Abstract:
When a channel between a transmission apparatus and a reception apparatus is distorted by multipath fading or other reasons, linear interpolation between pilot subcarriers produces a large estimation error, resulting in an increase in an equalization error and a decrease in reception performance. The present invention allows feedback of a signal that undergoes error correction, reduction in the channel estimation error through repeated channel estimation, and improvement in the reception performance.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation-in-part application under 35 U.S.C §111 (a) of International Application No. PCT/JP2015/075324, filed Sep. 7, 2015, which claims priority to Japanese Patent Application No. 2014-219371, filed Oct. 28, 2014. The contents of these applications are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a reception apparatus. 
     BACKGROUND ART 
     In an equalization process of a reception apparatus in wireless communications, in general, in multi-carrier communication such as orthogonal frequency-division multiplexing (OFDM) scheme, a transmission apparatus inserts pilot subcarriers with amplitude and phase already known at specified intervals in a subcarrier direction, and the reception apparatus performs linear interpolation between the pilot subcarriers to estimate a channel between the transmission apparatus and the reception apparatus, and then multiplies a reception signal by an inverse characteristic of estimated channel information to perform equalization. 
     As a prior art literature, for example, Patent Literature 1 discloses an invention of new channel estimation that produces reliable channel estimation even when data values to be decoded are likely to produce detection errors. In addition, as another prior art literature, for example, Patent Literature 2 discloses an invention of a multi-carrier reception apparatus that provides an accurate channel estimation value even under environments where frequency selective fading occurs and dynamic fading occurs that undergoes Doppler frequency fluctuation. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent No. 4272665 
     Patent Literature 2: Japanese Patent No. 3876437 
     SUMMARY OF INVENTION 
     Technical Problem 
     According to conventional technologies, when the channel between the transmission apparatus and the reception apparatus is distorted by multipath fading or other reasons, linear interpolation between pilot subcarriers produces a large estimation error, resulting in increase in an equalization error and decrease in reception performance. 
     Solution to Problem 
     A reception apparatus of the present invention includes: a fast Fourier transform unit configured to convert a received time signal into a frequency signal; a pilot subcarrier extraction unit configured to extract a pilot subcarrier from the frequency signal; a channel estimation unit configured to estimate a channel between transmission and reception by performing a linear interpolation process on a pilot subcarrier extraction signal extracted by the pilot subcarrier extraction unit; an equalization unit configured to perform equalization using the frequency signal and a result of the channel estimation; a bit log likelihood ratio calculation unit configured to calculate a bit log likelihood ratio from output of the equalization; a deinterleave unit configured to rearrange the bit log likelihood ratio in original order, the deinterleave unit making a pair with an interleave section that rearranges the bit log likelihood ratio in order determined in advance on a transmission side; a soft-in soft-out decoding unit configured to perform error correction of soft-in soft-out on output of the deinterleave; an interleave unit configured to rearrange an encoded bit log likelihood ratio in order similar to order on a transmission side out of output of a section of the soft-in soft-out error correction; a soft symbol calculation unit configured to generate a soft symbol from output of the interleave; a hard decision unit configured to perform hard decision on the generated soft symbol; and a reliability evaluation unit configured to generate reliability information from the encoded bit log likelihood ratio of output of the interleave. 
     In addition, the reception apparatus of the present invention is the reception apparatus as described above, in which the channel estimation unit includes a linear interpolation unit configured to perform a linear interpolation process on output of the soft symbol hard decision when the reliability information is larger than a predetermined threshold. 
     Moreover, the reception apparatus of the present invention is the reception apparatus as described above, in which the channel estimation unit repeats the linear interpolation process until the reliability information becomes smaller than the predetermined threshold. 
     Advantageous Effects of Invention 
     The present invention allows feedback of a signal that undergoes error correction, reduction in the channel estimation error through repeated channel estimation, and improvement in the reception performance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a configuration of a reception apparatus according to one embodiment of the present invention; 
         FIG. 2  is a functional block diagram illustrating a configuration of a channel estimation unit of the reception apparatus according to one embodiment of the present invention; 
         FIG. 3  is a diagram illustrating linear interpolation of the channel estimation unit; and 
         FIG. 4  is a diagram illustrating repeated linear interpolation of the channel estimation unit of the reception apparatus according to one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     One embodiment of the present invention will be described below with reference to the drawings.  FIG. 1  is a block diagram illustrating a configuration of a reception apparatus according to one embodiment of the present invention. In  FIG. 1 , the reception apparatus includes an antenna  121 , a radio frequency (RF) unit  122 , an analog-to-digital converter (A/D) unit  123 , a fast Fourier transform (FFT) unit  101 , a channel estimation unit  102 , an equalization unit  103 , a log likelihood ratio (LLR) calculation unit  104 , which is a bit log likelihood ratio calculation unit, a deinterleave unit  105 , a soft-in soft-out (SISO) decoding unit  106 , a hard decision unit  107 , an interleave unit  108 , a soft symbol calculation unit  109 , and a hard decision unit  110 . 
     The antenna  121  receives a wireless signal transmitted from a transmission side, the RF unit  122  converts the reception signal into a baseband signal, and the A/D unit  123  converts the baseband signal into a digital signal. The reception signal is an OFDM signal, for example. The FFT unit  101  converts a received time domain signal y (m, n) into a frequency domain signal, and outputs the converted signal as a reception frequency signal Y (m, k) to the channel estimation unit  102  and the equalization unit  103 . Here, m denotes a symbol number, n denotes a sample number, and k denotes a subcarrier number. 
     Based on the incoming reception frequency signal Y (m, k), a frequency signal acquired by performing hard decision on a soft symbol that undergoes error correction and feedback, and a reception bit log likelihood ratio (LLR) L D  (m, c k ) that undergoes error correction and feedback, the channel estimation unit  102  estimates a channel between the transmission apparatus and the reception apparatus, and outputs a channel estimation result to the equalization unit  103 . Here, c k  denotes a bit series number after an interleave process. 
     Next, an operation of the channel estimation unit  102  will be described with reference to  FIG. 2 . In  FIG. 2 , the channel estimation unit  102  includes a pilot subcarrier extraction unit  201 , a reliability evaluation unit  202 , a channel information calculation unit  203 , and a linear interpolation unit  204 . 
     The pilot subcarrier extraction unit  201  extracts a pilot subcarrier from the incoming reception frequency signal Y (m, k), and then outputs a reception frequency signal Y p  (m,—k) from which the pilot subcarrier is extracted to the linear interpolation unit  204 . The pilot subcarrier extraction unit  201  extracts the pilot subcarrier, for example, by replacing subcarriers other than the pilot subcarrier by ‘0’ among all the subcarriers. 
     [Mathematical Formula 1] 
     The reliability evaluation unit  202  evaluates reliability of an incoming signal {tilde over (X)}(m,k) that undergoes error correction and feedback based on a similarly incoming bit LLR L D  (m, c k ). When it is determined that the reliability is high, this signal will be {tilde over (X)}(m,k), and otherwise this signal will be zero. The reliability evaluation unit  202  outputs the signal after the reliability evaluation {tilde over (X)}′(m,k) and a reliability evaluation result signal rel (m, k) to the channel information calculation unit  203 . The signal after the reliability evaluation {tilde over (X)}′(m,k) is expressed by Mathematical Formula 1. 
     
