# Patent

## Title
Radio communication system, radio base station apparatus, user terminal and radio communication method

## Abstract

To reduce overhead of CSI feedback without decreasing throughput in applying CoMP transmission, a radio communication method of the invention is characterized by including, in a user terminal measuring quality information for each of cells to perform Coordinated Multi-Point transmission, and notifying a radio base station apparatus of the quality information, in the radio base station apparatus calculating a quality difference between the cells to perform Coordinated Multi-Point transmission, by using the quality information for each the cells from the user terminal, determining granularity of inter-cell channel state information for each of the cells based on the quality difference, and notifying the user terminal of the granularity, and, in that the user terminal generating inter-cell channel state information for each of the cells according to the granularity from the radio base station apparatus, and transmitting the inter-cell channel state information for each of the cells to radio base station apparatuses of cells to perform Coordinated Multi-Point transmission.

## Background
TECHNICAL FIELD
The present invention relates to a radio communication system, radio base station apparatus, user terminal and radio communication method applicable to a cellular system and the like.
BACKGROUND ART
In UMTS (Universal Mobile Telecommunications System) networks, for the purpose of improving spectral efficiency and further improving data rates, by adopting HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access), it is performed exploiting maximum features of the system based on W-CDMA (Wideband-Code Division Multiple Access). For the UMTS network, for the purpose of further increasing high-speed data rates, providing low delay and the like, Long Term Evolution (LTE) has been studied (Non-patent Literature 1).
In the 3G system, a fixed band of 5 MHz is substantially used, and it is possible to achieve transmission rates of approximately maximum 2 Mbps in downlink. Meanwhile, in the LTE system, using variable bands ranging from 1.4 MHz to 20 MHz, it is possible to achieve transmission rates of maximum 300 Mbps in downlink and about 75 Mbps in uplink. Further, in the UMTS network, for the purpose of further increasing the wide-band and high speed, successor systems to LTE have been studied (for example, LTE Advanced (LTE-A)).
CITATION LIST
Non-Patent Literature


    [Non-patent Literature 1] 3GPP, TR25.912 (V7.1.0), “Feasibility study for Evolved UTRA and UTRAN”, September 2006


