Patent Publication Number: US-8121654-B2

Title: Apparatus and method for removing an echo signal in a signal transmission/reception apparatus of a communication system

Description:
PRIORITY 
     This application claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed in the Korean Intellectual Property Office on Dec. 7, 2007 and assigned Serial No. 10-2007-0127179, the entire disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and method for transmitting/receiving a signal in a signal transmission/reception apparatus of a communication system. More particularly, the present invention relates to an apparatus and method for transmitting/receiving a signal after removing an echo signal in a signal transmission/reception apparatus of a communication system. 
     2. Description of the Related Art 
     In a conventional communication system, a signal is transmitted/received through a direct link between a Base Station (BS) and a Mobile Station (MS). However, in the conventional communication system, a location of the BS is fixed, so it is difficult to supply an effective communication service when there is a shadow region in a service area or when there is significant variation of a channel state. Therefore, in the conventional communication system, a Relay Station (RS) is used for amplifying a BS signal and extending a service area of a BS. 
     The conventional communication system can extend a cell service area and provide a channel with a better channel state to an MS using the RS. Further, the BS can provide a faster data channel to an MS using the RS in a cell boundary region that experiences a poor channel state. 
       FIG. 1  is a diagram illustrating a structure of a conventional communication system using an RS. 
     Referring to  FIG. 1 , the conventional communication system includes a BS  110 , an RS  130 , and an MS  150 . 
     The BS  110  can directly transmit data to the MS  150 , and can transmit data to the MS  150  using the RS  130 . The MS  150  can directly transmit data to the BS  110 , and can transmit data to the BS  110  using the RS  130 . 
     The RS  130  amplifies a signal received from the BS  110 , and transmits the amplified signal to the MS  150 . The RS  130  amplifies a signal received from the MS  110 , and transmits the amplified signal to the BS  150 . Further, the RS  130  includes at least one antenna for transmitting/receiving a signal. 
     Referring to  FIG. 1 , it will be assumed that the RS  130  includes two antennas, i.e., a transmission antenna for transmitting a signal and a reception antenna for receiving a signal. 
     Because the RS  130  transmits/receives signals, a signal transmitted through the transmission antenna of the RS  130  can be received by the reception antenna of the RS  130 . The signal transmitted through the transmission antenna of the RS  130  that is received by the reception antenna of the RS  130  is referred to as an “echo signal”. 
     The echo signal results in the occurrence of oscillation in the RS  130 . In addition, the signal received through the reception antenna is distorted due to the echo signal. When the signal received through the reception antenna is distorted, the quality of a signal transmitted by the RS  130  is decreased. 
     Therefore, there is a need for a signal transmission/reception apparatus that removes an echo signal in order to prevent a decrease in quality of a transmission signal due to the echo signal in the conventional communication system. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide an apparatus and method for removing an echo signal in a signal transmission/reception apparatus of a communication system. 
     Another aspect of the present invention is to provide an apparatus and method for removing an echo signal in order to prevent a decrease in quality of a transmission signal in a signal transmission/reception apparatus of a communication system. 
     A further aspect of the present invention is to provide an apparatus and method for transmitting/receiving a signal after removing an echo signal in a signal transmission/reception apparatus of a communication system. 
     In accordance with an aspect of the present invention, a method for removing an echo signal in a signal transmission/reception apparatus of a communication system is provided. The method includes estimating an input channel response using a training sequence, generating a first signal by removing the input channel response from a first reception signal, detecting an echo channel impulse response using the first signal, detecting an echo signal removing coefficient using the echo channel impulse response, generating a second signal in which an echo signal is removed by applying the echo signal removing coefficient to a second reception signal, removing the second signal from a third signal, wherein the first signal is received prior to receiving the second signal and the second signal is received prior to receiving the third signal. 
     In accordance with another aspect of the present invention, an apparatus to remove an echo signal in a signal transmission/reception apparatus of a communication system is provided. The apparatus includes an input channel estimator to estimate an input channel response using a training sequence, a first mixer to generate a first signal by removing the input channel response from a first reception signal, an echo channel estimator to detect an echo channel impulse response using the first signal, and detect an echo signal removing coefficient using the echo channel impulse response, an echo signal remover to generate a second signal in which an echo signal is removed by applying the echo signal removing coefficient to a second reception signal, and a second mixer to remove the second signal from a third signal, wherein the first signal is received prior to receiving the second signal and the second signal is received prior to receiving the third signal. 
     Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a diagram illustrating a structure of a conventional communication system using an RS; 
         FIG. 2  is a diagram illustrating a structure of a signal transmission/reception apparatus of a communication system according to an exemplary embodiment of the present invention; 
         FIG. 3  is a diagram illustrating a structure of an echo signal remover according to an exemplary embodiment of the present invention; and 
         FIG. 4  is a flowchart illustrating an operation of a training sequence detector according to an exemplary embodiment of the present invention. 
     
