Patent Publication Number: US-11026157-B2

Title: User terminal, base station and radio communication method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application and, thereby, claims benefit under 35 U.S.C. § 120 to U.S. patent application Ser. No. 15/308,890, filed on Nov. 4, 2016, and entitled “USER TERMINAL, BASE STATION AND RADIO COMMUNICATION METHOD.” U.S. patent application Ser. No. 15/308,890 is a national phase application of PCT Application No. PCT/JP2015/061894, filed on Apr. 17, 2015, which claims priority to Japanese Patent Application No. 2014-097143, filed on May 8, 2014. The contents of these applications are incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a user terminal, a base station and a radio communication method in a next-generation mobile communication system. 
     BACKGROUND 
     In the UMTS (Universal Mobile Telecommunications System) network, the specifications of long term evolution (LTE) have been drafted for the purpose of further increasing high speed data rates, providing lower delays and so on (see non-patent literature 1). In LTE, as multiple-access schemes, a scheme that is based on OFDMA (Orthogonal Frequency Division Multiple Access) is used in downlink channels (downlink), and a scheme that is based on SC-FDMA (Single Carrier Frequency Division Multiple Access) is used in uplink channels (uplink). 
     Furthermore, successor systems of LTE (referred to as, for example, “LTE-advanced” or “LTE enhancement” (hereinafter referred to as “LTE-A”)) have been under study for the purpose of achieving further broadbandization and increased speed beyond LTE. In the LTE-A system, a HetNet (Heterogeneous Network), in which small cells (for example, pico cells, femto cells and so on), each having a local coverage area of a radius of approximately several tens of meters, are formed inside a macro cell having a wide coverage area of a radius of approximately several kilometers, is under study (see, for example, non-patent literature 2). Also, in relationship to the HetNet, a study is in progress to use carriers of different frequency bands between a macro cell (macro base station) and a small cell (small base station), in addition to the same frequency band. 
     CITATION LIST 
     Non-Patent Literature 
     
         
         Non-Patent Literature 1: 3GPP TS 36. 300 “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall Description; Stage 2” 
         Non-Patent Literature 2: 3GPP TR 36.814 “E-UTRA Further Advancements for E-UTRA Physical Layer Aspects” 
       
    
     SUMMARY 
     In the above HetNet, many small cells may be placed in the macro cell. In this case, it may be possible to arrange small cells in a localized manner in places where the traffic is heavy, so as to achieve an off-loading effect between the cells. In an environment like this, a new small cell detection mechanism (“small cell discovery”) is under study for introduction as a method of allowing a user terminal to efficiently detect and connect with small cells. “Detection” (discovery) in this context refers generally to operations including, at least, cell and transmission point (TP) identification, coarse time/frequency synchronization, and RRM (Radio Resource Management) measurement with the connecting cell in the same frequency and in different frequencies (intra/inter-frequency RRM measurement). 
     In small cell detection (small cell discovery), a user terminal may be in the state of being connected with a network—for example, a macro cell—so that, when necessary, the network can provide assist information for small cell detection. However, since the assist information that can be provided may vary depending on the network structure, implementation, and so on, when small cell detection (small cell discovery) operations are carried out using a DRS (Discovery Reference Signal), which includes a PSS/SSS/CRS/CSI-RS, it is preferable to design each user terminal operation to be executable whether or not there is assist information from the network. 
     The present invention has been made in view of the above, and it is therefore an object of the present invention to provide a user terminal, a base station and a radio communication method, whereby small cell detection operations can be carried out adequately based on whether or not there is assist information from the network. 
     The user terminal of the present invention is provided in the form of a user terminal that communicates with a base station forming a small cell and a base station forming a macro cell having a coverage area in which the small cell is included, and this user terminal has a receiving section that receives a small cell detection command, which is transmitted from the macro cell and which includes timing information and assist information, a detection section that detects a synchronization signal, which is included in a small cell detection reference signal transmitted from the small cell, in accordance with the timing information, and a measurement section that measures a measurement reference signal, which is included in the small cell detection reference signal, in accordance with the detected synchronization signal and the assist information, and the values of the offset, the period and the duration of the small cell detection reference signal are included in the timing information, and the values of a physical cell ID of the small cell or a transmission point, a scrambling ID that is used for the measurement reference signal, a time/frequency resource configuration index and a subframe offset are included in the assist information. 
