to the minimum RSRQ) among the plurality of other base stations. 
         [0046]    In ST  115 , transmitting section  105  reports the ABS configuration used in the base station of transmitting section  105  determined by determining section  104  in ST  113  or ST  114  to the OMC. 
         [0047]    As described above, HeNB  2  (HeNB  100 ) illustrated in  FIG. 3  regards a base station detectable by HeNB  2  (e.g., HeNB  1 ), as a base station possibly interfered by HeNB  2  (HeNB  100 ), so that HeNB  2  (HeNB  100 ) sets an ABS configuration different from the ABS configuration used in this detectable base station, as the ABS configuration to be used in HeNB  2  (HeNB  100 ). As illustrated in  FIG. 5 , if base stations which interfere with each other (HeNB  1  and HeNB  2 ) use the same ABS configuration, non-transmission subframes completely overlap with each other between the base stations. As a result, interference between the base stations interfering with each other cannot be avoided. In this respect, different ABS configurations are used between base stations interfering with each other (e.g., HeNB  1  and HeNB  2 ) as illustrated in Embodiment 1. As a result, a period during which one of the base stations uses a transmission subframe while the other base station uses a non-transmission subframe can be surely secured, and interference between the base stations can be avoided. Accordingly, the interfered base station no longer has to be interfered, and the throughput of UEs located within a cell provided by the interfered base station can be improved, 
         [0048]    Moreover, when no usable ABS configuration is found as a result of neighboring base station search (ST  112 : Yes), HeNB  100  sets the ABS configuration (C MIN ) of the base station corresponding to the minimum RSRQ to be the ABS configuration to be used in HeNB  100  (HeNB  2 ). To put it more specifically, when an ABS configuration different from that of a neighboring base station cannot be set in HeNB  100 , HeNB  100  sets the ABS configuration of a neighboring base station having the smallest interference with HeNB  100  (ABS configuration of the base station corresponding to the minimum RSRQ) to be used in HeNB  100 . Accordingly, although HeNB  100  and the neighboring base station use the same ABS configuration, the interference between the base stations can be kept as low as possible. 
         [0049]    Note that, HeNB  100  may determine the ABS configuration for the HeNB  100  in such a way that the ABS configuration set in HeNB  100  is different from the ABS configuration set in a base station detected by a neighboring base station search (base station that may interfere with HeNB  100 ). Stated differently, HeNB  100  may set an ABS configuration which is identical to the one used in a base station that has not been detected by a neighboring base station search (base station which does not cause interference with HeNB  100 ). Specifically, HeNB  100  (determining section  104 ) may determine, as the ABS configuration (ABS pattern) to be used in HeNB  100 , an ABS configuration (ABS pattern) which is identical to the ABS configuration (ABS pattern) used in a base station other than a base station found by the search by HeNB  100  (search section  103 ) (apparatus that causes interference with HeNB  100 ) among a plurality of base stations (MeNB and HeNB) located in the network system (communication system). For example, N units of HeNB are located in the communication area of the MeNB, and if the N units of HeNB do not interfere with each other (none of HeNBs detects another HeNB), the same ABS configuration may be set in each of the N units of HeNB. 
         [0050]    More specifically, a description will be provided regarding a network system in which HeNBs  1  to  4  are installed in the communication area of an MeNB as illustrated in  FIG. 8 , for is example. In  FIG. 8 , HeNB  1  may interfere with the MeNB but does not interfere with HeNBs  2  to  4 . In this respect, HeNB  1  sets an ABS configuration (ABS Conf. #1) different from an ABS configuration (ABS Conf. #0) of the MeNB. In this case, HeNB  1  may set the same ABS configuration as that set in HeNBs  2  to  4  (ABS Conf. #1 set in HeNB  2  and HeNB  4  in the case of  FIG. 8 ), which do not interfere with HeNB  1 . Likewise, in  FIG. 8 , HeNB  4  may set the same ABS configuration as that set in HeNB  1  and HeNB  2  which do not interfere with HeNB  4  (ABS Conf. #1 of HeNB  1  and HeNB  2  in  FIG. 8 ). Likewise, the other base stations (MeNB, HeNB  2 , and HeNB  3 ) may also set an ABS configuration for the base stations in the same manner. 
         [0051]    As described above, in this embodiment, each base station determines an ABS configuration to be used in the base station from among a plurality of ABS configurations (ABS patterns) on the basis of the presence or absence of interference between the base station and the other base stations. 
         [0052]    Specifically, in this embodiment, each base station identifies a neighboring base station detectable by the base station as an apparatus that causes interference with the base station and sets, in the base station, an ABS configuration different from the ABS configuration used in the neighboring base station. Accordingly, each base station independently sets the most appropriate ABS configuration for the base station. As a result, interference between the base stations can be kept as low as possible. In addition, unlike NPL 1, applying ABSs not only to an MeNB but also to HeNBs makes it possible to decrease the number of non-transmission subframes in the MeNB and thus to improve the throughput of the whole network. 
