Abstract:
An embodiment of the present invention provides a method of using differential precoding feedback for correlated channels, comprising, transmitting by a mobile station (MS) as feedback an index angle for a differential discrete Fourier transform (DFT) codeword corresponding to a shift of a dominant beam represented by a base codeword, where the feedback corresponds to a precoding vector V(t)=Q({circumflex over (θ)}) for index angle {circumflex over (θ)}, where 
               Q   ⁡     (     θ   ^     )       =     [         1             ⅇ     j   ⁢           ⁢   2   ⁢           ⁢   π   ⁢           ⁢     m   /   32                   ⅇ     j   ⁢           ⁢   4   ⁢           ⁢   π   ⁢           ⁢     m   /   32                   ⅇ     j   ⁢           ⁢   6   ⁢           ⁢   π   ⁢           ⁢     m   /   32               ]           
and m=−16 cos({circumflex over (θ)}).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 12/757,109, filed Apr. 9, 2010, entitled “TECHNIQUES USING DIFFERENTIAL PRECODING FOR HIGHLY CORRELATED CHANNELS IN WIRELESS NETWORKS,” which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     In orthogonal frequency multiple access—multiple input multiple output (OFDMA-MIMO) based broadband radio systems such as, but not limited to, those that conform to the institute for electronic and electrical engineers (IEEE) 802.16m or LTE (Long Term Evolution—3GPP 4G technology) systems, beamforming is an effective method to improve the receiving signal to noise ratio (SNR). When the base station&#39;s (BS&#39;s) transmit antennas are closely mounted, the antenna beam pattern usually has low spatial selectivity, and thus the principle Eigen mode dominates the capacity. In addition, the elements of the ideal precoding vectors are usually constant modulus for highly correlated antennas. 
     Thus, a strong need exists for techniques using differential precoding for highly correlated channels in wireless networks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which: 
         FIG. 1  provides an antenna array response of base codeword and the differential codewords according to embodiments of the present invention; and 
         FIG. 2  illustrates a system according to embodiments of the present invention. 
     
    
    
     It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding or analogous elements. 
     DETAILED DESCRIPTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the preset invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention. 
     Although embodiments of the invention are not limited in this regard, discussions utilizing terms such as, for example, “processing,” “computing,” “calculating,” “determining,” “establishing”, “analyzing”, “checking”, or the like, may refer to operation(s) and/or process(es) of a computer, a computing platform, a computing system, or other electronic computing device, that manipulate and/or transform data represented as physical (e.g., electronic) quantities within the computer&#39;s registers and/or memories into other data similarly represented as physical quantities within the computer&#39;s registers and/or memories or other information storage medium that may store instructions to perform operations and/or processes. 
     Although embodiments of the invention are not limited in this regard, the terms “plurality” and “a plurality” as used herein may include, for example, “multiple” or “two or more”. The terms “plurality” or “a plurality” may be used throughout the specification to describe two or more components, devices, elements, units, parameters, or the like. For example, “a plurality of stations” may include two or more stations. 
     Embodiments of the present invention provide a differential codebook optimized for highly correlated antennas and may be adapted to the “downloadable” codebook in LTE. Compared with existing differential precoding schemes, the proposed codebook of the present invention has better performance, especially for a high speed environment and lower complexity at a mobile station (MS). 
     Assuming there are N t  transmit antenna, V(t−1) is the precoding vector used at the last frame and V(0) is the base codeword selected at the first frame of a differential period (e.g. 4 frames). Firstly, an MS needs to measure the short term channel covariance matrix
 
R=H H H.  (1)
 
     Secondly, the MS needs to determine the feedback with the following criteria: 
     
       
         
           
             
               
                 
                   
                     
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     which is the angle corresponding to the maximum of antenna array responds; θ s  is a set of predefined angles in a range of [−α,α] degree (α=15 is recommended). Then the index of {circumflex over (θ)} is feedback to a base station (BS). 
     Based on the {circumflex over (θ)} that is feedback from the MS, the BS reconstruct precoding vector as following:
 
 V ( t )= Q ({circumflex over (θ)})  (6)
 
     Table I below compares the performance of the existing differential precoding scheme and the differential precoding scheme according to embodiments of the present invention. The performance when only a base codebook is used is also listed for comparison. For a base codebook only scheme, 6 bits is fed back every frame by MS; for the two differential schemes, 4 bits is feedback from MS at the first frame in each reset period (4 frames) to select the 16 DFT codeword in base codebook, and 4 bits is feedback for differential codebook selection in the succeed frames. It can be seen that embodiments of the present invention outperforms a 16 m differential scheme except when the 5 degree 16 m differential codebook is used and MS speed is low. Moreover, the 20 degree 16 m differential codebook cannot track the changes of a channel when an MS moves with a high speed. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Performance comparisons of differential precoding schemes 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 3 km/h 20 degree 
                 30 km/h 20 degree 
                 120 km/h 20 degree 
               