    
    
     Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features and structures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     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 exemplary embodiments described herein may 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. 
     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. 
     It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
     Exemplary embodiments of the present invention provide an apparatus and method for removing an echo signal in a signal transmission/reception apparatus of a communication system. Exemplary embodiments of the present invention also provide an apparatus and method for removing an echo signal to prevent a decrease in quality of a transmission signal in a signal transmission/reception apparatus of a communication system. Further, exemplary embodiments of the present invention provide an apparatus and method for transmitting/receiving a signal after removing an echo signal in a signal transmission/reception apparatus of a communication system. 
     In exemplary embodiments of the present invention, a signal transmission/reception apparatus of a communication system uses at least one antenna for transmitting/receiving a signal. Hereinafter, it will be assumed that the signal transmission/reception apparatus of a communication system uses two antennas, i.e., a transmission antenna and a reception antenna. 
     Although a description of exemplary embodiments of the present invention will be given herein with reference to a Relay Station (RS) as an example of the signal transmission/reception apparatus, the present invention may be used not only in the RS but also in other signal transmission/reception apparatus in which an echo signal may occur. Herein, the RS relays signals between a Base Station (BS) and a Mobile Station (MS). 
     First, a structure of a communication system, according to an exemplary embodiment of the present invention, is identical to a structure of a communication system as illustrated in  FIG. 1 , so a detailed description thereof will be omitted herein. 
       FIG. 2  is a diagram illustrating a structure of a signal transmission/reception apparatus of a communication system according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 2 , the signal transmission/reception apparatus includes a Radio Frequency (RF) receiver  211 , a frequency down converter  213 , an Analog to Digital converter (A/D converter)  215 , an echo signal remover  217 , a frequency up converter  219 , a Digital to Analog converter (D/A converter)  221 , and an RF transmitter  223 . 
     If the signal transmission/reception apparatus is an RS, the RS may transmit a signal received from a BS to an MS, and transmit a signal received from the MS to the BS. 
     The RF receiver  211  is connected to a reception antenna, receives a signal using the reception antenna, and outputs the received signal to the frequency down converter  213 . 
     The frequency down converter  213  down converts the signal output from the RF receiver  211  into a base band signal or an Inter Frequency (IF) band signal, and outputs the frequency down converted signal to the A/D converter  215 . 
     The A/D converter  215  converts the frequency down converted signal from an analog signal into a digital signal, and outputs the digital signal to the echo signal remover  217 . 
     The echo signal remover  217  acquires an echo channel response by extracting a training sequence from the digital signal, and acquires an echo channel response coefficient using the acquired echo channel response. The echo signal remover  217  removes an echo signal included in the received signal using an echo channel removing coefficient filter according to the echo channel response coefficient, and outputs the signal, with the echo signal removed, to the frequency up converter  219 . 
     An exemplary structure of an echo signal remover  217  will be described below. 
     The frequency up converter  219  up converts the signal output from the echo signal remover  217 , and outputs the up converted signal to the D/A converter  221 . 
     The D/A converter  221  converts the up converted signal as a digital signal into an analog signal, and outputs the analog signal to the RF transmitter  223 . 
     The RF transmitter  223  transmits the analog signal using a transmission antenna. 
       FIG. 3  is a diagram illustrating a structure of an echo signal remover according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 3 , the echo signal remover  217  includes a first mixer  311 , a switch  313 , a second mixer  315 , a delay generator  317 , an echo signal removing unit  319 , and an echo channel coefficient generator  327 . 
     The echo channel coefficient generator  327  includes an echo channel estimator  321 , an input channel estimator  323 , and a training sequence detector  325 . 
     The first mixer  311  removes a signal output from the echo signal removing unit  319  from an input signal, i.e., a signal output from the A/D converter  215 , and outputs the signal, in which the signal output from the echo signal removing unit  319  is removed, to the delay generator  317 . 
     The delay generator  317  delays the signal output from the first mixer  311  according to a preset time, and outputs the delayed signal. Here, the preset time is determined according to a preset delay value. 
     The training sequence detector  325  may detect or acquire a training sequence using an input signal, for example, the signal output from the A/D converter  215 . Herein, the training sequence is a discrete time training sequence that occurred according to a sampling frequency. For example, the training sequence may include a preamble sequence, a midamble sequence, and a postamble sequence. 
     The input channel estimator  323  filters the training sequence generated by the training sequence detector  325  using an input channel estimation filter included in the input channel estimator  323 . 
     In this case, an output of the input channel estimation filter may be expressed by Equation (1) below. 
     
