JPWO2017026407A1 - Base station and wireless terminal - Google Patents

Abstract

A base station according to one embodiment supports FD-MIMO (Full-Dimension MIMO). The base station includes an antenna array having antenna elements arranged in two dimensions in the horizontal direction and the vertical direction, and a reference signal used for measurement of channel characteristics in the two dimensions, and is generated individually for each radio terminal. And a control unit that performs a process of transmitting a reference signal. The control unit performs a process of selecting a specific target wireless terminal from target wireless terminals connected to the base station, and transmitting the dedicated reference signal only to the specific target wireless terminal.

Description

  The present invention relates to a base station and a wireless terminal used in a wireless communication system.

  Introduction of MIMO (Multi-Input Multi-Output) communication using an antenna array having antenna elements arranged in two dimensions in the horizontal direction and the vertical direction in 3GPP (Third Generation Partnership Project), which is a standardization project for wireless communication systems Is being considered. Such MIMO communication is called FD-MIMO (Full-Dimension MIMO). According to FD-MIMO, directivity control not only in the horizontal direction but also in the vertical direction can be performed by arranging the antenna elements in two dimensions.

  A base station according to an embodiment includes a control unit that performs processing of transmitting an individual reference signal that is a reference signal used for measurement of channel characteristics and is generated for each wireless terminal. The control unit performs a process of selecting a specific target wireless terminal from target wireless terminals connected to the base station, and transmitting the dedicated reference signal only to the specific target wireless terminal.

  A wireless terminal according to one embodiment includes a control unit. The control unit is configured to receive, from the base station, a terminal-specific reference signal that is a demodulation reference signal that is transmitted from the base station to each radio terminal, and channel state information generated by channel estimation using the terminal-specific reference signal. Is transmitted to the base station.

  A base station according to one embodiment includes a control unit. The control unit is configured to transmit a terminal-specific reference signal, which is a demodulation reference signal transmitted to each radio terminal, to the radio terminal, and channel state information generated by the radio terminal by channel estimation using the terminal-specific reference signal. Is received from the wireless terminal.

  The terminal-specific reference signal is included in an assigned radio resource assigned to the radio terminal by the base station. The terminal-specific reference signal may be used not only for demodulating downlink data included in the allocated radio resource but also for generating the channel state information.

  A processor according to one embodiment controls a wireless terminal. The processor is configured to receive, from the base station, a terminal-specific reference signal that is a demodulation reference signal transmitted by the base station individually for each radio terminal, and channel state information generated by channel estimation using the terminal-specific reference signal. And processing to transmit to the base station.

It is a figure which shows the structure of a LTE system (wireless communication system). It is a block diagram of UE (radio terminal). It is a block diagram of eNB (base station). It is a figure which shows an example of the antenna (antenna array) with which eNB is equipped. FIG. 3 is a protocol stack diagram of a radio interface in the LTE system. It is a block diagram of the radio | wireless frame used in a LTE system. It is a figure for demonstrating the outline | summary of downlink MIMO. It is a figure which shows the operation | movement flow of eNB which concerns on 1st Embodiment. It is a flowchart which shows an example of the setting and update method of the performance parameter | index which concerns on 1st Embodiment. It is a figure which shows an example of the operation | movement sequence which concerns on 1st Embodiment. It is a figure which shows the operation | movement flow of eNB which concerns on the example 1 of a change of 1st Embodiment. It is a figure which shows the operation | movement flow of eNB which concerns on the modification 2 of 1st Embodiment. It is a figure which shows the example 1 of a change of the flow shown in FIG. It is a figure which shows the example 2 of a change of the flow shown in FIG. It is a figure which shows the outline | summary of the operation | movement which concerns on 2nd Embodiment. It is a block diagram of UE (radio terminal). It is a block diagram of eNB (base station). It is a sequence diagram which shows an example of the operation | movement sequence which concerns on 2nd Embodiment. It is a figure which shows an example of the array antenna which concerns on other embodiment.

[First Embodiment]
(Outline of the first embodiment)
The base station according to the first embodiment includes a control unit that performs a process of transmitting an individual reference signal that is a reference signal used for measurement of channel characteristics and is generated for each wireless terminal. The control unit performs a process of selecting a specific target wireless terminal from target wireless terminals connected to the base station, and transmitting the dedicated reference signal only to the specific target wireless terminal.

  The base station may further include an antenna array that supports FD-MIMO (Full-Dimension MIMO) and has antenna elements arranged in two dimensions in the horizontal direction and the vertical direction.

  The control unit performs a process of transmitting a common reference signal that is a reference signal used for measurement of channel characteristics and is common to the target radio terminal, and determines the specific target radio terminal based on a result of the process May be elected.

  The individual reference signal may be a reference signal used for measuring channel characteristics in two dimensions in the horizontal direction and the vertical direction, and the common reference signal may be a reference signal used for measuring channel characteristics in one dimension. .

  The control unit receives channel state information fed back based on the common reference signal from the target wireless terminal, compares downlink data transmission performance derived based on the channel state information with a performance index, and transmits the data transmission performance. A target wireless terminal whose value is lower than the performance index may be selected as the specific target wireless terminal.

  The performance index may be an average downlink data transmission performance of all the target wireless terminals.

  The control unit may determine a radio terminal supporting the FD-MIMO as the target radio terminal based on capability information of each radio terminal connected to the base station.

  The control unit may select a wireless terminal that has transmitted request information for requesting transmission of the dedicated reference signal to the base station as the specific target wireless terminal.

  The wireless terminal according to the first embodiment transmits, to a base station, request information for requesting transmission of an individual reference signal that is a reference signal used for measurement of channel characteristics and is generated for each wireless terminal. The control part which performs a process is provided.

  The individual reference signal may be a reference signal used for measuring channel characteristics in two dimensions in a horizontal direction and a vertical direction.

  The base station according to the first embodiment includes a control unit that performs a process of transmitting setting information of a specific TM (Transmission Mode) to a wireless terminal. The specific TM supports channel state information feedback using a dedicated reference signal and channel state information feedback using a common reference signal. The dedicated reference signal is a reference signal used for measuring channel characteristics and is a reference signal generated individually for each wireless terminal. The common reference signal is a reference signal used for measuring channel characteristics, and is a reference signal common to a plurality of wireless terminals.

  The specific TM may be a TM for FD-MIMO (Full-Dimension MIMO).

  The individual reference signal may be a reference signal used for measuring channel characteristics in two dimensions in the horizontal direction and the vertical direction, and the common reference signal may be a reference signal used for measuring channel characteristics in one dimension. .

  The control unit, after transmitting the setting information, issues a switching instruction for switching between feedback of channel state information using the common reference signal and feedback of channel state information using the dedicated reference signal. You may perform the process transmitted to a terminal.

  The control unit may perform a process of transmitting the setting information to the radio terminal by RRC layer signaling and a process of transmitting the switching instruction to the radio terminal by signaling lower than the RRC layer. Good.

  The radio | wireless terminal which concerns on 1st Embodiment is provided with the control part which performs the process which receives the setting information of specific TM (Transmission Mode) from a base station. The specific TM supports channel state information feedback using a dedicated reference signal and channel state information feedback using a common reference signal. The dedicated reference signal is a reference signal used for measuring channel characteristics, and is a reference signal generated individually for each wireless terminal. The common reference signal is a reference signal used for measuring channel characteristics, and is a reference signal common to a plurality of wireless terminals.

  The specific TM may be a TM for FD-MIMO (Full-Dimension MIMO).

  The individual reference signal may be a reference signal used for measuring channel characteristics in two dimensions in the horizontal direction and the vertical direction, and the common reference signal may be a reference signal used for measuring channel characteristics in one dimension. .

  The control unit, after receiving the setting information, issues a switching instruction for switching between feedback of channel state information using the common reference signal and feedback of channel state information using the dedicated reference signal. You may perform the process received from a station.

  The control unit may perform a process of receiving the setting information from the base station by RRC layer signaling and a process of receiving the switching instruction from the base station by lower layer signaling than the RRC layer. Good.

(Configuration of wireless communication system)
FIG. 1 is a diagram illustrating a configuration of an LTE (Long Term Evolution) system that is a wireless communication system according to the first embodiment. As shown in FIG. 1, the LTE system includes a UE (User Equipment) 100, an E-UTRAN (Evolved-UMTS Terrestrial Radio Access Network) 10, and an EPC (Evolved Packet Core) 20.

  The UE 100 corresponds to a wireless terminal. The UE 100 is a mobile communication device and performs radio communication with the eNB 200.

  The E-UTRAN 10 corresponds to a radio access network. The E-UTRAN 10 includes an eNB 200 (evolved Node-B). The eNB 200 corresponds to a base station. The eNB 200 is connected to each other via the X2 interface.

  The eNB 200 manages one or a plurality of cells, and performs radio communication with the UE 100 that has established a connection with the own cell. The eNB 200 has a radio resource management (RRM) function, a routing function of user data (hereinafter simply referred to as “data”), a measurement control function for mobility control / scheduling, and the like. “Cell” is used as a term indicating a minimum unit of a radio communication area, and also as a term indicating a function of performing radio communication with the UE 100.