     According to embodiments of the present invention, small cell detection operations can be carried out adequately based on whether or not there is assist information from the network. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  provide diagrams to explain the DRS in small cell detection in accordance with one or more embodiments; 
         FIGS. 2A and 2B  provide diagrams to explain the DRS in small cell detection in accordance with one or more embodiments; 
         FIG. 3  is a diagram to show physical cell IDs (PCIs), scrambling IDs and resource configurations that are configured in small cells in accordance with one or more embodiments; 
         FIG. 4  is a diagram to show the configurations of the DRS of  FIGS. 1A and 1B  in accordance with one or more embodiments; 
         FIGS. 5A and 5B  provide diagrams to explain examples of measurements by user terminals in accordance with one or more embodiments; 
         FIGS. 6A and 6B  provide diagrams to explain examples of measurements by user terminals in accordance with one or more embodiments; 
         FIG. 7  is a diagram to explain examples of time/frequency resource configuration indices in accordance with one or more embodiments; 
         FIGS. 8A and 8B  provide diagrams to explain the CRS and the PRS in accordance with one or more embodiments; 
         FIG. 9  is a diagram to show an example of a schematic structure of a radio communication system according to one or more embodiments; 
         FIG. 10  is a diagram to show an example of an overall structure of a radio base station according to one or more embodiments; 
         FIG. 11  is a diagram to show an example of a functional structure of a macro base station according to one or more embodiments; 
         FIG. 12  is a diagram to show an example of a functional structure of a small base station according to one or more embodiments; 
         FIG. 13  is a diagram to explain an example of an overall structure of a user terminal according to one or more embodiments; and 
         FIG. 14  is a diagram to explain an example of a functional structure of a user terminal according to one or more embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Now, embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In small cell detection, DRS-based measurement can be configured for user terminals in RRC (Radio Resource Control) connected mode (RRC CONNECTED). Also, in each carrier, one measurement timing configuration (measurement timing configuration) is configured. This measurement timing configuration includes, at least, the offset and the period of the DRS. The DRS includes at least a PSS/SSS (Primary Synchronization Signal/Secondary Synchronization Signal), and, in addition, includes one or both of a CRS (Cell-specific Reference Signal) and a CSI-RS (Channel State Information-Reference Signal). 
     In small cell detection, a user terminal detects small cells by using assist from the network (which is, for example, a base station). To be more specific, a small cell transmits the DRS in a long period, the network reports timing information for measuring the DRS and so on to a user terminal, and the user terminal measures the DRS by using the reported timing and so on. Also, based on, for example, the receiving state (RSRP (Reference Signal Received Power)/RSRQ (Reference Signal Received Quality)) reported from the user terminal having detected the DRS, the network may control the connection of the user terminal with the small cells. 
     The measurement timing configuration is configured in a user terminal in RRC connected mode from the connecting cell, so that it is possible to efficiently carry out the DRS detection and measurement only in timings the DRS is transmitted. Also, even when the timings of the macro cell and the small cells do not match due to differences in propagation delay and so on, it is still possible to acquire the DRS timing by means of the PSS/SSS. 
       FIG. 1A  shows a system comprised of a small base station eNB 2 , which forms a small cell, a macro base station eNB 1 , which forms a macro cell, and the coverage area of which includes the small cell, and a user terminal UE, which can communicate with the macro base station eNB 1  and the small base station eNB 2 . The macro base station eNB 1  and the small base station eNB 2  are synchronized, or each knows the other&#39;s timing difference. 
     The macro base station eNB 1  commands the user terminal UE connected with the base station eNB 1  as to whether or not to carry out small cell detection using the DRS. When small cell detection is carried out, the macro base station eNB 1  reports timing information (measurement timing configuration) to the user terminal UE. Furthermore, the macro base station eNB 1  may also report assist information to the user terminal UE. 
     The macro base station eNB 1  commands the small cell detection to the user terminal through higher layer signaling (for example, RRC signaling). To be more specific, the macro base station eNB 1  includes information regarding whether or not small cell detection is carried out using the DRS in the information element to define the measurement object (hereinafter also referred to as “MeasObject”), and reports this to the user terminal. For example, the macro base station eNB 1  includes the small cell detection command (DiscoveryConfig) in MeasObject and reports this to the user terminal UE, and, meanwhile, includes timing information and assist information in, for example, the information element to define the small cell detection reference signal configuration (DiscoveryRS-Config), and reports this to the user terminal UE. 
     The user terminal, commanded small cell detection from the network, executes small cell detection using the DRS. 
     As shown in  FIG. 1B , assume that the duration until a small cell transmits the first DRS is the DRS offset. The DRS is transmitted in the DRS period. The DRS duration refers to the length of time in which the synchronization signals and the measurement reference signal included in the DRS are transmitted in one DRS period. Information pertaining to these DRS offset, DRS period and DRS duration are included in the timing information. 
     As for the design of the DRS, the two options illustrated in  FIGS. 2A and 2B  are under study. One purpose of introducing small cell detection is to make it possible to measure cells that are in the off state even when the small cells are on and off. 
     In option #1 of the DRS shown in  FIG. 2A , the PSS/SSS/CRS are transmitted in a long period, and in a short-term burst. During the regular on state, the DRS is a subset of a synchronization signal (SS) and a CRS, which are always transmitted. As shown in  FIG. 2A , in the on state, the PSS/SSS are transmitted in a 5 [ms] period, and the CRS is transmitted in a 1 [ms] cycle, and, furthermore, data signals are transmitted. In the off state, for example, the PSS/SSS and the CRS are transmitted for 1 [ms], in a 40 [ms] cycle. 
