Patent Publication Number: US-9893871-B2

Title: In-band full duplex transceiver

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2014-0150693, 10-2014-0160310 and 10-2015-0150168 filed in the Korean Intellectual Property Office on Oct. 31, 2014, Nov. 17, 2014 and Oct. 28, 2015, respectively, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to an in-band full duplex transceiver. 
     (b) Description of the Related Art 
     A current wireless communication system uses a half duplex method. The half duplex method transmits or receives signals by dividing time or frequency so orthogonality between transmitting and receiving may be maintained. However, the half duplex method wastes resources (time or frequency), has a problem in a multi-hop relay among mobile small cells, and requires additional overhead to solve a hidden node problem. 
     The in-band full duplex method is suggested as a solution for solving non-efficiency of the half duplex method. The in-band full duplex method represents a method for allowing simultaneous in-band transmitting/receiving. The in-band full duplex method may increase link capacity by twice to a maximum in a theoretical manner so it is an essential technique for achieving 1000 times the traffic capacity required by the 5G mobile communication. 
     However, the in-band full duplex method allows a self-transmitting signal to be input to a receiver so the self-transmitting signal functions as a self-interference signal very strongly compared to a valid received signal, which is a drawback. To cancel the self-interference (called self-interference cancellation (SIC)), an antenna region SIC technique for physically separating a transmitting antenna from a receiving antenna with a large distance therebetween has been provided. A technique for reducing a self-interference level by using the antenna region SIC technique, and canceling remaining self-interference in a digital region, is called an interference cancellation system (ICS) technique. A problem of the ICS technique is that it is impossible to be applied to a small device because of the physical separation between the transmitting and receiving antennas. 
     An electrical balance duplex (EBD) is one of SIC techniques in the in-band full duplex method, but the EBD technique deteriorates SIC performance or destabilizes it as the system bandwidth becomes wider. That is, the existing EBD technique has a problem in that an SIC gain is great for a specific frequency bandwidth and it becomes less in other frequency bandwidths. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in an effort to provide an in-band full duplex transceiver applicable to a wideband. 
     An exemplary embodiment of the present invention provides an in-band full duplex transceiver including: a transmitter for generating a transmitting signal; a hybrid transformer including a first end connected to an antenna, outputting the transmitting signal to the antenna, and outputting a received signal provided through the antenna to a receiver; and an impedance matching unit including a plurality of balance networks connected to a second end of the hybrid transformer, and matching impedance of the antenna. 
     A sum of impedances of the balance networks may correspond to impedance of the antenna. 
     The balance networks may be configured with at least one of a capacitor, an inductor, and a resistor, and the plurality of balance networks may tune different frequency bandwidths. 
     The hybrid transformer may transmit the transmitting signal to the impedance matching unit. 
     The hybrid transformer may include a receiving output end for outputting the received signal, and the in-band full duplex transceiver may further include a finite impulse response (FIR) filter for receiving the transmitting signal, and canceling a self-transmitting interference signal included in a signal output by the receiving output end. 
     The FIR filter may include: a plurality of delay units for receiving the transmitting signal and delaying the same; a plurality of attenuators connected to the delay units and attenuating a signal; and a controller for setting an attenuation degree of the attenuator so as to cancel the self-transmitting interference signal, and the controller may set the attenuation degree for minimizing the self-transmitting interference signal by using a signal generated by converting the self-transmitting interference signal into a frequency domain and a signal generated by converting the transmitting signal into the frequency domain. 
     The receiving output end may include a first receiving output end and a second receiving output end, and a first signal output by the first receiving output end and a second signal output by the second receiving output end may be phase-inverted signals from each other. 
     The in-band full duplex transceiver may further include a first combiner for combining the first signal and the second signal, and a second combiner for combining an output signal of the first combiner and an output signal of the FIR filter, and outputting a resultant signal to the receiver, wherein the FIR filter may output a signal for canceling a self-transmitting interference signal included in the output signal of the first combiner to the second combiner. 
     The FIR filter may include a first FIR filter for receiving the transmitting signal and canceling the self-transmitting interference signal included in the first signal and a second FIR filter for receiving the transmitting signal and canceling the self-transmitting interference signal included in the second signal, and the in-band full duplex transceiver may further include a first combiner for combining the first signal and an output signal of the first FIR filter, and outputting a resultant signal to the receiver, and a second combiner for combining the second signal and an output signal of the second FIR filter, and outputting a resultant signal to the receiver. 
     The in-band full duplex transceiver may further include: a first combiner for combining the first signal and the second signal; a second combiner for combining an output signal of the first combiner and an output signal of the second end of the hybrid transformer; and a third combiner for combining an output signal of the second combiner and an output signal of the FIR filter, and outputting a resultant signal to the receiver, wherein the FIR filter may output a signal for canceling a self-transmitting interference signal included in the output signal of the second combiner to the third combiner. 
     The in-band full duplex transceiver may further include a first combiner for combining the first signal and an output signal of the second end of the hybrid transformer and a second combiner for combining the second signal and an output signal of the second end of the hybrid transformer, the FIR filter may include a first FIR filter for receiving the transmitting signal and canceling the self-transmitting interference signal included in the output signal of the first combiner and a second FIR filter for receiving the transmitting signal and canceling the self-transmitting interference signal included in the output signal of the second combiner, and the in-band full duplex transceiver may further include a third combiner for combining the output signal of the first combiner and the output signal of the first FIR filter and a fourth combiner for combining the output signal of the second combiner and the output signal of the second FIR filter. 
     The hybrid transformer may include a receiving output end for outputting the received signal, and the in-band full duplex transceiver may further include a finite impulse response (FIR) filter for receiving a signal of the second end of the hybrid transformer, and canceling a self-transmitting interference signal included in a signal output by the receiving output end. 
     The receiving output end may include a first receiving output end and a second receiving output end, and a first signal output by the first receiving output end and a second signal output by the second receiving output end may be phase-inverted signals from each other. 
     The in-band full duplex transceiver may further include a first combiner for combining the first signal and the second signal, and a second combiner for combining an output signal of the first combiner and an output signal of the FIR filter, and outputting a resultant signal to the receiver, wherein the FIR filter may output a signal for canceling the self-transmitting interference signal included in the output signal of the first combiner to the second combiner. 
     The FIR filter may include a first FIR filter for receiving a signal of the second end of the hybrid transformer and canceling the self-transmitting interference signal included in the first signal and a second FIR filter or receiving a signal of the second end of the hybrid transformer and canceling the self-transmitting interference signal included in the second signal, and the in-band full duplex transceiver may further include a first combiner for combining the first signal and an output signal of the first FIR filter and outputting a resultant signal to the receiver, and a second combiner for combining the second signal and an output signal of the second FIR filter and outputting a resultant signal to the receiver. 
     Another embodiment of the present invention provides an in-band full duplex transceiver. The in-band full duplex transceiver may include: a power amplifier for outputting a transmitting signal; a transformer including a primary coil having a first end connected to an antenna and a middle tab for receiving an output signal of the power amplifier, and a secondary coil for inducing a received signal provided through the antenna; an impedance matching unit including a plurality of balance networks connected to a second end of the primary coil and matching impedance of the antenna; and a finite impulse response (FIR) filter for receiving the transmitting signal and canceling a self-transmitting interference signal included in signals output by respective ends of the secondary coil. 
     A sum of impedances of the balance networks may be determined corresponding to impedance of the antenna. 
     The balance networks may be configured with at least one of a capacitor, an inductor, and a resistor, and the balance networks may tune different frequency bandwidths. 
     The FIR filter may include: a plurality of delay units for receiving the transmitting signal and delaying the same; a plurality of attenuators connected to the delay units and attenuating a signal; and a controller for setting an attenuation degree of the attenuators so as to cancel the self-transmitting interference signal, and the controller may set the attenuation degree for minimizing the self-transmitting interference signal by using a signal generated by converting the self-transmitting interference signal into the frequency domain and a signal generated by converting the transmitting signal into the frequency domain. 
     The networks may be coupled in parallel with each other. 
     According to an exemplary embodiment of the present invention, the SIC gain may be improved in the wideband by matching the impedance by use of a plurality of balance networks. 
     According to another exemplary embodiment of the present invention, the finite impulse response (FIR) filter is used to cancel the self-transmitting interference signal, thereby reducing the wideband and the quantization error. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an in-band full duplex transceiver according to an exemplary embodiment of the present invention. 
         FIG. 2  shows a divider according to an exemplary embodiment of the present invention. 
