Abstract:
The present invention relates to a method and an apparatus for controlling a rank information (RI) report. According to an embodiment of the present invention, a method for controlling an RI report of a User Equipment (UE) in a base station of a Long Term Evolution (LTE) mobile communications network is provided. The method includes the following steps: determining a Channel State Information (CSI) report set for a UE, where the CSI report set includes multiple CSI feedback modes (CSI processes); sending to the UE Radio Resource Control (RRC) signaling indicating the CSI report set; and sending to the UE signaling indicating a precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set. According to the above embodiment, rank selection and high rank transmission are supported for coordinated multi-point (CoMP) transmission in the LTE mobile communications network.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to mobile communications technologies, and more particularly, to Rank Information (RI) feedback. 
         [0003]    2. Description of the Prior Art 
         [0004]    In a modern mobile communications system such as Long Term Evolution (LTE), a base station determines a transmission format, a transmission block size, a modulation and coding scheme, and a Multiple Input Multiple Output (MIMO) transmission mode to be used in a downlink (DL) and an uplink (UL). To perform such determination for the DL, the base station needs information about the performance of a current DL channel from a User Equipment (UE), and said information is generally referred to as Channel State Information (CSI). 
         [0005]    Frequency Division Duplexing (FDD) DL coordinated multi-point (CoMP) transmission has potential in improving the coverage area, cell edge throughput, and/or spectral efficiency. In the CoMP transmission, multiple CSI reference signal resources (CSI-RS-resources) coordinate with each other to transmit data of one or more UEs. Because of the high space division multiplexing gain and/or better channel quality of high rank transmission, certain UEs may prefer high rank transmission to single rank transmission, especially in dynamic point selection (DPS) or dynamic point blanking (DPB). Therefore, the high rank transmission needs to be supported in the CoMP; however, a conventional RI report is no longer applicable. 
       SUMMARY OF THE INVENTION 
       [0006]    One objective of an embodiment of the present invention is to provide an RI report mode capable of supporting network configuration of the high rank transmission. 
         [0007]    According to an embodiment of the present invention, a method for controlling an RI report of a UE in a base station of an LTE mobile communications network is provided. The method includes the following steps: determining a CSI report set for a UE, where the CSI report set includes multiple CSI feedback modes (CSI processes); sending to the UE Radio Resource Control (RRC) signaling indicating the CSI report set; and sending to the UE signaling indicating a precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set. 
         [0008]    According to another embodiment of the present invention, a method for CSI feedback in a UE of an LTE mobile communications network is provided. The method includes the following steps: receiving signaling indicating a CSI report set, where the CSI report set includes multiple CSI feedback modes; receiving signaling indicating a precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set; and determining a rank characteristic of the CSI feedback according to the precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set. 
         [0009]    According to another embodiment of the present invention, a base station in an LTE mobile communications network is provided. The base station includes a processing unit and a transmitter. The processing unit is configured to determine a CSI report set for a UE, where the CSI report set includes multiple CSI feedback modes. The transmitter is configured to send to the UE RRC signaling indicating the CSI report set. The transmitter is further configured to send to the UE signaling indicating a precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set. 
         [0010]    According to another embodiment of the present invention, a UE used in an LTE mobile communications network is provided. The UE includes a receiver and a processing unit. The receiver is configured to receive signaling indicating a CSI report set, where the CSI report set includes multiple CSI feedback modes. The receiver is further configured to receive signaling indicating a precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set. The processing unit is configured to determine a rank characteristic of CSI feedback according to the precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set. 
         [0011]    According to the above embodiments, rank selection and high rank transmission are supported for CoMP transmission in the LTE mobile communications network. 
         [0012]    The technical features and advantages of the present invention are summarized above, so as to make the following detailed description of the present invention easier to understand. Other features and advantages of the present invention will be described in the following, which form the subject of the claims of the present invention. It should be understood by persons skilled in the art that the disclosed concepts and embodiments may be easily used as a basis for modifying or designing other structures or procedures used for implementing the same objective as the present invention. It should be further understood by persons skilled in the art that the equivalent construction does not depart from the spirit and scope of the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    With reference to the accompanying drawings, the following detailed description about the exemplary embodiments of the present invention are easier to understand. The present invention is illustrated through the examples and is not limited to the accompanying drawings. Similar drawing markers in the accompanying drawings indicate similar components. 
