Abstract:
The invention relates to a method of designing a codebook for a 3D antenna configuration, the method including the steps of: selecting a first codeword characterizing a first antenna array in the 3D antenna configuration from a first codebook; selecting a second codeword characterizing a second antenna array in the 3D antenna configuration from a second codebook; and obtaining a corresponding codeword in the codebook for the 3D antenna configuration by multiplying the first codeword with the second codeword. The solution of the invention has the advantage of being simple in design and can make use of existing LTE Rel-10 codebooks and thus have the advantage of good compatibility.

Description:
FIELD OF THE INVENTION 
       [0001]    The present disclosure relates to the wireless communication and particularly to a method of designing a codebook for a 3D antenna configuration. 
       BACKGROUND OF THE INVENTION 
       [0002]    The current LTE-A standard defines 2D channel modeling and 2D antenna configurations, which means that only horizontal azimuth angles of departure and arrival but no angles in the vertical direction are considered. 
         [0003]    Recently 3D channel modeling is attracting more and more attention, since it provides additional performance gain in the vertical direction in addition to the gain in the horizontal direction. In 3D channel modeling, a 3D antenna configuration is required to provide both horizontal coverage and vertical coverage. A typical application of the 3D antenna configuration is for the coverage of an urban environment with high densities of high buildings. Here a 3D antenna configuration refers to an antenna configuration in which a 3D beam can be generated. 
         [0004]    In a Frequency Division Duplex (FDD) system, a user equipment selects the best codeword in a predefined codebook representing the downlink Channel State Information (CSI) and feeds a Pre-coding Matrix Index (PMI) corresponding to the codeword back to a base station (eNodeB). The existing standard Rel-10 has offered codebooks for a 2D antenna configuration, but a design of codebooks for a 3D antenna configuration has been absent so far. 
       SUMMARY OF THE INVENTION 
       [0005]    An object of the invention is to propose a method of designing a codebook for a 3D antenna configuration in a simple design, which will be very advantageous. 
         [0006]    According to one aspect of the invention, there is provided a method of designing a codebook for a 3D antenna configuration, the method comprising the steps of: selecting a first codeword characterizing a first antenna array in the 3D antenna configuration from a first codebook; selecting a second codeword characterizing a second antenna array in the 3D antenna configuration from a second codebook; and obtaining a corresponding codeword in the codebook for the 3D antenna configuration by multiplying the first codeword with the second codeword. 
         [0007]    Here the 3D antenna configuration is an antenna configuration generating a 3D beam. 
         [0008]    Furthermore the 3D antenna configuration is a rectangular antenna array; and the first codebook is a horizontal codebook, and the second codebook is a vertical codebook; and the first codeword characterizes a horizontal linear antenna array in the rectangular antenna array, and the second codeword characterizes a vertical linear antenna array in the rectangular antenna array. 
         [0009]    According another aspect of the invention, there is proposed a method of determining a codeword in a base station of a communication system with a 3D antenna configuration, the base station being configured with a rectangular antenna array, and the method comprising the steps of: receiving information about a first index n of a first codeword in a first codebook, information about a second index m of a second codeword in a second codebook and information about a rank r from a user equipment; determining a target codeword corresponding to the received first index n, the second index m and the rank r, 
         [0010]    for the rank 1≦r≦N V , the target codeword being represented as: 
         [0000]        W   mN+n   (r)   =T   m,n   r   /∥T   m,n   r ∥;
 
         [0000]        T   m,n   r (:, k )=col( U   n   (r) (:, k )·( V   m   (1) ) T ),
       k=1, 2, . . . , r; and   m=0, 1, . . . , 2 M −1;   n=0, 1, . . . , 2 N −1;       
 
         [0014]    for the rank N V &lt;r≦N V N H , the target codeword being represented as: 
         [0000]        W   mN+n   (r)   =F   m,n   N     V     ┌r/N     V     ┐ (:,1: r )/∥ F   m,n   N     V     ┌r/N     V     ┐ (:,1: r )∥;
 
         [0000]        F   m,n   N     V     ┌r/N     V     ┐ (:, N   V ( k   1 −1)+ k   2 )=col( U   n   (N     V     ) (:, k   2 )·( V   m   (┌r/N     V     ┐) (:, k   1 )) T ),
       k 1 =1, 2, . . . , ┌r/N V ┐;   k 2 =1, 2, . . . , N V ;   m=0, 1, . . . , 2 M −1;   n=0, 1, . . . , 2 N −1,       
 
