Patent Publication Number: US-8982752-B2

Title: Base station apparatus and user terminal

Description:
TECHNICAL FIELD 
     The present invention relates to a radio communication system that communicates by allocating a plurality of fundamental frequency blocks (hereinafter referred to as “component carriers”) dynamically or semi-statically. More particularly, the present invention relates to a base station apparatus and a user terminal that transmit and receive downlink control channels under carrier aggregation. 
     BACKGROUND ART 
     The communication scheme to be a successor of W-CDMA (Wideband Code Division Multiple Access) and HSDPA (High Speed Downlink Packet Access), that is, long-term evolution (LTE), has been set forth by 3GPP, which is the standards organization of W-CDMA, and, for radio access schemes, OFDMA (Orthogonal Frequency Division Multiple Access) has been employed on the downlink and SC-FDMA (Single-Carrier Frequency Division Multiple Access) has been employed on the uplink. Presently, 3GPP is studying the successor system of LTE (referred to as “LTE-Advanced” including Release 10 and including versions after Release 10). LTE-Advanced hereinafter will be abbreviated as “LTE-A.” 
     The LTE system is a system to perform communication by sharing one, two, or a greater number of physical channels by a plurality of mobile stations UEs, on both the uplink and the downlink. A channel that is shared by a plurality of mobile stations UEs is generally referred to as a shared channel (or also referred to as “data channel”), and, in LTE, is the PUSCH (Physical Uplink Shared Channel) on the uplink or the PDSCH (Physical Downlink Shared Channel) on the downlink. 
     In a communication system using shared channels such as the LTE system, to which mobile stations UEs the above shared channels are allocated needs to be signaled per transmission time interval (TTI) (or per subframe in LTE). The PDCCH (Physical Downlink Control Channel) is defined as the downlink control channel to be used for the above signaling. A mobile station UE receives the PDCCH and performs blind decoding, thereby extracting downlink control information for that mobile station UE. In LTE, the search space, which defines the resource range where a mobile station has to perform blind decoding, is defined in order to reduce the load of blind decoding on the mobile station. The base station signals downlink control information for the mobile station by arranging the downlink control information in the search space. The mobile station UE does not subject the whole range of the PDCCH to blind decoding, and performs blind decoding only on the search space in the PDCCH, and acquires the downlink control information for the subject station. 
     CITATION LIST 
     Non-Patent Literature 
     
         
         Non-Patent Literature 1: 3GPP, TS36.211 (V.8.4.0), “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, September 2008 
         Non-Patent Literature 2: 3GPP, TS36.212 (V.8.4.0), “Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and Channel Coding (Release 8)”, September 2008 
         Non-Patent Literature 3: 3GPP, TS36.213 (V.8.4.0), “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer Procedures (Release 8)”, September 2008 
       
    
     SUMMARY OF INVENTION 
     Problem to be Solved by the Invention 
     Now, in LTE-A, which is presently under study by 3GPP, there is an agreement to widen the band by gathering and grouping a plurality of component carriers (carrier aggregation). 
     It is therefore an object of the present invention to provide a base station apparatus and a user terminal that can realize a search space configuration that is suitable to transmit and receive a downlink control channel in a communication system in which a plurality of component carriers are aggregated together into a wide band. 
     Means for Solving the Problem 
     A base station apparatus according to the present invention has: a selection section configured to select a downlink and uplink system band to be used in radio communication with a user terminal in units of fundamental frequency blocks; a downlink control information generation section configured to generate downlink control information for demodulating data channels that are sent in the respective selected fundamental frequency blocks, and arrange, in a downlink control channel of a specific fundamental frequency block among the fundamental frequency blocks constituting the system band, a search space in which the downlink control information of the fundamental frequency blocks is included; and a transmission section configured to transmit the downlink control channel in which the search space including the downlink control information is arranged by the downlink control information generation section. 
     Advantageous Effects of the Invention 
     According to the present invention, it is possible to provide a search space configuration that is suitable for a communication system in which a plurality of component carriers is aggregated into a wide band. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing a layered bandwidth configuration defined in LTE-A; 
         FIG. 2  provides conceptual diagrams of user-specific search spaces defined in LTE; 
         FIG. 3  provides diagrams showing a system band formed with four component carriers and search space configurations; 
         FIG. 4  provides diagrams showing search space configurations when component carriers are grouped; 
         FIG. 5  provides diagrams showing a system band formed with a plurality of component carriers and other search space configurations; 
         FIG. 6  provides diagrams showing a system band formed with a plurality of component carriers and other search space configurations; 
         FIG. 7  provides diagrams showing a system band configuration and search space arrangement where 3 types of search spaces are arranged; 
         FIG. 8  is a conceptual diagram to support fallback only in the anchor carrier; 
         FIG. 9  is a diagram showing a system band and search space arrangement including an asymmetric component carrier; 
         FIG. 10  is a diagram showing a configuration of a second search space for an asymmetric component carrier; 
         FIG. 11  is a diagram showing an example of arrangement of search spaces in the PDCCH when the number of carrier aggregations=1 component carrier; 
         FIG. 12  provides diagrams to explain mapping of search spaces for carrier aggregation; 
         FIG. 13  is a diagram showing search space configurations of component carriers where the DCI size is the same; 
         FIG. 14  is a diagram showing search space configurations when component carrier-specific offsets are applied to search spaces; 
         FIG. 15  is a diagram showing a search space configuration of each component carrier when an offset is applied; 
         FIG. 16  is an overview of a mobile communication system according to an embodiment; 
         FIG. 17  is a schematic configuration diagram of a base station apparatus according to an embodiment; 
         FIG. 18  is a schematic configuration diagram of a mobile terminal apparatus according to an embodiment; 
         FIG. 19  is a functional block diagram of a transmission processing section in a baseband signal processing section of a base station apparatus according to an embodiment; and 
         FIG. 20  is a functional block diagram of a baseband signal processing section provided in a mobile terminal apparatus according to an embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the communication system to which the present invention is applied, carrier aggregation to form a system band by adding or removing a plurality of component carriers is performed. Carrier aggregation will be described with reference to  FIG. 1 . 
       FIG. 1  is a diagram showing a layered bandwidth configuration that is agreed in LTE-A. The example shown in  FIG. 1  is a layered bandwidth configuration in the event the LTE-A system, which is a first mobile communication system to have a first system band formed with a plurality of component carriers (CCs), and the LTE system, which is a second mobile communication system to have a second system band formed with one component carrier, coexist. In the LTE-A system, radio communication is performed using, for example, a variable system bandwidth of maximum 100 MHz, and, in the LTE system, radio communication is performed using a variable system bandwidth of maximum 20 MHz. 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, and increases or decreases the number of component carriers dynamically or semi-statically. Aggregating a plurality of component carriers into a wideband 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 of the LTE system (base band: 20 MHz) is one component carrier. In  FIG. 1 , a mobile station UE (User Equipment) # 1  is a user terminal 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 user terminal 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 user terminal 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). 
     The present inventors have contemplated a search space arrangement to realize optimal PDCCH transmission/reception in the event a plurality of component carriers is aggregated in the LTE-A system, and arrived at the present invention. 
     According to one aspect of the present invention, in the LTE-A system, when the system band is formed with a plurality of component carriers, the search spaces of the plurality of component carriers to constitute the system band are mapped to the downlink control channel of one component carrier. The search spaces of all the component carriers may be mapped to one component carrier as well. Alternately, it is equally possible to divide the plurality of component carriers to constitute the system band into a plurality of groups, and map the search spaces of a plurality of component carriers in the same group to one component carrier in the same group. 
       FIG. 2  provides conceptual diagrams of user-specific search spaces (UE-specific search spaces) SS defined in LTE. Two search spaces of varying blind decoding sizes are defined. The blind decoding sizes may be determined according to the size (the DCI size) of downlink control information (DCI), and the DCI size is determined by the transmission mode and bandwidth of the component carriers. If the transmission mode is the same between aggregated component carriers, the DCI size is determined by the bandwidth of the component carriers. 
     LTE defines a plurality of DCI formats of varying DCI sizes (of varying types of blind decoding, in other words). One is DCI format 1 (shown as “D 0 ” in  FIG. 2 ), and the other one is DCI format 1A (shown as “D 0 ′” in  FIG. 2 ), which is a compact-type DCI format that makes “D 0 ” compact, and which is used mainly for users at cell edges. Also, for DCI for uplink assignment information, DCI format 0 (shown as “U 0 ” in  FIG. 2 ) of the same size as DCI format 1A, which is a DCI format for downlink compact assignment, is defined. 
