Patent Publication Number: US-10333570-B2

Title: Method and device for removing self-interference signal in environment using FDR mode

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage filing under 35 U.S.C. 371 of International Application No. PCT/KR2015/011141, filed on Oct. 21, 2015, which claims the benefit of U.S. Provisional Application Ser. No. 62/106,797 filed on Jan. 23, 2015, the contents of which are all hereby incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a wireless communication system, and more particularly, to a method for performing self-interference signal cancellation in an environment using an FDR scheme and device therefor. 
     BACKGROUND ART 
     Compared to conventional half duplex communication in which time or frequency resources are divided orthogonally, full duplex communication doubles a system capacity in theory by allowing a node to perform transmission and reception simultaneously. 
       FIG. 1  is a conceptual view of a UE and a Base Station (BS) which support Full Duplex Radio (FDR). 
     In the FDR situation illustrated in  FIG. 1 , the following three types of interference are produced. 
     Intra-device self-interference: Because transmission and reception take place in the same time and frequency resources, a desired signal and a signal transmitted from a BS or UE are received at the same time at the BS or UE. The transmitted signal is received with almost no attenuation at a Reception (Rx) antenna of the BS or UE, and thus with much larger power than the desired signal. As a result, the transmitted signal serves as interference. 
     UE to UE inter-link interference: An Uplink (UL) signal transmitted by a UE is received at an adjacent UE and thus serves as interference. 
     BS to BS inter-link interference: The BS to BS inter-link interference refers to interference caused by signals that are transmitted between BSs or heterogeneous BSs (pico, femto, and relay) in a HetNet state and received by an Rx antenna of another BS. 
     Among the tree types of interference, the intra-device self-interference (hereinafter referred to as self-interference (SI)) occurs only in the FDR system and it may result in performance degradation in the FDR system. Therefore, the SI is considered as a main problem for efficient operation of the FDR system. 
     DISCLOSURE OF THE INVENTION 
     Technical Task 
     An object of the present invention is to provide a method performed by a device supporting an FDR scheme for self-interference signal cancellation. 
     Another object of the present invention is to provide a device for not only supporting the FDR scheme but also performing the self-interference signal cancellation. 
     It will be appreciated by persons skilled in the art that the objects that could be achieved with the present invention are not limited to what has been particularly described hereinabove and the above and other objects that the present invention could achieve will be more clearly understood from the following detailed description. 
     Technical Solutions 
     In an aspect of the present invention, provided herein is a method for performing self-interference signal cancellation, the method performed by an apparatus supporting a full duplex communication (FDR) mode and including: transmitting a signal to a target node in a predetermined time interval; generating and storing a residual self-interference signal after cancelling an analog self-interference signal with respect to the signal at a radio frequency (RF) end of the device; receiving an acknowledgement/negative-acknowledgement (ACK/NACK) signal from the target node in response to the transmitted signal; and determining whether to use the stored residual self-interference signal later based on the ACK/NACK signal. When an ACK signal is received from the target node in response to the transmitted signal, the method may further include determining to discard the stored self-interference signal. On the other hand, when a NACK signal is received from the target node in response to the transmitted signal, the method may further include determining whether a signal received in the predetermined time interval is successfully decoded. In addition, the method may further include: retransmitting the signal to the target node; and when the signal received in the predetermined time interval is successfully decoded, using the stored residual self-interference signal in cancelling a digital self-interference signal associated with the retransmission. Moreover, the method may further include: retransmitting the signal to the target node; and when the signal received in the predetermined time interval is not successfully decoded, determining whether self-interference cancellation is successful. In this case, the success or failure of the self-interference cancellation may be determined based on whether a predetermined number or more of consecutive ACK signals are received from the target node. Furthermore, when it is determined that the self-interference cancellation is successful, the method may further include using the stored residual self-interference signal in cancelling a digital self-interference signal associated with the retransmission. On the contrary, when it is determined that the self-interference cancellation is not successful, the method may further include discarding the stored residual self-interference signal. 
     In another aspect of the present invention, provided herein is an apparatus performing self-interference signal cancellation, the apparatus supporting a full duplex communication (FDR) mode, the apparatus comprising: a transmitted configured to transmit a signal to a target node in a predetermined time interval; a radio frequency (RF) unit configured to generate and store a residual self-interference signal after cancelling an analog self-interference signal with respect to the signal; a receiver configured to receive an acknowledgement/negative-acknowledgement (ACK/NACK) signal from the target node in response to the transmitted signal; and a processor configured to determine whether to use the stored residual self-interference signal later based on the ACK/NACK signal. When an ACK signal is received from the target node in response to the transmitted signal, the processor may be configured to determine to discard the stored self-interference signal. On the other hand, when the receiver receives a NACK signal from the target node in response to the transmitted signal, the processor may be configured to determine whether a signal received in the predetermined time interval is successfully decoded. In addition, the transmitter may be configured to retransmit the signal to the target node. When the signal received in the predetermined time interval is successfully decoded, the RF unit may be configured to use the stored residual self-interference signal in cancelling a digital self-interference signal associated with the retransmission. 
     In the device, the transmitter may be configured to retransmit the signal to the target node. When the signal received in the predetermined time interval is not successfully decoded, the processor may be configured to determine whether self-interference cancellation is successful. In addition, the processor may be configured to determine the success or failure of the self-interference cancellation based on whether a predetermined number or more of consecutive ACK signals are received from the target node. Moreover, when it is determined that the self-interference cancellation is successful, the RF unit may be configured to use the stored residual self-interference signal in cancelling a digital self-interference signal for the retransmission. On the contrary, when it is determined that the self-interference cancellation is not successful, the processor may be configured to discard the stored residual self-interference signal. 
     Advantageous Effects 
     According to various embodiments of the present invention, self-interference cancellation complexity can be significantly reduced, thereby improving communication performance of the FDR scheme. 
     It will be appreciated by persons skilled in the art that the effects that can be achieved through the present invention are not limited to what has been particularly described hereinabove and other advantages of the present invention will be more clearly understood from the following detailed description. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
         FIG. 1  is a diagram illustrating an exemplary network supporting full-duplex/half-duplex radio modes of a user equipment according to the present invention. 
         FIG. 2  is a block diagram illustrating configurations of a base station  105  and a user equipment  110  in a wireless communication system  100 . 
         FIG. 3  is a conceptual diagram of self-interference (SI) and Tx/Rx links in an FDR communication environment. 
         FIG. 4  is a diagram illustrating positions within an RF transceiver (or RF front end) of a device to which three types of interference cancellation schemes are applied. 
         FIG. 5  is a block diagram of a self-interference cancellation (Self-IC) device included in a communication device available in an OFDM-based communication system environment in accordance with  FIG. 4 . 
         FIG. 6  is a diagram illustrating a procedure for self-interference cancellation performed by devices (e.g., user equipment, base station, etc.) in the FDR system. 
         FIG. 7  is a diagram illustrating another procedure for self-interference cancellation (Self-IC) different from that of  FIG. 6 . 
         FIG. 8  is a diagram illustrating a particular procedure for regenerating a digital SI signal and storing the digital SI signal in a memory unit based on three cases shown in Table 2. 
         FIG. 9  is a diagram for explaining a procedure for self-interference cancellation proposed in the present invention. 
         FIG. 10  is a block diagram of a device for performing the proposed method based on  FIG. 5 . 
         FIG. 11  illustrates an example in which success of self-interference cancellation (Self-IC) is expected based on consecutive HI information in accordance with embodiment 5-1. 
         FIG. 12  illustrates an example in which failure of self-interference cancellation (Self-IC) is expected based on consecutive HI information in accordance with embodiment 5-1. 
         FIG. 13  illustrates an example in which success of self-interference cancellation (Self-IC) is expected based on consecutive HI information in accordance with embodiment 5-2. 
         FIG. 14  illustrates an example in which failure of self-interference cancellation (Self-IC) is expected based on consecutive HI information in accordance with embodiment 5-2. 
         FIG. 15  illustrates an example in which success of self-interference cancellation (Self-IC) is expected based on consecutive HI information in accordance with embodiment 5-3. 
         FIG. 16  illustrates an example in which failure of self-interference cancellation (Self-IC) is expected based on consecutive HI information in accordance with embodiment 5-3. 
     