         FIG. 3  shows an FIR filter according to an exemplary embodiment of the present invention. 
         FIG. 4  shows an in-band full duplex transceiver according to another exemplary embodiment of the present invention. 
         FIG. 5  shows an in-band full duplex transceiver according to the other exemplary embodiment of the present invention. 
         FIG. 6  shows an in-band full duplex transceiver according to the other exemplary embodiment of the present invention. 
         FIG. 7  shows an in-band full duplex transceiver according to the other exemplary embodiment of the present invention. 
         FIG. 8  shows an in-band full duplex transceiver according to the other exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the following detailed description, only certain exemplary embodiments of the present invention have been shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
     Throughout the specification, a transceiver may indicate a terminal, a mobile terminal (MT), a mobile station (MS), an advanced mobile station (AMS), a high-reliability mobile station (HR-MS), a subscriber station (SS), a portable subscriber station (PSS), an access terminal (AT), and user equipment (UE), and it may include entire or partial functions of the terminal, MT, AMS, HR-MS, SS, PSS, AT, and UE. 
     The transceiver may represent a base station (BS), an advanced base station (ABS), a high-reliability base station (HR-BS), a node B, an evolved node B (eNodeB), an access point (AP), a radio access station (RAS), a base transceiver station (BTS), a mobile multihop relay (MMR)-BS, a relay station (RS) for functioning as a base station, and a high-reliability relay station (HR-RS) for functioning as a base station, and it may include entire or partial functions of the ABS, nodeB, eNodeB, AP, RAS, BTS, MMR-BS, RS, and HR-RS. 
       FIG. 1  shows an in-band full duplex transceiver  100  according to an exemplary embodiment of the present invention. 
     As shown in  FIG. 1 , the in-band full duplex transceiver  100  includes a power amplifier (PA)  110 , a divider  120 , an antenna  130 , a finite impulse response (FIR) filter  140 , a first combiner  150 , a second combiner  160 , and a low noise amplifier (LNA)  170 . 
     The PA  110  amplifies a radio frequency (RF) signal and outputs the same. In  FIG. 1 , a transmitting signal output by the PA  110  is denoted by w. The transmitting signal (w) is input to the divider  120  and the FIR filter  140 . The PA  110  configures part of the transmitter. 
     The divider  120  is connected to the antenna  130  to transmit the signal (w) to the antenna  130 . The divider  120  transmits a received signal provided by the antenna  130  to receiving output ends Rx 1  and Rx 2 . That is, the divider  120  transmits the transmitting signal to the antenna  130  and transmits the received signal to the receiver (e.g., LNA). 
       FIG. 2  shows a divider  120  according to an exemplary embodiment of the present invention. 
     As shown in  FIG. 2 , the divider  120  includes a hybrid transformer  121  and an impedance matching unit  122 . 
     The hybrid transformer  121  includes a primary coil L 1  and a secondary coil L 2 . The transmitting signal (w) is input to a middle tab of the primary coil L 1 , the antenna  130  is connected to a first end of the primary coil L 1 , and the impedance matching unit  122  is connected to a second end of the primary coil L 1 . The above-structured hybrid transformer  121  transmits the transmitting signal (w) to the antenna  130  and the impedance matching unit  122 . The hybrid transformer  121  transmits the received signal provided by the antenna  130  to the secondary coil L 2 , and outputs the received signal to the receiving output ends Rx 1  and Rx 2 . In this instance, the received signal output to the receiving output end Rx 1  has an opposite phase to that of the received signal output to the receiving output end Rx 2 . 
     The impedance matching unit  122  is set in a same or similar manner of impedance (Z ANT ) as the antenna  130 , and the transmitting signal is transmitted to the antenna  130  and the impedance matching unit  122 . That is, the impedance matching unit  122  prevents the transmitting signal from being input to the receiving end (i.e., the secondary coil L 2  of the hybrid transformer  121 ). The impedance matching unit  122  includes a plurality of balance networks (Z BN   _   1 -Z BN   _   K ) so as match the impedance in the wideband. As shown in  FIG. 2 , a plurality of balance networks (Z BN   _   1 -Z BN   _   K ) are respectively coupled in parallel to the second end of the primary coil L 1 . The impedances of the balance network are defined to be Z BN   _   1 , Z BN   _   2 , . . . , Z BN   _   K , the impedance of the antenna  130  is defined to be Z ANT , and the impedances of the respective balance networks are set so as to satisfy Z ANT =Z BN   _   1 +Z BN   _   2 , . . . , +Z BN   _   K . A method for setting the impedance will now be described. The impedance values of the balance networks are set by equally setting the impedance (i.e., Z ANT /K) or unequally setting the impedance but setting the entire impedances to be Z ANT . 
     The respective balance networks include a capacitor, an inductor, and a resistor that are passive elements, of which values are set to work for a specific frequency bandwidth. That is, the impedances of the balance networks are set, and capacitance, inductance, and resistance are set so that the balance networks may work for the different frequency bandwidths. The above-set balance networks tune different frequency bandwidths. For example, the balance network (Z BN   _   1 ) may be set to have a big SIC gain at a frequency f 1 , the balance network (Z BN   _   2 ) may be set to have a big SIC gain at a frequency f 2 , and the balance network (Z BN   _   K ) may be set to have a big SIC gain at a frequency fk. The impedance matching unit  122  may have various kinds of combinations through a plurality of balance networks, by which it may improve the SIC gain in the wideband. 
     The divider  120  shown in  FIG. 2  transmits the transmitting signal to the antenna  130  and the impedance matching unit  122 , and prevents the transmitting signal from being input to the secondary coil of the hybrid transformer  121 . That is, the impedance of the impedance matching unit  122  is set to correspond to impedance of the antenna  130 , thereby preventing the transmitting signal from being output to the receiving output ends Rx 1  and Rx 2 . The received signal provided through the antenna  130  is output to the receiving output ends Rx 1  and Rx 2  by the hybrid transformer  121 . 
     The divider  120  according to an exemplary embodiment of the present invention may be applied to another exemplary embodiment to be described. 
     An interference amount of the self-transmitting interference signal may be reduced by the divider  120  according to an exemplary embodiment of the present invention, but the reduced amount may not solve the quantization error problem generated in the digital region. To solve the problem, the in-band full duplex transceiver  100  according to an exemplary embodiment of the present invention includes the FIR filter  140 . 
     The antenna  130  simultaneously performs a transmission function and a receiving function. The transmitting signal is transmitted and the received signal is received through the antenna  130 . 
     The first combiner  150  combines received signals output by the receiving output end Rx 1  and the receiving output end Rx 2 . In this instance, since the received signal output by the receiving output end Rx 1  has the opposite phase of the received signal output by the receiving output end Rx 2 , the first combiner  150  subtracts the received signal output by the receiving output end Rx 2  from the received signal output by the receiving output end Rx 1  and combines the two signals. In  FIG. 1 , the combined received signal is denoted as x, and the received signal (x) includes a self-received signal of the in-band full duplex transceiver  100  and a self-transmitting interference signal of the in-band full duplex transceiver  100 . The transmitting signal (w) is prevented from being input to the receiving end (e.g., LNA) by the divider  120 , part thereof is applied to the receiving end (e.g., LNA) to function as an interference signal, and the interference signal signifies the self-transmitting interference signal. The self-transmitting interference signal will be denoted as {tilde over (x)}. In an exemplary embodiment of the present invention, the self-transmitting interference signal ({tilde over (x)}) will be canceled by using the FIR filter  140 . 
     The FIR filter  140  receives the transmitting signal (w), generates a signal for minimizing the self-transmitting interference signal ({tilde over (x)}), and outputs a resultant signal. A detailed configuration and an operation of the FIR filter  140  will be described in detail with reference to  FIG. 3 . 
     The second combiner  160  combines the received signal and the output signals of (x) of the FIR filter  140  and outputs a resultant signal to the LNA  170 . The second combiner  160  subtracts the signal output by the FIR filter  140  from the received signal (x) and combines the two signals. In this instance, the FIR filter  140  outputs a signal for minimizing the self-transmitting interference signal ({tilde over (x)}) so the second combiner  160  outputs a signal generated by canceling the self-transmitting interference signal ({tilde over (x)}) from the received signal (x) to the LNA  170 . 
     The LNA  170  receives the self-transmitting interference signal ({tilde over (x)})-canceled received signal from the second combiner  160 , removes noise from the input signal, and amplifies the resultant signal. The LNA  170  configures part of the receiver. 
       FIG. 3  shows an FIR filter  140  according to an exemplary embodiment of the present invention. 
     As shown in  FIG. 3 , the FIR filter  140  includes a plurality of delay units (d 1 -d N ), a plurality of attenuators (a 1 -a N ), a combiner  141 , and a controller  142 . 
     The delay units (d 1 -d N ) respectively include a fixed delay. Delay intervals among the delay units (d i  (i=1, 2, . . . , N)) may be equal or different and may be divided into a plurality of groups with a same delay interval. 
     The attenuators (a 1 -a N ) are connected to the delay units (d 1 -d N ) and attenuate signals. Attenuation degrees of the attenuators (a i  (i=1, 2, . . . , N)) are tunable, and are set by the controller  142 . 
     The controller  142  sets the attenuation degrees of the attenuators (a 1 -a N ) in a tunable manner. The controller  142  finds the attenuation degrees of a plurality of attenuators (a 1 -a N ) by using a signal ({tilde over (X)}(f)) acquired by converting the self-transmitting interference signal ({tilde over (x)}) into a frequency domain and a signal (W(f)) acquired by converting the transmitting signal (w) into the frequency domain. Here, the signal {tilde over (X)}(f) may be found by using frequency-domain subcarriers included in a head of a packet including the self-received signal or neighboring packets, which is known to a person skilled in the art and will not be described. 
     A method for the controller  142  to find the attenuation degrees of a plurality of attenuators (a 1 -a N ) will now be described. 
     A method for finding an attenuation degree a i  of the FIR filter  140  when the delay intervals of the delay units (d i  (i=1, 2, . . . , N)) are the same or different will now be described. The method may be expressed as Equation 1. 
     
