Abstract:
Disclosed herein is a signal processing device including: a correlation signal output unit configured to, about a received signal transmitted from another device, calculate a correlation value between the received signal and a pattern of a waveform of a known signal and output the calculated correlation value as a correlation signal in a time corresponding to one symbol; a known signal determiner configured to determine, based on the correlation signal, whether or not the received signal is modulated by the known signal in a first interval defined when the time corresponding to one symbol is divided into a plurality of intervals; and an identifier configured to identify a series of the symbol based on a result of the determination.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to signal processing device and method, and particularly to signal processing device and method that allow accurate detection of a subcarrier with a simple configuration. 
         [0003]    2. Description of the Related Art 
         [0004]    In recent years, a contactless IC card system employing a contactless IC card is becoming prevalent in a transportation system, a security system, an electronic money system, etc. 
         [0005]    In such a system employing a contactless IC card, upon the entering of the contactless IC card into a communication-allowed distance from a reader/writer, initially the reader/writer radiates electromagnetic waves to the contactless IC card via an antenna. In this state, the reader/writer transmits a signal for requesting data reply to the contactless IC card via the antenna. In response to this signal, the contactless IC card load-modulates the requested data as reply data and sends out the load-modulated signal to the reader/writer via an antenna part. 
         [0006]    The reader/writer receives the signal load-modulated by the contactless IC card and demodulates the signal to thereby acquire the reply data. Such demodulation processing is generally realized by a demodulator incorporated into the reader/writer. 
         [0007]    The demodulator is configured as e.g. a circuit incorporated into an IC chip. As described above, it is possible that the demodulator is incorporated into a reader/writer and demodulates a signal transmitted from a contactless IC card. In addition, it is also possible that the demodulator is incorporated into a contactless IC card and demodulates a signal transmitted from a reader/writer. 
         [0008]    Hereinafter, as the operating mode of demodulation processing by the demodulator, the operating mode when a signal transmitted from a reader/writer is received will be referred to as the card mode, and the operating mode when a signal transmitted from a contactless IC card is received will be referred to as the reader/writer mode. In the following, the demodulator that operates in the reader/writer mode will be mainly described. 
         [0009]    Furthermore, in the description of the present specification, the one-data period representing one-bit data in a demodulated signal will be referred to as the etu (Elementary Time Unit), and it is assumed that the data value of each bit is modulated by the bit coding system defined for each communication system. 
         [0010]    Presently, the format of a signal transmitted from a reader/writer to a contactless IC card or the format of a signal transmitted from a contactless IC card to a reader/writer is defined by the standards of e.g. ISO14443 and ISO18092. Three types exist as the types of the signal format defined by these standards. 
         [0000]    Specifically, the Type A system (ISO14443-A), the Type B system (ISO14443-B), and the Type C system (referred to also as the Felica system) exist. 
         [0011]    For example, a contactless IC card of the Type A system load-modulates a carrier (carrier wave) of 13.56 MHz by a subcarrier of 847 KHz (to be exact, 847.5 KHz) depending on reply data to a reader/writer, to thereby generate a reply signal. Specifically, in the Type A system, in 1 etu representing one-bit data, a data value “1” is represented by a symbol in which the subcarrier is superimposed during the first-half period of this 1 etu for example. Furthermore, in 1 etu representing one-bit data, a data value “0” is represented by a symbol in which the subcarrier is superimposed during the second-half period of this 1 etu. 
         [0012]    Moreover, the above-described three Types have frame header information and communication end information different from each other. For example, in the Type A system, at the communication start, a symbol of series D comes as the first-bit received signal (start of communication (SOC)). This serves as the frame header information. Furthermore, in the Type A system, the advent of a symbol of series F (without subcarrier modulation) indicates the communication end. 
         [0013]    For example, Japanese Patent Laid-open No. 2006-33281 (hereinafter, Patent Document 1) discloses a related-art reader/writer device that receives a signal load-modulated by the subcarrier in a contactless IC card of the Type A system. 
