Patent Publication Number: US-2023148110-A1

Title: Base station, terminal, and communication method

Description:
TECHNICAL FIELD 
     The present disclosure relates to a base station, a terminal, and a communication method. 
     BACKGROUND ART 
     A study is being conducted on a communication system called the fifth-generation mobile communication system (5G). In 5G, a study is being conducted on flexibly providing functions for each of use cases in which an increase in communication traffic, an increase in the number of terminals to be connected, high reliability, and low latency are respectively needed. Three use cases, that is, a use case for enhanced mobile broadband (eMBB: enhanced Mobile Broadband), a use case for large-scale communications/massive connection (mMTC: massive Machine Type Communications), and a use case for ultra-reliable and low-latency communication (URLLC: Ultra Reliable and Low Latency Communication), are available as typical use cases. 3GPP (the 3rd Generation Partnership Project), which is an international standardization group, is studying sophisticating a communication system from two embodiments of sophisticating an LTE system and a new RAT (Radio Access Technology) (see, for example, NPL 1). 
     CITATION LIST 
     Non Patent Literature 
     NPL 1: RP-161596, “Revision of SI: Study on New Radio Access Technology”, NTT DOCOMO, September 2016 
     NPL 2: R1-1702765, “DL control channel design”, Panasonic, February 2017 
     SUMMARY OF INVENTION 
     In the new RAT, a study is being conducted on setting, for a terminal (UE: user equipment), a plurality of control resource sets (hereinafter referred to as “CORESETs”) as a region in which a PDCCH (Physical Downlink Control Channel), which is a control signal channel including a DCI (downlink control indicator), is arranged. However, in the new RAT, no sufficient study has been conducted on a method for arranging CCEs (control channel elements) that constitute a search space in a CORESET. 
     One embodiment of the present disclosure facilitates providing a base station, a terminal, and a communication method that can appropriately arrange CCEs that constitute a search space in a CORESET. 
     A base station according to one embodiment of the present disclosure comprises: circuitry that allocates downlink control signals to a control channel region constituted by a plurality of control channel elements (CCEs); and a transmitter that transmits the downlink control signals. The number of resource element groups (REGs) that constitute the CCE is a power of 2, and a bundling size indicating the number of REGs included in the REGs that constitute the CCE and arranged in adjacent resource blocks is a power of 2. 
     A terminal according to one embodiment of the present disclosure comprises: a receiver that receives downlink control signals in a control channel region constituted by a plurality of control channel elements (CCEs); circuitry that decodes the downlink control signals. The number of resource element groups (REGs) that constitute the CCE is a power of 2, and a bundling size indicating the number of REGs included in the REGs that constitute the CCE and arranged in adjacent resource blocks is a power of 2. 
     In a communication method according to one embodiment of the present disclosure, downlink control signals are allocated to a control channel region constituted by a plurality of control channel elements (CCEs), and the downlink control signals are transmitted. The number of resource element groups (REGs) that constitute the CCE is a power of 2, and a bundling size indicating the number of REGs included in the REGs that constitute the CCE and arranged in adjacent resource blocks is a power of 2. 
     In a communication method according to one embodiment of the present disclosure, downlink control signals are received with a control channel region constituted by a plurality of control channel elements (CCEs), and the downlink control signals are decoded. The number of resource element groups (REGs) that constitute the CCE is a power of 2, and a bundling size indicating the number of REGs included in the REGs that constitute the CCE and arranged in adjacent resource blocks is a power of 2. 
     These general or specific embodiments may be realized by a system, an apparatus, a method, an integrated circuit, a computer program, or a recording medium or may be realized by an arbitrary combination of a system, an apparatus, a method, an integrated circuit, a computer program, and a recording medium. 
     Advantageous Effects of Invention 
     According to one embodiment of the present disclosure, it is possible to appropriately arrange CCEs that constitute a CORESET. 
     Additional benefits and advantages of one embodiment of the present disclosure will become apparent from the specification and drawings. Such benefits and/or advantages are individually obtained by some embodiments and features stated in the specification and drawings, which need not all be provided in order to obtain one or more of the same features. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1 A  illustrates a REG mapping example (frequency first mapping). 
         FIG.  1 B  illustrates a REG mapping example (frequency first mapping). 
         FIG.  1 C  illustrates a REG mapping example (time first mapping). 
         FIG.  1 D  illustrates a REG mapping example (time first mapping). 
         FIG.  2 A  illustrates one example of REG bundling. 
         FIG.  2 B  illustrates one example of the REG bundling. 
         FIG.  2 C  illustrates one example of the REG bundling. 
         FIG.  3    illustrates a REG mapping example when the number of REGs per CCE is 4, the number of symbols is 3, and a REG bundling size is 2. 
         FIG.  4    illustrates one example of the numbers of per-symbol REGs that constitute the same DCI for respective aggregation levels. 
         FIG.  5    illustrates the configuration of a portion of a base station according to a first embodiment. 
         FIG.  6    illustrates the configuration of a portion of a terminal according to the first embodiment. 
         FIG.  7    illustrates the configuration of the base station according to the first embodiment. 
         FIG.  8    illustrates the configuration of the terminal according to the first embodiment. 
         FIG.  9    illustrates an operation example of the base station and the terminal according to the first embodiment. 
         FIG.  10 A  illustrates a REG mapping example (when the number of symbols is 1) according to operation example 1-1 in the first embodiment. 
         FIG.  10 B  illustrates a REG mapping example (when the number of symbols is 2) according to operation example 1-1 in the first embodiment. 
         FIG.  10 C  illustrates a REG mapping example (when the number of symbols is 4) according to operation example 1-1 in the first embodiment. 
         FIG.  11    illustrates one example of the numbers of per-symbol REGs that constitute the same DCI for respective aggregation levels according to operation example 1-1 in the first embodiment. 
         FIG.  12    illustrates one example of the numbers of per-symbol REGs that constitute the same DCI for the respective aggregation levels according to operation example 1-1 in the first embodiment. 
         FIG.  13    illustrates a REG mapping example according to operation example 1-2 in the first embodiment. 