       
         
           
             
               
                 
                   
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     The received signal (x) is not used and the self-transmitting interference signal ({tilde over (x)}) is used. The received signal (x) of  FIG. 1  corresponds to a sum of the self-transmitting interference signal ({tilde over (x)}) and the self-received signal. Therefore, when x is used, the self-received signal may be attenuated as well as the self-transmitting interference signal at a front end of the LNA  170  so the self-transmitting interference signal ({tilde over (x)}) is used in Equation 1. It is not easy in a time domain to find a filter coefficient (i.e., a i ) of the time domain as expressed in Equation 1. Hence, the filter coefficient may be found by converting Equation 1 into the frequency domain. A method for finding a i  of the FIR filter  140  in the frequency domain is expressed in Equation 2. 
     
       
         
           
             
               
                 
                   
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     As expressed in Equation 2, the controller  142  may find the attenuation degrees of a plurality of attenuators (a 1 -a N ) satisfying Equation 2 by using the signal ({tilde over (X)}(f)) acquired by converting the self-transmitting interference signal ({tilde over (x)}) into the frequency domain and the signal (W(f)) acquired by converting the transmitting signal (w) into the frequency domain. In Equation 2, the portion ( ) 2  has a secondary equation on the attenuation degrees of the attenuators (a 1 -a N ) so a minimum value of the secondary equation may be found. A method of finding a 1 , a 2 , . . . , a N  satisfying Equation 2 is known to a person skilled in the art and a description thereof will be omitted. 
     A method for finding an attenuation degree a i  of the FIR filter  140  when two groups (a 1 -a L , a L+1 -a N ) with a same delay interval among the delay units (d i  (i=1, 2, . . . , N)) are provided will now be described. The method may be expressed in Equation 3 in the frequency domain. 
     