     In option #2 of the DRS shown in  FIG. 2B , the PSS/SSS/CSI-RS are transmitted in a long cycle, and in a short-term burst. In the DRS, the PSS/SSS are a subset of regular synchronization signals (SSs) in the on state, as in option #1. Meanwhile, the CSI-RS in the DRS is transmitted in the on and off state for discovery, apart from the regular CSI-RS for measuring CSI. As shown in  FIG. 2B , in the on state, PSS/SSS are transmitted in a 5 [ms] cycle, and, in a 1 [ms] cycle, the CRS and data signals are transmitted. The CSI-RS for discovery is, for example, transmitted for 1 [ms], in a 40 [ms] cycle, either in the on state or the off state. 
     The PSS/SSS that have been used since Rel. 8 LTE are designed so that a user terminal can autonomously search  504  physical cell IDs (PCIDs: Physical Cell IDs). Although there is a mechanism to report a list of nearby cells&#39; physical cell IDs to a user terminal, the user terminal can identify the physical cell IDs of nearby cells from the PSSs/SSSs which the user terminal receives. 
     With the CRS that is transmitted in DRS option #1, usually, the transmission sequence, the transmission resource location and so on are determined uniquely from the physical cell ID. Therefore, when a given physical cell ID is detected from a PSS/SSS, in which resource or sequence the corresponding cell&#39;s RSRP/RSRQ should be measured is determined uniquely. In DRS option #1, even if there is no assist information other than the measurement timing configuration, the user terminal can still detect and measure nearby small cells. 
     Meanwhile, when the CSI-RS for discovery is transmitted in DRS option #2, if no value is indicated to the user terminal from the network through higher layer signaling, the user terminal recognizes that that the same physical cell ID as that used for the PSS/SSS is used as the scrambling ID to determine the transmission sequence. Also, if a value is indicated from the network, the user terminal uses that value as the scrambling ID. However, the transmission resource location (determined by the CSI-RS configuration index, the CSI-RS subframe config., the CSI-RS subframe offset, and so on) is usually uncorrelated with the physical cell ID, and therefore needs to be reported from the network. 
     In this way, in the event of DRS option #2, unless a mechanism to report cell-specific resource configuration indices is introduced on a regular basis, a user terminal has no way of knowing in which resource the user terminal has to carry out detection and measurement. This is because  504  physical cell IDs and maximum twenty patterns of CSI-RS configurations give as many as approximately ten thousand combinations. Therefore, in the event of DRS option #2, which cell uses which CSI-RS configuration may be reported to a user terminal. 
     In the event of the CSI-RS for measuring CSI, only the information pertaining to the serving cell being the measurement object needs to be reported to the user terminal. However, in the event of the CSI-RS for discovery, the network must report the set of CSI-RS configurations with respect to all the nearby cells that are the candidates for detection, to the user terminal. 
     Generally speaking, it is difficult to properly configure and manage every nearby cell&#39;s assist information, which is, for example, a set of a scrambling ID and a CSI-RS configuration. For example, when a new cell is installed, which nearby cells&#39; information needs to be configured and managed as assist information poses a problem. Also, whether the assist information of the newly installed cell needs to supplied to all the nearby cells is another problem. 
     Meanwhile, in the event of a structure in which there are a plurality of RREs (Remote Radio Equipment) under one base station eNB, it is easy to configure and manage each RRE&#39;s information in one base station eNB. Same-cell-ID operation, in which the same physical cell ID is configured in all the PREs under a base station eNB, is one mode of operation in such a network structure. 
     However, if a measurement reference signal&#39;s sequence, resource location and so on are all determined from a physical cell ID as in DRS option #1, this raises the problem that, since completely identical reference signals may be transmitted between transmission points during same-cell-ID operation, each individual transmission point cannot be identified. If the CSI-RS is used as in DRS option #2, scrambling IDs that are different from the physical cell IDs, resource configurations and so on can be in each transmission point separately, so that it is possible to identify and measure the transmission points even during same-cell-ID operation. 
       FIG. 3  is a diagram to show the physical cell IDs (PCIs), scrambling IDs and resource configurations that are configured in small cells. As shown in  FIG. 3 , in same-cell-ID operation, the physical cell IDs (PCIs) and the scrambling IDs and resource configurations of the measurement reference signal in DRS option #2 can be separated. 
     In order to make it possible to identify transmission points during same-cell-ID operation, and to make it possible to detect and measure small cells based on DRSs even when assist information other than the measurement timing configuration is not available, the present inventors have arrived at defining the user terminal operations in small cell detection separately between when assist information other than the measurement timing configuration is available and when assist information other than the measurement timing configuration is not available. 
     For example, when a large number of RREs are provided as small cells under one base station eNB, the base station eNB reports the scrambling ID of the measurement reference signal, the corresponding time/frequency resource configuration, the corresponding physical cell ID and so on to a user terminal as assist information, and the user terminal detects and measures the DRS in accordance with the assist information that is configured. 