  The EPC 20 corresponds to a core network. The EPC 20 includes an MME (Mobility Management Entity) / S-GW (Serving-Gateway) 300. MME performs various mobility control etc. with respect to UE100. The S-GW performs data transfer control. The MME / S-GW 300 is connected to the eNB 200 via the S1 interface.

(Configuration of wireless terminal)
FIG. 2 is a block diagram of the UE 100 (wireless terminal). As illustrated in FIG. 2, the UE 100 includes a reception unit 110, a transmission unit 120, and a control unit 130.

  The receiving unit 110 performs various types of reception under the control of the control unit 130. The receiving unit 110 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 130. The transmission unit 120 performs various transmissions under the control of the control unit 130. The transmission unit 120 includes an antenna and a transmitter. The transmitter converts the baseband signal (transmission signal) output from the control unit 130 into a radio signal and transmits it from the antenna. The antenna provided in the UE 100 includes a plurality of antenna elements.

  The control unit 130 performs various controls in the UE 100. The control unit 130 includes a processor and a memory. The memory stores a program executed by the processor and information used for processing by the processor. The processor includes a baseband processor that performs modulation / demodulation and encoding / decoding of the baseband signal, and a CPU (Central Processing Unit) that executes various processes by executing a program stored in the memory. The processor may include a codec that performs encoding / decoding of an audio / video signal. The processor executes the above-described processing and processing described later.

(Base station configuration)
FIG. 3 is a block diagram of the eNB 200 (base station). As illustrated in FIG. 3, the eNB 200 includes a transmission unit 210, a reception unit 220, a control unit 230, and a backhaul communication unit 240.

  The transmission unit 210 performs various transmissions under the control of the control unit 230. The transmission unit 210 includes an antenna and a transmitter. The transmitter converts the baseband signal (transmission signal) output from the control unit 230 into a radio signal and transmits it from the antenna. The receiving unit 220 performs various types of reception under the control of the control unit 230. The receiving unit 220 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 230.

  The control unit 230 performs various controls in the eNB 200. The control unit 230 includes a processor and a memory. The memory stores a program executed by the processor and information used for processing by the processor. The processor includes a baseband processor that performs modulation / demodulation and encoding / decoding of the baseband signal, and a CPU (Central Processing Unit) that executes various processes by executing a program stored in the memory. The processor executes the above-described processing and processing described later.

  The backhaul communication unit 240 is connected to the neighboring eNB 200 via the X2 interface, and is connected to the MME / S-GW 300 via the S1 interface. The backhaul communication unit 240 is used for communication performed on the X2 interface, communication performed on the S1 interface, and the like.

  FIG. 4 is a diagram illustrating an example of an antenna (antenna array) provided in the eNB 200. As shown in FIG. 4, the antenna array has antenna elements (antenna ports) arranged in two dimensions in the horizontal direction and the vertical direction. FIG. 4 shows an example in which the antenna array has a total of 16 antenna elements, 4 in the horizontal direction and 4 in the vertical direction.

(Configuration of wireless interface)
FIG. 5 is a protocol stack diagram of a radio interface in the LTE system. As shown in FIG. 5, the radio interface protocol is divided into the first to third layers of the OSI reference model, and the first layer is a physical (PHY) layer. The second layer includes a MAC (Medium Access Control) layer, an RLC (Radio Link Control) layer, and a PDCP (Packet Data Convergence Protocol) layer. The third layer includes an RRC (Radio Resource Control) layer.

  The physical layer performs encoding / decoding, modulation / demodulation, antenna mapping / demapping, and resource mapping / demapping. Data and control signals are transmitted between the physical layer of the UE 100 and the physical layer of the eNB 200 via a physical channel.

  The MAC layer performs data priority control, retransmission processing by hybrid ARQ (HARQ), random access procedure, and the like. Data and control signals are transmitted between the MAC layer of the UE 100 and the MAC layer of the eNB 200 via a transport channel. The MAC layer of the eNB 200 includes a scheduler that determines an uplink / downlink transport format (transport block size, modulation / coding scheme (MCS)) and an allocation resource block to the UE 100.

  The RLC layer transmits data to the RLC layer on the receiving side using the functions of the MAC layer and the physical layer. Data and control signals are transmitted between the RLC layer of the UE 100 and the RLC layer of the eNB 200 via a logical channel.

  The PDCP layer performs header compression / decompression and encryption / decryption.

  The RRC layer is defined only in the control plane that handles control signals. Messages for various settings (RRC messages) are transmitted between the RRC layer of the UE 100 and the RRC layer of the eNB 200. The RRC layer controls the logical channel, the transport channel, and the physical channel according to establishment, re-establishment, and release of the radio bearer. When there is a connection (RRC connection) between the RRC of the UE 100 and the RRC of the eNB 200, the UE 100 is in the RRC connected mode, otherwise, the UE 100 is in the RRC idle mode.

  A NAS (Non-Access Stratum) layer located above the RRC layer performs session management, mobility management, and the like.

(Outline of LTE lower layer)
FIG. 6 is a configuration diagram of a radio frame used in the LTE system. In the LTE system, OFDMA (Orthogonal Frequency Multiple Access) is applied to the downlink, and SC-FDMA (Single Carrier Frequency Multiple Access) is applied to the uplink.

  As shown in FIG. 6, the radio frame is composed of 10 subframes arranged in the time direction. Each subframe is composed of two slots arranged in the time direction. The length of each subframe is 1 ms, and the length of each slot is 0.5 ms. Each subframe includes a plurality of resource blocks (RB) in the frequency direction and includes a plurality of symbols in the time direction. Each resource block includes a plurality of subcarriers in the frequency direction. One symbol and one subcarrier constitute one resource element (RE). Further, among radio resources (time / frequency resources) allocated to the UE 100, a frequency resource can be specified by a resource block, and a time resource can be specified by a subframe (or slot).

  In the downlink, the section of the first few symbols of each subframe is an area mainly used as a physical downlink control channel (PDCCH) for transmitting a downlink control signal. Details of the PDCCH will be described later. The remaining part of each subframe is an area that can be used as a physical downlink shared channel (PDSCH) for mainly transmitting downlink data.

  The eNB 200 basically transmits a downlink control signal (DCI: Downlink Control Information) to the UE 100 using the PDCCH, and transmits downlink data to the UE 100 using the PDSCH. The downlink control signal carried by the PDCCH includes uplink SI (Scheduling Information), downlink SI, and TPC bits. The uplink SI is scheduling information (UL grant) related to uplink radio resource allocation, and the downlink SI is scheduling information related to downlink radio resource allocation. The TPC bit is information instructing increase / decrease in uplink transmission power. In order to identify the UE 100 that is the transmission destination of the downlink control signal, the eNB 200 includes the CRC bits scrambled with the identifier (RNTI: Radio Network Temporary ID) of the transmission destination UE 100 in the downlink control signal. Each UE 100 performs blind decoding on the PDCCH for a downlink control signal that may be destined for the own UE, and detects a downlink control signal destined for the own UE. The PDSCH carries downlink data using downlink radio resources (resource blocks) indicated by the downlink SI.

  In the uplink, both end portions in the frequency direction in each subframe are regions mainly used as a physical uplink control channel (PUCCH) for transmitting an uplink control signal. The remaining part of each subframe is an area that can be used mainly as a physical uplink shared channel (PUSCH) for transmitting uplink data.

  The UE 100 basically transmits an uplink control signal (UCI: Uplink Control Information) to the eNB 200 using the PUCCH, and transmits uplink data to the eNB 200 using the PUSCH. Uplink control signals carried by the PUCCH include CQI (Channel Quality Indicator), PMI (Precoding Matrix Indicator), RI (Rank Indicator), scheduling request (SR: Scheduling Request), and HARQ ACK / NACK. CQI is an index indicating downlink channel quality, and is used for determining an MCS to be used for downlink transmission. PMI is an index indicating a precoder matrix that is preferably used for downlink transmission. The RI is an index indicating the number of layers (number of streams) that can be used for downlink transmission. SR is information for requesting allocation of PUSCH resources. HARQ ACK / NACK is delivery confirmation information indicating whether downlink data has been correctly received.

(Outline of downlink MIMO)
FIG. 7 is a diagram for explaining an overview of downlink MIMO.

  As shown in FIG. 7, a plurality of UEs 100 are connected to the eNB 200. Specifically, the plurality of UEs 100 are in the RRC connected mode in the cell of the eNB 200. The eNB 200 and each UE 100 have a plurality of antenna elements (antenna ports). The eNB 200 transmits downlink data to each UE 100 using a plurality of transmission antenna elements.

  The eNB 200 transmits a plurality of modulation symbol sequences to one UE 100 by SDM (Spatial Division Multiplexing) using the same radio resource (time / frequency resource). Such a method is called SU-MIMO (Single-User MIMO). Alternatively, the eNB 200 transmits a plurality of modulation symbol sequences to different UEs 100 by SDM using the same radio resource (time / frequency resource). Such a method is called MU-MIMO (Multi-User MIMO).

