Abstract:
An adaptive interference cancellation apparatus for cancelling a side lobe between neighboring cells installed in a repeater and capable of improving quality of a communication signal and preventing frequent handover by maintaining a serving base station signal among repeater signals and cancelling an interference signal from a neighboring cell is provided. The adaptive interference cancellation apparatus for a side lobe between neighboring cells, which is installed in a mobile communication repeater system comprising an omni-directional antenna, includes: three directional antennas installed around the omni-directional antenna; an output signal selector configured to select and output two of three directional antenna incident signals; and an adaptive noise canceller configured to adaptively filter the two directional antenna incident signals output from the output signal selector, estimate a noise reference signal, and extract a serving base station signal by subtracting the estimated noise reference signal from an omni-directional antenna incident signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2014-0057416, filed on May 13, 2014, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an adaptive interference cancellation apparatus for cancelling a side lobe between neighboring cells, and snore particularly, to an adaptive interference cancellation apparatus for cancelling a side lobe between neighboring cells which is installed in a repeater and can improve quality of a communication signal and prevent frequent handover by maintaining a serving base station signal among repeater signals and cancelling an interference signal from a neighboring cell. 
         [0004]    2. Discussion of Related Art 
         [0005]    In a mobile communication system, technology for cancelling interference of a base station signal between neighboring cells is very important technology for maintaining a quality of a serving base station signal 
         [0006]    Recently, as the prevalence of smart phones has expanded, wireless internet traffic is has been rapidly increasing. In order to solve this problem, more base stations are being installed, and cell radii of base stations are decreasing. Accordingly, as an intensity of a so-called side lobe which is an interference signal by a new base station input to a conventional repeater system which is installed in a radio wave shaded area such as an underground parking lot, etc. and reradiates by low-noise amplification of a base station signal increases, quality of the mobile communication signal is lowered. That is, since an interference signal by a newly-installed base station is mixed in the serving base station signal input to the conventional repeater system to cancel the radio wave shaded area, there is a problem in which the quality of a mobile communication signal is lowered and frequent handover occurs. 
         [0007]    Meanwhile, as a conventional art to cancel the interference signal from a neighboring cell, there is a method (refer to “conventional art 1” described below) of cancelling a pilot signal of a neighboring base station using characteristics of a pseudo noise (PN) code lowering an intensity of a signal other than its own signal at a very low level through inverse diffusion in a code division multiple access (CDMA)-based mobile communication system. 
         [0008]    In an orthogonal frequency division multiplexing access (OFDMA)-based mobile communication system, as a conventional art for cancelling an interference signal from a neighboring cell, there is a method (refer to “conventional art 2” described below) of using a carrier to noise interference ratio (CINR) of signals received from neighboring base stations and a serving base station. 
         [0009]    However, since conventional art 1 described above is technology using a unique PN code, conventional art 1 cannot be applied to an OFDMA-based mobile communication system such as a Long Term Evolution (LTE) system, and conventional art 2 is technology which is applied not to a repeater which is a transmitter but to a receiver, that is, a user terminal, to improve reception performance. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention is directed to an adaptive interference cancellation apparatus for cancelling a side lobe between neighboring cells which is installed in a repeater and can improve quality of a communication signal and prevent frequent handover by cancelling an interference signal from a neighboring cell while maintaining a serving base station signal among repeater signals. 
         [0011]    According to one aspect of the present invention, there is provided an adaptive interference cancellation apparatus for a side lobe between neighboring cells, which is installed in a mobile communication repeater system including an omni-directional antenna, including: three directional antennas installed around the omni-directional antenna; an output signal selector configured to select and output two of three directional antenna incident signals; and an adaptive noise canceller configured to adaptively filter the two directional antenna incident signals output from the output signal selector, estimate a noise reference signal, and extract a serving base station signal by subtracting the estimated noise reference signal from an omni-directional antenna incident signal. 
         [0012]    In the construction described above, the three directional antennas may be sector antennas installed at equal angles around the omni-directional antenna. 
         [0013]    The adaptive noise canceller may include: two adaptive filters configured to adaptively filter the two directional antenna incident signals and estimate the noise reference signal; an adder configured to subtract the noise reference signal output from each of the two adaptive filters from the omni-directional antenna incident signal; and a filter coefficient adjustment unit configured to adjust a filter coefficient of each of the two adaptive fillers to minimize an error signal which is the output signal of the adder. 
         [0014]    The filter coefficient adjustment unit may adjust the filter coefficient of each of the two adaptive filters through a least mean square algorithm to minimize the error signal. 
         [0015]    According to another aspect of the present invention, there is provided an adaptive interference cancellation apparatus for a side lobe between neighboring cells, which is installed in a mobile communication repeater system, including: three directional antennas; an output signal selector configured to select and output two of three directional antenna incident signals; and an adaptive noise canceller configured to adaptively filter the two directional antenna incident signals output from the output signal selector, estimate a noise reference signal, and extract a serving base station signal by subtracting the estimated noise reference signal from the one directional antenna incident signal which is not selected by the output signal selector. 
         [0016]    In the construction described above, the three directional antennas may be sector antennas installed at equal angles. 
