Patent Publication Number: US-9848392-B2

Title: Radio communication system, high-power base station, low-power base station, and communication control method

Description:
TECHNICAL FIELD 
     The present invention relates to a radio communication system, a high-power base station, a low-power base station, and a communication control method, to which a heterogeneous network is applied. 
     BACKGROUND ART 
     LTE (Long Term Evolution) and LTE Advanced which is an advanced type of LTE are next-generation systems providing communication which is faster and supports a larger capacity than the third-generation and the third-and-a-half generation cellular radio communication systems currently in operation. LTE and LTE Advanced are standardized by a standardization organization called 3GPP (3 rd  Generation Partnership Project). 
     In a downlink in the LTE systems (including LTE Advanced), a radio base station sends user data to a radio terminal by using a data transmission channel called PDSCH (Physical Downlink Shared Channel). Note that the downlink refers to communication in a direction from the radio base station to the radio terminal, whereas an uplink refers to communication from the radio terminal to the radio base station. 
     In LTE Advanced, provision of a heterogeneous network is under consideration. In a heterogeneous network, low-power base stations (so-called pico-cell base stations, femto-cell base stations, or relay nodes) are installed in a communication area of a high-power base station (a so-called macro-cell base station). A heterogeneous network is capable of distributing the load of the high-power base station to the low-power base stations. 
     However, since a radio terminal is generally connected to a radio base station which sends radio signals with the highest received power among multiple radio base stations, in the heterogeneous network the radio terminal may have a low chance of being connected to a low-power base station with low transmission power. 
     In consideration of such circumstances, there is proposed a method for expanding the coverage (communication area) of the low-power base station by performing control such that the radio terminal is connected to the low-power base station even when the received power from the low-power base station is not the highest (see, for example, Non-patent Document 1). 
     PRIOR ART DOCUMENT 
     Non-Patent Document 
     NON-PATENT DOCUMENT 1: 3GPP R1-093433 “Importance of Serving Cell Selection in Heterogeneous Networks” February, 2010. 
     SUMMARY OF THE INVENTION 
     When radio resources used as data transmission channels by neighboring radio base stations overlap each other, the data transmission channel of one of the radio base stations receives interference from the data transmission channel of the other base station, and therefore user data cannot be received normally from the one radio base station via its data transmission channel. 
     This problem is even more severe in the method of expanding the coverage of the low-power base station in the heterogeneous network since the data transmission channel of the low-power base station is highly likely to receive strong interference from the data transmission channel of the high-power base station. 
     Accordingly, the present invention has an objective of providing a radio communication system, a high-power base station, a low-power base station, and a communication control method capable of suppressing interference between base stations even when the coverage of the low-power base station is expanded. 
     The present invention has the following features in order to solve the aforementioned problem. First of all, a feature of the radio communication system is summarized as follows. A radio communication system comprises: a high-power base station (macro-cell base station MeNB); and a low-power base station (e.g. pico-cell base station PeNB) installed in a communication area of the high-power base station, having lower transmission power than the high-power base station, and expanded in its coverage, wherein the radio communication system further comprising a determination unit (bias value determination unit  123  or bias value determination unit  223 ) configured to determine a degree of expanding the coverage of the low-power base station, according to an amount of usable resources which are radio resources usable as a particular downlink channel (e.g. PDSCH) by the high-power base station. Here, the particular downlink channel is a downlink data transmission channel (PDSCH in LTE) for example. However, the particular downlink channel may be a downlink control information transmission channel (PDCCH in LTE) and so on, not apply only to downlink data transmission channel. The low-power base station is a pico-cell base station or a femto-cell base station for example. However, the low-power base station may be a relay node and so on, not apply only to the pico-cell base station or the femto-cell base station. 
     With the radio communication system according to the aforementioned feature, a degree of expanding the coverage of the low-power base station is determined according to an amount (i.e. possibility of interference occurrence) of usable resources which are radio resources usable as a particular downlink channel by the high-power base station. Thereby, the coverage of the low power base stations can be expanded appropriately considering the possibility of interference occurrence. Accordingly, interference between the base stations can be suppressed even if the coverage of the low power base stations is expanded. 
     Another feature of the radio communication system is summarized as follows. In the radio communication system according to the aforementioned feature, the determination unit determines the degree of expanding the coverage of the low-power base station such that the degree becomes larger as the usable resources decrease. 