SUMMARY OF THE INVENTION
Technical Problem
In addition, as one of promising techniques to further improve system performance of LTE system, there is inter-cell orthogonalization. For example, in the LTE-A system, intra-cell orthogonalization is achieved by orthogonal multiple access both in uplink and downlink. In other words, in downlink, user terminals UEs (User Equipments) are orthogonalized in the frequency domain. Meanwhile, for inter-cell, interference randomizing by 1-cell frequency reuse is a base as in W-CDMA.
Therefore, the 3GPP (3rd Generation Partnership Project) has studied Coordinated Multi-Point transmission/reception (CoMP) as techniques for actualizing inter-cell orthogonalization. In CoMP transmission/reception, a plurality of cells coordinates to perform signal processing of transmission and reception on a single or a plurality of user terminals UEs. For example, in downlink, studied are plurality-of-cell simultaneous transmission applying precoding, Coordinated Scheduling/Beamforming and the like. By applying these CoMP transmission/reception techniques, it is expected to improve throughput characteristics of user terminals UEs particularly positioned at the cell edge.
To apply CoMP transmission/reception techniques, it is necessary that a user terminal transmits channel state information (CSI) on a plurality of cells to a radio base station apparatus as feedback. Therefore, overhead of CSI feedback is large. When overhead of CSI feedback is simply decreased, it is not possible to effectively apply CoMP transmission/reception techniques, and throughput is not improved.
The present invention was made in view of such respects, and it is an object of the invention to provide a radio communication system, radio base station apparatus, user terminal and radio communication method that enable overhead of CSI feedback to be reduced without decreasing throughput in applying CoMP transmission.
Solution to Problem
A radio communication system of the present invention is a radio communication system provided with a plurality of radio base station apparatuses, and a user terminal configured to be able to perform Coordinated Multi-Point transmission/reception with the plurality of radio base station apparatuses, and is characterized in that the radio base station apparatus includes a calculation section that calculates, using quality information for each of cells to perform Coordinated Multi-Point transmission notified from the user terminal, a quality difference between the cells to perform Coordinated Multi-Point transmission and a determination section that determines granularity of inter-cell channel state information for each of the cells based on the quality difference, and that the user terminal includes a notification section that notifies the radio base station apparatus of quality information for each cell to perform Coordinated Multi-Point transmission, a generation section that generates inter-cell channel state information for each cell according to the granularity notified from the radio base station apparatus, and a transmission section that transmits the inter-cell channel state information for each cell to radio base station apparatuses of cells to perform Coordinated Multi-Point transmission.
A radio base station apparatus of the invention is a radio base station apparatus in a radio communication system provided with a plurality of radio base station apparatuses, and a user terminal configured to be able to perform Coordinated Multi-Point transmission/reception with the plurality of radio base station apparatuses, and is characterized by including a calculation section that calculates, using quality information for each of cells to perform Coordinated Multi-Point transmission notified from the user terminal, a quality difference between the cells to perform Coordinated Multi-Point transmission, and a determination section that determines granularity of inter-cell channel state information for each of the cells based on the quality difference.
A user terminal of the invention is a user terminal in a radio communication system provided with a plurality of radio base station apparatuses, and a user terminal configured to be able to perform Coordinated Multi-Point transmission/reception with the plurality of radio base station apparatuses, and is characterized by including a notification section that notifies the radio base station apparatus of quality information for each cell to perform Coordinated Multi-Point transmission, a generation section that generates inter-cell channel state information for each cell according to the granularity notified from the radio base station apparatus, and a transmission section that transmits the inter-cell channel state information for each cell to radio base station apparatuses of cells to perform Coordinated Multi-Point transmission.
A radio communication method of the invention is a radio communication method in a radio communication system provided with a plurality of radio base station apparatuses, and a user terminal configured to be able to perform Coordinated Multi-Point transmission/reception with the plurality of radio base station apparatuses, and is characterized by including the steps of in the user terminal, measuring quality information for each of cells to perform Coordinated Multi-Point transmission, and notifying the radio base station apparatus of the quality information, the steps of in the radio base station apparatus, calculating a quality difference between the cells to perform Coordinated Multi-Point transmission, by using quality information for each of the cells from the user terminal, determining granularity of inter-cell channel state information for each of the cells based on the quality difference, and notifying the user terminal of the granularity, and the steps of in the user terminal, generating inter-cell channel state information for each of the cells according to the granularity from the radio base station apparatus, and transmitting the inter-cell channel state information for each of the cells to radio base station apparatuses of cells to perform Coordinated Multi-Point transmission.
Technical Advantage of the Invention
According to the present invention, it is possible to reduce overhead of CSI feedback without decreasing throughput in applying CoMP transmission.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 contains diagrams to explain Coordinated Multi-Point transmission;
FIG. 2 contains schematic diagrams illustrating a configuration of a radio base station apparatus applied to Coordinated Multi-Point transmission;
FIG. 3 is a diagram showing a table where a quality difference between cells to perform Coordinated Multi-Point transmission and granularity of inter-cell channel state information for each cell are associated;
FIG. 4 is a sequence diagram illustrating a procedure between a radio base station apparatus and a user terminal;
FIG. 5 is a diagram to explain a system configuration of a radio communication system;
FIG. 6 is a diagram to explain an entire configuration of the radio base station apparatus;
FIG. 7 is a functional block diagram corresponding to a baseband processing section of the radio base station apparatus;
FIG. 8 is a diagram to explain an entire configuration of a user terminal; and
FIG. 9 is a functional block diagram corresponding to a baseband processing section of the user terminal.



