Patent Publication Number: US-2015071200-A1

Title: Communication system, base station apparatus and communication method

Description:
TECHNICAL FIELD 
     The present invention relates to a communication system, a base station apparatus and a communication method that are applicable to a cellular system and so on. 
     BACKGROUND ART 
     In a UMTS (Universal Mobile Telecommunications System) network, long-term evolution (LTE) has been under study for the purposes of further increasing high-speed data rates, providing low delay and so on (non-patent literature 1). In LTE, as multiple access schemes, a scheme that is based on OFDMA (Orthogonal Frequency Division Multiple Access) is used on downlink channels (downlink), and a scheme that is based on SC-FDMA (Single Carrier Frequency Division Multiple Access) is used on uplink channels (uplink). 
     Also, successor systems of LTE, referred to as “LTE-Advanced” or “LTE enhancement” (hereinafter “LTE-A”), are under study for the purposes of achieving further broadbandization and increased speed beyond LTE. In LTE-A (Rel-10), carrier aggregation to group a plurality of component carriers (CCs), where the system band of the LTE system is one unit, for broadbandization, is used. Also, in LTE-A, a HetNet (Heterogeneous Network) configuration to use an interference coordination technique (eICIC: enhanced Inter-Cell Interference Coordination) is under study. 
     CITATION LIST 
     Non-Patent Literature 
     
         
         Non-Patent Literature 1: 3GPP TR 25.913 “Requirements for Evolved UTRA and Evolved UTRAN” 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     Now, future systems (Rel-11 and later versions) anticipate carrier aggregation to take into account improvement of spectral efficiency and reduction of interference caused in a HetNet. Although carrier aggregation will also be expected to make effective use of conventional CRSs (Cell-specific Reference Signals), in this case, there is a threat that problems might arise from the perspective of reduction of interference to be caused. 
     The present invention has been made in view of the above, and it is therefore an object of the present invention to provide a communication system, a base station apparatus and a communication method that are suitable for carrier aggregation in a HetNet. 
     Solution to Problem 
     The communication system of the present invention is a communication system which provides a first transmission point and a plurality of second transmission points, and which controls carriers such that a mobile terminal apparatus communicates with the first transmission point using a first carrier and communicates with a second transmission point using a second carrier which is different from the first carrier, and, in this communication system, base station apparatuses to constitute the second transmission points comprise a transmission section that transmits a cell-specific reference signal in the second carrier, using the same frequency resource between the second transmission points, and the mobile terminal apparatus comprises a receiving section that receives a reference signal transmitted from the second transmission point by the second carrier. 
     Technical Advantage of the Invention 
     According to the present invention, it is possible to make the positions to arrange reference signals of additional carrier types different than heretofore, so that it is possible to reduce the interference from the reference signals. By this means, it is possible to make effective use of conventional systems and furthermore achieve a communication system, a base station apparatus and a communication method that are suitable for carrier aggregation in a HetNet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram to explain a system band in an LTE-A system; 
         FIG. 2  is a diagram to show an example of carrier aggregation in a HetNet; 
         FIG. 3  is a diagram to show an example of carrier aggregation using an additional carrier type; 
         FIG. 4  provides diagrams to show examples of radio resource allocation in an additional carrier type; 
         FIG. 5  provides a diagram to show examples of radio resource allocation at a base station apparatus in carrier aggregation; 
         FIG. 6  provides diagrams to show a first example of an additional carrier type; 
         FIG. 7  provides diagrams to show a second example of an additional carrier type; 
         FIG. 8  provides diagrams to show a third example of an additional carrier type; 
         FIG. 9  provides diagrams to show a fourth example of an additional carrier type; 
         FIG. 10  is a diagram to explain a system configuration of a radio communication system; 
         FIG. 11  is a diagram to explain an overall configuration of a base station apparatus; 
         FIG. 12  is a diagram to explain an overall configuration of a mobile terminal apparatus; 
         FIG. 13  is a functional block diagram of a baseband signal processing section provided in a base station apparatus and part of higher layers; and 
         FIG. 14  is a functional block diagram of a baseband signal processing section provided in a mobile terminal apparatus. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a diagram to show a layered bandwidth configuration defined in LTE-A. The example shown in  FIG. 1  is a layered bandwidth configuration that is used when an LTE-A system having a first system band formed with a plurality of fundamental frequency blocks (hereinafter “component carriers”) and an LTE system having a second system band formed with one component carrier coexist. In the LTE-A system, for example, radio communication is performed in a variable system bandwidth of 100 MHz or below, and, in the LTE system, for example, radio communication is performed in a variable system bandwidth of 20 MHz or below. The system band of the LTE-A system includes at least one component carrier, where the system band of the LTE system is one unit. Widening the band by way of gathering a plurality of component carriers in this way is referred to as “carrier aggregation.” 
     For example, in  FIG. 1 , the system band of the LTE-A system is a system band to include bands of five component carriers (20 MHz×5=100 MHz), where the system band (base band: 20 MHz) of the LTE system is one component carrier. In  FIG. 1 , mobile terminal apparatus UE (User Equipment) #1 is a mobile terminal apparatus to support the LTE-A system (and also support the LTE system), and is able to support a system band up to 100 MHz. UE #2 is a mobile terminal apparatus to support the LTE-A system (and also support the LTE system), and is able to support a system band up to 40 MHz (20 MHz×2=40 MHz). UE #3 is a mobile terminal apparatus to support the LTE system (and not support the LTE-A system), and is able to support a system band up to 20 MHz (base band). 
     Future systems (Rel-11 and later versions) anticipate extension of carrier aggregation, for specific use with respect to a HetNet. To be more specific, system configurations such as the one shown in  FIG. 2  may be possible.  FIG. 2  is a diagram to show an example of carrier aggregation in a HetNet. 
