Abstract:
Provided are a method and apparatus for estimating and correcting the phase error in 5G or pre-5G communication systems providing much higher data rates compared to existing 4G communication systems including LTE systems. The existing phase error estimation scheme using a cyclic prefix in the time domain may fail to prevent performance degradation due to inter-carrier interference. In the present invention, it is possible to enhance reception performance of the receiver by estimating and correcting the phase error multiple times within a symbol using a time domain signal and by reducing the influence of inter-carrier interference.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY 
       [0001]    The present application is related to and claims priority under 35 U.S.C. 119(a) to Korean patent application no. 10-2016-0010235, which was filed in the Korean intellectual property office on Jan. 27, 2016, and Korean patent application no. 10-2016-0049882, which was filed in the Korean intellectual property office on Apr. 25, 2016, the entire contents of each of which are incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a method and apparatus for estimating and correcting the phase error in a wireless communication system based on orthogonal frequency-division multiplexing (OFDM). 
       BACKGROUND 
       [0003]    To cope with the increasing demand for wireless data traffic after commercialization of 4G communication systems, active efforts are underway to develop enhanced 5G or pre-5G communication systems. As such, 5G or pre-5G communication systems are referred to as beyond 4G communication systems or post LTE systems. 
         [0004]    To achieve high data rates, use of the extremely high frequency (mmWave) band (e.g. 60 GHz band) is expected in a 5G communication system. To reduce propagation path loss and to increase propagation distance in the mmWave band, use of various technologies such as beamforming, massive MIMO, full dimensional MIMO (FD-MIMO), array antenna, analog beamforming and large scale antenna are under discussion for 5G communication systems. 
         [0005]    To enhance system networks, various technologies such as evolved or advanced small cell, cloud radio access network (cloud RAN), ultra-dense network, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP) and interference cancellation are under development for 5G communication systems. 
         [0006]    In addition, for 5G communication systems, hybrid FSK and QAM modulation (FQAM) and sliding window superposition coding (SWSC) are under development for advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA) and sparse code multiple access (SCMA) are under development for advanced access technologies. 
         [0007]    In an existing wireless communication system based on orthogonal frequency-division multiplexing (OFDM), for phase error estimation in the frequency domain, the common phase error (CPE) affecting all OFDM subcarriers is estimated and corrected using a reference signal, and, for inter-carrier interference (ICI) reduction in the time domain, the phase error is estimated and corrected on a per-symbol basis using a cyclic prefix. 
         [0008]    However, when the phase error rapidly changes in the symbol duration, phase error correction using a cyclic prefix may fail to significantly reduce the influence of inter-carrier interference. Particularly, in the case of a millimeter wave communication system, as the phase error occurs severely owing to characteristics of radio frequency integrated circuits (RFIC), phase error estimation on a per-symbol basis alone may fail to prevent performance degradation due to inter-carrier interference. 
       SUMMARY 
       [0009]    To address the above-discussed deficiencies, it is a primary object to provide a method and apparatus for estimating and correcting the phase error in a wireless communication system based on OFDM. In particular, an aspect of the present invention is to provide a method and apparatus for estimating the phase error within a symbol. 
         [0010]    In accordance with an aspect of the present invention, there is provided a method of phase error estimation and correction for a receiver. The method may include: receiving a time-domain signal; filtering the received time-domain signal; and estimating the phase error using the filtered time-domain signal, wherein a correlation regarding the time-domain signal is used for phase error estimation. 
         [0011]    In accordance with another aspect of the present invention, there is provided a receiver capable of phase error estimation and correction. The receiver may include: a receive circuitry to receive a time-domain signal; and a controller to control a process of filtering the received time-domain signal, and estimating the phase error using the filtered time-domain signal, wherein a correlation regarding the time-domain signal is used for phase error estimation. 
         [0012]    In accordance with another aspect of the present invention, there is provided a method for a transmitter to send a reference signal for phase error estimation. The method may include: generating sequences of a reference signal; determining a resource to which the reference signal is to be mapped; mapping the reference signal to the determined resource; and sending the mapped reference signal to a receiver, wherein the sequences of the reference signal are correlated in the time domain. 
         [0013]    In accordance with another aspect of the present invention, there is provided a transmitter capable of sending a reference signal for phase error estimation. The transmitter may include: a transmit circuitry to send a reference signal to a receiver; and a controller to control a process of generating sequences of the reference signal, determining a resource to which the reference signal is to be mapped, mapping the reference signal to the determined resource, and sending the mapped reference signal to the receiver, wherein the sequences of the reference signal are correlated in the time domain. 
