Abstract:
A method and an apparatus apply an adaptive weight in a wireless communication system. In the method, channel estimation is performed. A weighting factor that reduces a Mean Square Error (MSE) is determined with respect to a channel in a specific section. A channel estimate value is multiplied by the weighting factor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY 
       [0001]    The present application is related to and claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed in the Korean Intellectual Property Office on Nov. 4, 2010 and assigned Serial No. 10-2010-0109007, the entire disclosure of which is hereby incorporated by reference. 
       TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method and an apparatus for applying an optimized adaptive weight to an output of a channel estimator in order to raise accuracy of the channel estimator used for estimating a channel gain in a Pilot Interference Canceller (PIC). 
       BACKGROUND OF THE INVENTION 
       [0003]    A Code Division Multiple Access (CDMA) communication system uses a PIC technology that cancels a pilot in order to improve a cell performance of an uplink. 
         [0004]    However, a weight determining method used in the PIC cannot have an optimized performance in an aspect of a Mean Square Error (MSE). 
         [0005]    That is, since the weight determining method used in the PIC has a structure that uses a constant weighting factor, the method cannot apply an instantaneously optimized weighting factor and so an MSE gets large. 
         [0006]    Therefore, a method and an apparatus that can apply an instantaneously optimized weighting factor are required. 
       SUMMARY OF THE INVENTION 
       [0007]    To address the above-discussed deficiencies of the prior art, it is a primary aspect of the present invention is to provide a method and an apparatus for a PIC channel estimator considering a weight. 
         [0008]    Another aspect of the present invention is to provide a method and an apparatus for improving a performance of a channel estimator by applying a weighting factor that uses a weight algorithm having a minimum (or reduced) Mean Square Error (MSE) in a short term average to an output of the channel estimator in a CDMA communication system. 
         [0009]    In accordance with an aspect of the present invention, a method for applying an adaptive weight in a wireless communication system is provided. The method includes performing channel estimation, determining a weighting factor that reduces a Mean Square Error (MSE) with respect to a channel in a specific section, and multiplying a channel estimate value by the weighting factor. 
         [0010]    In accordance with another aspect of the present invention, an apparatus for applying an adaptive weight in a wireless communication system is provided. The apparatus includes a channel estimator for performing channel estimation, and a weighting block for determining a weighting factor that reduces a Mean Square Error (MSE) with respect to a channel in a specific section, and multiplying a channel estimate value by the weighting factor. 
         [0011]    In accordance with still another aspect of the present invention, a method for canceling a pilot interference in an uplink of a Code Division Multiple Access (CDMA) communication system is provided. The method includes over-sampling a received sample, generating a Pseudo Noise (PN) sequence, despreading the over-sampled sample by a Spreading Factor (SF) to generate a signal of a symbol basis, performing channel estimation on the signal of the symbol basis, and determining a weighting factor reducing a Mean Square Error (MSE) with respect to a channel in a specific section and multiplying a channel estimate value by the weighting factor. 
         [0012]    In accordance with further another aspect of the present invention, an apparatus for canceling a pilot interference in an uplink of a Code Division Multiple Access (CDMA) communication system is provided. The apparatus includes an interpolator for over-sampling a received sample, a Pseudo Noise (PN) generator for generating a Pseudo Noise (PN) sequence, a despreader for despreading the over-sampled sample by a Spreading Factor (SF) to generate a signal of a symbol basis, a channel estimator for performing channel estimation on the signal of the symbol basis, and a weighting block for determining a weighting factor reducing a Mean Square Error (MSE) with respect to a channel in a specific section and multiplying a channel estimate value by the weighting factor. 