     Two types of search spaces (search space SS 1  and search space SS 2 ) are assigned to the PDCCH of component carrier CC 0 . DCI format 1 (D 0 ) is arranged in search space SS 1 , and D 0 ′ and U 0  having the same bit size are arranged in common search space SS 2 . The DCI (Format 1) to be arranged in search space SS 1  is a control signal for demodulation of the PDSCH of component carrier CC 0 , and the DCI (Format 0) to be arranged in search space SS 2  is a control signal for demodulation of the uplink PUSCH of the same component carrier CC 0 . 
       FIG. 2A  is an example where downlink assignment information (D 0 ) is arranged in one search space SS 1  and uplink assignment information (U 0 ) is arranged in the other search space SS 2 .  FIG. 2B  shows a case where DCI format 1A, which is a DCI format for compact assignment, is selected for downlink assignment information, and “D 0 ′” and “U 0 ” are arranged in the common search space SS 2 . 
     A configuration to map the search spaces of a plurality of component carriers to constitute the system band to the downlink control channel of one component carrier will be described with reference to  FIG. 3  and  FIG. 4 . As for the component carriers shown as examples in  FIG. 3  and  FIG. 4 , all the component carriers have the same bandwidth and hold a symmetric relationship between the downlink component carriers and the uplink component carriers. 
       FIG. 3  shows a system band formed with four component carriers CC 0  to CC 3 , and shows downlink component carriers (DL), uplink component carriers (UL) and search spaces SSs arranged in the PDCCHs of the downlink component carriers.  FIG. 3A  shows an example of arranging search spaces according to the rules of LTE illustrated in  FIG. 2 . As shown in  FIG. 3A , search spaces SS 1  and SS 2  are arranged in the PDCCH of each of the downlink component carriers (DL). 
       FIG. 3B  shows a configuration in which the search spaces of a plurality of component carriers CC 0  to CC 3  are mapped to PDCCH 0  of one component carrier CC 0 . In one search space SS 1  mapped to PDCCH 0 , downlink assignment information D 0  to D 3  of all the component carriers CC 0  to CC 3  are arranged. Also, in the other search space SS 2  mapped to PDCCH 0 , uplink assignment information U 0  to U 3  of all the component carriers CC 0  to CC 3  are arranged. Note that, in the event compact-type uplink assignment information D 0 ′ to D 3 ′ are used, compact-type downlink assignment information D 0 ′ to D 3 ′ and uplink assignment information U 0  to U 3  are arranged in the search space SS 2 . 
     Now, the method of identifying downlink assignment information D 0  to D 3  (D 0 ′ to D 3 ′) and uplink assignment information U 0  to U 3  for a plurality of component carriers CC 0  to CC 3 , arranged in the same search space SS, will be described. 
     An identifier (hereinafter referred to as “carrier indicator”) that can specify the original component carrier (meaning the component carrier where the shared data channel (PDSCH) to be demodulated using each downlink assignment information (D 0  to D 3 )/(D 0 ′ to D 3 ′) is transmitted) is attached to DCI format 1/1A in which downlink assignment information (D 0  to D 3 )/(D 0 ′ to D 3 ′) is arranged. The field in which the carrier indicator is arranged on DCI format 1/1A may be referred to as the CIF (Carrier Indicator Field). A CIF to indicate the original component carrier is likewise provided in DCI format 0 where uplink assignment information U 0  to U 3  is arranged. 
     Consequently, when a user terminal having received PDCCH 0  of the component carrier CC 0  performs blind decoding of the search space SS 1  of PDCCH 0 , although D 0  to D 3  are all demodulated at the same time in one blind decoding, it is still possible to identify which component carrier each downlink assignment information is associated with, by analyzing the CIFs that are separately provided in D 0  to D 3 . Likewise, when blind decoding is performed for the search space SS 2  of PDCCH 0 , U 0  to U 3  are all demodulated at the same time in one blind decoding, it is still possible to identify which component carrier each uplink assignment information is associated with, by analyzing the CIFs of U 0  to U 3 . 
       FIG. 4  shows a system band formed with four component carriers CC 0  to CC 3 , showing an example where a plurality of component carriers to constitute the system band is divided into a plurality of groups and search spaces are mapped in group units.  FIG. 4A  shows the same subject matter as in  FIG. 3A .  FIG. 4B  shows an example of mapping search spaces for a plurality of component carriers in the same group to one component carrier in the same group, per group. To be more specific, the whole of the system band is divided into the first group of component carriers CC 0  and CC 1  and a second group of component carriers CC 2  and CC 3 . The search spaces of the component carriers CC 0  and CC 1  in the first group are mapped to PDCCH 0  of one component carrier CC 0  in the same group, and the search spaces of the component carriers CC 2  and CC 3  in the second group are mapped to PDCCH 2  of one component carrier CC 2  in the same group. 
     For example, with the example shown in  FIG. 3 , when the communication quality of the component carrier CC 0  is good and the communication quality of PDCCH 1  to PDCCH 3  of the other component carriers CC 1  to CC 3  is poor, uplink/downlink assignment information, which is important information, can be signaled using PDCCH 0  of the component carrier CC 0  of good communication quality. Also, as shown in  FIG. 4B , by dividing component carriers into a number of groups and specifying the component carrier to use to transmit downlink control information in each group, it is equally possible to prevent the increase in number of component carriers (which may also be referred to as the number of DCIs) to be arranged in one search space. 
     Next, a search space configuration that is suitable to a system band where component carriers of varying bandwidths coexist will be described.  FIG. 5  illustrates a system band formed with four component carriers CC 0  to CC 3 , where two component carriers CC 0  and CC 1  have the same bandwidth, and the other two component carriers CC 2  and CC 3  have the same bandwidth, and this bandwidth is different from the bandwidth of CC 0  and CC 1 . The uplink component carriers and downlink component carriers are symmetric. Note that the search space arrangement shown in  FIG. 5A  is the same arrangement as in  FIG. 3A  and  FIG. 4A . 
     For example, in the example of  FIG. 5  of carrier aggregation to transmit downlink control information including uplink/downlink assignment information using the PDCCHs of CCs, downlink assignment information D 0  and D 1  of the component carriers CC 0  and CC 1  having the same bandwidth has the same size, so that, as shown in  FIG. 5B , the common search space SS 1  (D 0 / 1 ) for CC 0  and CC 1  is formed in the search space SS 1  of CC 0 . Also, downlink assignment information D 2  and D 3  of the component carriers CC 2  and CC 3  having the same bandwidth has the same size, so that, as shown in  FIG. 5B , the common search space SS 1  (D 2 / 3 ) for CC 2  and CC 3  is formed in the search space SS 1 . Consequently, in the search space SS 1  for downlink assignment information, the two common search spaces SS 1  (D 0 / 1 ) and SS 1  (D 2 / 3 ) coexist. 
     Also, uplink assignment information U 0  and U 1  of the component carriers CC 0  and CC 1  has the same size, so that the common search space SS 2  (U 0 / 1 ) for CC 0  and CC 1  is formed in the search space SS 2 . Also, uplink assignment information U 2  and U 3  of the component carriers CC 2  and CC 3  has the same size, so that, as shown in  FIG. 5B , the common search space SS 2  (U 2 / 3 ) for CC 2  and CC 3  is formed in the search space SS 2 . Consequently, in the search space SS 2  for uplink assignment information, the two common search spaces SS 2  (U 0 / 1 ) and SS 2  (U 2 / 3 ) coexist. Note that D 0 ′/D 1 ′ and D 2 ′/D 3 ′ of compact type, having the same DCI size, may be arranged in common search spaces SS 2  (U 0 / 1 ) and SS 2  (U 2 / 3 ). “C” represents the CIFs attached separately to the downlink assignment information (D 0 , D 1 , D 2  and D 3 ) and uplink assignment information (U 0 , U 1 , U 2  and U 3 ). 
     It is equally possible to define one component carrier as an anchor carrier, in a communication system (for example, LTE-A) to aggregate a plurality of component carriers and secure a wide-range system band as a whole. The anchor carrier may be defined to constantly guarantee the same operations as in LTE. To guarantee the same operations as in LTE, the CIF cannot be included in the DCI format. Also, in the event the same operations as in LTE are not guaranteed, by defining a specific component carrier as a reference component carrier (anchor carrier) and identifying that reference component carrier between the base station apparatus and the user terminal, it is still possible to specify component carriers without attaching a CIF to the DCI of the reference component carrier. 
     Given this, when a plurality of component carriers are grouped together and a wide-range system band is secured on the whole, there is a possibility that there are component carriers in which CIFs are not attached to the DCIs. 