     Alternatively, when every small cell is under a different base station eNB and it is difficult to manage the assist information, the base stations eNB provide no assist to the connecting user terminals other than providing timing information. 
     In this case, advanced rules are defined based on physical cell IDs acquired from the PSSs/SSSs and/or the like, so that the user terminals can determine the sequence patterns and measurement resource locations of the measurement reference signals. 
     By defining user terminal operations in this way, even when it is not possible to manage and assist detailed information pertaining to each small cell&#39;s DRS configuration as a network, user terminals can still detect and measure DRSs autonomously. Alternatively, when detailed information pertaining to each small cell&#39;s DRS configuration can be managed and assisted as a network, by taking advantage of this, it is possible to allow user terminals to execute more efficient DRS detection and measurement. Furthermore, it is possible to provide support for, for example, transmission point identification during same-cell-ID operation. 
     &lt;Operation 1&gt; 
     A case will be discussed below where a common DRS offset, cycle and duration are reported to user terminals as timing information, and where, furthermore, all the DRS transmission-related information of each small cell or transmission point is managed properly as a network. 
     In this case, in addition to timing information, the physical cell ID of each small cell or transmission point, the time/frequency resource configuration index, and the scrambling ID for use for the measurement reference signal, are reported to user terminals as assist information. A plurality of time/frequency resource configuration indices may be configured in each small cell or transmission point. Furthermore, when the subframe to transmit the measurement reference signal can be shifted per small cell or transmission point, the subframe offset is also reported as assist information. 
       FIG. 4  is a diagram to show the DRS of  FIGS. 1A and 1B . With this DRS, the DRS offset, the DRS period and the DRS duration are reported as timing information. In the example shown in  FIG. 4 , the DRS duration is three subframes, in which the PSS/SSS are transmitted as synchronization signals in the top subframe, and the CSI-RS is transmitted as a measurement reference signal in the rest of the subframes. Furthermore, with this DRS, the physical cell ID (PCID), the scrambling ID, the CSI-RS configuration index (CSI-RS config.) and the subframe offset are reported as examples of assist information. 
     When these pieces of information are reported, the user terminal executes DRS measurement using these. The user terminal, when detecting a PSS/SSS in accordance with the timing information, may limit, or may not limit, the candidates to detect based on the physical cell IDs included in the list of nearby cells reported. The user terminal may measure the measurement reference signals of the small cells or transmission points corresponding to the physical cell IDs that are acquired by PSS/SSS detection, or measure all the measurement reference signals that are reported. 
     That is, a user terminal can use the PSS/SSS for timing synchronization alone, or use the PSS/SSS for timing synchronization and identification of physical cell IDs. 
     &lt;Operation 2&gt; 
     A case will be discussed below where timing information to include the DRS duration is reported to user terminals, but the subframe offset value of each small cell or transmission point is not included in the assist information. 
     In this case, a user terminal may carry out the measurement operation on the assumption that the measurement reference signal is placed in conventional subframe locations in the DRS duration. The conventional subframe locations where the measurement reference signal is assumed to be placed have only to be defined in the specifications, and may be, for example, subframes in which the PSS/SSS are not included (see  FIG. 5A ). Alternatively, the user terminal may carry out the measurement operation on the assumption that the measurement reference signal is placed in all subframe locations in the DRS duration. 
     In the example shown in  FIG. 5A , the DRS duration is four subframes, and the user terminal carries out the measurement operation on the assumption that the measurement reference signal (in  FIG. 5A , the CSI-RS) is placed subframes not including the PSS/SSS. 
     &lt;Operation 3&gt; 
     A case will be discussed below where the DRS duration is not included in the timing information that is reported to user terminals. 
     In this case, when a PSS/SSS is detected, a user terminal may carry out the measurement operation on the assumption that the measurement reference signal is placed in conventional subframe locations with reference to the subframe where that PSS/SSS is included. The conventional subframe locations with reference to the subframe including the PSS/SSS have only to be defined in the specifications, and can be, for example, the same subframe as the subframe including the PSS/SSS, the subframe that comes right after this subframe, and so on (see  FIG. 5B ). 
     The DRS duration is not reported in the example shown in  FIG. 5B , a user terminal carries out the measurement operation on the assumption the measurement reference signal (the CSI-RS in  FIG. 5B ) is placed in the same subframe as the subframe in which the PSS/SSS is included (the left example). Alternatively, the user terminal carries out the measurement operation on the assumption that the measurement reference signal (the CSI-RS in  FIG. 5B ) is placed in the subframe right after the subframe including the PSS/SSS (the right example). 
     &lt;Operation 4&gt; 
     A case will be discussed below where the physical cell ID of each small cell or transmission point is not included in the assist information to be reported to user terminals. 
     In this case, if a physical cell ID is acquired by PSS/SSS detection, a user terminal may assume that this physical cell ID is a scrambling ID for use for the measurement reference signal, and carry out measurement with respect to the time/frequency resource configuration corresponding to the detected physical cell ID (scrambling ID). 