  The eNB 200 transmits a reference signal for measurement of channel characteristics (that is, channel estimation) before transmitting user data. Such a reference signal is called CSI-RS (Channel State Information-Reference Signal). UE100 estimates a channel characteristic based on CSI-RS received from eNB200. The UE 100 generates channel state information (CSI) indicating a channel state based on the channel estimation result, and feeds back the generated CSI to the eNB 200. The CSI is at least one of CQI, PMI, and RI. The eNB 200 controls downlink data transmission (in particular, MCS) based on the CSI fed back from the UE 100.

  In general MIMO, the eNB 200 performs data transmission using each transmission antenna element arranged in one dimension in the horizontal direction. Moreover, eNB200 transmits CSI-RS using each transmission antenna element arranged in one dimension. Such CSI-RS is a reference signal common to a plurality of UEs 100. The reference signal is used for channel characteristic measurement (channel estimation) in one dimension. Hereinafter, such a common reference signal is referred to as “existing CSI-RS”. Note that “existing CSI-RS (existing CSI-RS)” is not limited to CSI-RS introduced in Release 10 of the 3GPP standard, but includes all reference signals up to Release 12, for example, DRS (Discovery Reference Reference) Signal)) is also included in the conventional CSI-RS concept.

  On the other hand, in FD-MIMO, the eNB 200 performs data transmission using each transmission antenna element arranged two-dimensionally in the horizontal direction and the vertical direction. Moreover, eNB200 transmits CSI-RS using the transmission antenna element arranged in two dimensions. Although FD-MIMO can improve data transmission performance compared to general MIMO, there is a problem that the amount of transmission of the reference signal (CSI-RS) for FD-MIMO, that is, overhead increases.

(Operation according to the first embodiment)
Hereinafter, the operation according to the first embodiment will be described.

(1) Operation Flow of eNB 200 The eNB 200 according to the first embodiment supports FD-MIMO. The eNB 200 generates and transmits a reference signal specific to the UE 100. The reference signal is a reference signal used for channel characteristic measurement (channel estimation) in two dimensions. Hereinafter, such an individual reference signal is referred to as “2D UE-specific CSI-RS”. “2D UE-specific CSI-RS” is a reference signal for FD-MIMO.

  The eNB 200 selects a specific target UE 100 from among a plurality of target UEs 100 connected to the own eNB 200, and transmits “2D UE-specific CSI-RS” only to the specific target UE 100. Thus, overhead can be suppressed compared with the case where “2D UE-specific CSI-RS” is transmitted to all target UEs 100.

  In 1st Embodiment, eNB200 receives the channel state information (CSI) fed back based on conventional CSI-RS from several target UE100, and selects specific target UE100 based on channel state information. Specifically, the eNB 200 selects a target UE 100 that needs to transmit “2D UE-specific CSI-RS” by MCS (Modulation and Coding Scheme) determined based on channel state information based on conventional CSI-RS. . Thereby, since eNB200 can transmit "2D UE-specific CSI-RS" only with respect to target UE100 which should improve data transmission performance, it can improve data transmission performance, suppressing an overhead. it can.

  FIG. 8 is a diagram illustrating an operation flow of the eNB 200 according to the first embodiment.

  As illustrated in FIG. 8, in step S1, the eNB 200 transmits the conventional CSI-RS to all UEs 100, and receives CSI fed back from all the UEs 100 based on the conventional CSI-RS. And eNB200 transmits downlink data to UE100 based on CSI. Step S1 is a process in accordance with the current LTE specification.

  In step S <b> 2, the eNB 200 determines the target UE 100 from among the UEs 100 connected to the eNB 200. Specifically, eNB200 acquires the terminal capability information (UE capability Information) of each UE100 connected to self-eNB200, and determines UE100 which supports FD-MIMO as object UE100. Whether or not FD-MIMO is supported may be indicated by UE category information (UE category). “UE capability information” is a type of RRC layer signaling (RRC signaling), and is information that the eNB 200 receives from the UE 100. Alternatively, “UE capability Information” can be acquired by the eNB 200 from the MME. Note that when it is assumed that all UEs 100 connected to the eNB 200 support FD-MIMO, the eNB 200 may determine all the UEs 100 as the target UEs 100.

  In step S3, the eNB 200 derives the downlink data transmission performance for each target UE 100 based on the CSI, compares the downlink data transmission performance with the performance index, and identifies the target UE 100 whose data transmission performance is lower than the performance index as a specific target. Elected as UE100. For the target UE 100 whose data transmission performance exceeds the performance index, the eNB 200 performs data transmission using the downlink MCS determined according to the CSI fed back from the UE 100 using the conventional CSI-RS as it is.

  In the first embodiment, the downlink data transmission performance is a data rate. The eNB 200 may calculate the data rate from the MCS, or may measure the data rate when data is actually transmitted according to the MCS. When calculating a data rate from MCS, eNB200 calculates | requires a corresponding TBS (Transport Block Size) index from the index of MCS determined by CSI fed back from UE100. Moreover, eNB200 calculates | requires TBS size per layer from the number of resource blocks used for transmission, and a TBS index. Here, the TBS size is the number of bits that can be transmitted in one subframe. And eNB200 calculates a data rate from the number of layers used for transmission, and TBS size.

  In the first embodiment, the performance index is an average data transmission performance (average data rate) of all target UEs 100. The performance index may be updated periodically. The performance index setting and updating method will be described later.

  In step S4, the eNB 200 transmits “2D UE-specific CSI-RS” to the specific target UE 100 selected in step S3. Here, the eNB 200 may generate and transmit “2D UE-specific CSI-RS” based on the CSI received from the specific target UE 100 in step S2. For example, the eNB 200 generates and transmits “2D UE-specific CSI-RS” based on the PMI included in the CSI. Or eNB200 may estimate the direction of specific target UE100 based on CSI, and may transmit "2D UE-specific CSI-RS" toward the estimated direction.

  Moreover, eNB200 receives CSI fed back from the specific object UE100 which measured the channel using "2D UE-specific CSI-RS." And eNB200 determines MCS again based on received CSI, and transmits downlink data to specific object UE100.

  Next, a performance index setting and updating method will be described. FIG. 9 is a flowchart showing an example of a performance index setting and updating method.

  As illustrated in FIG. 9, in step S11, the eNB 200 sets an initial value or a minimum value of the performance index. The initial value of the performance index is a predetermined data rate (for example, 3 Mbps). The minimum value of the performance index is the minimum data rate to be guaranteed.

  In step S12, the eNB 200 determines whether it is a performance index update timing. The performance index update timing is a timing according to a predetermined cycle (for example, 30 minutes). In the case of periodic update, the eNB 200 accumulates the data transmission performance (data rate) derived for each target UE 100 until the update timing. Alternatively, immediate updating may be used instead of periodic updating. In the case of immediate update, when a new data rate is derived for the target UE 100, the performance index is updated immediately.

  When it is the update timing of the performance index (step S12: Yes), in step S13, the eNB 200 calculates the average data rate of all the target UEs 100 in this cell, and updates the calculated average data rate as the performance index. Thereafter, the eNB 200 returns the process to step S12.

(2) TM according to the first embodiment
Hereinafter, TM (Transmission Mode) according to the first embodiment will be described. TM is information indicating a transmission method between the eNB 200 and the UE 100, and is set for each UE 100 by RRC signaling.

  When eNB200 and UE100 transmit / receive conventional CSI-RS, conventional TM (for example, TM9) is applied. On the other hand, when eNB200 and UE100 transmit / receive "2D UE-specific CSI-RS", it is thought that new TM different from the conventional TM is applied.

  Therefore, when switching from the conventional CSI-RS to “2D UE-specific CSI-RS”, a process of resetting the TM between the eNB 200 and the UE 100 may be required. Specifically, when the eNB 200 switches from the conventional CSI-RS transmission to the “2D UE-specific CSI-RS” transmission for the specific target UE 100, the RRC signaling (for example, “RRC Connection”, for example) is performed. Reconfiguration "message) is transmitted to the specific target UE 100.

  However, when performing such TM reconfiguration, there is a problem that a delay for switching from the conventional CSI-RS to “2D UE-specific CSI-RS” occurs and the signaling overhead increases.

  In the first embodiment, the TM for FD-MIMO supports CSI feedback using “2D UE-specific CSI-RS” and CSI feedback using conventional CSI-RS. As described above, the TM for FD-MIMO also supports CSI feedback using the conventional CSI-RS, thereby enabling CSI feedback using the conventional CSI-RS under the new TM for FD-MIMO. it can. Therefore, RRC signaling for TM switching as described above does not occur and efficient operation becomes possible.

(3) Operation Sequence An example of the operation sequence according to the first embodiment will be described below. FIG. 10 is a diagram illustrating an example of an operation sequence according to the first embodiment. A UE 100 illustrated in FIG. 10 is a target UE (a UE that supports FD-MIMO).