    
    
     BEST MODE FOR INVENTION 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. In the following detailed description of the invention includes details to help the full understanding of the present invention. Yet, it is apparent to those skilled in the art that the present invention can be implemented without these details. For instance, although the following descriptions are made in detail on the assumption that a mobile communication system includes 3GPP LTE system, the following descriptions are applicable to other random mobile communication systems in a manner of excluding unique features of the 3GPP LTE. 
     Occasionally, to prevent the present invention from getting vaguer, structures and/or devices known to the public are skipped or can be represented as block diagrams centering on the core functions of the structures and/or devices. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     Besides, in the following description, assume that a terminal is a common name of such a mobile or fixed user stage device as a user equipment (UE), a mobile station (MS), an advanced mobile station (AMS) and the like. And, assume that a base station (BS) is a common name of such a random node of a network stage communicating with a terminal as a Node B (NB), an eNode B (eNB), an access point (AP) and the like. Although the present specification is described based on IEEE 802.16 m system, contents of the present invention may be applicable to various kinds of other communication systems. 
     In a mobile communication system, a user equipment is able to receive information in downlink and is able to transmit information in uplink as well. Information transmitted or received by the user equipment node may include various kinds of data and control information. In accordance with types and usages of the information transmitted or received by the user equipment, various physical channels may exist. 
     The following descriptions are usable for various wireless access systems including CDMA (code division multiple access), FDMA (frequency division multiple access), TDMA (time division multiple access), OFDMA (orthogonal frequency division multiple access), SC-FDMA (single carrier frequency division multiple access) and the like. CDMA can be implemented by such a radio technology as UTRA (universal terrestrial radio access), CDMA 2000 and the like. TDMA can be implemented with such a radio technology as GSM/GPRS/EDGE (Global System for Mobile communications)/General Packet Radio Service/Enhanced Data Rates for GSM Evolution). OFDMA can be implemented with such a radio technology as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, E-UTRA (Evolved UTRA), etc. UTRA is a part of UMTS (Universal Mobile Telecommunications System). 3GPP (3rd Generation Partnership Project) LTE (long term evolution) is a part of E-UMTS (Evolved UMTS) that uses E-UTRA. The 3GPP LTE employs OFDMA in DL and SC-FDMA in UL. And, LTE-A (LTE-Advanced) is an evolved version of 3GPP LTE. 
     Moreover, in the following description, specific terminologies are provided to help the understanding of the present invention. And, the use of the specific terminology can be modified into another form within the scope of the technical idea of the present invention. 
       FIG. 2  is a block diagram for configurations of a base station  105  and a user equipment  110  in a wireless communication system  100 . 
     Although one base station  105  and one user equipment  110  (D2D user equipment included) are shown in the drawing to schematically represent a wireless communication system  100 , the wireless communication system  100  may include at least one base station and/or at least one user equipment. 
     Referring to  FIG. 2 , a base station  105  may include a transmitted (Tx) data processor  115 , a symbol modulator  120 , a transmitter  125 , a transceiving antenna  130 , a processor  180 , a memory  185 , a receiver  190 , a symbol demodulator  195  and a received data processor  197 . And, a user equipment  110  may include a transmitted (Tx) data processor  165 , a symbol modulator  170 , a transmitter  175 , a transceiving antenna  135 , a processor  155 , a memory  160 , a receiver  140 , a symbol demodulator  155  and a received data processor  150 . Although the base station/user equipment  105 / 110  includes one antenna  130 / 135  in the drawing, each of the base station  105  and the user equipment  110  includes a plurality of antennas. Therefore, each of the base station  105  and the user equipment  110  of the present invention supports an MIMO (multiple input multiple output) system. And, the base station  105  according to the present invention may support both SU-MIMO (single user-MIMO) and MU-MIMO (multi user-MIMO) systems. 
     In downlink, the transmitted data processor  115  receives traffic data, codes the received traffic data by formatting the received traffic data, interleaves the coded traffic data, modulates (or symbol maps) the interleaved data, and then provides modulated symbols (data symbols). The symbol modulator  120  provides a stream of symbols by receiving and processing the data symbols and pilot symbols. 
     The symbol modulator  120  multiplexes the data and pilot symbols together and then transmits the multiplexed symbols to the transmitter  125 . In doing so, each of the transmitted symbols may include the data symbol, the pilot symbol or a signal value of zero. In each symbol duration, pilot symbols may be contiguously transmitted. In doing so, the pilot symbols may include symbols of frequency division multiplexing (FDM), orthogonal frequency division multiplexing (OFDM), or code division multiplexing (CDM). 
     The transmitter  125  receives the stream of the symbols, converts the received stream to at least one or more analog signals, additionally adjusts the analog signals (e.g., amplification, filtering, frequency upconverting), and then generates a downlink signal suitable for a transmission on a radio channel. Subsequently, the downlink signal is transmitted to the user equipment via the antenna  130 . 
     In the configuration of the user equipment  110 , the receiving antenna  135  receives the downlink signal from the base station and then provides the received signal to the receiver  140 . The receiver  140  adjusts the received signal (e.g., filtering, amplification and frequency downconverting), digitizes the adjusted signal, and then obtains samples. The symbol demodulator  145  demodulates the received pilot symbols and then provides them to the processor  155  for channel estimation. 
     The symbol demodulator  145  receives a frequency response estimated value for downlink from the processor  155 , performs data demodulation on the received data symbols, obtains data symbol estimated values (i.e., estimated values of the transmitted data symbols), and then provides the data symbols estimated values to the received (Rx) data processor  150 . The received data processor  150  reconstructs the transmitted traffic data by performing demodulation (i.e., symbol demapping, deinterleaving and decoding) on the data symbol estimated values. 
     The processing by the symbol demodulator  145  and the processing by the received data processor  150  are complementary to the processing by the symbol modulator  120  and the processing by the transmitted data processor  115  in the base station  105 , respectively. 
     In the user equipment  110  in uplink, the transmitted data processor  165  processes the traffic data and then provides data symbols. The symbol modulator  170  receives the data symbols, multiplexes the received data symbols, performs modulation on the multiplexed symbols, and then provides a stream of the symbols to the transmitter  175 . The transmitter  175  receives the stream of the symbols, processes the received stream, and generates an uplink signal. This uplink signal is then transmitted to the base station  105  via the antenna  135 . 
     In the base station  105 , the uplink signal is received from the user equipment  110  via the antenna  130 . The receiver  190  processes the received uplink signal and then obtains samples. Subsequently, the symbol demodulator  195  processes the samples and then provides pilot symbols received in uplink and a data symbol estimated value. The received data processor  197  processes the data symbol estimated value and then reconstructs the traffic data transmitted from the user equipment  110 . 
     The processor  155 / 180  of the user equipment/base station  110 / 105  directs operations (e.g., control, adjustment, management, etc.) of the user equipment/base station  110 / 105 . The processor  155 / 180  may be connected to the memory unit  160 / 185  configured to store program codes and data. The memory  160 / 185  is connected to the processor  155 / 180  to store operating systems, applications and general files. 
     The processor  155 / 180  may be called one of a controller, a microcontroller, a microprocessor, a microcomputer and the like. And, the processor  155 / 180  may be implemented using hardware, firmware, software and/or any combinations thereof. In the implementation by hardware, the processor  155 / 180  may be provided with such a device configured to implement the present invention as ASICs (application specific integrated circuits), DSPs (digital signal processors), DSPDs (digital signal processing devices), PLDs (programmable logic devices), FPGAs (field programmable gate arrays), and the like. 
     Meanwhile, in case of implementing the embodiments of the present invention using firmware or software, the firmware or software may be configured to include modules, procedures, and/or functions for performing the above-explained functions or operations of the present invention. And, the firmware or software configured to implement the present invention is loaded in the processor  155 / 180  or saved in the memory  160 / 185  to be driven by the processor  155 / 180 . 
     Layers of a radio protocol between a user equipment/base station and a wireless communication system (network) may be classified into 1st layer L1, 2nd layer L2 and 3rd layer L3 based on 3 lower layers of OSI (open system interconnection) model well known to communication systems. A physical layer belongs to the 1st layer and provides an information transfer service via a physical channel. RRC (radio resource control) layer belongs to the 3rd layer and provides control radio resourced between UE and network. A user equipment and a base station may be able to exchange RRC messages with each other through a wireless communication network and RRC layers. 
     In the present specification, although the processor  155 / 180  of the user equipment/base station performs an operation of processing signals and data except a function for the user equipment/base station  110 / 105  to receive or transmit a signal, for clarity, the processors  155  and  180  will not be mentioned in the following description specifically. In the following description, the processor  155 / 180  can be regarded as performing a series of operations such as a data processing and the like except a function of receiving or transmitting a signal without being specially mentioned. 
     The present invention proposes an information utilization method for self-interference cancellation (Self-IC) in a full-duplex radio (FDR) system. Particularly, the invention discloses a method of storing information used in previous Self-IC for the purpose of reusing and reprocessing the stored information for current Self-IC. Considering that data to be transmitted is known in the FDR system, it is possible to implement a method of performing Self-IC using existing information such as self-interference (SI) or self-interference channel gain (or self-channel gain) when hybrid automatic repeat request (HARQ) is received. More particularly, the invention proposes a method for obtaining additional information for Self-IC using HARQ indicator (HI) information of both transmitting and receiving ends and determining information used for the Self-IC. 
       FIG. 3  is a conceptual diagram of self-interference (SI) and Tx/Rx links in an FDR communication environment. 
     Referring to  FIG. 3 , the SI can be divided into direct interference, which is caused when a signal transmitted from a transmit (Tx) antenna is received at an Rx antenna of the same device without path attenuation, and reflected interference, which is caused when a signal transmitted from a Tx antenna is reflected on a surrounding object and then received at an Rx antenna of the same device. In addition, the strength of the SI is extremely higher than that of a desired signal due to a physical distance difference. Thus, the SI should be cancelled for efficient operation of the FDR system. 
     Table 1 shows requirements of the Self-IC in accordance with a maximum Tx power of a device for the efficient operation of the FDR system. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Self-IC requirements when the FDR is applied to a mobile communication system 
               
