Abstract:
Provided is a receiving device which, even when a temporary degradation occurs in the propagation path environment during an AFC tracking operation, does not require an initial lead-in operation when the propagation path environment returns to normal and which has lower power consumption and shortened time during which communication is not possible; also provided are a receiving method and a computer program. This receiving device calculates the variance of the phase rotation angle relative to a reference signal, compensating for frequency error if the rotation angle variance is less than a pre-set threshold value (TH), and stopping frequency error compensation if the rotation angle variance is greater than or equal to the threshold value (TH).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a U.S. national stage of application No. PCT/JP2012/054430, filed on Feb. 23, 2012. Priority under 35 U.S.C. §119(a) and 35 U.S.C. §365(b) is claimed from Japanese Patent Applications No. 2011-038469 filed on Feb. 24, 2011, the disclosure of which is also incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a receiver, a receiving method, and a computer program. 
       BACKGROUND ART 
       [0003]    In a wireless communication, Auto Frequency Control (hereinafter, called “AFC”) is generally used as a technique for compensating a frequency error between a base station and a terminal, which is caused owing to a product variation and a temperature change with respect to a crystal oscillator (such as a Temperature-Compensated Crystal Oscillator (TCXO) and the like). 
         [0004]    In a wireless communication system of Long Term Evolution (LTE) and the like, standardized by 3rd Generation Partnership Project (3GPP); considered is a method in which the amount of error is measured for correction by using the amount of phase rotation on a time axis of a synchronization signal and a reference signal. 
         [0005]    In the method, a traceable range of the amount of phase rotation is limited to a range of +/−180 degrees. Long time intervals of signals, with which the amount of phase rotation is measured, lead to the large amount of phase rotation so that a range, in which a frequency error can be compensated, is narrowed. On the other hand, short time intervals mean measuring a small variation under a stable condition so that a measuring accuracy is reduced. 
         [0006]    Therefore, conducted is a two-stage control; that is to say, since a frequency error is large in general at a stage before starting a communication between a terminal and a base station, measuring time intervals are narrowed in order to enable a larger acquisition (initial acquisition stage); and meanwhile, the measuring time intervals are widened after starting the communication stably, in order to increase an accuracy (tracking stage). 
         [0007]    Traditionally, sometimes a statistics processing unit executes a variance computation process for monitoring a variation in a phase difference of a phase difference signal coming from a phase difference measuring unit; and when a variance value exceeds a criterion variance value, it is judged that most probably a wrong AFC control is conducted, and then changing an error signal for AFC is interrupted so as to stop an AFC function for maintaining an error signal based on phase difference information just before the stop (for example, refer to Patent Literature PTL 1). 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL1: JP 2001-223610 A 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0009]    Unfortunately, there is a possibility that, in the case where a temporary degradation of a propagation path environment happens during a tracking operation after a start of communication, a frequency error cannot properly be measured any more so that a wrong compensation is carried out and an error exceeds a tracking range. In such a case, it is needed to execute an initial acquisition again. At the time, because an execution of the initial acquisition, there arise problems, such as the generation of a time period in which no communication can be done, an increase in power consumption, and the like. 
         [0010]    Thus, it is an objective of the present invention to give a solution to the issue described above; namely to provide a receiver, a receiving method, and a computer program that eliminate the need of an initial acquisition after a propagation path environment restores its good condition even in the case where a temporary degradation of a propagation path environment happens during a tracking operation of AFC, in such a way as to make it possible to further reduce power consumption, and to further shorten a time period in which no communication can be done. 
       Solution to Problem 
       [0011]    In order to give a solution to the issue described above, a receiving device according to the present invention includes: a variance calculation means for calculating a variance of inter-reference-signal phase rotation angles; and a frequency compensation means that compensates a frequency error if the variance of rotation angles is less than a predetermined first threshold, and interrupts an operation of compensating a frequency error if the variance of rotation angles is equal to or greater than the first threshold. 