       
         
           
             
               
                 
                   
                     
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     In Equation (1), n represents a current time index, m represents an impulse response index of the input channel estimation filter, s[n] represents a reference training sequence used in a current sample, ĥ[m] represents a coefficient of the input channel estimation filter consisted of L h +1 taps, and {circumflex over (x)}[n] represents an input channel response of the input channel estimation filter. 
     The switch  313  switches the signal output from the first mixer  311  to the second mixer  315  using a controller (not shown) included in the RS. 
     The second mixer  315  removes the input channel response output from the input channel estimator  323  from a signal switched through the switch  313 . 
     The echo channel estimator  321  estimates an echo channel using the signal output from the second mixer  315 . The echo channel estimator  321  may be simplified in the form of a complex gain, or may be implemented in the form of a filter structure when it will be assumed that a multipath component is ignored. 
     The echo channel estimator  321  removes an input signal using the training sequence. Here, the echo channel estimator  321  may extract an echo signal component by removing the input signal, and may estimate an echo channel without considering a correlation relationship of a signal. 
     The echo channel estimator  321  acquires an echo signal removing coefficient using the estimated echo channel, i.e., an echo channel impulse response. 
     The echo signal removing unit  319  receives a signal delayed by the delay generator  317  and the echo signal removing coefficient, and acquires an echo signal removing filter response using the echo signal removing coefficient. 
     The echo signal removing unit  319  may include a filter for removing an echo signal, and removes an echo signal by applying the echo signal removing coefficient to the echo signal removing filter. Here, the echo signal removing coefficient is an estimated value of the echo channel impulse response. 
     In this case, an operation of the echo signal remover  217  in a training sequence time duration may be expressed by Equation (2) below. 
     
       
         
           
             
               
                 
                   
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     In Equation (2), n represents a current time index, y[n] represents an output of the echo signal remover  217 , m represents an impulse response index of the echo signal removing filter, f[m] represents an impulse response of an echo channel, and {circumflex over (f)}[m] represents an output of the echo signal removing filter with a length L f . Herein, the echo signal removing filter uses the estimation value of the echo channel impulse response. In addition, w[n] represents a noise additionally received in an input side of the RS as an additive noise, and d represents a delay value of the delay generator  317 . 
     As expressed in Equation (2), if each of the echo channel estimator  321  and the input channel estimator  323  perfectly estimates a corresponding channel, the output of the echo signal remover  217  includes only the additive noise w[n]. Hereinafter, the output of the echo signal remover  217  will be referred to as an ‘interference removing output signal’. 
     Therefore, the echo signal remover  217  may estimate an echo channel and an input channel filter coefficient to minimize a power sum of the interference removing output signal or an average power of the interference removing output signal using one of a Least Square Estimation (LSE) scheme, and a Minimum Mean Square Error Estimation (MMSE) scheme, etc. 
     In addition, the echo signal remover  217  may use an adaptive algorithm to estimate the echo channel and input channel filter coefficient in a real time. Here, the adaptive algorithm may be one of a Least Mean Square (LMS) algorithm, a Recursive Least Square (RLS) algorithm, etc. 
     The signal transmission/reception apparatus, i.e., the RS applies the estimated echo channel response for the training sequence time duration to a time duration except for the training sequence time duration when a time variant characteristic of the echo channel is equal to or less than a threshold time variant characteristic. 
     In this case, the RS transmits an interference removing output signal expressed by Equation (3) below. 
     