     Another feature of the radio communication system is summarized as follows. In the radio communication system according to the aforementioned feature, the usable resources decrease, the determination unit determines the degree of expanding the coverage of the low-power base station such that the degree is made larger than the degree determined before the usable resources decrease. 
     Another feature of the radio communication system is summarized as follows. The radio communication system according to the aforementioned feature further comprising: a selector (connection target selector  121 , connection target selector  221 ) configured to select, as a connection target of a radio terminal, the base station providing the highest reception quality value on the basis of a first reception quality value (RSRP MeNB ) indicating reception quality of a radio signal that the radio terminal receives from the high-power base station, a second reception quality value (RSRP PeNB ) indicating reception quality of a radio signal that the radio terminal receives from the low-power base station, and a correction value (bias value) for correcting the second reception quality value to a larger value, wherein the correction value indicates the degree of expanding the coverage of the low-power base station, and the determination unit determines the correction value according to the amount of usable resources. 
     Another feature of the radio communication system is summarized as follows. In the radio communication system according to the aforementioned feature, the particular downlink channel is a data transmission channel for transmitting user data to a radio terminal. 
     Another feature of the radio communication system is summarized as follows. In the radio communication system according to the aforementioned feature, the usable resources are at least part of a total downlink frequency band (total resource blocks). 
     Another feature of the radio communication system is summarized as follows. In the radio communication system according to the aforementioned feature, the usable resources are at least part of a time range in a total downlink time frame (subframe or radio frame). 
     A feature of a high-power base station is summarized as follows. A high-power base station comprises: a determination unit (bias value determination unit  123 ) configured to determine a degree of expanding coverage of a low-power base station (e.g. pico-cell base station PeNB) according to an amount of usable resources which are radio resources usable as a particular downlink channel (e.g. PDSCH) by the high-power base station, the low-power base station installed in a communication area of the high-power base station and having lower transmission power than the high-power base station; and a transmitter (X2 interface communication unit  140 ) configured to transmit, to the low-power base station, information indicating the expansion degree determined by the determination unit. 
     A feature of a low-power base station is summarized as follows. A low-power base station (e.g. pico-cell base station PeNB) installed in a communication area of a high-power base station and having lower transmission power than the high-power base station, comprises: a determination unit (bias value determination unit  223 ) configured to determine a degree of expanding coverage of the low-power base station according to an amount of usable resources which are radio resources usable as a particular downlink channel by the high-power base station. 
     A feature of a communication control method is summarized as follows. A communication control method comprises: determining a degree of expanding coverage of a low-power base station according to an amount of usable resources which are radio resources usable as a particular downlink channel by a high-power base station, wherein the low-power base station installed in a communication area of the high-power base station and having lower transmission power than the high-power base station. 
     Another feature of a communication control method is summarized as follows. A communication control method comprises: causing a high-power base station to transmit, to a low-power base station, information indicating an amount of usable resources which are radio resources usable as a particular downlink channel by the high-power base station, the low-power base station installed in a communication area of the high-power base station and having lower transmission power than the high-power base station; causing the low-power base station to determine a degree of expanding coverage of the low-power base station based on the information indicating the amount of usable resources received from the high-power base station, and to transmit, to the high-power base station, information indicating the determined degree of expanding the coverage of the low-power base station; and causing the high-power base station to receive the information indicating the degree of expanding the coverage of the low-power base station, which is transmitted from the low-power base station. 
     The present invention can provide a radio communication system, a high-power base station, a low-power base station, and a communication control method capable of suppressing interference between the base stations even when the coverage of the low-power base station is expanded. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating the overview of an LTE system according to a first embodiment and a second embodiment. 
         FIG. 2  is a frame configuration diagram showing a frame configuration used when a FDD scheme is employed. 
         FIG. 3  is a schematic configuration diagram of a radio communication system according to the first embodiment. 
         FIG. 4  is a diagram illustrating interference control according to the first embodiment and the second embodiment. 
         FIG. 5  is a block diagram showing the configuration of a macro-cell base station according to the first embodiment. 
         FIG. 6  is a block diagram showing the configuration of a pico-cell base station according to the first embodiment. 
         FIG. 7  is an operation sequence diagram showing the operations of the radio communication system according to the first embodiment. 
         FIG. 8  is a block diagram showing the configuration of a macro-cell base station according to the second embodiment. 
         FIG. 9  is a block diagram showing the configuration of a pico-cell base station according to the second embodiment. 
         FIG. 10  is an operation sequence diagram showing the operations of a radio communication system according to the second embodiment. 