           [0014]      FIG. 1  is a flow chart of a method running between an eNB  10  and a UE  20  in an LTE mobile communications network according to an embodiment of the present invention; 
           [0015]      FIG. 2  is a structural block diagram of an eNB in an LTE mobile communications network according to an embodiment of the present invention; and 
           [0016]      FIG. 3  is a structural block diagram of a UE in an LTE mobile communications network according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The detailed descriptions of the accompanying drawings are intended to illustrate the current exemplary embodiments of the present invention, rather than to represent the only implementation form of the present invention. It should be understood that the same or equivalent function may be completed by different embodiments that are intended to be included within the spirit and scope of the present invention. 
         [0018]    It should be understood by persons skilled in the art that the means and functions described herein may be implemented by using software functions combining a program control microprocessor and a general-purpose computer, and/or implemented by using an application-specific integrated circuit (ASIC). It should also be understood that, although the present invention is mainly illustrated in the form of methods and apparatuses, the present invention may also be embodied by a computer program product and a system including a computer processor and a memory connected to the processor, where the memory is encoded by using one or more programs that may complete the functions disclosed herein. 
         [0019]    It should be understood by persons skilled in the art that different protocol standards have different technical terms to represent a base station. For example, a base station in an LTE system or an LTE-Advanced (LTE-A) system is referred to as Node B or evolved Node B (eNB). The base station in the present invention may be but is not limited to an eNB in an LTE-A system. 
         [0020]    Generally, a CSI report is sent by a UE to a serving eNB thereof, and the serving eNB needs to indicate configuration of the CSI report. In CoMP transmission, multiple CSI-RS-resources coordinate with each other to transmit data of one or more UEs. Certain UEs may prefer high rank transmission to single rank transmission, especially in DPS or DPB. 
         [0021]      FIG. 1  is a flow chart of a method for a CSI report supporting high rank transmission and between an eNB  10  and a UE  20  in an LTE mobile communications network according to an embodiment of the present invention. As shown in  FIG. 1 , the method includes steps  110  and  120  at the side of the eNB  10  and steps  210  and  220  at the side of the UE  20 . 
         [0022]    In step  110 , the eNB  10  determines a CSI report set for the UE  20 , where the CSI report set includes multiple CSI feedback modes (CSI processes). It should be noted that, each CSI feedback mode is defined as including a Non-zero Power (NZP) reference signal resource part used for channel measurement and an Interference Measurement Resource (IMR) part used for interference measurement. 
         [0023]    In step  120 , the eNB  10  sends to the UE  20  RRC signaling indicating the CSI report set. 
         [0024]    In some embodiments, the method at the side of the eNB  10  further includes a step of sending additional Downlink Control Indicator (DCI) signaling to the UE  20 . The RRC signaling and/or additional DCI signaling sent in step  120  further indicate a precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set. 
         [0025]    In step  210 , the UE  20  receives signaling indicating a CSI report set, where the CSI report set includes multiple CSI feedback modes. 
         [0026]    In certain embodiments, the method at the side of the eNB  20  further includes a step of receiving additional DCI signaling from the eNB  10 . As mentioned above, the RRC signaling and/or additional DCI signaling received in step  210  further indicate a precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set. 
         [0027]    In step  220 , the UE  20  determines a rank characteristic of CSI feedback according to the precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set. 
         [0028]    In an embodiment, the RRC signaling sent in step  120  further indicates the precoding codebook subset restriction of each CSI feedback mode in the CSI report set through a bit map parameter, for example, parameter codebookSubsetRestriction. Preferably, such bit map parameter may be configured for each CSI feedback mode. A precoding codebook has multiple elements (or subsets) corresponding to different ranks, so an available element (subset) corresponding to a rank greater than 1 means support for high rank transmission. Two exemplary configurations of a CSI report set with a precoding codebook subset restriction are shown in Table 1-1 and Table 1-2. 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1-1 
               