     The system shown in  FIG. 2  is configured in layers with a macro base station apparatus eNB (eNodeB) and a plurality of base station apparatus RRHs (Remote Radio Heads). Inside the cell of the macro base station apparatus eNB (first transmission point), small cells are formed locally by the base station apparatus RRHs (second transmission points). A mobile terminal apparatus UE is located in the small cell of base station apparatus RRH #1, and communicates with the macro base station apparatus eNB and base station apparatus RRH #1 by means of carrier aggregation. For example, carrier aggregation is executed using component carrier CC #1 of the macro base station apparatus eNB as a P-cell (Primary cell) and using component carrier CC #2 of base station apparatus RRH #1 as an S-cell. 
     To carry out carrier aggregation, the mobile terminal apparatus UE needs to find (detect) a base station apparatus RRH (S-cell) by inter-frequency measurement, while being connected with the macro base station apparatus eNB. After having captured synchronization with a PSS/SSS (Primary Synchronization Signal/Secondary Synchronization Signal), which are synchronization signals, a mobile terminal apparatus UE of Rel-10 or earlier versions measures the inter-frequency received quality from each base station apparatus RRH based on CRSs. Then, the measured signal quality of each base station apparatus RRH and a predetermined target value are compared, and a base station apparatus RRH (S-cell) of good received quality is detected. 
     Now, in Rel-11, carriers without compatibility with legacy component carriers of carrier aggregation are under study, and these may be effective in a HetNet where carrier aggregation is applied. A carrier without compatibility with legacy component carriers may be referred to as an “additional carrier type” or may be referred to as an “extension carrier.” 
       FIG. 3  is a diagram to show an example of carrier aggregation using an additional carrier type. Note that, in  FIG. 3 , CC #1 of the macro base station apparatus eNB is set in a legacy carrier type and CC #2 of a base station apparatus RRH is set in an additional carrier type. Note that  FIG. 3  only shows CRSs, a PDCCH (Physical Downlink Control Channel), and a PDSCH (Physical Downlink Shared Channel), for ease of explanation. Also, the bandwidth of an additional carrier type does not have to use the system band of the LTE system (base band: 20 MHz) as one unit, and can be changed as appropriate. 
     As shown in  FIG. 3 , in the legacy carrier type, a PDCCH is set over three symbols from the top of one RB (Resource Block) defined in LTE. Also, in the legacy carrier type, in one RB, CRSs are set not to overlap with user data and other reference signals such as DM-RSs (Demodulation-Reference Signals). The CRSs are used to demodulate user data, and, besides, used to measure downlink channel quality information (CQI: Channel Quality Indicator) for scheduling and adaptive control, and used to measure the average downlink propagation path state for a cell search and handover (mobility measurement) and so on. 
     By contrast with this, an additional carrier type is able to make the CRSs and PDCCH subject to non-transmission, for example. This additional carrier type is not supported by conventional mobile terminal apparatus UEs (Rel-10 and earlier versions) and is expected to be supported only by new mobile terminal apparatus UEs (Rel-11 and later versions). Also, the additional carrier type is expected to be used primarily in S-cells (Secondary cells). 
     In this way, by executing carrier aggregation to make CRSs and PDCCH subject to non-transmission in an additional carrier type, it is possible to reduce the interference by the CRSs. That is, by making CRSs subject to non-transmission in an additional carrier type, it is possible to reduce the interference caused by the CRSs from neighboring base station apparatus RRHs. Also, it is possible to transmit user data and so on using the CRS and PDCCH radio resources, so that it is possible to improve spectral efficiency as well. 
     By contrast with this, from the perspective of making effective re-use of conventional (Rel-10 and earlier versions) resources (hardware and software), there is a demand to actively utilize CRSs, instead of making CRSs subject to non-transmission, even in an additional carrier type. However, when conventional CRSs are applied to an additional carrier type on an as-is basis, the interference which the CRSs cause may grow, and there is a threat of creating disadvantages in terms of transmission power and so on. So, reducing the radio resources to use to transmit CRSs in an additional carrier type compared with a legacy carrier type is under study. 
       FIG. 4  provides diagrams to show examples of radio resource allocation in an additional carrier type.  FIG. 4A  shows an example where the radio resources for CRSs are reduced in the time direction, and  FIG. 4B  shows an example where the radio resources for CRSs are reduced in the frequency direction. Note that  FIG. 4  only shows CRSs, PDSCHs, and PSSs/SSSs for ease of explanation. 
     In  FIG. 4A , the PSSs/SSSs, which are synchronization signals, are transmitted every four subframes (that is, in a five-subframe cycle), and the CRSs are also transmitted every four subframes (in a five-subframe cycle). The CRSs are not transmitted in the other subframes. The PDSCHs are allocated to the first subframe, the second subframe, and the third to sixth subframes, and are used to transmit user data and so on. In this case, CRSs are not transmitted outside predetermined subframes, so that it is possible to reduce the interference caused by the CRSs and furthermore reduce transmission power. Note that the transmission cycle of CRSs is by no means limited to this. 
     In  FIG. 4B , CRSs are transmitted in six RBs in the center, and are not transmitted in the other frequency resources (frequency positions). The PDSCHs are allocated to the first subframe, the second subframe and the third to sixth subframes, and are used to transmit user data and so on. In this case, CRSs are not transmitted in frequency resources other than the six RBs in the center, so that it is possible to reduce the interference caused by the CRSs and furthermore reduce transmission power. Note that CRSs do not necessarily have to be transmitted in six RBs in the center. CRSs may be transmitted in other frequency ranges as well. 
       FIG. 5  provides diagrams to show examples of radio resource allocation at base station apparatus RRHs in carrier aggregation. For ease of explanation,  FIG. 5  only shows radio resources of base station apparatus RRH #1 used as an S-cell and radio resources of base station apparatus RRH #2 that is close to base station apparatus RRH #1. Also, as in  FIG. 4A , a case is shown here as an example where an additional carrier type to reduce the radio resources for CRSs in the time direction is applied. 