  As illustrated in FIG. 10, in step S101, the eNB 200 transmits a “RRC Connection Reconfiguration” message including a TM (Transmission Mode) for FD-MIMO to the UE 100. Note that the “RRC Connection Reconfiguration” message is dedicated RRC signaling addressed to the UE 100. The TM setting information is included in, for example, “AntennaInfo IE”.

  Moreover, eNB200 transmits the setting parameter for "2D UE-specific CSI-RS" and the setting parameter for conventional CSI-RS to UE100. The setting parameter is information indicating the signal configuration of the reference signal, for example. Such reference signal setting parameters (RS configuration) are included in "CQI-Report Config IE", "CSI-RS-Config IE", and the like. For example, a new setting parameter for “2D UE-specific CSI-RS” is added in “CSI-RS-Config IE”.

  The configuration parameter may include information regarding the configuration of the CSI report. Since the CSI report of “2D UE-specific CSI-RS” according to the first embodiment is normally an aperiodic report (Aperiodic CSI report), it may include aperiodic CSI report configuration information.

  The UE 100 that has received the “RRC Connection Reconfiguration” message stores the FD-MIMO TM. Moreover, UE100 memorize | stores the setting parameter for "2D UE-specific CSI-RS" and the setting parameter for conventional CSI-RS. When the TM for FD-MIMO is selected, the UE 100 can use the setting parameter for “2D UE-specific CSI-RS” in addition to the setting parameter for conventional CSI-RS.

  UE100 applies the setting parameter for conventional CSI-RS by default. Alternatively, TM setting information for FD-MIMO may include information that explicitly specifies a reference signal to be used for channel measurement among conventional CSI-RS and “2D UE-specific CSI-RS”. In this case, UE100 applies the setting parameter for conventional CSI-RS based on the information which designates conventional CSI-RS.

  In step S102, the eNB 200 transmits the conventional CSI-RS. UE100 measures a channel using conventional CSI-RS, and produces | generates CSI based on conventional CSI-RS.

  In step S103, the UE 100 transmits CSI to the eNB 200 (CSI feedback). The eNB 200 receives the CSI. The UE 100 may further request the eNB 200 to transmit “2D UE-specific CSI-RS”. This request will be described in the first modification of the first embodiment.

  In step S104, eNB200 determines MCS based on CSI, and transmits downlink data to UE100. The UE 100 receives downlink data.

  In step S105, the eNB 200 calculates or measures the downlink data transmission performance (data rate) of the target UE 100.

  In step S106, the eNB 200 compares the downlink data transmission performance of the target UE 100 with the performance index. When the downlink data transmission performance of the target UE 100 exceeds the performance index, the eNB 200 maintains the MCS determined by the conventional CSI-RS, continues the downlink data transmission of the target UE 100, and “2D UE-specific CSI-RS”. Do not switch to sending.

  On the other hand, when the downlink data transmission performance of the UE 100 is lower than the performance index, in step S107, the eNB 200 switches from CSI feedback using the conventional CSI-RS to CSI feedback using “2D UE-specific CSI-RS”. To the UE 100. Alternatively, the eNB 200 may issue a switching instruction in response to a request from the UE 100. The UE 100 receives the switching instruction. Here, the eNB 200 issues a switching instruction by signaling in a lower layer (for example, a MAC layer or a physical layer) than the RRC. For example, the eNB 200 may issue a switching instruction by MAC CE (Control Element) or may issue a switching instruction by DCI.

  Also, the eNB 200 and the UE 100 may determine that switching of the reference signal used for CSI feedback occurs when a predetermined period (for example, 8 subframes) elapses from the timing (subframe) at which the switching instruction is transmitted / received. In that case, the UE 100 may switch to the setting parameter for “2D UE-specific CSI-RS” when a predetermined period has elapsed from the timing at which the switching instruction is transmitted and received. The eNB 200 may start transmission of “2D UE-specific CSI-RS” from the timing (subframe) at which the reference signal used for CSI feedback is switched.

  In step S108, eNB200 requests | requires the feedback (report) of CSI using "2D UE-specific CSI-RS" to UE100.

  In step S109, the eNB 200 transmits “2D UE-specific CSI-RS” to the UE 100. UE100 measures a channel using "2D UE-specific CSI-RS", and produces | generates CSI based on "2D UE-specific CSI-RS."

  In step S110, the UE 100 transmits CSI to the eNB 200 (CSI feedback). The eNB 200 receives the CSI.

  In step S111, eNB200 determines MCS based on CSI, and transmits downlink data to UE100. The UE 100 receives downlink data.

  In this sequence, switching from CSI feedback using conventional CSI-RS to CSI feedback using “2D UE-specific CSI-RS” has been described. However, the eNB 200 may instruct the UE 100 performing CSI feedback using “2D UE-specific CSI-RS” to switch to CSI feedback using conventional CSI-RS. The eNB 200 and the UE 100 may determine that a reference signal used for CSI feedback is switched when a predetermined period (for example, 8 subframes) elapses from the timing (subframe) at which the switching instruction is transmitted and received. The eNB 200 may stop the transmission of “2D UE-specific CSI-RS” from the timing (subframe) at which the reference signal used for CSI feedback is switched.

  In addition, an example in which CSI feedback using “2D UE-specific CSI-RS” is an aperiodic method (aperiodic CSI report) has been described, but “2D UE-specific CSI report” is a “2D UE-specific CSI report”. CSI using “CSI-RS” may be fed back.

[First Modification of First Embodiment]
In 1st Embodiment mentioned above, eNB200 demonstrated an example which elects specific target UE100 by comparing downlink data transmission performance for every target UE100 with a performance parameter | index. However, eNB200 may transmit "2D UE-specific CSI-RS" to the said UE100 according to the request | requirement from UE100 instead of such a method.

  For example, when the CSI is worse than a predetermined value / expected value, when the UE 100 determines that the deviation of the measurement result is large from the propagation state, or when the CSI greatly fluctuates from the previous measurement result, the UE 100 “2D UE-specific” It is determined that channel estimation based on “CSI-RS” is necessary, and request information for requesting transmission of “2D UE-specific CSI-RS” is transmitted to eNB 200. Or UE100 requests | requires transmission of "2D UE-specific CSI-RS" according to the condition of the service which self-UE100 uses (for example, when it is necessary to receive a large amount of data such as a video signal at high speed). Request information is transmitted to eNB200.

  FIG. 11 is a diagram illustrating an operation flow of the eNB 200 according to the first modification of the first embodiment. Here, differences from the above-described first embodiment will be mainly described.

  As illustrated in FIG. 11, in step S21, the eNB 200 transmits the conventional CSI-RS to all UEs 100, and receives the CSI fed back from the UE 100 based on the conventional CSI-RS. And eNB200 transmits downlink data to UE100 based on CSI.

  In step S <b> 22, the eNB 200 selects, as the specific target UE 100, the UE 100 that has transmitted the request information requesting transmission of “2D UE-specific CSI-RS”. The eNB 200 continues to use the downlink MCS determined by the conventional CSI-RS for the UE 100 that has not transmitted the request information.

  In Step S23, the eNB 200 transmits “2D UE-specific CSI-RS” to the specific target UE 100 selected in Step S22. The CSI fed back from the specific target UE 100 that measured the channel using “2D UE-specific CSI-RS” is received. And eNB200 determines MCS based on received CSI, and transmits downlink data to specific object UE100.

[Modification 2 of the first embodiment]
In 1st Embodiment mentioned above, eNB200 demonstrated an example which updates a performance parameter | index periodically or immediately. By repeating such an update, the performance index gradually increases, and as a result, transmission of “2D UE-specific CSI-RS” may be required for all target UEs 100. As a result, the purpose of suppressing overhead may not be achieved. In the second modification of the first embodiment, a method for solving this problem will be described.

  FIG. 12 is a diagram illustrating an operation flow of the eNB 200 according to the second modification of the first embodiment.

  As illustrated in FIG. 12, the eNB 200 updates the overhead and the corresponding throughput when the performance index is updated or when a new data rate is derived (ie, when there is a new MCS) (step S31: Yes). Are compared (step S32). Here, the “increase state” is, for example, an increase speed or an increase amount. The “overhead” is CSI based on “2D UE-specific CSI-RS”, “2D UE-specific CSI-RS”, setting information of “2D UE-specific CSI-RS”, and the like. The “appropriate throughput” is the throughput of the target UE 100, and is calculated by, for example, “performance index × number of target UEs 100”. Or it is good also considering the sum of the transmission performance (data rate) of all the object UE100 as an applicable throughput.

  When the overhead increase state is equal to or higher than the corresponding throughput increase state (step S32: Yes), the eNB 200 stops updating the performance index (step S33).

  On the other hand, when the increase state of overhead is less than the increase state of the corresponding throughput (step S32: No), the eNB 200 restarts the update of the performance index (step S34).

  FIG. 13 is a diagram illustrating a first modification of the flow illustrated in FIG. As illustrated in FIG. 13, the eNB 200 is in a performance index update stop state when the performance index is updated or when a new data rate is derived (that is, when there is a new MCS) (step S41: Yes). If so (step S42: Yes), the overhead increase state is compared with the corresponding throughput increase state (step S43). If the overhead increase state is equal to or higher than the corresponding throughput increase state (step S43: Yes), the eNB 200 lowers the performance index by multiplying the performance index by a coefficient less than 1 (step S44).