         [0019]    wherein U m   (r)  and V m   (r)  represent the first codeword and the second codeword with the index m at the rank r respectively, N H  represents an amount of the horizontal linear antenna arrays, N V  represents an amount of the vertical linear antenna arrays, a size of the horizontal codebook is N bits, a size of the vertical codebook is M bits, col(A) represents a transformation of a matrix A into a column vector, A(:,k) represents the k th  column of the matrix A, T m,n   r  is a N H N V ×r matrix, and F m,n   r  is a N H N V ×r matrix. 
         [0020]    According to another aspect of the invention, there is proposes a method, in a user equipment of a communication system with a 3D antenna configuration, of providing a base station with codeword index information, the base station being configured with a rectangular antenna array, and the method comprising the step of: transmitting information about a first index n of a first codeword in a first codebook, information about a second index m of a second codeword in a second codebook and information about a rank r to the base station according to a predetermined criterion, wherein the first codeword characterizes a first antenna array in the 3D antenna configuration, and the second codeword characterizes a second antenna array in the 3D antenna configuration. 
         [0021]    The solution of the invention has the advantage of being simple in design and can make use of existing LTE Rel-10 codebooks and thus have the advantage of good compatibility. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0022]    Other features, objects and advantages of the invention will become more apparent upon review of the following detailed description of non-limiting embodiments taken with reference to the drawings in which: 
           [0023]      FIG. 1  illustrates a rectangular antenna array deployed at a base station; 
           [0024]      FIG. 2  illustrates a flow chart of a method of designing a codebook for a 3D antenna configuration according to an embodiment of the invention; and 
           [0025]      FIG. 3  illustrates a flow chart of a method determining a codeword in a base station and a user equipment of a communication system applying a 3D antenna configuration according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0026]    Embodiments of the invention will be described below in details with reference to the drawings. 
         [0027]    A rectangular antenna array is a typical 3D antenna configuration in which a 3D beam can be generated. A method of the invention will be set forth in the following description in connection with a rectangular antenna array. 
         [0028]    For a geographically localized rectangular antenna array, each row antenna array has a similar spatial direction, and each column antenna array also has a similar spatial direction. This means that for each row/column antenna array, the same codeword could be used to quantize corresponding horizontal/vertical channel state information. The horizontal codebook characterizes the row antenna array, and the vertical codebook characterizes the column antenna array. The horizontal codebook and a vertical codebook can be newly designed codebooks or codebooks composed of a part or all of codewords in an existing Rel-10 codebook, since the Rel-10 codebook is designed for such a row/column antenna array. 
         [0029]    If both a horizontal codebook and a vertical codebook are composed of a part or all of codewords in an existing Rel-10 codebook, then different codebook subset restrictions could be done for various antenna types, such as a co-polarized antennas and a cross-polarized antenna. That&#39;s because only a part of the codewords may be suitable for the different antenna types. Since an existing Rel-10 codebook can be adopted, the method of designing a codebook for a 3D antenna configuration proposed in this invention has the advantage of good compatibility. 
         [0030]      FIG. 1  illustrates a rectangular antenna array deployed at a base station. As illustrated, the rectangular antenna array in  FIG. 1  is composed of N H  horizontal linear arrays and N V  vertical linear arrays, and the total number of antennas is N H ×N V . In the current LTE-A standard, N H =1, and in the invention, N H &gt;1 for a 3D antenna configuration, for example, N H =5 and N V =4. 
         [0031]      FIG. 2  illustrates a flow chart of a method of designing a codebook for a 3D antenna configuration according to an embodiment of the invention. The method of the invention will be illustratively described below in detail with reference to  FIG. 2 . 
         [0032]    This method starts with the step S 110 , and in the step S 120 , a first codeword characterizing a first antenna array in the 3D antenna configuration is selected from a first codebook. In the step S 130 , a second codeword characterizing a second antenna array in the 3D antenna configuration is selected from a second codebook. 
         [0033]    In the present embodiment, the 3D antenna configuration adopts the rectangular antenna array illustrated in  FIG. 1 . The first codebook is a horizontal codebook characterizing a row antenna array, and the second codebook is a vertical codebook characterizing a column antenna array. Thus in the step S 120 , particularly a horizontal codeword characterizing a horizontal linear antenna array in the rectangular antenna array is selected from the horizontal codebook as the first codeword; and a vertical codeword characterizing a vertical linear antenna array in the rectangular antenna array is selected from the vertical codebook as the second codeword. 
         [0034]    In the step S 140 , the first codeword is multiplied with the second codeword to obtain a corresponding codeword in the codebook for the 3D antenna configuration. The method ends in the step S 150 . 
         [0035]    Particularly the first codeword is multiplied with the second codeword according to a predetermined rule which will be described below in details. 
         [0036]    Let U m   (r)  and V m   (r)  represent a horizontal codeword and a vertical codeword with an index m at the rank-r respectively. The sizes of the horizontal codebook and the vertical codebook are N bits and M bits respectively, and their codewords are typically unitary matrixes, that is, respective columns are orthogonal to each other. 
         [0037]    For the rank 1, by the multiplication of horizontal codeword and vertical codeword, the 3D codeword for all antennas could be easily derived. 
         [0038]    For a higher rank, each column of a 3D codeword can be obtained by multiplying certain columns of a horizontal codeword with a vertical codeword. Since an angle spread is much larger in a horizontal plane than in a vertical plane, the direction of a primary beam is preferably selected to be in the horizontal plane. For rank 1≦r≦N V , a 3D codeword is built from a horizontal codebook at the rank r and a vertical codebook at the rank 1. Particularly a 3D codeword with the index (mN+n) at the rank r can be obtained as below using a vertical codeword with an index of m at the rank 1 and a horizontal codeword with an index of n at the rank r: 
         [0000]        W   mN+n   (r)   =T   m,n   r   /∥T   m,n   r ∥;
 