         [0013]    Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
           [0015]      FIG. 1  is a view illustrating a general PIC structure applied to an uplink of a CDMA communication network according to an exemplary embodiment of the present invention; 
           [0016]      FIG. 2  is a block diagram illustrating a weighting block according to an exemplary embodiment of the present invention; 
           [0017]      FIG. 3  is a block diagram illustrating a weighting block according to an exemplary embodiment of the present invention; 
           [0018]      FIG. 4  is a block diagram illustrating an IIR filter for an averaging unit according to an exemplary embodiment of the present invention; and 
           [0019]      FIG. 5  is a flowchart illustrating a process for determining a weighting factor according to an exemplary embodiment of the present invention. 
       
    
    
       [0020]    Throughout the drawings, like reference numerals will be understood to refer to like parts, components and structures. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIGS. 1 through 5 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless communication system. 
         [0022]    The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
         [0023]    The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
         [0024]    Exemplary embodiments of the present invention provide a method and an apparatus for a PIC channel estimator considering a weight. 
         [0025]    Exemplary embodiments of the present invention use a PIC that cancels a pilot in order to improve a cell performance of an uplink in a CDMA system. 
         [0026]      FIG. 1  is a view illustrating a general PIC structure applied to an uplink of a CDMA communication network according to an exemplary embodiment of the present invention. 
         [0027]    Referring to  FIG. 1 , the PIC includes an interpolator  101 , a despreader  102 , a Pseudo Noise (PN) generator  103 , a channel estimator  104 , a weighting block  105 , a pilot regenerator  106 , a pilot canceller  107 , and a summation  108 . 
         [0028]    The interpolator  101  over-samples a received sample r(k) by I time. Here, “I” denotes an over-sampling ratio of the interpolator  101 . 
         [0029]    The PN generator  103  generates a PN sequence used for an i-th user to provide the same to the despreader  102  and the pilot regenerator  106 . 
         [0030]    The despreader  102  despreads a signal received from the interpolator  101  by a Spreading Factor (SF) to generate a signal x i,j (n) of a symbol basis and provides the generated signal to the channel estimator  104 . 
         [0031]    The channel estimator  104  filters the signal x i,j (n) provided by the despreader  102  to generate an estimated channel gain ĥ i,j (n) and provides the same to the weighting block  105 . 
         [0032]    The weighting block  105  applies a weight to an estimated channel gain ĥ i, j (n) by a generated weighting factor. An example of a weighting factor generating block used for a general PIC is illustrated in  FIG. 2 . A channel estimate value α f     α   ĥ i,j (n) to which a weight has been applied using a constant weighting factor α(n)=α f     α    as in  FIG. 2  is provided to the pilot regenerator  106 . 
         [0033]    The pilot regenerator  106  generates a pilot y i,j (k) to be cancelled every user using a channel estimate value α i,j (n)ĥ i,j (n) to which a weight has been applied, a pilot pattern, and a PN sequence. 
         [0034]    The summation  108  sums all of pilots y i,j (k) of every user generated by the pilot regenerator  106  and provides the same to the pilot canceller  107 . 
         [0035]    The pilot canceller  107  cancels pilots y i,j (k) of every user from a received signal r(k) to generate a sample r p (k) where a pilot has been cancelled. 
         [0036]      FIG. 2  is a block diagram illustrating a weighting block according to an exemplary embodiment of the present invention. 
         [0037]    Referring to  FIG. 2 , the weighting block includes a multiplier  202 . The multiplier  202  multiplies a received channel estimate value of the channel estimator  104  and a constant weighting factor α(n)=α f     α    and outputs a result value α f     α   ĥ i,j (n) 
         [0038]    The weighting block provides the channel estimate value α f     α   ĥ i,j (n) to which a weight has been applied to the pilot regenerator  106 . 
         [0039]      FIG. 3  is a block diagram illustrating a weighting block according to an exemplary embodiment of the present invention. 
         [0040]    Referring to  FIG. 3 , the weighting block includes a complex conjugate multiplier  302 , a power calculator  303 , averaging units  304  and  305 , a real number unit  306 , a noise canceller  307 , a divider  308 , and a multiplier  309 . The complex conjugate multiplier  302  includes a complex conjugate unit  302 - 1  for outputting a complex conjugate of an input value. 