       
         
           
             
               
                 
                   
                     
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     For convenience of description, an example (when the delay intervals are the same or different) of the first delay interval will be described, and examples of the second delay interval and other delay intervals are applicable. 
     As described above, the in-band full duplex transceiver  100  according to an exemplary embodiment of the present invention may improve the frequency characteristic (i.e., applicable to the wideband) and may solve the quantization error problem in the digital region by applying the divider  120  and the FIR filter  140 . 
       FIG. 4  shows an in-band full duplex transceiver  100   a  according to another exemplary embodiment of the present invention. 
     As shown in  FIG. 4 , the in-band full duplex transceiver  100   a  includes a power amplifier (PA)  110 , a divider  120 , an antenna  130 , a first finite impulse response (FIR) filter  140   a , a second FIR filter  140   a ′, a first combiner  160   a , a second combiner  160   a ′, and a low noise amplifier (LNA)  170 . The in-band full duplex transceiver  100   a  shown in  FIG. 4  corresponds to the in-band full duplex transceiver  100  shown in  FIG. 1  except that it uses two FIR filters and combines signals. No repeated descriptions will be provided. 
     The transmitting signal (w) output by the PA  110  is input to the divider  120 , the first FIR filter  140   a , and the second FIR filter  140   a′.    
     As described with reference to  FIG. 1  and  FIG. 2 , the divider  120  phase-inverts the received signal provided by the antenna  130  to separate the same, and outputs the separated received signals to the receiving output end Rx 1  and the receiving output end Rx 2 . In  FIG. 4 , a combined received signal (a sum of a self-received signal and a self-transmitting interference signal) output to the receiving output end Rx 1  is indicated by x 1 , and a combined received signal (a sum of a self-received signal and a self-transmitting interference signal) output to the receiving output end Rx 2  is indicated by x 2 . The signals x 1  and x 2  are phase-inverted from each other. Regarding the signal x 1 , the self-transmitting interference signal will be shown to be ( ), and regarding the signal x 2 , the self-transmitting interference signal will be shown to be ( ). The self-transmitting interference signal ( ) is canceled by the first FIR filter  140   a , and the self-transmitting interference signal ( ) is canceled by the second FIR filter  140   a′.    
     The first FIR filter  140   a  receives the transmitting signal (w), generates a signal for minimizing the self-transmitting interference signal ( ), and outputs the same. The second FIR filter  140   a ′ receives the transmitting signal (w), generates a signal for minimizing the self-transmitting interference signal ( ), and outputs the same. Configurations of the first FIR filter  140   a  and the second FIR filter  140   a ′ are similar to that of the FIR filter  140  of  FIG. 3 , and operations thereof are similar to that of the FIR filter  140  of  FIG. 3 . 
     The first combiner  160   a  combines the received signal x 1  and the output signal of the first FIR filter  140   a  and outputs a resultant signal to the LNA  170 . The first combiner  160   a  subtracts the signal output by the first FIR filter  140   a  from a signal (i.e., —x 1 ) generated by inverting the received signal x 1 , and combines the two signals. Here, the first combiner  160   a  inverts the received signal x 1  because the received signal x 1  is a phase-inverted received signal. In this instance, the first FIR filter  140   a  outputs the signal for minimizing the self-transmitting interference signal ( ), so the first combiner  160   a  outputs a signal generated by canceling the self-transmitting interference signal ( ) from the inverted received signal −x 1  to the LNA  170 . 
     The second combiner  160   a  combines the received signal x 2  and an output signal of the second FIR filter  140   a  and outputs a resultant signal to the LNA  170 . The second combiner  160   a  subtracts the signal output by the second FIR filter  140   a  from the received signal x 2  and combines the two signals. In this instance, the second FIR filter  140   a  outputs the signal for minimizing the self-transmitting interference signal ( ), so the second combiner  160   a  outputs a signal generated by canceling the self-transmitting interference signal ( ) from the received signal x 2  to the LNA  170 . 
     The LNA  170  receives a received signal from which the self-transmitting interference signal ( ) is canceled from the first combiner  160   a  and a received signal from which self-transmitting interference signal ( ) is canceled from the second combiner  160   a , cancels noise from the two received signals, and amplifies the same. Alternatively, the LNA  170  receives a signal generated by combining the received signal from which the self-transmitting interference signal ( ) is canceled from the first combiner  160   a  and the received signal from which the self-transmitting interference signal ( ) is canceled from the second combiner  160   a ′, cancels noise from the two signals, and amplifies the same. 
     A method for the first FIR filter  140   a  and the second FIR filter  140   a ′ to find attenuation degrees of a plurality of attenuators (a 1 -a N ) will now be described. A method for the first FIR filter  140   a  and the second FIR filter  140   a ′ to find the attenuation degrees a i  when the delay intervals among the delay units (d i  (i=1, 2, . . . , N)) are the same or different will now be described. The method is expressed in Equation 4. 
     
       
         
           
             
               
                 
                   
                     
                       min 
                       
                         
                           a 
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                                 = 
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                             ⁢ 
                             
                               
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                                 i 
                               
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                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ) 
                 
               
             
           
         
       
     
     It is not easy to find a filter coefficient (i.e., a i ) in the time domain expressed in Equation 4 
     Therefore, the filter coefficient may be found by converting Equation 4 into the frequency domain. A method for finding the filter coefficient (a i ) of the first FIR filter  140   a  and the second FIR filter  140   a ′ in the frequency domain may be expressed in Equation 5. 
     
       
         
           
             
               
                 
                   
                     
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                       2 
                     
                   
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                       min 
                       
                         
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                                 m 
                                 = 
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                               N 
                             
                             ⁢ 
                             
                               
                                 a 
                                 m 
                               
                               ⁢ 
                               
                                 W 
                                 ⁡ 
                                 
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                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ) 
                 
               
             
           
         
       
     
     As expressed in Equation 5, the first FIR filter  140   a  may find the attenuation degrees of a plurality of attenuators (a 1 -a N ) satisfying Equation 5 by using the signal (− (f)) generated by converting the self-transmitting interference signal (− ) into the frequency domain and the signal (W(f)) generated by converting the transmitting signal (w) into the frequency domain. The second FIR filter  140   a ′ may find the attenuation degrees of a plurality of attenuators (a 1 -a N ) satisfying Equation 5 by using the signal ( (f)) generated by converting the self-transmitting interference signal ( ) into the frequency domain and the signal (W(f)) generated by converting the transmitting signal (w) into the frequency domain. 
       FIG. 5  shows an in-band full duplex transceiver  100   b  according to the other exemplary embodiment of the present invention. 
     As shown in  FIG. 5 , the in-band full duplex transceiver  100   b  includes a power amplifier  110 , a divider  120 , an antenna  130 , an FIR filter  140   b , a first combiner  150   b , a second combiner  150   b ′, a third combiner  160   b , and a low noise amplifier  170 . A configuration of combiners of the in-band full duplex transceiver  100   b  shown in  FIG. 5  is similar to that shown in  FIG. 1  except for different portions. Therefore, no repeated descriptions will be provided. 
     The divider  120  includes a hybrid transformer  121  and an impedance matching unit  122 . The hybrid transformer  121  transmits a transmitting signal (w) to the antenna  130  and the impedance matching unit  122 . A signal corresponding to a signal at a rear end of the PA  110  or a transmitting signal of the antenna  130  is output to a node (a balance point (BP) hereinafter) of the hybrid transformer  121  and the impedance matching unit  122 . The impedance matching unit  122  is configured with passive elements, and controls the impedance applied to the antenna  130  and the impedance applied to the impedance matching unit  122  to be the same. The received signal provided by the antenna  130  is phase-inverted and separated by the hybrid transformer  121 , and the separated received signal is output to the receiving output end Rx 1  and the receiving output end Rx 2 . The signal provided by the antenna  130  is output to the balance point (BP). Therefore, part of the received signal as well as part of the transmitting signal is output to the balance point (BP). 
     The first combiner  150   b  combines the received signals output by the receiving output end Rx 1  and the receiving output end Rx 2 . In this instance, the received signal output by the receiving output end Rx 1  and the received signal output by the receiving output end Rx 2  have opposite phases to each other, so the first combiner  150   b  subtracts the received signal output by the receiving output end Rx 2  from the received signal output by the receiving output end Rx 1 , and combines the two signals. 
     The second combiner  150   b ′ combines the signal output by the first combiner  150   b  and the signal output by the balance point (BP). In this instance, the signal output by the first combiner  150   b  and the signal output by the balance point (BP) have a same phase so the second combiner  150   b ′ combines the two signals. Referring to  FIG. 5 , the signal output by the second combiner  150   b ′ is indicated by xb, and the signal xb includes a self-received signal of the in-band full duplex transceiver  100   b  and a self-transmitting interference signal of the in-band full duplex transceiver  100   b . The self-transmitting interference signal among the signal xb will be shown as  . According to an exemplary embodiment of the present invention, the self-transmitting interference signal ( ) is canceled by using the FIR filter  140   b.    
     The FIR filter  140   b  receives a transmitting signal (w), generates a signal for minimizing the self-transmitting interference signal ( ), and outputs a resultant signal. A configuration of the FIR filter  140   b  is similar to that of the FIR filter  140  described with reference to  FIG. 3 , and an operation thereof is similar to that of the FIR filter  140  described with  FIG. 3 . 
     The third combiner  160   b  combines the output signal (xb) of the second combiner  150   b ′ and the output signal of the FIR filter  140   b , and outputs a resultant signal to the LNA  170 . The third combiner  160   b  subtracts the signal output by the second FIR filter  140   b  from the signal xb and combines the two signals. In this instance, the third FIR filter  140   b  outputs a signal for minimizing the self-transmitting interference signal ( ) so the third combiner  160   b  outputs the signal generated by canceling the self-transmitting interference signal ( ) from the signal xb to the LNA  170 . 
     A method for the FIR filter  140   b  to find the attenuation degrees of a plurality of attenuators (a 1 -a N ) will now be described. The method for the FIR filter  140   b  to find the attenuation degree a i  when delay intervals of the delay units (d i  (i=1, 2, . . . , N)) are the same or different will now be described. The method is expressed in Equation 6. 
     