         [0014]    Before undertaking the DETAILED DESCRIPTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
           [0016]      FIG. 1  illustrates an example representation of a transmission signal according to embodiments of the present disclosure; 
           [0017]      FIG. 2  illustrates an example allocation of TRSs to different frequencies on a per-symbol basis according to embodiments of the present disclosure; 
           [0018]      FIG. 3  illustrates an example first illustration of a time-domain signal {x(t)} according to embodiments of the present disclosure; 
           [0019]      FIG. 4  illustrates an example second illustration of a time-domain signal {x(t)} according to embodiments of the present disclosure; 
           [0020]      FIG. 5  illustrates an example frequency-domain signal {X(f)} corresponding to the time-domain signal {x(t)} according to embodiments of the present disclosure; 
           [0021]      FIG. 6  illustrates a flowchart of a method for signal transmission of the transmitter according to embodiments of the present disclosure; 
           [0022]      FIG. 7A  illustrates a flowchart of a method for signal reception of the receiver according to embodiments of the present disclosure; 
           [0023]      FIG. 7B  illustrates a flowchart describing detailed receive operations performed by the estimator according to embodiments of the present disclosure; 
           [0024]      FIGS. 7C and 7D  illustrate a flowchart describing detailed receive operations performed by the corrector according to embodiments of the present disclosure; 
           [0025]      FIG. 8  illustrates an example estimated phase error values in the time domain according to embodiments of the present disclosure; 
           [0026]      FIG. 9  illustrates an example phase error values for subcarriers on a per-symbol basis after phase error correction in the time domain and FFT are applied in sequence according to embodiments of the present disclosure; 
           [0027]      FIG. 10  illustrates an example transmitter according to embodiments of the present disclosure; 
           [0028]      FIG. 11  illustrates an example receiver according to embodiments of the present disclosure; and 
           [0029]      FIG. 12  illustrates another example receiver according to embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]      FIGS. 1 through 12 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged electronic device. 
         [0031]    Hereinafter, exemplary embodiments of the present invention are described in detail with reference to the accompanying drawings. Descriptions of functions and structures well known in the art and not directly related to the present invention may be omitted to avoid obscuring the subject matter of the present invention. The terms and words used in the following description are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the present invention. Hence, these terms should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art. 
         [0032]    It should be understood by those skilled in the art that the subject matter of the present invention is applicable to other systems having similar technical backgrounds without significant modifications departing from the scope of the present invention. 
         [0033]    The aspects, features and advantages of certain embodiments of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings. The description of the various embodiments is to be construed as exemplary only and does not describe every possible instance of the present invention. It should be apparent to those skilled in the art that the following description of various embodiments of the present invention is provided for illustration purpose only and not for the purpose of limiting the present invention as defined by the appended claims and their equivalents. The same reference symbols are used throughout the description to refer to the same parts. 
         [0034]    For phase error estimation in an existing wireless communication system based on OFDM, the common phase error (CPE) is estimated and corrected using a reference signal in the frequency domain. However, in the frequency domain, the receiver can correct common phase error components only but cannot correct random phase error components affecting inter-carrier interference (ICI). In the time domain, the receiver may reduce ICI by estimating and correcting the phase error on a per-symbol basis using a cyclic prefix. 
         [0035]    As the existing error estimation scheme uses a cyclic prefix in the time domain, it can estimate the phase error only on a per-symbol basis. However, when the phase error rapidly changes in the symbol duration, phase error correction using a cyclic prefix may fail to significantly reduce the influence of inter-carrier interference. Particularly, in the case of a millimeter wave communication system, as the phase error occurs severely owing to characteristics of RFIC, phase error estimation on a per-symbol basis alone may fail to prevent performance degradation due to inter-carrier interference. 
         [0036]    The present invention proposes a method and apparatus for estimating and correcting the phase error in a wireless communication system based on OFDM. In particular, it is possible to optimize the reception performance of the receiver by estimating the phase error within a symbol. To this end, the following description focuses on schemes for transmission and reception. 
         [0037]      FIG. 1  illustrates an example representation of a transmission signal. 