             
             
               
                   
               
               
                 Exemplary configuration of CSI report set with precoding codebook subset 
               
               
                 restriction 
               
             
          
           
               
                 CSI  
                   
                 Interference 
                 Precoding codebook subset 
               
               
                 process 
                 Channel part 
                 part 
                 restriction 
               
               
                   
               
               
                 1 
                 NZP CSI-RS #1 
                 IMR #1 
                 codebookSubsetRestriction #1 
               
               
                 2 
                 NZP CSI-RS #2 
                 IMR #2 
                 codebookSubsetRestriction #2 
               
               
                 3 
                 NZP CSI-RS #3 
                 IMR #3 
                 codebookSubsetRestriction #3 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1-2 
               
             
             
               
                   
               
               
                 Exemplary configuration of CSI report set with precoding codebook subset 
               
               
                 restriction 
               
             
          
           
               
                 CSI 
                   
                 Interference 
                 Precoding codebook subset 
               
               
                 process 
                 Channel part 
                 part 
                 restriction 
               
               
                   
               
               
                 1 
                 NZP CSI-RS #1 
                 IMR #1 
                 codebookSubsetRestriction #1 
               
               
                 2 
                 NZP CSI-RS #1 
                 IMR #2 
                 codebookSubsetRestriction #2 
               
               
                   
               
             
          
         
       
     
         [0029]    In another embodiment, the RRC signaling sent in step  120  further indicates a validity period of each CSI feedback mode in the CSI report set. The validity period is represented by, for example, a numeral indicating the number of continuous subframes. An exemplary configuration of the CSI report set with validity period information and a precoding codebook subset restriction is shown in Table 2. 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Exemplary configuration of CSI report set with validity period information 
               
               
                 and precoding codebook subset restriction 
               
             
          
           
               
                 CSI 
                   
                 Interference 
                 Precoding codebook subset 
               
               
                 process 
                 Channel part 
                 part 
                 restriction 
               
               
                   
               
               
                 1 
                 NZP CSI-RS #1 
                 IMR #1 
                 codebookSubsetRestriction #1 
               
               
                   
                   
                   
                 and validity period #1 
               
               
                 2 
                 NZP CSI-RS #2 
                 IMR #2 
                 codebookSubsetRestriction #2 
               
               
                   
                   
                   
                 and validity period #2 
               
               
                 3 
                 NZP CSI-RS #3 
                 IMR #3 
                 codebookSubsetRestriction #3 
               
               
                   
                   
                   
                 and validity perrod #3 
               
               
                   
               
             
          
         
       
     
         [0030]    In an embodiment, the RRC signaling sent in step  120  further indicates the rank restriction of each CSI feedback mode in the CSI report set. The rank restriction may separately indicate no restriction or restriction to a specific rank for each CSI feedback mode. As for the specific rank, for example, when the rank is 1, the rank may be marked as “eNB preferential rank”. An exemplary configuration of the rank restriction of each CSI feedback mode is shown in Table 3. 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Exemplary configuration of rank restriction of CSI report set 
               
             
          
           
               
                   
                   
                 Interference 
                   
               
               
                 CSI process 
                 Channel part 
                 part 
                 Rank restriction 
               
               
                   
               
               
                 1 
                 NZP CSI-RS #1 
                 IMR #1 
                 ‘No restriction’ 
               
               
                 2 
                 NZP CSI-RS #2 
                 IMR #2 
                 ‘eNB preferential rank’ 
               
               
                 3 
                 NZP CSI-RS #3 
                 IMR #3 
                 ‘eNB preferential rank’ 
               
               
                   
               
             
          
         
       
     
         [0031]    In an embodiment, the additional DCI signaling sent by the eNB  10  to the UE  20  indicates the rank restriction of each CSI feedback mode in the CSI report set. A configuration of the CSI report set indicated by RRC signaling and a configuration of the rank restriction corresponding to each CSI feedback mode in the CSI report set indicated by the DCI signaling are shown in Table 4-1 and Table 4-2. In some cases, the DCI signaling may be used to update the rank restriction of each CSI feedback mode in the CSI report set. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 4-1 
               
             
             
               
                   
               
               
                 Exemplary configuration of CSI report set indicated by RRC signaling 
               
             
          