         FIG.  14    illustrates one example of the numbers of per-symbol REGs that constitute the same DCI for the respective aggregation levels according to operation example 1-2 in the first embodiment. 
         FIG.  15    illustrates a DMRS mapping example (when the number of symbols is 2) according to operation example 1-3 in the first embodiment. 
         FIG.  16 A  illustrates a DMRS mapping example (when the number of symbols is 4) according to operation example 1-3 in the first embodiment. 
         FIG.  16 B  illustrates a DMRS mapping example (when the number of symbols is 4) according to operation example 1-3 in the first embodiment. 
         FIG.  17 A  illustrates a REG mapping example (when the number of symbols is 2) according to a second embodiment. 
         FIG.  17 B  illustrates a REG mapping example (when the number of symbols is 4) according to the second embodiment. 
         FIG.  17 C  illustrates a REG mapping example (when the number of symbols is 3) according to the second embodiment. 
         FIG.  17 D  illustrates a REG mapping example (when the number of symbols is 6) according to the second embodiment. 
         FIG.  18    illustrates one example (when the number of symbols is 2) of the numbers of per-symbol REGs that constitute the same DCI for the respective aggregation levels according to the second embodiment. 
         FIG.  19    illustrates one example (when the number of symbols is 4) of the numbers of per-symbol REGs that constitute the same DCI for the respective aggregation levels according to the second embodiment. 
         FIG.  20    illustrates one example (when the number of symbols is 3) of the numbers of per-symbol REGs that constitute the same DCI for the respective aggregation levels according to the second embodiment. 
         FIG.  21    illustrates one example (when the number of symbols is 6) of the numbers of per-symbol REGs that constitute the same DCI for the respective aggregation levels according to the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present disclosure will be described below in detail with reference to the drawings. 
     As described above, in the new RAT, a study is being conducted on a technology in which a CORESET, which is a control channel region including a Das that are control signals, is set for a UE, and the UE monitors (blind-decodes) a search space in the set CORESET to detect Das addressed to the UE. Also, a study is being conducted on using CCEs to define the search space in which the Das are arranged. 
     In this case, in the new RAT, a number between 4 and 8 is considered as the number of REGs (resource element groups) per CCE. Also, a number from one symbol to all symbols in a slot or a subframe is conceivable as the number of symbols in which a CORESET is set. 
     However, the numbers of per-symbol REGs in respective CCEs become unequal, depending on a combination of the number of REGs per CCE and the number of symbols in which a CORESET is set. When the numbers of REGs across symbols are not equal in CCEs, there is a problem in that variations occur in reception SINRs (Signal to Interference and Noise Ratios) for the respective symbols, thus making it difficult to adjust power across the CCEs. 
     A description will be given below in more detail. 
     A DCI is transmitted with one or more CCEs. The number of CCEs used for one DCI is herein referred to as an “aggregation level”. That is, the aggregation level indicates the amount of resources for transmitting a DCI. When a DCI is transmitted with a CORESET (a PDCCH region), for example, the DCI is transmitted with one CCE in aggregation level  1 , and the DCI is transmitted with two CCEs in aggregation level  2 . 
     Also, setting one symbol in one PRB (physical resource block) as a “REG” is considered as one form of a plurality of REGs that constitute a CCE (see, for example,  FIG.  1 A ). 
     In the case of such a form, a case in which REGs arranged in the same symbol constitute a CCE (which may also be referred to as an “NR-CCE”), as illustrated in  FIGS.  1 A and  1 B , and a case in which REGs arranged in a plurality of symbols constitute a CCE, as illustrated in  FIGS.  1 C and  1 D , are conceivable as CCE configurations. 
     The case in which REGs that constitute one CCE are arranged in the same symbol is called “frequency first mapping”. The frequency first mapping has advantages in that the number of symbols occupied by CCEs is reduced, and the amount of resources allocated to a PDSCH (Physical Downlink Shared Channel) increases. On the other hand, the case in which REGs that constitute one CCE are arranged in a plurality of symbols is called “time first mapping”. The time first mapping has advantages in that CCEs can be transmitted with a plurality of symbols (see, for example, NPL 2) when there is a restriction on transmission power that can be used for each symbol, and thus the transmission power can be improved. 
     In the new RAT, a method called “REG bundling” is also studied. The REG bundling refers to a method in which a plurality of REGs that constitute the same CCE are arranged in adjacent PRBs, and the REGs share reference signals (DMRSs: Demodulation Reference Signals) arranged in the adjacent PRBs, to thereby improve the channel estimation accuracy. 
       FIGS.  2 A to  2 C  illustrate REG mapping examples (arrangement examples) when a bundling number (hereinafter referred to as a “REG bundling size”), which is the number of REGs arranged in adjacent PRBs, is 2. As illustrated in  FIGS.  2 A to  2 C , two REGs of the REGs that constitute the same CCE are arranged in adjacent PRBs. As a result, even when a plurality of DMRSs is not arranged in the time direction (symbol direction), channel estimation can be interpolated in the frequency direction, thus making it possible to improve the channel estimation accuracy. 
     In this case, when the number of REGs per CCE is 4 or 8, and the number of symbols in a CORESET is 3, it is difficult to equally arrange REGs for each CCE in the three symbols.  FIG.  3    illustrates a REG mapping example when the number of REGs per CCE is 4, the number of symbols in a CORESET is 3, and the REG bundling size is 2. Also,  FIG.  4    illustrates, in the REG mapping example illustrated in  FIG.  3   , the numbers of per-symbol REGs that constitute the same DCI for respective aggregation levels (which may be referred to as “AL”). 
     For example, in the case of aggregation level  1  (AL  1 ) in  FIG.  3   , that is, in a case in which the number of REGs in which a DCI is arranged is four, two REGs are arranged in each of two symbols of three symbols, and no REG is arranged in the remaining symbol. Also, in  FIG.  3   , in the case of aggregation level  2  (AL  2 ), that is, in a case in which the number of REGs in which a DCI is arranged is  8 , four REGs are arranged in one symbol (symbol # 0  in the example in  FIG.  3   ) of three symbols, and two REGs are arranged in each of the remaining two symbols (symbols # 1  and # 2  in the example  FIG.  3   ). The same applies to aggregation levels  4  and  8  (AL  4  and AL  8 ). 