  FIG. 14 is a diagram illustrating a second modification example of the flow illustrated in FIG. 12. As illustrated in FIG. 14, when the eNB 200 derives a performance index update timing or newly derives a data rate (that is, when there is a new MCS) (step S51: Yes), the overhead is a specified value (for example, an operator). (Step S52: Yes), the performance index is lowered by multiplying the performance index by a coefficient less than 1 (step S53).

[Modification 3 of the first embodiment]
In the first embodiment described above, the target UE 100 may be classified into a plurality of groups according to the priority or category, and the flow of FIG. 8 may be applied to each group. Specifically, the eNB 200 sets and updates the performance index for each group, and calculates the transmission performance (data rate) for the target UE 100 for each group. And eNB200 compares a transmission performance with a performance parameter | index for every group, and controls CSI feedback.

  Also, the eNB 200 may select the target UE 100 having a high priority or category as the specific target UE 100 and transmit “2D UE-specific CSI-RS” unconditionally (ignoring overhead).

[Other changes]
In 1st Embodiment mentioned above, the example which switches to CSI feedback using "2D UE-specific CSI-RS" from the CSI feedback using conventional CSI-RS about specific object UE100 was demonstrated. Further, an example in which the conventional CSI-RS is a common reference signal for one-dimensional channel estimation and “2D UE-specific CSI-RS” is an individual reference signal for two-dimensional channel estimation has been described.

  However, the conventional CSI-RS may be read as “common reference signal for channel estimation”, and “2D UE-specific CSI-RS” may be read as “individual reference signal for channel estimation”. Alternatively, the conventional CSI-RS may be read as “first channel estimation reference signal”, and “2D UE-specific CSI-RS” may be read as “second channel estimation reference signal”. In this case, the first channel estimation reference signal and the second channel estimation reference signal may share the same antenna port (number) or may be defined as different antenna ports (numbers).

  In the first embodiment described above, the LTE system is exemplified as the wireless communication system. However, the present invention is not limited to LTE systems. The present invention may be applied to a wireless communication system other than the LTE system.

[Additional Notes of First Embodiment]
(1. Introduction)
Elevation Beamforming / Full Dimension (FD) MIMO WI was approved in RAN Plenary # 68. It is one of the goals of this WI to specify CSI reporting improvements, including extensions of the Rel-12 CSI reporting mechanism, for both periodic and aperiodic CSI reporting. This appendix describes the considerations for CSI measurement and reporting.

(2. Problems when considering CSI reporting)
LTE SI Elevation Beamforming / Full Dimension (FD) MIMO results are summarized in TR 36.897.

  From TR 36.897, it can be seen that there are three main types of functional enhancement of CSI-RS for FDD. The first is a beamformed CSI-RS based scheme. The second is a non-precoded CSI-RS based scheme. The third is enhancement of hybrid beamforming CSI-RS and non-precoded CSI-RS. Some specific schemes have been further studied at the SI stage for each type of CSI-RS enhancement.

  For example, for beamformed CSI-RS-based enhancements, some schemes configure a single CSI process and single or multiple NZP CSI-RS resources in the UE, and the UE uses CSI (PMI , CQI, RI, or beam index) according to each specific scheme (in some schemes, multiple CSI processes are set up for the UE), the UE reports the preferred CSI.

  For non-precoding CSI-RS based schemes, sub-sampling in all port mapping, time, frequency, and / or space domain where all MIMO channels can be measured at the UE, as well as separate vertical and horizontal CSI-RS reports Partial port mapping has been considered.

  Two CSI processes are typically required for enhancement of hybrid beamforming CSI-RS and non-precoded CSI-RS. One is for the first step non-precoded CSI measurement and reporting, and the other is in response to the non-precoded CSI reporting first step for further reporting of beamformed CSI measurements. Is for.

  Although the advantages and disadvantages of these schemes are not compared, the trade-off between overhead and measurement / transmission performance needs to be considered when considering enhancement of CSI-RS. It is usually not difficult to improve performance with increasing overhead. What is important is to make an initial effort to improve performance without increasing overhead or with a slight increase.

  Proposal 1: Should be considered first to improve performance without increasing overhead or with a slight increase.

  Intuitively, it can be understood that transmission performance, eg throughput, can be improved by using FD-MIMO. However, all UEs need to further improve the throughput. For example, some UEs very close to the eNB may be able to achieve sufficient throughput using a conventional one-dimensional antenna array CSI process.

  Consideration 1: Some UEs may suffice with conventional one-dimensional antenna array CSI processing to achieve sufficient throughput.

  In addition, the legacy UE may not be able to cope with the FD-MIMO CSI process depending on the WI result as a matter of course. Thus, compatibility with legacy UEs should be considered when considering CSI-RS measurement and reporting.

  Consideration 2: Legacy UE may not be able to cope with the extended function of CSI-RS of FD-MIMO, and compatibility is a problem.

(3.2 step CSI report)
Based on the discussion in Section 2, we propose that the two-step CSI report introduced below should be considered.

  As described in Section 2, conventional one-dimensional antenna array CSI processing should be sufficient for some UEs to achieve sufficient throughput, and compatibility with legacy UEs should also be considered. Considering certain points, standardized legacy CSI processing for one-dimensional antenna arrays can be used as the first step CSI measurement and reporting. That is, the eNB first transmits a conventional one-dimensional antenna array CSI-RS. The UE measures the channel and feeds back CSI (PMI, CQI, RI) to the eNB based on the measurement result. Based on the CSI reported by the UE, the eNB determines the MCS and starts transmitting data.

  Since the first step CSI measurement and reporting only uses the legacy CSI process, no additional overhead is introduced and the legacy UE can operate normally.

  Then, as a second step CSI measurement and report, from among UEs capable of processing 2D AAS CSI-RS (indicated by RRC UE capability information exchange) and / or UEs requesting higher throughput data transmission ENB selects UEs according to some predefined criteria (which can be the expected throughput or simply the average throughput of the cell), requests aperiodic CSI reports from those selected UEs, Based on the CSI report in the first step, a 2D AAS UE-specific CSI-RS is transmitted to the selected UE. The associated UE measures the channel again in response to the 2D AAS UE specific CSI-RS and reports the new CSI to the eNB. Based on the new second step CSI report, the eNB determines a new MCS and transmits data using the new MCS.

  For the second step CSI reporting, 2D AAS UE specific CSI-RS, which usually results in higher resolution measurements, is used, so that improved transmission characteristics in a statistical sense can be achieved. . The advantages of 2-step CSI measurement and reporting are summarized as follows.

  -Compatibility with legacy UEs, ie legacy UEs can operate normally.

  No additional overhead is introduced for UEs that do not require -2D AAS CSI-RS.

  -The overhead can be increased slightly and the required transmission performance can be expected, i.e. the best trade-off between performance and overhead is achieved.

  Proposal 2: Two-step CSI reporting should be considered to achieve the best trade-off between performance and overhead, and to achieve compatibility with legacy UEs.

[Second Embodiment]
In the following, the difference between the second embodiment and the first embodiment will be mainly described.

(Outline of the second embodiment)
In mobile communication systems, a MIMO (Multi-Input Multi-Output) technique for transmitting signals using a plurality of transmission antennas and a plurality of reception antennas is widely used.

  In downlink MIMO, a base station transmits a plurality of data sequences to one radio terminal by SDM (Spatial Division Multiplexing) using the same radio resource (time / frequency resource). Such a method is called SU-MIMO (Single-User MIMO). Alternatively, the base station transmits a plurality of data sequences to different wireless terminals by SDM using the same wireless resource. Such a method is called MU-MIMO (Multi-User MIMO).

  In addition, the base station transmits a reference signal for feedback of channel state information (CSI) from each antenna. Such a reference signal is called CSI-RS (Channel State Information-Reference Signal). CSI-RS is a reference signal common to a plurality of radio terminals in the cell of the base station.

  The wireless terminal performs channel estimation using the CSI-RS received from the base station, thereby generating CSI related to the downlink channel state, and transmits (feeds back) the generated CSI to the base station. The base station controls downlink data transmission based on CSI fed back from the wireless terminal.

  However, the arrangement of CSI-RS is sparse. For example, in an LTE (Long Term Evolution) system, the CSI-RS is transmitted in a long cycle of about once in a plurality of subframes. Therefore, CSI feedback using CSI-RS has a problem that it is difficult to obtain highly accurate CSI.

  In addition, when performing large-scale (massive) MIMO in which a large number of antennas are provided in a base station, there is a problem that the amount of CSI-RS to be transmitted increases, that is, overhead increases.

  Therefore, the second embodiment aims to provide a wireless terminal, a base station, and a processor that can obtain highly accurate CSI while suppressing an increase in overhead.

