Patent Publication Number: US-2019173709-A1

Title: Apparatus and method for reducing signal distortion

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 10-2017-0166149, filed Dec. 5, 2017, which is hereby incorporated by reference in its entirety into this application. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to full-duplex communication technology, and more particularly, to technology for reducing signal distortion occurring in full-duplex communication. 
     2. Description of the Related Art 
     When a base station and a terminal communicate with each other using uplink/downlink channels, a typical communication system prevents interference from occurring between uplink and downlink signals by dividing a frequency or time band. 
     Here, uplink/downlink communication for dividing a frequency band is referred to as “Frequency Division Duplex (FDD)”, and uplink/downlink communication for dividing a time band is referred to as “Time Division Duplex (TDD)”. 
     Since FDD or TDD transmits uplink/downlink signals, frequency use efficiency may be decreased. However, full-duplex communication, which is a scheme for simultaneously transmitting uplink/downlink signals, can overcome the disadvantage of decreased frequency use efficiency. Since full-duplex communication is a not a scheme for dividing a frequency or time band and transmitting signals, frequency efficiency may be effectively improved. 
     However, such full-duplex communication may have limitations in that, during a procedure for receiving a signal from a transmitting end while transmitting a self-signal, the self-signal is applied as interference. Therefore, the full-duplex communication needs a procedure for effectively modeling and eliminating a self-signal that is fed back from the transmitting end in order for a receiving end to effectively detect a signal transmitted from the transmitting end. 
     Further, since the self-signal is amplified by a High-Power Amplifier (HPA) and applied from the transmitting antenna of the same system, the self-signal has very high power compared to a desired reception signal. Moreover, when a self-interference signal is an Orthogonal Frequency Division Multiplexing (OFDM) signal, a time-domain OFDM signal is composed of independently modulated subcarriers. Therefore, when these subcarriers are added in phase, a higher-intensity signal is produced, and thus a high Peak-to-Average-Power-Ratio (PAPR) appears. 
     Therefore, full-duplex communication requires a procedure for eliminating a PAPR to effectively model a self-interference signal. 
     Meanwhile, Korean Patent Application Publication No. 10-2015-0119263 discloses technology entitled “Method and Apparatus for Managing Interference in Full-Duplex Communication”. This technology discloses an apparatus and method that receive an intended signal from a first wireless device operating in a full-duplex mode and receive an interfering signal from a second wireless device for communicating with the first wireless device, and that project a matrix of the received intended signal onto a space associated with the interfering signal, thus reducing interference of the signal caused by the interfering signal. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to reduce signal distortion in an Orthogonal Frequency Division Multiplexing (OFDM) transmission signal in a full-duplex communication system. 
     Another object of the present invention is to reduce signal distortion by decreasing a Peak-to-Average-Power-Ratio (PAPR) occurring in a procedure for performing an IFFT on an OFDM signal. 
     A further object of the present invention is to reduce the size of side information required for selective mapping (SLM) so as to effectively decrease a PAPR. 
     In accordance with an aspect of the present invention to accomplish the above objects, there is provided an apparatus for reducing signal distortion, including a signal input unit for receiving Orthogonal Frequency Division Multiplexing (OFDM) signals; a symbol selection unit for selecting an OFDM symbol having a lowest Peak-to-Average-Power-Ratio (PAPR) by applying selective mapping (SLM) to the OFDM signals; an information generation unit for generating side information that includes information about a phase-shift sequence of the selected OFDM symbol; and a signal output unit for outputting a resulting signal by adding the side information to the selected OFDM symbol. 
     The signal output unit may add the side information to a message block of a Physical Link Channel (PLC) frame in the selected OFDM symbol. 
     The signal output unit may primarily transmit an OFDM symbol including the side information and secondarily output an OFDM symbol corresponding to the side information. 
     The signal input unit may output the OFDM signals such that the OFDM signals are divided into at least two OFDM signal groups. 
     The symbol selection unit may generate OFDM symbol groups by applying selective mapping to each of the at least two OFDM signal groups. 
     The symbol selection unit may calculate average PAPR values of OFDM symbols included in the at least two OFDM symbol groups and select OFDM symbols having a lowest average PAPR. 
     The symbol selection unit may calculate an average PAPR of OFDM symbols which are included in the at least two OFDM symbol groups and which are generated using an identical phase-shift sequence. 