         [0012]    Moreover, a receiving method according to the present invention includes: a variance calculation step for calculating a variance of inter-reference-signal phase rotation angles; and a frequency compensation step that compensates a frequency error if the variance of rotation angles is less than a predetermined first threshold, and interrupts an operation of compensating a frequency error if the variance of rotation angles is equal to or greater than the first threshold. 
         [0013]    Furthermore, a computer program according to the present invention to operate a computer for an operation includes a variance calculation step for calculating a variance of inter-reference-signal phase rotation angles; and a frequency compensation step that compensates a frequency error if the variance of rotation angles is less than a predetermined first threshold, and interrupts an operation of compensating a frequency error if the variance of rotation angles is equal to or greater than the first threshold. 
       Advantageous Effects of Invention 
       [0014]    According to the present invention, it becomes possible to provide a receiver, a receiving method and a computer program that make it possible to further shorten a time period in which no communication can be done, and to further reduce power consumption. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is a block diagram showing a configuration example of a receiver. 
           [0016]      FIG. 2  is a block diagram showing a configuration example of an AFC control unit  16 . 
           [0017]      FIG. 3  is a flowchart for explaining an example of a process of TCXO control. 
           [0018]      FIG. 4  is a block diagram showing another configuration example of an AFC control unit  16 . 
           [0019]      FIG. 5  is a flowchart for explaining another example of a process of TCXO control. 
           [0020]      FIG. 6  is a block diagram showing a configuration example of hardware of a computer. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    A receiver according to an exemplary embodiment of the present invention is explained below with reference to  FIG. 1  through  FIG. 6 . 
         [0022]      FIG. 1  is a block diagram showing a configuration example of a receiver. A receiver  10  receives a signal by way of a communication method specified by LTE. The receiver  10  includes a Radio Frequency (RF) unit  11 , a Fast Fourier Transform (FFT) unit  12 , a channel estimation unit  13 , a demodulation unit  14 , a channel decoding unit  15 , and an AFC control unit  16 . 
         [0023]    A signal received by a receiving antenna (not shown) is supplied to the RF unit  11  of the receiver  10  (the signal received is hereinafter called a “received signal”). The RF unit  11  A/D-converts (to convert from Analog to Digital) the received signal, and supplies a digital signal obtained as a result of the A/D conversion, to the FFT unit  12 . The FFT unit  12  transforms the digital signal into a datum of frequency components by way of a Fourier transform. Then, the FFT unit  12  supplies the datum of frequency components to the channel estimation unit  13 . 
         [0024]    The channel estimation unit  13  estimates a channel estimation matrix that expresses the channel status, by using a reference signal out of the datum of frequency components, the reference signal being mapped in advance on a frequency resource. Then, the channel estimation unit  13  supplies the channel estimation matrix to the demodulation unit  14  and the AFC control unit  16 . 
         [0025]    The demodulation unit  14  demodulates an I-component and Q-component into likelihood information, on the basis of the received signal, the channel estimation matrix estimated by the channel estimation unit  13 , and the like. Then, the demodulation unit  14  supplies the likelihood information to the channel decoding unit  15 . The channel decoding unit  15  carries out error correction decoding and error detection, and then supplies an obtained result to a higher-level layer. 
         [0026]    Then, the AFC control unit  16  in the receiver  10  measures a frequency error on the basis of the channel estimation matrix estimated by the channel estimation unit  13 , controls a crystal oscillator, and outputs a TCXO control value. 
         [0027]      FIG. 2  is a block diagram showing a configuration example of the AFC control unit  16 . The AFC control unit  16  includes a correlation calculation unit  21 , a rotation angle calculation unit  22 , a measuring time correction unit  23 , a time average processing unit  24 , a TCXO control unit  25 , and a variance measuring unit  26 . 
         [0028]    The correlation calculation unit  21  calculates a correlation of RS according to Formula (1), wherein ‘a’ is a receiving antenna, ‘b’ is a transmission antenna, ‘t’ is a time-wise direction index of a reference signal (hereinafter, also called “RS”), T is a frequency-wise direction index of RS, and ‘h(a, b, t, i)’ is a channel estimation value corresponding to the above parameters. 
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         [0029]    Incidentally, an averaging number in a frequency-wise direction of RS is expressed by Formula (2). 
         [0000]      {Math. 2} 
         [0000]        N   AFC   Formula (2)
 