               
                 (BW = 20 MHz) 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Receiver 
                   
               
               
                   
                 Max. Tx 
                 Thermal Noise. 
                   
                 Thermal Noise 
                 Self-IC Target 
               
               
                 Node Type 
                 Power (P A ) 
                 (BW = 20 MHz) 
                 Receiver NF 
                 Level 
                 (P A -TN-NF) 
               
               
                   
               
               
                 Macro 
                 46 dBm 
                 −101 dBm 
                 5 dB 
                 −96 dBm 
                 142 dB 
               
               
                 eNB 
                   
                   
                 (for eNB) 
               
               
                 Pico eNB 
                 30 dBm 
                   
                   
                   
                 126 dB 
               
               
                 Femto 
                 23 dBm 
                   
                   
                   
                 119 dB 
               
               
                 eNB, 
               
               
                 WLAN AP 
               
               
                 UE 
                 23 dBm 
                   
                 9 dB 
                 −92 dBm 
                 115 dB 
               
               
                   
                   
                   
                 (for UE) 
               
               
                   
               
            
           
         
       
     
     Referring to Table 1, it may be noted that to effectively operate the FDR system in a 20-MHz BW, a UE needs 119-dBm Self-IC performance. A thermal noise value may be changed to N 0,BW =−174 dBm+10×log 10 (BW) to the BW of a mobile communication system. In Table 3, the thermal noise value is calculated on the assumption of a 20-MHz BW. In relation to Table 3, for Receiver Noise Figure (NF), a worst case is considered referring to the 3GPP specification requirements. Receiver Thermal Noise Level is determined to be the sum of a thermal noise value and a receiver NF in a specific BW. 
     Types of Self-IC Schemes and Methods for Applying the Self-IC Schemes 
       FIG. 4  is a view illustrating positions at which three Self-IC schemes are applied, in a Radio Frequency (RF) Tx and Rx end (or an RF front end) of a device. Now, a brief description will be given of the three Self-IC schemes. 
     Antenna Self-IC: 
     Antenna Self-IC is a Self-IC scheme that should be performed first of all Self-IC schemes. SI is cancelled at an antenna end. Most simply, transfer of an SI signal may be blocked physically by placing a signal-blocking object between a Tx antenna and an Rx antenna, the distance between antennas may be controlled artificially, using multiple antennas, or a part of an SI signal may be canceled through phase inversion of a specific Tx signal. Further, a part of an SI signal may be cancelled by means of multiple polarized antennas or directional antennas. 
     Analog Self-IC: 
     Interference is canceled at an analog end before an Rx signal passes through an Analog-to-Digital Convertor (ADC). An SI signal is canceled using a duplicated analog signal. This operation may be performed in an RF region or an Intermediate Frequency (IF) region. SI signal cancellation may be performed in the following specific method. A duplicate of an actually received SI signal is generated by delaying an analog Tx signal and controlling the amplitude and phase of the delayed Tx signal, and subtracted from a signal received at an Rx antenna. However, due to the analog signal-based processing, the resulting implementation complexity and circuit characteristics may cause additional distortion, thereby changing interference cancellation performance significantly. 
     Digital Self-IC: 
     Interference is canceled after an Rx signal passes through an ADC. Digital Self-IC covers all IC techniques performed in a baseband region. Most simply, a duplicate of an SI signal is generated using a digital Tx signal and subtracted from an Rx digital signal. Or techniques of performing precoding/postcoding in a baseband using multiple antennas so that a Tx signal of a UE or an eNB may not be received at an Rx antenna may be classified into digital Self-IC. However, since digital Self-IC is viable only when a digital modulated signal is quantized to a level enough to recover information of a desired signal, there is a need for the prerequisite that the difference between the signal powers of a designed signal and an interference signal remaining after interference cancellation in one of the above-described techniques should fall into an ADC range, to perform digital Self-IC. 
       FIG. 5  is a block diagram of a Self-IC device in a proposed communication apparatus in an OFDM communication environment based on  FIG. 4 . 
     While  FIG. 5  shows that digital Self-IC is performed using digital SI information before Digital to Analog Conversion (DAC) and after ADC, it may be performed using a digital SI signal after Inverse Fast Fourier Transform (IFFT) and before Fast Fourier Transform (FFT). Further, although  FIG. 5  is a conceptual view of Self-IC though separation of a Tx antenna from an Rx antenna, if antenna Self-IC is performed using a single antenna, the antenna may be configured in a different manner from in  FIG. 5 . A functional block may be added to or removed from an RF Tx end and an RF Rx end shown in  FIG. 5  according to a purpose. 
     Signal Modeling in the FDR System 
     A signal received at a device (e.g., UE, BS, etc.) in the FDR system can be modeled as shown in Equation 1. 
     
       
         
           
             
               
                 
                   
                     
                       y 
                       ⁡ 
                       
                         [ 
                         n 
                         ] 
                       
                     
                     = 
                     
                       
                         
                           ∑ 
                           
                             
                               k 
                               = 
                               1 
                             
                             , 
                             … 
                             ⁢ 
                             
                                 
                             
                             , 
                             K 
                           
                           
                               
                           
                         
                         ⁢ 
                         
                           
                             
                               h 
                               
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                             ⁡ 
                             
                               [ 
                               n 
                               ] 
                             
                           
                           ⁢ 
                           
                             
                               x 
                               SI 
                               k 
                             
                             ⁡ 
                             
                               [ 
                               n 
                               ] 
                             
                           
                         
                       
                       + 
                       
                         
                           
                             h 
                             D 
                           
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                             [ 
                             n 
                             ] 
                           
                         
                         ⁢ 
                         
                           
                             x 
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                           ⁡ 
                           
                             [ 
                             n 
                             ] 
                           
                         
                       
                       + 
                       
                         z 
                         ⁡ 
                         
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                           n 
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                   , 
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equation 1, x SI [n] indicates data transmitted from an RF transmitter of the device, h SI [n] indicates a self-interference channel (self-channel) gain of the data transmitted from the RF transmitter, x D [n] indicates data that an RF receiver of the device desires to receive, h D [n] indicates a desired channel gain of the data that the RF receiver desires to receive, and z[n] indicates Additive White Gaussian Noise (AWGN). In this case, k may be considered to have a value of 5 or 7 (i.e., k=5 or 7). 
     For the above-described analog or digital Self-IC, it is necessary to estimate the self-interference channel (self-channel). In this case, after completion of the Self-IC using a gain of the estimated analog and/or digital self-interference channel, ĥ SI,k  [n], for k=1, . . . , K, the received signal of the device can be expressed as shown in Equation 2. 
     