       
         
           
             
               
                 
                   
                     
                       
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     Here, rel (m, k) denotes the reliability evaluation result signal, and the reliability evaluation result signal rel (m, k) is expressed by Mathematical Formula 2, for example. 
     [Mathematical Formula 2] 
     
       
         
           
             
               
                 
                   
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     Here, B denotes the number of modulation bits in the modulation scheme, whereas thr denotes a reliability evaluation threshold. As the reliability evaluation threshold thr, for example, a value of size that causes no hard decision error is set. If there is even one bit with low likelihood (LLR is close to zero), an infinite product of LLRs is calculated to decrease an absolute value of rel (m, k). 
     [Mathematical Formula 3] 
     The channel information calculation unit  203  calculates channel information Ĥ FB (m,k) obtained from reliable (rel (m, k)=1) fed-back data subcarrier by using the incoming reception frequency signal Y (m, k), the signal that undergoes error correction and feedback {tilde over (X)}′(m,k), and the reliability evaluation result signal rel (m, k). The channel information calculation unit  203  then outputs the channel information Ĥ FB (m,k) and the reliability evaluation result signal rel (m, k) to the linear interpolation unit  204 . The channel information calculation unit  203  calculates the channel information Ĥ FB (m,k), for example, by Mathematical Formula 3. 
     