         [0030]    Meanwhile, a correction value on an amplitude &amp; phase according to sampling of AGC &amp; FFT is expressed by Formula (3). 
         [0000]      {Math. 3} 
         [0000]      Θ  Formula (3)
 
         [0031]    The correlation calculation unit  21  supplies the correlation of RS: V(a, b, t) to the rotation angle calculation unit  22 . 
         [0032]    The rotation angle calculation unit  22  calculates an inter-RS rotation angle: θ(a, b, t) according to Formula (4), by using the correlation value: V(a, b, t) calculated by the correlation calculation unit  21 . 
         [0000]      {Math. 4} 
         [0000]      θ( a,b,t )=arctan  Im ( V ( a,b,t ))/ Re ( V ( a,b,t ))  Formula (4)
 
         [0033]    Incidentally, Im(c) and Re(c) in Formula (4) represent an imaginary part and a real part of a complex number c, respectively. 
         [0034]    The rotation angle calculation unit  22  supplies the inter-RS rotation angle: θ(a, b, t) to the measuring time correction unit  23 . 
         [0035]    By using RS of a time-wise direction index t, a time difference T(t) of RS of a time-wise direction index t+1, and a criterion time T, the measuring time correction unit  23  adjusts the rotation angle: θ(a, b, t) with the criterion time T according to Formula (5). 
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         [0036]    The measuring time correction unit  23  supplies the rotation angle adjusted with the criterion time T to the time average processing unit  24  and the variance measuring unit  26 . 
         [0037]    With respect to N sets of data of the rotation angle; 
         [0000]      {Math. 6} 
         [0000]      θ T ( a,b,t )
 
         [0000]    the time average processing unit  24  calculates an average of their data according to Formula (6). 
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         [0038]    The average of N sets of data of the rotation angle; 
         [0000]      {Math. 8} 
         [0000]      θ T ( a,b,t )
 
         [0000]    is supplied to the TCXO control unit  25  by the time average processing unit  24 . 
         [0039]    With respect to N sets of data of the rotation angle; 
         [0000]      {Math. 9} 
         [0000]      θ T ( a,b,t )
 
         [0000]    the variance measuring unit  26  calculates a variance of their data according to Formula (7). 
         [0000]    
       
         
           
             
               
                 
                   
                       
                   
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         [0040]    The variance of N sets of data of the rotation angle; 
         [0000]      {Math. 11} 
         [0000]      θ T ( a,b,t )
 
         [0000]    is supplied to the TCXO control unit  25  by the variance measuring unit  26 . 
         [0041]    If the variance presented below, which the variance measuring unit  26  has calculated, is less than a threshold TH; 
         [0000]      {Math. 12} 
         [0000]      σ 2 ( a,b,t )
 
         [0000]    the TCXO control unit  25  calculates a TCXO control value, and conducts TCXO control. 
         [0042]    In other words, the TCXO control unit  25  calculates a TCXO control value in this case by using the average of the rotation angle represented below, which the time average processing unit  24  has calculated; 
         [0000]      {Math. 13} 
         [0000]        θ   T ( a,b,t ) 
         [0000]    and conducts TCXO control. 
         [0043]    On the other hand, if the variance presented below is equal to or greater than the threshold TH; 
         [0000]      {Math. 14} 
         [0000]      σ 2 ( a,b,t )
 