           
               
                 CSI process 
                 Channel part 
                 Interference part 
               
               
                   
               
               
                 1 
                 NZP CSI-RS #1 
                 IMR #1 
               
               
                 2 
                 NZP CSI-RS #2 
                 IMR #2 
               
               
                 3 
                 NZP CSI-RS #3 
                 IMR #3 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 4-2 
               
             
             
               
                   
               
               
                 Exemplary configuration of rank restriction of CSI feedback mode 
               
               
                 indicated by DCI signaling 
               
             
          
           
               
                 CSI process 
                 Rank restriction 
               
               
                   
               
               
                 1 
                 ‘No restriction’ 
               
               
                 2 
                 ‘eNB preferential rank’ 
               
               
                 3 
                 ‘eNB preferential rank’ 
               
               
                   
               
             
          
         
       
     
         [0032]    In an embodiment, the RRC signaling sent in step  120  further indicates the same rank restriction for all CSI feedback modes in the CSI report set. Such a rank restriction may indicate no restriction or indicate a common rank for the CSI report set, where the common rank is, for example, 1. An exemplary configuration of the rank restriction of the CSI report set is shown in Table 5-1. 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 5-1 
               
             
             
               
                   
               
               
                 Exemplary configuration of rank restriction of CSI report set 
               
             
          
           
               
                   
                   
                 Interference 
                   
               
               
                 CSI process 
                 Channel part 
                 part 
                 Rank restriction 
               
               
                   
               
               
                 1 
                 NZP CSI-RS #1 
                 IMR #1 
                 ‘No restriction’ or 
               
               
                 2 
                 NZP CSI-RS #2 
                 IMR #2 
                 ‘common rank’ 
               
               
                 3 
                 NZP CSI-RS #3 
                 IMR #3 
               
               
                   
               
             
          
         
       
     
         [0033]    In another embodiment, the additional DCI signaling sent by the eNB  10  to the UE  20  indicates the same rank restriction for all CSI feedback modes in the CSI report set. The rank restriction may indicate no restriction or indicate a common rank for the CSI report set, where the common rank is, for example, 1. A configuration of the CSI report set indicated by the RRC signaling and a configuration of the rank restriction of the CSI report set indicated by the DCI signaling are shown in Table 5-2 and Table 5-3. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 5-2 
               
             
             
               
                   
               
               
                 Exemplary configuration of CSI report set indicated by RRC signaling 
               
             
          
           
               
                 CSI process 
                 Channel part 
                 Interference part 
               
               
                   
               
               
                 1 
                 NZP CSI-RS #1 
                 IMR #1 
               
               
                 2 
                 NZP CSI-RS #2 
                 IMR #2 
               
               
                 3 
                 NZP CSI-RS #3 
                 IMR #3 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 5-3 
               
             
             
               
                   
               
               
                 Exemplary configuration of rank restriction of CSI report set indicated by 
               
               
                 DCI signaling 
               
             
          
           
               
                   
                 CSI process 
                 Rank restriction 
               
               
                   
                   
               
               
                   
                 1 
                 ‘No restriction’ or ‘common rank’ 
               
               
                   
                 2 
               
               
                   
                 3 
               
               
                   
                   
               
             
          
         
       
