Abstract:
The present invention discloses a method for selecting one or more downlink beamforming vectors for a wireless channel to create beamformed signals. The method comprises estimating a downlink channel covariance matrix by using an uplink covariance matrix of the wireless channel, computing a plurality of downlink eigenvectors of a downlink channel by using the downlink channel covariance matrix, generating a plurality of downlink beamforming weighting vectors by using the plurality of downlink eigenvectors of the downlink channel, and selecting one or more dominant downlink beamforming weighting vectors from the plurality of downlink beamforming weighting vectors based on feedback from a mobile station (MS), wherein the downlink beamformed signals are created by applying the corresponding one or more dominant beamforming weighting vectors to an antenna array of a base transceiver station.

Description:
CROSS REFERENCE 
       [0001]    The present application claims the benefit of U.S. Provisional Application Ser. 60/854,217, which was filed on Oct. 24, 2006. 
     
    
     BACKGROUND 
       [0002]    In a wireless communications network employing a beamforming method, the quality of downlink signals received by a mobile station (MS) is determined by beamforming weighting vectors of a downlink channel, which are created from the covariance matrix of the downlink channel. However, lack of information about channel coefficients of a downlink channel makes it difficult for a base transceiver station (BTS) to obtain optimal downlink beamforming weighting vectors, especially in a fast fading environment employing frequency division duplex (FDD) network or time division duplex (TDD). 
         [0003]    A downlink beamforming weighting vector can be computed using a downlink channel covariance matrix, which is obtained from an uplink channel covariance matrix. Since a BTS transmits signals to an MS using the downlink beamforming weighting vector, it needs the MS to provide constant feedback on the performance of the network. The feedback received from the MS helps the BTS to decide how to modify the downlink beamforming weighting vectors in order to maintain or enhance the performance of the wireless network. 
         [0004]    In a conventional wireless network, the BTS applies a beamforming weighting vector to the antennas on an MS before transmitting signals to the MS. The BTS continues transmitting signals with the same beamforming weighting vector. Since channel condition is not static, the weight vector may not consistently yield the same level of network performance. 
         [0005]    As such, what is desired is a method for improving the performance of the wireless network utilizing a set of beamforming weighting vectors according to the feedback received from an MS. 
       SUMMARY 
       [0006]    The present invention discloses a method for selecting one or more downlink beamforming vectors for a wireless channel to create beamformed signals. The method comprises estimating a downlink channel covariance matrix by using an uplink covariance matrix of the wireless channel, computing a plurality of downlink eigenvectors of a downlink channel by using the downlink channel covariance matrix, generating a plurality of downlink beamforming weighting vectors by using the plurality of downlink eigenvectors of the downlink channel, and selecting one or more dominant downlink beamforming weighting vectors from the plurality of downlink beamforming weighting vectors based on feedback from a mobile station (MS), wherein the downlink beamformed signals are created by applying the corresponding one or more dominant beamforming weighting vectors to an antenna array of a base transceiver station. 
         [0007]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0008]    The drawings accompanying and forming part of this specification are included to depict certain aspects of the invention. The invention may be better understood by reference to one or more of these drawings in combination with the description presented herein. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale. 
           [0009]      FIG. 1  is a flow chart diagram illustrating a method in accordance with the present invention. 
       
    
    
     DESCRIPTION 
       [0010]    The following detailed description of the invention refers to the accompanying drawings. The description includes exemplary embodiments, not excluding other embodiments, and changes may be made to the embodiments described without departing from the spirit and scope of the invention. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
         [0011]    The present invention discloses a method for selecting one or more dominant downlink beamforming weighting vectors that yield the best performance for a wireless communication. The selection of the one or more dominant downlink beamforming weighting vectors is based on a probing-and-feedback method. In other words, out of a set of downlink beamforming weighting vectors, a base transceiver station (BTS) chooses at least one vector that yields the best performance based on feedback received from a mobile station (MS). 
         [0012]    The present invention significantly improves the efficiency of generating beamforming weighting vectors of a downlink channel in Macrocell/Microcell systems without incurring high computational complexity. The present invention is presented in a network employing frequency division duplex (FDD) Orthogonal Frequency Division Multiple Access (OFDMA). Nonetheless, it can be easily extended to wireless networks employing TDD/FDD Code Division Multiple Access (CDMA) or TDD/FDD OFDMA due to the fact that in these methods an uplink channel covariance matrix is also used to generate a downlink channel covariance matrix from which downlink beamforming weighting vectors are calculated. 
         [0013]    In an FDD system, a downlink channel covariance matrix is obtained by having a predetermined speculative transformation matrix multiplied by an uplink channel covariance matrix. A predetermined speculative transformation matrix is a function of system parameters of a wireless network. The parameters include the number of antennas, the spacing of antennas, the number of sectors, and uplink and downlink carrier frequencies. By contrast, in a TDD system an uplink channel covariance matrix is used as a downlink channel covariance matrix 
         [0014]    After downlink beamforming weighting vectors are generated from a downlink channel covariance matrix, they are ranked according to some predetermined rules. A predetermined number of highest-ranking downlink beamforming weighting vectors are selected for generating beamformed signals. Each of the selected beamforming weighting vectors is applied to the antenna array of the BTS and the beamformed signals are transmitted to an MS. The MS sends the BTS feedback about the performance of the network regularly. The BTS uses the feedback to determine the selection of one or more beamforming weighting vectors for subsequent transmissions of downlink signals. 
         [0015]      FIGURE 1  is a flow chart diagram illustrating a method in accordance with one embodiment of the present invention. In step  110 , channel coefficients of an uplink channel for a desired wireless station is estimated to be H ul =[H ul,1  H ul,2  . . . H ul,M ] T , where operator [*] T  represents vector transposition operation and M is the number of antennas in a BTS. Let R ul  be an uplink channel covariance matrix. An uplink instantaneous channel covariance matrix is computed according to the following equation: R ul =H ul (H ul ) H , where operator [*] H  represents Hermitian operation. An uplink average channel covariance matrix is computed according to the following equation: 
         [0000]    
       