     That is, as illustrated in  FIG.  4   , in AL  1 , REGs that constitute one CCE used for transmitting a DCI are arranged in only two symbols and are not arranged in one symbol. Also, as illustrated in  FIG.  4   , in AL  2 , AL  4 , and AL  8 , REGs that constitute a plurality of CCEs used for transmitting a DCI are unequally arranged across the symbols. Thus, since the numbers of REGs across symbols are not equal in one or more CCEs used for transmitting a DCI, variations occur in the reception SINRs for the respective symbols, thus making it difficult to adjust power across the CCEs. 
     Accordingly, the following description will be given of a method in which REGs that constitute a plurality of CCEs used for transmitting a DCI are uniformly arranged across symbols to thereby reduce variations in the reception SINRs for respective symbols and make it easy to adjust power across the CCEs. 
     (First Embodiment) 
     [Overview of Communication System] 
     A communication system according to each embodiment of the present disclosure includes a base station  100  and a terminal  200  (UE). 
       FIG.  5    is a block diagram illustrating the configuration of a portion of the base station  100  according to an embodiment of the present disclosure. In the base station  100  illustrated in  FIG.  5   , a signal allocating unit  105  allocates downlink control signals (DCI) to a control channel region (CORESET) constituted by a plurality of control channel elements (CCEs). A transmitting unit  106  transmits the downlink control signals. 
       FIG.  6    is a block diagram illustrating the configuration of a portion of the terminal  200  according to the embodiment of the present disclosure. In the terminal  200  illustrated in  FIG.  6   , a receiving unit  201  receives downlink control signals (DCI) in a control channel region (CORESET) constituted by a plurality of control channel elements (CCEs). A DCI receiving unit  203  decodes (blind-decodes) the downlink control signals. 
     In this case, the number of resource element groups (REGs) that constitute a CCE is a power of 2, and the bundling size indicating the number of REGs included in the REGs that constitute the CCE and arranged in adjacent resource blocks is a power of 2. 
     [Configuration of Base Station] 
       FIG.  7    is a block diagram illustrating the configuration of the base station  100  according to the present embodiment. In  FIG.  7   , the base station  100  has a CORESET setting unit  101 , a DCI generating unit  102 , an error-correction encoding unit  103 , a modulating unit  104 , the signal allocating unit  105 , the transmitting unit  106 , a receiving unit  107 , a signal separating unit  108 , a demodulating unit  109 , and an error-correction decoding unit  110 . 
     The CORESET setting unit  101  sets a CORESET for each terminal  200  (UE). The setting (definition) of the CORESET includes, for example, the number of PRBs in which each CORESET is set, a PRB number, a symbol number, the number of symbols, an ID used for scrambling the CORESET, a REG (Resource Element Group) mapping method (localized or distributed), Quasi collocation (QCL), and so on. The CORESET setting unit  101  generates higher-layer signaling (for example, an SIB (System Information Block) or dedicated RRC (Radio Resource Control)) including CORESET-setting information indicating the setting of the CORESET. The CORESET setting unit  101  outputs higher-layer signaling to the error-correction encoding unit  103  and outputs the CORESET-setting information to the signal allocating unit  105 . 
     The DCI generating unit  102  generates a DCI including resource allocation information (DL (Downlink) allocation information or UL (Uplink) allocation information) for DL data signals or UL data signals and outputs the DCI to the signal allocating unit  105 . Also, the DCI generating unit  102  outputs the DL allocation information in the generated DCI to the signal allocating unit  105  and outputs the UL allocation information therein to the signal separating unit  108 . 
     The error-correction encoding unit  103  performs error-correction encoding on transmission data signals (DL data signals) and the higher-layer signaling (CORESET-setting information) input from the CORESET setting unit  101  and outputs the encoded signals to the modulating unit  104 . 
     The modulating unit  104  performs modulation processing on the signals received from the error-correction encoding unit  103  and outputs the modulated signals to the signal allocating unit  105 . 
     Based on the DL allocation information input from the DCI generating unit  102 , the signal allocating unit  105  allocates the signals (the DL data signals and the higher-layer signaling) received from the modulating unit  104  to downlink resources. Also, in accordance with the CORESET-setting information input from the DCI generating unit  102 , the signal allocating unit  105  allocates the DCI input from the DCI generating unit  102  to resources (one or more CCEs in the CORESET). For example, the signal allocating unit  105  may change REG mapping or CCE mapping to a search space in accordance with the number of symbols in which a CORESET is set, the number being indicated in the CORESET-setting information. The transmission signals are formed as described above. The formed transmission signals are output to the transmitting unit  106 . 
     The transmitting unit  106  performs wireless transmission processing, such as up-conversion, on the transmission signals input from the signal allocating unit  105  and transmits the transmission signals to the terminal  200  via an antenna. 
     The receiving unit  107  receives, via the antenna, signals transmitted from the terminal  200 , performs wireless reception processing, such as down-conversion, on the reception signals and outputs the reception signal to the signal separating unit  108 . 
     The signal separating unit  108  separates UL data signals from the reception signals received from the receiving unit  107 , based on the UL allocation information input from the DCI generating unit  102 , and outputs signals to the demodulating unit  109 . 
     The demodulating unit  109  performs demodulation processing on the signals input from the signal separating unit  108  and outputs resulting signals to the error-correction decoding unit  110 . 
     The error-correction decoding unit  110  decodes the signals input from the demodulating unit  109  to obtain the reception data signals (UL data signals) from the terminal  200 . 
     [Configuration of Terminal] 
       FIG.  8    is a block diagram illustrating the configuration of the terminal  200  according to the present embodiment. In  FIG.  8   , the terminal  200  has the receiving unit  201 , a signal separating unit  202 , the DCI receiving unit  203 , a demodulating unit  204 , an error-correction decoding unit  205 , a setting-information receiving unit  206 , an error-correction encoding unit  207 , a modulating unit  208 , a signal allocating unit  209 , and a transmitting unit  210 . 