       FIG. 5C  shows a search space arrangement when a CIF is not attached to downlink assignment information D 0  and uplink assignment information U 0  of the component carrier CC 0 . Since the CIF is removed from the DCI of the component carrier CC 0 , the DCI size varies between D 0 /D 0 ′ and D 1 /D 1 ′. Likewise, the DCI sizes of U 0  and U 1  vary. The search space arrangement shown in  FIG. 5C  is a configuration to separate a search space every DCI size. The search space SS 1  in which downlink assignment information is arranged is formed with a search space SS 1  (D 0 ) where D 0  without a CIF is arranged, a search space SS 1  (D 1 ) where D 1  with a CIF is arranged, and a common search space SS 1  (D 2 / 3 ) where D 2  and D 3  of the same size with CIFs are arranged. The search space SS 2  in which uplink assignment information is arranged is formed with a search space SS 2  (U 0 ) where U 0  without a CIF is arranged, a search space SS 2  (U 1 ) where U 1  with a CIF is arranged, and a common search space SS 2  (U 2 / 3 ) where U 2  and U 3  of the same size with CIFs are arranged. Note that it is equally possible to arrange D 0 ′, D 1 ′, D 2 ′ and D 3 ′ of compact type, that are the same size as uplink assignment information U 1  and so on, in corresponding places in the search space SS 2 . 
       FIG. 6  illustrates a system band formed with four component carriers CC 0  to CC 3 , where two component carriers CC 0  and CC 1  have the same bandwidth, and the other two component carriers CC 2  and CC 3  have the same bandwidth, and this bandwidth is different from the bandwidth of CC 0  and CC 1 . Note that the search space arrangement shown in  FIG. 6A  is the same search space arrangement as in  FIG. 5A . 
     Downlink assignment information D 0  and D 1  of the component carriers CC 0  and CC 1  having the same bandwidth is arranged in the search spaces SS 1 / 2  of PDCCH 0  of the component carrier CC 0  as shown in  FIGS. 6B and 6C , and downlink assignment information D 2  and D 3  of the component carriers CC 2  and CC 3  that have the same bandwidth is arranged in the search spaces SS 1 / 2  of PDCCH 2  of the component carrier CC 2  as shown in  FIGS. 6B and 6C . 
       FIG. 6B  shows a search space arrangement in the event CIFs are attached to all of downlink assignment information D 0  to D 3  (D 0 ′ to D 3 ′) and uplink assignment information U 0  to U 3 . Common search space SS 1  (D 0 / 1 ) for CC 0  and CC 1  is arranged in the search space SS 1  of PDCCH 0 , and common search space SS 1  (D 2 / 3 ) for CC 2  and CC 3  is arranged in the search space SS 1  of PDCCH 2 . Common search space SS 2  (U 0 / 1 ) for CC 0  and CC 1  is arranged in the search space SS 2  of PDCCH 0 , and common search space SS 2  (U 2 / 3 ) for CC 2  and CC 3  is arranged in search space SS 2  of PDCCH 2 . 
       FIG. 6C  is the same as the search space configuration shown in  FIG. 6B  in grouping search spaces SS 1  and SS 2  into anchor carriers CC 0  and CC 2  of the respective groups (which may be referred to as “reference component carriers”), but is different in not attaching CIFs to assignment information D 0  and U 0  of one anchor carrier CC 0 . A configuration is equally possible where CIFs are not attached to assignment information D 2  and U 2  of the other anchor carrier CC 2 , and, in this case, it is preferable to use the same configuration as the search spaces of assignment information D 0  and U 0  of the one anchor carrier CC 0 . 
     Since SC-FDMA is employed as the uplink radio access scheme in LTE, the DCI size of uplink assignment information is made the same as the DCI size of compact-type downlink assignment information (DCI format 1A). On, the other hand, in LTE-A, there is an agreement to employ clustered DFT-spread OFDM, which allocates a plurality of clusters, as the uplink radio access scheme. Since the volume of information of uplink resource allocation information becomes large in clustered DFT-spread OFDM, a larger DCI size than DCI format 1A is preferable. Also, in LTE-A, there is an agreement to apply MIMO transmission to the uplink, and, from that aspect, too, uplink resource allocation information increases. 
     So, apart from DCI format 0, which is made to match the DCI size of compact-type downlink assignment information (DCI format 1A), a DCI format (hereinafter referred to as “DCI format 0A”) having an expanded number of resource allocation bits over DCI format 0 is defined. In the PDCCH, a search space to arrange DCI format 0A is set. 
     According to another aspect of the present invention, in the LTE-A system, search spaces to support the three DCI sizes of DCI format 1, DCI format 0/1A and DCI format 0A are arranged in the PDCCH, and the user terminal performs blind decoding of the three types of DCI formats. 
       FIG. 7  illustrates a system band configuration and search space arrangement where three types of search spaces are arranged. This drawing illustrates a system band formed with four component carriers CC 0  to CC 3 , where two component carriers CC 0  and CC 1  have the same bandwidth, and the other two component carriers CC 2  and CC 3  have the same bandwidth, and this bandwidth is different from the bandwidth of CC 0  and CC 1 . In the PDCCH of each of the component carriers CC 0  to CC 3 , three types of search spaces SS 1 , SS 2  and SS 3  are arranged. For example, the search spaces arranged in the PDCCH of the component carrier CC 0  will be described as an example. The first search space SS 1  in which downlink assignment information D 0  of DCI format 1 having a first DCI size is arranged, the second search space SS 2  in which downlink assignment information D 0 ′ and U 0 ′ of DCI format 1A or DCI format 0 having a second DCI size are arranged, and the third search space SS 3  in which uplink assignment information U 0  of DCI format 0A having a third DCI size is arranged, are arranged in the PDCCH. 
     In  FIG. 7 , although downlink assignment information D 0 ′ and uplink assignment information U 0 ″ are not allocated in the second search space SS 2 , when a downlink control signal is signaled to a user terminal at a cell edge or to a user terminal with little control information, downlink assignment information D 0 ′ and uplink assignment information U 0 ″ are allocated in the second search space SS 2 . 
     In this way, if the three types of search spaces SS 1 , SS 2  and SS 3  are applied on a selective basis, it is possible to make signaling by utilizing the second search space when the volume of information of downlink control signals needs to be reduced as for cell edge users in LTE, and make signaling by utilizing the third search space when the volume of information of uplink assignment information is large. 
     As shown in  FIG. 8 , it is equally possible to support fallback to mode (DCI format 1A or DCI format 0) to use the second search space only in the anchor carrier (CC 0 ). In the component carriers (CC 1  to CC 3 ) other than the anchor carrier (CC 0 ), blind decoding of the second search space is not performed at the user terminal. In the event a band of good communication quality is allocated to the anchor carrier (CC 0 ), it is possible to utilize the second search space of a small DCI size effectively, and reduce the load of the user terminal since two types of blind decoding are sufficient for carriers other than the anchor carrier (CC 0 ). 
     Next, a search space arrangement that is suitable for a system band including a component carrier, in which only a downlink component carrier is assigned and an uplink component carrier is not assigned, and which is therefore asymmetric (hereinafter referred to as “asymmetric component carrier”), will be described. 
       FIG. 9  shows a system band and search space arrangement including an asymmetric component carrier. One component carrier CC 0  is assigned a pair of an uplink component carrier and a downlink component carrier, but the other component carrier CC 1  is assigned only a downlink component carrier and no uplink component carrier is assigned, thus constituting an asymmetric component carrier. As with component carrier CC 0 , a search space SS 1  for downlink assignment information (DCI format 1), and a second search space SS 2  for a compact size, where downlink assignment information D 0 ″ (DCI format 0), which has a compact size compared to the downlink assignment information D 0  (DCI format 1), and uplink assignment information U 0  (DCI format 0) of the same size, can be allocated in pair, are arranged in the PDCCH. On the other hand, in the event of the asymmetric component carrier CC 1 , for information to place in the second search space SS 2  for a compact size, there is only downlink assignment information D 1 ′ of a compact size compared to downlink assignment information D 1 . 
     The present inventors have carefully considered what should be arranged or should not be arranged in pair with compact-size downlink assignment information D 1 ′ in the second search space SS 2 , and, as a result, arrived at the present invention. 
     According to another aspect of the present invention, when, in the LTE-A system, a plurality of component carriers to constitute the system band includes an asymmetric component carrier, the search space of the asymmetric component carrier is formed with a search space SS 1  for downlink assignment information D 1  (DCI format 1) and a second search space SS 2  for a compact size, in which only downlink assignment information D 1 ′ (DCI format 0) of a compact size compared to the downlink assignment information D 1  is arranged. That is to say, the second search space SS 2  for a compact size is not allocated a pair of downlink assignment information (DL) for the downlink component carrier and uplink assignment information (UL) for the uplink component carrier, and only the compact-type downlink assignment information D 1 ′ (DCI format 1A) for the downlink component carrier is arranged (option  2 ). 