         [0000]    the TCXO control unit  25  abandons a measurement result, and controls in such a way as not to change a frequency of the TCXO. 
         [0044]      FIG. 3  is a flowchart for explaining an example of a process of TCXO control. At Step S 11 , the correlation calculation unit  21  calculates a correlation of RS according to Formula (1), by using a receiving antenna: a, a transmission antenna: b, a time-wise direction index of an RS: t, a frequency-wise direction index of RS: i, and a channel estimation value corresponding to the above parameters: h(a, b, t, i). 
         [0045]    At Step S 12 , the rotation angle calculation unit  22  calculates an inter-RS rotation angle according to Formula (4), by using the correlation value calculated by the correlation calculation unit  21 . 
         [0046]    At Step S 13 , by using RS of a time-wise direction index t, a time difference T(t) of RS of a time-wise direction index t+1, and a criterion time T, the measuring time correction unit  23  adjusts the rotation angle with the criterion time T according to Formula (5). 
         [0047]    At Step S 14 , the time average processing unit  24  calculates an average of N sets of data of the rotation angle, according to Formula (6). 
         [0048]    At Step S 15 , the variance measuring unit  26  calculates a variance of N sets of data of the rotation angle, according to Formula (7). 
         [0049]    At Step S 16 , the TCXO control unit  25  makes a judgment on whether or not the variance of N sets of data of the rotation angle is less than a threshold TH. If it is judged at Step S 16  that the variance of N sets of data of the rotation angle is less than the threshold TH, operation progresses to Step S 17  so that the TCXO control unit  25  calculates a TCXO control value by using the average of N sets of data of the rotation angle. At Step S 18 , the TCXO control unit  25  conducts TCXO control, and then the TCXO control ends. 
         [0050]    If it is judged at Step S 16  that the variance of N sets of data of the rotation angle is equal to or greater than the threshold TH, operation progresses to Step S 19  so that the TCXO control unit  25  abandons a measurement result. Then, at Step S 20 , the TCXO control unit  25  controls in such a way as not to change a frequency of the TCXO, and the TCXO control ends. 
         [0051]    Incidentally, the TCXO control may be conducted with reference to power of a correlation vector of a reference signal. 
         [0052]      FIG. 4  is a block diagram showing another configuration example of an AFC control unit  16 . The AFC control unit  16  shown in  FIG. 4  includes a correlation calculation unit  21 , a rotation angle calculation unit  22 , a measuring time correction unit  23 , a time average processing unit  24 , a TCXO control unit  25 , a variance measuring unit  26 , and a power measuring unit  41 . Since the correlation calculation unit  21  through the variance measuring unit  26  are the same as those in the case that  FIG. 2  shows, explanation on them is skipped. 
         [0053]    The correlation calculation unit  21  supplies a correlation of RS, calculated according to Formula (1), to the rotation angle calculation unit  22  and the power measuring unit  41 . 
         [0054]    By using the correlation vector presented below, which has been calculated by the correlation calculation unit  21 ; 
         [0000]      {Math. 15} 
         [0000]        V ( a,b,t ) 
         [0000]    the power measuring unit  41  calculates a power average according to Formula (8). 
         [0000]    
       
         
           
             
               
                 
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         [0055]    The power measuring unit  41  supplies the power average: P(a, b, t) to the TCXO control unit  25 . 
         [0056]    With respect to the variance presented below, which has been calculated by the variance measuring unit  26 ; 
         [0000]      {Math. 17} 
         [0000]      σ 2 ( a,b,t )
 
         [0000]    only if the variance is less than a threshold TH and the power average: P(a, b, t) calculated by the power measuring unit  41  is greater than a power threshold THP, the TCXO control unit  25  calculates a TCXO control value and conducts TCXO control. 
         [0057]    In other words, the TCXO control unit  25  calculates a TCXO control value in this case by using the average of the rotation angle represented below, which the time average processing unit  24  has calculated; 
         [0000]      {Math. 18} 
         [0000]        θ   T ( a,b,t ) 
         [0000]    and conducts TCXO control. 
         [0058]    If the variance presented below is equal to or greater than the threshold TH; 
         [0000]      {Math. 19} 
         [0000]      σ 2 ( a,b,t )
 