     The receiving unit  201  receives reception signals via the antenna, performs reception processing, such as down-conversion, on the reception signals, and then outputs the reception signals to the signal separating unit  202 . The reception signals include, for example, DL data signals, higher-layer signaling (including the CORESET-setting information), a DCI (including resource allocation information and so on), and so on. 
     The signal separating unit  202  separates uplink signals from the reception signals and outputs signals to the demodulating unit  204 . Also, the signal separating unit  202  identifies resources corresponding to a CORESET to be monitored (a CORESET to be separated) from the reception signals received from the receiving unit  201 , based on information indicating the CORESET setting input from the setting-information receiving unit  206 , separates signals arranged in the resources, and outputs signals to the DCI receiving unit  203 . Also, the signal separating unit  202  separates DL data signals from the reception signals, based on the DL allocation information input from the DCI receiving unit  203 , and outputs signals to the demodulating unit  204 . 
     The DCI receiving unit  203  attempts to decode the signals input from the signal separating unit  202  and arranged in resources corresponding to the CORESET and detects (receives) a DCI addressed to the local terminal. The DCI receiving unit  203  outputs the UL allocation information indicated by the received DCI to the signal allocating unit  209  and outputs the DL allocation information to the signal separating unit  202 . 
     The demodulating unit  204  demodulates the signals input from the signal separating unit  202  and outputs the demodulated signals to the error-correction decoding unit  205 . 
     The error-correction decoding unit  205  decodes the demodulated signals received from the demodulating unit  204 , outputs resulting reception data signals, and outputs resulting higher-layer signaling to the setting-information receiving unit  206 . 
     Based on the CORESET-setting information included in the higher-layer signaling output from the error-correction decoding unit  205 , the setting-information receiving unit  206  identifies the setting of the CORESET for each terminal  200 . The setting-information receiving unit  206  then outputs the identified information to the signal separating unit  202 . 
     The error-correction encoding unit  207  performs error-correction encoding on the transmission data signals (UL data signals) and outputs the encoded data signals to the modulating unit  208 . 
     The modulating unit  208  modulates the data signals input from the error-correction encoding unit  207  and outputs the modulated data signals to the signal allocating unit  209 . 
     Based on the UL allocation information input from the DCI receiving unit  203 , the signal allocating unit  209  identifies resources to which the UL data is to be allocated. The signal allocating unit  209  then allocates the data signals input from modulating unit  209  to the identified resources and outputs signals to the transmitting unit  210 . 
     The transmitting unit  210  performs transmission processing, such as up-conversion, on the signals input from the signal allocating unit  209  and transmits transmission signals via the antenna. 
     [Operations of Base station  100  and Terminal  200 ] 
     The operations of the base station  100  and the terminal  200  having the above-described configurations will be described in detail. 
       FIG.  9    is a sequence diagram illustrating the operations of the base station  100  and the terminal  200 . 
     The base station  100  sets a CORESET for each terminal  200  (ST 101 ). The base station  100  transmits setting information of the set CORESET to the terminal  200  by using higher-layer signaling (ST 102 ). Next, the base station  100  generates a DCI including resource allocation information and so on (ST 103 ). The base station  100  arranges the generated DCI in any of search spaces in the CORESET set in ST 101  and transmits the DCI to the terminal  200  (ST 104 ). Details of a mapping method (an arrangement method) for CCEs (REGs) that constitute a CORESET are described later. 
     Meanwhile, based on the CORESET-setting information included in the higher-layer signaling received in ST 102 , the terminal  200  monitors the CORESET (search space) to detect a DCI addressed to the local terminal  200  (ST 105 ). 
     Next, a description will be given of details of a mapping method for CCEs (REGs) that constitute a CORESET. 
     Operation examples 1-1 to 1-3 according to the present embodiment will be individually described below. 
     OPERATION EXAMPLE 1-1 
     In operation example 1-1, with respect to a CCE and REG mapping to a CORESET, the number of REGs that constitute a CCE (the number of REGs per CCE) is a power of 2, and the REG bundling size is a power of 2. 
     In addition, in operation example 1-1, the number of symbols in which a CORESET is set is a power of 2. 
     With such an arrangement, even when the number of symbols in a CORESET set for the terminal  200  differs, mapping of REGs that constitute a CCE in the CORESET becomes common, and the mapping of the REGs becomes simple. 
     Also, setting the REG bundling size to a power of 2 makes it easy to perform adjustment when different subcarrier spacings (numerology) are allocated to the same slot or when interference control is performed between cells. 
       FIGS.  10 A to  10 C  illustrate REG mapping examples when the number of REGs per CCE is 4 (=2 2 ), and the REG bundling size is 2 (=2 1 ). 
     In  FIGS.  10 A to  10 C , the REG mapping is the time first mapping. That is, REGs that constitute one CCE are arranged in the time direction (symbols) prior to the frequency direction (PRBs) in units of the REG bundling size. Also, in  FIGS.  10 A to  10 C , the CCE mapping to the search space is also the time first mapping. That is, the base station  100  allocates a DCI in the time direction (symbols) prior to the frequency direction (PRBs) in units of a CCE. As a result, REGs that constitute a CCE used for transmitting a DCI are arranged in different symbols as much as possible in units of the REG bundling size. 
       FIG.  10 A  illustrates a REG mapping example when the number of symbols in a CORESET is 1 (=2 0 . 
     As illustrated in  FIG.  10 A , when the number of symbols in a CORESET is  1 , all REGs that constitute a CCE are arranged in the same symbol (symbol # 0 ) even in the case of the time first mapping. Accordingly, for example, setting the number of symbols in a CORESET to 1 makes it possible to realize frequency first mapping with a design that is equivalent to the design for the time first mapping even without additionally defining mapping in the frequency first mapping. 
     In particular, changing a beam (precoding) for each symbol is conceivable in a high frequency band, for example, in a millimeter wave band. In such a case, when the terminal  200  (UE) monitors a plurality of CORESETs, each CORESET ( FIG.  10 A ) being arranged in one symbol, by using a plurality of symbols, it is easy to perform time division multiplexing. Accordingly, in a high frequency band, limiting the number of symbols in a CORESET to 1 is also effective. 
       FIG.  10 B  illustrates a REG mapping example when the number of symbols in a CORESET is 2 (=2 1 ). 