     Alternatively, in the timing the PSS/SSS is detected, the user terminal may carry out measurement with respect to all sets of scrambling IDs for use for the measurement reference signal and time/frequency resource configurations that are reported as assist information. In this case, the user terminal does not assume that a physical cell ID is a scrambling ID to use for the measurement reference signal. 
     In the example shown in  FIG. 6A , when the scrambling IDs to be used for the measurement reference signal are reported to a user terminal as assist information, the user terminal may measure all sets of these scrambling IDs that are reported and time/frequency resource configuration indices (in  FIG. 6A , CSI-RS configuration indices). Alternatively, the user terminal may assume that a physical cell ID that is detected in PSS/SSS detection is a scrambling ID for use for the measurement reference signal, and carry out measurement only with respect to the time/frequency resource configuration index that corresponds to the physical cell ID that is detected. 
     &lt;Operation 5&gt; 
     A case will be discussed below where the scrambling IDs for use for the measurement reference signal of each small cell or transmission point are not included in the assist information that is reported to user terminals. 
     In this case, if a physical cell ID is acquired by PSS/SSS detection, a user terminal may assume that this physical cell ID is a scrambling ID for use for the measurement reference signal, and carry out measurement with respect to the corresponding time/frequency resource configuration. 
     Alternatively, the user terminal may assume that the scrambling IDs that are reported as assist information are the physical cell IDs, and carry out measurement with respect to all the time/frequency resource configurations that are assisted in the timings detected from the PSSs/SSSs. 
     &lt;Operation 6&gt; 
     A case will be discussed below where neither the physical cell IDs of small cells or transmission points nor the scrambling IDs for use for the measurement reference signal are included in the assist information that is reported to user terminals. 
     In this case, if a physical cell ID is acquired by PSS/SSS detection, a user terminal may assume that this physical cell ID is a scrambling ID, and measure the combinations with all the time/frequency resource configurations that are reported. Alternatively, the user terminal may carry out measurement with respect to the time/frequency resource configurations that include the value that is acquired from the detected physical cell ID based on a conventional rule. The conventional rule has only to be defined in the specifications, and can be, for example, mod (PCID,  5 ). 
     In the example shown in  FIG. 6B , a user terminal may assume that the physical cell ID acquired by PSS/SSS detection is the scrambling ID, and measure the combinations with all the time/frequency resource configuration indices that are reported (in  FIG. 6B , CSI-RS configuration indices). Alternatively, the user terminal may measure only the time/frequency resource configuration indices including mod (PCID,  5 ). For example, when the physical cell ID that is detected is 10, mod (PCID,  5 )=0 holds, so that the user terminal measures the time/frequency resource configuration index including 0 in  FIG. 6B . 
     As for the number of time/frequency resource configurations to configure in each small cell or transmission point, it is possible to report the number simultaneous configurations in the measurement configuration, which is assist information that is common to all cells, and report only one index value for the individual assist information for each small cell or transmission point (see  FIG. 7 ). The “number of CSI reference signals configured” in TS 36. 211 Table 6. 10. 5. 2-1 corresponds to the number simultaneous configurations in this context. 
     When the above number of simultaneous configurations is not reported, the user terminal may interpret the time/frequency resource configuration that is reported based on the assumption that number simultaneous configurations=1. Alternatively, the user terminal may assume that the maximum number of time/frequency resource configurations included in the assist information of each small cell or transmission point is the number simultaneous configurations. 
     &lt;Operation 7&gt; 
     A case will be discussed below where information such as time/frequency resource configurations are not reported to user terminals as assist information, and only the timing information and the cell list (physical cell IDs) are reported. 
     In this case, a user terminal may limit, or may not limit, the object of PSS/SSS detection based on the cell list. The user terminal may assume that a physical cell ID acquired by PSS/SSS detection is a scrambling ID for use for the measurement reference signal, and measure the combinations with all the candidate time/frequency resource configurations. Alternatively, the user terminal may assume that a physical cell ID that is reported is a scrambling ID for use for the measurement reference signal, and measure the combinations with all the time/frequency resource configuration indices. 
     Alternatively, the user terminal may carry out measurement based on the assumption that a value that is acquired from a physical cell ID and a conventional rule is the time/frequency resource configuration index to correspond to that physical cell ID. The conventional rule has only to be defined in the specifications, and can be, for example, mod (PCID, X). As for the value of X, if the number of simultaneous configurations is reported as common assist information, this value may be used. If the number of simultaneous configurations is not reported as common assist information, the time/frequency resource configuration index may be derived based on the assumption that X=20 in the event of normal CPs (Cyclic Prefixes) and X=16 in the event of extended CPs. The CP length may be derived by way of PSS/SSS detection, or may be reported in advance from the network. 
     &lt;Operation 8&gt; 
     A case will be described below where no assist information is reported to user terminals other than timing information. 