DESCRIPTION OF EMBODIMENTS
An Embodiment of the present invention will specifically be described below with reference to accompanying drawings.
Downlink CoMP transmission will be described first with reference to FIG. 1. As downlink CoMP transmission, there are Coordinated Scheduling/Coordinated Beamforming and Joint processing. Coordinated Scheduling/Coordinated Beamforming is a method for transmitting a shared data channel to one user terminal UE from only one cell, where radio resources in the frequency/spatial domain are allocated with consideration given to interference from another cell and interference to another cell as shown in FIG. 1A. Meanwhile, Joint processing is a method for transmitting shared data channels from a plurality of cells at the same time by applying precoding, and has Joint transmission in which a plurality of cells transmits shared data channels to one user terminal UE as shown in FIG. 1B, and Dynamic Point Selection (DPS) in which one cell is instantaneously selected and transmits a shared data channel as shown in FIG. 1C.
As a configuration to actualize CoMP transmission/reception, for example, there are a configuration (centralized control based on the RRE configuration) including a plurality of remote radio equipments (RREs) connected to a radio base station apparatus (radio base station apparatus eNB) with optical fibers or the like as shown in FIG. 2A, and another configuration (autonomous decentralized control based on the independent base station configuration) of a radio base station apparatus (radio base station apparatus eNB) as shown in FIG. 2B. In addition, FIG. 2A illustrates the configuration including a plurality of remote radio equipments RREs, and may be of a configuration including only a single remote radio equipment RRE as shown in FIG. 1.
In the configuration (RRE configuration) as shown in FIG. 2A, the radio base station apparatus eNB controls the remote radio equipments RRE1 and RRE2 in a centralized manner. In the RRE configuration, since the radio base station apparatus eNB (centralized base station) that performs baseband signal processing and control of a plurality of remote radio equipments RREs and each cell (i.e. each remote radio equipment RRE) are connected with baseband signals using optical fibers, the centralized base station is capable of collectively performing radio resource control among cells. In other words, the problems of delay of signaling between radio base station apparatuses eNBs and overhead are small, which become problems in the independent base station configuration, and high-speed radio resource control among cells is relatively made ease. Accordingly, in the RRE configuration, in downlink, it is possible to apply the method using high-speed inter-cell signal processing such as plurality-of-cell simultaneous transmission.
Meanwhile, in the configuration (independent base station configuration) as shown in FIG. 2B, each of a plurality of radio base station apparatuses eNBs (or RREs) performs radio resource allocation control such as scheduling. In this case, by using an X2 interface between the radio base station apparatus eNB of a cell 1 and the radio base station apparatus eNB of a cell 2, when necessary, timing information and radio resource allocation information such as scheduling is transmitted to any one of the radio base station apparatuses eNBs to perform coordination between cells.
CoMP transmission is applied to improve throughput of a user terminal existing at the cell edge. Therefore, it is controlled to apply CoMP transmission when a user terminal exists at the cell edge. In this case, a radio base station apparatus obtains a difference of quality information (for example, RSRP (Reference Signal Received Power)) for each cell from a user terminal, and in the case where the difference is equal to or less than a threshold i.e. in the case where the quality difference between cells is small, determines that the user terminal exists at the cell edge to apply CoMP transmission. Meanwhile, in the case where the difference of quality information for each cell exceeds the threshold i.e. in the case where the quality difference between cells is large, since the user terminal is close to a radio base station apparatus of either cell, the radio base station apparatus determines that the user terminal exists near the center of the cell, and does not apply CoMP transmission.
In the case of applying CoMP transmission, the user terminal transmits channel state information for each of a plurality of cells to the radio base station apparatus (radio base station apparatus of the serving cell) as feedback. Meanwhile, in the case of not applying CoMP transmission, the user terminal transmits the channel state information of the serving cell to the radio base station apparatus as feedback. Thus, when CoMP transmission is applied, since the user terminal transmits the channel state information for each of a plurality of cells as feedback, overhead of feedback information increases.
Herein, as the channel state information, there is channel state information for each cell, and inter-cell channel state information. In Joint transmission type CoMP, radio base station apparatuses of a plurality of cells transmit the same data with the same phase to a user terminal at the same timing. In this case, it is necessary to transmit inter-cell information as feedback so that the user terminal is capable of receiving the same data with the same phase at the same timing, together with the channel state information of each cell. As the channel state information for each cell, there are PMI (Precoding Matrix Indicator), CDI (Channel Distribution Information), CQI (Channel Quality Indicator) and the like, and as the inter-cell channel state information, there is phase difference information, amplitude difference information and the like.