         [0038]    Referring to  FIG. 1 , resources of a typical OFDM-based wireless communication system may be represented in the time and frequency domains. In  FIG. 1 , the horizontal axis  100  indicates time, and the vertical axis  101  indicates frequency. A RB indicated by indicia  120  is used for a reference signal for phase error estimation (referred to as time-domain reference signal or TRS). A RB indicated by indicia  110  is used for data transmission. In the present invention, the property is used that a signal dense in the frequency domain can be transformed through filtering into a signal spread in the time domain. More specifically, length N frequency-domain signal {X(f)}, f=0,1, . . . ,N−1 allocated to TRS RBs  120  may be converted through filtering and Inverse Fast Fourier Transform (IFFT) into time-domain signal {x(t)}, t=0,1, . . . ,N−1 evenly distributed among symbols  140 . Then, it is possible to estimate the phase error within a symbol on the basis of the time-domain signal {x(t)}. 
         [0039]    In  FIG. 1 , it should be noted that the position in frequency of TRS may be changed. For example, TRS may be positioned in the center of the bandwidth or in another portion thereof. 
         [0040]      FIG. 2  illustrates an example allocation of TRSs to different frequencies on a per-symbol basis. RBs  220  for TRSs are assigned to different frequencies on a per-symbol basis, and the remaining resources are RBs  210  for data. It is possible to perform channel estimation for multiple frequency bands by use of TRSs allocated as shown in  FIG. 2 . 
         [0041]      FIG. 3  illustrates an example first illustration of a time-domain signal {x(t)}. 
         [0042]    As shown in  FIG. 3 , it is possible to compose a time-domain signal {x(t)} by repeating a length L sequence x(0), x(1), . . . , x(L−1) R times. That is, a length L sequence is repeated R times to form a length N time-domain signal {x(t)} (N=L*R). Here, indicia  300 ,  310  and  320  indicate repetition  1 , repetition  2 , and repetition R, respectively. In this case, the receiver may obtain the phase difference by use of delay correlation between length L sequences or obtain the phase error by use of autocorrelation of a length L sequence, and correct the phase error on a cycle of L. 
         [0043]      FIG. 4  illustrates an example second illustration of a time-domain signal {x(t)}. 
         [0044]    As shown in  FIG. 4 , it is possible to compose a time-domain signal {x(t)} by concatenating length L sequences. That is, length N time-domain signal {x(t)} . . . ,N−1)=x(0), x(1), . . . , x(L−1), x(L), x(L+1), . . . , x(2L−1), . . . , x(N−1). Here, indicia  400 ,  410  and  420  indicate length L sequences, respectively. Such sequences are an example of a time-domain signal. The time-domain signal {x(t)} may be composed of partially correlated sequences. In this case, the receiver may correct the phase error by obtaining the same number of phase differences as the number of partial correlations. For example, the receiver may use quarter correlated sequences to thereby correct the phase error on a cycle of N/4. 
         [0045]    Binary sequences may be used to form a time-domain signal. For example, assuming that N=16 and L=4, {x(0), x(1), x(2), x(3)}=[1, 1, 1, −1] in  FIG. 3 . In  FIG. 4 , {x(0), x(1), x(2), x(3)}=[−1, 1, 1, 1], {x(4), x(5), x(6), x(7)}=[1, −1, 1, 1], {x(8), x(9), x(10), x(11)}=−{x(0), x(1), x(2), x(3)}, {x(12), x(13), x(14), x(15)}={x(4), x(5), x(6), x(7)}. Complex sequences such as Zadoff-Chu sequences may also be used. However, the present invention is not limited to binary or complex sequences, and other types of sequences may be used for a time-domain signal to obtain a gain in performance of the receiver. In  FIG. 3 or 4 , the number of correlations for phase error estimation may be reduced if necessary. 
         [0046]    A time-domain signal {x(t)} may be transformed through Fast Fourier Transform (FFT) into a frequency-domain signal {X(f)}, which may be allocated by the transmitter to RBs  120  as shown in  FIG. 1 . For filtering, the transmitter may not allocate the frequency-domain signal to the border resources in the frequency domain (dotted resources not corresponding to X(1), X(2), . . . , X(N−1) in  FIG. 1 ) or the central resource therein. 
         [0047]    The receiver may estimate the common phase error by use of a frequency-domain signal {X(f)}.  FIG. 5  illustrates an example frequency-domain signal {X(f)} corresponding to the time-domain signal {x(t)} of  FIG. 3 . In  FIG. 5 , the receiver may estimate the common phase error by use of M frequency-domain signal values mapped to specific frequency resources among N values of the frequency-domain signal {X(f)} (M≦N). Similarly, in the case of the time-domain signal {x(t)} of  FIG. 4 , the receiver may estimate the common phase error by use of M frequency-domain signal values mapped to specific frequency resources among N values of the frequency-domain signal {X(f)} (M≦N). 
         [0048]    Accordingly, the receiver may use a time-domain signal {x(t)} on the basis of TRSs sent by the transmitter to estimate and correct the phase error within a symbol, reducing the influence of inter-carrier interference. In addition, the receiver may use a frequency-domain signal {X(f)} to correct the common phase error. 
         [0049]      FIG. 6  illustrates a flowchart of a method for signal transmission of the transmitter. 
         [0050]    Referring to  FIG. 6 , at step  600 , the transmitter generates sequences of TRS. At step  610 , the transmitter determines a resource to which TRS is to be mapped. At step  620 , the transmitter maps TRS to the determined resource. At step  630 , the transmitter sends TRS to the receiver. The TRS sequences may be obtained by applying FFT to a time-domain signal or obtained by using a preset sequence. The transmitter may determine the resource to which TRS is to be mapped as shown in  FIG. 1 or 2 . The transmitter may map data to be sent to those resources to which TRS is not mapped. 