     Each base station apparatus RRH transmits CRSs in varying frequency resources so as not to interfere with each other. In systems of Rel-10 and earlier versions, each base station apparatus RRH transmits CRSs using frequency resources that are shifted by a predetermined amount, in the frequency domain, with respect to the frequency resources of reference signals transmitted from the macro base station apparatus eNB. That is to say, CRSs that are transmitted from each base station apparatus RRH are shifted in the frequency direction from the CRS of the macro base station apparatus eNB. The amount of shift, V shift , is determined based on cell-specific IDs (cell IDs) (V shift =(cell ID mod 6)). 
     In this case, as shown in  FIG. 5 , the CRS of base station apparatus RRH #1 and the CRS of base station apparatus RRH #2 are transmitted in different frequency resources, and therefore interference is not produced between the CRSs. However, with this additional carrier type, cases might occur where the PDSCH in base station apparatus RRH #1 is allocated in a way to match the frequency resources where the CRS in the base station apparatus RRH #2 is transmitted. Consequently, there is a threat that the CRS of base station apparatus RRH #2 interferes with the PDSCH of base station apparatus RRH #1. Likewise, in an additional carrier type in which the radio resources for CRSs are reduced in the frequency direction, there is a threat that the CRS of a base station apparatus RRH and the PDSCH of another base station apparatus RRH interfere with each other. 
     In view of this problem, the present inventors have focused on the fact that, in an additional carrier type, it is not strictly necessary to use CRSs in data demodulation, and made the present invention. Unless CRSs are used to demodulate data, interference between the CRSs can be tolerated to an extent, so that it is possible to allow flexibility in the arrangement of CRSs. That is, a gist of the present invention is to reduce the interference between the CRS of a base station apparatus RRH and the PDSCH of another base station apparatus RRH by making the arrangement of CRSs of an additional carrier type different than heretofore. 
     Now, additional carrier types to provide new CRS transmission patterns will be described below with reference to  FIG. 6  to  FIG. 9 .  FIG. 6  provides diagrams to show a first example of an additional carrier type.  FIG. 7  provides diagrams to show a second example of an additional carrier type.  FIG. 8  provides diagrams to show a third example of an additional carrier type.  FIG. 9  provides diagrams to show a fourth example of an additional carrier type. Note that, although  FIG. 6  to  FIG. 9  show only radio resources for base station apparatus RRH #1 that is used as an S-cell and radio resources for base station apparatus RRH #2 that is close to base station apparatus RRH #1, for ease of explanation, there may be other base station apparatus RRHs to form small cells.  FIG. 6  to  FIG. 9  show only CRS, PDSCH, and PSS/SSS allocation patterns schematically. 
     As shown in  FIG. 6 , in the first example, an additional carrier type to reduce the radio resources for CRSs in the time direction is applied (see  FIG. 4A ). In the first example, the frequency resources to transmit CRSs are set independently of cell IDs, and the CRSs are transmitted in the same frequency resources in all the base station apparatus RRHs. For example, by making the amount of shift to apply to the CRS of the macro base station apparatus eNB fixed regardless of cell IDs (V shift =C: C is a constant), it is possible to transmit CRSs in the same frequency resources. 
     To be more specific, in base station apparatus RRH #1 and base station apparatus RRH #2, the CRSs are transmitted using the same radio resources that overlap in the time direction and in the frequency direction. Also, the CRSs are transmitted at four-subframe intervals (in a five-subframe cycle). The PSSs/SSSs are transmitted in the same subframe with the CRSs. That is to say, the PSSs/SSSs are also transmitted at four-subframe intervals (in a five-subframe cycle). The PDSCH is allocated to the first subframe to the tenth subframe in base station apparatus RRH #1, and is allocated to the ninth subframe and the tenth subframe in base station apparatus RRH #2. However, the CRS transmission interval (transmission cycle) and the PDSCH arrangement are by no means limited to these. 
     With the first example, CRSs are transmitted using the same radio resources that overlap in the time direction and in the frequency direction in all the additional carrier types, so that the CRS of a base station apparatus RRH and the PDSCH of another base station apparatus RRH are never transmitted in the same radio resources. Consequently, it is possible to reduce the interference from the CRS of the base station apparatus RRH to the PDSCH of the other base station apparatus RRH. In this case, although there is a threat that the CRS of the base station apparatus RRH and the CRS of the other base station apparatus RRH interfere with each other, DM-RSs can be used in data demodulation, so that no problem arises in this regard. CRSs that are transmitted in this additional carrier type are received in a mobile terminal apparatus UE, and can be used, for example, in symbol synchronization and channel quality measurement. 
     Note that when interference from CRSs to PDSCHs poses no problem, the radio resources to use to transmit the CRSs may be changed per base station apparatus RRH. In this case, for example, the subframe numbers and frequency positions of the CRSs are reported to the mobile terminal apparatus UE by higher layer signaling. By this means, it is possible to allow flexibility in CRS transmission, so that it is possible to reduce the interference from the CRS of a base station apparatus RRH to the PDSCH of another base station apparatus RRH, and, furthermore, reduce the interference from the CRS of the base station apparatus RRH to the CRS of the other base station apparatus RRH. 
     As shown in  FIG. 7 , in the second example, an additional carrier type to reduce the radio resources for CRSs in the frequency direction is applied (see  FIG. 4B ). In the second example, too, the frequency resources to transmit CRSs are set independently of cell IDs, and the CRSs are transmitted in the same frequency resources in all the base station apparatus RRHs. Similar to the first example, assume that the amount of shift to apply to the CRS of the macro base station apparatus eNB is fixed regardless of cell IDs (V shift =C: C is a constant). 