     By this means, upon trying to arrange DCI in a pair of DL and UL in the second search space SS 2  for a compact size as stipulated in LTE, if a pair is formed using uplink assignment information (UL) of another component carrier of a varying bandwidth, there arises a process of matching the size of the compact-type downlink assignment information in the asymmetric component carrier with the uplink assignment information (UL) of the other component carrier. With the present invention, such process can be prevented from arising. 
     Also, it is equally possible not to provide any second search space SS 2  for a compact size in the search space for the asymmetric component carrier (option  3 ). 
     Also, it is equally possible to form a pair using uplink assignment information of another component carrier apart from the asymmetric component carrier (for example, uplink assignment information U 0  of the component carrier CC 0 ) (option  1 ). Although this raises a process of matching the size of compact-type downlink assignment information D 1 ′ in the asymmetric component carrier with the uplink assignment information (U 0 ) of the other component carrier, as described above, there is an advantage of increasing the redundancy of signaling of uplink assignment information (for example, U 0 ) of the other component carrier. 
       FIG. 10  shows specific examples of above options  1  to  3 , illustrating configuration examples of the second search space SS 2  for a compact size for asymmetric component carrier CC 1  illustrated in  FIG. 9 . 
     In option  1 , the second search space SS 2  is defined, in which a pair of compact-type downlink assignment information D 1 ′ for the asymmetric component carrier CC 1  and uplink assignment information U 0  of the component carrier CC 0  other than the asymmetric component carrier CC 1 , can be arranged. 
     Uplink assignment information U 0  of the component carrier CC 0  has a bigger bit size than compact-type downlink assignment information D 1 ′ of the asymmetric component carrier CC 1 . To make uplink assignment information U 0  (CC 0 ) and downlink assignment information D 1 ′ the same in blind decoding size to be allowed to be arranged in the second search space SS 2 , padding bits are added to the smaller downlink assignment information D 1 ′ so that its bit size matches with that of the bigger uplink assignment information U 0 . When the downlink assignment information D 1 ′ is arranged in the second search space SS 2 , the bit size is adjusted by adding padding bits to the downlink assignment information D 1 ′. 
     By this means, it is possible to signal uplink assignment information U 0  of the component carrier CC 0  using the second search space of another component carrier CC 1 , and consequently it is possible to increase the redundancy of the uplink assignment information U 0 . 
     Also, if uplink assignment information UL is recycled from another component carrier having the same bandwidth (and the same transmission mode) as the asymmetric component carrier CC 1 , the compact-type downlink assignment information D 1 ′ and uplink assignment information UL become the same size, and the process of adding padding bits is not required. 
     In option  2 , the second search space SS 2  for a compact size, in which compact-type downlink assignment information D 1 ′ of the asymmetric component carrier CC 1  alone is arranged, is defined. Since uplink assignment information U 0  (CC 0 ) of a different bit size is not recycled from another component carrier, the process of adding padding bits for matching the bit size of both does not arise, and it is therefore possible to simplify the process. 
     In option  3 , the second search space SS 2  for a compact size is not arranged in the asymmetric component carrier. By this means, it is possible to realize even more simplified process than option  2 . 
     Next, a plurality of search space arrangements for a plurality of component carriers on the PDCCH of one component carrier will be described.  FIG. 11  shows an example of arrangement of a search space in the PDCCH when the number of carrier aggregations=1 component carrier. 
     In LTE, rate matching of downlink control information (DCI) to one of 72, 144, 288 and 576 bits (where the cases of 72 bits and 576 bits correspond to the coding rates of ⅔ and 1/12) is performed according to the reception quality of a user terminal. 72 bits is defined as the basic unit (CCE: Control Channel Element), and an optimal number of CCEs is determined according to reception quality, from 4 types of numbers of CCEs={1, 2, 4, 8}. The number of CCE aggregations is smaller for a user terminal of better reception quality, and the number of CCE aggregations is made bigger for a user terminal of poorer reception quality such as a user terminal located at a cell edge. In this way, the number of CCE aggregations (resources) for transmitting downlink control information (DCI) is determined per user terminal. 
       FIG. 11  shows an example of forming the PDCCH of a component carrier with 50 CCEs. When the number of CCEs=1, a search space SS is arranged in 6 CCEs (the range of CCE numbers 17 to 22), and, when the number of CCEs=2, a search space SS is arranged in 6 CCEs (the range of CCE numbers 1 to 6). When the number of CCEs=4, a search space SS is arranged in 2 CCEs (the range of CCE numbers 2 and 3), and, when the number of CCEs=8, a search space SS is arranged in 2 CCEs (the range of CCE numbers 0 and 1). 
     The present inventors have contemplated a search space arrangement which can control adequate search space arrangement according to the number of component-carrier aggregations and which has high compatibility to switch the search space arrangement adequately when the PDSCH is activated/deactivated, and, as a result, arrived at the present invention. Deactivation of the PDSCH refers to controlling the transmission power of the PDSCH to 0 or to a value close to 0, or controlling the transmission data of the PDSCH to be 0 or minimum information. Activation of the PDSCH refers to making the transmission power or transmission data of the PDSCH greater than a predetermined level. 
     According to another aspect of the present invention, when the search spaces in which downlink control information (DCI) for individual component carriers to constitute the system band is arranged are mapped to the downlink control channel of one component carrier, the search spaces for the individual component carriers are arranged in a consecutive manner based on the starting position of the search space for the component carrier to send the PDCCH. 
     By this means, only by signaling the component carrier number (CC number) to send the PDCCH and the CC number to send the PDSCH among a plurality of component carriers to constitute the system band, the user terminal is able to specify the search space of each component carrier. Also, since the search spaces are arranged in component carrier units, it is easy to deactivate only the search space of a component carrier with a deactivated PDSCH. 
     Now, search space mapping to arrange the search spaces of component carriers in a consecutive manner based on the starting position of the search space of the component carrier to send the PDCCH, will be described in detail with reference to  FIG. 12 . 
     The component carrier to send the PDCCH is CC 1 , and the PDCCH of the component carrier CC 1  is formed with a bandwidth of 50 CCEs. In the PDCCH of the component carrier CC 1 , the search spaces of other component carriers CC 2  and CC 3  are arranged. When the number of CCE aggregations=1 and 2, the search spaces are formed with 6 CCEs, and, when the number of CCE aggregations=4 and 8, the search spaces are formed with 2 CCEs. 
     For example, as shown in  FIG. 12A , when the number of CCE aggregations=1, the search space of the component carrier CC 1  to send the PDCCH starts from the CCE number 17. Following the search space of CC 1 , the search space of CC 2  is arranged, and, following the search space of CC 2 , the search space of CC 3  is arranged. The same applies to cases of other numbers of CCE aggregations. 
     That is to say, if the starting position of the search space of the component carrier to send the PDCCH, the order of component carriers for which search spaces are arranged, and the size of the search spaces are learned, it is possible to specify the individual search spaces even when the search spaces of a plurality of CCs are arranged in the PDCCH of one CC. The size of the search spaces is determined by the number of CCE aggregations, so that it is not necessary to signal the search space size separately. It then follows that, only by newly signaling the CC number to send the PDCCH (in the above case, CC 1 ) and the CC numbers to send the PDSCH (in the above case, CC 2  and CC 3 ), the user terminal is able to specify the search spaces of the component carriers (CC 1  to CC 3 ). 
       FIG. 12B  shows the search space arrangement in the event CC 2  is deactivated. As shown in this drawing, the search space of deactivated CC 2  (for example, if the number of CCE aggregations=1, the CCE numbers 23 to 28) is deactivated. Only the search space of CC 2  is deactivated, without influencing the positions of the search spaces of activated CC 1  and CC 3 . When CC 2  is activated again, it is only necessary to arrange the downlink control information (DCI) of CC 2  in the original CC 2  search space (CCE numbers 23 to 28). 
     In this way, a search space arrangement to arrange the search spaces of a plurality of component carriers in a consecutive manner readily supports the activation/deactivation of the PDSCH. Considering the false detection probability, when user-specific search spaces are mapped to different component carriers as described above, a configuration is preferable whereby the user-specific search spaces can also be deactivated. 
     As described above, in the event the search spaces of a plurality of component carriers are arranged in a consecutive manner, there are cases where the number of CCEs to constitute the search space needs not be increased in proportion to the number of component carriers. If the DCI size is the same between a plurality of component carriers, it is not necessary to increase the number of CCEs in proportion to the number of component carriers. 