     In this case, a user terminal may assume that a physical cell ID that is acquired by PSS/SSS detection is a scrambling ID, and measure the combinations with all the candidate time/frequency resource configurations. Alternatively, the user terminal may carry out measurement based on the assumption that a value that is acquired from a physical cell ID and a conventional rule is the time/frequency resource configuration index to correspond to that physical cell ID. The conventional rule has only to be defined in the specifications, and can be, for example, mod (PCID,  20 ). 
     &lt;Examples of User Terminal Operations&gt; 
     A user terminal can use the PSS/SSS only in timing synchronization, and, as for the measurement reference signal, measure all the candidates that are reported as assist information. Alternatively, the user terminal can use the PSS/SSS in timing synchronization and in physical cell ID identification, and, as for the measurement reference signal, measure the candidates corresponding to physical cell IDs that are detected. 
     First, the user terminal searches for the PSS/SSS in accordance with the timing information. When physical cell IDs are assisted, the detection candidates may be limited or may not be limited. As a result of the search, 0 or more combinations of synchronizing timings and physical cell IDs are found. 
     Next, the user terminal measures the RSRP/RSRQ by using the measurement reference signal, in accordance with the synchronizing timings that are detected. The user terminal may limit, or may not limit, the candidates of the measurement reference signal based on the physical cell IDs that are detected as a set with the synchronizing timings. When there is assist information of the time/frequency resource configurations, it is possible to make only those corresponding to the detected physical cell IDs the measurement object, or make all those that assisted the measurement object. When there is not assist information of the time/frequency resource configurations, it is possible to determine the time/frequency resource configurations to be the object of measurement based on the detected physical cell IDs, according to a conventional rule, or make all the possible candidates the object of measurement. 
     Although the present embodiment has been described with examples in which, in option #2 of the DRS, the PSS/SSS/CSI-RS are transmitted in a long cycle and in a short-term burst, it is possible to apply the above-described operation 1 to operation 8 likewise even when, instead of the CSI-RS, the CRS (see  FIG. 8A ), the PRS (Positioning Reference Signal (see  FIG. 8B )) and so on are used. 
     In the event the conventional CRS and PRS are used, physical cell ID-based scrambling IDs and frequency shifts are applied. Although, in this case, small cells or transmission points can be measured based on physical cell IDs acquired from the PSS/SSS, without assist, it is not possible, for example, to identify transmission points during same-cell-ID operation. 
     As an example of enhancement of the CRS or the PRS for the DRS, it is possible to support identifying transmission points during same-cell-ID operation by applying scrambling IDs and frequency shifts based on VCIDs (Virtual Cell IDs) reported from the network, instead of physical cell IDs. It is also possible, during the DRS duration, to allow each cell to carry out transmission only in part of the subframes and assume non-transmission (muting) in the other subframes. However, if the CRS is not transmitted regularly in cells in the on state, this has an impact on conventional user terminals (legacy UEs), and therefore the above-described enhancements are presumed to be implemented in frequency carriers where there are no conventional user terminals. 
     In this case, the same user terminal operations as those in the above-described operations 2, 3 and 8 may be possible depending on whether or not the scrambling IDs are reported, whether or not the DRS duration or the subframe offset are reported, and so on. 
     (Structure of Radio Communication System) 
     Now, a structure of a radio communication system according to the present embodiment will be described. 
       FIG. 9  is a schematic structure diagram to show an example of a radio communication system according to the present embodiment. As shown in  FIG. 9 , a radio communication system  1  is comprised of a plurality of radio base stations  10  ( 11  and  12 ), and a plurality of user terminals  20  that are present within cells formed by each radio base station  10  and that are configured to be capable of communicating with each radio base station  10 . The radio base stations  10  are each connected with a higher station apparatus  30 , and are connected to a core network  40  via the higher station apparatus  30 . 
     In  FIG. 9 , the radio base station  11  is, for example, a macro base station having a relatively wide coverage, and forms a macro cell C 1 . The radio base stations  12  are, for example, small base stations having local coverages, and form small cells C 2 . Note that the number of radio base stations  11  and  12  is not limited to that shown in  FIG. 9 . 
     In the macro cell C 1  and the small cells C 2 , the same frequency band may be used, or different frequency bands may be used. Also, the macro base stations  11  and  12  are connected with each other via an inter-base station interface (for example, optical fiber, the X2 interface, etc.). 
     Between the radio base station  11  and the radio base stations  12 , between the radio base station  11  and other radio base stations  11 , or between the radio base stations  12  and other radio base stations  12 , dual connectivity (DC) or carrier aggregation (CA) may be employed. 
     The user terminals  20  are terminals to support various communication schemes such as LTE, LTE-A and so on, and may include both mobile communication terminals and stationary communication terminals. The user terminals  20  can communicate with other user terminals  20  via the radio base stations  10 . 
     Note that the higher station apparatus  30  may be, for example, an access gateway apparatus, a radio network controller (RNC), a mobility management entity (MME) and so on, but is by no means limited to these. 