     To be more specific, in both base station apparatus RRH #1 and base station apparatus RRH #2, CRSs are transmitted using six RBs in the center. That is, CRSs are transmitted using the same radio resources that overlap in the time direction and in the frequency direction in all the additional carrier types. However, the frequency range of the radio resources to use to transmit CRSs is by no means limited to this. The arrangement of the PSS/SSS and the PDSCH is the same as in  FIG. 6 . However, the PDSCH arrangement is by no means limited to this. 
     With the second example, too, CRSs are transmitted using the same radio resources that overlap in the time direction and in the frequency direction in all the additional carrier types, so that the CRS of a base station apparatus RRH and the PDSCH of another base station apparatus RRH are never transmitted in the same radio resources. Consequently, similar to the first example, it is possible to reduce the interference from the CRS of the base station apparatus RRH to the PDSCH of the other base station apparatus RRH. CRSs that are transmitted in this additional carrier type are received in a mobile terminal apparatus UE, and can be used, for example, in symbol synchronization and channel quality measurement. With the second example, too, when interference from CRSs to PDSCHs poses no problem, the radio resources to use to transmit the CRSs may be changed per base station apparatus RRH. 
     This second example may be combined with the first example and used. That is to say, it is possible to use an additional carrier type that reduces the radio resources for CRSs in the time direction and in the frequency direction. In this case, too, by setting the frequency resources to transmit CRSs independently of cell IDs, it is possible to transmit CRSs in the same frequency resources in all the base station apparatus RRHs. By this means, it is possible to reduce the interference from the CRS of a base station apparatus RRH to the PDSCH of another base station apparatus RRH. 
     As shown in  FIG. 8 , with the third example, an additional carrier type to reduce the radio resources for CRSs in the time direction and in the frequency direction is applied. With the third example, CRSs are arranged in accordance with a reference signal resource arrangement pattern that is common between each base station apparatus RRH. The CRSs are transmitted using frequency resources in part of the reference signal resource arrangement pattern and are arranged such that the frequency resources do not overlap between the base station apparatus RRHs. Zero-power CRSs are arranged in frequency resources in the reference signal resource arrangement pattern where CRSs are not arranged, and CRSs are not transmitted there. 
     For example, in base station apparatus RRH #1, the CRS is transmitted in six RBs in the center and is not transmitted in the other frequency resources. Also, in base station apparatus RRH #2, the CRS is transmitted in different six RBs and is not transmitted in the other frequency resources. In the reference signal resource arrangement pattern, frequency resources where the CRSs are subject to non-transmission are not used to transmit other signals. That is to say, these frequency resources are subject to zero-power transmission (that is, zero-power reference signals are arranged in these frequency resources). In the third example, CRSs are transmitted at four-subframe intervals (in a five-subframe cycle). However, the transmission frequency and the transmission cycle of the CRSs are by no means limited to these. The arrangement of the PSS/SSS and the PDSCH is the same as in  FIG. 6  and so on. However, for example, the PDSCH arrangement is by no means limited to this. 
     With the third example, the CRS of each base station apparatus RRH is shifted in units of six-RB, such that the frequency resources for CRSs do not overlap between the base station apparatus RRHs. Frequency resources where the CRS is transmitted in a given base station apparatus RRH overlaps with frequency resources that are subject to zero-power transmission in another base station apparatus RRH. To be more specific, the frequency resources where the CRS is transmitted at base station apparatus RRH #1 and the frequency resources that are subject to zero-power transmission at base station apparatus RRH #2 overlap. Also, the frequency resources in which the CRS is transmitted at base station apparatus RRH #2 and the frequency resources that are subject to zero-power transmission at base station apparatus RRH #1 overlap. 
     In this way, with the third example, the frequency resources to transmit the CRS at a base station apparatus RRH and the frequency resources to be subject to zero-power transmission at another base station apparatus RRH overlap, so that the CRS of the base station apparatus RRH and the PDSCH of the other base station apparatus are never transmitted in the same frequency resources. By this means, it is possible to reduce the interference from the CRS of a base station apparatus RRH to the PDSCH of another base station apparatus RRH. Also, since the frequency resources to transmit the CRSs do not overlap between the base station apparatus RRHs, interference between the CRSs can be reduced. 
     The subframe numbers and frequency positions of the radio resources where CRSs are transmitted in each base station apparatus RRH are reported to a mobile terminal apparatus UE through higher layer signaling. Also, the amount of shift to apply to CRSs at each base station apparatus RRH can be determined, for example, based on following equation 1: 
       [1] 
       6·(N ID   cell  mod[N RB   DL /6])  (Equation 1)
 
     As shown in  FIG. 9 , the fourth example is equivalent to a modification of the third example. That is to say, although, with the third example, the PSSs/SSSs are transmitted in the same frequency resources in all the additional carrier types, with the fourth example, the PSSs/SSSs are transmitted in varying frequency resources. To be more specific, the frequency resources to transmit the PSSs/SSSs are selected to match the six RBs to transmit CRSs in each base station apparatus RRH. However, it is equally possible to set the frequency positions of the PSSs/SSSs independently of the frequency positions where CRSs are transmitted. 
     With the fourth example, too, similar to the third example, the CRS of each base station apparatus RRH is shifted in units of six-RB, such that the frequency resources where a given base station apparatus RRH transmits the CRS overlap with the frequency resources that are subject to zero-power transmission at another base station apparatus RRH. That is to say, the CRS of one base station apparatus RRH and the PDSCH of another base station apparatus are never transmitted in the same frequency resources, so that it is possible to reduce the interference from the CRS of the base station apparatus RRH to the PDSCH of the other base station apparatus RRH. Also, since the frequency resources to transmit CRSs do not overlap between the base station apparatus RRHs, it is possible to reduce the interference between the CRSs. 