       FIG. 13  is a diagram showing a search space configuration for component carriers having the same DCI size. With respect to the numbers of CCs (Ncc) from Ncc=1 to Ncc=5, the search space is shown by hatching. When the number of CCs is from Ncc=1 to Ncc=3, the number of CCEs of the search space increases in proportion to the number of CCs. 
     For example, a case will be described here where the number of CCE aggregations is 1 CCE. When the number of CCs is Ncc=1, the number of CCEs=6 is allocated to the search space of 1 CC. Furthermore, when Ncc=2, the number of CCEs=12, which is double of 1 CC, is allocated in association with the search spaces for 2 CCs. When Ncc=3, the number of CCEs=18, which is triple of 1 CC, is allocated in association with the search spaces for 3 CCs. Up to Ncc=3, the same case applies as shown in  FIG. 12A . 
     In the example shown in  FIG. 13 , the number of CCEs of the search spaces is maximum 18 CCEs. When the search space size is 18 CCEs at a maximum, it is possible to arrange DCIs for 5 CCs not to interfere with each other. Consequently, even when the number of CCs increases to Ncc=4 and Ncc=5, the search space is fixed to the number of CCEs=18 and does not increase in proportion to the number of CCs. By this means, if the DCI size is the same, it is possible to allocate DCIs in any position in the search space and therefore reduce the number of CCEs of the search space, by attaching a CIF to the DCI of each component carrier. 
     Also, by making the search spaces SSs partly overlap between component carriers CCs, it is possible to prevent increase in number of CCEs of the search spaces SSs even if the number of CC aggregations increases. 
       FIG. 14  is an example of arrangement in which the search spaces SSs partly overlap between component carriers CCs by applying component carrier CC-specific offsets to the search spaces SSs. Assuming that the number of CCE aggregations is N_level={1, 2, 4, 8} and the CCE size to correspond to the number of CCE aggregations is N_size={6, 12, 8, 16}, the offset amount=(N_size/N_level)/2 is calculated. Now, the unit of offset is the number of search spaces SSs at each level. This offset amount is designed so that the search spaces of neighboring CCs overlap approximately by half. 
     For example, to examine the case of Nlevel=1, when Ncc=1, there is 1 CCE, so that 6 CCEs from the CCE numbers 17 to 22 become the search space. When Ncc=2, the offset amount=3 CCEs, so that the CCE number 20 to CCE number 25 become the search space of the second CC. The search space to be secured in the PDCCH becomes the range of CCE numbers 17 to 25. When Ncc=3, the offset amount=3 CCEs, so that the CCE number 23 to CCE number 28 become the search space of the third CC. The search space to be secured in the PDCCH becomes the range of CCE numbers 17 to 28. When Ncc=4, the offset amount=3 CCEs, so that the CCE number 26 to CCE number 31 become the search space of the fourth CC. The search space to be secured in the PDCCH becomes the range of CCE numbers 17 to 31. When Ncc=5, the offset amount=3 CCEs, so that the CCE number 29 to CCE number 34 become the search space of the fourth CC. The search space to be secured in the PDCCH becomes the range of CCE numbers 17 to 34. 
     In this way, although the search space size increases in proportion to the number of CC aggregations, by applying component carrier-specific offsets to the search spaces, it is possible to reduce the increases of the search space size when the number of CC aggregations increases. 
     As described above, by making the search spaces SSs partly overlap between component carriers CCs, it is possible to suppress the increase of the number of CCEs of the search space SS even if the number of CC aggregations increases. Now, mapping to reduce the overlap of search spaces SSs between component carriers CCs will be described. 
     Assuming that the number of CCEs to match the bandwidth of the PDCCH is N CCE , the size of the search space SS at each level of N_level={1, 2, 4, 8} is N_size={6, 12, 8, 16}, and the number of CCs is Ncc, the number of CCEs is sufficient when N CCE  is greater than N_size×Ncc, and the search spaces SSs of individual component carriers CCs are mapped not to overlap. For example, the offset amount N_offset then is N_offset=N_size/N_level. 
     Also, when N CCE  is smaller than N_size×Ncc, the number of CCEs is insufficient, and therefore the following amount of offset N_offset is calculated. 
     
       
         
           
             
               
                 
                   
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     Here, the unit of offset is the number of search spaces SSs at each level. 
     For example, a case of mapping search spaces SSs for the number of CCs Ncc=5 when N CCE =41 will be examined as follows. When N_level=2 and 8, N_size×Ncc becomes 60 and 80 (&gt;41), so that the offsets are calculated using the above equation. The offset amounts then are N_offset=4 and 1.  FIG. 15  shows the search space of each component carrier when these offsets are applied. To examine the number of CCE aggregations=2, although, when Ncc=1, the starting position of the search space is SS number 1, when Ncc=2, the starting position of the search space is SS number 5, which is where an offset amount=4 is added to SS number 1. That is to say, the overlap of search spaces between Ncc=1 and Ncc=2 is the number of SSs=2. Between other CCs, the overlap of search spaces is the number of SSs=2. Also, to examine the number of CCE aggregations=8, although, when Ncc=1, the starting position of the search space is SS number 0, when Ncc=2, the starting position of the search space is SS number 1, which is where offset amount=1 is added to SS number 0. That is to say, the overlap of search spaces between Ncc=1 and Ncc=2 is the number of SSs=1. 
     Also, when there is no transmission data in the PDSCH of component carrier CCN, deactivation to reduce or make 0 the transmission power of the PDSCH is performed for the purpose of saving power. Even when the PDSCH is deactivated, only the PDCCH is activated (transmission of downlink control information is maintained at required transmission power). 
     According to another aspect of the present invention, the PDCCH of a component carrier is provided with ON/OFF functions for activation/deactivation. In the event the PDSCH of a given component carrier CC_N is deactivated, the PDCCH of that component carrier CC_N is also deactivated. Also, such design may also be possible where, given one component carrier CC_N alone, the PDSCH is ON (activated) but the PDCCH is OFF. 
     In the event a CIF is attached to DCI, when the PDSCH is OFF (deactivated), it is preferable to deactivate the user-specific search space for that PDSCH. 
     Now, an embodiment of the present invention will be described below in detail with reference to the accompanying drawings. Although a case of using base stations and mobile stations to support the LTE-A system will be described here, the present invention is also applicable to communication systems other than LTE. 
     Referring to  FIG. 16 , a mobile communication system  1  having a mobile station (UE)  10  and a base station (Node B)  20  according to an embodiment of the present invention will be described.  FIG. 16  is a diagram for explaining the configuration of the mobile communication system  1  having mobile stations  10  and a base station  20  according to the present embodiment. Note that the mobile communication system  1  illustrated in  FIG. 16  is a system to accommodate, for example, the LTE system or SUPER 3G. Also, this mobile communication system  1  may be referred to as IMT-Advanced or may be referred to as 4G. 
     As illustrated in  FIG. 16 , the mobile communication system  1  is configured to include a base station apparatus  20  and a plurality of mobile terminal apparatuses  10  ( 10   1 ,  10   2 ,  10   3 , . . .  10   n , where n is an integer to satisfy n&gt;0) that communicate with this base station apparatus  20 . The base station apparatus  20  is connected with a higher station apparatus  30 , and this higher station apparatus  30  is connected with a core network  40 . The mobile terminal apparatuses  10  are able to communicate with the base station apparatus  20  in a cell  50 . Note that the higher station apparatus  30  includes, 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. The higher station apparatus  30  may be included in the core network  40 . 
     The mobile terminal apparatuses ( 10   1 ,  10   2 ,  10   3 , . . .  10   n ) include LTE terminals and LTE-A terminals, and, the following descriptions will be given with respect to “mobile terminal apparatus  10 ,” unless specified otherwise. Also, although the mobile terminal apparatus  10  performs radio communication with the base station apparatus  20  for ease of explanation, more generally, user apparatuses (UE: User Equipment) including mobile terminal apparatuses and fixed terminal apparatuses may be used as well. 
     In the mobile communication system  1 , as radio access schemes, OFDMA (Orthogonal Frequency Division Multiple Access) is applied to the downlink, and SC-FDMA (Single-Carrier Frequency-Division Multiple Access) and clustered DFT-spread OFDM are 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, a system band into bands formed with one or continuous resource blocks, and allowing a plurality of terminals to use mutually different bands. Clustered DFT-spread OFDM is a scheme to realize uplink multiple access by allocating groups (clusters) of discontinuous, clustered subcarriers to one mobile station UE and applying discrete Fourier transform spread OFDM to each cluster. 