     In the radio communication system  1 , a downlink shared channel (PDSCH: Physical Downlink Shared Channel), which is used by each user terminal  20  on a shared basis, downlink control channels (PDCCH (Physical Downlink Control Channel), EPDCCH (Enhanced Physical Downlink Control Channel), etc.), a broadcast channel (PBCH) and so on are used as downlink channels. User data, higher layer control information and predetermined SIBs (System Information Blocks) are communicated in the PDSCH. Downlink control information (DCI) is communicated by the PDCCH and the EPDCCH. 
     In the radio communication system  1 , an uplink shared channel (PUSCH: Physical Uplink Shared Channel), which is used by each user terminal  20  on a shared basis, and an uplink control channel (PUCCH: Physical Uplink Control Channel) are used as uplink channels. User data and higher layer control information are communicated by the PUSCH. 
       FIG. 10  is a diagram to show an overall structure of a radio base station  10  according to the present embodiment. As shown in  FIG. 10 , the radio base station  10  has a plurality of transmitting/receiving antennas  101  for MIMO communication, amplifying sections  102 , transmitting/receiving sections  103 , a baseband signal processing section  104 , a call processing section  105  and an interface section  106 . 
     User data to be transmitted from the radio base station  10  to a user terminal  20  on the downlink is input from the higher station apparatus  30 , into the baseband signal processing section  104 , via the interface section  106 . 
     In the baseband signal processing section  104 , a PDCP layer process, division and coupling of user data, RLC (Radio Link Control) layer transmission processes such as an RLC retransmission control transmission process, MAC (Medium Access Control) retransmission control, including, for example, an HARQ transmission process, scheduling, transport format selection, channel coding, an inverse fast Fourier transform (IFFT) process and a pre-coding process are performed, and the result is forwarded to each transmitting/receiving section  103 . Furthermore, downlink control signals are also subjected to transmission processes such as channel coding and an inverse fast Fourier transform, and forwarded to each transmitting/receiving section  103 . 
     Each transmitting/receiving section  103  converts the downlink signals, which are pre-coded and output from the baseband signal processing section  104  on a per antenna basis, into a radio frequency band. The amplifying sections  102  amplify the radio frequency signals having been subjected to frequency conversion, and transmit the signals through the transmitting/receiving antennas  101 . 
     On the other hand, as for uplink signals, radio frequency signals that are received in the transmitting/receiving antennas  101  are each amplified in the amplifying sections  102 , converted into baseband signals through frequency conversion in each transmitting/receiving section  103 , and input in the baseband signal processing section  104 . 
     In the baseband signal processing section  104 , the user data that is included in the input uplink signals is subjected to an FFT process, an IDFT process, error correction decoding, a MAC retransmission control receiving process and RLC layer and PDCP layer receiving processes, and the result is forwarded to the higher station apparatus  30  via the interface section  106 . The call processing section  105  performs call processing such as setting up and releasing communication channels, manages the state of the radio base station  10  and manages the radio resources. 
     The interface section  106  transmits and receives signals to and from neighboring radio base stations (backhaul signaling) via an inter-base station interface (for example, optical fiber, the X2 interface, etc.). Alternatively, the interface section  106  transmits and receives signals to and from the higher station apparatus  30  via a predetermined interface. 
       FIG. 11  is a diagram to show a functional structure of a macro base station  11  according to the present embodiment. Note that the following functional structure is formed with the baseband signal processing section  104  provided in the macro base station  11  and so on. 
     As shown in  FIG. 11 , the macro base station  11  is comprised at least of a scheduler (control section)  301 , a DRS parameter determining section  302 , a small cell detection commanding section  303 , a DL signal generating section  304  and a measurement report acquiring section  305 . 
     The scheduler  301  allocates the radio resources for the DL signals to transmit to the user terminals  20  and the radio resources for the UL signals to transmit from the user terminals  20  (scheduling). For example, the scheduler  301  commands the user terminals  20  to generate assist information in the DL signal generating section  304  when small cell detection is carried out. 
     In this case, as has been shown with the above embodiment, the scheduler  301  can include a small cell detection command (DiscoveryConfig) in MeasObject and report this to the user terminals  20 , and, meanwhile, execute control so that the timing information and the assist information are included in the information element (DiscoveryRS-Config) to define the configuration of the detection reference signal. 
     The small cell detection commanding section  303  controls commands for small cell detection using the DRS, for the user terminals  20 . For example, the small cell detection commanding section  303  gives commands to user terminals (connected UEs) connected with the macro base station  11  as to whether or not to carry out small cell detection in a predetermined frequency. 
     When commanding the user terminals  20  to carry out small cell detection using the DRS, the DRS parameter determining section  302  determines information related to the DRS parameters. The information related to the DRS parameters may include assist information. The information that is determined in the DRS parameter determining section  302  is output to the scheduler  301  and the DL signal generating section  304 . 