             
          
           
               
                 SNR(dB) 
                 0 
                 4 
                 8 
                 12 
                 0 
                 4 
                 8 
                 12 
                 0 
                 4 
                 8 
                 12 
               
               
                   
               
               
                 Base_only 
                 2.8553  
                 4.0224 
                 5.2765 
                 6.5729 
                 2.8099 
                 3.971 
                 5.2217 
                 6.5165  
                 2.807 
                 3.9688 
                 5.22 
                 6.5151 
               
               
                 Diff_16 m  
                 2.8556  
                 4.0227  
                 5.2768 
                 6.5732 
                 2.8042 
                 3.9646 
                 5.2149 
                 6.5094 
                 2.8003 
                 3.9613 
                 5.2121 
                 6.507 
               
               
                 Diff_new  
                 2.8657  
                 4.0341 
                 5.2888 
                 6.5855 
                 2.8182 
                 3.9803 
                 5.2316 
                 6.5265 
                 2.8173 
                 3.9802 
                 5.232 
                 6.5274 
               
               
                   
               
             
          
           
               
                   
                 3 km/h 5 degree 
                 30 km/h 5 degree 
                 120 km/h 5 degree 
               
             
          
           
               
                 SNR(dB) 
                 0 
                 4 
                 8 
                 12 
                 0 
                 4 
                 8 
                 12 
                 0 
                 4 
                 8 
                 12 
               
               
                   
               
               
                 Base_only 
                 2.8553 
                 4.0224 
                 5.2765 
                 6.5729 
                 2.8099 
                 3.971 
                 5.2217 
                 6.5165 
                 2.807 
                 3.9688 
                 5.22 
                 6.5151 
               
               
                 Diff_16 m 
                 2.8665  
                 4.0349 
                 5.2898 
                 6.5865 
                 2.8137 
                 3.9753 
                 5.2263 
                 6.5211 
                 2.8112 
                 3.9734 
                 5.2249 
                 6.5201 
               
               
                 Diff_new  
                 2.8657 
                 4.0341 
                 5.2888 
                 6.5855 
                 2.8182 
                 3.9803 
                 5.2316 
                 6.5265 
                 2.8173 
                 3.9802 
                 5.232 
                 6.5274 
               
               
                   
               
             
          
         
       
     
     The principle of the present invention may be similar to the existing 802.16m precoding scheme. However, embodiments of the present invention provide a difference in equation (2) set forth above, where in the existing 16 m scheme U=[V(t−1) V(t−1) ⊥ ] H , and Q is the predefined differential codebook where the differential codeword around the center codeword with a fixed degree, e.g. 20 degree. 
     For highly correlated antennas, the elements of precoding codeword are constant modulus in most case. The proposed differential codebook realizes this property by rotating the elements of the DFT codeword in a base codebook with some corresponding angles. This will keep the constant modulus property of DFT codeword after differential operation. This is preferred for power amplifier at the radio chains. 
     With a properly designed differential codebook, the dominate beam of an antenna array response of differential codeword may shift within a predefined angle compared with the beam of the corresponding base codeword. This property will guarantee a maximum gain in the principle Eigen mode of the channel in the viewpoint of an antenna array response. 
     Looking now at  FIG. 1 , shown generally as  100 , shows this property clearly as Gain vs. AoD. Line  110  shows the antenna array response of a base codeword and lines  120  are the antenna array response after the proposed differential scheme. Lines  130  are the antenna array response after the 16 m differential scheme. 
     In addition, the Q operation is only needed to be performed one time in a reset period at the MS instead of being calculated every frame if the shifting angle range is designed properly (for example, but not limited to [−20 20] degrees may be recommended). Thus the complexity is reduced at MS. 
     Turning to  FIG. 2  at  200  provides a system diagram according to embodiments of the present invention, in which an 802.16 MS  220  and applications  230  are collocated in a mobile user terminal  210 , such as, but not limited to mobile phone, laptop, PDA etc. MS and a base station (BS)  240  may communicate wirelessly. Both MS and BS may utilize transceivers that operate according to the embodiments set forth herein. 
     A further embodiment of the present invention may provide a computer readable medium encoded with computer executable instructions, which when accessed, cause a machine to perform operations comprising using differential precoding for highly correlated channels in a wireless network, comprising by using a differential codebook optimized for highly correlated antennas. 
     While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents may occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.