         [0041]    The complex conjugate unit  302 - 1  of the complex conjugate multiplier  302  obtains a complex conjugate h* i,ĵ (n) of an output h i,ĵ (n) of the channel estimator  104 , and the complex conjugate multiplier  302  multiplies the obtained complex conjugate h* i,ĵ (n) and a signal x i,j (n) to be input to the channel estimator  104  to provide a result value thereof to the averaging unit E( )  304 . 
         [0042]    The averaging unit  304  obtains an average E[x i,j (n)h i,ĵ (n)*] for a short time from an output value of the complex conjugate multiplier  302  to provide the same to the real number unit Re( )  306 . 
         [0043]    The real number unit Re( )  306  selects only a real part Re{E[x i,j (n)h i,ĵ (n)*]} from the output E[x i,j (n)h h i,ĵ (n)*] of the averagi unit  304  and provides the same to the noise canceller  307 . 
         [0044]    The noise canceller  307  cancels a noise from the output Re{E[x i,j (n)h i,ĵ (n)*]} provided by the real number unit  306  and provides a result thereof Re{E[x i,j (n)h i,ĵ (n)*]}−w(0)σ i,j   2  to the divider  308 . 
         [0045]    The power calculator  303  determines a square |ĥ i,j (n)| 2  of an absolute value of an output of the channel estimator  104 and provides the same to the averaging unit E( )  305 . 
         [0046]    The averaging unit  305  determines an average E[|h i,j (n)| 2  ] for a short time from the value |ĥ i,j (n) provided by the power calculator  303  and provides the same to the divider  309 . 
         [0047]    Here, the averaging units E( )  304  and  305  perform a function of determining an average for a short time, that is, determining an instantaneous average. The averaging units  304  and  305  may use an IIR filter having a coefficient of α IIR  as in  FIG. 4  which will be described later. 
         [0048]    The divider  308  outputs a value 
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         [0000]    obtained by dividing an output Re{E[x i,j (n)h i,ĵ (n)*]}−w(0)σ i,j   2  of the noise canceller  307  by an output E[|ĥ i,j (n)| 2 ] of the averaging unit  305 . 
         [0049]    The multiplier  309  outputs a value α i,j (n)h i,ĵ (n) obtained by multiplying an output value α i,j (n) of the divider  308  by an output value h i,ĵ (n) of the channel estimator  104 . 
         [0050]      FIG. 4  is a block diagram illustrating an IIR filter for an averaging unit according to an exemplary embodiment of the present invention. 
         [0051]    Referring to  FIG. 4 , the averaging units  304  and  305  include a multiplier  1   401 , a multiplier  2   402 , and a shift register  403 . 
         [0052]    The multiplier  1   401  multiplies an input signal by α IIR , and the multiplier  2   402  multiplies an output of the shift register  403  by 1−α IIR . 
         [0053]    The output of the multiplier  1   401  and the output of the multiplier  2   402  are added to each other, and the added value is input to the shift register  403 . 
         [0054]    Generally, the channel estimate value estimated by the channel estimator  104  may be expressed by Equation (1). 
         [0000]        h   i,j ( n )= Ŵ   T   X   i,j ( n )   (1)
 
         [0055]    where W T  is a matrix represented by approximation using a FIR filter having a sufficiently large Tap number (=2L+1), and may be expressed by Equation (2). 
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         [0056]    In Equation (2), w(1) is an 1-th channel estimate coefficient when approximation has been performed using a FIR filter. 
         [0057]    In addition, in Equation (1), X i,j (k) is an input symbol signal x i,j (n) represented in terms of a matrix and has a structure as in Equation (3). 
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         [0058]    In Equation (3), x i,j (n) may be expressed by Equation (4). 
         [0000]        x   i,j ( n )= h   i,j ( n )+ g   i,j ( n )   (4)
 
         [0059]    In Equation (4), h i, j (n)is a channel matrix, and g i,j (n) is a Gaussian noise having a dispersion of σ i,j   2 . 