     In accordance with another aspect of the present invention to accomplish the above objects, there is provided a method for reducing signal distortion, the method being performed using a signal distortion reduction apparatus, the method including receiving Orthogonal Frequency Division Multiplexing (OFDM) signals; selecting an OFDM symbol having a lowest Peak-to-Average-Power-Ratio (PAPR) by applying selective mapping (SLM) to the OFDM signals; generating side information that includes information about a phase-shift sequence of the selected OFDM symbol; and outputting a resulting signal by adding the side information to the selected OFDM symbol. 
     Outputting the resulting signal may be configured to add the side information to a message block of a Physical Link Channel (PLC) frame in the selected OFDM symbol. 
     Outputting the resulting signal may be configured to primarily transmit an OFDM symbol including the side information and secondarily output an OFDM symbol corresponding to the side information. 
     Receiving the OFDM signals may be configured to output the OFDM signals such that the OFDM signals are divided into at least two OFDM signal groups. 
     Selecting the OFDM symbol may be configured to generate OFDM symbol groups by applying selective mapping to each of the at least two OFDM signal groups. 
     Selecting the OFDM signal may be configured to calculate average PAPR values of OFDM symbols included in the at least two 01-DM symbol groups and select OFDM symbols having a lowest average PAPR. 
     Selecting the OFDM signal may be configured to calculate an average PAPR of OFDM symbols which are included in the at least two OFDM symbol groups and which are generated using an identical phase-shift sequence. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram illustrating an uplink/downlink communication scheme in frequency-division duplex and time-division duplex; 
         FIG. 2  is a diagram illustrating overlapping between subcarriers of an OFDM signal; 
         FIG. 3  is a graph illustrating overlapping between subcarriers of an OFDM signal; 
         FIG. 4  is a block diagram illustrating a full-duplex communication device according to an embodiment of the present invention; 
         FIG. 5  is a block diagram illustrating an apparatus for reducing signal distortion according to an embodiment of the present invention; 
         FIG. 6  is a diagram illustrating selective mapping according to an embodiment of the present invention; 
         FIG. 7  is a diagram illustrating the structure of OFDM symbols selected using selective mapping according to an embodiment of the present invention; 
         FIG. 8  is a diagram illustrating the structure of an OFDM channel in a DOCSIS 3.1 system according to an embodiment of the present invention; 
         FIG. 9  is a diagram illustrating a PLC frame included in the OFDM channel of the DOCSIS 3.1 system according to an embodiment of the present invention; 
         FIG. 10  is a diagram illustrating the message block of the DOCSIS 3.1 system according to an embodiment of the present invention; 
         FIG. 11  is a diagram illustrating selective mapping for reducing the size of side information according to an embodiment of the present invention; 
         FIG. 12  is a diagram illustrating the structure of OFDM symbols selected using selective mapping for reducing the size of side information according to an embodiment of the present invention; 
         FIG. 13  is an operation flowchart illustrating a method for reducing signal distortion according to an embodiment of the present invention; and 
         FIG. 14  is an operation flowchart illustrating a signal distortion reduction method for reducing the size of side information according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described in detail below with reference to the accompanying drawings. Repeated descriptions and descriptions of known functions and configurations which have been deemed to make the gist of the present invention unnecessarily obscure will be omitted below. The embodiments of the present invention are intended to fully describe the present invention to a person having ordinary knowledge in the art to which the present invention pertains. Accordingly, the shapes, sizes, etc. of components in the drawings may be exaggerated to make the description clearer. 
     In the present specification, it should be understood that terms such as “include” or “have” are merely intended to indicate that features, numbers, steps, operations, components, parts, or combinations thereof are present, and are not intended to exclude the possibility that one or more other features, numbers, steps, operations, components, parts, or combinations thereof will be present or added. 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. 
       FIG. 1  is a diagram illustrating an uplink/downlink communication scheme in frequency-division and time-division communication. 
     Referring to  FIG. 1 , it can be seen that Frequency-Division Duplex (FDD) and Time-Division Duplex (TDD) uplink/downlink communication schemes are depicted. 
     However, since uplink/downlink signals are transmitted by dividing a frequency band or a time band, frequency use efficiency is deteriorated. Therefore, in order to overcome this disadvantage, a Full-Duplex (DD) communication scheme, which is a simultaneous uplink/downlink transmission scheme, is required. 