     As illustrated in  FIG.  10 B , when the number of symbols in a CORESET is 2, two REGs of REGs that constitute a CCE are arranged in symbol # 0 , and the remaining two REGs are arranged in symbol # 1 . That is, two of the REGs that constitute a CCE are arranged in each of two symbols in units of the REG bundling size. 
       FIG.  10 C  illustrates a REG mapping example when the number of symbols in a CORESET is 4 (=2 2 ). 
     As illustrated in  FIG.  10 C , when the number of symbols in a CORESET is 4, REGs that constitute one CCE are arranged in two symbols in units of the REG bundling size (2 REGs). Also, for example, REGs that constitute two CCEs used for transmitting a DCI in aggregation level  2  are arranged in four different symbols. 
     When REGs in each CCE are arranged as illustrated in  FIGS.  10 B and  10 C , REGs that constitute one or more CCEs used for transmitting a DCI are equally arranged in each symbol. 
     For example,  FIG.  11    illustrates one example of the numbers of per-symbol REGs for respective aggregation levels (AL  1 , AL  2 , AL  4 , and AL  8 ) when the number of REGs per CCE is 4, the number of symbols is 2, and the REG bundling number is 2 (see, for example,  FIG.  10 B ). As illustrated in  FIG.  11   , it can be understood that the numbers of per-symbol REGs that constitute a CCE used for transmitting the same DCI are the same in any of the aggregation levels. 
     Also,  FIG.  12    illustrates one example of the numbers of per-symbol REGs for the respective aggregation levels when the number of REGs per CCE is 4, the number of symbols is 4, and the REG bundling number is 2 (see, for example  FIG.  10 C ). As illustrated in  FIG.  12   , in AL  1 , four REGs that constitute one CCE used for transmitting a DCI are uniformly arranged in two symbols. Also, as illustrated in  FIG.  12   , the numbers of per-symbol REGs that constitute a CCE used for transmitting the same DCI are equal to each other in any aggregation level of AL  2 , AL  4 , and AL  8 . 
     Thus, since a number of REGs that constitute each CCE, the number being a power of 2, are arranged in symbols in units of the REG bundling size that is a power of 2, the numbers of REGs that constitute a CCE used for transmitting a DCI become equal for the respective symbols, thus making it easy to adjust power across the CCEs. 
     Also, since mapping of REGs and mapping of CCEs to a search space are based on the time first mapping, a PDCCH (DCI) is arranged in a plurality of symbols when the aggregation level is high, thus providing an advantage that it is easy to perform power boosting. 
     Also, setting the REG bundling size to a power of 2 makes it possible to align spacings of PRBs in a frequency domain even when a terminal  200  having a different subcarrier spacing exists, thus making it possible to improve the use efficiency of resources. 
     OPERATION EXAMPLE 1-2 
     Cases in which the number of symbols in a CORESET is a power of 2 have been described in operation example 1-1. In contrast, a case in which the number of symbols in a CORESET is a value other than a power of 2 will be described in operation example 1-2. 
     For example, when the number of symbols in a CORESET is a number of symbols other than a power of 2, REG mapping is set as a reference of REG mapping for the number of symbols which is larger than the number of symbols in a CORESET and which is a power of 2 closest thereto. 
     Specifically, when the number of symbols in a CORESET is 3, the last symbol is punctured or rate-matched in REG mapping (see, for example,  FIG.  10 C ) in which the number of symbols is 4 (=2 2 ), which is described above in operation example 1-1, to thereby set REG mapping for the symbols in the CORESET, as illustrated in  FIG.  13   . 
     With such an arrangement, the number of REGs that are actually used differs from that in the reference REG mapping, but when the aggregation level is 2, 4, or 8, the numbers of per-symbol REGs that constitute a CCE used for transmitting the same DCI become equal to each other. Also, since the design of common REG mapping can be used for all symbols, there is an advantage in that the design becomes simple. 
     For example,  FIG.  14    illustrates the numbers of per-symbol REGs for the respective aggregation levels when the number of REGs per CCE is 4, the number of symbols is 3, and the REG bundling number is 2.  FIG.  14    illustrates the numbers of per-symbol REGs when the last symbol is punctured or rate-matched based on REG mapping (see, for example,  FIG.  10 C or  12   ) when the number of REGs per CCE is 4, the number of symbols is 4, and the REG bundling number is 2. As illustrated in  FIG.  14   , in AL  2 , AL  4 , and AL  8 , it can be understood that the numbers of per-symbol REGs that constitute a CCE used for transmitting the same DCI are equal to each other in any aggregation level. 
     Also, in AL  1 , there are a case in which REGs that constitute one CCE used for transmitting a DCI are arranged in two symbols and a case in which the REGs are arranged in one symbol, as illustrated in  FIG.  14   . In AL  1 , when the number of symbols in which REGs are arranged is one, that is, when the number of REGs is half the normal values (4 REGs), there may be cases in which sufficient reception quality cannot be expected. Accordingly, for AL  1 , the terminal  200  may be limited to monitor only CCEs arranged in two symbols. 
     The description has been given of a case in which when the number of symbols in a CORESET is the number of symbols other than a power of 2, a symbol is punctured or rate-matched based on a REG mapping design for the number of symbols which is larger than the number of symbols in a CORESET. However, when the number of symbols in a CORESET is the number of symbols other than a power of 2, a symbol may be repeated based on the REG mapping design for the number of symbols which is smaller than the number of symbols in a CORESET. For example, when the number of symbols in a CORESET is 5, it is also possible to perform an operation in which REG mapping when the number of symbols is 5 is set by repeating the last symbol (symbol # 3 ) or the front-end symbol (symbol # 0 ) based on the REG mapping (see, for example,  FIG.  10 C ) used when the number of symbols in a CORESET is 4. 
     Thus, according to operation example 1-2, even when the number of symbols in a CORESET is a number other than a power of 2, the number of REGs that constitute a CCE used for transmitting a DCI becomes equal for each symbol, making it easy to adjust power across CCEs. 
     The present disclosure is not limited to a case in which the number of symbols in a CORESET is 3 or 5 in operation example 1-2. 
