Patent Publication Number: US-2020304193-A1

Title: Method of transmitting channel state information-reference signal (csi-rs), base station, and user equipment

Description:
TECHNICAL FIELD 
     One or more embodiments disclosed herein relate to a method of transmitting a Channel State Information (CSI)-Reference Signal (RS) for beam management in a wireless communication system, a base station, and a user equipment. 
     BACKGROUND 
     New Radio (NR; fifth generation (5G) radio access technology) supports the following configurations for P1/P2/P3 beam management (BM) schemes. 
     In the P1 (or joint P2 and P3) BM scheme, multiple CSI-RS resource sets for BM are configured in a resource setting. The resource setting includes a parameter “repetition” set to “ON” or “OFF.” For each CSI-RS resource set, repetition must be set to “ON.” If repetition is set to “ON,” a user equipment (UE) reports, to a base station (BS), at least a CSI-RS resource set indicator for CSI-RS Resource Indicator (CRI) feedback. Whether set ID(s) are local within a resource setting or global across all resource settings has been studied. 
     In the P2 BM scheme, one resource set is configured in a resource setting with repetition “OFF” and the UE reports, to the BS, at least a local CSI-RS resource indicator within the resource set for CRI feedback. 
     In the P3 BM scheme, one resource set is configured in a resource setting with repetition “ON” and the UE does not report CRI related information. 
     As shown in  FIG. 1 , in the P2 BM scheme, TX beam sweeping with a fixed RX beam may be performed, and then a refined TX beam may be founded. 
     In a CSI-RS configuration (P2), one resource set is configured in a resource setting with repetition “OFF.” A UE assumes that all CSI-RS resources within a resource set are not spatially Quasi-Co-Location (QCL)ed and different TX beams are applied to different CSI-RS resources within the resource set. 
     As shown in  FIG. 2 , in the P3 procedure, RX beam sweeping with a fixed TX beam may be performed, and then a refined RX beam may be founded. 
     In a CSI-RS configuration (P3), one resource set is configured in a resource setting with repetition “ON.” A UE assumes that all CSI-RS resources within a resource set are spatially QCLed and the same TX beams are applied to different CSI-RS resources within the resource set. 
     As shown in  FIG. 3 , in the P1 beam management procedure, TX beam sweeping and RX beam sweeping may be performed, and then a refined TX/RX beam pairs may be founded. In CSI-RS configuration (P1) requirements, it shall enable TX beam refinement with TX beam sweeping and a fixed RX beam, it shall enable RX beam refinement with RX beam sweeping and a fixed TX beam, and it shall reuse the configuration of repetition indicator “ON/OFF.” 
       FIGS. 4A and 4B  are diagrams showing an example of the P1 BM scheme where multiple CSI-RS resource sets are configured in a resource setting. In  FIGS. 4A and 4B , the TX and the RX may be a BS and a UE, respectively. A CSI-RS resource index identifies each TX beam in each CSI-RS resource set. A CSI-RS resource set index identifies each CSI-RS resource set. In the P1 BM, as shown in  FIG. 4B , the same TX beam for different CSI-RS resources is within a resource set. For example, in an example of  FIG. 4B , the same four TX beams “TxBF-1” is with in CSI-RS resource set 1. In a similar manner, the same four TX beams “TxBF-2” is with in CSI-RS resource set 2. The UE applies different RX beams for different CSI-RS resources within a resource set, and then performing RX beam sweeping within a resource set. The different TX beams are used for different CSI-RS resource sets. TX beam sweeping is performed across different resource sets. The UE reports at least a CSI-RS resource set indicator for CSI-RS Resource Indicator (CRI) feedback. In the P1 BM scheme of  FIGS. 4A and 4B , the UE may sweep multiple RX beams with in a CSI-RS resource set. As a result, configurations of the UE may become complexity and limit the number of beams used by the UE. 
     CITATION LIST 
     Non-Patent Reference 
     
         
         [Non-Patent Reference 1] 3GPP, TS 36.211 V 14.4.0 
         [Non-Patent Reference 2] 3GPP, TS 36.213 V14.4.0 
       
    
     SUMMARY 
     One or more embodiments of the present invention relate to a novel CSI-RS configuration that configures a single set of CSI-RS with different TX beams for different CSI-RSs within the set. A semi-persistent transmission of the set may allow the UE to apply different UE RX beams for different sets. The UE reports the selected CSI-RS index according to some predefined criteria, e.g., best quality. 