       FIGS. 2 and 3  are a diagram and a graph respectively illustrating overlapping between subcarriers of an OFDM signal. 
     Referring to  FIG. 2 , it can be seen that overlapping between subcarriers of an OFDM signal is depicted. 
     Pieces of input data that are respectively received in series are converted into a number of parallel data symbols identical to the number of subcarriers. The length of the converted parallel data symbols in a time domain may be extended by a multiple of the number of subcarriers. In this case, it can be seen that, in data symbols, a high Peak-to-Average-Power-Ratio (PAPR) occurs due to an increase in peak power attributable to overlapping between subcarriers. 
     Referring to  FIG. 3 , the square root of the PAPR to the number of subcarriers of an OFDM signal is depicted. 
     As a plurality of signals are added in phase, peak power is increased by a multiple of the number of subcarriers compared to average power. In this case, the same subcarriers are modulated in the same initial phase. 
       FIG. 4  is a block diagram illustrating a full-duplex communication device according to an embodiment of the present invention. 
     Referring to  FIG. 4 , an OFDM-based full-duplex communication device  100  according to an embodiment of the present invention includes a modulation unit  110 , an Inverse Fast Fourier Transform (IFFT) unit  120 , a Digital-to-Analog Conversion (DAC) unit  130 , a transmission unit  140 , a first Analog-to-Digital Conversion (ADC) unit  150 , a second ADC unit  160 , an interference signal estimation unit  170 , a Fast Fourier Transform (FFT) unit  180 , and a demodulation unit  190 . 
     The modulation unit  110  may generate and modulate a signal. 
     The IFFT unit  120  may perform an IFFT on the modulated signal. 
     The DAC unit  130  may perform DAC on the IFFT-transformed signal. 
     The transmission signal  140  may transmit an uplink transmission signal, converted into an analog signal, to a base station or an additional terminal device through a transmitting end. 
     Here, the transmission signal  140  may amplify the transmission signal using a High-Power Amplifier (HPA). 
     The transmission signal amplified by the HPA may have power much higher than that of a reception signal. 
     Here, when the transmission frequency of the transmission signal that is to be transmitted and the reception frequency of the reception signal that is received are identical to each other, self-interference may occur at the receiving end of the full-duplex communication device  100  if the transmission signal is added to the reception signal. 
     The self-interference signal may be a high-power signal having passed through the HPA. 
     However, the reception signal may be a low-power signal received after being transmitted from a relatively long distance away. 
     Therefore, when the self-interference signal is an OFDM signal, the OFDM signal in a time domain may be composed of independently modulated subcarriers. When subcarriers are added in phase, a large-intensity signal is produced, and a high Peak-to-Average-Power-Ratio (PAPR) may occur. 
     Therefore, when a self-interference signal having high power has a high PAPR, quantization noise occurring in the ADC unit of the full-duplex communication device  100  may be greatly increased. 
     The first ADC unit  150  may perform ADC on the transmission signal. 
     The second ADC unit  160  may perform ADC on the reception signal. 
     The interference signal estimation unit  170  may estimate a self-interference signal from the transmission signal and the reception signal, converted into digital signals. 
     Here, the interference signal estimation unit  170  may cancel the estimated self-interference signal from the reception signal. 
     The FFT  180  may perform a Fast Fourier Transform (FFT) on the self-interference signal-cancelled reception signal. 
     The demodulation unit  190  may demodulate the FFT-transformed reception signal. 
     Here, an apparatus for reducing signal distortion according to an embodiment of the present invention may be coupled to the full-duplex communication device to reduce a PAPR occurring in an IFFT procedure, or may be included in the IFFT unit  120  to directly reduce a PAPR. 
       FIG. 5  is a diagram illustrating an apparatus for reducing signal distortion according to an embodiment of the present invention. 
     Referring to  FIG. 5 , the signal distortion reduction apparatus according to the embodiment of the present invention includes a signal input unit  10 , a symbol selection unit  20 , an information generation unit  30 , and a signal output unit  40 . 
     The signal input unit  10  may receive an Orthogonal Frequency Division Multiplexing (OFDM) signal. 
     The symbol selection unit  20  may select an OFDM symbol having the lowest Peak-to-Average-Power-Ratio (PAPR) by applying selective mapping (SLM) to the OFDM signal. 
     Here, when a self-interference signal is X, the symbol selection unit  20  may represent the output of an II-FT-transformed self-interference signal by the following Equation (1): 
         x   i ( n )=IFFT[ X   i ( k )]  (1)
 