         [0051]      FIGS. 7A to 7D  illustrate a flowchart describing a method for signal reception of the receiver. 
         [0052]      FIG. 7A  illustrates a flowchart of a signal reception method for the receiver. 
         [0053]    Referring to  FIG. 7A , the receiver receives time-domain samples. Here, a sample is a unit of a time-domain signal, and received samples may be used interchangeably with a received signal. At step  700 , the estimator of the receiver estimates the phase error on the basis of the received time-domain samples. At step  710 , the corrector of the receiver corrects the phase error according to the estimated phase error. 
         [0054]      FIG. 7B  illustrates a flowchart describing detailed receive operations performed by the estimator. 
         [0055]    Referring to  FIG. 7B , at step  720 , the filter of the estimator obtains a time-domain signal of TRS by filtering the received time-domain samples. At step  730 , the phase error estimation unit of the estimator estimates the phase error on the basis of the filtered time-domain samples. The receiver may estimate the phase error by use of at least one of delay correlation, autocorrelation and partial correlation regarding the filtered time-domain samples. Which correlation to use may be determined according to the characteristics of the time-domain signal sent by the transmitter. The correlation to be used may be set in advance, or the transmitter may notify the receiver of the correlation type. 
         [0056]      FIGS. 7C and 7D  illustrate a flowchart describing detailed receive operations performed by the corrector. In  FIG. 7C , the phase error is corrected on the basis of time-domain samples. In  FIG. 7D , the phase error is corrected on the basis of frequency-domain samples. 
         [0057]    Referring to  FIG. 7C , at step  740 , the phase error corrector of the corrector corrects the phase error on the basis of the estimated phase error and time-domain samples. Here, linear interpolation may be used to correct the estimated phase error. Phase error estimation and correction may be repeated. At step  750 , the FFT of the corrector transforms the phase-error corrected time-domain samples to frequency-domain samples by applying FFT. At step  760 , the common phase error corrector of the corrector extracts a frequency-domain signal {X(f)} from resources to which TRS is mapped among the FFT output, and estimates and corrects the common phase error on the basis of the extracted frequency-domain signal. 
         [0058]    Referring to  FIG. 7D , at step  770 , the FFT of the corrector transforms the estimated phase error and time-domain samples respectively to the estimated frequency-domain phase error and frequency-domain samples by applying FFT. At step  780 , the phase error corrector of the corrector corrects the phase error on the basis of the estimated frequency-domain phase error and frequency-domain samples. Here, circular convolution may be used to correct the estimated frequency-domain phase error. At step  790 , the common phase error corrector of the corrector extracts a signal {X(f)} from resources to which TRS is mapped among the phase error corrected frequency-domain signals, and estimates and corrects the common phase error on the basis of the extracted signal. 
         [0059]    In  FIGS. 7A and 7D , steps are described in connection with specific components of the receiver. However, the present invention is not limited thereto or thereby. 
         [0060]      FIG. 8  illustrates an example estimated phase error values in the time domain. 
         [0061]    In  FIG. 8 , each symbol is sampled 8192 times and 7 symbols are shown. Thin solid line  800  represents the phase error for received time-domain samples in  FIG. 7 . Thick solid line  810  represents phase error correction values obtained when phase error estimation is performed using correlations on a per-symbol basis and linear interpolation is applied. As the phase error changes greatly within a symbol, performing phase error correction once may result in a large variance as indicated by thick solid line  810 . However, in the present invention, it is possible to perform phase error estimation multiple times within a symbol, effectively reducing the error. Thin broken line  820 , thick broken line  830 , and solid line  840  represent phase error correction values obtained when phase error estimation is performed within a symbol and linear interpolation is applied two times, four times, and eight times, respectively. It can be seen that the phase error correction values approach the phase error values indicated by thin solid line  800  with the increasing number of phase error estimations. 
         [0062]      FIG. 9  illustrates an example phase error values for subcarriers on a per-symbol basis after phase error correction in the time domain and FFT are applied in sequence. 
         [0063]    In  FIG. 9 , thick solid line  900  represent the common phase error estimated through a frequency-domain signal {X(f)}. It can be seen that the common phase error is estimated quite well. 
         [0064]    Table 1 represents reception performance of the receiver when the present invention is applied. Here, a millimeter wave phase noise model is applied to the transmitter and the receiver, and the error vector magnitude (EVM) is used as the reception performance index. Binary sequences are used as TRS, and phase error estimation within a symbol is performed eight times. It can be seen from Table 1 that an existing scheme can produce a performance gain of about 2 dB and the scheme of the present invention can produce an additional performance gain of about 7 dB compared with the existing scheme. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Correction 
                 EVM(dB) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 None 
                 −23.12 
               