       
         
           
             
               
                 
                   
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                         - 
                         
                           
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                           ⁢ 
                           
                             
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                     2 
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ) 
                 
               
             
           
         
       
     
     It is not easy in the time domain to find the filter coefficient (i.e., a i ) of the time domain expressed in Equation 6. Therefore, the filter coefficient may be found by converting Equation 6 into the frequency domain. A method for finding a i  of the FIR filter  140   b  in the frequency domain is expressed in Equation 7. 
     
       
         
           
             
               
                 
                   
                     min 
                     
                       
                         a 
                         1 
                       
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                       , 
                       
                           
                       
                       ⁢ 
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                   ⁢ 
                   
                     
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                               ( 
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                               ) 
                             
                           
                           
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                               - 
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                         - 
                         
                           
                             ∑ 
                             
                               m 
                               = 
                               1 
                             
                             N 
                           
                           ⁢ 
                           
                             
                               a 
                               m 
                             
                             ⁢ 
                             
                               W 
                               ⁡ 
                               
                                 ( 
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                                 ) 
                               
                             
                             ⁢ 
                             
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                     2 
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     7 
                   
                   ) 
                 
               
             
           
         
       
     
     As expressed in Equation 7, the FIR filter  140   b  may find the attenuation degrees of a plurality of attenuators (a 1 -a N ) satisfying Equation 7 by using the signal ( (f)) generated by converting the self-transmitting interference signal ( ) into the frequency domain and the signal (W(f)) generated by converting the transmitting signal (w) into the frequency domain. 
       FIG. 6  shows an in-band full duplex transceiver  100   c  according to the other exemplary embodiment of the present invention. 
     As shown in  FIG. 6 , the in-band full duplex transceiver  100   c  includes a power amplifier  110 , a divider  120 , an antenna  130 , a first FIR filter  140   c , a second FIR filter  140   c ′, a first combiner  150   c , a second combiner  150   c ′, a third combiner  160   c , a fourth combiner  160   c ′, and a low noise amplifier  170 . The in-band full duplex transceiver  100   c  of  FIG. 6  is similar to the in-band full duplex transceiver  100   b  of  FIG. 5  except that it uses two FIR filters and combines signals. Therefore, no repeated descriptions will be provided. 
     The transmitting signal (w) output by the PA  110  is input to the divider  120 , the first FIR filter  140   c , and the second FIR filter  140   c′.    
     As described with reference to  FIG. 4 , the received signal provided by the antenna  130  is phase-inverted by the hybrid transformer  121  and is then separated, and the separated received signal is output to the receiving output end Rx 1  and the receiving output end Rx 2 . The signal provided by the antenna  130  is output to the balance point (BP). Therefore, part of the received signal as well as part of the transmitting signal is output to the balance point (BP). 
     The first combiner  150   c  combines the signal output by the receiving output end Rx 1  and the signal output to the balance point (BP). In this instance, the signal output by the receiving output end Rx 1  and the signal output by the balance point (BP) have opposite phases from each other, so the first combiner  150   c  subtracts the signal output to the receiving output end Rx 1  from the signal output to the balance point (BP) and combines the two signals. In  FIG. 6 , the signal output by the first combiner  150   c  is indicated by xc 1 , and the signal xc 1  includes a self-received signal of the in-band full duplex transceiver  100   c  and a self-transmitting interference signal of the in-band full duplex transceiver  100   c . A self-transmitting interference signal from among the signal xc 1  will be denoted as  . The self-transmitting interference signal ( ) will be canceled by use of the first FIR filter  140   c.    
     The second combiner  150   c ′ combines the signal output by the receiving output end Rx 2  and the signal output by the balance point (BP). The signal output by the receiving output end Rx 2  and the signal output to the balance point (BP) have a same phase, so the second combiner  150   c ′ combines the two signals. In  FIG. 5 , the signal output by the second combiner  150   c ′ is indicated by xc 2 , and the signal xc 2  includes a self-received signal of the in-band full duplex transceiver  100   c  and a self-transmitting interference signal of the in-band full duplex transceiver  100   c . The self-transmitting interference signal from among xc 2  will be denoted as  . The self-transmitting interference signal ( ) is canceled by using the second FIR filter  140   c′.    
     The first FIR filter  140   c  receives a transmitting signal (w), generates a signal for minimizing the self-transmitting interference signal ( ), and outputs the same. The second FIR filter  140   c ′ receives a transmitting signal (w), generates a signal for minimizing the self-transmitting interference signal ( ), and outputs the same. Configurations of the first FIR filter  140   c  and the second FIR filter  140   c ′ correspond to that of the FIR filter  140  of  FIG. 3 , and operations thereof are similar to that of the FIR filter  140  of  FIG. 3 . 
     The third combiner  160   c  combines the output signal xc 1  of the first combiner  150   c  and the output signal of the first FIR filter  140   c , and outputs a resultant signal to the LNA  170 . The third combiner  160   c  subtracts the signal output by the first FIR filter  140   c  from the signal xc 1 , and combines the two signals. In this instance, the third FIR filter  140   c  outputs a signal for minimizing the self-transmitting interference signal ( ), so the third combiner  160   c  outputs a signal generated by canceling the self-transmitting interference signal ( ) from the signal xc 1  to the LNA  170 . 
     The fourth combiner  160   c ′ combines an output signal xc 2  of the second combiner  150   c ′ and an output signal of the second FIR filter  140   c ′, and outputs a resultant signal to the LNA  170 . The LNA  170  receives a signal generated by combining the self-transmitting interference signal canceled received signal by the third combiner  160   c  and the self-transmitting interference signal canceled received signal by the fourth combiner  160   c ′, cancels noise from the received signals, and amplifies the same. 
     The fourth combiner  160   c ′ subtracts the signal output by the second FIR filter  140   c ′ from the signal xc 2  and combines the two signals. In this instance, the fourth FIR filter  140   c ′ outputs a signal for minimizing the self-transmitting interference signal ( ), so the fourth combiner  160   c ′ outputs a signal generated by canceling the self-transmitting interference signal ( ) from the signal xc 2  to the LNA  170 . 
     A method for the first FIR filter  140   c  and the second FIR filter  140   c ′ to find an attenuation degree of a plurality of attenuators (a 1 -a N ) will now be described. The method for the first FIR filter  140   c  and the second FIR filter  140   c ′ to find the attenuation degree a i  when the delay intervals of the delay units (d i  (i=1, 2, . . . , N)) are the same or different will now be described. The method is expressed in Equation 8. 
     