         FIG. 11  is a diagram illustrating a case where PDSCH resources are divided by time. 
         FIG. 12  is a diagram illustrating another case of dividing PDSCH resources by time. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Descriptions are given of a first embodiment, a second embodiment, and other embodiments of the present invention. In the drawings referred to by the following embodiments, the same or similar parts are given the same or similar reference numerals. 
     [Overview of the LTE System] 
     Before describing the first embodiment and the second embodiment, an overview of an LTE system is described, on points related to the present invention. 
       FIG. 1  is a diagram illustrating the overview of the LTE system. As shown in  FIG. 1 , multiple radio base stations eNB configure E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network). Each of the various radio base stations eNB forms a cell which consists of a communication area where a radio terminal UE is to be provided with services. 
     The radio terminal UE is a radio communication device that is owned by the user, and is also called a user device. The radio terminal UE is configured to connect to a radio base station eNB from which the radio terminal UE measures the highest reference signal received power (RSRP) among the multiple radio base stations eNB. Note that it is not limited to the RSRP, and other reception quality indices, such as SNR (Signal to Noise ratio), may be used instead. 
     The radio base stations eNB are capable of communicating with each other via X2 interfaces which are logical communication paths providing communications between the base stations. Each of the multiple radio base stations eNB can communicate with EPC (Evolved Packet Core), or more specifically, MME (Mobility Management Entity)/S-GW (Serving Gateway), via an S1 interface. 
     In radio communication between each radio base station eNB and the radio terminal UE, an OFDMA (Orthogonal Frequency Division Multiple Access) scheme is employed as the multiplexing scheme for the downlink, and an SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme is employed as the multiplexing scheme for the uplink. Further, an FDD (Frequency Division Duplex) scheme or a TDD (Time Division Duplex) scheme is employed as the duplexing scheme. 
       FIG. 2( a )  is a frame configuration diagram showing the configuration of a downlink radio frame used when the FDD scheme is employed.  FIG. 2( b )  is a frame configuration diagram showing the configuration of a downlink subframe. 
     As shown in  FIG. 2( a ) , the downlink radio frame is configured with ten downlink subframes, and each downlink subframe is configured with two downlink slots. Each downlink subframe is 1 ms long, and each downlink slot is 0.5 ms long. Each downlink slot contains seven OFDM symbols in the time-axis direction (time domain), and contains multiple resource blocks (RB) in the frequency-axis direction (frequency domain) as shown in  FIG. 2( b ) . Each RB contains 12 sub-carriers. 
     As shown in  FIG. 2 ( b ) , each downlink subframe contains two successive downlink slots. A maximum of three OFDM symbols from the top of the first downlink slot of each downlink subframe is a control region configuring radio resources used as PDCCH (Physical Downlink Control Channel) for transmitting control information. The control information corresponds to information such as uplink and downlink scheduling information (i.e., information on allocated radio resources). 
     The rest of the OFDM symbols composing the downlink subframe is a data region configuring radio resources used as PDSCH (Physical Downlink Shared Channel) for transmitting user data. The radio terminal UE can identify user data transmitted via PDSCH by decoding the control information transmitted via PDCCH. 
     First Embodiment 
     The first embodiment of the present invention is described next. The first embodiment is described using, as an example, a heterogeneous network deployment in which pico-cell base stations PeNB which are low-power base stations (low-output base stations) are installed inside a communication area (i.e., a macro cell) of a macro-cell base station MeNB which is a high-power base station (a high-output base station). 
     In the following first embodiment, descriptions are given of (1) the configuration of a radio communication system, (2) interference control, (3) the configuration of a macro-cell base station, (4) the configuration of a pico-cell base station, (5) operations of the radio communication system, and (6) advantageous effects of the first embodiment. 
     (1) Configuration of the Radio Communication System 
       FIG. 3  is a diagram of the schematic configuration of a radio communication system  1  according to the first embodiment. 
     As shown in  FIG. 3 , the radio communication system  1  includes a macro-cell base station MeNB, a radio terminal MUE connected to the macro-cell base station MeNB, pico-cell base stations PeNB1 to PeNB3 which are installed within a macro cell MC formed by the macro-cell base station MeNB and are adjacent to the macro-cell base station MeNB, and radio terminals PUE in pico cells PC formed by the pico-cell base stations PeNB1 to PeNB3 and are connected to the pico-cell base stations PeNB, respectively. Below, the pico-cell base stations PeNB1 to PeNB3 are simply called pico-cell base stations PeNB when no particular differentiation is necessary among them. The macro-cell base station MeNB and the pico-cell base stations PeNB use a common frequency band. In addition, the pico cells PC formed by the pico-cell base stations PeNB are called “hot zones” below. 