  The wireless terminal according to the second embodiment includes a control unit. The control unit is configured to receive, from the base station, a terminal-specific reference signal that is a demodulation reference signal that is transmitted from the base station to each radio terminal, and channel state information generated by channel estimation using the terminal-specific reference signal. Is transmitted to the base station.

  The channel state information may be channel quality information.

  The channel state information may be interference information.

  The terminal-specific reference signal is included in an assigned radio resource assigned to the radio terminal by the base station. In the radio terminal, the control unit performs the process of receiving the terminal-specific reference signal from the base station together with downlink data included in the allocated radio resource, and the channel estimation using the terminal-specific reference signal, In addition to demodulating downlink data, processing for generating the channel state information may be performed.

  In the wireless terminal, the control unit includes processing for receiving downlink control information from the base station via a physical downlink control channel, and instruction information indicating a transmission instruction for the channel state information in the downlink control information. If included, a process of transmitting the channel state information to the base station may be performed.

  The downlink control information includes allocation information indicating allocated radio resources allocated by the base station to the radio terminal. In the wireless terminal, when the allocation information and the instruction information are included in the downlink control information, the control unit generates the channel estimation using the terminal-specific reference signal included in the allocated wireless resource. You may perform the process which transmits channel state information to the said base station.

  In the wireless terminal, the control unit transmits the response information indicating whether or not the downlink data has been successfully received from the base station to the base station, and the channel state at the transmission timing of the response information. And processing for transmitting information to the base station.

  In the wireless terminal, the control unit includes information indicating a difference between past channel state information transmitted to the base station before the current timing and current channel state information corresponding to the current timing, You may perform the process transmitted to the said base station as channel state information.

  The base station according to the second embodiment includes a control unit. The control unit is configured to transmit a terminal-specific reference signal, which is a demodulation reference signal transmitted to each radio terminal, to the radio terminal, and channel state information generated by the radio terminal by channel estimation using the terminal-specific reference signal. Is received from the wireless terminal.

  The channel state information may be channel quality information.

  The channel state information may be interference information.

  The terminal-specific reference signal is included in an assigned radio resource assigned to the radio terminal by the base station. The terminal-specific reference signal may be used not only for demodulating downlink data included in the allocated radio resource but also for generating the channel state information.

  In the base station, the control unit includes a process of including instruction information indicating a transmission instruction of the channel state information in downlink control information, and the downlink control information including the instruction information via a physical downlink control channel. And processing to transmit to the wireless terminal.

  In the base station, the control unit may perform a process of including the instruction information in the downlink control information together with allocation information indicating an allocated radio resource allocated by the base station to the radio terminal.

  In the base station, the control unit, in the process of receiving response information indicating whether or not the downlink data transmitted by the base station has been successfully received from the wireless terminal, and the reception timing of the response information, And processing for receiving channel state information from the wireless terminal.

  In the base station, the control unit includes information indicating a difference between past channel state information received from the wireless terminal before the current timing and current channel state information corresponding to the current timing, You may perform the process received from the said radio | wireless terminal as channel state information.

  The processor according to the second embodiment controls the wireless terminal. The processor is configured to receive, from the base station, a terminal-specific reference signal that is a demodulation reference signal transmitted by the base station individually for each radio terminal, and channel state information generated by channel estimation using the terminal-specific reference signal. And processing to transmit to the base station.

(Outline of operation according to second embodiment)
The LTE system according to the second embodiment supports downlink MIMO. In SU-MIMO, the eNB 200 transmits a plurality of data sequences to one UE 100 by SDM using the same radio resource (time / frequency resource). In MU-MIMO, the eNB 200 transmits a plurality of data sequences to different UEs 100 by SDM using the same radio resource.

  Further, the eNB 200 transmits a terminal-specific reference signal (UE-specific RS), which is a demodulation reference signal used for channel estimation for data demodulation, from each antenna (each antenna port). The UE-specific RS is a reference signal that the eNB 200 transmits to each UE. The UE-specific RS is included in the allocated radio resource (PDSCH resource) allocated to the UE 100 by the eNB 200. That is, UE-specific RS is transmitted in the antenna and the allocated radio resource used for transmitting downlink data to UE 100.

  FIG. 15 is a diagram illustrating an outline of an operation according to the second embodiment. As illustrated in FIG. 15, the UE 100 receives a UE-specific RS, which is a demodulation reference signal transmitted from the eNB 200 for each UE, from the eNB 200. UE100 transmits CSI produced | generated by the channel estimation using UE-specific RS to eNB200. eNB200 receives CSI which UE100 produced | generated by channel estimation using UE-specific RS from UE100. CSI is, for example, channel quality information (CQI). Although details will be described later, one CQI may be generated for the entire resources allocated for the PDSCH.

  Alternatively, the CSI may be interference information. The interference indicated by the interference information includes interference due to signals in other layers from the eNB 200 transmitting the UE-specific RS, interference due to signals from the surroundings, noise, and the like. Note that CSI may include PMI and RI.

  The UE-specific RS is included in the PDSCH resource (resource block) allocated to the UE 100 by the eNB 200. For this reason, when eNB200 allocates PDSCH resource to UE100 frequently, since UE100 receives UE-specific RS frequently, it can obtain highly accurate CSI using UE-specific RS.

  Moreover, when Massive MIMO which provides many antennas in eNB200 is implemented, UE-specific RS can be transmitted from each antenna used for transmission of the downlink data with respect to UE100. Therefore, by using UE-specific RS for CSI feedback, additional CSI-RS can be made unnecessary even when Massive MIMO is performed.

  Therefore, according to the second embodiment, highly accurate CSI can be obtained while suppressing an increase in overhead.

(Wireless terminal according to the second embodiment)
Next, the configuration of the UE 100 (wireless terminal) will be described. FIG. 16 is a block diagram of the UE 100. As illustrated in FIG. 16, the UE 100 includes a reception unit 110, a transmission unit 120, and a control unit 130.

  The receiving unit 110 performs various types of reception under the control of the control unit 130. The receiving unit 110 includes a plurality of antennas and a receiver. The receiver converts radio signals received by the plurality of antennas into baseband signals (reception signals) and outputs the baseband signals to the control unit 130. The transmission unit 120 performs various transmissions under the control of the control unit 130. The transmission unit 120 includes a plurality of antennas and a transmitter. The transmitter converts a baseband signal (transmission signal) output from the control unit 130 into a radio signal and transmits it from a plurality of antennas.

  The control unit 130 performs various controls in the UE 100. The control unit 130 includes a processor and a memory. The memory stores a program executed by the processor and information used for processing by the processor. The processor includes a baseband processor that performs modulation / demodulation and encoding / decoding of the baseband signal, and a CPU (Central Processing Unit) that executes various processes by executing a program stored in the memory. The processor may include a codec that performs encoding / decoding of an audio / video signal. The processor executes the above-described processing and processing described later.

  In the second embodiment, the control unit 130 receives the CSI generated by the process of receiving the UE-specific RS, which is a demodulation reference signal transmitted from the eNB 200 for each UE, from the eNB 200, and channel estimation using the UE-specific RS. and processing to transmit to the eNB 200. Specifically, the control unit 130 only demodulates the downlink data by the process of receiving the UE-specific RS from the eNB 200 together with the downlink data included in the assigned PDSCH resource, and channel estimation using the UE-specific RS. And processing for generating CSI as well.

  In 2nd Embodiment, the control part 130 transmits CSI to eNB200, when the instruction | indication information which shows the process which receives DCI from eNB200 via PDCCH, and the instruction information which shows the transmission instruction | indication of CSI based on UE-specific RS is contained in DCI. You may perform the process to perform. In other words, the control part 130 transmits CSI based on UE-specific RS to eNB200 by the trigger by DCI. Thereby, eNB200 can obtain CSI as needed. The instruction information is, for example, a 1-bit flag. Alternatively, a new DCI format for CSI feedback using UE-specific RS may be introduced. In this case, information indicating the new DCI format may be regarded as instruction information. Alternatively, instead of transmitting the instruction information by DCI, the instruction information may be transmitted by signaling of a higher layer (such as MAC or RRC) than the physical layer. The instruction information by MAC or RRC may be information instructing to perform CSI feedback using UE-Specific RS whenever there is PDSCH assignment using UE-Specific RS.

  DCI includes allocation information (downlink SI) indicating allocated PDSCH resources allocated to the UE 100 by the eNB 200. In the second embodiment, when the allocation information and the instruction information are included in the DCI, the control unit 130 may perform a process of transmitting CSI based on the UE-specific RS included in the allocation PDSCH resource indicated by the allocation information to the eNB 200. Good. Thereby, CSI regarding the channel state within the range of the PDSCH resource allocated to the UE 100 can be obtained. For this reason, the subband CSI (subband CQI) used in a general LTE system may be unnecessary.

  When performing such an operation, the control unit 130 does not use the wideband CSI (wideband CQI) or the subband CSI (subband CQI) used in a general LTE system, but the CSI for the entire PDSCH resource range or a subset thereof. Feedback may be provided. Regarding this “subset”, the UE 100 may be notified from the eNB 200 through DCI or by signaling in MAC / RRC.