     OPERATION EXAMPLE 1-3 
     In operation example 1-3, a REG bundling in the same CCE is arranged in the same PRB, in addition to operation example 1-1 described above. That is, REGs that constitute one CCE are arranged in a plurality of symbols at the same frequency in units of the REG bundling size. 
     Also, at this point, DMRSs used for demodulating the CCE are arranged in the front-end symbol of a plurality of symbols in which the CCE is arranged and are not arranged in the remaining symbols. 
     With such an arrangement, it is possible to reduce the number of DMRSs. 
       FIG.  15    illustrates a DMRS and REG mapping example when the number of symbols in a CORESET is 2, the number of REGs per CCE is 4, and the REG bundling size is 2. 
     In  FIG.  15   , since the REG bundling size is 2, REGs that constitute the same CCE are arranged in two adjacent PRBs. Also, the REGs that constitute the same CCE are arranged in two symbols # 0  and # 1  at the same frequency (2 PRBs) in units of the REG bundling size. 
     At this point, as illustrated in  FIG.  15   , DMRSs are arranged in front-end symbol # 0  of symbols in which REGs that constitute a CCE used for transmitting the same DCI and that are arranged at the same frequency in units of the REG bundling size are arranged. That is, no DMRS is arranged in symbol # 1 , as illustrated in  FIG.  15   . In this case, for symbol # 1 , the terminal  200  performs channel estimation by using the DMRSs in symbol # 0  in which the REGs that constitute the same CCE are arranged. 
     Next,  FIGS.  16 A and  16 B  illustrate DMRS and REG mapping examples when the number of symbols in a CORESET is 4, the number of REGs per CCE is  4 , and the REG bundling size is 2. 
     In  FIGS.  16 A and  16 B , since the REG bundling size is 2, REGs that constitute the same CCE are arranged in two adjacent PRBs, as in  FIG.  15   . 
     In  FIG.  16 A , the REGs that constitute the same CCE are arranged in the same PRBs ( 2  PRBs) in units of the REG bundling. 
     At this point, DMRSs are arranged in the front-end symbol of symbols in which REGs that constitute a CCE used for transmitting the same DCI and that are arranged at the same frequency in units of the REG bundling size are arranged. 
     For example, in  FIG.  16 A , in the case of aggregation level  2 , two CCEs used to transmit the same DCI are arranged in four symbols # 0  to # 3  in the same PRBs (see, for example, PRBs # 0  and # 1 ). In this case, for PRBs # 0  and # 1 , the DMRSs are arranged in front-end symbol # 0  and are not arranged in remaining symbols # 1  to # 3 . Accordingly, for the PRBs (PRBs # 0  and # 1  in  FIG.  16 A ) in which two CCEs used for a DCI in aggregation level  2  are arranged, the terminal  200  can also use DMRSs in symbol # 0  to perform demodulation in symbols # 2  and # 3  in addition to symbol # 1 . 
     On the other hand, when the aggregation level is  1 , the DMRSs are arranged in the front-end symbol (symbol # 0  or symbol # 2  in  FIG.  16 A ) of the symbols in which one CCE (REGs) used for transmitting the same DCI is arranged, as in PRBs # 8  and # 9  and PRBs # 12  and # 13  in  FIG.  16 A . That is, in the case of aggregation level  1 , the DMRSs are arranged in the front-end symbol of the symbols in which REGs that constitute each CCE are arranged. 
     Also, in  FIG.  16 B , in the case of aggregation level  2 , two CCEs used for transmitting the same DCI are arranged in different PRBs (for example, PRBs # 0  and # 1  and PRBs # 8  and # 9 ). In this case, for each frequency ( 2  PRBs) in which the REGs are arranged in units of the REG bundling size, DMRSs are arranged in the front-end symbols of the symbols in which the REGs that constitute the CCEs are arranged. For example, when two CCEs used for transmitting the same DCI are a CCE arranged in symbols # 0  and # 1  in PRBs # 0  and # 1  and a CCE arranged in symbols # 2  and # 3  in PRBs # 8  and # 9 , the DMRSs are respectively arranged in front-end symbols # 0  and # 2  of the symbols in which the CCEs in PRBs # 0  and # 1  and PRBs # 8  and # 9  are arranged. In this case, for the CCE arranged in symbols # 0  and # 1  in  FIG.  16 B , the terminal  200  performs channel estimation by using the DMRSs arranged in symbol # 0 , and for the CCE arranged in symbols # 2  and # 3 , the terminal  200  performs channel estimation by using the DMRSs arranged in symbol # 2 . 
     Thus, according to operation example 1-3, a DMRS is shared in a CCE arranged in a plurality of symbols in the same PRB. Specifically, since DMRSs are arranged in the front-end symbol and are not arranged in the remaining symbols, it is possible to reduce the number of DMRSs. Also, by using the DMRSs arranged in the front-end symbol, the terminal  200  can quickly demodulate a DCI arranged in subsequent symbols. 
     When sharing DMRSs between a plurality of UEs is defined, the terminal  200  may be adapted to perform, even when two CCEs are arranged in different PRBs, channel estimation by using DMRSs arranged in the front-end PRBs of each CCE, in the case of aggregation level  2 , as illustrated in  FIG.  16 B . With such an arrangement, although the precoding cannot be changed for each CCE, the amount of DMRS resources can be reduced by an amount corresponding to the DMRSs arranged in the subsequent PRBs. Also, when spatial or code multiplexing is performed on DMRSs, one part of the DMRSs multiplexed in the front-end symbols may be used for the frontward symbols, and another part of the DMRSs may be used for the rearward symbols. 
     The above description has been given of operation examples 1-1 to 1-3. 
     Thus, in the present embodiment, with respect to mapping of CCEs and REGs to be arranged in a CORESET, the number of REGs per CCE is a power of 2, and the REG bundling size is a power of 2. 
     That is, a plurality of REGs that constitute a CCE are separated into a power of 2 in units of the REG bundling size. As a result, the REG mapping design becomes simple. For example, when the number of symbols in a CORESET is a power of 2, the REGs in each CCE are uniformly arranged in the symbols in units of the REG bundling. Hence, the numbers of REGs across the symbols become equal in one or more CCEs used for transmitting a DCI, thus making it possible to prevent variations in the reception SINRs for the respective symbols and making it possible to simplify the adjustment of power across the CCEs. 