         [0053]    As described above, according to Embodiment 1, the interference between base stations (MeNB and HeNBs) is kept as low as possible, and the throughput of the whole network can be thus improved. 
         [0054]    Embodiment 1 has been described with a case where search section  103  searches for a neighboring base station, first (performs cell search first), and then, determining section  104  monitors the ABS configuration of the neighboring base station as illustrated in  FIG. 7 . However, HeNB  100  may monitor an ABS configuration of a neighboring base station in parallel with search for a neighboring base station. In this manner, the processing amount of the neighboring base station search processing and the ABS configuration monitoring processing can be further reduced. 
       Embodiment 2 
       [0055]    While the HeNBs perform search for a neighboring base station in Embodiment 1, HeNBs do not perform search for a neighboring base station in Embodiment 2. 
         [0056]    Hereinafter, Embodiment 2 will be specifically described. 
         [0057]    The HeNBs according to Embodiment 2 are different from HeNB  100  according to Embodiment 1 illustrated in  FIG. 4  in that the HeNBs according to Embodiment 2 are each configured without control section  102  and search section  103 , for example. Determining section  104  according to Embodiment 2 first sets an ABS configuration having the smallest number (ABS configuration=0 in  FIG. 5 ), for example, among ABS configurations that can be set in the HeNB according to Embodiment 2 (ABS configurations=0 to 7 in  FIG. 5 ) to be the ABS configuration candidate for the HeNB. Next, determining section  104  measures the amount of interference from another base station (MeNB or HeNB) using the downlink signal received by receiving section  101 . When the measured amount of interference is equal to or greater than a previously set threshold (allowable amount of interference), determining section  104  increments the currently set ABS configuration candidate and sets a new ABS configuration candidate. Meanwhile, when the measured amount of interference is less than the threshold, determining section  104  sets the currently set ABS configuration candidate to be the ABS configuration for the base station. Accordingly, determining section  104  determines an ABS configuration (ABS pattern) which makes the amount of interference from a base station other than the base station of determining section  104  less than the threshold (allowable value) to be the ABS configuration (ABS pattern) to be used in the base station thereof from among a plurality of ABS configurations (ABS patterns). 
         [0058]    In addition, transmitting section  105  reports the ABS configuration finally determined by determining section  104  to the OMC. 
         [0059]    As described above, in Embodiment 2, each base station determines an ABS configuration to be used in the base station from among a plurality of ABS configurations (ABS patterns) on the basis of the presence or absence of interference between the base station and the other base stations. Specifically, in Embodiment 2, each base station uses an ABS configuration which involves a small amount of interference with the other base stations (the amount of interference is less than the threshold). Thus, as in the case of Embodiment 1, the state in which one of the base stations uses a transmission subframe while another base station uses a non-transmission subframe can be surely secured, and the interference between base stations can be limited. Accordingly, the interfered base station no longer has to be interfered, and the throughput of UEs located in the cell provided by the interfered base station can be improved. 
         [0060]    In Embodiment 2, when the amount of interference becomes equal to or greater than the threshold with all the ABS configurations, HeNB  100  may set an ABS configuration corresponding to the smallest amount of interference to be the ABS configuration used in the base station. Specifically, even when the HeNB cannot set the amount of interference with another base station to be less than the allowable value, the HeNB can keep the interference with the other base station as low as possible by setting the ABS configuration corresponding to the lowest amount of interference. 
         [0061]    As described above, according to Embodiment 2, possible to keep interference between base stations (MeNB and HeNB) as low as possible and thus to improve the throughput of the whole network. 
         [0062]    Each of the embodiments of the present invention has been described above. 
         [0063]    Note that, in each of the embodiments, a description has been provided with the assumption that an HeNB is configured to acquire neighboring base station information when the HeNB is turned on. However, the HeNB may be configured to acquire the neighboring base station information periodically, such as daily. 
         [0064]    In addition, each of the embodiments has been described with a case where HeNBs report an ABS configuration to the OMC. However, each HeNB may report an ABS pattern consisting of a total of 40 bits as illustrated in  FIG. 5  directly to the OMC, for example. 
         [0065]    The disclosure of the specification, the drawing, and the abstract of Japanese Patent Application No. 2011-027449, filed on Feb. 10, 2011, is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0066]    The present invention is suitable for mobile communication system including an MeNB, an MUE, an HeNB and an HUE. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           100  HeNB 
           101  Receiving section 
           102  Control section 
           103  Search section 
           104  Determining section 
           105  Transmitting section