     The pico-cell base stations PeNB (also called hot-zone nodes) are low-power base stations with lower transmission power than the macro-cell base station MeNB, and are installed in high-traffic zones of the macro cell. In the heterogeneous network, the pico-cell base stations PeNB have low transmission power. Accordingly, when a maximum received power standard (called an RP standard below), which is a connection target selection standard where the radio terminal UE selects and connects to the radio base station eNB having the highest RSRP, is employed, the coverage of the pico-cell base stations PeNB might decrease. Especially when the pico-cell base stations PeNB are located close to the macro-cell base station MeNB, the coverage of the pico-cell base stations PeNB is so decreased that the pico-cell base stations PeNB cannot be used effectively. 
     The following two methods can be mainly used to allow the coverage of each pico-cell base station PeNB to expand without increasing the transmission power of the pico-cell base station PeNB. 
     In the first method, instead of using the RP standard where the radio base station eNB transmitting radio signals of the highest RSRP is selected as a connection target of the radio terminal UE, the radio base station eNB having the smallest propagation loss (path loss) with the radio terminal UE is selected as a connection target of the radio terminal UE. In this way, the radio base station eNB closest to the radio terminal UE is for example selected as the connection target, allowing expansion of the coverage of the pico-cell base stations PeNB. Such a connection target selection standard is referred to as a minimum path-loss standard (called a PL standard below). 
     In the second method, when the radio terminal UE can receive radio signals from each of the macro-cell base station MeNB and the pico-cell base stations PeNB, before comparing the RSRP of the macro-cell base station MeNB and the RSRPs of the pico-cell base stations PeNB, a bias value is added to each of the RSRPs of the pico-cell base stations PeNB. By giving bias to the RSRPs of the pico-cell base stations PeNB (i.e., adding a bias value to the RSRPs of the pico-cell base stations PeNB), the RSRPs given the bias are more likely to exceed the RSRP of the macro-cell base station MeNB. Consequently, the pico-cell base stations PeNB are preferentially selected as the connection target, achieving expansion of the coverage of the pico-cell base stations PeNB. Such a connection target selection standard is referred to as a range expansion standard (called an RE standard below). By making the bias value equal to the difference in transmission power between the macro-cell base station MeNB and the pico-cell base station PeNB (e.g., 16 dB), the RE standard becomes a connection target selection standard equivalent to the PL standard. 
     In the first embodiment, the coverage of the pico-cell base station PeNB is expanded using the RE standard. For example, the connection target of the radio terminal UE is selected by the radio terminal UE when the radio terminal UE is in standby (an idle state), and selected by the radio base station eNB when the radio terminal UE is in communication (an active state). In the active state, the radio terminal UE periodically gives RSRP measurement values to the radio base station eNB to which the radio terminal UE is connected. Accordingly, the radio base station eNB to which the radio terminal UE is connected can select the next connection target of the radio terminal UE and hand-over the radio terminal UE to the next connection target. 
     The macro-cell base station MeNB uses a PDSCH to transmit user data to the radio terminal MUE. The pico-cell base station PeNB uses a PDSCH to transmit user data to the radio terminal PUE. When the frequency bands of these PDSCHs overlap each other, the PDSCHs of the macro-cell base station MeNB and the pico-cell base station PeNB interfere with each other. 
     When the coverage of the pico-cell base station PeNB is expanded, the radio terminal PUE connected to the pico-cell base station PeNB sometimes receives higher power from the macro-cell base station MeNB than from the pico-cell base station PeNB. In this case, the PDSCH of the pico-cell base station PeNB receives strong interference from the PDSCH of the macro-cell base station MeNB, making the radio terminal PUE unable to receive (decode) user data. 
     (2) Interference Control 
     In the downlink of the heterogeneous network, if the coverage is expanded by giving bias according to the RE standard so that the coverage may be larger than the hot zone formed by the RP standard, the difference in transmission power between the macro-cell base station MeNB and the pico-cell base station PeNB causes interference power to be greater than desired signal power. 
     Then, the radio terminal UE not having an optimal SINR is consequently contained in the hot zone. Such a radio terminal UE basically suffers strong interference from the macro-cell base station MeNB having high transmission power, so that the SINR becomes very low. 
     To avoid this, the following interference control is performed in the first embodiment.  FIG. 4  is a diagram illustrating interference control according to the first embodiment. 