The inventors of the present invention have noted the inter-cell channel state information in the above-mentioned channel state information. As described above, in the case where the difference in the quality information for each cell is small, it is determined that the user terminal exists at the cell edge, and CoMP transmission is applied. Meanwhile, in the case where the difference in the quality information for each cell is large, it is determined that the user terminal exists near the center of the cell, and CoMP transmission is not applied. Thus, as the difference in the quality information for each cell is smaller, the throughput improvement effect due to CoMP is exhibited. In other words, as the difference in the quality information for each cell is larger, it is conceivable that a contribution of the throughput improvement effect due to CoMP is smaller. Under such consideration, in the case of applying Joint transmission type CoMP, when the difference in the quality information for each cell is small, it is conceivable that inter-cell channel state information with higher accuracy (granularity) is required. Meanwhile, when the difference in the quality information for each cell is large, it is conceivable that inter-cell channel state information does not require such higher accuracy (granularity). Herein, the granularity of the inter-cell channel state information means fineness (accuracy) of the inter-cell channel state information, and as the granularity is higher, the accuracy is higher. Accordingly, when the granularity is replaced with an information amount of the inter-cell channel state information, as the granularity of the inter-cell channel state information is higher, the information amount (the number of bits) of the inter-cell channel state information is larger, and as the granularity of the inter-cell channel state information is lower, the information amount (the number of bits) of the inter-cell channel state information is smaller.
Further, the inventors of the invention have actually measured how much the relative capacity changes due to the difference (quality difference) in the quality information for each cell, by varying the granularity of the inter-cell channel state information. As a result, it is understood that a gap of the relative capacity is large due to the difference in the quality information for each cell. More specifically, when the quality difference was small, the relative capacity differed significantly by varying the granularity of the inter-cell channel state information. Meanwhile, when the quality difference was large, the relative capacity did not differ significantly by varying the granularity of the inter-cell channel state information. Accordingly, when the quality difference is relatively large, it is understood that it is possible to maintain the relative capacity even by lowering the granularity of the inter-cell channel state information i.e. decreasing the information amount.
Therefore, the inventors of the invention have found out that it is possible to reduce overhead of the channel state information to feed back by varying accuracy (granularity) of the inter-cell channel state information, and arrived at the invention.
In other words, it is the gist of the invention that granularity of inter-cell channel state information for each cell is determined based on a quality difference between cells calculated using quality information for each cell (coordination candidate cell) to perform Coordinated Multi-Point transmission notified from a user terminal, the user terminal transmits inter-cell channel state information generated according to the granularity as feedback, and that in applying CoMP transmission, overhead of CSI feedback is thereby reduced without decreasing throughput.
In the invention, when a quality difference in quality information for each cell (coordination candidate cell) to perform Coordinated Multi-Point transmission is small, granularity of inter-cell channel state information to feed back in applying CoMP is determined to be higher. Meanwhile, when a quality difference in quality information for each cell (coordination candidate cell) to perform Coordinated Multi-Point transmission is large, granularity of inter-cell channel state information to feed back in applying CoMP is determined to be lower.
In control (control for varying accuracy (granularity) of inter-cell channel state information corresponding to a difference in quality information for each cell) in the invention, a table is prepared which associates a difference (quality difference) in quality information for each cell with granularity of inter-cell channel state information, a quality difference for each cell is obtained, and granularity is determined from the quality difference by referring to the table. As the table, for example, there is a table as shown in FIG. 3. In FIG. 3, when the quality difference is relatively small (0.5 dB, 5 dB), the granularity of inter-cell channel state information is increased (2 bits). Meanwhile, when the quality difference is relatively large (10 dB, 15 dB), the granularity of inter-cell channel state information is decreased (0 bit). Further, as the quality difference increases (0 dB, 5 dB)→(5 dB, 10 dB)→(10 dB, 15 dB), the granularity of inter-cell channel state information is lower (2 bits→1 bit→0 bit). In addition, the inter-cell channel state information of 0 bit means that inter-cell channel state information is not transmitted as feedback. Further, as a matter of course, a table that associates the quality difference with the granularity is not limited to the table as shown in FIG. 3.
As the quality information for each cell, there is RSRP, RSRQ (Reference Signal Received Quality), temporal long-term channel quality information, temporal short-term channel quality information or the like.
Herein, a procedure of control according to the present invention will be described with reference to FIG. 4. FIG. 4 is a sequence diagram illustrating the procedure between a radio base station apparatus and a user terminal.
First, a user terminal measures quality information (for example, RSRP) for each cell to perform Coordinated Multi-Point (ST11). The user terminal notifies a radio base station apparatus of the quality information by higher layer signaling, for example.
Next, the radio base station apparatus calculates a quality difference between cells to perform Coordinated Multi-Point transmission using the quality information for each cell from the user terminal (ST12). Next, the radio base station apparatus determines granularity of inter-cell channel state information for each cell based on the calculated quality difference (ST13). For example, by referring to the table as shown in FIG. 3, the apparatus determines granularity from the quality difference. Subsequently, the radio base station apparatus notifies the user terminal of the granularity (CSI granularity information), for example, by higher layer signaling (RRC signaling, broadcast information).