     where x i  (n) may denote an n-th sample of an i-th OFDM symbol. The i-th OFDM symbol composed of N samples may be represented by the following Equation (2): 
         x   i =[ x   i (0) x   i (1) . . .  x   i ( n ) . . .  x   i ( N− 1)]  (2)
 
     In this case, the symbol selection unit  20  may calculate a PAPR of the OFDM signal using the following Equation (3): 
     
       
         
           
             
               
                 
                   PAPR 
                   = 
                   
                     
                       max 
                        
                       
                         
                            
                           
                             
                               x 
                               i 
                             
                              
                             
                               ( 
                               n 
                               ) 
                             
                           
                            
                         
                         2 
                       
                     
                     
                       E 
                        
                       
                         [ 
                         
                           
                              
                             
                               
                                 x 
                                 i 
                               
                                
                               
                                 ( 
                                 n 
                                 ) 
                               
                             
                              
                           
                           2 
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     In Equation (3), max|x i (n)| 2  may denote the maximum peak envelope power of the i-th OFDM symbol. E[|x i (n)| 2 ] may denote average power. When the number of subcarriers is N, the maximum PAPR that the OFDM symbol can have may be N. Meanwhile, a Complementary Cumulative Distribution Function (CCDF) may be used as the index of signal distortion reduction performance, and may be represented by the following Equation (4): 
         F   x ( x )= Pr [ x&gt;x   0 ]  (4)
 
     In Equation (4), when x is set to the PAPR of the signal and x 0  is set to the threshold of the PAPR, the CCDF of the PAPR may be the probability that the PAPR will be greater than the threshold. 
     The symbol selection unit  20  may reduce the PAPR of the signal using selective mapping (SLM). 
     Here, before the IFFT operation of converting the OFDM signal from a frequency-domain signal into a time-domain signal is performed, the symbol selection unit  20  may multiply a plurality of phase-shift sequences by the frequency-domain signal, and may then perform an IFFT on multiplied signals, thus calculating a PAPR. 
     Here, the symbol selection unit  20  may multiply V phase-shift sequences having phase shift vectors by the input OFDM signal, as given by the following Equation (5): 
         P =[ P   (1)   P   (2)    . . . P   (v)    . . . P   (V) ]  (5)
 
     The v-th sequence P (v)  in Equation (5) may be represented by the following Equation (6): 
         P   (v)=[   p   0   (v)   p   1   (v)    . . . p   n   (v)    . . . p   N-1   (v) ],  P   n   (v)∈{±1,±j}   (6)
 
     Here, the symbol selection unit  20  may output signals to which the phase-shift sequences have been applied, as represented by the following Equation (7): 
         X   i   (v)   =X   i   ·P   (v) , 1≤ v≤V   (7)
 
     Here, the symbol selection unit  20  may perform an IFFT on the signals to which the phase-shift sequences have been applied. 
     The symbol selection unit  20  may calculate PAPR values for the signals on which IFFT has been performed. 
     Here, the symbol selection unit  20  may select an OFDM symbol having the lowest PAPR, as represented by the following Equation (8). 
     