       
         
           
             
               
                 
                   
                     
                       min 
                       
                         
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                       2 
                     
                   
                   ⁢ 
                   
                     
 
                   
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                               ⁢ 
                               
                                 
                                   w 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       t 
                                       - 
                                       
                                         d 
                                         i 
                                       
                                     
                                     ) 
                                   
                                 
                                 
                                   ︸ 
                                   
                                     Tapped 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     signal 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     of 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     reference 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     signal 
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     8 
                   
                   ) 
                 
               
             
           
         
       
     
     It is not easy in the time domain to find the filter coefficient (i.e., a i ) of the time domain expressed in Equation 8. Therefore, the filter coefficient may be found by converting Equation 8 into the frequency domain. A method for finding a i  of the first FIR filter  140   c  and the second FIR filter  140   c ′ in the frequency domain is expressed in Equation 9. 
     
       
         
           
             
               
                 
                   
                     
                       min 
                       
                         
                           a 
                           1 
                         
                         , 
                         
                           a 
                           2 
                         
                         , 
                         
                             
                         
                         ⁢ 
                         … 
                         ⁢ 
                         
                             
                         
                         , 
                         
                           a 
                           N 
                         
                       
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           
                             
                               ⁢ 
                               
                                 ( 
                                 f 
                                 ) 
                               
                             
                             
                               ︸ 
                               
                                 Self 
                                 - 
                                 Interference 
                               
                             
                           
                           - 
                           
                             
                               ∑ 
                               
                                 m 
                                 = 
                                 1 
                               
                               N 
                             
                             ⁢ 
                             
                               
                                 a 
                                 m 
                               
                               ⁢ 
                               
                                 W 
                                 ⁡ 
                                 
                                   ( 
                                   f 
                                   ) 
                                 
                               
                               ⁢ 
                               
                                 e 
                                 
                                   
                                     - 
                                     j 
                                   
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   π 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     d 
                                     m 
                                   
                                   ⁢ 
                                   f 
                                 
                               
                             
                           
                         
                         ) 
                       
                       2 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       min 
                       
                         
                           a 
                           1 
                         
                         , 
                         
                           a 
                           2 
                         
                         , 
                         
                             
                         
                         ⁢ 
                         … 
                         ⁢ 
                         
                             
                         
                         , 
                         
                           a 
                           N 
                         
                       
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           
                             
                               ⁢ 
                               
                                 ( 
                                 f 
                                 ) 
                               
                             
                             
                               ︸ 
                               
                                 Self 
                                 - 
                                 Interference 
                               
                             
                           
                           - 
                           
                             
                               ∑ 
                               
                                 m 
                                 = 
                                 1 
                               
                               N 
                             
                             ⁢ 
                             
                               
                                 a 
                                 m 
                               
                               ⁢ 
                               
                                 W 
                                 ⁡ 
                                 
                                   ( 
                                   f 
                                   ) 
                                 
                               
                               ⁢ 
                               
                                 e 
                                 
                                   
                                     - 
                                     j 
                                   
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   π 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     d 
                                     m 
                                   
                                   ⁢ 
                                   f 
                                 
                               
                             
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     9 
                   
                   ) 
                 
               
             
           
         
       
     
     As expressed in Equation 9, the FIR filter  140   c  may find the attenuation degree of a plurality of attenuators (a 1 -a N ) satisfying Equation 9 by using the signal (DeletedTexts) generated by converting the self-transmitting interference signal (DeletedTexts) into the frequency domain and the signal (W(f)) generated by converting the transmitting signal (w) into the frequency domain. The second FIR filter  140   c ′ may find the attenuation degrees of a plurality of attenuators (a 1 -a N ) satisfying Equation 9 by using the signal (DeletedTexts) generated by converting the self-transmitting interference signal (DeletedTexts) into the frequency domain and the signal (W(f)) generated by converting the transmitting signal (w) into the frequency domain. 
       FIG. 7  shows an in-band full duplex transceiver  100   d  according to the other exemplary embodiment of the present invention. 
     As shown in  FIG. 7 , the in-band full duplex transceiver  100   d  includes a power amplifier  110 , a divider  120 , an antenna  130 , an FIR filter  140   d , a first combiner  150   d , a second combiner  160   d , and a low noise amplifier  170 . 
     As described above, the hybrid transformer  121  transmits the transmitting signal (w) to the antenna  130  and the impedance matching unit  122 . That is, a signal corresponding to a signal at a rear end of the PA  110  or a transmitting signal of the antenna  130  is output to the balance point (BP). In  FIG. 7 , the signal output to the balance point (BP) is indicated by y. The received signal provided by the antenna  130  is phase-inverted by the hybrid transformer  121  and is then separated, and the separated received signal is output to the receiving output end Rx 1  and the receiving output end Rx 2 . 
     The first combiner  150   d  combines the received signals output by the receiving output end Rx 1  and the receiving output end Rx 2 . In this instance, the received signal output by the receiving output end Rx 1  and the received signal output by the receiving output end Rx 2  have opposite phases from each other, so the first combiner  150   d  subtracts the received signal output by the receiving output end Rx 2  from the received signal output by the receiving output end Rx 1  and combines the two signals. The signal output by the first combiner  150   d  corresponds to the signal x of  FIG. 1  so it is denoted as x. The signal x includes a self-received signal of the in-band full duplex transceiver  100   d  and a self-transmitting interference signal of the in-band full duplex transceiver  100 . In a like manner of  FIG. 1 , the self-transmitting interference signal from among the signal x is denoted as {tilde over (x)}. The self-transmitting interference signal ({tilde over (x)}) is canceled by using the FIR filter  140   d.    
     The FIR filter  140   d  receives an output signal (y) of the balance point (BP), generates a signal for minimizing the self-transmitting interference signal ({tilde over (x)}) and outputs the same. A configuration of the FIR filter  140   d  corresponds to that of the FIR filter  140  of  FIG. 3 , and an operation thereof is similar to that of the FIR filter  140  of  FIG. 3 . As described above, a signal corresponding to the transmitting signal (w) is output to the balance point (BP). Instead of directly using the transmitting signal (w), the FIR filter  140   d  uses the signal that corresponds to the transmitting signal (w) to generate the signal for minimizing the self-transmitting interference signal ({tilde over (x)}). 
     The second combiner  160   d  combines the output signal (x) of the first combiner  150   d  and the output signal of the FIR filter  140   d , and outputs a resultant signal to the LNA  170 . The second combiner  160   d  subtracts the signal output by the FIR filter  140   d  from the signal x and combines the two signals. In this instance, the FIR filter  140   d  outputs a signal for minimizing the self-transmitting interference signal ({tilde over (x)}) so the second combiner  160   d  outputs a signal generated by canceling the self-transmitting interference signal ({tilde over (x)}) from the signal x to the LNA  170 . 
     A method for the FIR filter  140   d  to find an attenuation degree of a plurality of attenuators (a 1 -a N ) will now be described. The method for the FIR filter  140   d  to find the attenuation degree a i  when delay intervals of the delay units (d i  (i=1, 2, . . . , N)) are the same or different will now be described. The method is expressed in Equation 10. 
     