       
         
           
             
               
                 
                   
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     When an echo channel is perfectly estimated, there is no interference that occurs due to an echo signal. Therefore, in the RS, a multi carrier input signal x[n] and an additive noise occurs according to the transmission of the multi carrier input signal x[n]. 
     On the other hand, if it is not possible to ignore the time variant characteristic of the echo channel because there is a plurality of mobile dispersion objects near the RS, that is, if the time variant characteristic of the echo channel is greater than the threshold time variant characteristic, a quality of the interference removing output signal may become worse due to a channel variance between the training sequence time duration and the time duration except for the training sequence time duration. 
     To prevent a case from occurring where the quality of the interference removing output signal becomes worse due to the channel variance between the training sequence time duration and the time duration except for the training sequence time duration, the RS may determine a current echo channel impulse response using previous echo channel estimation values. 
       FIG. 4  is a flowchart illustrating an operation of a training sequence detector according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 4 , in step  411 , the training sequence detector  325  acquires time synchronization using a reception signal, i.e., a signal output from the A/D converter  215 . In step  413 , the training sequence detector  325  performs a Discrete Fourier Transform (DFT) on the reception signal. While not illustrated in  FIG. 4 , the training sequence detector  325  may perform a Fast Fourier Transform (FFT) on the reception signal in step  413  according to an exemplary embodiment of the invention. 
     In step  415 , the training sequence detector  325  correlates the discrete Fourier transformed signal with each of preset reference training sequences. Here, the reception signal as a time domain signal is converted into a frequency domain signal after performing the DFT, so the preset reference training sequences are frequency domain reference training sequences. In addition, each of the preset reference training sequences is uniquely allocated to a corresponding cell or a corresponding sector. Thus, each of the preset reference training sequences may be used as information identifying the corresponding cell or the corresponding sector. That is, when the training sequence detector  325  detects a specific training sequence as a final training sequence, the training sequence detector  325  determines a cell or a sector to which the final training sequence is uniquely allocated as a cell or a sector in which the training sequence detector  325  is located. 
     In step  417 , the training sequence detector  325  detects a maximum correlation value among a plurality of correlation values according to the correlation. In step  419 , the training sequence detector  325  detects information on a cell or a sector in which the training sequence detector  325  is located by detecting a reference training sequence with the maximum correlation value. Hereinafter, the information on the cell or the sector in which the training sequence detector  325  is located will be referred to as ‘cell or sector information’. The training sequence detector  325  determines a cell or a sector to which the detected training sequence is uniquely allocated as the cell or the sector in which the training sequence detector  325  is located. 
     In order to increase accuracy for detection of the cell or sector information, an operation in step  411  to step  415  may be performed during a plurality of training sequence time durations. That is, the training sequence detector  325  accumulates correlation values for each of the preset reference training sequences during the plurality of training sequence time durations, detects averaging values of the accumulated correlation values, and uses the averaging values, thereby increasing accuracy for detection of the cell or sector information. 
     Further, the RS may only perform an operation for receiving a signal while the training sequence detector  325  detects the cell or sector information. In this case, the RS may not perform an operation for transmitting a signal. Thus, an error probability for detecting the cell or sector information due to an echo signal is decreased. 
     In step  421 , the training sequence detector  325  generates a training sequence using the detected cell or sector information. Here, the generated training sequence is identical to a training sequence transmitted by a transmitter, for example, a BS. Thus, the training sequence detector  325  may generate the training sequence, transmitted by the BS, without noise. 
     In step  423 , the training sequence detector  325  performs an Inverse Discrete Fourier Transform (IDFT) on the generated training sequence. While not described in  FIG. 4 , the training sequence detector  325  may perform an Inverse Fast Fourier Transform (IFFT) on the generated training sequence in step  423  according to an exemplary embodiment of the invention. 
     In step  425 , the training sequence detector  325  acquires a time domain training sequence. 
     In an exemplary embodiment of the present invention, a training sequence is used for an operation of the echo channel estimator  321  as illustrated in  FIG. 4 . As described above, when the training sequence is used, time synchronization and frequency synchronization may be acquired. In addition, the training sequence is used for identifying a cell or a sector, and is modulated using a different frequency tone and a different frequency sequence in each cell or each sector. 
     Therefore, when the RS is initially established, or a configuration of a cell or a sector of a BS is changed, the RS may detect a location and an echo signal of the RS using the training sequence by periods. 
     As is apparent from the foregoing description, according to exemplary embodiments of the present invention, a signal transmission/reception apparatus of a communication system may remove an echo signal. Further, the signal transmission/reception apparatus may remove the echo signal using the training sequence. Thus, an oscillation in the signal transmission/reception apparatus and a case where a quality of a signal transmitted by the signal transmission/reception apparatus becomes worse, are prevented from occurring. 
     While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.