     The DL signal generating section  304  generates DL signals based on commands from the scheduler  301  and the DRS parameter determining section  302 . The signals generated in the DL signal generating section  304  are transmitted to the user terminals  20  via the transmitting/receiving sections  103 . 
     The measurement report acquiring section  305  acquires measurement reports that are reported from the user terminals. The measurement report acquiring section  305  receives the results of small cell detection carried out in the user terminals by using the DRS, in the form of measurement reports. Then, the measurement report acquiring section  305  controls the connection of the user terminals  20  with the small cells based on the measurement reports. 
       FIG. 12  is functional structure diagram of a small base station  12  according to the present embodiment. Note that the following functional structure is formed with the baseband signal processing section  104  provided in the small base station  12  and so on. 
     As shown in  FIG. 12 , a small base station  12  is comprised at least of a UL signal detection section  311 , a scheduler  312  and a DL signal generating section  313 . 
     The UL signal detection section  311  detects the UL signals transmitted from the user terminals  20 . The UL signal detection section  311  can receive measurement reports that are reported from the user terminals  20 . 
     The scheduler  313  allocates the radio resources for the DL signals to transmit to the user terminals  20  (scheduling). Also, the scheduler  312  controls the transmission of the DRS. 
     The DL signal generating section  313  generates DL signals based on commands from the scheduler  312 . For example, the DL signal generating section  313  generates synchronization signals (PSS/SSS), reference signals (CRS), small cell detection reference signals (DRS), control signals, data signals and so on. The signals generated in the DL signal generating section  313  are transmitted to the user terminals  20  via the transmitting/receiving sections  103 . 
       FIG. 13  is a diagram to show an overall structure of a user terminal  20  according to the present embodiment. As shown in  FIG. 13 , a user terminal  20  has a plurality of transmitting/receiving antennas  201  for MIMO communication, amplifying sections  202 , transmitting/receiving sections (receiving sections)  203 , a baseband signal processing section  204  and an application section  205 . 
     As for downlink data, radio frequency signals that are received in the plurality of transmitting/receiving antennas  201  are each amplified in the amplifying sections  202 , and subjected to frequency conversion and converted into the baseband signal in the transmitting/receiving sections  203 . This baseband signal is subjected to an FFT process, error correction decoding, a retransmission control receiving process and so on in the baseband signal processing section  204 . In this downlink data, downlink user data is forwarded to the application section  205 . The application section  205  performs processes related to higher layers above the physical layer and the MAC layer, and so on. Furthermore, in the downlink data, broadcast information is also forwarded to the application section  205 . 
     Meanwhile, uplink user data is input from the application section  205  to the baseband signal processing section  204 . The baseband signal processing section  204  performs a retransmission control (HARQ (Hybrid ARQ)) transmission process, channel coding, pre-coding, a DFT process, an IFFT process and so on, and the result is forwarded to each transmitting/receiving section  203 . The baseband signal that is output from the baseband signal processing section  204  is converted into a radio frequency band in the transmitting/receiving sections  203 . After that, the amplifying sections  202  amplify the radio frequency signal having been subjected to frequency conversion, and transmit the resulting signal from the transmitting/receiving antennas  201 . 
       FIG. 14  is a diagram to show a principle functional structure of the baseband signal processing section  204  provided in the user terminal  20 . As shown in  FIG. 14 , the baseband signal processing section  204  provided in the user terminal  20  at least has a cell detection/measurement section  401 , a measurement report transmission control section  402  and a UL signal generating section  403 . 
     The cell detection/measurement section  401  carries out small cell detection by using the DRSs that are transmitted from the small base stations  12 . Also, the cell detection/measurement section  401  receives information related to small cell detection commands from the macro base station  11  through RRC signaling. Also, when small cell detection is carried out, the cell detection/measurement section  401  detects the DRSs based on information related to DRS parameters, which is reported from the macro base station  11  (assist information). 
     Based on the result of small cell detection carried out in the cell detection/measurement section  401 , the measurement report transmission control section  402  controls the transmission of measurement reports. For example, the measurement report transmission control section  402  acquires the cell identity/RSRP/RSRQ, which correspond to a predetermined cell, and which are acquired by small cell detection. In this case, the measurement report transmission control section  402  controls the network (base station) to report the measurement result as a measurement report. 
     The measurement report transmission control section  402  controls the transmission of measurement reports based on the result acquired by small cell detection. 
     The UL signal generating section  403  generates measurement reports and so on based on commands from the measurement report transmission control section  402 . Furthermore, the UL signal generating section  403  also generates uplink control signals such as delivery acknowledgement signals and so on, and uplink data signals. 
     Note that the present invention is by no means limited to the above-described embodiment and can be implemented with various changes. The sizes and shapes illustrated in the accompanying drawings in relationship to the above embodiment are by no means limiting, and may be changed as appropriate within the scope of optimizing the effects of the present invention. Besides, implementations with various appropriate changes may be possible without departing from the scope of the object of the present invention.