     As shown in  FIG. 4( a ) , only part of the PDSCH resources (corresponding to the data region shown in  FIG. 2( b ) ) of the macro-cell base station MeNB is usable, and the rest is not used. Thereby, the unused part is offered to the radio terminal PUE having a low SINR in the hot zone. Herein, the PDSCH resources usable by the macro-cell base station MeNB are also called “usable PDSCH resources,” and the PDSCH resources not usable by the macro-cell base station MeNB are also called “unusable PDSCH resources.” In the first embodiment, the usable PDSCH resources are at least part of the total downlink resource blocks, and the unusable PDSCH resources are the rest of the total downlink resource blocks, i.e., resource blocks other than the part described above. The PDSCH resources can be divided in any manner, but due to the LTE specifications, they are divided according to the resolution of a fed-back CQI. 
     As shown in  FIG. 4( b ) , the radio resources which correspond to the unusable PDSCH resources do not receive interference from the macro-cell base station MeNB. Accordingly, the pico-cell base station PeNB allocates such interference-free PDSCH resources to the radio terminal PUE of a low SINR. To be more specific, the radio terminal PUE periodically feeds back a measurement result of reception quality as a channel quality indicator (CQI) to the pico-cell base station PeNB, and the pico-cell base station PeNB can allocate the interference-free PDSCH resources preferentially to the radio terminal PUE in response to feedback of a favorable CQI for the interference-free PDSCH resources. 
     Alternatively, the pico-cell base station PeNB can be given the interference-free PDSCH resources by being notified of the unusable PDSCH resources by the macro-cell base station MeNB. In this case, the pico-cell base station PeNB can allocate the interference-free PDSCH resources preferentially to the radio terminal PUE without waiting for feedback of a favorable CQI for the interference-free PDSCH resources. In the first embodiment, the macro-cell base station MeNB notifies the pico-cell base station PeNB of the unusable PDSCH resources. 
     The amount of usable PDSCH resources of the macro-cell base station MeNB (herein, an “amount” includes the concept of “percentage”) is determined according to, for example, a message from the pico-cell base station PeNB or another macro-cell base station. In the LTE system, a message for restricting usage of the PDSCH resources can be exchanged between the base stations via the X2 interfaces, and therefore the macro-cell base station MeNB determines the amount of usable PDSCH resources according to the received message. 
     Alternatively, the amount of usable PDSCH resources of the macro-cell base station MeNB is determined according to the traffic load of the macro-cell base station MeNB (e.g., the number of terminals being connected). More specifically, the amount of usable PDSCH resources of the macro-cell base station MeNB is decreased when the macro-cell base station MeNB has low traffic load, whereas the amount of usable PDSCH resources of the macro-cell base station MeNB is increased when the macro-cell base station MeNB has high traffic load. 
     In the first embodiment, the macro-cell base station MeNB determines the bias value of the RE standard according to the amount of its usable PDSCH resources. Specifically, when the amount of usable PDSCH resources of the macro-cell base station MeNB is small, even a large bias value of the RE standard would not cause the radio terminal PUE connected to the pico-cell base station PeNB to easily receive interference from the macro-cell base station MeNB. On the other hand, when the amount of usable PDSCH resources of the macro-cell base station MeNB is large, a large bias value of RE standard would cause the radio terminal PUE connected to the pico-cell base station PeNB to easily receive interference from the macro-cell base station MeNB. Accordingly, when the usable PDSCH resources of the macro-cell base station MeNB decrease, the macro-cell base station MeNB correspondingly increases the bias value of the RE standard, and when the usable PDSCH resources of the macro-cell base station MeNB increase, the macro-cell base station MeNB correspondingly decreases the bias value of the RE standard. In this way, even when the coverage of the pico-cell base station PeNB is expanded, interference between the PDSCHs of the base stations can be suppressed. Further, when the usable PDSCH resources of the macro-cell base station MeNB are updated as appropriate, it is desirable that the bias value of the RE standard be newly set according to the update of the usable PDSCH resources of the macro-cell base station MeNB. 
     (3) Configuration of the Macro-Cell Base Station 
     The configuration of the macro-cell base station MeNB is described next.  FIG. 5  is a block diagram showing the configuration of the macro-cell base station MeNB according to the first embodiment. 
     As shown in  FIG. 5 , the macro-cell base station MeNB includes an antenna unit  101 , a radio communication unit  110 , a controller  120 , a storage unit  130 , and an X2 interface communication unit  140 . 