       
         
           
             
               
                 
                   
                     
                       v 
                       ~ 
                     
                     i 
                   
                   = 
                   
                     arg 
                      
                     
                       
                         min 
                         
                           1 
                           ≤ 
                           v 
                           ≤ 
                           V 
                         
                       
                        
                       
                         [ 
                         
                           
                             max 
                             
                               1 
                               ≤ 
                               n 
                               ≤ 
                               N 
                             
                           
                            
                           
                              
                             
                               
                                 x 
                                 i 
                                 
                                   ( 
                                   v 
                                   ) 
                                 
                               
                                
                               
                                 ( 
                                 n 
                                 ) 
                               
                             
                              
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     In Equation (8), {tilde over (v)} i  denotes the index of an OFDM symbol having the lowest PAPR, among V independent OFDM symbols for the i-th OFDM symbol. Therefore, the OFDM symbol may be represented by x i   ({tilde over (v)}     i     ) . 
     The information generation unit  30  may generate side information including information about the phase-shift sequence of the selected OFDM symbol. 
     The receiving end requires side information about the phase-shift sequences P ({tilde over (v)}     i     )  for all selected OFMD symbols so as to restore the reception signal. 
     Here, the information generation unit  30  may generate side information in which the number of bits to be transmitted for each selected OFDM symbol is log 2  V. 
     The signal output unit  40  may output a signal by adding the side information to each selected OFDM symbol. 
     The signal output unit  40  may add the side information to the message block of a Physical Link Channel (PLC) frame in each selected OFDM symbol. 
     Here, the signal output unit  40  may primarily transmit OFDM symbols including the side information and secondarily output OFDM symbols corresponding to the side information. 
     The signal output unit  40  may include the side information in the corresponding OFDM symbols without applying a PAPR cancellation technique to the OFDM symbols including the side information. 
     Further, the signal input unit  10  may divide the input OFDM signal into at least two OFDM signal groups and then output the OFDM signal groups. 
     In this case, when the number of OFDM symbols to be transmitted is equal to or greater than a preset number, or when the input OFDM signal is composed of OFDM symbols in which the length of one frame is very large, the signal input unit  10  may output the input OFDM signal by dividing the OFDM signal into at least two OFDM signal groups. 
     Here, the symbol selection unit  20  may generate OFDM symbol groups by applying selective mapping (SLM) to each of the at least two OFDM signal groups. 
     The symbol selection unit  20  may calculate the average PAPR of the OFDM symbols which are included in at least two OFDM symbol groups and which are generated using the same phase-shift sequence. 
     Here, the symbol selection unit  20  may calculate the average PAPR values of the OFDM symbols included in the at least two OFDM symbol groups, and may select OFDM symbols having the lowest average PAPR. 
     Therefore, assuming that one frame is composed of K OFDM symbols, a total of K·log 2  V bits may be required. In this case, when an available band that enables side information to be transmitted is present in a PAPR header, the optimal PAPR may be achieved when all of the side information is transmitted. Conversely, when there is no available band, it may be more effective to use a method for reducing the number of bits required for side information. 
     The symbol selection unit  20  may select OFDM symbols having the lowest average PAPR, as given by the following Equation (9): 
     
       
         
           
             
               
                 
                   
                     v 
                     ~ 
                   
                   = 
                   
                     arg 
                      
                     
                       
                         min 
                         
                           1 
                           ≤ 
                           v 
                           ≤ 
                           V 
                         
                       
                        
                       
                         [ 
                         
                           
                             1 
                             G 
                           
                            
                           
                             
                               ∑ 
                               
                                 i 
                                 = 
                                 1 
                               
                               G 
                             
                              
                             
                               
                                 max 
                                 
                                   1 
                                   ≤ 
                                   n 
                                   ≤ 
                                   N 
                                 
                               
                                
                               
                                  
                                 
                                   
                                     x 
                                     i 
                                     
                                       ( 
                                       v 
                                       ) 
                                     
                                   
                                    
                                   
                                     ( 
                                     n 
                                     ) 
                                   
                                 
                                  
