Abstract:
An embodiment of the present invention provides a method used in transmit beam forming, comprising, providing differential feedback by using variable differential codebooks across a plurality of feedbacks wherein the differential codebooks may have different shapes, and/or spans, and/or the numbers of codewords across feedbacks.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of U.S. provisional application Ser. No. 61/142,582, filed Jan. 5, 2009, entitled “ADVANCED WIRELESS COMMUNICATION SYSTEMS AND TECHNIQUES” the entire content of which is incorporated by reference herein. 
    
    
     BACKGROUND 
     Transmit beamforming, which may improve wireless communications, requires the feedback of beamforming matrixes in a frequency division duplexing (FDD) mode. The feedback can be differential with respect to the previous feedback. Differential feedback exploits the correlation between the ideal beamforming matrixes across time in order to reduce feedback overhead and increase beamforming accuracy. A quantization codebook may be used to quantize the differential matrix for the feedback. Previously, quantization codebook is constant across time. This is not optimal. 
     Thus, a strong need exists for improvements in providing differential feedback used in transmit beamforming. 
    
    
     
       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  depicts differential codebook variation across feedbacks; 
         FIG. 2  depicts full quantization space and polar cap according to embodiments of the present invention; and 
         FIG. 3  shows channel capacity increases as differential feedback goes for constant and variable differential codebooks, respectively, 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. 
     As mentioned above, in the prior art of differential feedback, a constant differential codebook is used across feedbacks. For the example as shown as  100  of  FIG. 1  the feedback starts at  110  with an initial feedback for frame  0 , which uses a non-differential codebook. For frame  1   120 , the differential matrix with respect to the beamforming matrix of frame  0   110  is computed and quantized by a differential codebook, i.e. differential codebook  1 . Thus, previously the same differential codebook is used for the subsequent frames as seen by frame  2   130  and frame  3   140 . 
     However, this is not optimal. Embodiments of the present invention provide varying the differential codebook across a plurality of feedbacks. In practice, it is desirable to use the same number of feedback bits across frames. For the same number of bits, the initial feedback has an error greater than the latter differential feedbacks, because the initial feedback can not exploit any correlation (in time or frequency). To compensate for the large error, the differential codebook for frame  1   120  should have a span greater than the differential codebook for the latter frames, i.e. frame  2  and  3 . The span can be quantified by polar cap size in angle for a given codebook. As illustrated generally as  200  of  FIG. 2 , the greater the size of the ring, the greater the polar cap  220  size is. For example, for frame  1   120 , the polar cap  220  size may be 30 degrees and it reduces to 20 degrees for frame  2   130 , and then remains at 20 degrees for frame  3   140 , where the angle is measured between the center codeword and the edge codeword. The differential codebook may have a shape other than a ring with a center  220 . In general, the differential codebook may vary its shape, and/or span, and/or the number of codewords across feedbacks. 
     Looking now at  300  of  FIG. 3 , it is seen that channel capacity  310  increases as differential feedback  320  goes for constant and variable differential codebooks, respectively.  FIG. 3  shows simulation results for variable  330  and constant  340  polar caps, respectively. Although not limited in this respect, embodiments of the present invention show in  FIG. 3  a simulated MIMO configuration with 4 transmit and 2 receive antennas sending 1 spatial stream. The channel model exemplified is eITU with 3 km/h mobile speed. The transmit antenna spacing is half wavelength, which is desirable for multi-user MIMO. The channel capacity increases as the differential feedback goes. The capacity of a variable polar cap increases faster than that of a constant polar cap, and 1-3% performance gain can be achieved by using variable polar cap. For the variable polar cap, the polar cap sizes are 20, 15, and 5 degrees for the first, second, and third differential feedbacks, respectively, although the present invention is not limited in this respect. In contrast, for the constant polar cap, the polar cap size is always 5 degrees, where the 5 degree is the optimal cap size for the stead state of the differential feedback in this MIMO channel. 
     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.