     The radio communication unit  110  is configured with, for example, a radio frequency (RF) circuit, a base band (BB) circuit, or the like, and configured to exchange radio signals with the radio terminal PUE via the antenna unit  101 . The radio communication unit  110  is also configured to modulate transmission signals and demodulate received signals. 
     The controller  120  is configured with, for example, a CPU, and is configured to control various functions of the macro-cell base station MeNB. The storage unit  130  is configured with a memory for example, and is configured to store various pieces of information used for the control of the macro-cell base station MeNB, or the like. The X2 interface communication unit  140  is configured to use an X2 interface to perform communications with other radio base stations. 
     The controller  120  includes a connection target selector  121 , a usable resource determination unit  122 , a bias value determination unit  123 , and a resource allocator  124 . 
     The connection target selector  121  is configured to select the radio base station to which the radio terminal MUE is to be connected next, based on RSRP information (i.e., a measurement report) informed of by the radio terminal MUE. When the radio terminal MUE receives reference signals from the macro-cell base station MeNB and the pico-cell base station PeNB, before comparing RSRP MeNB  of the macro-cell base station MeNB and RSRP PeNB  of the pico-cell base station PeNB, the connection target selector  121  gives bias to RSRPPeNB. When RSRPPeNB thus given bias is higher than RSRPMeNB, the connection target selector  121  performs hand-over control, switching the connection target of the radio terminal MUE to the pico-cell base station PeNB. 
     The usable resource determination unit  122  is configured to determine usable PDSCH resources according to messages from other radio base stations, which messages are for limiting use of PDSCH resources and received by the X2 interface communication unit  140 . Alternatively, the usable resource determination unit  122  is configured to determine the usable PDSCH resources according to the traffic load of the macro-cell base station MeNB (e.g., the number of terminals being connected). 
     The bias value determination unit  123  is configured to determine a bias value of the RE standard for each of the pico-cell base stations PeNB according to the amount of usable PDSCH resources determined by the usable resource determination unit  122 . To be more specific, when the usable PDSCH resources of the macro-cell base station MeNB decrease, the bias value determination unit  123  correspondingly increases the bias value of the RE standard, and when usable PDSCH resources of the macro-cell base station MeNB increase, the bias value determination unit  123  correspondingly decreases the bias value of the RE standard. 
     The bias value of the RE standard determined by the bias value determination unit  123  may be the same for all the pico-cell base stations PeNB, or may be different for each of the pico-cell base stations PeNB. For example, a relatively-large bias value of the RE standard is set for the pico-cell base station PeNB which is located close to the macro-cell base station MeNB (or has small path loss) and therefore is likely to be affected by the interference. Conversely, a relatively-small bias value of the RE standard is set for the pico-cell base station PeNB which is away from the macro-cell base station MeNB (or has large path loss) and therefore is unlikely to be affected by the interference. 
     The resource allocator  124  is configured to allocate radio resources (resource blocks) to the radio terminal MUE from the usable PDSCH resources determined by the usable resource determination unit  122 . For example, the resource allocator  124  allocates radio resources (resource blocks) to the radio terminal MUE from the usable PDSCH resources based on the CQIs fed back from the radio terminal MUE and by using a scheduling algorithm such as proportional fairness (PF). 
     (4) Configuration of the Pico-Cell Base Station 
     The configuration of the pico-cell base station PeNB is described next.  FIG. 6  is a block diagram showing the configuration of the pico-cell base station PeNB according to the first embodiment. 
     As shown in  FIG. 6 , the pico-cell base station PeNB includes an antenna unit  201 , a radio communication unit  210 , a controller  220 , a storage unit  230 , and an X2 interface communication unit  240 . 
     The radio communication unit  110  is configured with, for example, a radio frequency (RF) circuit, a base band (BB) circuit, or the like, and configured to exchange radio signals with the radio terminal PUE via the antenna unit  201 . The radio communication unit  210  is also configured to modulate transmission signals and demodulate received signals. 
     The controller  220  is configured with, for example, a CPU, and is configured to control various functions of the pico-cell base station PeNB. The storage unit  230  is configured with a memory for example, and is configured to store various pieces of information used for the control of the pico-cell base station PeNB, or the like. The X2 interface communication unit  240  is configured to use the X2 interface to perform communications with other radio base stations. 
     The controller  220  includes a connection target selector  221  and a resource allocator  222 . 