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     The information generation unit  30  may generate side information including information about the phase-shift sequences of the selected OFDM symbols. 
     The receiving end requires side information about the phase-shift sequences P ({tilde over (v)}     i     )  of all selected OFDM symbols in order to restore the reception signal. 
     Here, the information generation unit  30  may generate side information in which the number of bits to be transmitted for each selected OFDM symbol is log 2  V. 
     The signal output unit  40  may add the side information to the selected OFDM symbols, and may then output a resulting signal. 
     Here, the signal output unit  40  may add the side information to the message block of the Physical Link Channel (PLC) frame of each selected OFDM symbol. 
     The signal output unit  40  may primarily transmit OFDM symbols including the side information and secondarily output OFDM symbols corresponding to the side information. 
     Here, the signal output unit  40  may include the side information in the corresponding OFDM symbols without applying a PAPR cancellation technique to the OFDM symbols including the side information. 
       FIG. 6  is a diagram illustrating selective mapping according to an embodiment of the present invention. 
     Referring to  FIG. 6 , it can be seen that an OFDM symbol having the lowest PAPR is selected using selective mapping according to an embodiment of the present invention. 
     First, in selective mapping, phase conversion may be performed in such a way as to receive an OFDM signal x i , multiply the OFDM signal by phase-shift sequences P (v) , and then convert a frequency-domain signal into a time-domain signal. 
     Here, OFDM symbols may be generated by performing an IFFT on phase-converted signals for respective phase-shift sequences. 
     The PAPR values may be calculated for respective OFDM symbols. 
     Here, the selection of an OFDM symbol may be performed in such a way as to compare PAPR values of OFDM symbols and select an OFDM symbol having the lowest PAPR, and the selected OFDM symbol x i   ({tilde over (v)}     i     )  may be transmitted through the transmitting end. 
       FIG. 7  is a diagram illustrating the structure of OFDM symbols selected using selective mapping according to an embodiment of the present invention. 
     Referring to  FIG. 7 , it can be seen that the structure of OFDM symbols selected using selective mapping according to an embodiment of the present invention is depicted. 
     The OFDM symbols may include PAPR headers  51  and  53  and a PAPR frame  52 . 
     The PAPR headers  51  and  53  may correspond to one or two OFDM symbols. 
     In this case, the PAPR headers  51  and  53  may include side information, and a PAPR reduction technique may not be applied to the PAPR headers  51  and  53 . 
     The PAPR frame  52  may be composed of K OFDM symbols, and a PAPR reduction technique may be applied to the PAPR frame  52 . 
     Here, when a total of K OFDM symbols are transmitted, K·log e  V bits may be required for the side information. 
       FIG. 8  is a diagram illustrating the structure of an OFDM channel in a Data Over Cable Service Interface Specification (DOCSIS) 3.1 system according to an embodiment of the present invention.  FIG. 9  is a diagram illustrating a PLC frame included in the OFDM channel of the DOCSIS 3.1 system according to an embodiment of the present invention.  FIG. 10  is a diagram illustrating the message block of the DOCSIS 3.1 system according to an embodiment of the present invention. 
     Referring to  FIG. 8 , the structure of the OFDM channel in the DOCSIS 3.1 system according to an embodiment of the present invention is depicted. 
     As illustrated in  FIG. 8 , the structure of the OFDM channel in the DOCSIS 3.1 system according to the embodiment of the present invention may include a Physical Link Channel (PCL). 
     Referring to  FIG. 9 , a PLC frame included in the OFDM channel of the DOCSIS 3.1 system according to the embodiment of the present invention is depicted. 
     Here, side information may be included in the message block of the PLC frame. 
     In detail, a PAPR-related message block (MB) may be added to a specific portion of the PLC frame. 
     Referring to  FIG. 10 , the message block of the DOCSIS 3.1 system according to the embodiment of the present invention may be a generic format of a message block for future use. 
     Therefore, PAPR-related signaling may be added to the corresponding MB. 
     Information included in the message block to which side information is to be added according to an embodiment of the present invention may be described as shown in Table 1. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Field 
                 Size 
                 Value 
                 Description 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Message Block Type 
                 4 
                 bits 
                 5 
                   
               
               
                 R 
                 3 
                 bits 
                 0 
                 Reserved 
               
               
                 Message Body Size 
                 9 
                 bits 
                 64 
                 The length of the 
               
               
                   
                   
                   
                   
                 Message Body field 
               
               
                   
                   
                   
                   
                 specified in octets. 
               