     Next, the radio communication system according to the present embodiment will be described.  FIG. 10  is a diagram to explain a system configuration of a radio communication system according to the present embodiment. Note that the radio communication system shown in  FIG. 10  is a system to accommodate, for example, an LTE system or its successor system. In this radio communication system, carrier aggregation to group a plurality of fundamental frequency blocks as one, where the system band of the LTE system is one unit, is used. Also, this radio communication system may be referred to as “IMT-Advanced” or may be referred to as “4G.” 
     As shown in  FIG. 10 , the radio communication system is a HetNet, where a base station apparatus  20 A (first transmission point) of a cell C1, and a plurality of base station apparatuses  20 B (second transmission points) of cells C2 that are provided in the cell C1 build a layered network. The base station apparatus  20 A is commonly referred to as a macro base station apparatus, and covers the large cell C1. The base station apparatuses  20 B are base station apparatuses (commonly referred to as RRH base station apparatuses), and form the small cells C2, locally, inside the cell C1. The base station apparatus  20 A and each base station apparatus  20 B are connected with each other by wire connection or by wireless connection. The mobile terminal apparatuses  10  are able to communicate with the base station apparatuses  20 A and  20 B in the cell C1 and the cell C2, respectively. Also, the base station apparatus  20 A is connected with a core network  30  via a higher station apparatus. 
     Note that the higher station apparatus may be, for example, an access gateway apparatus, a radio network controller (RNC), a mobility management entity (MME) and so on, but is by no means limited to these. Each mobile terminal apparatus  10  may be either a conventional mobile terminal apparatus (Rel-10 or earlier versions) or a new mobile terminal apparatus (Rel-11 or later versions), but the following description will be given simply with respect to a mobile terminal apparatus, unless specified otherwise. Also, it is each mobile terminal apparatus  10  that will be described to perform radio communication with the base station apparatuses  20 A and  20 B for ease of explanation, more generally, user equipment (UE), which includes both mobile terminal apparatuses and fixed terminal apparatuses, may be used as well. 
     This radio communication system supports carrier aggregation specialized for a HetNet. In this case, a mobile terminal apparatus  10  captures synchronization with the PSS/SSS from each base station apparatus  20 B and receives the CRSs, while being connected with the base station apparatus  20 A. The scrambling code to apply to the CRS varies between base station apparatuses  20 B (between RRHs), and the scrambling code can be determined from the cell ID acquired from the PSS/SSS. Consequently, it is possible to identify the CRS from each base station apparatus  20 B (RRH) based on cell IDs. The mobile terminal apparatus  10  measures the signal quality from each base station apparatus  20 B based on the CRSs received, and feeds back the measurement result to the base station apparatus  20 A. Then, in accordance with the feedback from the mobile terminal apparatus  10 , the base station apparatus  20 A detects a base station apparatus  20 B of good received quality as an S-cell, and executes carrier aggregation. 
     In the radio communication system, as radio access schemes, OFDMA (Orthogonal Frequency Division Multiple Access) is applied to the downlink, and SC-FDMA (Single-Carrier Frequency-Division Multiple Access) is applied to the uplink. OFDMA is a multi-carrier transmission scheme to perform communication by dividing a frequency band into a plurality of narrow frequency bands (subcarriers) and mapping data to each subcarrier. SC-FDMA is a single carrier transmission scheme to reduce interference between terminals by dividing, per terminal, the system band into bands formed with one or continuous resource blocks, and allowing a plurality of terminals to use mutually different bands. 
     Here, communication channels will be described. Downlink communication channels include a PDSCH that is used by each mobile terminal apparatus  10  on a shared basis, and downlink L1/L2 control channels (PDCCH, PCFICH, PHICH). User data and higher control information are transmitted by the PDSCH. PDSCH and PUSCH (Physical Uplink Shared Channel) scheduling information and so on are transmitted by the PDCCH. The number of OFDM symbols to use for the PDCCH is transmitted by the PCFICH (Physical Control Format Indicator Channel). HARQ ACK and NACK for the PUSCH are transmitted by the PHICH (Physical Hybrid-ARQ Indicator Channel). 
     Uplink communication channels include a PUSCH, which is used by each mobile terminal apparatus  10  on a shared basis as an uplink data channel, and a PUCCH (Physical Uplink Control Channel), which is an uplink control channel. User data and higher control information are transmitted by the PUSCH. Also, downlink channel quality information (CQI), ACK/NACK and so on are transmitted by the PUCCH. 
     An overall configuration of the base station apparatuses  20 A and  20 B according to the present embodiment will be described with reference to  FIG. 11 . Note that the baseband processing is not executed in the base station apparatus  20 B, and the base station apparatus  20 B receives a baseband signal from the base station apparatus  20 A and reports this to the mobile terminal apparatus  10 . 
     The base station apparatus  20 A has a transmitting/receiving antenna  201 A, an amplifying section  202 A, a transmitting/receiving section  203 A, a baseband signal processing section  204 A, a call processing section  205 A, and a transmission path interface  206 A. Also, the base station apparatus  20 B has a transmitting/receiving antenna  201 B, an amplifying section  202 B, and a transmitting/receiving section  203 B. Transmission data to be transmitted from the base station apparatuses  20 A and  20 B to the mobile terminal apparatus  10  on the downlink is input from the higher station apparatus into the baseband signal processing section  204 A via the transmission path interface  206 A. 
     In the baseband signal processing section  204 A, a signal of a downlink data channel is subjected to a PDCP layer process, division and coupling of user data, RLC (Radio Link Control) layer transmission processes such as an RLC retransmission control transmission process, MAC (Medium Access Control) retransmission control, including, for example, an HARQ transmission process, scheduling, transport format selection, channel coding, an inverse fast Fourier transform (IFFT) process, and a precoding process. Furthermore, a signal of a downlink control channel is also subjected to transmission processes such as channel coding and an inverse fast Fourier transform. 