         
           
             
               
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         [0000]    where N e  is the number of samples and N e  is between [1,∞). 
         [0016]    In step  120 , a downlink channel covariance matrix is computed by using an uplink channel covariance matrix. In an FDD system the computation is based on the following equation: R dl =R ul C T , where R dl  is a downlink channel covariance matrix; R ul  is an uplink channel covariance matrix; and C T  is a predetermined speculative transformation matrix. The speculative transformation matrix C T  is an M by M matrix. On the other hand, in a TDD system the computation is based on the following equation: R dl =R ul    
         [0017]    In step  130 , by using singular value decomposition (SVD), a sorted list of M eigenvalues is obtained from the downlink channel covariance matrix. Let {λ 1 ,λ 2 , . . . λ M } denote M eigenvalues of a downlink channel covariance matrix R dl  with |λ 1 |≧|λ 2 |≧ . . . ≧|λ M |, where |λ i | is the absolute value of the ith eigenvalue λ i . 
         [0018]    The SVD process also yields a set of M eigenvectors that corresponds to the set of M eigenvalues. Let the M eigenvectors be {U 1 , U 2 , . . . , U M }, where an M by 1 vector U i  is normalized as the Euclidean norm of vector U i , i.e. ∥U i ∥=√{square root over (U i   H U i )}=1. 
         [0019]    According to a predetermined rule, downlink beamforming weighting vectors are generated from the set of eigenvectors and ranked (see step  140 ). The table below shows a set of beamforming weighting vectors, and the first column represents the rank of a beamforming weighting vector. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Priority level 
                 Weighting Vector W i  (M by 1) 
               
               
                   
                   
               
             
             
               
                   
                 1 
                 W 1  = U 1   
               
               
                   
                 2 
                 W 2  = U 2   
               
               
                   
                 3 
                 W 3  = (aU 1  + bU 2 )/((aU 1  + bU 2 ) H  (aU 1  + bU 2 )) 
               
               
                   
                 4 
                 W 4  = H UL   
               
               
                   
                 5 
                 W 5  = U 3   
               
               
                   
                 6 
                 W 6  = U 4   
               
               
                   
                   
               
             
          
         
       
     
         [0020]    The coefficients a and b of a beamforming weighting vector W 3  are predetermined according to channel condition. 
         [0021]    The BTS selects one or more highest-ranking downlink beamforming weighting vectors as candidates for creating beamformed signals. It applies one vector at a time to the antenna array of the BTS (step  150 ), and the beamformed signals are transmitted to a MS. 
         [0022]    The MS that receives transmitting signals from the BTS assesses the quality of the receiving signals and returns feedback to the BTS. The feedback from the MS is sent in one or a combination of the following forms: an acknowledgement (ACK) or a negative acknowledgement (NAK) message depending on whether signal quality exceeds a predetermined threshold and a grading message indicating the grade of receiving signals. The grades for receiving signals are good, fair and bad in a three-level grading system. On the other hand, the grades are very good, good, fair, bad and very bad in a five-level grading system. 
         [0023]    One embodiment of the present invention is that feedback is sent as an ACK or NAK message. According to the ranking of beamforming weighting vectors, the BTS chooses a first dominant beamforming weighting vector from the set of the beamforming weighting vectors and creating beamformed signals. If the BTS receives an ACK message from the MS, the BTS continues using the first dominant beamforming weighting vector until the MS returns a NAK message indicating that the quality of signals is below a predetermined threshold. Consequently, the BTS selects a second dominant beamforming weighting vector according to the rank from the set of the beamforming weighting vectors to create beamformed signals. This embodiment of the present invention incurs no additional overhead and it only uses one beamforming weighting vector at a time to transmit signals. 
         [0024]    Another embodiment of the present invention is that feedback from the MS is sent as a grading message. The BTS chooses multiple dominant beamforming weighting vectors from the set of beamforming weighting vectors. The BTS uses the dominant beamforming weighting vectors sequentially to create beamformed signals. The MS assesses the quality of the receiving signals transmitted using each dominant beamforming weighting vector and returns a grading message to the BTS. The BTS uses the feedback from the MS and determines how to use the dominant beamforming weighting vectors according to a predetermined rule. 
         [0025]    For example, if one beamforming weighting vector has a grade better than the rest, the BTS uses the dominant beamforming weighting vector to transmit signals. However, if multiple beamforming weighting vectors have a grade better than the rest, the BTS uses these multiple dominant beamforming weighting vectors to create beamformed signals according to a predetermined coding method, which increases the diversity and the coding gain of the wireless network. 
         [0026]    Still another embodiment of the present invention is that the feedback from the MS includes an ACK message and a grading message. Based on the information contained in both ACK and grading messages, the BTS selects one or more dominant beamforming weighting vectors to transmit signals. 
         [0027]    The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
         [0028]    Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.