     One or more embodiments of the present invention relate to a method of transmitting CSI-RSs in a wireless communication system that includes a BS and a UE. The method includes transmitting, from the BS to the UE, first multiple CSI-RSs in a CSI-RS resource set that consists of CSI-RS resources. The first multiple CSI-RSs are transmitted using the CSI-RS resources, respectively. The CSI-RS resource set is repeated in a time domain or a frequency domain. The transmitting transmits second multiple CSI-RSs in the repeated CSI-RS resource set. 
     One or more embodiments of the present invention relate to a BS in a wireless communication system that includes a transmitter that transmits, to the UE, first multiple CSI-RSs in a CSI-RS resource set that consists of CSI-RS resources. The first multiple CSI-RSs are transmitted using the CSI-RS resources, respectively. The CSI-RS resource set is repeated in a time domain or a frequency domain. The transmitter transmits second multiple CSI-RSs in the repeated CSI-RS resource set. 
     One or more embodiments of the present invention relate to a UE in a wireless communication system that includes a receiver that receives, from a BS, first multiple CSI-RSs in a CSI-RS resource set that consists of CSI-RS resources. The first multiple CSI-RSs are transmitted from the BS using the CSI-RS resources, respectively. The CSI-RS resource set is repeated in a time domain or a frequency domain. The receiver receives second multiple CSI-RSs in the repeated CSI-RS resource set. 
     One or more embodiments of the present invention can reduce the UE complexity, increase the implementation flexibility to support different types of UE with different RX beam sweeping capabilities. 
     Other embodiments and advantages of the present invention will be recognized from the description and figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing an example of a P2 beam management scheme and a CSI-RS Configuration for the P2 beam management. 
         FIG. 2  is a diagram showing an example of a P3 beam management scheme and a CSI-RS Configuration for the P3 beam management. 
         FIG. 3  is a diagram showing an example of a P1 beam management scheme and a CSI-RS Configuration for the P1 beam management. 
         FIGS. 4A and 4B  are diagrams showing an example of a P1 beam management scheme and a CSI-RS Configuration for the P1 beam management. 
         FIG. 5  is a diagram showing a configuration of a wireless communication system according to one or more embodiments of the present invention. 
         FIGS. 6A and 6B  are diagrams showing an example of a beam management scheme and a CSI-RS Configuration according to one or more embodiments of the present invention. 
         FIG. 7  is a sequence diagram showing a BM operation according to one or more embodiments of the present invention. 
         FIGS. 8A and 8B  are diagrams showing an example of a beam management scheme and a CSI-RS Configuration according to one or more embodiments of another example of the present invention. 
         FIG. 9  is a diagram showing a schematic configuration of the gNB according to one or more embodiments of the present invention. 
         FIG. 10  is a diagram showing a schematic configuration of the UE according to one or more embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be described in detail below, with reference to the drawings. In embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid obscuring the invention. 
       FIG. 5  is a diagram showing a wireless communications system  1  according to one or more embodiments of the present invention. The wireless communication system  1  includes a base station (BS)  20 , a user equipment (UE)  10 , and a core network  30 . In one or more embodiments of the present invention, the BS  20  and the UE  10  may be referred to as a transceiver (TX) and a receiver (RX), respectively. The wireless communication system  1  may be a New Radio (NR) system. The wireless communication system is not limited to the specific configurations described herein and may be any type of wireless communication system such as a Long Term Evolution (LTE) system or an LTE-Advanced (LTE-A) system. 
     The BS  20  may communicate uplink (UL) and downlink (DL) signals with the UE  10  in a cell of the BS  20 . The DL and UL signals may include control information and user data. The BS  20  may communicate DL and UL signals with the core network  30  through backhaul links  31 . The BS  20  may be a gNodeB (gNB) in a NR system. The BS  20  may be referred to as a transmission and reception point (TRP). For example, when the wireless communications system is an LTE system, the BS  20  may be an evolved NodeB (eNB). 
     The BS  20  includes antennas, a communication interface to communicate with an adjacent BS  20  (for example, X2 interface), a communication interface to communicate with the core network (for example, S1 interface), and a CPU (Central Processing Unit) such as a processor or a circuit to process transmitted and received signals with the UE  10 . Operations of the BS  20  may be implemented by the processor processing or executing data and programs stored in a memory. However, the BS  20  is not limited to the hardware configuration set forth above and may be realized by other appropriate hardware configurations as understood by those of ordinary skill in the art. Numerous BSs  20  may be disposed so as to cover a broader service area of the wireless communication system. 