     
         [0034]    As mentioned above, in step  220 , the UE  20  determines the rank characteristic of the CSI feedback according to the precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set indicated by the RRC signaling received in step  210 . The precoding codebook subset and/or rank restriction of each CSI feedback mode may be different, so the rank characteristic determined by the UE  20  may change accordingly. 
         [0035]    In an embodiment, the precoding codebook subset and/or rank restriction indicates that a CSI feedback mode has no precoding codebook subset or rank restriction, and the UE  20  selects a UE preferential rank for the CSI feedback mode. 
         [0036]    In another embodiment, the precoding codebook subset and/or rank restriction indicates that a CSI feedback mode is restricted to a specific rank (for example, the rank marked as ‘eNB preferential rank’) but has no precoding codebook subset restriction, and the UE  20  selects the specific rank for the CSI feedback mode. 
         [0037]    In another embodiment, the precoding codebook subset and/or rank restriction indicates that a CSI feedback mode has a precoding codebook subset restriction but has no rank restriction, and the UE  20  selects a UE preferential rank for the CSI feedback mode among ranks corresponding to an available precoding codebook subset. 
         [0038]    In another embodiment, the precoding codebook subset and/or rank restriction indicates that a CSI feedback mode has a precoding codebook subset restriction and is restricted to a specific rank, and the UE  20  selects a rank of the CSI feedback for the CSI feedback mode according to a relationship between the specific rank and an available precoding codebook subset indicated by the restriction. If the available precoding codebook subset includes the specific rank, the UE  20  selects the specific rank for the CSI feedback mode. If the specific rank is less than a minimum rank in the available precoding codebook subset, the UE  20  selects the minimum rank for the CSI feedback mode. If the specific rank is greater than a maximum rank in the available precoding codebook subset, the UE  20  selects the maximum rank for the CSI feedback mode. If the available precoding codebook subset does not include the specific rank and the specific rank is between the minimum rank and the maximum rank in the available precoding codebook subset, the UE  20  selects for the CSI feedback mode the UE preferential rank or a rank, in the available precoding codebook subset, less than and closest to the specific rank. 
         [0039]    In all the above embodiments, the DCI signaling may use an undefined item of a CSI request field. 
         [0040]      FIG. 2  is a structural block diagram of an eNB  10  in an LTE mobile communications network according to an embodiment of the present invention.  FIG. 3  is a schematic block diagram of a UE  20  in an LTE mobile communications network according to an embodiment of the present invention. As shown in  FIG. 2  and  FIG. 3 , the eNB  10  includes a processing unit  1010  and a transmitter (or transceiver)  1020 , and the UE  20  includes a receiver (or transceiver)  2010  and a processing unit  2020 . 
         [0041]    According to some embodiments of the present invention, the eNB  10  and the UE  20  support high rank transmission used in CoMP. 
         [0042]    The processing unit  1010  is configured to determine a CSI report set for the UE  20 , where the CSI report set includes multiple CSI feedback modes (CSI processes). It should be noted that, each CSI feedback mode is defined as including an NZP reference signal resource part used for channel measurement and an IMR part used for interference measurement. 
         [0043]    The transmitter  1020  is configured to send to the UE  20  RRC signaling indicating the CSI report set. 
         [0044]    In some embodiments, the transmitter  1020  may be further configured to send additional DCI signaling to the UE  20 . The RRC signaling and/or additional DCI signaling sent to the UE  20  further indicates a precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set. 
         [0045]    The receiver  2010  is configured to receive signaling indicating a CSI report set, where the CSI report set includes multiple CSI feedback modes. 
         [0046]    In some embodiments, the receiver  2010  may be further configured to receive additional DCI signaling from the eNB  10 . As mentioned above, the RRC signaling and/or additional DCI signaling received from the eNB  10  further indicates the precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set. 
         [0047]    The processing unit  2020  is configured to determine a rank characteristic of CSI feedback according to the precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set. 
         [0048]    In an embodiment, the RRC signaling sent by the eNB  10  to the UE  20  further indicates a precoding codebook subset restriction of each CSI feedback mode in the CSI report set through a bit map parameter, for example, parameter codebookSubsetRestriction. Preferably, such bit map parameter may be configured for each CSI feedback mode. A precoding codebook has multiple elements (or subsets) corresponding to different ranks, so an available element (subset) corresponding to a rank greater than 1 means support for high rank transmission. Two exemplary configurations of a CSI report set with a precoding codebook subset restriction are shown in Table 1-1 and Table 1-2, as described above. 