       
         
           
             
               
                 
                   
                     
                       y 
                       
                         Self 
                         ⁢ 
                         
                           - 
                         
                         ⁢ 
                         IC 
                       
                     
                     ⁡ 
                     
                       [ 
                       n 
                       ] 
                     
                   
                   = 
                   
                     
                       
                         
                           h 
                           D 
                         
                         ⁡ 
                         
                           [ 
                           n 
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                       ⁢ 
                       
                         
                           x 
                           D 
                         
                         ⁡ 
                         
                           [ 
                           n 
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                     + 
                     
                       
                         ∑ 
                         
                           
                             k 
                             = 
                             1 
                           
                           , 
                           
                               
                           
                           ⁢ 
                           … 
                           ⁢ 
                           
                               
                           
                           , 
                           K 
                         
                         
                             
                         
                       
                       ⁢ 
                       
                         
                           
                             ( 
                             
                               
                                 
                                   h 
                                   
                                     SI 
                                     , 
                                     k 
                                   
                                 
                                 ⁡ 
                                 
                                   [ 
                                   n 
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                               - 
                               
                                 
                                   
                                     h 
                                     ^ 
                                   
                                   
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                                     , 
                                     k 
                                   
                                 
                                 ⁡ 
                                 
                                   [ 
                                   n 
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                             ) 
                           
                           ResidualSI 
                         
                         ⁢ 
                         
                           
                             x 
                             SI 
                             k 
                           
                           ⁡ 
                           
                             [ 
                             n 
                             ] 
                           
                         
                       
                     
                     + 
                     
                       
                         z 
                         ⁡ 
                         
                           [ 
                           n 
                           ] 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     When the received signal is decoded using the estimated gain of the described channel, ĥ D  [n], the received signal can be expressed as shown in Equation 3. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             
                               
                                 
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                                 D 
                                 * 
                               
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                             2 
                           
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                 
                                   
                                     
                                       h 
                                       ^ 
                                     
                                     D 
                                     * 
                                   
                                   ⁡ 
                                   
                                     [ 
                                     n 
                                     ] 
                                   
                                 
                                 ⁢ 
                                 
                                   
                                     h 
                                     D 
                                   
                                   ⁡ 
                                   
                                     [ 
                                     n 
                                     ] 
                                   
                                 
                               
                               
                                 
                                    
                                   
                                     
                                       
                                         h 
                                         ^ 
                                       
                                       D 
                                     
                                     ⁡ 
                                     
                                       [ 
                                       n 
                                       ] 
                                     
                                   
                                    
                                 
                                 2 
                               
                             
                             ⁢ 
                             
                               
                                 x 
                                 D 
                               
                               ⁡ 
                               
                                 [ 
                                 n 
                                 ] 
                               
                             
                           
                           + 
                           
                             
                               
                                 
                                   
                                     h 
                                     ^ 
                                   
                                   D 
                                   * 
                                 
                                 ⁡ 
                                 
                                   [ 
                                   n 
                                   ] 
                                 
                               
                               ⁢ 
                               
                                 
                                   z 
                                   ′ 
                                 
                                 ⁡ 
                                 
                                   [ 
                                   n 
                                   ] 
                                 
                               
                             
                             
                               
                                  
                                 
                                   
                                     
                                       h 
                                       ^ 
                                     
                                     D 
                                   
                                   ⁡ 
                                   
                                     [ 
                                     t 
                                     ] 
                                   
                                 
                                  
                               
                               2 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         
                           = 
                             
                           ⁢ 
                           
                             
                               
                                 x 
                                 D 
                               
                               ⁡ 
                               
                                 [ 
                                 n 
                                 ] 
                               
                             
                             + 
                             
                               
                                 
                                   
                                     
                                       h 
                                       ^ 
                                     
                                     D 
                                     * 
                                   
                                   ⁡ 
                                   
                                     [ 
                                     n 
                                     ] 
                                   
                                 
                                 ⁢ 
                                 
                                   e 
                                   ⁡ 
                                   
                                     [ 
                                     n 
                                     ] 
                                   
                                 
                               
                               
                                 
                                    
                                   
                                     
                                       
                                         h 
                                         ^ 
                                       
                                       D 
                                     
                                     ⁡ 
                                     
                                       [ 
                                       n 
                                       ] 
                                     
                                   
                                    
                                 
                                 2 
                               
                             
                             + 
                             
                               
                                 
                                   
                                     
                                       h 
                                       ^ 
                                     
                                     D 
                                     * 
                                   
                                   ⁡ 
                                   
                                     [ 
                                     n 
                                     ] 
                                   
                                 
                                 ⁢ 
                                 
                                   
                                     z 
                                     ′ 
                                   
                                   ⁡ 
                                   
                                     [ 
                                     n 
                                     ] 
                                   
                                 
                               
                               
                                 
                                    
                                   
                                     
                                       
                                         h 
                                         ^ 
                                       
                                       D 
                                     
                                     ⁡ 
                                     
                                       [ 
                                       n 
                                       ] 
                                     
                                   
                                    
                                 
                                 2 
                               
                             
                           
                         
                         , 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equation 3, 
                 z   ′     ⁡     [   n   ]       =         ∑       k   =   1     ,           ⁢   …   ⁢           ,   K               ⁢       (         h     SI   ,   k       ⁡     [   n   ]       -         h   ^       SI   ,   k       ⁡     [   n   ]         )     ⁢       x   SI   k     ⁡     [   n   ]           +     z   ⁡     [   n   ]               
and
 
 e[n]=ĥ   D    [n]−h   D   [n].  
 
     According to the initial digital self-interference cancellation technique, linear components of the interference signal are modeled for digital self-interference cancellation. In recent years, efforts are made to implement a digital self-interference cancellation technique of using not only linear components of interference signal information but also non-linear components of the interference signal information for feasible operation of the FDR system. As shown in Equation 1, linear and non-linear components of SI information may be determined by transmitted data and an interference channel gain of the transmitted data. Therefore, interference signal information needs to be calculated and updated every transmission. However, if an SI signal including linear and nonlinear components is calculated in real time whenever transmission is performed, it may increase the amount of calculation and complexity. Therefore, a method for reducing the complexity of the digital self-interference cancellation while efficiently operating the system needs to be developed. 
       FIG. 6  is a diagram illustrating a procedure for self-interference cancellation performed by devices (e.g., user equipment, base station, etc.) in the FDR system. 
     Referring to  FIG. 6 , a device initiates FDR operation [S 610 ] and cancels an SI signal using the antenna Self-IC scheme and analog Self-IC scheme [S 620 ]. After cancelling the SI signal by applying the antenna Self-IC scheme and analog Self-IC scheme, the device can obtain the signal of Equation 1, 
                 y   ⁡     [   n   ]       =         ∑       k   =   1     ,   …   ⁢           ,   K               ⁢         h     SI   ,   k       ⁡     [   n   ]       ⁢       x   SI   k     ⁡     [   n   ]           +         h   D     ⁡     [   n   ]       ⁢       x   D     ⁡     [   n   ]         +     z   ⁡     [   n   ]           ,   .         
Next, the device can generate a digital SI signal from the signal of Equation 1 [S 630 ] and then obtain the signal of Equation 2 by performing the digital Self-IC using the generated digital SI signal [S 640 ]. After performing the digital Self-IC on a block which is expected to contain a desired signal (i.e., a desired signal detection block), the device can detect the desired signal shown in Equation 3 through decoding [S 650 ]. After detection of the desired signal, the device can terminate the FDR operation [S 660 ].
 