         [0017]    The adaptive noise canceller may include: two adaptive filters configured to adaptively filter the two directional antenna incident signals and estimate the noise reference signal; an adder configured to subtract the noise reference signal output from each of the two adaptive filters from the one directional antenna incident signal; and a filter coefficient adjustment unit configured to adjust a filter coefficient of each of the two adaptive filters to minimize an error signal which is the output signal of the adder. 
         [0018]    The filter coefficient adjustment unit may adjust the filter coefficient of each of the two adaptive filters through a least mean square algorithm to minimize the error signal. 
         [0019]    According to still another aspect of the present invention, there is provided an adaptive interference cancellation apparatus for a side lobe between neighboring cells, including: three directional antennas; three adaptive interference cancellation units, each comprising an adaptive noise canceller configured to adaptively niter two of three directional antenna incident signals, estimate a noise reference signal, and extract a serving base station signal by subtracting the estimated noise reference signal from one directional antenna incident signal; and a multiplexer configured to selectively output one having the smallest value among the output signals of the three adaptive interference cancellation units. 
         [0020]    In the construction described above, each of the three adaptive interference cancellation units may include: an output signal selector configured to selectively output two of three directional antenna incident signals; and an adaptive noise canceller configured to adaptively filter the two directional antenna incident signals output from the output signal selector, estimate a noise reference signal, and extract a serving base station signal by subtracting the estimated noise reference signal from the one directional antenna incident signal which, is not selected by the output signal selector. 
         [0021]    The three directional antennas may be sector antennas installed at equal angles. 
         [0022]    Each of the adaptive noise canceller may include: two adaptive filters configured to adaptively filter the two directional antenna incident signals and estimate the noise reference signal; an adder configured to subtract the noise reference signal output from each of the two adaptive filters from the one directional antenna incident, signal; and a filter coefficient adjustment unit configured to adjust a filter coefficient of each of the two adaptive filters to minimize an error signal which is the output signal of the adder. 
         [0023]    The filter coefficient adjustment, unit may adjust the filter coefficient of each of the two adaptive filters through a least mean square algorithm to minimize the error signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The above and other objects, features and advantages of the present invention will, become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the accompanying drawings, in which: 
           [0025]      FIG. 1  is a block, diagram illustrating an adaptive interference cancellation apparatus for cancelling a side lobe between neighboring cells according to one exemplary embodiment of the present invention; 
           [0026]      FIG. 2  is a block diagram illustrating an adaptive interference cancellation apparatus for cancelling a side lobe between neighboring cells according to another exemplary embodiment of the present invention; and 
           [0027]      FIG. 3  is a block diagram illustrating a repeater system to which the adaptive interference cancellation apparatus shown in  FIG. 2  is applied. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0028]    Exemplary embodiments of the present invention will be described in detail below with reference to the accompanying drawings. While the present invention is shown and described in connection with exemplary embodiments thereof, it will be apparent to those skilled in the art that various modifications cars be made without departing from the spirit and scope of the invention. 
         [0029]      FIG. 1  is a block diagram illustrating an adaptive interference cancellation apparatus for cancelling a side lobe between neighboring cells according to one exemplary embodiment of the present invention, and illustrates an example which is applied to a conventional mobile communication repeater system. 
         [0030]    As shown in  FIG. 1 , a conventional mobile communication repeater system may perform low-noise amplification on a serving base station signal, and reradiate the low-noise amplified signal to a radio wave shaded area such as an underground parking lot etc. through an omni-directional antenna  20 . Accordingly, every base station signal of the serving cell as well as the neighboring cells may be incident to the omni-directional antenna  20 . 
         [0031]    Meanwhile, in this process, a side lobe excluding a main beam radiated from a new base station antenna may be mixed with the serving base station signal through the omni-directional antenna  20 , and to cancel this side lobe in the apparatus of the present invention, three directional antennas  10  for receiving base station signals of the serving cell and the neighboring cells, for example, three sector antennas (S/A 1 , S/A 2 , and S/A 3 )  12 ,  14  and  16 , may be further installed. 
         [0032]    In  FIG. 1 , a dotted oval represents a horizontal radiation (incident) pattern of each of the sector antennas  12 ,  14 , and  16 , and a conventional sector antenna may have a beam width of 66°, In the present invention, three sector antennas  12 ,  14  and  16  may be installed at equal angles of 120° around the omni-directional antenna  20  in a radio wave shaded area, for example, in an upper space of the omni-directional antenna  20 , and receive base station signals incident in a corresponding direction. 
         [0033]    In  FIG. 1 , reference numeral  30  designates an output signal selector for selecting only two among the three sector antenna incident signals X s1 , X s2  and X s3 , and the two incident signals selected by the output signal selector (MUX)  30  may be input to an adaptive noise canceller  40  as an interference signal. With reference to  FIG. 1 , an example in which the incident signals X s1  and X s2  of the first and second sector antennas  12  and  14  are selected as the interference signals will be described below. 
         [0034]    Meanwhile, the adaptive noise canceller  40  may adaptively filter the two sector antenna incident signals X s1  and X s2  output from the output signal selector  30 , estimate a noise reference signal, and extract only the serving base station signal by subtracting the estimated noise reference signal from an incident signal X m  of the omni-directional antenna  20 . The following Equation 1 is for explaining an operation of the adaptive noise canceller  40 . 
         [0000]        E ( n )= X   m ( n )−[ W   1,1   *   *X   s1 ( n )+ W   2,1   *   *X   s2 ( n )]  [Equation 1]
 