     The UE  10  may communicate DL and UL signals that include control information and user data with the BS  20  using Multi Input Multi Output (MIMO) technology. The UE  10  may be a mobile station, a smartphone, a cellular phone, a tablet, a mobile router, or information processing apparatus having a radio communication function such as a wearable device. The wireless communication system may include one or more UEs  10 . 
     The UE  10  includes a CPU such as a processor, a RAM (Random Access Memory), a flash memory, and a radio communication device to transmit/receive radio signals to/from the BS  20  and the UE  10 . For example, operations of the UE  10  described below may be implemented by the CPU processing or executing data and programs stored in a memory. However, the UE  10  is not limited to the hardware configuration set forth above and may be configured with, e.g., a circuit to achieve the processing described below. 
       FIGS. 6A and 6B  are diagrams showing an example of a BM scheme and a CSI-RS Configuration according to one or more embodiments of the present invention. 
     As shown in  FIG. 6A , before performing BM, the BS  20  may transmit a signal using TX spatially QCL (SQCL)-1 and the UE  10  may receive a signal using RX SQCL-1. The TX SQCL-1 and the RX SQCL-1 are not beamformed. 
     In the BM according to one or more embodiments of the present invention, a single CSI-RS resource set may be configured in a resource setting. For example, the BS  20  may use different TX beams for different CSI-RS resources within a CSI-RS resource set. The BS  20  may perform TX beam sweeping within a CSI-RS resource set. In one or more embodiments of the present invention, the beams used for the transmission of the CSI-RSs may be referred to CSI-RS resources. 
     According to one or more embodiments of the present invention, the UE  10  applies the same RX beam for different CSI-RS resources within a CSI-RS resource set. The same CSI-RS resource set is repeated multiple times, and the UE  10  applies different RX beams per each repetition. Then, RX beam sweeping may be performed. For example, as shown in  FIG. 6B , when the BS  20  performs TX beam sweeping, the BS  20  may transmit multiple CSI-RSs using different Tx beams (e.g., TX beamformed (BF) beams—1 to 4) within a CSI-RS resource set 1. Each beam within the CSI-RS resource set is associated with a CSI-RS resource index. The CSI-RS resource index identifies a resource used for transmission of each of the multiple CSI-RSs. The BS  20  may transmit multiple CSI-RSs using TX BF-1 to 4 within a CSI-RS resource set 1 repeatedly. Thus, the CSI-RS resource set is repeated in a time domain or a frequency domain. 
     In the BM, the UE  10  may select at least a CSI-RS resource index and report the selected CSI-RS resource index as CSI feedback. The CSI feedback includes the same information as P2 (or P3 if necessary). As shown in  FIG. 6A , the BS  20  and the UE  10  may use beamformed TX SQCL-1 and RX SQCL-1, respectively, based on a result of the BM. 
       FIG. 7  is a sequence diagram showing a BM operation according to one or more embodiments of the present invention. 
     As shown in  FIG. 7 , at step S 11 , the BS  20  may transmit resource setting information indicating a CSI-RS resource set using higher layer signaling such as a Radio Resource Control signaling. The resource setting information indicates that the CSI-RS resource set is repeated in a time domain or in a frequency domain. The UE  10  may be configured with the notified resource setting information and receive CSI-RSs based on the resource setting information. 
     At step S 12 , the BS  20  may transmit CSI-RSs using different TX beams 1-4. The CSI-RS resource set 1 consists of the TX beams 1-4 (CSI-RS resources). The CSI-RSs are transmitted using the TX beams 1-4 (CSI-RS resources), respectively. In one or more embodiments of the present invention, the number of TX beams is not limited to four and may be at least two. 
     At step S 13 , the UE  10  may receive the CSI-RSs using a RX beam 1. 
     At step S 14 , the BS  20  may transmit CSI-RSs using TX beams 1-4 repeatedly. The CSI-RS resource set 1 consists of the TX beams 1-4 (CSI-RS resources). The CSI-RSs are transmitted using the TX beams 1-4 (CSI-RS resources), respectively. Thus, the CSI-RS resource set 1 is repeated in a time domain or a frequency domain. For example, when the semi-persistent CSI-RS resource/resource set/resource setting transmission or periodic CSI-RS resource/resource set/resource setting transmission is applied to transmission of CSI-RSs, the CSI-RS resource set is repeated. 
     At step S 15 , the UE  10  may receive the CSI-RSs using a RX beam 2. 
     After the step S 15 , the BS  20  may transmit the CSI-RSs using TX beams 1-4 one or more times. The UE  10  may receive the CSI-RSs from the BS  20  using another RX beam. 