       FIG. 7  is a diagram illustrating another procedure for self-interference cancellation (Self-IC) different from that of  FIG. 6 . 
     Referring to  FIG. 7 , steps S 710  to S 750  are similar to the steps S 610  to S 650  of  FIG. 6 . However, the procedure of  FIG. 7  is different from that of  FIG. 6  in that a block unit for regenerating a digital SI signal and storing the regenerated digital SI signal in a memory unit can be added to an RF chain (or RF end) and used for the Self-IC. In the FDR system, if the same data is retransmitted according to a retransmission request from a UE or BS that currently performs communication, a signal identical to the existing SI signal may be received at an RF receiver as an interference signal. Thus, if the interference signal used in the last or previous digital Self-IC or a reconfigured SI signal is reused, it may reduce or eliminate complexity in generating a new SI signal. That is, before the RF transmitter retransmits the identical signal, the device detects the desired signal that the RF receiver desires to receive [S 750 ]. Thereafter, the device regenerates the digital SI signal and then stores it in the memory unit [S 760 ]. The stored digital SI signal can be used for cancelling interference that occurs when the same data is retransmitted according to the retransmission request. 
     Meanwhile, signal detection performance significantly depends on accuracy of an estimated SI channel (ĥ SI,k [n]) and a desired channel (signal) (ĥ D  [n]). That is, the success or failure of the Self-IC can be determined according to whether the final received signal is successfully detected or not. Table 2 shows a relationship between the success or failure of the Self-IC and the success or failure of detecting the received signal (number of cases according to the success or failure of Self-IC and the success or failure of detecting the received signal). 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Success of Self-IC 
                 Failure of Self-IC 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Success of detecting 
                 Case 1 
                 None 
               
               
                   
                 received signal 
               
               
                   
                 Failure of detecting 
                 Case 2 
                 Case 3 
               
               
                   
                 received signal 
               
               
                   
                   
               
            
           
         
       
     
     In Table 2, the case 1 is a case in which the device successfully receives the desired signal after completion of the digital Self-IC, the case 2 is a case in which in spite of the successful digital Self-IC, the device fails to detect the received signal due to erroneous estimation of the desired channel or a low-quality link, and the case 3 is a case in which due to the failure of the Self-IC, the device fails to detect the received signal. In case of the failure of the Self-IC, since the strength of the interference signal is extremely higher than that of the received signal, it is determined that the device cannot detect the received signal. In  FIG. 8 , a particular procedure for regenerating a digital SI signal and storing the regenerated digital SI signal in a memory unit based on the above-described three cases. 
       FIG. 8  is a diagram illustrating a particular procedure for regenerating a digital SI signal and storing the digital SI signal in a memory unit based on the three cases shown in Table 2. 
     Basically, the success of detecting the received signal is highly related to the success or failure of the Self-IC. In addition, considering that the previous (same) data is transmitted when a target node (e.g., a UE or BS) requests retransmission, a high Self-IC success rate is expected even though variables used in the previous Self-IC are used for the current Self-IC (or the variables used in the previous Self-IC may not be changed or be modified). Therefore, the present invention proposes a method for performing the Self-IC based on ACK/NACK in response to transmitted and received signals. 
     For example, a BS transmits, to a UE, an HARQ indicator (HI) including an ACK/NACK signal in response to a signal transmitted from the UE through a physical hybrid-ARQ indicator channel (PHICH) and the UE receives the HI [S 810 ] (On the other hand, the UE may transmit ACK/NACK information through a physical uplink control channel (PUCCH)). The UE determines whether the HI information received from the BS indicates ACK or NACK [S 820 ]. If it is determined that the HI information indicates the ACK, the UE can discard the previously generated digital SI information with respect to the transmitted signal to which the response is the ACK [S 830 ]. On the other hand, if it is determined that the HI information indicates the NACK, the UE determines whether the previously desired received signal is decoded [S 840 ]. That is, for example, if the UE receives, from the BS, HI information corresponding to a NACK signal in response to a signal transmitted in a subframe n, the UE needs to determine whether a signal received in the subframe n is decoded or not. This corresponds to the case 1 of Table 2. If the received signal is decoded, the UE regenerate the digital SI signal and stores the regenerated digital SI signal in the memory unit [S 850 ]. On the contrary, if the received signal is not decoded, the UE determines whether the Self-IC is successful [S 860 ]. If the Self-IC is successful, this corresponds to the case 2 of Table 2. The UE regenerates the digital SI signal and then stores the regenerated digital SI signal in the memory unit. If the Self-IC is not successful, the UE generates a digital SI signal together with a partial SI signal [S 870 ]. The above-described Self-IC method can be applied to not only a UE but also all wireless communication devices using the FDR mode such as a BS. Moreover, to secure a capacity of the memory, the UE can determine whether to continuously store SI signals as shown in  FIG. 9 . That is, the UE may determine whether to store the SI signals based on the HI information received from the BS. 
       FIG. 9  is a diagram for explaining a procedure for self-interference cancellation proposed in the present invention. 
     Referring to  FIG. 9 , a device (e.g., UE) regenerate the digital SI signal [S 910 ] and then the regenerated digital SI signal in the memory unit [S 920 ]. Thereafter, the device determines whether HI information received from a target node (e.g., BS) indicates ACK or NACK [S 930 ]. If it is determined that the HI information indicates the ACK, the device discards SI information with respect to the transmitted signal to which the response is the ACK [S 940 ]. 
       FIG. 10  is a block diagram of a device for performing the proposed method based on  FIG. 5 . 
     To compensate and store an SI signal used for a digital Self-IC scheme, a controller  1010  with a memory block is installed in a cancellation block. Operation of the controller  1010  with the memory block will be described in detail. 
     Embodiment 1: Method for Storing a Previously Used SI Signal in the Memory Without Any Change 
     According to the embodiment 1 of the present invention, a BS/UE stores a previously estimated digital SI signal (or an SI signal of a digital end) (e.g., residual SI signal after analog self IC) for the purpose of reusing a digital SI signal. The BS/UE stores the signal in the memory to reuse an estimated SI signal, 
                     SI   ^     1     ⁡     [   n   ]       =       ∑       k   =   1     ,   …   ⁢           ,   K               ⁢           h   ^       SI   ,   k       ⁡     [   n   ]       ⁢       x   SI   k     ⁡     [   n   ]             ,         
which is used to obtain Equation 2. In this case, instead of separately storing an estimated SI channel gain (self-channel gain), ĥ SI,k [n] and transmitted data information, x SI [n], the BS/UE can store a completed digital SI signal obtained by adding a product of the Tx data and the estimated SI channel gain (self-channel gain). In addition, the BS/UE can discard the stored digital SI signal after receiving an ACK signal received from a target node (e.g., UE/BS) after elapse of a predetermined time. Alternatively, the BS/UE can maintain the stored digital SI signal after receiving an NACK signal.
 
     Embodiment 2: Method for Storing a Previously Used SI Signal in the Memory after Eliminating Encoded Tx Data Portion from the SI Signal 
     According to the embodiment 2 of the present invention, it is proposed that the BS/UE stores an SI signal estimated by subtracting information on decoded desired received data from the received signal for the purpose of reusing the digital SI signal. After successfully decoding the received signal, the BS/UE can generate the digital SI signal to be reused by using the decoded received data and an estimated desired channel gain. Generally, non-linear characteristics frequently occur in hardware due to the high strength of the SI signal and it may significantly affect a value of residual SI, 
                 ∑       k   =   1     ,           ⁢   …   ⁢           ,   K               ⁢       (         h     SI   ,   k       ⁡     [   n   ]       -         h   ^       SI   ,   k       ⁡     [   n   ]         )     ⁢       x   SI   k     ⁡     [   n   ]           ,         
which is created by a deviation of the SI signal. Thus, a method for reducing a deviation is also proposed. The new SI signal obtained by subtracting the decoded received data from the received signal can be expressed as shown in Equation 4.
 