  In addition, when PDSCH allocation using UE-specific RS is performed at multiple antenna ports (that is, when multiple layers are allocated to UE 100), control unit 130 assigns each UE-specific RS to each UE-specific RS. A corresponding CSI may be generated. Particularly in the case of CQI, the CQI feedback can take the following four patterns.

  (1) The CQI is fed back one by one for each UE-specific RS.

  (2) One CQI is fed back to one UE-specific RS, and one differential CQI is fed back to another UE-specific RS based on the one CQI.

  (3) Only one CQI is fed back to a plurality of UE-specific RSs.

  (4) Feed back one CQI for each codeword. Note that one codeword includes a layer corresponding to one or a plurality of UE-specific RSs.

  In the second embodiment, the control unit 130 transmits CSI to the eNB 200 in the process of transmitting HARQ ACK / NACK indicating whether or not the downlink data has been successfully received from the eNB 200, and the HARQ ACK / NACK transmission timing. And the process of transmitting to. That is, the control unit 130 aligns the timing of CSI feedback using UE-specific RS with the transmission timing of HARQ ACK / NACK. As a result, the eNB 200 can predict the timing of CSI feedback. In the case of FDD, HARQ ACK / NACK transmission timing is four subframes after the subframe in which downlink data is received. In the case of TDD, the transmission timing of HARQ ACK / NACK depends on the TDD radio frame configuration.

  In 2nd Embodiment, the control part 130 is the process which transmits the information which shows the difference between the past CSI transmitted to eNB200 before the present timing, and the present CSI corresponding to the present timing to eNB200 as CSI. May be performed. That is, the control unit 130 performs CSI differential feedback. Thereby, the information amount of CSI can be reduced. As an example, when the CSI related to differential feedback is 1 bit, two patterns (such as + 1 / −1) can be expressed. When the CSI related to differential feedback is 2 bits, four patterns (+ 1/0 / −1 / −3, etc.) can be expressed.

(Base station according to the second embodiment)
Next, the configuration of the eNB 200 (base station) will be described. FIG. 17 is a block diagram of the eNB 200. As illustrated in FIG. 17, the eNB 200 includes a transmission unit 210, a reception unit 220, a control unit 230, and a backhaul communication unit 240.

  The transmission unit 210 performs various transmissions under the control of the control unit 230. The transmission unit 210 includes a plurality of antennas and a transmitter. The transmitter converts a baseband signal (transmission signal) output from the control unit 230 into a radio signal and transmits it from a plurality of antennas. The receiving unit 220 performs various types of reception under the control of the control unit 230. The receiving unit 220 includes a plurality of antennas and a receiver. The receiver converts radio signals received by the plurality of antennas into baseband signals (reception signals) and outputs the baseband signals to the control unit 230.

  The control unit 230 performs various controls in the eNB 200. The control unit 230 includes a processor and a memory. The memory stores a program executed by the processor and information used for processing by the processor. The processor includes a baseband processor that performs modulation / demodulation and encoding / decoding of the baseband signal, and a CPU (Central Processing Unit) that executes various processes by executing a program stored in the memory. The processor executes the above-described processing and processing described later.

  The backhaul communication unit 240 is connected to the neighboring eNB 200 via the X2 interface, and is connected to the MME / S-GW 300 via the S1 interface. The backhaul communication unit 240 is used for communication performed on the X2 interface, communication performed on the S1 interface, and the like.

  In the second embodiment, the control unit 230 transmits the CSI generated by the UE 100 through the process of transmitting the UE-specific RS, which is a demodulation reference signal to be transmitted to each UE, to the UE 100, and channel estimation using the UE-specific RS. The process received from UE100 is performed. The UE-specific RS is used not only for demodulating downlink data included in the assigned PDSCH resource, but also for generating CSI.

  The eNB 200 controls downlink data transmission based on the CSI fed back from the UE 100. For example, the eNB 200 generates a transmission precoding weight that suppresses interference between UEs based on CSI, and multiplies downlink data (data sequence) and UE-specific RS by the transmission precoding weight, thereby performing beamforming. Transmits downlink data by. In that case, eNB200 allocates the resource block corresponding to CSI to UE100, and performs downlink data transmission. Moreover, eNB200 determines MCS based on CSI and performs downlink data transmission using the determined MCS.

  In the second embodiment, the control unit 230 includes a process of including instruction information indicating a CSI transmission instruction based on UE-specific RS in the DCI, and a process of transmitting DCI including the instruction information to the UE 100 via the PDCCH. You may go. Moreover, the control part 230 may perform the process which includes instruction information in DCI with the allocation information which shows the allocation PDSCH resource which eNB200 allocated to UE100. That is, when assigning a PDSCH resource (resource block) requiring CSI feedback to the UE 100, the eNB 200 instructs the UE 100 to perform CSI feedback on the PDSCH resource. Thereby, eNB200 can obtain CSI about a desired resource block.

  Alternatively, instead of transmitting the instruction information by DCI, the instruction information may be transmitted by signaling of a higher layer (such as MAC or RRC) than the physical layer. The instruction information by MAC or RRC may be information instructing to perform CSI feedback using UE-Specific RS whenever there is PDSCH assignment using UE-Specific RS.

  In addition, as described above, CSI feedback is not wideband CSI (wideband CQI) or subband CSI (subband CQI) used in a general LTE system, but CSI feedback for the entire PDSCH resource range or a subset thereof. There may be. The control unit 230 may notify the UE 100 of this “subset” during DCI or by signaling in MAC / RRC.

  In 2nd Embodiment, the control part 230 is CSI in the process which receives HARQ ACK / NACK which shows whether reception of the downlink data which eNB200 transmitted succeeded from UE100, and the reception timing of HARQ ACK / NACK. May be performed from the UE 100.

  In the second embodiment, the control unit 230 receives information indicating the difference between the past CSI received from the UE 100 before the current timing and the current CSI corresponding to the current timing from the UE 100 as CSI. May be performed. The control unit 230 derives the current CSI by accumulating the CSI related to the differential feedback based on such CSI differential feedback.

(Operation sequence according to the second embodiment)
Next, an example of an operation sequence according to the second embodiment will be described. FIG. 18 is a sequence diagram illustrating an example of an operation sequence according to the second embodiment. In the initial state of FIG. 18, the UE 100 is in the RRC connected mode in the cell of the eNB 200.

  As illustrated in FIG. 18, in step S201, the eNB 200 transmits setting information related to CSI feedback using the UE-specific RS to the UE 100 by RRC signaling. UE100 memorize | stores the setting information received from eNB200, and performs CSI feedback using UE-Specific RS based on setting information.

  Specific examples of the setting information include the following information (1) to (3).

  (1) Information requesting CSI feedback using UE-Specific RS. This information is, for example, a transmission mode (eg, TM 9a) for CSI feedback using UE-Specific RS, or an information element (IE) that requests CSI feedback using UE-Specific RS. When such information is set, the UE 100 performs CSI feedback based on the UE-Specific RS at all times or when triggered by DCI.

  (2) Information specifying a PUCCH resource for CSI feedback. When performing feedback on the PUCCH, since transmission is not possible in the conventional PUCCH format, a new PUCCH (eg, PUCCH format 5) is defined. And eNB200 sets the information (for example, resource block, cyclic shift etc.) for designating this resource to UE100.

  (3) Setting information indicating whether one CSI feedback is made for the entire PDSCH allocation resource, whether each subset is divided into one CSI feedback, or both are fed back. In addition, if CSI feedback is performed in a subset, information for designating the subset (for example, partitioning in 6 RB units) may be required. However, this may be a predetermined value depending on the specification.

  In step S202, eNB200 transmits DCI to UE100 via PDCCH. DCI includes allocation information indicating allocated PDSCH resources (resource blocks) allocated to the UE 100 by the eNB 200, and instruction information indicating a CSI transmission instruction based on the UE-specific RS. The DCI may also include information that designates a PUSCH resource for feeding back CSI. The information specifying the PUSCH resource may be information corresponding to UL grant. Alternatively, the information specifying the PUSCH resource may be a part of the information such as RB in common with the PDSCH resource allocation information and only other information. UE100 receives DCI from eNB200, and specifies allocation PDSCH resource based on allocation information.

  In step S203, eNB200 transmits downlink data and UE-specific RS to UE100 using allocation PDSCH resource. UE100 receives UE-specific RS from eNB200 with the downlink data contained in allocation PDSCH resource.

  In step S204, the UE 100 not only demodulates downlink data but also generates CSI by channel estimation using the UE-specific RS. Moreover, UE100 decodes downlink data and produces | generates HARQ ACK / NACK which shows decoding success or failure.

  In step S205, UE100 transmits CSI to eNB200 in the transmission timing of HARQ ACK / NACK. Specifically, UE100 transmits ACK / NACK and CSI to eNB200 via PUCCH or PUSCH. Such CSI feedback may be the differential feedback described above. The eNB 200 receives ACK / NACK and CSI.