Next, the user terminal generates inter-cell channel state information for each cell using the granularity (for example, the number of bits) notified from the radio base station apparatus (ST14). Then, the user terminal transmits channel state information including thus generated inter-cell channel state information for each cell to radio base station apparatuses of cells that perform Coordinated Multi-Point transmission for each cell as feedback (ST15). According to such control according to the invention, it is possible to achieve a maximum 30 percentage reduction in overhead of feedback information while maintaining throughput in applying CoMP.
A radio communication system according to the Embodiment of the invention will specifically be described below. FIG. 5 is an explanatory diagram of a system configuration of the radio communication system according to this Embodiment. In addition, the radio communication system as shown in FIG. 5 is a system including the LTE system or SUPER 3G, for example. In the radio communication system, used is carrier aggregation for integrating a plurality of base frequency blocks with a system band of the LTE system as a unit. Further, the radio communication system may be called IMT-Advanced or may be called 4G.
As shown in FIG. 5, the radio communication system 1 includes radio base station apparatuses 20A and 20B, and a plurality of first and second user terminals 10A and 10B that communicate with the radio base station apparatuses 20A and 20B, and is comprised thereof. The radio base station apparatuses 20A and 20B are connected to an upper station apparatus 30, and the upper station apparatus 30 is connected to a core network 40. Further, the radio base station apparatuses 20A and 20B are mutually connected by wired connection or wireless connection. The first and second user terminals 10A and 10B are capable of communicating with the radio base station apparatuses 20A and 20B in cells C1 and C2. In addition, for example, the upper station apparatus 30 includes an access gateway apparatus, radio network controller (RNC), mobility management entity (MME), etc., but is not limited thereto. In addition, among cells, when necessary, a plurality of base stations performs control of CoMP transmission.
The first and second user terminals 10A and 10B include LTE terminals and LTE-A terminals, and are described as first and second user terminals unless otherwise specified in the following description. Further, for convenience in description, the description is given while assuming that equipments that perform radio communications with the radio base station apparatuses 20A and 20B are the first and second user terminals 10A and 10B, and more generally, the equipments may be user equipments (UEs) including user terminals and fixed terminals.
In the radio communication system 1, as a radio access scheme, OFDMA (Orthogonal Frequency Division Multiple Access) is applied in downlink, while SC-FDMA (Single Carrier-Frequency Division Multiple Access) is applied in uplink, and the uplink radio access scheme is not limited thereto. OFDMA is a multicarrier transmission scheme for dividing a frequency band into a plurality of narrow frequency bands (subcarriers), and mapping data to each subcarrier to perform communications. SC-FDMA is a single-carrier transmission scheme for dividing the system band into bands comprised of a single or consecutive resource blocks for each terminal so that a plurality of terminals uses mutually different bands, and thereby reducing interference among the terminals.
The downlink communication channels have the PDSCH (Physical Downlink Shared Channel) as a downlink data channel shared among the first and second user terminals 10A and 10B, and downlink L1/L2 control channels (PDCCH, PCFICH, PHICH). Transmission data and higher control information is transmitted on the PDSCH. Scheduling information of the PDSCH and PUSCH and the like is transmitted on the PDCCH (Physical Downlink Control Channel). The number of OFDM symbols used in the PDCCH is transmitted on the PCFICH (Physical Control Format Indicator Channel). ACK/NACK of HARQ to the PUSCH is transmitted on the PHICH (Physical Hybrid-ARQ Indicator Channel).
The uplink communication channels have the PUSCH (Physical Uplink Shared Channel) as an uplink data channel shared among the user terminals, and the PUCCH (Physical Uplink Control Channel) that is a control channel in uplink. Transmission data and higher control information is transmitted on the PUSCH. Further, on the PUCCH is transmitted downlink reception quality information (CQI), ACK/NACK and the like.
Referring to FIG. 6, described is the entire configuration of the radio base station apparatus according to this Embodiment. In addition, the radio base station apparatuses 20A and 20B have the same configuration, and therefore, are described as the radio base station apparatus 20. Further, the first and second user terminals 10A and 10B described later also have the same configuration, and therefore, are described as the user terminal 10. The radio base station apparatus 20 is provided with transmission/reception antennas 201, amplifying sections 202, transmission/reception sections (notification section) 203, baseband signal processing section 204, call processing section 205 and transmission path interface 206. The transmission data to transmit from the radio base station apparatus 20 to the user terminal in downlink is input to the baseband signal processing section 204 via the transmission path interface 206 from the upper station apparatus 30.
The baseband signal processing section 204 performs, on the downlink data channel signal, PDCP layer processing, segmentation and concatenation of the transmission data, RLC (Radio Link Control) layer transmission processing such as transmission processing of RLC retransmission control, MAC (Medium Access Control) retransmission control e.g. HARQ transmission processing, scheduling, transmission format selection, channel coding, Inverse Fast Fourier Transform (IFFT) processing and precoding processing. Further, on a signal of the Physical Downlink Control Channel that is a downlink control channel, the section 204 also performs transmission processing of channel coding, Inverse Fast Fourier Transform and the like.