     
       
         
           
             
               
                 
                   
                     
                       
                         SI 
                         ^ 
                       
                       Sub 
                     
                     ⁡ 
                     
                       [ 
                       n 
                       ] 
                     
                   
                   = 
                   
                     
                       
                         y 
                         ⁡ 
                         
                           [ 
                           n 
                           ] 
                         
                       
                       - 
                       
                         
                           
                             
                               h 
                               ^ 
                             
                             D 
                           
                           ⁡ 
                           
                             [ 
                             n 
                             ] 
                           
                         
                         ⁢ 
                         
                           
                             x 
                             D 
                           
                           ⁡ 
                           
                             [ 
                             n 
                             ] 
                           
                         
                       
                     
                     = 
                     
                       
                         
                           ∑ 
                           
                             
                               k 
                               = 
                               1 
                             
                             , 
                             … 
                             ⁢ 
                             
                                 
                             
                             , 
                             K 
                           
                           
                               
                           
                         
                         ⁢ 
                         
                           
                             
                               h 
                               
                                 SI 
                                 , 
                                 k 
                               
                             
                             ⁡ 
                             
                               [ 
                               n 
                               ] 
                             
                           
                           ⁢ 
                           
                             
                               x 
                               SI 
                               k 
                             
                             ⁡ 
                             
                               [ 
                               n 
                               ] 
                             
                           
                         
                       
                       + 
                       
                         
                           ( 
                           
                             
                               
                                 h 
                                 D 
                               
                               ⁡ 
                               
                                 [ 
                                 n 
                                 ] 
                               
                             
                             - 
                             
                               
                                 
                                   h 
                                   ^ 
                                 
                                 D 
                               
                               ⁡ 
                               
                                 [ 
                                 n 
                                 ] 
                               
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           
                             x 
                             D 
                           
                           ⁡ 
                           
                             [ 
                             n 
                             ] 
                           
                         
                       
                       + 
                       
                         
                           z 
                           ⁡ 
                           
                             [ 
                             n 
                             ] 
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
     The digital SI signal stored in the memory of the embodiment 1 is expressed as ŜI 1 [n] and the digital SI signal stored in the memory of the embodiment 2 is expressed as ŜI sub [n]. Since the signal ŜI sub  [n] contains even non-estimated non-linear components unlike the digital SI signal ŜI 1 [n] stored in the memory of the embodiment 1, it can contain not only an increase in Tx power but also complex non-linear components of SI occurring in the MIMO FDR system. In addition, since calculation complexity of ĥ D [n]x D [n] is significantly lower than that of the SI signal, it is possible to generate a new SI signal in which the complex non-linear components are reflected without an immense increase in the complexity. An updated SI signal obtained by combining the SI signal that reflects the non-linear components obtained in Equation 2 and the digital SI signal ŜI 1 [n] of the embodiment 1 can be expressed as shown in Equation 5.
 
 ŜI   2   [n]=α×ŜI   1   [n ]+(1−α)× ŜI   Sub   [n],   [Equation 5]
 
     In Equation 5, α is a value for determining a ratio of a previously estimated SI value to an SI value estimated by reflecting statistical characteristics. If α is 1, the SI value of the embodiment 1 is equal to that of the embodiment 2. The value of α is determined according to whether the Self-IC is successful and thus, it can be changed. The reference for determining α can be configured as follows. If a device operating in the FDR mode is fixed like a BS and located at a place without scattering, α may be set to 1 because there is no SI channel variation between store and transmission times. On the contrary, if the device operating in the FUR mode moves like a UE or located at a place with scattering, α may be set to have a value less than 1 (e.g., 0.5) because the SI channel is significantly changed between the store and transmission times. Moreover, if a new SI channel is made due to a significant change in the surrounding environment, the previously stored SI channel information may cause a negative effect on the Self-IC. Thus, if a channel value obtained by performing subtraction after decoding by setting α to 0 is used, it is possible to obtain better Self-IC results. 
     Embodiment 3: Method for Storing a Previously Used SI Signal in the Memory after Eliminating Encoded Tx Data Portion from the SI Signal 
     According to the embodiment 3 of the present invention, it is proposed that the BS/UE estimates the previous SI signal in consideration of statistical characteristics of the received signal for the purpose of reusing the digital SI signal. After successfully decoding the received signal, the BS/UE can generate the digital SI signal to be reused by using the decoded received data, the estimated SI channel gain (self-channel gain), and statistical characteristics of the desired channel gain. A minimum mean square error (MMSE) filter value can be calculated as shown in Equation 6 using x SI [n] corresponding to information known by transmission or decoding to a transmitter, power strength P x     SI    of x D [n], and distribution values σ h     SI     2  and σ h     D     2  between P x     D    and the estimated self-channel gain and desired-channel gain. 
     
       
         
           
             
               
                 
                   
                     
                       W 
                       MMSE 
                     
                     = 
                     
                       
                         
                           P 
                           
                             X 
                             SI 
                           
                         
                         ⁢ 
                         
                           σ 
                           
                             h 
                             SI 
                           
                           2 
                         
                       
                       
                         
                           
                             P 
                             
                               X 
                               SI 
                             
                           
                           ⁢ 
                           
                             σ 
                             
                               h 
                               SI 
                             
                             2 
                           
                         
                         + 
                         
                           
                             P 
                             
                               X 
                               D 
                             
                           
                           ⁢ 
                           
                             σ 
                             
                               h 
                               D 
                             
                             2 
                           
                         
                         + 
                         
                           σ 
                           z 
                           2 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     6 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equation 6, σ z   2  indicates a noise variance. If the MMSE filter factor calculated in Equation 6 is multiplied with the received signal, it is possible to obtain the SI signal in which the statistical characteristics are reflected. The SI signal can be expressed as shown in Equation 7.
 
 ŜI   MMSE   [n]=W   MMSE   y[n].   [Equation 7]
 
     The updated SI signal obtained by combining the SI signal of Equation 7 that reflects the statistical characteristics and the signal of the embodiment 1 can be expressed as shown in Equation 8.
 
 ŜI   3   [n]=α×ŜI   1   [n ]+(1−α)× ŜI   MMSE   [n],   [Equation 8]:
 
     In Equation 8, α is a value for determining a ratio of a previously estimated SI signal value to an SI signal value estimated by reflecting the statistical characteristics. If α is 1, the SI signal value of the embodiment 3 is equal to that of the embodiment 1. The value of α is determined according to whether the Self-IC is successful and thus, it can be changed. The reference for determining α can be configured as follows. If a device operating in the FDR mode is fixed like a BS and located at a place without scattering, α may be set to 1 because there is no SI channel variation between store and transmission times. On the contrary, if the device operating in the FDR mode moves like a UE or located at a place with scattering, α may be set to have a value less than 1 (e.g., 0.5) because the SI channel is significantly changed between the store and transmission times. Moreover, if a new SI channel is made due to a significant change in the surrounding environment, the previously stored SI channel information may cause a negative effect on the Self-IC. Thus, if a statistical channel value is used after setting α to 0, it is possible to obtain better Self-IC results. 
     Embodiment 4: Method for Using the SI Signal Stored in the Memory in Accordance with the Embodiments 1 to 3 in the Case of Non-adaptive HARQ 
     According to the embodiment 4 of the present invention, it is proposed that the BS/UE performs the digital Self-IC by reusing the stored SI signal when the BS/UE retransmits the same data according to a non-adaptive HARQ request. Specifically, according to the embodiment 4, if the BS/UE needs to retransmit the same data due to the non-adaptive HARQ request after elapse of a predetermined time (in case of uplink (UL), the predetermined time may be set to 8 subframes, i.e., the retransmission may be performed in a subframe (n+8), whereas in case of downlink (DL), it may be changed), the BS/UE can skip the procedure for creating the new SI signal for the Self-IC and perform the Self-IC through a digital Self-IC block using ŜI 1 [n]ŜI 2 [n] or ŜI 3 [n] stored in the memory in accordance with the embodiments 1 to 3 because a change in the SI channel gain depending on time is relatively smaller than that in the desired channel when the BS/UE operates in the FUR mode. 
     In detail, the BS/UE determines whether a signal received in a subframe n before the retransmission is decoded by using HARQ information transmitted from a target node (UE/BS). That is, in the case of the data retransmission, the BS/UE reuses the SI signal ŜI 1 [n]ŜI 2 [n] or ŜI 3 [n] in the subframe, which is stored in the memory, for the Self-IC only when successfully decoding the received signal. 
     For example, when ŜI 1 [n] is reused in the case of the data retransmission, signal modeling can be implemented as follows. If the same data is transmitted in a subframe (n+n R ) according to the retransmission request from the target UE/BS, the received signal can be modeled in the FUR system as shown in Equation 9. 
     