     The connection target selector  221  is configured to select the radio base station to which the radio terminal PUE is to be connected next, based on the RSRPs informed of by the radio terminal PUE connected to the pico-cell base station PeNB. When the radio terminal PUE receives reference signals from the macro-cell base station MeNB and the pico-cell base stations PeNB, before comparing the RSRPMeNB of the macro-cell base station MeNB and the RSRPPeNB of each of the pico-cell base stations PeNB, the connection target selector  221  gives bias to the RSRPPeNB. When the RSRPPeNB thus given bias is lower than the RSRPMeNB, the connection target selector  221  performs hand-over control, switching the connection target of the radio terminal PUE to the macro-cell base station MeNB. 
     The resource allocator  222  is configured to allocate radio resources (resource blocks) to the radio terminal PUE. For example, the resource allocator  222  allocates radio resources (resource blocks) to the radio terminal MUE from the usable PDSCH resources, based on the CQIs fed back from the radio terminal PUE and by using a scheduling algorithm such as proportional fairness (PF). In a case where the unusable PDSCH resources are notified of by the macro-cell base station MeNB, the resource allocator  222  allocates interference-free PDSCH resources which correspond to the unusable PDSCH resources (see  FIG. 4 ) preferentially to the radio terminal PUE without waiting for feedback of a favorable CQI for the interference-free PDSCH resources. 
     (5) Operations of the Radio Communication System 
       FIG. 7  is an operation sequence diagram showing the operations of the radio communication system  1  according to the first embodiment. 
     In Step S 11 , the usable resource determination unit  122  of the macro-cell base station MeNB determines its usable PDSCH resources. 
     In Step S 12 , the bias value determination unit  123  of the macro-cell base station MeNB determines a bias value of the RE standard for each of the pico-cell base stations PeNB according to the amount of usable PDSCH resources determined by the usable resource determination unit  122 , and stores the bias values in the storage unit  130 . The bias values stored in the storage unit  130  are thereafter referred to by the connection target selector  121 . 
     In Step S 13 , the X2 interface communication unit  140  of the macro-cell base station MeNB sends the pico-cell base stations PeNB information indicating the bias values determined by the bias value determination unit  123  and information indicating the unusable PDSCH resources determined by the usable resource determination unit  122 . The X2 interface communication unit  240  of each pico-cell base station PeNB receives the information indicating the bias values and the information indicating the unusable PDSCH resources. 
     In Step S 14 , the resource allocator  124  of the macro-cell base station MeNB allocates radio resources (resource blocks) to the radio terminal MUE from the usable PDSCH resources determined by the usable resource determination unit  122 . 
     In Step S 15 , the storage unit of  230  of each pico-cell base station PeNB stores the information indicating the bias values received by the X2 interface communication unit  240 . The bias values are thereafter referred to by the connection target selector  221 . 
     In Step S 16 , the resource allocator  222  of each pi co-cell base station PeNB allocates radio resources (resource blocks) to the radio terminal PUE. Based on the information indicating the unusable PDSCH resources received by the X2 interface communication unit  240 , the resource allocator  222  allocates interference-free PDSCH resources corresponding to the unusable PDSCH resources (see  FIG. 4 ) preferentially to the radio terminal PUE of a low SINR. 
     (6) Advantageous Effects of First Embodiment 
     As described above, according to the amount of its usable PDSCH resources, the macro-cell base station MeNB determines the bias values indicating the degree of expanding the coverage of the pico-cell base stations PeNB. Thereby, the coverage of the pico-cell base stations PeNB can be expanded appropriately considering the high possibility of interference occurrence. Accordingly, interference between the base stations can be suppressed even if the coverage of the pico-cell base stations PeNB is expanded. 
     Second Embodiment 
     In the first embodiment, the macro-cell base station MeNB determines the bias values, but in the second embodiment, each pico-cell base station PeNB determines its bias value. In the following, points different from the first embodiment are described, while overlapping points are not described again. 
       FIG. 8  is a block diagram showing the configuration of a macro-cell base station MeNB according to the second embodiment. As shown in  FIG. 8 , the macro-cell base station MeNB according to the second embodiment does not have the bias value determination unit  123  described in the first embodiment. 
       FIG. 9  is a block diagram showing the configuration of each pico-cell base station PeNB according to the second embodiment. As shown in  FIG. 9 , the pico-cell base station PeNB according to the second embodiment includes a bias value determination unit  223 . The bias value determination unit  223  is configured to determine a bias value of the RE standard. The method of determining the bias value is the same as in the first embodiment. 