         [0060]    When an MSE is minimized (or reduced) in a short term, an MSE is minimized (or reduced) in a long term, so that a weighting factor α(n) minimizing (or reducing) an MSE for a short term can be determined. An MSE for a short term may be expressed by Equation (5). Here, a unit of a short term denotes a unit of n in Equation. 
         [0000]      MSE i,j ( n )= E[|h   i,j ( n )−α i,j ( n ) h   i,ĵ ( n )   (5)
 
         [0061]    Equation (5) may be changed into Equation (6). 
         [0000]      MSE i,j ( n )= E[|   i,j ( n )| 2 ]−2α i,j ( n )Re{ E[h   i,j ( n ) h   i,j ( n )*]}+α i,ĵ ( n ) 3   E[|h   i,ĵ ( n )| 2 ]  (6)
 
         [0062]    Equation (6) is differentiated to determine α i, j (n) minimizing (or reducing) MSE(n) as in Equation (7). 
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         [0063]    In Equation (7), w(0) is a zero-th coefficient of W T . 
         [0064]    A flowchart for Equation (7) is illustrated in  FIG. 5  which will be described below. 
         [0065]      FIG. 5  is a flowchart illustrating a process for determining a weighting factor according to an exemplary embodiment of the present invention. 
         [0066]    Referring to  FIG. 5 , when a weighting factor determining algorithm of the weighting block starts, the channel estimator  104  performs channel estimation using a received signal x i,j (n) and outputs a result value h i,ĵ (n) (step  502 ). 
         [0067]    The power calculator  303  determines a square (power) |h i,ĵ (n)| 2  of an output value of the channel estimator  104  (step  503 ), and the averaging unit  305  obtains an instantaneous average value E[|h i,ĵ (n)| 2 ] (step  504 ). 
         [0068]    Simultaneously (or at about a similar time) with the processes of steps  503  and  504 , a process for determining a numerator of a weighting factor using Equation (7) may be performed (steps  505 - 508 ). 
         [0069]    The complex conjugate multiplier  302  multiplies an input value x i,j (n) for the channel estimator  104  by a complex conjugate of a channel estimate value h i,ĵ (n) and outputs a result value x i,j (n)h i,ĵ (n)* (step  505 ). 
         [0070]    The averaging unit  304  obtains an instantaneous average E[x i,j (n)h i,ĵ (n)*] of an output value of the complex conjugate multiplier  302  (step  506 ). 
         [0071]    The real number unit  306  selects only a real part Re{E[x i,j (n)h i,ĵ (n)*]} from the instantaneous average (step  507 ). 
         [0072]    The noise canceller  307  outputs a value Re{E[x i,j (n)h i,ĵ (n)*]}−w(0)σ i,j   2  obtained by canceling a noise from an output value of the real number unit  306  (step  508 ). 
         [0073]    The divider  308  divides the output value Re{E[x i,j (n)h i,ĵ (n)*]}−w(0)σ i,j  of the noise canceller  307  by the output E[|h i,ĵ (n)| 2 ] of the averaging unit  305  and outputs a result value 
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         [0000]    (step  509 ). 
         [0074]    The multiplier  309  multiplies the output h i,ĵ (n) of the channel estimator  104  by the determined weighting factor α(n) and outputs a result thereof α i,j (n)h i,ĵ (step  510 ). 
         [0075]    Since exemplary embodiments of the present invention use a structure that applies an instantaneously optimized (or improved) weighting factor without using a structure that uses a constant weighting factor, they may prevent a problem that an MSE gets large. In addition, exemplary embodiments of the present invention reduce remaining pilot power when canceling a pilot interference by improving an MSE performance of a channel estimator used in a PIC. Since quality of a signal from which a pilot has been cancelled improves, a cell throughput of an uplink improves. 
         [0076]    Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.