     The UE  10  may select at least a TX beam from the TX beams 1-4. Each Tx beam may be identified by the CRI or beam index (BI). For example, the UE  10  may measure reception quality of the received CSI-RSs and determine at least a TX beam (or CRI) for the CSI reporting based on the measurement results. 
     At step S 16 , the UE  10  may perform CSI reporting including the selected CRI(s) to the BS  20 . 
     Thus, according to one or more embodiments of the present invention, the BS  20  may transmit, to the UE, multiple CSI-RSs in a CSI-RS resource set that consists of CSI-RS resources (e.g., TX beams). The multiple CSI-RSs may be transmitted using the CSI-RS resources, respectively. The CSI-RS resource set may be repeated in a time domain or a frequency domain. The BS  20  may transmit, to the UE, second multiple CSI-RSs in the repeated CSI-RS resource set. 
     According to one or more embodiments of the present invention, when semi-persistent CSI-RS resource/resource set/resource setting transmission or periodic CSI-RS resource/resource set/resource setting transmission is applied to transmission of CSI-RSs, the CSI-RS resource set may be repeated. 
     According to one or more embodiments of the present invention, the CSI-RS resources (e.g., TX beams) in the CSI-RS resource set may be different from each other. In other words, the CSI-RS resources in the CSI-RS resource set have different CSI-RS resource IDs. 
     As another example of repetition of the CSI-RS resource set, multiple CSI-RS resource setting may be configured and each CSI-RS resource setting consists of the same CSI-RS resource ID. By configuring either periodic semi-persistent CSI-RS resource/resource set/resource setting transmission or periodic CSI-RS resource/resource set/resource setting transmission for the multiple CSI-RS resource settings, the periodic or semi-persistence repeat of CSI-RS resource set is realized. Even by configuring aperiodic CSI-RS resource/resource set/resource setting transmission for the multiple CSI-RS resource settings, a one-time repeat of the CSI-RS resource set within multiple CSI-RS resource settings can be realized. Thus, the CSI-RS resource set that consists of the CSI-RS resources having the same CSI-RS resource ID may be configured as periodic, semi-persistent, or aperiodic. 
     According to one or more embodiments of another example of the present invention, as shown in  FIGS. 8A and 8B , multiple CSI-RS resource sets are configured in a resource setting. Different TX beams are used for different CSI-RS resources within a resource set. TX beam sweeping may be performed within a resource set. The UE  10  applies the same RX beam for different CSI-RS resources within a resource set. The different RX beams are used for different CSI-RS resource sets. The RX beam sweeping may be performed across different resource sets. According to one or more embodiments of another example of the present invention, the UE  10  reports CSI-RS resource set indicator(s) and/or CRI for CRI feedback. If the same CRI can be selected from a single resource set, CRI only reporting may be sufficient. If the same CRI is selected from multiple resource sets, the resource set index may be needed. 
     The method according to one or more embodiments of the present invention may relax UE implementation complexity (allow longer RX beam switching time) and support different UE categories (beam sweeping is not constrained by # of symbols per slot). Furthermore, in one or more embodiments of the present invention, more unified design as the other beam sweeping/selection procedure (CRI only feedback) and simplified signaling design (single resource set with repetition) may be provided. 
     The conventional method requires the UE to change RX beams per CSI-RS time unit, which increase the UE implementation complexity and is only suitable for high-end UE. One or more embodiments of the present invention only require UE to change RX beams per CSI-RS set time unit, which greatly relaxes the UE complexity. 
     The conventional method may limit the number of CSI-RS within a CSI-RS set due to the time domain configuration. For example, if a CSI-RS set is a slot which contains 4 possible symbols and each symbol is carrying one CSI-RS, then it allows a UE to have 4 different RX beams. In addition, in a network, different UEs may need to sweep different number of RX beams. The method according to embodiments of the present invention accommodates UE RX beam sweeping by configuring different number of CSI-RS sets. As the time unit per set is much a super set of time unit of CSI-RS. It is easier to flexibly change the set number to accommodate different UEs with different required beam sweeping number. 
     The conventional method needs to report CSI-RS set index, whereas the method according to embodiments of the present invention only reports CSI-RS index. The method according to embodiments of the present invention is fully compatible with the existing system design with respect to the reporting type. 
     (Configuration of BS) 
     The BS  20  according to one or more embodiments of the invention will be described below with reference to  FIG. 8 .  FIG. 8  shows a schematic configuration of the BS  20  according to one or more embodiments of the invention. The BS  20  may include a plurality of antennas (antenna element group)  201 , amplifier  202 , transceiver (transmitter/receiver)  203 , a baseband signal processor  204 , a call processor  205 , and a transmission path interface  206 . 