       FIG. 10  is an operation sequence diagram showing the operations of a radio communication system  1  according to the second embodiment.  FIG. 10  shows an example of an operation sequence performed between one pico-cell base station PeNB and the macro-cell base station MeNB. 
     In Step S 21 , the usable resource determination unit  122  of the macro-cell base station MeNB determines the usable PDSCH resources of the macro-cell base station MeNB. 
     In Step S 22 , the X2 interface communication unit  140  of the macro-cell base station MeNB sends the pico-cell base station PeNB information indicating the usable PDSCH resources (or its amount) determined by the usable resource determination unit  122 . The X2 interface communication unit  240  of the pico-cell base station PeNB receives the information indicating the usable PDSCH resources (or its amount) of the macro-cell base station MeNB. 
     In Step S 23 , based on the information received by the X2 interface communication unit  240 , the bias value determination unit  223  of the pico-cell base station PeNB determines a bias value of the RE standard for the pico-cell base station PeNB according to the amount of usable PDSCH resources of the macro-cell base station MeNB. The bias value determination unit  223  then stores the bias value in the storage unit  230 . The bias value stored in the storage unit  230  is thereafter referred to by the connection target selector  221 . 
     In Step S 24 , the X2 interface communication unit  240  of the pico-cell base station PeNB sends the macro-cell base station MeNB information indicating the bias value determined by the bias value determination unit  223 . The X2 interface communication unit  140  of the macro-cell base station MeNB receives the information indicating the bias value. 
     In Step S 25 , the resource allocator  222  of the pico-cell base station PeNB allocates radio resources (resource blocks) to the radio terminal PUE. The resource allocator  222  allocates interference-free PDSCH resources corresponding to the unusable PDSCH resources of the macro-cell base station MeNB (see  FIG. 4 ) preferentially to the radio terminal PUE with a low SINR. 
     In Step S 26 , the storage unit  130  of the macro-cell base station MeNB stores the information indicating the bias value received by the X2 interface communication unit  140 . The bias value thus stored is thereafter referred to by the connection selector  121 . 
     In Step S 27 , based on the information indicating unusable PDSCH resources received by the X2 interface communication unit  140 , the resource allocator  124  of the macro-cell base station MeNB allocates radio resources (resource blocks) to the radio terminal MUE from the usable PDSCH resources. 
     As described, the second embodiment can offer advantageous effects similar to those in the first embodiment. 
     Other Embodiments 
     As described above, the present invention has been disclosed by using the embodiments. However, it should not be understood that the description and drawings which constitute part of this disclosure limit the present invention. From this disclosure, various alternative embodiments, examples, and operation techniques will be easily found by those skilled in the art. 
     In the embodiments described above, the PDSCH resources are divided by frequency, but the PDSCH resources may be divided by time.  FIG. 11  is a diagram showing a case of dividing the PDSCH resources by time. Any unit can be set for the time division, but due to the LTE specifications, the resources are divided in a unit of OFDM symbol. Instead of time-dividing a subframe in a unit of OFDM symbol, a radio frame shown in  FIG. 2  may be divided by time in a unit of subframe.  FIG. 12  shows a case of dividing a radio frame by time in a unit of subframe. 
     In the embodiments described above, the resource division involves division of PDSCH resources (i.e., division of a data region). However, the present invention is not limited to PDSCH, and also applicable to division of PDCCH resources (i.e., division of a control region). Either of the frequency division or the time division may be used for the division of PDCCH resources. 
     In LTE Advanced, a relay node which is a radio base station configuring a wireless backhaul is expected to be employed, and the X2 interface is to be employed for the relay node as well. Accordingly, the relay node can be the low-power base station according to the present invention. 
     Further, the present invention is applied to the LTE system in the embodiments described above, but may be applied to other radio communication systems such as a radio communication system based on WiMAX (IEEE 802.16). 
     As described above, the present invention naturally includes various embodiments which are not described herein. Accordingly, the technical scope of the present invention should be determined only by the matters to define the invention in the scope of claims regarded as appropriate based on the description. 
     This application claims the benefit of priority from Japanese Patent Application No. 2010-95546 (filed on Apr. 16, 2010), the entire contents of which are incorporated herein by reference. 
     INDUSTRIAL APPLICABILITY 
     As described, the radio communication system, the high-power base station, the low-power base station, and the communication control method according to the present invention are capable of suppressing interference between the base stations even when the coverage of the low-power base station is expanded, and therefore are useful in radio communication such as mobile communication.