     Also, the baseband signal processing section  204 A reports control information for allowing the mobile terminal apparatuses  10  to perform radio communication with the base station apparatuses  20 A and  20 B, to the mobile terminal apparatuses  10  connected to the same cell, by a broadcast channel. The information for communication in the cell includes, for example, the uplink or downlink system bandwidth, root sequence identification information (root sequence index) for generating random access preamble signals in the PRACH (Physical Random Access Channel), and so on. 
     In this case, the baseband signal of CC #1 is output from the baseband signal processing section  204 A to the transmitting/receiving section  203 A, and the baseband signal of CC #2 is output from the baseband signal processing section  204 A to the transmitting/receiving section  203 B of the base station apparatus  20 B via optical fiber. The baseband signals that are output from the baseband signal processing section  204 A are converted into a radio frequency band in the transmitting/receiving sections  203 A and  203 B. The amplifying sections  202 A and  202 B amplify the radio frequency signals having been subjected to frequency conversion, and transmit the results from the transmitting/receiving antennas  201 A and  201 B. 
     Meanwhile, as for data to be transmitted from the mobile terminal apparatus  10  to the base station apparatuses  20 A and  20 B on the uplink, radio frequency signals received in the transmitting/receiving antennas  201 A and  201 B of the base station apparatuses  20 A and  20 B are amplified in the amplifying sections  202 A and  202 B, converted into baseband signals through frequency conversion in the transmitting/receiving sections  203 A and  203 B and input in the baseband signal processing section  204 A. 
     The baseband signal processing section  204 A applies, to the transmission data included in the baseband signal received as input, an fast Fourier transform (FFT) process, an inverse discrete Fourier transform (IDFT) process, error correction decoding, a MAC retransmission control receiving process, and RLC layer and PDCP layer receiving processes. The baseband signals are transferred to the higher station apparatus via the transmission path interface  206 A. The call processing section  205 A performs call processing such as setting up and releasing communication channels, manages the state of the base station apparatuses  20 A and  20 B and manages the radio resources. 
     Next, an overall configuration of a mobile terminal apparatus according to the present embodiment will be described with reference to  FIG. 12 . The mobile terminal apparatus  10  has a transmitting/receiving antenna  101 , an amplifying section  102 , a transmitting/receiving section  103 , a baseband signal processing section  104 , and an application section  105 . 
     As for downlink data, a radio frequency signal that is received in the transmitting/receiving antenna  101  is amplified in the amplifying section  102 , and subjected to frequency conversion and converted into a baseband signal in the transmitting/receiving section  103 . This baseband signal is subjected to receiving processes such as an FFT process, error correction decoding and retransmission control, in the baseband signal processing section  104 . In this downlink data, downlink user data is transferred to the application section  105 . The application section  105  performs processes related to higher layers above the physical layer and the MAC layer. Also, in the downlink data, broadcast information is also transferred to the application section  105 . 
     Meanwhile, uplink transmission data is input from the application section  105  to the baseband signal processing section  104 . The baseband signal processing section  104  performs a mapping process, a retransmission control (H-ARQ) transmission process, channel coding, a discrete Fourier transform (DFT) process, and an IFFT process. The baseband signal that is output from the baseband signal processing section  104  is converted into a radio frequency band in the transmitting/receiving section  103 , and, after that, amplified in the amplifying section  102  and transmitted from the transmitting/receiving antenna  101 . 
       FIG. 13  is a functional block diagram of a baseband signal processing section  204 A provided in the base station apparatus  20 A according to the present embodiment and part of the higher layers, and primarily illustrates the function blocks for transmission processes in the baseband signal processing section  204 A. Transmission data for the mobile terminal apparatus  10  under the base station apparatus  20 A is transferred from the higher station apparatus to the base station apparatus  20 A. Note that  FIG. 13  shows a case where the base station apparatus  20 A uses two of CC #1 and CC #2. Obviously, the number of CCs each base station apparatus  20  uses is not limited to this. Also, assume that CC #1 of the base station apparatus  20 A is set in a legacy carrier type, and CC #2 is set in an additional carrier type. 
     A control information generating section  300  generates, per user, higher control information to report to the mobile terminal apparatus  10  through higher layer signaling. The higher control information may include information about the radio resources to use to transmit CRSs in an additional carrier type. For example, in the first example or the second example, it is possible to include information such as the subframe numbers and frequency positions of the radio resources to use to transmit CRSs. Also, it is equally possible to include information related to V shift  in the higher control information. In particular, when changing the radio resources to use to transmit CRSs on a per base station apparatus basis, it is preferable to include information about the radio resources to use to transmit CRSs in higher control information. Also, in the third example or the fourth example, it is possible to include information such as the subframe numbers and frequency positions of the radio resources to use to transmit CRSs. In this way, by including information about the radio resources to use to transmit CRSs in higher control information, the transmitting source of the CRSs can be identified. 
     A data generating section  301  outputs transmission data transferred from the higher station apparatus, as user data, on a per user basis. A component carrier selection section  302  selects, on a per mobile terminal apparatus  10  basis, the component carriers to use for radio communication with the mobile terminal apparatus  10 . When carrier aggregation is performed, CC #1 of the base station apparatus  20 A is a P-cell and an S-cell is selected from CC #2 of other base station apparatuses  20 B connected via optical fiber  319 . An increase/decrease of component carriers is reported from the base station apparatus  20 A to the mobile terminal apparatus  10  by higher layer signaling, and a message of completion of application is received from the mobile terminal apparatus  10 . 