         [0035]    In Equation 1, E(n), X m (n), X s1 (n), and X s2 (n) represent an output signal of the adaptive noise canceller  40 , an output signal of the omni-directional antenna  20 , and output signals of the first and the second sector antennas  12  and  14  output by being sampled in an arbitrary sample index n, respectively. That is, the adaptive noise canceller  40  may adjust filter coefficients W *   1,1  and W *   2,1  of the adaptive filters (F 1  and F 2 )  42  and  44 , for example, through a least mean square algorithm, to minimize the error signal E(n) obtained by subtracting the incident signals X s1 (n) and X s2 (n) sampled by the two sector antennas from the incident signal X m (n) sampled by the omni-direetional antenna  20  using an adder  46 . The filter coefficient adjustment algorithm maybe implemented in software or by a hardware operator. 
         [0036]    In Equation 1, the filter coefficients W *   1,1  and W *   2,1  obtained by feeding the error signal E(n) back to the adaptive filters  42  and  44  may be obtained by the following Equations 2 and 3, respectively. 
         [0000]        W   1,i+1 =(1−α)* W   1,i   +α*W   1 ( t )  [Equation 2]
 
         [0000]        W   2,i+1 =(1−α)* W   2,1   +α*W   2 ( t )  [Equation 3]
 
         [0037]    Meanwhile, in Equations 2 and 3, a designates a weight value reduction coefficient in an exponentially weighted moving average (EWMA) method which is a kind of smooth weight value calculation method, is given as a value which is larger than 0 and smaller than 1, and reduces exponentially in proportion to the age of data. 
         [0038]    In Equations 2 and 3, W *   1,i  and W *   2,i  calculated in an arbitrary time frame i may be obtained by dividing added cross-correlation values by a sampling size N, respectively, after adding cross-correlation values of an incident signal X m,i (n) sampled by the omni-directional antenna  20  and incident signals X *   s1,i (n) and X *   s2,i (n) sampled by the sector antennas  12  and  14 , as shown in the following Equations 4 and 5, and the sampling size N may be given as 2048 for each time frame i. 
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         [0039]    Meanwhile, in Equations 4 and 5, the incident signal X m,i  sampled by the omni-directional antenna  20  and the incident signals X s1,i  and X s2,i  sampled by the sector antennas  12  and  14  may be obtained by the following Equations 6 to 8, respectively. 
         [0000]        X   m,i   =[X   m,i ( n ) . . .  X   m,i ( n+N− 1)] T   [Equation 6]
 
         [0000]        X   s1,i   =[X   s1,i ( n ) . . .  X   s1,i ( n+N− 1)] T   [Equation 7]
 
         [0000]        X   s2,i   =[X   s2,i ( n ) . . .  X   s2,i ( n+N− 1)] T   [Equation 8]
 