     Here, the communication channels in the LTE and LTE-A systems will be described. The downlink communication channels include the PDSCH, which is used by each mobile terminal apparatus  10  on a shared basis, and downlink L1/L2 control channels (including the PDCCH, PCFICH and PHICH). This PDSCH transmits user data and higher control signals. The higher control signals include RRC signaling to report the increase/decrease of the number of carrier aggregations, the uplink radio access scheme (SC-FDMA/clustered DFT-spread OFDM) to be applied to each component carrier, and so on, to the mobile terminal apparatus  10 . Also, in the event the mode to activate/deactivate the PDSCH and/or PDCCH is supported, signaling to turn the activation/deactivation of the PDSCH and PDCCH ON/OFF is included on a per-component carrier basis. 
     The uplink communication channels include PUSCH, which is used by each mobile terminal apparatus  10  on a shared basis, and the PUCCH (Physical Uplink Control Channel), which is an uplink control channel. User data is transmitted by this PUSCH. The PUCCH transmits downlink radio quality information (CQI: Channel Quality Indicator), ACK/NACK and so on, and, although intra-subframe frequency hopping applies in SC-FDMA, in clustered DFT-spread OFDM, intra-subframe frequency hopping does not apply, because a frequency scheduling effect can be achieved without intra-subframe frequency hopping. 
     According to the present embodiment, an overall configuration of the base station apparatus  20  will be described with reference to  FIG. 17 . The base station apparatus  20  has a transmission/reception antenna  201 , an amplifying section  202 , a transmission/reception section  203 , a baseband signal processing section  204 , a call processing section  205 , and a transmission path interface  206 . 
     User data to be transmitted from the base station apparatus  20  to the mobile terminal apparatus  10  on the downlink is input from the higher station apparatus  30  into the baseband signal processing section  204 , via the transmission path interface  206 . 
     In the baseband signal processing section  204 , PDCP layer processing, division and coupling of user data, RLC (Radio Link Control) layer transmission processing such as RLC retransmission control transmission processing, MAC (Medium Access Control) retransmission control, including, for example, HARQ (Hybrid Automatic Repeat reQuest) transmission processing, scheduling, transport format selection, channel coding, inverse fast Fourier transform (IFFT) processing, and precoding processing, are performed. Furthermore, as with signals of the physical downlink control channel, which is a downlink control channel, transmission processing such as channel coding and inverse fast Fourier transform is performed. 
     Also, the baseband signal processing section  204  notifies control information for allowing each mobile terminal apparatus  10  to communicate with the base station apparatus  20 , to the mobile terminal apparatuses  10  connected to the same cell  50 , by a broadcast channel. Broadcast information for communication in the cell  50  includes, for example, the uplink or downlink system bandwidth, identification information of a root sequence (root sequence index) for generating random access preamble signals in the PRACH, and so on. 
     In the transmission/reception section  203 , the baseband signal output from the baseband signal processing section  204  is subjected to frequency conversion into a radio frequency band. The amplifying section  202  amplifies the transmission signal subjected to frequency conversion, and outputs the result to the transmission/reception antenna  201 . 
     Meanwhile, as for signals to be transmitted on the uplink from the mobile terminal apparatus  10  to the base station apparatus  20 , a radio frequency signal that is received in the transmission/reception antenna  201  is amplified in the amplifying section  202 , subjected to frequency conversion and converted into a baseband signal in the transmission/reception section  203 , and is input to the baseband signal processing section  204 . 
     The baseband signal processing section  204  performs FFT processing, IDFT processing, error correction decoding, MAC retransmission control reception processing, and RLC layer and PDCP layer reception processing of the user data included in the baseband signal that is received on the uplink. The decoded signal is transferred to the higher station apparatus  30  through the transmission path interface  206 . 
     The call processing section  205  performs call processing such as setting up and releasing a communication channel, manages the state of the base station apparatus  20  and manages the radio resources. 
     Next, referring to  FIG. 18 , an overall configuration of the mobile terminal apparatus  10  according to the present embodiment will be described. An LTE terminal and an LTE-A terminal have the same hardware configurations in the principle parts, and therefore will be described indiscriminately. The mobile terminal apparatus  10  has a transmission/reception antenna  101 , an amplifying section  102 , a transmission/reception section  103 , a baseband signal processing section  104 , and an application section  105 . 
     As for downlink data, a radio frequency signal received in the transmission/reception antenna  101  is amplified in the amplifying section  102 , and subjected to frequency conversion and converted into a baseband signal in the transmission/reception section  103 . This baseband signal is subjected to reception processing such as FFT processing, error correction decoding and retransmission control and so on 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 processing 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 . 
     On the other hand, uplink user data is input from the application section  105  to the baseband signal processing section  104 . In the baseband signal processing section  104 , retransmission control (H-ARQ (Hybrid ARQ)) transmission processing, channel coding, DFT processing, IFFT processing and so on are performed. The baseband signal output from the baseband signal processing section  104  is converted into a radio frequency band in the transmission/reception section  103 , and, after that, amplified in the amplifying section  102  and transmitted from the transmission/reception antenna  101 . 
       FIG. 19  is a functional block diagram of the baseband signal processing section  204  and part of the higher layers provided in the base station apparatus  20  according to the present embodiment, and primarily illustrates the function blocks of the transmission processing section in the baseband signal processing section  204 .  FIG. 19  illustrates an example of a base station configuration which can support maximum M (CC # 1  to CC #M) component carriers. Transmission data for the mobile terminal apparatus  10  under the base station apparatus  20  is transferred from the higher station apparatus  30  to the base station apparatus  20 . 
     A control information generation section  300  generates higher control signals for performing higher layer signaling (for example, RRC signaling), on a per-user basis. The higher control signals may include a command to request addition/removal of component carriers CC. 
     A data generation section  301  outputs the transmission data transferred from the higher station apparatus  30  separately as user data. 
     A component carrier selection section  302  selects component carriers to use in radio communication with the mobile terminal apparatus  10  on a per-user basis. As described above, addition/removal of component carriers is reported from the base station apparatus  20  to the mobile terminal apparatus  10  by RRC signaling, and a complete message is received from the mobile terminal apparatus  10 . As this complete message is received, assignment (addition/removal) of component carriers to that user is fixed, and the fixed component carrier assignment is set in the component carrier selection section  302  as component carrier assignment information. In accordance with the component carrier assignment information that is set in the component carrier selection section  302  on a per-user basis, higher control signals and transmission data are allocated to a component carrier channel coding section  303  of the applicable component carrier. Also, a specific component carrier (hereinafter referred to as “SS-grouping component carrier”), in which search spaces from a plurality of component carriers are grouped, is selected from the component carriers to be used in radio communication with the mobile terminal apparatus  10 . 
     A scheduling section  310  controls assignment of component carriers to a serving mobile terminal apparatus  10  according to overall communication quality of the system band. The scheduling section  310  determines addition/removal of component carriers to assign for communication with the mobile terminal apparatus  10 . A decision result related to addition/removal of component carriers is reported to the control information generation section  300 . Also, an SS-grouping component carrier is determined from the component carriers selected per user terminal. The SS-grouping component carrier may be switched dynamically or may be switched semi-statically. 
     Also, the scheduling section  310  controls resource allocation in component carriers CC # 1  to CC #M. The LTE terminal user and the LTE-A terminal user are scheduled separately. Also, the scheduling section  310  receives as input the transmission data and retransmission command from the higher station apparatus  30 , and also receives as input the channel estimation values and resource block CQIs from the reception section having measured an uplink received signal. The scheduling section  310  schedules downlink assignment information, uplink assignment information and uplink/downlink shared channel signals, with reference to the retransmission command, channel estimation values and CQIs that are received as input from the higher station apparatus  30 . A propagation path in mobile communication varies differently per frequency, due to frequency selective fading. So, upon transmission of user data to the mobile terminal apparatus  10 , resource blocks of good communication quality are assigned to each mobile terminal apparatus  10 , on a per subframe basis (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 and assigned. Consequently, the scheduling section  310  assigns resource blocks, with which improvement of throughput is anticipated, using the CQI of each resource block, fed back from each mobile terminal apparatus  10 . Also, the number of CCE aggregations is controlled according to the propagation path conditions with the mobile terminal apparatus  10 . The number of CCE aggregations is increased with respect to cell edge users. Also, the MCS (Coding rate and Modulation Scheme) to fulfill a required block error rate with the assigned resource blocks is determined. Parameters to fulfill the MCS (Coding rate and Modulation Scheme) determined by the scheduling section  310  are set in channel coding sections  303 ,  308  and  312 , and in modulation sections  304 ,  309  and  313 . 