Further, the baseband signal processing section 204 notifies user terminals 10 connected to the same cell of control information for each user terminal 10 to perform radio communications with the radio base station apparatus 20 on the broadcast channel. For example, the information for communications in the cell includes the system bandwidth in uplink or downlink, identification information (Root Sequence Index) of a root sequence to generate a signal of a random access preamble on the PRACH (Physical Random Access Channel), etc.
The transmission/reception section 203 converts the frequency of the baseband signal output from the baseband signal processing section 204 into a radio frequency band. The amplifying section 202 amplifies a radio frequency signal subjected to frequency conversion to output to the transmission/reception antenna 201. In addition, the transmission/reception section 203 constitutes reception means for receiving an uplink signal including information of a phase difference among a plurality of cells and PMI, and transmission means for performing Coordinated Multi-Point transmission on a transmission signal.
Meanwhile, with respect to signals transmitted from the user terminal 10 to the radio base station apparatus 20 in uplink, a radio frequency signal received in the transmission/reception antenna 201 is amplified in the amplifying section 202, subjected to frequency conversion in the transmission/reception section 203, thereby converted into a baseband signal, and is input to the baseband signal processing section 204.
The baseband signal processing section 204 performs FFT processing, IDFT processing, error correcting decoding, reception processing of MAC retransmission control, and reception processing of RLC layer and PDCP layer on transmission data included in the baseband signal received in uplink. The decoded signal is transferred to the upper station apparatus 30 via the transmission path interface 206.
The call processing section 205 performs call processing such as setting and release of the communication channel, status management of the radio base station apparatus 20, and management of radio resources.
FIG. 7 is a block diagram illustrating a configuration of the baseband signal processing section in the radio base station apparatus as shown in FIG. 6. The baseband signal processing section 204 is mainly comprised of a layer 1 processing section 2041, MAC processing section 2042, RLC processing section 2043, quality difference calculation section 2044, and CSI granularity determining section 2045.
The layer 1 processing section 2041 mainly performs processing concerning the physical layer. For example, on a signal received in uplink, the layer 1 processing section 2041 performs processing such as channel decoding, Discrete Fourier Transform (DFT), frequency demapping, Inverse Fast Fourier Transform (IFFT), and data demodulation. Further, on a signal to transmit in downlink, the layer 1 processing section 2041 performs processing such as channel coding, data modulation, frequency mapping and Inverse Fast Fourier Transform (IFFT).
The MAC processing section 2042 performs processing such as retransmission control in the MAC layer on the signal received in uplink, scheduling in uplink/downlink, selection of a transmission format of PUSCH/PDSCH, and selection of a resource block of PUSCH/PDSCH.
On packets received in uplink and packets to transmit in downlink, the RLC processing section 2043 performs segmentation of packets, concatenation of packets, retransmission control in the RLC layer and the like.
The quality difference calculation section 2044 calculates a quality difference between cells to perform CoMP transmission, using the quality information (for example, RSRP) for each cell from the user terminal. The quality difference calculation section 2044 outputs information of the calculated quality difference to the CSI granularity determining section 2045.
The CSI granularity determining section 2045 determines granularity of inter-cell CSI for each cell based on the calculated quality difference. The CSI granularity determining section 2045 determines granularity of inter-cell CSI transmitted as feedback in applying CoMP to be higher when the quality difference in the quality information for each cell to perform CoMP transmission is small, while determining granularity of inter-cell CSI transmitted as feedback in applying CoMP to be lower when the quality difference for each cell to perform CoMP transmission is large. For example, by referring to the table as shown in FIG. 3, the granularity is determined from the quality difference. The granularity is notified to the user terminal by higher layer signaling, for example.
The entire configuration of the user terminal according to this Embodiment will be described next with reference to FIG. 8. The LTE terminal and the LTE-A terminal have the same configuration of principal part of hardware, and are not distinguished to describe. The user terminal 10 is provided with transmission/reception antennas 101, amplifying sections 102, transmission/reception sections (reception section) 103, baseband signal processing section 104 and application section 105.
With respect to data in downlink, a radio frequency signal received in the transmission/reception antenna 101 is amplified in the amplifying section 102, subjected to frequency conversion in the transmission/reception section 103, and is converted into a baseband signal. The baseband signal is subjected to FFT processing, error correcting decoding, reception processing of retransmission control, etc. in the baseband signal processing section 104. Among the data in downlink, the transmission data in downlink is transferred to the application section 105. The application section 105 performs processing concerning layers higher than the physical layer and MAC layer and the like. Further, among the data in downlink, the broadcast information is also transferred to the application section 105.