       
         
           
             
               
                 
                   
                     
                       
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     The linear interpolation unit  204  estimates the channel between the transmission apparatus and the reception apparatus by using the pilot subcarrier-extracted incoming reception frequency signal Y p  (m, k), the channel information obtained from the fed-back signal Ĥ FB (m,k), and the reliability evaluation result signal rel (m, k). The linear interpolation unit  204  then outputs the channel estimation result Ĥ(m,k) to the equalization unit  103 . 
     Next, typical linear interpolation of wireless communications will be described with reference to  FIG. 3 .  FIG. 3  is a diagram illustrating linear interpolation of the channel estimation unit  102 . In  FIG. 3 , the channel estimation unit  102  estimates the channel by performing linear interpolation between the pilot subcarriers. However, variations between the pilot subcarriers cause a channel estimation error in the actual channel. 
     Next, linear interpolation, which is one embodiment of the present invention, will be described with reference to  FIG. 4 .  FIG. 4  is a diagram illustrating repeated linear interpolation of the channel estimation unit  102  of the reception apparatus according to one embodiment of the present invention. 
     [Mathematical Formula 4] 
       FIG. 4  illustrates one example of linear interpolation when there is feedback, which is one embodiment of the present invention. In addition to each pilot subcarrier, linear interpolation is performed using the channel information Ĥ FB (m,k) obtained from the reliable (rel (m, k)=1) fed-back data subcarrier, and thus the channel estimation error can be reduced as compared with a conventional scheme. 
     The equalization unit  103  calculates equalization output {circumflex over (X)}(m,k), for example, by Mathematical Formula 4 using the incoming reception frequency signal Y (m, k) and the interpolated channel estimation result Ĥ(m,k). 
     
       
         
           
             
               
                 
                   
                     
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     The LLR calculation unit  104  calculates a reception bit LLR L E  (m, c k ), for example, by calculating a distance between the incoming equalization output {circumflex over (X)}(m,k) and an ideal point in each bit. The LLR calculation unit  104  then outputs the reception bit LLR L E  (m, c k ) to the deinterleave unit  105 . The LLR calculation unit  104  calculates the reception bit LLR, for example, by Mathematical Formula 5. 
     
       
         
           
             
                 
             
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     Here, X(i) denotes a mapping value of the modulation scheme at mapping number i, whereas Q denotes the number of mapping points of the modulation scheme. Here, an approximate expression is Mathematical Formula 6. 
     
       
         
           
             
               
                 
                   
                       
                   
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     Approximation of Mathematical Formula 5 with Mathematical Formula 6 produces Mathematical Formula 7. 
     
       
         
           
             
               
                 
                   
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     By the above Mathematical Formula, the reception bit LLR is calculated using a minimum distance between 1 and the bit to which attention is paid and a minimum distance between 0 and the bit. 
     The deinterleave unit  105  returns a series of the incoming reception bit LLR L E  (m, c k ) obtained through rearrangement in order determined in advance on a transmission side to original order, and then outputs the reception bit LLR L E  (m, c′ k ) rearranged to the original order to the SISO decoding unit  106 . Here, c′ k  is a bit information series number before the interleave process. A unit of the interleave and deinterleave process of this example is in agreement with a unit of an OFDM symbol or an integral submultiple of the OFDM symbol (that is, a total number of bits assigned to data subcarrier in one OFDM symbol is a natural multiple of an interleave length). 
     The SISO decoding unit  106  performs the error correction process of soft-in soft-out on the incoming reception bit LLR L E  (m, c′ k ), outputs the information bit LLR L (m, c′ k ) that undergoes the error correction to the hard decision unit  107 , and outputs an encoded bit LLR L D  (m, c′ k ) that undergoes the error correction to the interleave unit  108 . The SISO error correction process can be implemented, for example, by a method using maximum aposteriori probability based on a BCJR (Bahl, Cocke, Jelinek, Raviv: designers&#39; initials) algorithm or a method based on SOVA (Soft Output Viterbi Algorithm). 
     The hard decision unit  107  performs the hard decision process on the incoming information bit LLR L (m, c′ k ) and outputs a reception information bit d′ (m, c′ k ) that undergoes the hard decision process. The hard decision process is performed, for example, by Mathematical Formula 8. 
     
       
         
           
             
               
                 
                   
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     The interleave unit  108  rearranges the incoming encoded bit LLR L D  (m, c′ k ) in order determined in advance similar to order on a transmission side, and then outputs the rearranged encoded bit LLR L D  (m, c k ) to the soft symbol calculation unit  109  and the channel estimation unit  102 . 
     [Mathematical Formula 9] 
     The soft symbol calculation unit  109  calculates the soft symbol {tilde over (S)}(m,k) by using the incoming encoded bit LLR L D  (m, c k ), and then outputs the calculated soft symbol {tilde over (S)}(m,k) to the hard decision unit  110 . The soft symbol calculation unit  109  calculates the soft symbol {tilde over (S)}(m,k), for example, by the following procedure. First, the soft symbol calculation unit  109  calculates bit probability p 0  (m, c k ) of 0 and p 1  (m, c k ) of 1 by Mathematical Formula 9 and Mathematical Formula 10 from the encoded bit LLR L D  (m, c k ). 
     