               
                   
                 Existing scheme 
                 −25.07 
               
               
                   
                 Present invention 
                 −32.03 
               
               
                   
                   
               
             
          
         
       
     
         [0065]      FIG. 10  illustrates an example transmitter. 
         [0066]    Referring to  FIG. 10 , the transmitter may include a transmit circuitry  1000  and a controller  1010 . The controller  1010  may generate sequences of TRS, determine a resource to which TRS is to be mapped, map TRS to the determined resource, and control the transmit circuitry  1000  to send TRS to the receiver. The transmit circuitry  1000  may send TRS and data to the receiver under the control of the controller  1010 . 
         [0067]      FIG. 11  illustrates an example receiver. 
         [0068]    Referring to  FIG. 11 , the receiver  1180  may include a receive circuitry  1100 , a filter  1110 , a phase tracker  1130  (TRS based phase tracker), a phase corrector  1140 , a resource demapper  1160 , a FFT  1150 , and a common phase corrector  1170  (TRS based common phase corrector). Although not shown, the above components may operate under the control of a controller. The filter  1110  and phase tracker  1130  may constitute an estimator  1102 . The phase corrector  1140 , FFT  1150 , resource demapper  1160 , and common phase corrector  1170  may constitute a corrector  1104 . 
         [0069]    The receive circuitry  1100  may receive time-domain samples. The filter  1110  may filter the received time-domain samples. The phase tracker  1130  may estimate the phase error of the time domain in several sections within a symbol on the basis of the synchronized filtered samples (corresponding to the time-domain signal {x(t)} in  FIG. 1 ). 
         [0070]    The estimated phase error is input to the phase corrector  1140 . The phase corrector  1140  corrects the estimated phase error on the basis of the synchronized samples. The phase corrector  1140  may utilize simple linear interpolation to correct the estimated phase error. The FFT  1150  transforms phase-error corrected time-domain samples into frequency-domain samples by applying FFT. The resource demapper  1160  extracts a signal (corresponding to the frequency-domain signal {X(f)} in  FIG. 1 ) from resources to which TRS is mapped among the FFT output. The common phase corrector  1170  estimates and corrects the common phase error on the basis of the extracted frequency-domain signal, preventing performance degradation due to the phase error. Alternatively, the FFT  1150  may transform the estimated phase error and time-domain samples respectively to the estimated frequency-domain phase error and frequency-domain samples by applying FFT. The phase corrector  1140  may correct the phase error on the basis of the estimated frequency-domain phase error and frequency-domain samples. Here, the phase corrector  1140  may utilize circular convolution to correct the estimated frequency-domain phase error. The resource demapper  1160  may extract TRS from the phase error corrected frequency-domain signals. The common phase corrector  1170  may estimate and correct the common phase error on the basis of the extracted signal. 
         [0071]      FIG. 12  illustrates an example receiver. 
         [0072]    Referring to  FIG. 12 , the receiver  1220  may include a receive circuitry  1200  and a controller  1210 . The receive circuitry  1200  receives time-domain samples. The controller  1210  filters the time-domain samples, estimates the phase error on the basis of the filtered time-domain samples, and corrects the phase error on the basis of the estimated phase error. For phase error correction, the controller  1210  may use linear interpolation to correct the phase of time-domain samples, or may use circular convolution to correct the phase of frequency-domain samples after applying FFT. Estimation and correction of the phase error may be repeated. The controller  1210  may extract a signal from resources to which TRS is mapped among the FFT output, and estimate and correct the common phase error on the basis of the extracted signal. 
         [0073]    In a feature of the present invention, the receiver may optimize signal reception performance through phase error estimation within a symbol. 
         [0074]    Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.