               
                 PAPR Cancellation- 
                 512 
                 bits 
                 TBD 
                 TBD 
               
               
                 related 
               
               
                 Message Body 
               
               
                 CRC 
                 3 
                 bytes 
                   
                 CRC-24-D 
               
               
                   
               
            
           
         
       
     
     Further, the PAPR cancellation-related message body field described in Table 1 may be stated in detail, as shown in Table 2. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Field 
                 Size 
                 Value 
                 Description 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 PAPR ON 
                 1 
                 bits 
                   
                 PAPR on/off selection 
               
               
                 PAPR Mode 
                 7 
                 bits 
                 0-SLM 
                 PAPR Mode 
               
               
                   
                   
                   
                 1-PTS 
               
               
                   
                   
                   
                 2-TBD 
               
               
                   
                   
                   
                 (others) 
               
               
                 PAPR reduction value 
                 63 
                 bytes 
                 0-503 
                 PAPR Reduction Value 
               
               
                   
               
            
           
         
       
     
     In this case, an example of the message body stated in Table 2 may be changed by a user. 
       FIG. 11  is a diagram illustrating selective mapping for reducing the size of side information according to an embodiment of the present invention. 
     Referring to  FIG. 11 , selective mapping for reducing the size of side information is depicted. 
     For this, selective mapping is configured to output an input OFDM signal by dividing the input OFDM signal into at least two OFDM signal groups. 
     Here, at least two OFDM symbol groups may be generated by applying phase-shift sequences to each of the at least two OFDM signal groups and by performing an IFFT on the applied results. 
     Here, OFDM symbols having the lowest average PAPR may be selected by calculating the average PAPR values of OFDM symbols included in the at least two OFDM symbol groups. 
     Here, it can be seen that the OFDM symbols having the lowest average PAPR are selected by calculating the average PAPR values of OFDM symbols which are included in the at least two OFDM symbol groups and which are generated using the same phase-shift sequence. 
       FIG. 12  is a diagram illustrating the structure of OFDM symbols selected using selective mapping for reducing the size of side information according to an embodiment of the present invention. 
     Referring to  FIG. 12 , it can be seen that the structure of OFDM symbols selected using selective mapping for reducing the size of side information according to the embodiment of the present invention is depicted. 
     The OFDM symbols may include a PAPR header  61  and a PAPR frame  62 . 
     The PAPR header  61  may correspond to one or two OFDM symbols. 
     The PAPR header  61  may include side information, and a PAPR reduction technique may not be applied to the PAPR header  61 . 
     The PAPR frame  62  may be composed of G OFDM symbols, and the PAPR reduction technique may be applied to the PAPR frame  62 . 
     In this case, when a total of K OFDM symbols are transmitted, K·log 2  V bits may be required for the side information. 
     In this case, the number of bits of side information required for G OFDM symbols is log 2  V. When G&gt;1, this value is less than that of the case where the number of bits of side information required in a typical PAPR cancellation technique is G·log 2  V. When the total length of the entire frame is K, and L=K/G is assumed, the number of bits of side information required in the PAPR cancellation technique for one frame is L·log 2  V. In contrast, the number of bits of side information required in the typical PAPR cancellation technique is K·log 2  V. 
       FIG. 13  is an operation flowchart illustrating a method for reducing signal distortion according to the embodiment of the present invention. 
     Referring to  FIG. 13 , the signal distortion reduction method according to the embodiment of the present invention may receive a signal at step S 210 . 
     That is, at step S 210 , an Orthogonal Frequency Division Multiplexing (OFDM) signal may be received. 
     Next, the signal distortion reduction method according to the embodiment of the present invention may perform an IFFT at step S 220 . 
     That is, at step S 220 , V phase-shift sequences having phase shift vectors may be multiplied by the input OFDM signal, as shown in Equation (5). 
     At step S 220 , signals to which the phase-shift sequences have been applied may be output, as shown in Equation (7). 
     At step S 220 , the IFFT may be performed on the signals to which the phase-shift sequences have been applied. 
     Next, the signal distortion reduction method according to the embodiment of the present invention may calculate a PAPR at step S 230 . 
     That is, at step S 230 , the PAPR of each signal on which the IFFT has been performed may be calculated using Equation (3). 
     Then, the signal distortion reduction method according to the embodiment of the present invention may select an OFDM symbol at step S 240 . 