       
         
           
             
               
                 
                   
                     
                       y 
                       ⁡ 
                       
                         [ 
                         
                           n 
                           + 
                           
                             n 
                             R 
                           
                         
                         ] 
                       
                     
                     = 
                     
                       
                         
                           ∑ 
                           
                             
                               k 
                               = 
                               1 
                             
                             , 
                             … 
                             ⁢ 
                             
                                 
                             
                             , 
                             K 
                           
                           
                               
                           
                         
                         ⁢ 
                         
                           
                             
                               h 
                               
                                 SI 
                                 , 
                                 k 
                               
                             
                             ⁡ 
                             
                               [ 
                               
                                 n 
                                 + 
                                 
                                   n 
                                   R 
                                 
                               
                               ] 
                             
                           
                           ⁢ 
                           
                             
                               x 
                               
                                 SI 
                                 , 
                                 
                                   NA 
                                   ⁢ 
                                   
                                     - 
                                   
                                   ⁢ 
                                   HARQ 
                                 
                               
                               k 
                             
                             ⁡ 
                             
                               [ 
                               
                                 n 
                                 + 
                                 
                                   n 
                                   R 
                                 
                               
                               ] 
                             
                           
                         
                       
                       + 
                       
                         
                           
                             h 
                             D 
                           
                           ⁡ 
                           
                             [ 
                             
                               n 
                               + 
                               
                                 n 
                                 R 
                               
                             
                             ] 
                           
                         
                         ⁢ 
                         
                           
                             x 
                             D 
                           
                           ⁡ 
                           
                             [ 
                             
                               n 
                               + 
                               
                                 n 
                                 R 
                               
                             
                             ] 
                           
                         
                       
                       + 
                       
                         n 
                         ⁡ 
                         
                           [ 
                           
                             n 
                             + 
                             
                               n 
                               R 
                             
                           
                           ] 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     9 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equation 9, x SI,NA-HARQ [n+n R ] indicates data transmitted by the BS/UE in the subframe (n+n R ) in a non-adaptive HARQ manner according the retransmission request. In addition, since the data x SI,NA-HARQ [n+n R ] is transmitted using the same modulation and coding scheme (MCS) level and resource blocks (RBs) as those used in the previous data transmission, the data x SI,NA-HARQ [n+n R ] is identical to the data x SI [n] transmitted in the subframe n, i.e., x SI,NA-HARQ [n+n R ]=x SI [n] (since simple retransmission is performed, the transmitted data is the same). 
     As described above, the BS/UE performs the Self-IC on the subframe (n+n R ) through the digital Self-IC block by reusing the digital SI signal information ŜI 1 [n] that is estimated in the subframe n through the digital Self-IC block. In this case, a received signal passing through the digital Self-IC step can be expressed as shown in Equation 10. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             y 
                             
                               Self 
                               ⁢ 
                               
                                 - 
                               
                               ⁢ 
                               IC 
                             
                           
                           ⁡ 
                           
                             [ 
                             
                               n 
                               + 
                               
                                 n 
                                 R 
                               
                             
                             ] 
                           
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               ∑ 
                               
                                 
                                   k 
                                   = 
                                   1 
                                 
                                 , 
                                 … 
                                 ⁢ 
                                 
                                     
                                 
                                 , 
                                 K 
                               
                             
                             ⁢ 
                             
                               
                                 
                                   h 
                                   
                                     SI 
                                     , 
                                     k 
                                   
                                 
                                 ⁡ 
                                 
                                   [ 
                                   
                                     n 
                                     + 
                                     
                                       n 
                                       R 
                                     
                                   
                                   ] 
                                 
                               
                               ⁢ 
                               
                                 
                                   x 
                                   
                                     SI 
                                     , 
                                     
                                       NA 
                                       ⁢ 
                                       
                                         - 
                                       
                                       ⁢ 
                                       HARQ 
                                     
                                   
                                   k 
                                 
                                 ⁡ 
                                 
                                   [ 
                                   
                                     n 
                                     + 
                                     
                                       n 
                                       R 
                                     
                                   
                                   ] 
                                 
                               
                             
                           
                           - 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             
                               
                                 SI 
                                 1 
                               
                               ⁡ 
                               
                                 [ 
                                 n 
                                 ] 
                               
                             
                             + 
                             
                               
                                 
                                   h 
                                   D 
                                 
                                 ⁡ 
                                 
                                   [ 
                                   
                                     n 
                                     + 
                                     
                                       n 
                                       R 
                                     
                                   
                                   ] 
                                 
                               
                               ⁢ 
                               
                                 
                                   x 
                                   D 
                                 
                                 ⁡ 
                                 
                                   [ 
                                   
                                     n 
                                     + 
                                     
                                       n 
                                       R 
                                     
                                   
                                   ] 
                                 
                               
                             
                             + 
                             
                               n 
                               ⁡ 
                               
                                 [ 
                                 
                                   n 
                                   + 
                                   
                                     n 
                                     R 
                                   
                                 
                                 ] 
                               
                             
                           
                           , 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                 h 
                                 D 
                               
                               ⁡ 
                               
                                 [ 
                                 
                                   n 
                                   + 
                                   
                                     n 
                                     R 
                                   
                                 
                                 ] 
                               
                             
                             ⁢ 
                             
                               
                                 x 
                                 D 
                               
                               ⁡ 
                               
                                 [ 
                                 
                                   n 
                                   + 
                                   
                                     n 
                                     R 
                                   
                                 
                                 ] 
                               
                             
                           
                           + 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             ∑ 
                             
                               
                                 k 
                                 = 
                                 1 
                               
                               , 
                               
                                   
                               
                               ⁢ 
                               … 
                               ⁢ 
                               
                                   
                               
                               , 
                               K 
                             
                             
                                 
                             
                           
                           ⁢ 
                           
                             ( 
                             
                               
                                 
                                   h 
                                   
                                     SI 
                                     , 
                                     k 
                                   
                                 
                                 ⁡ 
                                 
                                   [ 
                                   
                                     n 
                                     + 
                                     
                                       n 
                                       R 
                                     
                                   
                                   ] 
                                 
                               
                               - 
                               
                                 
                                   h 
                                   
                                     SI 
                                     , 
                                     k 
                                   
                                 
                                 ⁡ 
                                 
                                   [ 
                                   n 
                                   ] 
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           
                             
                               x 
                               
                                 SI 
                                 , 
                                 
                                   NA 
                                   ⁢ 
                                   
                                     - 
                                   
                                   ⁢ 
                                   HARQ 
                                 
                               
                               k 
                             
                             ⁡ 
                             
                               [ 
                               
                                 n 
                                 + 
                                 
                                   n 
                                   R 
                                 
                               
                               ] 
                             
                           
                           + 
                           
                             
                               n 
                               ⁡ 
                               
                                 [ 
                                 
                                   n 
                                   + 
                                   
                                     n 
                                     R 
                                   
                                 
                                 ] 
                               
                             
                             . 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     10 
                   
                   ] 
                 
               
             
           
         
       
     
     It can be seen that a residual SI signal in Equation 10 has a value different from that of the residual SI signal in Equation 2. However, in the environment in which the SI channel is not or slightly changed depending on time, since a difference between 
               ∑       k   =   1     ,   …   ⁢           ,   K       ⁢         h     SI   ,   k       ⁡     [     n   +     n   R       ]       ⁢       x     SI   ,     NA   ⁢     -     ⁢   HARQ       k     ⁡     [     n   +     n   R       ]       ⁢           ⁢   and                   ∑       k   =   1     ,   …   ⁢           ,   K               ⁢         h     SI   ,   k       ⁡     [   n   ]       ⁢       x   SI   k     ⁡     [   n   ]               
is small enough to be handled by the digital Self-IC, the successful previous Self-IC results can be reused through the digital SI information stored in the memory, thereby achieving excellent Self-IC performance in the case of the retransmission.
 