     The baseband signal processing section  204  has channel coding sections  303 , modulation sections  304 , and mapping sections  305 , to match the maximum number of users to be multiplexed, N, in one component carrier. The channel coding section  303  performs channel coding of the shared data channel (PDSCH), formed with user data (including part of higher control signals) that is output from the data generation section  301 , on a per-user basis. The modulation section  304  modulates user data having been subjected to channel coding, on a per-user basis. The mapping section  305  maps the modulated user data to radio resources. 
     Also, the baseband signal processing section  204  has a downlink control information generation section  306  that generates downlink shared data channel control information, which is user-specific downlink control information, and a downlink shared channel control information generation section  307  that generates downlink shared control channel control information, which is user-common downlink control information. 
     Downlink assignment information (D 0 ) of DCI format 1 is downlink shared data channel control information. The downlink control information generation section  306  generates downlink control information (for example, DCI format 1), formed with downlink assignment information, from the resource allocation information, MCS information, HARQ information, PUCCH transmission power control command, and so on, determined on a per-user basis. The downlink control information (for example, DCI format 1) is arranged in the search space according to the present invention. 
     The baseband signal processing section  204  has channel coding sections  308  and modulation sections  309  to match the maximum number of users to be multiplexed, N, in one component carrier. The channel coding section  308  performs channel coding of control information generated in the downlink control information generation section  306  and the downlink shared channel control information generation section  307 , on a per-user basis. The modulation section  309  modulates the downlink control information after channel coding. 
     Also, the baseband signal processing section  204  has an uplink control information generation section  311  that generates, on a per-user basis, uplink shared data channel control information, which is control information for controlling the uplink shared data channel (PUSCH), a channel coding section  312  that performs, on a per-user basis, channel coding of uplink shared data channel control information generated, and a modulation section  313  that modulates, on a per-user basis, uplink shared data channel control information having been subjected to channel coding. 
     The downlink control information (U 0 ) formed with the uplink assignment information of DCI format 0 is uplink shared data channel control information. The uplink control information generation section  311  generates uplink assignment information from uplink resource allocation information (single carrier/cluster) that is determined per user, MCS information and redundancy version (RV), an identifier (new data indicator) to identify between new data and retransmission data, a PUCCH transmission power control command (TPC), cyclic shift for the demodulation reference signal (CS for DMRS), CQI request, and so on. In subframes (component carriers) where SC-FDMA is selected for the uplink radio access scheme, downlink control information (U 0 ) formed with uplink assignment information of DCI format 0 is generated according to the rules defined in LTE. The downlink control information (for example, DCI format 0) is arranged in the search space according to the present invention. 
     The control information that is modulated on a per-user basis in the above modulation sections  309  and  313  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 section  305  are input in an IFFT section  316  as downlink channel signals. The IFFT section  316  converts the downlink channel signal from a frequency domain signal into a time sequence signal by performing an inverse fast Fourier transform. A cyclic prefix insertion section  317  inserts cyclic prefixes in the time sequence signal of the downlink channel signal. Note that a cyclic prefix functions as a guard interval for cancelling the differences in multipath propagation delay. The transmission data to which cyclic prefixes are added is transmitted to the transmission/reception section  203 . 
       FIG. 20  is a functional block diagram of the baseband signal processing section  104  provided in the mobile terminal apparatus  10 , illustrating function blocks of an LTE-A terminal which supports LTE-A. First, the downlink configuration of the mobile terminal apparatus  10  will be described. 
     A CP removing section  401  removes the CPs from a downlink signal received from the radio base station apparatus  20  as received data. The downlink signal, from which the CPs have been removed, is input into an FFT section  402 . The FFT section  402  performs a fast Fourier transform (FFT) on the downlink signal, converts the time-domain signal into a frequency domain signal, and inputs the frequency domain signal in a demapping section  403 . The demapping section  403  demaps the downlink signal, and extracts, from the downlink signal, 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 . 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 control information demodulation section  405  that demodulates control information, a data demodulation section  406  that demodulates downlink shared data, and a channel estimation section  407 . The control information demodulation section  405  includes a shared control channel control information demodulation section  405   a  that demodulates downlink shared control channel control information from the downlink control channel, an uplink shared data channel control information demodulation section  405   b  that demodulates uplink shared data channel control information by performing blind decoding of the search space according to the present invention from the downlink control channel, and a downlink shared data channel control information demodulation section  405   c  that demodulates downlink shared data channel control information by performing blind decoding of the search space according to the present invention from the downlink control channel. The data demodulation section  406  includes a 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 shared control channel control information demodulation section  405   a  extracts shared control channel control information, which is user-common control information, by the blind decoding process, demodulation process, channel decoding process and so on of the common search space of the downlink control channel (PDCCH). The shared control channel control information includes downlink channel quality information (CQI), and therefore is input in a mapping section  115  (described later), and mapped as part of transmission data for the radio base station apparatus  20 . 
     The uplink shared data channel control information demodulation section  405   b  extracts uplink shared data channel control information, which is user-specific uplink assignment information, by the blind decoding process, demodulation process, channel decoding process and so on, of the user-specific search spaces of the downlink control channel (PDCCH). In particular, as for the user-specific search spaces, as described above, the search spaces of a plurality of component carriers are grouped in the PDCCH of the SS-grouping component carrier, so that of which component carrier&#39;s control information the demodulated DCI is, is determined using the CIFs. The uplink assignment information is used to control the uplink shared data channel (PUSCH), and is input into the downlink shared channel data demodulation section  406   b.    
     The downlink shared data channel control information demodulation section  405   c  extracts downlink shared data channel control information, which is user-specific downlink control signals, by the blind decoding process, demodulation process, channel decoding process and so on, of the user-specific search spaces of the downlink control channel (PDCCH). In particular, as for the user-specific search spaces, as described above, the search spaces of a plurality of component carriers are grouped in the PDCCH of the SS-grouping component carrier, so that which component carrier&#39;s control information demodulated DCI is, is determined using the CIFs. The downlink shared data channel control information is used to control the downlink shared data channel (PDSCH), and is input into the downlink shared data demodulation section  406 . 
     Also, the downlink shared data channel control information demodulation section  405   c  performs the blind decoding process of the user-specific search spaces, based on information which relates to the PDCCH and PDSCH and which is included in the higher control signals demodulated in the downlink shared data demodulation section  406   a . Information that relates to user-specific search spaces (which may include ON and OFF of activation/deactivation of the PDSCH/PDCCH) is signaled by higher control signals. 
     The downlink shared data demodulation section  406   a  acquires the user data, higher control information and so on, based on the downlink shared data channel control information received as input from the downlink shared data channel control information demodulation section  405   c . The higher control information (including mode information) is output to the channel estimation section  407 . The downlink shared channel data demodulation section  406   bc  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.    
     The channel estimation section  407  performs channel estimation using common reference signals. The estimated channel variation is output 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 . These demodulation sections demodulate downlink allocation information using the estimated channel variation and demodulation reference signals. 
     The baseband signal processing section  104  has, as function blocks of the transmission processing system, a data generation section  411 , a channel coding section  412 , a modulation section  413 , a DFT section  414 , a mapping section  415 , an IFFT section  416 , and a CP insertion section  417 . The data generation 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 processing such as error correction to the transmission data, and the modulation section  413  modulates the transmission data subjected to 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 the subcarrier positions designated by the base station apparatus. The IFFT section  416  performs an inverse fast Fourier transform on input data to match the system band and converts the input data into time sequence data, and the CP insertion section  417  inserts cyclic prefixes in the time sequence data per data segment. 
     Next, the control of search spaces to arrange downlink control information (DCI) of CC 0  to CC 3  in the event a plurality of component carriers CC 0  to CC 3  are allocated to the system band to use in radio communication between the mobile station apparatus  10  and the base station apparatus  20  will be described in detail. 
     The operation of arranging downlink control information (DCI) for CC 0  to CC 3  in the search spaces shown in  FIG. 5B  will be described. The control information generation section  300  (UE # 1 ) for UE # 1  signals the component carriers CC 0  to CC 3  to constitute the system band, to the mobile station apparatus  10 , by RRC signaling, with higher control signals. Also, the control information generation section  300  (UE # 1 ) signals SS-grouping component carrier CC 0 , which groups the search spaces of a plurality of component carriers, to the mobile station apparatus  10 , by RRC signaling, with higher control signals. Then, as shown in  FIG. 12A , when the search spaces of the component carriers are arranged based on the starting position of the search space of component carrier CC 1  to send the PDCCH, RRC signaling is made using the component carrier number (CC 1 ) to send the PDCCH, the component carrier numbers (CC 2 , CC 3 ) to send the PDSCH, and higher control signals. 