     Also, in the present embodiment, REG mapping when a CORESET has a number of symbols which is other than a power of 2 can be realized through symbol puncturing, repetition, or rate matching, based on REG mapping setting when the number of symbols is a power of 2 and by using the REG mapping setting. As a result, even when the number of symbols in a CORESET is not a power of 2, the numbers of REGs across the symbols become equal to each other, thus making it possible to prevent variations in the reception SINRs for the respective symbols and making it possible to simplify the adjustment of power across CCEs. 
     As described above, according to the present embodiment, it is possible to appropriately arrange CCEs that constitute a CORESET. 
     Although a REG mapping example when the number of REGs per CCE is 4 (=2 2 ) and the REG bundling size is 2 (=2 1 ) has been described above, the REG bundling size may be 4 (=2 2 ). In this case, REGs that constitute a CCE are arranged in the same symbol. Also, when the number of REGs that constitute a CCE (the number of REGs per CCE) is 8 (=2 3 ), the REG bundling size can be set to 2 (=2 1 ),  4  (=2 2 ), or 8 (=2 3 ). 
     The above description has been given of a case in which the base station  100  performs higher-layer signaling to notify the terminal  200  of the setting information of a CORESET to be set. The setting information of a CORESET, however, may be specified by the base station  100  and the terminal  200 . In this case, the CORESET setting notification using the higher-layer signaling becomes unnecessary. 
     (Second Embodiment) 
     Since a base station and a terminal according to the present embodiment are common to the basic configuration of the base station  100  and the terminal  200  according to the first embodiment, a description will be given using  FIGS.  7  and  8   . 
     In the present embodiment, in CCE and REG mapping to a CORESET, the number of REGs per CCE is 6, and the REG bundling size is changed according to the number of symbols in the CORESET. 
     With such an arrangement, even when the number of REGs per CCE is 6, the numbers of REGs that are arranged in the respective symbols can be made equal even when the number of symbols in a CORESET is 2, 3, or 4. 
     Operation examples according to the present embodiment will be specifically described below. 
     In the following, the number of REGs per CCE is 6, the REG bundling size is 3 when the number of symbols in a CORESET is a power of 2 (1, 2, 4, 8, . . . ), and the REG bundling size is 2 when the number of symbols in a CORESET is 3 or  6 . Also, the aggregation level is a power of 2. 
       FIGS.  17 A to  17 D  illustrate a REG mapping example according to the present embodiment. 
     &lt;Case in Which Number of Symbols in CORESET is Power of 2&gt; 
       FIGS.  17 A and  17 B  illustrate a REG mapping example when the number of symbols in a CORESET is a power of 2. Specifically,  FIG.  17 A  illustrates a REG mapping example when a CORESET has two symbols, and  FIG.  17 B  illustrates a REG mapping example when a CORESET has four symbols. 
     As illustrated in  FIGS.  17 A and  17 B , the REG bundling size is 3 when the number of symbols in a CORESET is a power of 2. 
     In this case, when the mapping is time first mapping, each CCE is arranged in two symbols, as illustrated in  FIGS.  17 A and  17 B . Also, when a CORESET has four symbols, three of REGs that constitute two CCEs used for transmitting a DCI are arranged in each of the four symbols, in aggregation level  2 , as illustrated in  FIG.  17 B . Also, although not illustrated, symbol arrangement is similarly possible with the REG bundling size  3  even when the number of symbols in a CORESET is another value (one symbol or eight symbols) of a power of 2. 
     &lt;Case in Which Number of Symbols in CORESET is 3 or 6&gt; 
       FIG.  17 C  illustrates a REG mapping example when a CORESET has three symbols, and  FIG.  17 D  illustrates a REG mapping example when a CORESET has six symbols. 
     When the number of symbols in a CORESET is 3 or 6, the REG bundling size is 2, as illustrated in  FIGS.  17 C and  17 D . 
     In this case, when the mapping is time first mapping, each CCE is arranged in three symbols, as illustrated in  FIGS.  17 C and  17 D . Also, when the CORESET has six symbols, two of REGs that constitute two CCEs used for transmitting a DCI are arranged in each of six symbols, in aggregation level  2 , as illustrated in  FIG.  17 D . 
       FIGS.  18 ,  19 ,  20 , and  21    illustrate REG mapping examples for aggregation levels (AL  1 , AL  2 , AL  4 , and AL  8 ) when the number of REGs per CCE is 6, and the numbers of symbols in a CORESET are 2, 4, 3, and 6 (see  FIGS.  17 A to  17 D ). 
     The numbers of REGs arranged in the symbols in all aggregation levels are equal to each other when the number of symbols in a CORESET is 2, which is illustrated in  FIG.  18   , and the number of symbols in a CORESET is 3, which is illustrated in  FIG.  20   . 
     Also, the numbers of REGs arranged in the symbols in aggregation level  2  or higher are equal to each other when the number of symbols in a CORESET is  4 , which is illustrated in  FIG.  19   , and when the number of symbols in a CORESET is 6, which is illustrated in  FIG.  21   . Also, in aggregation level  1 , when the number of symbols in a CORESET is 4, which is illustrated in  FIG.  19   , REGs that constitute a DCI are arranged in two symbols, and when the number of symbols in a CORESET is 6, which is illustrated in  FIG.  21   , REGs that constitute a DCI are arranged in three symbols. 
     As described above, in the present embodiment, the number of REGs per CCE is 6, and the REG bundling size is changed according to the number of symbols in a CORESET. As a result, in each CCE, REGs are uniformly arranged in each symbol in units of the REG bundling. Hence, the numbers of REGs across the symbols become equal in one or more CCEs used for transmitting a DCI, thus making it possible to prevent variations in the reception SINRs for the respective symbols and making it possible to simplify the adjustment of power across the CCEs. 
     The numbers of symbols, 5 and 7, in a CORESET, which are not described in the above examples may be extended to an actual number of symbols in a CORESET, by using puncturing, rate matching, or repetition based on a mapping design for the number of symbols (for example, the number of symbols, 2, 3, 4, or 6) which is the closest to the actual number of symbols in the CORESET, as in operation example 1-3. 