         [0000]    the TCXO control unit  25  abandons a measurement result, and controls in such a way as not to change a frequency of the TCXO. 
         [0059]    Moreover, if the power average: P(a, b, t) calculated by the power measuring unit  41  is equal to or less than the power threshold THP, the TCXO control unit  25  abandons a measurement result, and controls in such a way as not to change a frequency of the TCXO. 
         [0060]      FIG. 5  is a flowchart for explaining another example of a process of TCXO control. Since procedures of Step S 41  through Step S 45  are the same as those of Step S 11  through Step S 15  in  FIG. 3 , respectively, explanation on them is skipped. 
         [0061]    At Step S 46 , the power measuring unit  41  calculates a power average: P(a, b, t) according to Formula (8), by using a correlation vector calculated by the correlation calculation unit  21 . 
         [0062]    At Step S 47 , the TCXO control unit  25  makes a judgment on whether or not the variance of N sets of data of the rotation angle is less than a threshold TH. If it is judged at Step S 47  that the variance of N sets of data of the rotation angle is less than the threshold TH, operation progresses to Step S 48  so that the TCXO control unit  25  makes a judgment on whether or not the power average: P (a, b, t) is greater than the power threshold THP. 
         [0063]    If it is judged at Step S 48  that the power average: P(a, b, t) is greater than the power threshold THP, operation progresses to Step S 49 , and the TCXO control unit  25  calculates a TCXO control value by using the average of N sets of data of the rotation angle. At Step S 50 , the TCXO control unit  25  conducts TCXO control, and then the TCXO control ends. 
         [0064]    If it is judged at Step S 47  that the variance of N sets of data of the rotation angle is equal to or greater than the threshold TH, or if it is judged at Step S 48  that the power average: P (a, b, t) is equal to or less than the power threshold THP, operation progresses to Step S 51  so that the TCXO control unit  25  abandons a measurement result. Then, at Step S 52 , the TCXO control unit  25  controls in such a way as not to change a frequency of the TCXO, and the TCXO control ends. 
         [0065]    Incidentally, in order to reduce the amount of processing operation, the variation may be calculated only in the case of power being great. 
         [0066]    Even in the case where a temporary degradation of a propagation path environment happens during a tracking operation of AFC, an initial acquisition, after a propagation path environment restores its good condition, becomes unnecessary so that it becomes possible to reduce power consumption, and to shorten a time period in which no communication can be done. 
         [0067]    Thus, reliability (variance) of the amount of phase rotation measured is calculated, and a compensating operation of frequency is interrupted if the calculated value is equal to or greater than a threshold. 
         [0068]    As described above, it is detected that reliability of the frequency error measurement is lowered due to a temporary degradation of a propagation path environment, and a control operation of frequency compensation in such a case is interrupted. Thus, a frequency error is kept away from becoming large, and a tracking operation can be done without executing an initial acquisition again, after a propagation path environment is restored. 
         [0069]    The series of processes described above may be executed by means of hardware, and may also be executed by way of software. For executing the series of processes by way of software, a computer program constituting the software is installed into a computer, which is built in exclusive-use hardware, from a recording medium; or the software is installed from a recording medium, for example, into a general-purpose personal computer that can execute various functions with various computer programs being installed. 
         [0070]      FIG. 6  is a block diagram showing a configuration example of hardware of a computer that executes the series of processes described above by way of a computer program. 
         [0071]    In the computer; a central processing unit (CPU)  101 , a read only memory (ROM)  102 , and a random access memory (RAM)  103  are interconnected by using a bus  104 . 
         [0072]    Moreover, an I/O interface  105  is connected to the bus  104 . Connected to the I/O interface  105  are; an input unit  106  including a keyboard, a mouse, a microphone, and the like; an output unit  107  including a display, a speaker, and the like; a storage unit  108  including a hard disc, a non-volatile memory, and the like; a communication unit  109  including a network interface and the like; and a drive  110  for driving a removable medium  111  such as a magnetic disc, an optical disc, a magnetic optical disc, or a semiconductor memory. 
         [0073]    In the computer configured as described above, the CPU  101  loads a computer program, for example, stored in the storage unit  108 , to the RAM  103  by way of the I/O interface  105  and the bus  104 , and executes the program in order to carry out the series of processes described above. 
         [0074]    The computer program to be executed by the computer (the CPU  101 ) is recorded, for being provided, in the removable medium  111  as a package medium; such as, for example, a magnetic disc (including a flexible disc), an optical disc (Compact Disc-Read Only Memory (CD-ROM), Digital Versatile Disc (DVD), and the like), a magnetic optical disc, or a semiconductor memory; or the computer program is provided via a wired or wireless transmission medium such as a local area network, the Internet, or digital satellite broadcasting. 
         [0075]    Then, the computer program can be installed in the computer by way of being stored in the storage unit  108  through the I/O interface  105 , while the removable medium  111  being mounted on the drive  110 . Alternatively, the computer program can be installed in the computer by way of being stored in the storage unit  108 , while being received in the communication unit  109  by the intermediary of a wired or wireless transmission medium. In another way, the computer program can previously be installed in the computer by way of storing the program in advance in the ROM  102  or the storage unit  108 . 
         [0076]    Incidentally, the program to be executed by the computer may be a program with which processes are carried out in chronological order along the sequence explained in this specification document, or may be a computer program with which processes are carried out in parallel or at the time as required, such as, in response to a call. 
         [0077]    Furthermore, an embodiment of the present invention is not limited only to the embodiment described above, and various other variations may be made without departing from the concept of the present invention.