         [0000]        T   m,n (:, k )=col( U   n   (r) (:, k )·( V   m   (1) ) T ),   (1)
       k=1, 2, . . . , r;   m=0, 1, . . . , 2 M −1;   n=0, 1, . . . , 2 N −1;       
 
         [0042]    Where col(A) represents a transformation of a matrix A into a column vector, and A(:,k) represents the k th  column of the matrix A. T m,n   r  is a N H N V ×r matrix. 
         [0043]    For rank N V &lt;r≦N V N H , a horizontal codebook at the rank N V  is used since the number of horizontal antennas is N V . To obtain another rank (r−N V ), higher rank vertical codebook should be used for 3D codebook construction. In the present embodiment, a rank number of vertical codebook is selected as ┌r/N V ┐, since the direction of a primary beam is selected to be in a horizontal plane. 
         [0044]    A 3D codeword with the index (mN+n) at the rank r can be obtained in the following equation using a vertical codeword with an index of m at the rank ┌r/N V ┐ and a horizontal codebook with an index of n at the rank N V : 
         [0000]        W   mN+n   (r)   =T   m,n   N     V     ┌r/N     V     ┐ (:,1: r )/∥ T   m,n   N     V     ┌r/N     V     ┐ (:,1: r )∥;
 
         [0000]        T   m,n   N     V     ┌r/N     V     ┐ (:, N   V ( k   1 −1)+ k   2 )=col( U   n   (N     V     ) (:, k   2 )·( V   m   (┌r/N     V     ┐) (:, k   1 )) T ),  (2)
       k 1 =1, 2, . . . , ┌r/N V ┐;   k 2 =1, 2, . . . , N V ;   m=0, 1, . . . , 2 M −1;   n=0, 1, . . . , 2 N −1
 
Where F m,n   r  is a N H N V ×r matrix.
       
 
         [0049]    As can be apparent from the foregoing description, the obtained 3D codewords have the attribute of a unitary matrix. 
         [0050]      FIG. 3  illustrates a flow chart of a method determining a codeword in a base station and a user equipment of a communication system with a 3D antenna configuration according to an embodiment of the invention. In the step S 310  of the method, the user equipment transmits information about a first index n of a first codeword in a first codebook, information about a second index m of a second codeword in a second codebook and information about a rank r to the base station according to a predetermined criterion, where the first codeword characterizes a first antenna array in the 3D antenna configuration, and the second codeword characterizes a second antenna array in the 3D antenna configuration. The predetermined criterion is a maximum capacity criterion or another criterion which can occur to those skilled in the art. The base station receives the foregoing information transmitted from the user equipment in the step S 310  and determines a target codeword corresponding to the received first index n, second index m and rank r in Equation (1) and Equitation (2) in the step S 320 . 
         [0051]    Those skilled in the art shall appreciate that the foregoing embodiments are illustrative but not limiting. Different technical features appearing in different embodiments can be combined to advantage. Those skilled in the art shall appreciate and implement other variant embodiments of the disclosed embodiments upon reviewing the drawings, the description and the claims. In the claims, the term “comprising” will not preclude another step(s); the indefinite article “a/an” will not preclude plural; and the terms “first”, “second”, etc., are intended to designate a name but not to represent any specific order. Some technical features appearing in different dependent claims will not indicate that these technical features can not be combined to advantage.