     Also, the number of symbols in a CORESET may be limited to the number of symbols, 1, 2, 3, 4, 6, or 8, which makes it easy to allocate six REGs, per CCE. 
     The above description has been given of each embodiment of the present disclosure. 
     Although, in the embodiments described above, physical mapping has been described as one example for frequency domains (PRB #), the present disclosure can also be applied to logical mapping. In the case of the logical mapping, the mapping is changed from the logical mapping to the physical mapping, and thus, even frequency domains that are contiguous in the logical mapping are arranged at physically discrete positions, thus obtaining a frequency diversity effect. 
     Also, although an example in which REGs that constitute each CCE are arranged in different PRBs for each REG bundling in order to obtain the frequency diversity effect has been described above, the mapping of REGs that constitute each CCE is not limited thereto. 
     The control resource set (CORESET) may also be called a search space (search space). 
     Also, a plurality of CORESETs may be set for a UE. For example, although, in the embodiments described above, symbol # 0  has been described as being the front-end symbol in which a CORESET is set, another CORESET may be set from a rearward symbol. 
     Also, the higher-layer signaling may be replaced with MAC signaling. In the case of the MAC signaling, the frequency of changes of a case set for a UE can be increased, compared with the RRC signaling. 
     Also, the above-described DMRSs may be reference signals (Reference Signals) having a different name. 
     Also, the first and second embodiments described above may be combined together. That is, depending on a case in which the number of REGs per CCE is a power of 2 (the first embodiment) and a case in which the number of REGs per CCE is 6 (the second embodiment), the base station  100  and the terminal  200  may determine the REG bundling size or the number of symbols in a CORESET and may set REG mapping. 
     The present disclosure can be realized by software, hardware, or software in cooperation with hardware. Each functional block used in the description of each embodiment above can be partly or entirely realized by an LSI, which is an integrated circuit, and each process described in each embodiment above may be controlled partly or entirely by one LSI or a combination of LSIs. The LSI may be individually formed as chips or may be formed by one chip so as to include a part or all of the functional blocks. The LSI may comprise an input and an output of data. The LSI may be referred to as an IC, a system LSI, a super LSI, or an ultra LSI depending on a difference in the degree of integration. The technique of the circuit integration is not limited to the LSI and may be realized by using a dedicated circuit, a general-purpose processor, or a special-purpose processor. Also, an FPGA (field programmable gate array) that can be programmed after the manufacture of the LSI or a reconfigurable processor in which the connections and the settings of circuit cells arranged inside the LSI can be reconfigured may be used. The present disclosure may be realized as digital processing or analogue processing. In addition, when a technology for circuit integration that replaces LSI becomes available with the advancement of semiconductor technology or another derivative technology, such a technology may also naturally be used to integrate the functional blocks. Application of biotechnology or the like is possible. 
     A base station in the present disclosure comprises: circuitry that allocates downlink control signals to a control channel region constituted by a plurality of control channel elements (CCEs); and a transmitter that transmits the downlink control signals. The number of resource element groups (REGs) that constitute the CCE is a power of 2, and a bundling size indicating the number of REGs included in the REGs that constitute the CCE and arranged in adjacent resource blocks is a power of 2. 
     In the base station in the present disclosure, the number of symbols in which the control channel region is arranged is a power of 2. 
     In the base station in the present disclosure, the REGs that constitute one of the CCEs are arranged in a time direction prior to a frequency direction in units of the bundling size. 
     In the base station in the present disclosure, the circuit allocates the downlink control signals in a time direction prior to a frequency direction in units of the CCE. 
     In the base station in the present disclosure, REGs that constitute one of the CCEs are arranged in a plurality of symbols at a same frequency in units of the bundling size. 
     In the base station in the present disclosure, a reference signal is arranged in a front-end symbol of the plurality of symbols, and a reference signal is not arranged in a remaining symbol. 
     A station in the present disclosure comprises: circuitry that allocates downlink control signals to a control channel region constituted by a plurality of control channel elements (CCEs); and a transmitter that transmits the downlink control signals. The number of resource element groups (REGs) that constitute the CCE is 6. When the number of symbols in which the control channel region is arranged is a power of 2, a bundling size indicating the number of REGs included in the REGs that constitute the CCE and arranged in adjacent resource blocks is 3, and when the number of symbols in which the control channel region is arranged is 3 or 6, the bundling size is 2. 
     A terminal in the present disclosure comprises: a receiver that receives downlink control signals in a control channel region constituted by a plurality of control channel elements (CCEs); and circuitry that decodes the downlink control signals. The number of resource element groups (REGs) that constitute the CCE is a power of 2, and a bundling size indicating the number of REGs included in the REGs that constitute the CCE and arranged in adjacent resource blocks is a power of 2. 
     In a communication method in the present disclosure, downlink control signals are allocated to a control channel region constituted by a plurality of control channel elements (CCEs), and the downlink control signals are transmitted. The number of resource element groups (REGs) that constitute the CCE is a power of 2, and a bundling size indicating the number of REGs included in the REGs that constitute the CCE and arranged in adjacent resource blocks is a power of 2. 
     In a communication method in the present disclosure, downlink control signals are received with a control channel region constituted by a plurality of control channel elements (CCEs), and the downlink control signals are decoded. The number of resource element groups (REGs) that constitute the CCE is a power of 2, and a bundling size indicating the number of REGs included in the REGs that constitute the CCE and arranged in adjacent resource blocks is a power of 2. 
     One embodiment of the present disclosure is useful for a mobile communications system. 
     REFERENCE SIGNS LIST 
     
         
           100  base station 
           101  CORESET setting unit 
           102  DCI generating unit 
           103 ,  207  error-correction encoding unit 
           104 ,  208  modulating unit 
           105 ,  209  signal allocating unit 
           106 ,  210  transmitting unit 
           107 ,  201  receiving unit 
           108 ,  202  signal separating unit 
           109 ,  204  demodulating unit 
           110 ,  205  error-correction decoding unit 
           200  terminal 
           203  DCI receiving unit 
           206  setting-information receiving unit