         [0049]    In another embodiment, the RRC signaling sent by the eNB  10  to the UE  20  further indicates a validity period of each CSI feedback mode in the CSI report set. The validity period is represented by, for example, a numeral indicating the number of continuous subframes. An exemplary configuration of the CSI report set with validity period information and a precoding codebook subset restriction is shown in Table 2, as described above. 
         [0050]    In an embodiment, the RRC signaling sent by the eNB  10  to the UE  20  further indicates the rank restriction of each CSI feedback mode in the CSI report set. The rank restriction may separately indicate no restriction or restriction to a specific rank for each CSI feedback mode. As for the specific rank, for example, when the rank is 1, the rank may be marked as “eNB preferential rank”. An exemplary configuration of the rank restriction of each CSI feedback mode is shown in Table 3, as described above. 
         [0051]    In an embodiment, the additional DCI signaling sent by the eNB  10  to the UE  20  indicates the rank restriction of each CSI feedback mode in the CSI report set. A configuration of the CSI report set indicated by RRC signaling and a configuration of the rank restriction corresponding to each CSI feedback mode in the CSI report set indicated by the DCI signaling are shown in Table 4-1 and Table 4-2, as described above. In some cases, the DCI signaling may be used to update the rank restriction of each CSI feedback mode in the CSI report set. 
         [0052]    In another embodiment, the RRC signaling sent by the eNB  10  to the UE  20  further indicates the same rank restriction for all CSI feedback modes in the CSI report set. Such a rank restriction may indicate no restriction or indicate a common rank for the CSI report set, where the common rank is, for example, 1. An exemplary configuration of the rank restriction of the CSI report set is shown in Table 5-1, as described above. 
         [0053]    In another embodiment, the additional DCI signaling sent by the eNB  10  to the UE  20  indicates the same rank restriction for all CSI feedback modes in the CSI report set. The rank restriction may indicate no restriction or indicate a common rank for the CSI report set, where the common rank is, for example, 1. A configuration of the CSI report set indicated by the RRC signaling and a configuration of the rank restriction of the CSI report set indicated by the DCI signaling are shown in Table 5-2 and Table 5-3, as described above. 
         [0054]    As mentioned above, the processing unit  2020  is configured to determine the rank characteristic of the CSI feedback according to the precoding codebook subset and/or rank restriction of each CSI feedback mode in the CSI report set indicated by the received RRC signaling. The precoding codebook subset and/or rank restriction of each CSI feedback mode may be different, so the determined rank characteristic may change accordingly. 
         [0055]    In an embodiment, the precoding codebook subset and/or rank restriction indicates that the CSI feedback mode has no precoding codebook subset or rank restriction, and the processing unit  2020  selects a UE preferential rank for the CSI feedback mode. 
         [0056]    In another embodiment, the precoding codebook subset and/or rank restriction indicates that a CSI feedback mode is restricted to a specific rank (for example, the rank marked as ‘eNB preferential rank’) but has no precoding codebook subset restriction, and the processing unit  2020  selects the specific rank for the CSI feedback mode. 
         [0057]    In another embodiment, the precoding codebook subset and/or rank restriction indicates that a CSI feedback mode has a precoding codebook subset restriction but has no rank restriction, and the processing unit  2020  selects a UE preferential rank for the CSI feedback mode among ranks corresponding to an available precoding codebook subset. 
         [0058]    In another embodiment, the precoding codebook subset and/or rank restriction indicates that a CSI feedback mode has a precoding codebook subset restriction and is restricted to a specific rank, and the processing unit  2020  selects a rank of the CSI feedback for the CSI feedback mode according to a relationship between the specific rank and an available precoding codebook subset indicated by the restriction. If the available precoding codebook subset includes the specific rank, the processing unit  2020  selects the specific rank for the CSI feedback mode. If the specific rank is less than a minimum rank in the available precoding codebook subset, the processing unit  2020  selects the minimum rank for the CSI feedback mode. If the specific rank is greater than a maximum rank in the available precoding codebook subset, the processing unit  2020  selects the maximum rank for the CSI feedback mode. If the available precoding codebook subset does not include the specific rank and the specific rank is between the minimum rank and the maximum rank in the available precoding codebook subset, the processing unit  2020  selects for the CSI feedback mode the UE preferential rank or a rank, in the available precoding codebook subset, less than and closest to the specific rank. 
         [0059]    In all the above embodiments, the DCI signaling may use an undefined item of a CSI request field. 
         [0060]    Different embodiments of the present invention being illustrated and described above, the present invention is not limited to these embodiments. Many modifications, alterations, variations, replacements and equivalences are obvious to persons skilled in the art without departing from the spirit and scope of the present invention described in the claims.