     That is, at step S 240 , the OFDM symbol having the lowest PAPR may be selected, as shown in Equation (8). 
     Thereafter, the signal distortion reduction method according to the embodiment of the present invention may output a signal at step S 250 . 
     That is, at step S 250 , side information including information about the phase-shift sequence of the selected OFDM symbol may be generated. 
     A receiving end requires side information about the phase-shift sequences P ({tilde over (v)}     i     )  for all selected OFDM symbols so as to restore a reception signal 
     At step S 250 , a resulting signal may be output by adding the side information to the selected OFDM symbol. 
     At step S 250 , the side information may be added to the message block of a Physical Link Channel (PLC) frame in the selected OFDM symbol. 
     Further, at step S 250 , OFDM symbols including the side information may be primarily transmitted, and OFDM symbols corresponding to the side information may be secondarily output. 
     Here, at step S 250 , the side information may be included in the corresponding symbols without applying a PAPR cancellation technique to the OFDM symbols including the side information. 
       FIG. 14  is an operation flowchart illustrating a method for reducing signal distortion in an OFDM signal group according to an embodiment of the present invention. 
     Referring to  FIG. 14 , the signal distortion reduction method according to the embodiment of the present invention may receive signals at step S 310 . 
     That is, at step S 310 , OFDM signals may be received. 
     Further, the signal distortion reduction method according to the embodiment of the present invention may set OFDM symbol groups at step S 320 . 
     In detail, at step S 320 , the received OFDM signals may be output such that they are divided into at least two OFDM signal groups. 
     For example, at step S 320 , when the number of OFDM symbols to be transmitted is equal to or greater than a preset number or when the input OFDM signal is composed of OFDM symbols in which the length of one frame is very large, the received OFDM signals may be output such that they are divided into at least two OFDM signal groups. 
     Next, the signal distortion reduction method according to the embodiment of the present invention may perform an IFFT at step S 330 . 
     That is, at step S 330 , V phase-shift sequences having phase shift vectors may be multiplied by each signal included in the set OFDM signal groups, as shown in Equation (5). 
     Here, at step S 330 , signals to which the phase-shift sequences have been applied may be output, as shown in Equation (7). 
     At step S 330 , an IFFT may be performed on the signals to which the phase-shift sequences have been applied. 
     In this case, at step S 330 , OFDM symbol groups may be generated by performing an IFFT on each of the at least two OFDM signal groups. 
     Next, the signal distortion reduction method according to the embodiment of the present invention may calculate an average PAPR at step S 340 . 
     That is, at step S 340 , the PAPR values of the signals on which the IFFT has been performed may be calculated using Equation (3). 
     Here, at step S 340 , the average PAPR of the OFDM symbols which are included in at least two OFDM symbol groups and which are generated using the same phase-shift sequence may be calculated. 
     Then, the signal distortion reduction method according to the embodiment of the present invention may select OFDM symbols at step S 350 . 
     That is, at step S 350 , OFDM symbols having the lowest average PAPR may be selected by calculating the average PAPR values of the OFDM symbols included in the at least two OFDM symbol groups. 
     Thereafter, the signal distortion reduction method according to the embodiment of the present invention may output a signal at step S 360 . 
     That is, at step S 360 , side information including information about the phase-shift sequences of the selected OFDM symbols may be generated. 
     The receiving end requires side information about the phase-shift sequences P ({tilde over (v)}     i     )  of all selected OFDM symbols in order to restore a reception signal. 
     Here, at step S 360 , the side information may be added to the selected OFDM symbols, and then a resulting signal may be output. 
     Here, at step S 360 , the side information may be added to the message block of the Physical Link Channel (PLC) frame in each selected OFDM symbol. 
     Further, at step S 360 , OFDM symbols including the side information may be primarily transmitted, and OFDM symbols corresponding to the side information may be output. 
     Here, at step S 360 , the side information may be included in the corresponding OFDM symbols without a PAPR cancellation technique being applied to the OFDM symbols including the side information. 
     Accordingly, the present invention may reduce signal distortion in an Orthogonal Frequency Division Multiplexing (OFDM) transmission signal in a full-duplex communication system. 
     Further, the present invention may reduce signal distortion by decreasing a PAPR occurring in a procedure for performing an IFFT on an OFDM signal. 
     Furthermore, the present invention may reduce the size of side information required for selective mapping (SLM) so as to effectively decrease a PAPR. 
     As described above, in the apparatus and method for reducing signal distortion according to the present invention, the configurations and schemes in the above-described embodiments are not limitedly applied, and some or all of the above embodiments can be selectively combined and configured such that various modifications are possible.