         [0014]    According to Patent Document 1, the reader/writer repeatedly transmits a signal for requesting data reply to the contactless IC card of the Type A system (this will be referred to as the polling processing). 
         [0015]    If the contactless IC card of the Type A system exists near the reader/writer, the reader/writer receives a reply sent out from this contactless IC card. At this time, the reader/writer extracts a subcarrier component from the received signal. Furthermore, the in-phase component (I-signal) and the quadrature component (Q-signal) of the subcarrier component are detected to be supplied to a demodulator. 
         [0016]    The demodulator squares the I-signal and the Q-signal to add the squaring result and take the square root of the addition result. Then the demodulator supplies a calculation result signal obtained as the result to a moving average unit. In the calculation result signal, the signal level of the subcarrier component superimposed on the received signal appears. 
         [0017]    The calculation result signal is integrated by the moving average unit in units of ½ etu, and the obtained integral result signals are sequentially supplied to a shift register. 
         [0018]    A timing generator takes timing synchronization in units of ½ etu with the received signal and generates an internal clock rising at the end timing of the ½-etu cycle to output the internal clock to the shift register and a subcarrier signal level detector. Furthermore, the timing generator takes timing synchronization in units of 1 etu with the received signal and generates an internal clock rising at the end timing of the etu cycle to output the internal clock to the subcarrier signal level detector. 
         [0019]    The timing synchronization in units of 1 etu and in units of ½ etu between the internal clock and the received signal can be taken by detecting the SOC. Specifically, the SOC detected from the received frame is regarded as the start point and an enable is generated at each of the cycles of 1 etu and ½ etu defined by the standards. 
         [0020]    The shift register sequentially latches the results of the moving average of the ½-etu interval in synchronization with the input clock of ½ etu. Thereby, the shift register stores the integral results each corresponding to a respective one of first-half ½ etu and second-half ½ etu of the unit symbol interval. 
         [0021]    A subcarrier signal determiner reads out, from the shift register, the integral results each corresponding to a respective one of first-half ½ etu and second-half ½ etu of the unit symbol interval as the subcarrier component signal level of each ½ etu, in synchronization with the clock of 1 etu. Subsequently, a threshold determination is made twice every 1 etu for the obtained subcarrier component signal level of each ½ etu and thereby whether a subcarrier component is present or absent is determined. 
         [0022]    In the threshold determination, if the signal level of the subcarrier component in a ½-etu period surpasses a predetermined threshold, it is determined that the subcarrier exists in this ½-etu period. If the integral result signal indicating the signal level of the subcarrier component in the ½-etu period is equal to or lower than the predetermined threshold, it is determined that the subcarrier does not exist in this ½-etu period. 
         [0023]    In the threshold determination, if it is determined that the subcarrier exists in the first-half period and the subcarrier does not exist in the second-half period, it is determined that this one-data period is equivalent to series D (data value “1”). If it is determined that the subcarrier does not exist in the first-half period and the subcarrier exists in the second-half period, it is determined that this one-data period is equivalent to series E (data value “0”). 
         [0024]    If it is determined that the subcarrier exists in neither the first-half period nor the second-half period, it is determined that this one-data period is equivalent to series F (e.g. non-modulation period). If it is determined that the subcarrier exists in both the first-half period and the second-half period, it is determined that this one-data period is equivalent to collision. 
         [0025]    Due to the above-described scheme, the demodulator can reproduce data returned from the contactless IC card of the Type A system one bit by one bit and also can make determinations relating to the frame end and collision. 
       SUMMARY OF THE INVENTION 
       [0026]    When demodulation is carried out by the technique of Patent Document 1, the subcarrier is detected by regarding the value resulting from the integral of the IQ amplitude for each ½ etu interval as the signal level, and collision determination and decoding are performed. Furthermore, a threshold determination is made regarding whether a subcarrier signal is present or absent in accordance with the magnitude of the amplitude of the received signal, irrespective of the magnitude of the correlation between the received waveform and the desired subcarrier waveform. 