Meanwhile, with respect to transmission data in uplink, the application section 105 inputs the data to the baseband signal processing section 104. The baseband signal processing section 104 performs mapping processing, transmission processing of retransmission control (HARQ), channel coding, DFT processing and IFFT processing. The transmission/reception section 103 converts the frequency of the baseband signal output from the baseband signal processing section 104 into a radio frequency band. Then, the amplifying section 102 amplifies the radio frequency signal subjected to frequency conversion to transmit from the transmission/reception antenna 101. In addition, the transmission/reception section 103 constitutes transmission means for transmitting information of a phase difference, information of a connected cell, selected PMI and the like to radio base station apparatuses eNBs of a plurality of cells and reception means for receiving a downlink signal.
FIG. 9 is a block diagram illustrating a configuration of the baseband signal processing section in the user terminal as shown in FIG. 8. The baseband signal processing section 104 is mainly comprised of a layer 1 processing section 1041, MAC processing section 1042, RLC processing section 1043, feedback information generating section 1044, and quality measuring section 1045.
The layer 1 processing section 1041 mainly performs processing concerning the physical layer. For example, on a signal received in downlink, the layer 1 processing section 1041 performs processing such as channel decoding, Discrete Fourier Transform (DFT), frequency demapping, Inverse Fast Fourier Transform (IFFT), and data demodulation. Further, on a signal to transmit in uplink, the layer 1 processing section 2041 performs processing such as channel coding, data modulation, frequency mapping and Inverse Fast Fourier Transform (IFFT).
The MAC processing section 1042 performs retransmission control (HARQ) in the MAC layer on the signal received in downlink, analysis of downlink scheduling information (identification of a transmission format of the PDSCH, identification of a resource block of the PDSCH) and the like. Further, the MAC processing section 1042 performs processing such as MAC retransmission control on a signal to transmit in uplink, and analysis of uplink scheduling information (identification of a transmission format of the PUSCH, identification of a resource block of the PUSCH).
On packets received in downlink and packets to transmit in uplink, the RLC processing section 1043 performs segmentation of packets, concatenation of packets, retransmission control in the RLC layer and the like.
The feedback information generating section 1044 generates CSI (feedback information). As the CSI, there are CSI (PMI, CDI, CQI) for each cell, inter-cell CSI (phase difference information, amplitude difference information), RI (Rank Indicator) and the like. Particularly, for the inter-cell CSI, the feedback information generating section 1044 generates CSI with granularity (for example, the number of bits) notified from the radio base station apparatus. These pieces of CSI are transmitted to the radio base station apparatus as feedback on the PUCCH and PUSCH.
The quality measuring section 1045 measures quality information for each cell to perform CoMP transmission. As the quality information, there is RSRP, RSRQ, temporal long-term channel quality information, temporal short-term channel quality information or the like. The quality information is notified to the radio base station apparatus by higher layer signaling.
In the radio communication system having the above-mentioned configuration, first, the quality measuring section 1045 in the user terminal measures quality information (for example, RSRP) for each cell to perform CoMP transmission. For example, the user terminal notifies the radio base station apparatus of the quality information by higher layer signaling.
Next, the quality difference calculation section 2044 in the radio base station apparatus calculates a quality difference between cells to perform CoMP transmission, using the quality information for each cell from the user terminal. Next, the CSI granularity determining section 2045 determines granularity of inter-cell CSI for each cell based on the calculated quality difference. At this point, the section 2045 determines granularity of inter-cell CSI transmitted as feedback in applying CoMP to be higher when the quality difference is small, while determining granularity of inter-cell CSI transmitted as feedback in applying CoMP to be lower when the quality difference is large. For example, by referring to the table as shown in FIG. 3, the granularity is determined from the quality difference. Subsequently, the radio base station apparatus notifies the user terminal of the granularity, for example, by higher layer signaling.
Next, the feedback information generating section 1044 in the user terminal generates inter-cell CSI for each cell according to the granularity (for example, the number of bits) notified from the radio base station apparatus. Then, thus generated inter-cell CSI for each cell is transmitted to radio base station apparatuses of cells to perform CoMP transmission as feedback for each cell together with the other CSI. By this means, it is possible to achieve reductions in overhead of feedback information while maintaining throughput in applying CoMP transmission.
In the above-mentioned descriptions, the present invention is specifically described using the above-mentioned Embodiment, but it is obvious to a person skilled in the art that the invention is not limited to the Embodiment described in the Description. The invention is capable of being carried into practice as modified and changed aspects without departing from the subject matter and scope of the invention defined by the descriptions of the scope of the claims. Accordingly, the descriptions of the Description are intended for illustrative explanation, and do not have any restrictive meaning to the invention.
The present application is based on Japanese Patent Application No. 2011-243022 filed on Nov. 7, 2011, entire content of which is expressly incorporated by reference herein.