       
         
           
             
               
                 
                   
                     min 
                     
                       
                         a 
                         1 
                       
                       , 
                       
                         a 
                         2 
                       
                       , 
                       
                           
                       
                       ⁢ 
                       … 
                       ⁢ 
                       
                           
                       
                       , 
                       
                         a 
                         N 
                       
                     
                   
                   ⁢ 
                   
                     
                       ( 
                       
                         
                           
                             
                               x 
                               ~ 
                             
                             ⁡ 
                             
                               ( 
                               t 
                               ) 
                             
                           
                           
                             ︸ 
                             
                               Self 
                               - 
                               Interference 
                             
                           
                         
                         - 
                         
                           
                             ∑ 
                             
                               i 
                               = 
                               1 
                             
                             N 
                           
                           ⁢ 
                           
                             
                               a 
                               i 
                             
                             ⁢ 
                             
                               
                                 y 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     t 
                                     - 
                                     
                                       d 
                                       i 
                                     
                                   
                                   ) 
                                 
                               
                               
                                 ︸ 
                                 
                                   Tapped 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   signal 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   of 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   reference 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   signal 
                                 
                               
                             
                           
                         
                       
                       ) 
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     10 
                   
                   ) 
                 
               
             
           
         
       
     
     Equation 10 corresponds to Equation 1 except for the substitution of w for y. 
     It is not easy in the time domain to find the filter coefficient (i.e., a i ) of the time domain expressed in Equation 10. Therefore, the filter coefficient may be found by converting Equation 10 into the frequency domain. A method for finding a i  of the FIR filter  140   d  in the frequency domain is expressed in Equation 11. 
     
       
         
           
             
               
                 
                   
                     min 
                     
                       
                         a 
                         1 
                       
                       , 
                       
                         a 
                         2 
                       
                       , 
                       
                           
                       
                       ⁢ 
                       … 
                       ⁢ 
                       
                           
                       
                       , 
                       
                         a 
                         N 
                       
                     
                   
                   ⁢ 
                   
                     
                       ( 
                       
                         
                           
                             
                               X 
                               ~ 
                             
                             ⁡ 
                             
                               ( 
                               f 
                               ) 
                             
                           
                           
                             ︸ 
                             
                               Self 
                               - 
                               Interference 
                             
                           
                         
                         - 
                         
                           
                             ∑ 
                             
                               m 
                               = 
                               1 
                             
                             N 
                           
                           ⁢ 
                           
                             
                               a 
                               m 
                             
                             ⁢ 
                             
                               Y 
                               ⁡ 
                               
                                 ( 
                                 f 
                                 ) 
                               
                             
                             ⁢ 
                             
                               e 
                               
                                 
                                   - 
                                   j 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                                 ⁢ 
                                 π 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   d 
                                   m 
                                 
                                 ⁢ 
                                 f 
                               
                             
                           
                         
                       
                       ) 
                     
                     2 
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     11 
                   
                   ) 
                 
               
             
           
         
       
     
     As expressed in Equation 11, the FIR filter  140   d  may find an attenuation degree of a plurality of attenuators (a 1 -a N ) satisfying Equation 11 by using the signal ({tilde over (X)}(f)) generated by converting the self-transmitting interference signal ({tilde over (x)}) into the frequency domain and the signal (Y(f)) generated by converting the output signal (y) of the balance point (BP) into the frequency domain. 
       FIG. 8  shows an in-band full duplex transceiver  100   e  according to the other exemplary embodiment of the present invention. 
     As shown in  FIG. 8 , the in-band full duplex transceiver  100   e  includes a power amplifier (PA)  110 , a divider  120 , an antenna  130 , a first finite impulse response (FIR) filter  140   e , a second FIR filter  140   e ′, a first combiner  160   e , a second combiner  160   e ′, and a low noise amplifier (LNA)  170 . The in-band full duplex transceiver  100   e  of  FIG. 8  is similar to the in-band full duplex transceiver  100   d  of  FIG. 7  except that it uses two FIR filters and combines signals. Therefore, no repeated descriptions will be provided. 
     Referring to  FIG. 8 , a signal output by the receiving output end Rx 1  corresponds to the signal x 1  of  FIG. 4  so it is denoted as x 1 , and a signal output by the receiving output end Rx 2  corresponds to the signal x 2  of  FIG. 4  so it is denoted as x 2 . A self-transmitting interference signal from among the signal x 1  is denoted as  , and a self-transmitting interference signal from among the signal x 2  is denoted as  . The self-transmitting interference signal   is canceled by the first FIR filter  140   e , and the self-transmitting interference signal   is canceled by the second FIR filter  140   e′.    
     The first FIR filter  140   e  receives an output signal (y) of the balance point (BP), generates a signal for minimizing the self-transmitting interference signal ( ), and outputs the same. The second FIR filter  140   e ′ receives the output signal (y) of the balance point (BP), generates a signal for minimizing the self-transmitting interference signal ( ), and outputs the same. 
     The first combiner  160   e  combines the output signal x 1  of the receiving output end Rx 1  and the output signal of the first FIR filter  140   e , and outputs a resultant signal to the LNA  170 . The first combiner  160   e  subtracts the signal output by the first FIR filter  140   e  from a signal (i.e., −x 1 ) generated by inverting the output signal x 1  of the receiving output end Rx 1  and combines the two signals. Here, the first combiner  160   e  inverts the output signal x 1  of the receiving output end Rx 1  because it is a phase-inverted received signal. In this instance, the first FIR filter  140   e  outputs a signal for minimizing the self-transmitting interference signal ( ), so the first combiner  160   e  outputs a signal generated by canceling the self-transmitting interference signal ( ) from the inverted output signal −x 1  of the receiving output end Rx 1  to the LNA  170 . 
     The second combiner  160   e ′ combines an output signal x 2  of the receiving output end Rx 2  and an output signal of the second FIR filter  140   e ′, and outputs a resultant signal to the LNA  170 . The second combiner  160   e ′ subtracts the signal output by the first FIR filter  140   e  from the output signal x 1  of the receiving output end Rx 1  and combines the two signals. In this instance, the second FIR filter  140   e ′ outputs a signal for minimizing the self-transmitting interference signal ( ) so the second combiner  160   e ′ outputs a signal generated by canceling the self-transmitting interference signal ( ) from the output signal x 2  of the receiving output end Rx 2  to the LNA  170 . The LNA  170  receives a signal generated by combining the self-transmitting interference signal canceled received signal from the first combiner  160   e  and the self-transmitting interference signal canceled received signal from the second combiner  160   e ′, cancels noise from the two signals, and amplifies the same. 
     A method for the first FIR filter  140   e  and the second FIR filter  140   e ′ to find an attenuation degree of a plurality of attenuators (a 1 -a N ) will now be described. A method for the first FIR filter  140   e  and the second FIR filter  140   e ′ to find an attenuation degree a i  when delay intervals of the delay units (d i  (i=1, 2, . . . , N)) are the same or different will now be described. The method is expressed in Equation 12. 
     