     Also, if a component carrier to deactivate the PDSCH is included, the control information generation section  300  (UE # 1 ) signals the CC number of the component carrier to be deactivated, by RRC signaling. Higher control signals to be signaled by RRC signaling are arranged in the PDSCH and sent. 
     In the baseband processing section  204 , the downlink control information generation section  306  (UE # 1 ) for component carriers CC 0  to CC 3  generates control information D 0  to D 3 , and the uplink control information generation section  311  (UE # 1 ) generates control information U 0  to U 3 . A CIF is attached to each of control information D 0  to D 3  and U 0  to U 3 . The generate control information are passed to the downlink control information generation section  306  (UE # 1 ) and uplink control information generation section  311  (UE # 1 ) of the SS-grouping component carrier (CC 0 ). The downlink control information generation section  306  (UE # 1 ) and uplink control information generation section  311  (UE # 1 ) of the SS-grouping component carrier (CC 0 ) arrange control information D 0  to D 3  and U 0  to U 3  in the search spaces formed as shown in  FIG. 5B . The search spaces shown in  FIG. 5B  are two types: normal-size SS 1  and compact-size SS 2 . 
     Also, downlink control information generation section  306  (UE # 1 ) and uplink control information generation section  311  (UE # 1 ) determine the sizes of the search spaces by applying one of the schemes from  FIG. 12  to  FIG. 15 . With the scheme shown in  FIG. 12 , the basic size (6 CCEs) is simply made N-fold by the number of component carriers N. With the scheme shown in  FIG. 13 , when the number of CCs is 1 to 3, the size making the basic size (6 CCEs) N-fold by the number of component carriers N is used, and, when the number of CCs is greater than that, the size at the number of CCs=3 is fixed. With the scheme shown in  FIG. 14 , the search space sizes are configured to partly overlap between component carriers by applying CC-specific offsets. With the scheme shown in  FIG. 15 , mapping is performed to reduce the overlap between component carriers. 
     Control information (D 0  or D 0 ′) that is generated in the downlink control information generation section  306  (UE # 1 ) and control information (U 0  or U 0 ′) that is generated in the uplink control information generation section  311  (UE # 1 ) are multiplexed in the control channel multiplexing section  314  not to overlap, and the state of search space arrangement shown in  FIG. 5B  is assumed. In this way, the PDCCH of the SS-grouping component carrier CC 0 , in which control information D 0  to D 3  and U 0  to U 3  are arranged in the search spaces, is transmitted. 
     Also, in the event a component carrier to deactivate the PDSCH is included, the search space to arrange the control information for that deactivated component carrier is also deactivated.  FIG. 12B  shows a state in which the component carrier CC 2  is deactivated. The downlink control information generation section  306  controls not to arrange control information or allocate transmission power to the search space of the component carrier CC 2  where PDSCH is deactivated. 
     Note that it is also possible to deactivate the PDCCH of the component carrier in which the PDSCH is deactivated concurrently. The control information generation section  300  generates channel “OFF” information with respect to the deactivated PDSCH and/or PDCCH and signals this to the mobile terminal apparatus  10  by RRC signaling. 
     Meanwhile, the mobile terminal apparatus  10  to be user UE # 1  receives the PDCCH on the downlink. The deinterleaving section  404  de-interleaves the PDCCH mapped to the first through third OFDM symbols at the top of the subframe. The rate matching parameter (the number of CCEs) and the CCE starting position are not clear in the mobile terminal apparatus  10 , and therefore the control information demodulation section  405  performs blind decoding per CCE and searches for a CCE where the CRC masked by the user ID is “OK”. 
     The downlink shared data channel control information demodulation section  405   a  searches for shared data channel control information for the subject apparatus by performing blind decoding of search space SS 1  of the PDCCH. Then, since SS-grouping component carrier CC 0  has already been reported, PDCCH blind decoding is not performed with respect to the component carriers CC 1  to CC 3  where the PDCCH is not transmitted. Control information D 0  to D 3  is demodulated by performing blind decoding of the search space SS 1 . Control information of component carriers CC 1  to CC 3  is specified based on the CIFs attached to the control information D 0  to D 3 . 
     The uplink shared data channel control information demodulation section  405   b  searches for shared data channel control information for the subject apparatus by performing blind decoding of the search space SS 2  of the PDCCH. For component carriers CC 1  to CC 3  where the PDCCH is not sent, PDCCH blind decoding is not performed. Control information U 0  to U 3  is demodulated by performing blind decoding of the search space SS 2 . Control information of component carriers CC 1  to CC 3  is specified based on the CIFs attached to the control information U 0  to U 3 . 
     The uplink shared data channel control information demodulation section  405   b  interprets the searched control information U 0  to U 3  for the subject apparatus. Then, resource allocation information and other parameters (MCS information and so on) are extracted from DCI format 0. The resource allocation information is given to the mapping section  415 , and the other parameters are given to the applicable blocks such as the channel coding section  412 , modulation section  413  and so on. 
     When downlink control information (DCIs) for CC 0  to CC 3  are arranged in the search spaces shown in  FIG. 5C , the component carrier CC 0  is signaled as the anchor carrier. The anchor carrier is recognized between the mobile station apparatus  10  and the base station apparatus  20 . The downlink control information generation section  306  (UE # 1 ) and the uplink control information generation section  311  (UE # 1 ) arrange control information D 0  and U 0  of the anchor carrier in the search spaces without attaching CIFs. 
     In the mobile terminal apparatus  10 , the downlink shared data channel control information demodulation section  405   a  and the uplink shared data channel control information demodulation section  405   b  can recognize that control information D 0  and U 0  without CIFs are control information for the component carrier CC 0  (anchor carrier). 
     Note that, as shown in  FIGS. 6B and 6C , when a plurality of component carriers CC 1  to CC 3  to constitute the system band is grouped according to the DCI size, the SS-grouping component carrier is determined on a per-group basis, and the search space control shown in  FIG. 5B  or  FIG. 5C  is performed on a per-group basis. Then, the CC numbers of the SS-grouping component carrier, which becomes the CC to send the PDSCH, and the CC to send to PDSCH, are signaled by RRC signaling, on a per-group basis. 
     Also, as shown in  FIG. 7 , it is equally possible to define three types of blind decoding and provide a search space per blind decoding type. 
       FIG. 7  provides three types of search spaces SS 1 , SS 2  and SS 3 , per component carrier. In the event multiple frequency bands are used for an uplink radio access scheme, control information (U 0 ), formed with uplink assignment information, is generated in DCI format 0A of a larger size than compact-type DCI format 0. If DCI format 0A of a large size is commanded from the scheduling section  310 , the uplink control information generation section  311  generates control information (U 0 ), formed with uplink assignment information, in DCI format 0A. Control information (U 0 ) generated in DCI format 0A of a large size is arranged in the third search space SS 3 . 
     In the mobile terminal apparatus  10 , the uplink shared data channel control information demodulation section  405   b  performs blind decoding of the third search space SS 3  and demodulates the control information (U 0 ) generated in DCI format 0A. 
     Note that, as shown in  FIG. 8 , it is equally possible to support the compact-type second search space SS 2  only in the anchor carrier (CC 0 ). The anchor carrier (CC 0 ) adopts a search space configuration in which fallback from DCI format 0A of a large size to DCI format 0 of a compact size is possible. If fallback to DCI format 0 is commanded from the scheduling section  310 , the uplink control information generation section  311  switches the generation of control information, including uplink assignment information, to compact-size DCI format 0, and arranges compact-size control information (U 0 ′) in the second search space SS 2 . 
     Also, when allocation is asymmetric between the uplink and the downlink as with component carrier CC 1  shown in  FIGS. 9 and 10 , it is preferable to select one from above options  1  to  3 . For example, a case of selecting option  2  in  FIG. 10  will be explained. The downlink control information generation section  306  (UE # 1 ) uses the search space shown in option  2  in  FIG. 10  when control information (D 1 ′) is generated in compact-size DCI format 1A. That is to say, the compact-size search space SS 2 , where uplink control information (UL) is not allocated and control information (D 1 ′) alone is allocated, is arranged in the PDCCH of the asymmetric component carrier CC 1 . 
     In the mobile terminal apparatus  10 , the downlink shared data channel control information demodulation section  405   a  performs blind decoding of the second search space SS 2  and demodulates control information (D 0 ′) generated in DCI format 1A. 
     Note that, when option  3  of  FIG. 10  is selected, control information (D 0 ′) is not arranged in the search space SS 2 , and DCI format 1 of a normal size alone is arranged in the first search space SS 1  and sent. 
     The disclosure of Japanese Patent Application No. 2010-087383, filed on Apr. 5, 2010, including the specification, drawings, and abstract, is incorporated herein by reference in its entirety.