       
         
           
             
               
                 
                   
                     
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                     ⁢ 
                     
                         
                     
                     ⁢ 
                     10 
                   
                   ) 
                 
               
             
           
         
       
     
     Next, the soft symbol calculation unit  109  calculates mapping probability P (m, i) by Mathematical Formula 10 using the bit probability P 0  (m, c k ) and P 1  (m, c k ). 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       Mathematical 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Formula 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       10 
                     
                     ] 
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     P 
                     ⁡ 
                     
                       ( 
                       
                         m 
                         , 
                         i 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       ∏ 
                       
                         b 
                         = 
                         1 
                       
                       B 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         p 
                         
                           g 
                           ⁡ 
                           
                             ( 
                             
                               i 
                               , 
                               
                                 B 
                                 - 
                                 b 
                               
                             
                             ) 
                           
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           m 
                           , 
                           
                             
                               B 
                               · 
                               k 
                             
                             + 
                             b 
                             - 
                             1 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     11 
                   
                   ) 
                 
               
             
           
         
       
     
     Here, g (i, z) is a z-th bit value when counted from a least significant bit when a mapping point number i is expressed in binary. 
     [Mathematical Formula 11] 
     Finally, the soft symbol calculation unit  109  calculates) the soft symbol {tilde over (S)}(m,k) by Mathematical Formula 12 using the mapping probability p (m, i) and mapping value X (i). 
     
       
         
           
             
               
                 
                   
                     
                       S 
                       ~ 
                     
                     ⁡ 
                     
                       ( 
                       
                         m 
                         , 
                         k 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         0 
                       
                       
                         Q 
                         - 
                         1 
                       
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         P 
                         ⁡ 
                         
                           ( 
                           
                             m 
                             , 
                             i 
                           
                           ) 
                         
                       
                       · 
                       
                         X 
                         ⁡ 
                         
                           ( 
                           i 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Mathematical 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     12 
                   
                   ) 
                 
               
             
           
         
       
     
     The hard decision unit  110  performs hard decision on the) incoming soft symbol {tilde over (S)}(m,k), and then outputs a result of the hard decision {tilde over (X)}(m,k) to the channel estimation unit  102 . In the hard decision, the hard decision unit  110  selects, for example, a point with a minimum distance between the point and the soft symbol {tilde over (S)}(m,k) among all mapping points. 
     The above-described embodiment allows the reception apparatus to feedback the data subcarrier that undergoes error correction to the channel estimation unit  102 , handles the reliable feedback signal on a par with the pilot subcarrier, and performs repeated channel estimation, thereby enabling reduction in the channel estimation error and improvement in reception performance. The channel estimation unit  102  of this example, which performs initialization to channel estimation (linear interpolation) based on only the pilot subcarrier for each OFDM symbol, is suitable for mobile communications with intense channel variations. 
     The reception apparatus according to the embodiment of the present invention allows feedback of a signal that undergoes error correction, reduction in the channel estimation error through repeated channel estimation, and improvement in the reception performance. The number of repetitions of channel estimation may be determined to be, for example, twice or three times in advance, or a threshold is defined in advance and channel estimation may be repeated until reliability information becomes equal to or less than the threshold. When the channel variation is gentle, the reception apparatus may store the channel estimation result of past OFDM symbol, smooth the channel estimation result of past OFDM symbol and the estimation result from the pilot subcarrier of current OFDM symbol in a time direction, and then the reception apparatus may use the smoothed estimation result as an initial value for channel estimation this time. 
     INDUSTRIAL APPLICABILITY 
     While the present invention has been described in detail above, the present invention is not limited to the reception apparatus described here, and may be widely applicable to reception apparatuses other than the above-described reception apparatus, for example, reception apparatuses that perform block equalization in frequency regions such as discrete Fourier transform spread (DFTS) OFDM, single-carrier frequency-division multiple access (SC-FDMA), and single carrier block transmission with cyclic prefix (CP-SC). 
     REFERENCE SIGNS LIST 
     
         
           101 : FFT unit 
           102 : channel estimation unit 
           103 : equalization unit 
           104 : bit log likelihood ratio calculation unit 
           105 : deinterleave unit 
           106 : soft-in soft-out decoding unit 
           107 : hard decision unit 
           108 : interleave unit 
           109 : soft symbol calculation unit 
           110 : hard decision unit 
           121 : antenna 
           122 : RF unit 
           123 : A/D unit 
           201 : pilot subcarrier extraction unit 
           202 : reliability evaluation unit 
           203 : channel information calculation unit 
           204 : linear interpolation unit