## Claims

1. A radio communication system comprising:
a plurality of radio base station apparatuses, wherein
at least one radio base station apparatus of the plurality of radio base station apparatuses includes:
a transmission section that performs signal processing on Coordinated Multi-Point transmissions of a downlink signal sent to a user terminal;
a calculation section that calculates a quality difference between the cells performing Coordinated Multi-Point transmission by using quality information associated with each cell from the user terminal; and
a determination section that determines granularity of inter-cell channel state information for each cell based on the quality difference between the cells performing Coordinated Multi-Point transmission; and

the user terminal including:
a reception section that receives a downlink Coordinated Multi-Point transmission signal from a radio base station apparatus;
a notification section that notifies the at least one radio base station apparatus of quality information associated with each cell performing Coordinated Multi-Point transmission;
a generation section that generates inter-cell channel state information for each cell according to the granularity notified from the at least one radio base station apparatus; and
a transmission section that transmits the inter-cell channel state information for each cell to the radio base station apparatus associated with Coordinated Multi-Point transmission.

2. The radio communication system according to claim 1, wherein the determination section determines the granularity to be high when the quality difference between cells to perform the Coordinated Multi-Point transmission is small.

3. The radio communication system according to claim 2, wherein the determination section has a table where the quality difference and the granularity are associated with each other, and determines the granularity from the quality difference by referring to the table.

4. The radio communication system according to claim 1, wherein the determination section has a table where the quality difference and the granularity are associated with each other, and determines the granularity from the quality difference by referring to the table.

5. The radio communication system according to claim 1, wherein the quality information is Received Signal Received Power (RSRP), Reference Signal Received Quality (RSRQ), temporal long-term channel quality information, or temporal short-term channel quality information.

6. The radio communication system according to claim 1, wherein the CoMP transmission is Joint transmission type CoMP transmission.

7. A radio base station apparatus comprising:
a transmission section that performs signal processing on Coordinated Multi-Point transmissions of a downlink signal sent to a user terminal;
a calculation section that calculates a quality difference between the cells performing Coordinated Multi-Point transmission by using quality information associated with each cell from the user terminal; and
a determination section that determines granularity of inter-cell channel state information for each cell based on the quality difference between the cells performing Coordinated Multi-Point transmission.

8. A user terminal comprising:
a reception section that receives a downlink Coordinated Multi-Point transmission signal from a radio base station apparatus;
a notification section that notifies the at least one radio base station apparatus of quality information associated with each cell to perform Coordinated Multi-Point transmission;
a generation section that generates inter-cell channel state information for each cell according to granularity notified from the radio base station apparatus; and
a transmission section that transmits the inter-cell channel state information for each cell to the radio base station apparatus associated with Coordinated Multi-Point transmission.

9. A radio communication method comprising:
measuring, by a user terminal, quality information associated with each cell to perform Coordinated Multi-Point transmission, and
notifying, by the user terminal, at least one radio base station apparatus of the radio base station apparatuses of the quality information;
performing, by the at least one radio base station apparatus, signal processing on Coordinated Multi-Point transmissions of a downlink signal sent to the user terminal;
calculating, by the at least one radio base station apparatus, a quality difference between the cells performing Coordinated Multi-Point transmission, by using quality information associated with each cell from the user terminal;
determining, by the at least one radio base station apparatus, granularity of inter-cell channel state information for each cell based on the quality difference between the cells performing Coordinated Multi-Point transmission; and
notifying, by the at least one radio base station apparatus, the user terminal of the granularity;
generating, by the user terminal, inter-cell channel state information for each of the cells according to the granularity from the at least one radio base station apparatus; and
transmitting, by the user terminal, the inter-cell channel state information for each cell to the radio base station apparatus associated with Coordinated Multi-Point transmission.