     A scheduling section  310  controls the allocation of component carriers to a serving mobile terminal apparatus  10  according to the overall communication quality of the system band. The scheduling section  310  performs scheduling separately between LTE terminal users and LTE-A terminal users. The scheduling section  310  receives as input the data to transmit and retransmission commands from the higher station apparatus, and also receives as input the channel estimation values and resource block CQIs from the receiving section having measured an uplink signal. 
     Also, the scheduling section  310  schedules downlink control channel signals and downlink shared channel signals with reference to the retransmission commands, the channel estimation values and the CQIs received as input. A propagation path in radio communication varies differently per frequency, due to frequency selective fading. So, the scheduling section  310  designates resource blocks (mapping positions) of good communication quality, on a per subframe basis, with respect to the downlink data for each mobile terminal apparatus  10  (which is referred to as “adaptive frequency scheduling”). In adaptive frequency scheduling, for each resource block, a mobile terminal apparatus  10  of good propagation path quality is selected. Consequently, the scheduling section  310  designates resource blocks (mapping positions), using the CQI of each resource block, fed back from each mobile terminal apparatus  10 . 
     Likewise, the scheduling section  310  designates resource blocks of good communication quality, on a per subframe basis, with respect to the control information and so on transmitted by the PDCCH by adaptive frequency scheduling. Consequently, the scheduling section  310  designates resource blocks (mapping positions) using the CQI of each resource block fed back from each mobile terminal apparatus  10 . Also, the MCS (coding rate and modulation scheme) to fulfill a predetermined block error rate with the allocated resource blocks is determined. Parameters to fulfill the MCS (coding rate and modulation scheme) determined in the scheduling section  310  are set in channel coding sections  303  and  308 , and modulation sections  304  and  309 . Note that adaptive frequency scheduling is applied not only to the base station apparatus  20 A but is also applied to the base station apparatuses  20 B as well via the optical fiber  319 . 
     When carrier aggregation is performed, the scheduling section  310  designates the radio resources for transmitting the CRS in an S-cell in accordance with the additional carrier type that is applied. For example, when the additional carrier type according to the first example is applied, the scheduling section  310  commands that CRSs be transmitted at predetermined subframe intervals using frequency resources that overlap between all the base station apparatuses  20 B. When the additional carrier type according to the second example is applied, the scheduling section  310  commands that CRSs be transmitted in a predetermined frequency range using frequency resources that overlap between all the base station apparatuses  20 B. 
     Also, when the additional carrier type according to the third example or the fourth example is applied, the scheduling section  310  commands that CRSs be arranged in part of a reference signal resource arrangement pattern that is common between the base station apparatuses  20 B. Also, the scheduling section  310  commands that the frequency resources for CRSs not overlap between the base station apparatuses  20 B. Furthermore, the scheduling section  310  commands that the resources in the reference signal resource arrangement pattern of each base station apparatus  20 B where CRSs are not arranged be made subject to zero-power transmission (zero-power CRSs). 
     The baseband signal processing section  204 A has channel coding sections  303 , modulation sections  304  and mapping sections  305  to support the maximum number of users to multiplex, N, in one CC. The channel coding sections  303  perform channel coding of the downlink shared data channel (PDSCH), which is formed with downlink data (including part of higher control signals) that is output from the data generating section  301 , on a per user basis. The modulation sections  304  modulate user data having been subjected to channel coding, on a per user basis. The mapping sections  305  map the modulated user data to radio resources. 
     Also, the baseband signal processing section  204 A has a downlink control information generating section  306  that generates downlink control information, channel coding sections  308 , and modulation sections  309 . In the downlink control information generating section  306 , an uplink shared data channel control information generating section  306   b  generates uplink scheduling grants (UL grants) for controlling an uplink data channel (PUSCH). The uplink scheduling grants are generated on a per user basis. 
     Also, a downlink shared data channel control information generating section  306   c  generates downlink scheduling assignments (DL assignments) for controlling a downlink data channel (PDSCH). The downlink scheduling assignments are generated on a per user basis. Also, a shared channel control information generating section  306   a  generates shared control channel control information, which is downlink control information that is common between users. 
     Control information that is modulated in the modulation sections  309  on a per user basis is multiplexed in a control channel multiplexing section  314  and furthermore interleaved in an interleaving section  315 . A control signal that is output from the interleaving section  315  and user data that is output from the mapping sections  305  are input in an IFFT section  316  as downlink channel signals. 
     The baseband signal processing section  204 A (CC #2) for the base station apparatus  20 B has a reference signal generating section (generating section)  318  that generates downlink reference signals. The reference signal generating section  318  generates the CRSs that are transmitted at each base station apparatus  20 B. Note that the reference signal generating section  318  may generate DM-RSs for downlink demodulation, CSI-RSs for CSI measurement, and so on. 
     The IFFT section  316  receives as input control signals from the interleaving section  315  and receives as input user data from the mapping sections  305 , as downlink channel signals. Furthermore, the IFFT section  316  (CC #2) for the base station apparatus  20 B receives as input the downlink reference signals from the reference signal generating section  318 . The IFFT section  316  performs an inverse fast Fourier transform of the downlink channel signal and the downlink reference signal and converts frequency domain signals into time sequence signals. A cyclic prefix inserting section  317  inserts cyclic prefixes in the time sequence signal of the downlink channel signals. Note that a cyclic prefix functions as a guard interval for cancelling the differences in multipath propagation delay. Transmission data, to which cyclic prefixes have been added, is transmitted to the transmitting/receiving sections  203 A and  203 B. 
     Note that, in  FIG. 13 , in CC #2, all the subframes may be set in an additional carrier type, or predetermined subframes may be set in an additional carrier type and the rest of the subframes may be set in a legacy carrier type. In this case, it is possible to connect not only new mobile terminal apparatuses (Rel-11 and later versions) to CC #2 of the base station apparatus  20 B, but it is also possible to connect conventional mobile terminal apparatuses (Rel-10 or earlier versions) as well. 