       
         
           
             
               
                 
                   
                     
                       min 
                       
                         
                           a 
                           1 
                         
                         , 
                         
                           a 
                           2 
                         
                         , 
                         
                             
                         
                         ⁢ 
                         … 
                         ⁢ 
                         
                             
                         
                         , 
                         
                           a 
                           N 
                         
                       
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           
                             
                               
                                 - 
                               
                               ⁢ 
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                             
                               ︸ 
                               
                                 Self 
                                 - 
                                 Interference 
                               
                             
                           
                           - 
                           
                             
                               ∑ 
                               
                                 i 
                                 = 
                                 1 
                               
                               N 
                             
                             ⁢ 
                             
                               
                                 a 
                                 i 
                               
                               ⁢ 
                               
                                 
                                   y 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       t 
                                       - 
                                       
                                         d 
                                         i 
                                       
                                     
                                     ) 
                                   
                                 
                                 
                                   ︸ 
                                   
                                     Tapped 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     signal 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     of 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     reference 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     signal 
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                       2 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       min 
                       
                         
                           a 
                           1 
                         
                         , 
                         
                           a 
                           2 
                         
                         , 
                         
                             
                         
                         ⁢ 
                         … 
                         ⁢ 
                         
                             
                         
                         , 
                         
                           a 
                           N 
                         
                       
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           
                             
                               ⁢ 
                               
                                 ( 
                                 t 
                                 ) 
                               
                             
                             
                               ︸ 
                               
                                 Self 
                                 - 
                                 Interference 
                               
                             
                           
                           - 
                           
                             
                               ∑ 
                               
                                 i 
                                 = 
                                 1 
                               
                               N 
                             
                             ⁢ 
                             
                               
                                 a 
                                 i 
                               
                               ⁢ 
                               
                                 
                                   y 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       t 
                                       - 
                                       
                                         d 
                                         i 
                                       
                                     
                                     ) 
                                   
                                 
                                 
                                   ︸ 
                                   
                                     Tapped 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     signal 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     of 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     reference 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     signal 
                                   
                                 
                               
                             
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     12 
                   
                   ) 
                 
               
             
           
         
       
     
     It is not easy in the time domain to find the filter coefficient (i.e., a i ) of the time domain expressed in Equation 12. Therefore, the filter coefficient may be found by converting Equation 12 into the frequency domain. A method for finding a i  of the first FIR filter  140   e  and the second FIR filter  140   e ′ in the frequency domain is expressed in Equation 13. 
     
       
         
           
             
               
                 
                   
                     
                       min 
                       
                         
                           a 
                           1 
                         
                         , 
                         
                           a 
                           2 
                         
                         , 
                         
                             
                         
                         ⁢ 
                         … 
                         ⁢ 
                         
                             
                         
                         , 
                         
                           a 
                           N 
                         
                       
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           
                             
                               - 
                               
                                 
                                   ⁢ 
                                   
                                     ( 
                                     f 
                                     ) 
                                   
                                 
                                 ︸ 
                               
                             
                             
                               Self 
                               - 
                               Interference 
                             
                           
                           - 
                           
                             
                               ∑ 
                               
                                 m 
                                 = 
                                 1 
                               
                               N 
                             
                             ⁢ 
                             
                               
                                 a 
                                 m 
                               
                               ⁢ 
                               
                                 Y 
                                 ⁡ 
                                 
                                   ( 
                                   f 
                                   ) 
                                 
                               
                               ⁢ 
                               
                                 e 
                                 
                                   
                                     - 
                                     j 
                                   
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   π 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     d 
                                     m 
                                   
                                   ⁢ 
                                   f 
                                 
                               
                             
                           
                         
                         ) 
                       
                       2 
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       min 
                       
                         
                           a 
                           1 
                         
                         , 
                         
                           a 
                           2 
                         
                         , 
                         
                             
                         
                         ⁢ 
                         … 
                         ⁢ 
                         
                             
                         
                         , 
                         
                           a 
                           N 
                         
                       
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           
                             
                               ⁢ 
                               
                                 ( 
                                 f 
                                 ) 
                               
                             
                             
                               ︸ 
                               
                                 Self 
                                 - 
                                 Interference 
                               
                             
                           
                           - 
                           
                             
                               ∑ 
                               
                                 m 
                                 = 
                                 1 
                               
                               N 
                             
                             ⁢ 
                             
                               
                                 a 
                                 m 
                               
                               ⁢ 
                               
                                 Y 
                                 ⁡ 
                                 
                                   ( 
                                   f 
                                   ) 
                                 
                               
                               ⁢ 
                               
                                 e 
                                 
                                   
                                     - 
                                     j 
                                   
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   π 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     d 
                                     m 
                                   
                                   ⁢ 
                                   f 
                                 
                               
                             
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     13 
                   
                   ) 
                 
               
             
           
         
       
     
     As expressed in Equation 13, the first FIR filter  140   e  may find an attenuation degree of a plurality of attenuators (a 1 -a N ) satisfying Equation 13 by using the signal (−{tilde over (X)}(f)) generated by converting the self-transmitting interference signal (−{tilde over (x)}) into the frequency domain and the signal (Y(f)) generated by converting the output signal (y) of the balance point (BP) into the frequency domain. The second FIR filter  140   e ′ may find an attenuation degree of a plurality of attenuators (a 1 -a N ) satisfying Equation 13 by using the signal ( (f)) generated by converting the self-transmitting interference signal ( ) into the frequency domain and the signal (Y(f)) generated by converting the output signal (y) of the balance point (BP) into the frequency domain. 
     The in-band full duplex transceiver according to exemplary embodiments of the present invention described with reference to  FIG. 1  to  FIG. 8  may be applied to a multi-input multi-output (MIMO) transceiver. A method for applying the in-band full duplex transceiver to the MIMO transceiver is known to a person skilled in the art, so no detailed description thereof will be provided. 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.