     User data that is transmitted on the DL from the BS  20  to the UE  20  is input from the core network  30 , through the transmission path interface  206 , into the baseband signal processor  204 . 
     In the baseband signal processor  204 , signals are subjected to Packet Data Convergence Protocol (PDCP) layer processing, Radio Link Control (RLC) layer transmission processing such as division and coupling of user data and RLC retransmission control transmission processing, Medium Access Control (MAC) retransmission control, including, for example, HARQ transmission processing, scheduling, transport format selection, channel coding, inverse fast Fourier transform (IFFT) processing, and precoding processing. Then, the resultant signals are transferred to each transceiver  203 . As for signals of the DL control channel, transmission processing is performed, including channel coding and inverse fast Fourier transform, and the resultant signals are transmitted to each transceiver  203 . 
     The baseband signal processor  204  notifies each UE  10  of control information (system information) for communication in the cell by higher layer signaling (e.g., RRC signaling and broadcast channel). Information for communication in the cell includes, for example, UL or DL system bandwidth. 
     In each transceiver  203 , baseband signals that are precoded per antenna and output from the baseband signal processor  204  are subjected to frequency conversion processing into a radio frequency band. The amplifier  202  amplifies the radio frequency signals having been subjected to frequency conversion, and the resultant signals are transmitted from the antennas  201 . 
     As for data to be transmitted on the UL from the UE  10  to the BS  20 , radio frequency signals are received in each antenna  201 , amplified in the amplifier  202 , subjected to frequency conversion and converted into baseband signals in the transceiver  203 , and are input to the baseband signal processor  204 . 
     The baseband signal processor  204  performs FFT processing, IDFT processing, error correction decoding, MAC retransmission control reception processing, and RLC layer and PDCP layer reception processing on the user data included in the received baseband signals. Then, the resultant signals are transferred to the core network  30  through the transmission path interface  206 . The call processor  205  performs call processing such as setting up and releasing a communication channel, manages the state of the BS  20 , and manages the radio resources. 
     (Configuration of User Equipment) 
     The UE  10  according to one or more embodiments of the invention will be described below with reference to  FIG. 9 .  FIG. 9  shows a schematic configuration of the UE  10  according to one or more embodiments of the invention. The UE  10  has a plurality of UE antennas  101 , amplifiers  102 , the circuit  103  comprising transceiver (transmitter/receiver)  1031 , the controller  104 , and an application  105 . 
     As for DL, radio frequency signals received in the UE antennas  101  are amplified in the respective amplifiers  102 , and subjected to frequency conversion into baseband signals in the transceiver  1031 . These baseband signals are subjected to reception processing such as FFT processing, error correction decoding and retransmission control and so on, in the controller  104 . The DL user data is transferred to the application  105 . The application  105  performs processing related to higher layers above the physical layer and the MAC layer. In the downlink data, broadcast information is also transferred to the application  105 . 
     On the other hand, UL user data is input from the application  105  to the controller  104 . In the controller  104 , retransmission control (Hybrid ARQ) transmission processing, channel coding, precoding, DFT processing, IFFT processing and so on are performed, and the resultant signals are transferred to each transceiver  1031 . In the transceiver  1031 , the baseband signals output from the controller  104  are converted into a radio frequency band. After that, the frequency-converted radio frequency signals are amplified in the amplifier  102 , and then, transmitted from the antenna  101 . 
     One or more embodiments of the present invention may be used for the FL DM-RS and the additional DM-RS. 
     One of more embodiments of the present invention may be used for DM-RS for CP-OFDM uplink and downlink. If it is applied for the downlink, the technologies for the signaling can be also used to UE assumption for DM-RS measurement. 
     One or more embodiments of the present invention may be used for each of the uplink and the downlink independently. One or more embodiments of the present invention may be also used for both of the uplink and the downlink in common. The uplink channel and signal may be replaced with the downlink signal channel and signal. 
     Although the present disclosure mainly described examples of a channel and signaling scheme based on NR, the present invention is not limited thereto. One or more embodiments of the present invention may apply to another channel and signaling scheme having the same functions as NR such as LTE/LTE-A and a newly defined channel and signaling scheme. 
     The above examples and modified examples may be combined with each other, and various features of these examples can be combined with each other in various combinations. The invention is not limited to the specific combinations disclosed herein. 
     Although the disclosure has been described with respect to only a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that various other embodiments may be devised without departing from the scope of the present invention. Accordingly, the scope of the invention should be limited only by the attached claims.