       FIG. 14  is a functional block diagram of the baseband signal processing section  104  in the mobile terminal apparatus  10 , and shows the function blocks of an LTE-A terminal that supports an additional carrier type. 
     Downlink signals that are received as received data from the base station apparatuses  20 A and  20 B have the CPs removed in a CP removing section  401 . The downlink signals, from which the CPs have been removed, are input in an FFT section  402 . The FFT section  402  performs a fast Fourier transform on the downlink signals, converts the time domain signals into frequency domain signals and inputs the signals in a demapping section  403 . The demapping section  403  demaps the downlink signals, and extracts, from the downlink signals, multiplex control information in which a plurality of pieces of control information are multiplexed, user data and higher control signals. Note that the demapping process by the demapping section  403  is performed based on higher control signals that are received as input from the application section  105 . The multiplex control information that is output from the demapping section  403  is deinterleaved in a deinterleaving section  404 . 
     Also, the baseband signal processing section  104  has a downlink control information demodulation section  405  that demodulates downlink control information, a data demodulating section  406  that demodulates downlink shared data, and a channel estimation section  407 . The downlink control information demodulation section  405  includes a shared channel control information demodulation section  405   a  that demodulates shared control channel control information from the multiplex control information, an uplink shared data channel control information demodulation section  405   b  that demodulates uplink shared data channel control information from the multiplex control information, and a downlink shared data channel control information demodulation section  405   c  that demodulates downlink shared data channel control information from the multiplex control information. 
     The shared channel control information demodulation section  405   a  extracts shared control channel control information, which is control information that is common between users, by, for example, performing a blind decoding process of the common search space in the downlink control channel (PDCCH), a demodulation process, and a channel decoding process and so on. The shared control channel control information includes downlink channel quality information (CQI), and therefore is input in a mapping section  415  and mapped as part of transmission data for the base station apparatus  20 . 
     The uplink shared data channel control information demodulation section  405   b  extracts uplink shared data channel control information (for example, UL grants), by, for example, performing a blind decoding process of the user-specific search spaces of the downlink control channel (PDCCH), a demodulation process, and a channel decoding process and so on. The demodulated uplink shared data channel control information is input in the mapping section  415  and is used to control the uplink shared data channel (PUSCH). 
     The downlink shared data channel control information demodulation section  405   c  extracts user-specific downlink shared data channel control information (for example, DL assignments) by performing a blind decoding process of the user-specific search spaces of the downlink control channel (PDCCH), a demodulation process, a channel decoding process and so on. The demodulated downlink shared data channel control information is input in the data demodulation section  406  and used to control the downlink shared data channel (PDSCH), and input in a downlink shared data demodulation section  406   a.    
     The data demodulation section  406  has the downlink shared data demodulation section  406   a  that demodulates the user data and higher control signals, and a downlink shared channel data demodulation section  406   b  that demodulates downlink shared channel data. 
     The downlink shared data demodulation section  406   a  acquires user data and higher control information based on the downlink shared data channel control information that is input from the downlink shared data channel control information demodulation section  405   c . The downlink shared channel data demodulation section  406   b  demodulates downlink shared channel data based on the uplink shared data channel control information that is input from the uplink shared data channel control information demodulation section  405   b . In this case, the data demodulation section  406  performs derate matching by switching the rate matching pattern depending on the carrier type of the component carriers. For example, with component carriers of an additional carrier type, the demodulation process is performed adequately taking into account the user data allocated to the CRS and PDCCH resources. 
     The channel estimation section  407  performs channel estimation using user-specific reference signals (DM-RSs) or cell-specific reference signals (CRSs). The channel estimation section  407  outputs the estimated channel variation to the shared control channel control information demodulation section  405   a , the uplink shared data channel control information demodulation section  405   b , the downlink shared data channel control information demodulation section  405   c  and the downlink shared data demodulation section  406   a . In these demodulation sections, the demodulation process is performed using the estimated channel variation and the reference signals for demodulation. 
     Also, the baseband signal processing section  104  has, as function blocks of the transmission processing system, a data generating section  411 , a channel coding section  412 , a modulation section  413 , a DFT section  414 , a mapping section  415 , an IFFT section  416 , and an CP inserting section  417 . The data generating section  411  generates transmission data from bit data that is received as input from the application section  105 . The channel coding section  412  applies channel coding processes such as error correction to the transmission data, and the modulation section  413  modulates the transmission data after the channel coding by QPSK and so on. 
     The DFT section  414  performs a discrete Fourier transform on the modulated transmission data. The mapping section  415  maps the frequency components of the data symbols after the DFT to subcarrier positions designated by the base station apparatuses  20 A and  20 B. The IFFT section  416  converts the input data, which corresponds to the system band, into time sequence data, by performing an inverse fast Fourier transform, and the CP inserting section  417  inserts cyclic prefixes in the time sequence data in data units. 
     As described above, with the communication system according to the present embodiment, the arrangement of CRSs of additional carrier types is made different than heretofore, so that it is possible to reduce the interference from the CRSs to the PDSCH. Consequently, at a mobile terminal apparatus  10 , the received quality of signals transmitted in the PDSCH such as user data improves. In this way, it is possible to make effective use of conventional systems and furthermore achieve a communication system, a base station apparatus and a communication method that are suitable for carrier aggregation in a HetNet. 
     The present invention is by no means limited to the above embodiment and can be implemented in various modifications. For example, without departing from the scope of the present invention, it is possible to adequately change the number of carriers, the bandwidth of carriers, the signaling method, the types of additional carrier types, the number of processing sections, the order of processing steps in the above description, and implement the present invention. Besides, the present invention can be implemented with various changes, without departing from the scope of the present invention. 
     The disclosure of Japanese Patent Application No. 2012-062745, filed on Mar. 19, 2012, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.