[Other changes]
In the second embodiment described above, the antenna configuration used for Massive MIMO was not particularly mentioned. The eNB 200 may use an array antenna having antennas (antenna ports) arranged in two dimensions in the horizontal direction and the vertical direction, as an antenna configuration used for Massive MIMO. FIG. 19 is a diagram illustrating an example of an array antenna. In the example shown in FIG. 19, the antenna array has a total of 16 antennas, 4 in the horizontal direction and 4 in the vertical direction, but may have more antennas. MIMO using such an antenna array is called FD-MIMO (Full-Dimension MIMO), and directivity control not only in the horizontal direction but also in the vertical direction is possible.

  The second embodiment can be implemented in combination with the first embodiment. For example, some configurations according to the second embodiment may be added to the first embodiment, or some configurations according to the second embodiment may be replaced with some configurations according to the first embodiment. Also good.

  In 2nd Embodiment mentioned above, the LTE system was illustrated as a mobile communication system. However, the present invention is not limited to LTE systems. The present invention may be applied to a mobile communication system other than the LTE system.

[Cross-reference]
This application claims the priority of US Provisional Application No. 62/203622 (filed on August 11, 2015) and Japanese Patent Application No. 2015-170172 (filed on August 31, 2015). The entire contents are incorporated herein.

  The present invention is useful in the communication field.

Claims (36)

A control unit that performs a process of transmitting an individual reference signal that is a reference signal used for measurement of channel characteristics and is generated individually for each wireless terminal;
The control unit is a base station that performs a process of selecting a specific target radio terminal from target radio terminals connected to the base station and transmitting the dedicated reference signal only to the specific target radio terminal. The base station supports FD-MIMO (Full-Dimension MIMO),
The base station according to claim 1, further comprising an antenna array having antenna elements arranged two-dimensionally in a horizontal direction and a vertical direction. The controller is
A reference signal used for measurement of channel characteristics, and a process of transmitting a common reference signal common to the target wireless terminals,
The base station according to claim 1, wherein the specific target wireless terminal is selected based on a result of the processing. The individual reference signal is a reference signal used for measurement of channel characteristics in two dimensions in a horizontal direction and a vertical direction,
The base station according to claim 1, wherein the common reference signal is a reference signal used for measuring channel characteristics in one dimension. The controller is
Receiving channel state information fed back based on the common reference signal from the target wireless terminal;
Comparing the downlink data transmission performance derived based on the channel state information with a performance index,
The base station according to claim 3, wherein a target wireless terminal whose data transmission performance is lower than the performance index is selected as the specific target wireless terminal.   The base station according to claim 5, wherein the performance index is an average downlink data transmission performance of all the target wireless terminals.   The base station according to claim 2, wherein the control unit determines a radio terminal supporting the FD-MIMO as the target radio terminal based on capability information of each radio terminal connected to the base station.   The base station according to claim 1, wherein the control unit selects, as the specific target radio terminal, a radio terminal that has transmitted request information for requesting transmission of the dedicated reference signal to the base station.   A wireless terminal comprising a control unit that performs processing for transmitting request information for requesting transmission of an individual reference signal, which is a reference signal used for measurement of channel characteristics and generated individually for each wireless terminal, to a base station.   The radio terminal according to claim 9, wherein the individual reference signal is a reference signal used for measurement of channel characteristics in two dimensions in a horizontal direction and a vertical direction. A control unit that performs processing to transmit setting information of a specific TM (Transmission Mode) to the wireless terminal;
The specific TM supports channel state information feedback using a dedicated reference signal and channel state information feedback using a common reference signal,
The dedicated reference signal is a reference signal used for measuring channel characteristics, and is a reference signal generated individually for each radio terminal,
The common reference signal is a base signal that is a reference signal used for measurement of channel characteristics and is a reference signal common to a plurality of wireless terminals.   The base station according to claim 11, wherein the specific TM is a TM for FD-MIMO (Full-Dimension MIMO). The individual reference signal is a reference signal used for measurement of channel characteristics in two dimensions in a horizontal direction and a vertical direction,
The base station according to claim 11, wherein the common reference signal is a reference signal used for measuring channel characteristics in one dimension.   The control unit, after transmitting the setting information, issues a switching instruction for switching between feedback of channel state information using the common reference signal and feedback of channel state information using the dedicated reference signal. The base station according to claim 11, which performs a process of transmitting to a terminal. The controller is
Processing for transmitting the setting information to the wireless terminal by RRC layer signaling;
The base station according to claim 14, wherein the base station performs processing for transmitting the switching instruction to the wireless terminal by signaling in a layer lower than the RRC layer. A control unit that performs processing for receiving setting information of a specific TM (Transmission Mode) from the base station;
The specific TM supports channel state information feedback using a dedicated reference signal and channel state information feedback using a common reference signal,
The dedicated reference signal is a reference signal used for measurement of channel characteristics, and is a reference signal generated for each wireless terminal,
The common reference signal is a reference signal used for measurement of channel characteristics and is a reference signal common to a plurality of radio terminals.   The wireless terminal according to claim 16, wherein the specific TM is a TM for FD-MIMO (Full-Dimension MIMO). The individual reference signal is a reference signal used for measurement of channel characteristics in two dimensions in a horizontal direction and a vertical direction,
The wireless terminal according to claim 16, wherein the common reference signal is a reference signal used for measuring channel characteristics in one dimension.   The control unit, after receiving the setting information, issues a switching instruction for switching between feedback of channel state information using the common reference signal and feedback of channel state information using the dedicated reference signal. The wireless terminal according to claim 16, which performs a process of receiving from a station. The controller is
Receiving the setting information from the base station by RRC layer signaling;
The wireless terminal according to claim 19, wherein the wireless terminal performs a process of receiving the switching instruction from the base station by signaling in a layer lower than the RRC layer. A wireless terminal comprising a control unit,
The controller is
A process of receiving, from the base station, a terminal-specific reference signal, which is a demodulation reference signal transmitted by the base station for each wireless terminal;
And a process of transmitting channel state information generated by channel estimation using the terminal-specific reference signal to the base station.   The wireless terminal according to claim 21, wherein the channel state information is channel quality information.   The radio terminal according to claim 21, wherein the channel state information is interference information. The terminal-specific reference signal is included in an allocated radio resource allocated to the radio terminal by the base station,
The controller is
Processing for receiving the terminal-specific reference signal from the base station together with downlink data included in the allocated radio resource;
The radio terminal according to claim 21, wherein the channel estimation using the terminal-specific reference signal performs not only the demodulation of the downlink data but also the generation of the channel state information by the channel estimation. The controller is
Receiving downlink control information from the base station via a physical downlink control channel;
The radio terminal according to claim 21, wherein when the instruction information indicating an instruction to transmit the channel state information is included in the downlink control information, a process of transmitting the channel state information to the base station is performed. The downlink control information includes allocation information indicating allocated radio resources allocated to the radio terminal by the base station,
When the allocation information and the instruction information are included in the downlink control information, the control unit generates the channel state information generated by the channel estimation using the terminal-specific reference signal included in the allocated radio resource. 26. The wireless terminal according to claim 25, wherein processing for transmitting to a base station is performed. The controller is
Processing for transmitting response information indicating whether or not the downlink data has been successfully received from the base station to the base station;
The wireless terminal according to claim 21, wherein the wireless terminal performs a process of transmitting the channel state information to the base station at a transmission timing of the response information.   The control unit uses, as the channel state information, information indicating a difference between past channel state information transmitted to the base station before the current timing and current channel state information corresponding to the current timing as the channel state information. The wireless terminal according to claim 21, wherein the wireless terminal performs processing for transmission to a base station. A base station comprising a control unit,
The controller is
A process of transmitting a terminal-specific reference signal, which is a demodulation reference signal to be transmitted to each wireless terminal, to the wireless terminal;
A base station that performs processing for receiving, from the wireless terminal, channel state information generated by the wireless terminal by channel estimation using the terminal-specific reference signal.   The base station according to claim 29, wherein the channel state information is channel quality information.   The base station according to claim 29, wherein the channel state information is interference information. The terminal-specific reference signal is included in an allocated radio resource allocated to the radio terminal by the base station,
The base station according to claim 29, wherein the terminal-specific reference signal is used not only for demodulating downlink data included in the allocated radio resource but also for generating the channel state information. The controller is
A process of including instruction information indicating a transmission instruction of the channel state information in downlink control information;
The base station according to claim 29, wherein the base station performs a process of transmitting the downlink control information including the instruction information to the wireless terminal via a physical downlink control channel.   The base station according to claim 33, wherein the control unit performs a process of including the instruction information in the downlink control information together with allocation information indicating an allocated radio resource allocated to the radio terminal by the base station. The controller is
A process of receiving response information indicating whether or not the downlink data transmitted by the base station has been successfully received from the wireless terminal;
The base station according to claim 29, wherein a process of receiving the channel state information from the wireless terminal is performed at the reception timing of the response information.   The control unit uses, as the channel state information, information indicating a difference between past channel state information received from the wireless terminal before the current timing and current channel state information corresponding to the current timing as the channel state information. 30. The base station according to claim 29, which performs a process of receiving from a wireless terminal.

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