         [0040]    For example, two output signals of the output signal selector  30  in an interference cancellation apparatus according to one exemplary embodiment shown in  FIG. 1  may be obtained by selecting two sector antenna incident signals when a value of the output signal of the adaptive noise canceller  40  is the smallest after obtaining every output signal of the adaptive noise canceller  40  while excluding one sector antenna incident signal. 
         [0041]      FIG. 2  is a block diagram illustrating an adaptive interference cancellation apparatus for cancelling a side lobe between, neighboring cells according to another exemplary embodiment of the present invention. As shown in  FIG. 2 , an apparatus of the present invention applied to a newly installed mobile communication repeater system may use only three sector antennas (S/A 1 , S/A 2 , and S/A 3 )  12 ,  14 , and  16  without requiring the omni-directional antenna  20 . That is, one of the three antenna incident signals X s1 , X s2  and X s3 , for example, X s3 , may be set as a serving base station signal, and the remaining two signals, for example, X s1  and X s2 , may be set as interference signals from a neighboring cell. 
         [0042]    Accordingly, the output signal selector  30  may output the two sector antenna incident signals X s1  and X s2  among the three antenna incident signals X s1 , X s2  and X s3 . 
         [0043]    Meanwhile, an adaptive noise canceller  40 ′ may adaptively filter the two sector antenna incident signals X s1  and X s2  output from the output signal selector  30 , estimate a noise reference signal, and extract only a serving base station signal by subtracting the estimated noise reference signal from one sector antenna incident signal X s3 , The following Equation 9 is for explaining an operation of the adaptive noise canceller  40 ′. 
         [0000]        E ( n )= X   s3 ( n )−[ W   1,1   *   *X   s1 ( n )+ W   2,1   *   *X   s2 ( n )]  [Equation 9]
 
         [0044]    In Equation 9, E(n), X s1 (n), X s2 (n), and X s3 (n) represent an output signal of the adaptive noise canceller  40 ′, and output signals of the first to third sector antennas  12 ,  14 , and  16  output by being sampled in an arbitrary sample index n, respectively. That is, the adaptive noise canceller  40 ′ may adjust filter coefficients W *   1,i  and W *   2,i  of the adaptive filters (F 1  and F 2 )  42  and  44 , for example, through a least mean square algorithm, to minimize the error signal E(n) obtained by subtracting the incident signals X s1 (n) and X s2 (n) sampled by the two sector antennas from the incident signal X s3 (n) sampled by the sector antenna  16  using an adder  46 . 
         [0045]    In Equation 9, the filter coefficients W *   1,i  and W *   2,i  obtained by feeding the error signal E(n) back to the adaptive filters  42  and  44  may be obtained by Equations 2 and 3 described above. 
         [0046]    In Equations 2 and 3. W *   1,i  and W *   2,i  calculated in an arbitrary time frame i may be obtained by dividing each of added cross-correlation values by a sampling size N after adding cross-correlation values of the incident signal X s3 (n) sampled by the sector antenna  16  and the incident signals X *   s1,i (n) and X *   s2,i (n) sampled by the sector antennas  12  and  14 , as shown, in the following Equations 10 and 11, and the sampling size N may be given as 2048 for each time frame i. 
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         [0047]    Meanwhile, in Equations 10 and 11, the incident signals X *   s1,i (n) and X *   s2,i (n) sampled by the sector antennas  12  and  14  may each be obtained by determinants of Equations 6 to 8 described above, and the incident signal X s3,i  sampled by the sector antenna  16  may be obtained by a determinant of the following Equation 12. 
         [0000]        X   s3,i   =[X   s3,i ( n ) . . .  X   s3,i ( n+N− 1)] T   [Equation 12]
 
         [0048]    For example, two output signals of the output signal selector  30  in an interference cancellation apparatus according to another exemplary embodiment shown in  FIG. 2  may be obtained by selecting two sector antenna incident signals when a value of the output signal of the adaptive noise canceller  40 ′ is the smallest after obtaining every output signal of the adaptive noise canceller  40 ′ while setting one sector antenna incident signal as the serving base station signal. 
         [0049]      FIG. 3  is a block diagram illustrating a repeater system to which the adaptive interference cancellation apparatus shown in  FIG. 2  is applied. A mobile communication repeater system shown in  FIG. 3  may include three adaptive noise cancellers  40   a ,  40   b , and  40   c  setting each of three sector antenna incident signals as a serving base station signal, and may selectively output only an output signal of the adaptive noise canceller outputting the smallest value among real-time output signals of the adaptive noise cancellers  40   a ,  40   b , and  40   c  though a multiplexer (MUX)  50 . 
         [0050]    The adaptive interference cancellation apparatus for cancelling the side lobe between the neighboring cells of the present invention may be applied to a previously installed or newly installed mobile communication repeater system in a radio wave shaded area, and can provide a high-quality mobile communication signal for a user by effectively cancelling only the side lobe which is an interference signal from the neighboring signal in a base station signal of a serving cell through a simple structure even in an area in which base stations are densely installed. 
         [0051]    It will be apparent to those skilled in the art that various modifications can be made to the above-described exemplary embodiments of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover all such modifications provided they come within the scope of the appended claims and their equivalents.