     In addition, according to a sub-embodiment (embodiment 4-1) of the embodiment 4, it is proposed that when intending to perform the retransmission, the BS/UE operating in the FDR mode performs the retransmission based on the non-adaptive HARQ using the same MCS level and RBs as those used in the previous transmission and reception based on chase combining for above-mentioned reasons. 
     Moreover, according to another sub-embodiment (embodiment 4-2) of the embodiment 4, when intending to perform the retransmission, the BS/UE operating in the FDR mode can store the digital SI signal information in the memory in each or all of the 8 subframes for above-mentioned reasons. 
     Furthermore, according to a further sub-embodiment (embodiment 4-3) of the embodiment 4, when performing a transmission time interval (TTI) bundling transmission method, the BS/UE operating in the FDR mode can continuously use the SI signal information used in the previous transmission for above-mentioned reasons. Considering that the TTI bundling method is used to transmit ACK/NACK information at once for the purpose of reducing feedback overhead, the BS/UE can continuously use the SI signal information and SI channel information, which is stored in the memory for the retransmission. 
     Embodiment 5: Method Performed by an FDR Device for Determining Whether Self-IC is Successful 
     In the above embodiments, it has been described how the FDR device determines whether the Self-IC is successful. In the present embodiment, it is described how the FDR device determines whether the Self-IC is successful based on a window-based control signal when the FDR device fails to decode the received signal. 
     In the case 2 of Table 2, a device fails to decode a received signal in spite of successful Self-IC. In general, if the device fails to decode the received signal, the device cannot know whether the Self-IC is successful or not because the device cannot know which steps in the decoding and Self-IC processes are problematic. However, in this case, the SI signal can be reused as follows. 
     According to the embodiment 5, it is proposed that the BS/UE determines whether to use the previously used SI signal or SI channel by accumulating or processing information on whether the received signal is successfully decoded before the retransmission during a predetermined time. If the SI channel gain is changed, the BS/UE operating in the FDR mode anticipates whether the SI channel gain is changed by accumulating or processing the information on whether the received signal is successfully decoded before the retransmission during the predetermined time to compensate a change in the SI channel gain. Based on the anticipated results, the BS/UE determines whether to reuse the stored SI signal (i.e., ŜI 1 [n]ŜI 2 [n] or ŜI 3 [n]) or the SI channel gain (ĥ SI,k [n], for k=1, . . . , K). 
     According to a sub-embodiment (embodiment 5-1) of the embodiment 5, when the UE operating in the FDR mode performs uplink transmission, it is possible to accumulate HI information included in one PHICH or PHICH group during W subframes before the subframe n and use the accumulated HI information to obtain information on whether the decoding is successful. 
       FIG. 11  illustrates an example in which success of self-interference cancellation (Self-IC) is expected based on consecutive HI information in accordance with the embodiment 5-1. 
     Referring to  FIG. 11 , if W is, for example, 6, it is possible to expect the success of the Self-IC based on 6 consecutive pieces of ACK information before the subframe n. 
       FIG. 12  illustrates an example in which failure of self-interference cancellation (Self-IC) is expected based on consecutive HI information in accordance with the embodiment 5-1. 
     If the number of accumulated ACKs is less than a threshold, it is possible to expect the failure of the Self-IC. As shown in  FIG. 12 , if the number of accumulated ACKs is equal to or less than 5 subframes, it is possible to expect the failure of the Self-IC. 
     According to another sub-embodiment (embodiment 5-2) of the embodiment 5, when the UE operating in the FDR mode performs uplink transmission, it is possible to accumulate HI information included in one PHICH or PHICH group during W subframes including the subframe n and use the accumulated HI information to obtain information on whether the decoding is successful. 
       FIG. 13  illustrates an example in which success of self-interference cancellation (Self-IC) is expected based on consecutive HI information in accordance with the embodiment 5-2. 
     As shown in  FIG. 13 , if W is, for example, 6, it is possible to expect the success of the Self-IC based on 6 consecutive pieces of ACK information after the subframe n. 
       FIG. 14  illustrates an example in which failure of self-interference cancellation (Self-IC) is expected based on consecutive HI information in accordance with the embodiment 5-2. 
     As shown in  FIG. 14 , if the number of accumulated ACKs is less than a threshold, it is possible to expect the failure of the Self-IC. That is, if the number of ACKs accumulated during 6 subframes after the subframe n is equal to or less than 5 subframes, it is possible to expect the failure of the Self-IC. 
     According to a further sub-embodiment (embodiment 5-3) of the embodiment 5, when the UE operating in the FDR mode performs uplink transmission, it is possible to accumulate HI information included in one PHICH or PHICH group during W subframes before and after the subframe n and use the accumulated HI information to obtain information on whether the decoding is successful. 
       FIG. 15  illustrates an example in which success of self-interference cancellation (Self-IC) is expected based on consecutive HI information in accordance with the embodiment 5-3. 
     As shown in  FIG. 15 , if W is, for example, 6, it is possible to expect the success of the Self-IC based on 6 consecutive pieces of ACK information except the subframe n. 
       FIG. 16  illustrates an example in which failure of self-interference cancellation (Self-IC) is expected based on consecutive HI information in accordance with embodiment 5-3. 
     If the number of ACKs accumulated during 6 consecutive subframes except the subframe n is less than a threshold, it is possible to expect the failure of the Self-IC. For example, if the number of ACKs accumulated during the 6 consecutive subframes except the subframe n is equal to or less than 5 subframes as shown in  FIG. 16 , it is possible to expect the failure of the Self-IC. 
     As described with the embodiment 5, whether the Self-IC is successful can be estimated based on the accumulated HI information. In addition, if it is determined that the Self-IC is successfully performed, it is possible to reduce complexity in generating the SI signal to be used for the Self-IC by reusing the SI signal as mentioned with reference to the embodiments 1 to 4. 
     Further, when the FDR device performs the retransmission after determining that the Self-IC is successful based on the accumulated HI information as described above with reference to the embodiment 5, the FDR device may have been failed to perform the Self-IC. This is because information on the success of the Self-IC may not be accurate even though the accumulated HI information is used or the channel may be changed at a time when the retransmission is performed. In this case, after determining that accuracy of the stored SI signal in accordance with the embodiments 1 to 4 is low, the FDR device can discard the information stored in the memory and then perform estimation of a self-channel and reconfiguration of an SI signal. 
     The embodiments of the present invention described hereinabove are combinations of elements and features of the present invention. The elements or features may be considered selective unless otherwise mentioned. Each element or feature may be practiced without being combined with other elements or features. Further, an embodiment of the present invention may be constructed by combining parts of the elements and/or features. Operation orders described in the embodiments of the present invention may be rearranged. Some constructions of any one embodiment may be included in another embodiment and may be replaced with corresponding constructions of another embodiment. It is obvious that claims that are not explicitly cited in each other in the appended claims may be presented in combination as an embodiment of the present invention or included as a new claim by subsequent amendment after the application is filed. 
     It will be apparent to those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit and essential characteristics of the invention. Thus, the above embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the invention should be determined by reasonable interpretation of the appended claims and all change which comes within the equivalent scope of the invention are included in the scope of the invention. 
     INDUSTRIAL APPLICABILITY 
     A device capable of supporting an FDR scheme and performing self-interference signal cancellation according to the present invention can be industrially applied to various wireless communication systems including the 3GPP LTE/LTE-A system.