         [0027]    However, generally large noise is frequently included in a transmitted signal from the contactless IC card side. Therefore, determining whether a subcarrier signal is present or absent as shown in Patent Document 1 involves a problem that, when a signal including large noise is subjected to the threshold determination, the signal is erroneously detected as a subcarrier signal. 
         [0028]    For example, a consideration will be made below about the case in which a reader/writer receives a signal that originally has a non-modulation period (series F) as its one-data period but includes noise in the second-half period of its waveform. 
         [0029]    As described above, in the related-art system, a subcarrier determination is made by making a threshold determination about the power (the value of integral) for each ½ bit. Therefore, if a signal that is originally a signal of series F but includes noise in the second-half period of its waveform is received, it is determined that the subcarrier does not exist in the first-half period but the subcarrier exists in the second-half period, and it is determined that this one-data period is equivalent to series E (data value “0”). As a result, the transmitted waveform whose data period is the non-modulation period (series F) is erroneously regarded as series E (data value “0”). 
         [0030]    Furthermore, a consideration will be made below about the case in which a reader/writer receives a signal in which noise is included in the second-half period of the transmitted waveform in which originally a subcarrier exists in the first-half period and the subcarrier does not exist in the second-half period. 
         [0031]    In the related-art system, a subcarrier determination is made by making a threshold determination about the power for each ½ etu. Therefore, it is determined that the subcarrier exists in the first-half period and the subcarrier exists in the second-half period, and it is erroneously determined that this one-data period is equivalent to collision. 
         [0032]    As just described, the related-art technique involves a problem that oscillation in the waveform due to noise or the like is erroneously regarded as subcarrier modulation and as a result erroneous data or the like is demodulated. 
         [0033]    There is a desire for the present invention to allow accurate detection of a subcarrier with a simple configuration. 
         [0034]    According to one embodiment of the present invention, there is provided a signal processing device including correlation signal output means for, about a received signal transmitted from another device, calculating a correlation value between the received signal and the pattern of the waveform of a known signal and outputting the calculated correlation value as a correlation signal in a time corresponding to one symbol, and known signal determination means for determining, based on the correlation signal, whether or not the received signal is modulated by the known signal in a first interval defined when the time corresponding to one symbol is divided into a plurality of intervals, and identifying means for identifying a series of the symbol based on a result of the determination. 
         [0035]    According to another embodiment of the present invention, there is provided a signal processing method including the steps of, about a received signal transmitted from another device, calculating a correlation value between the received signal and the pattern of the waveform of a known signal and outputting the calculated correlation value as a correlation signal in a time corresponding to one symbol by correlation signal output means, and determining, based on the correlation signal, whether or not the received signal is modulated by the known signal in a first interval defined when the time corresponding to one symbol is divided into a plurality of intervals by known signal determination means, and identifying a series of the symbol based on a result of the determination by an identifier. 
         [0036]    In the embodiments of the present invention, about the received signal transmitted from another device, the correlation value between the received signal and the pattern of the waveform of the known signal is calculated and the calculated correlation value is output as the correlation signal in the time corresponding to one symbol. Based on the correlation signal, whether or not the received signal is modulated by the known signal in the first interval defined when the time corresponding to one symbol is divided into a plurality of intervals is determined. Subsequently, the series of the symbol is identified based on the result of the determination. 
         [0037]    The embodiments of the present invention allow accurate detection of a subcarrier with a simple configuration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]      FIG. 1  is a block diagram showing a configuration example of a communication system composed of a contactless IC card and a reader/writer in a related art; 
           [0039]      FIG. 2  is a diagram for explaining bit coding in communication by the Type A system; 
           [0040]      FIG. 3  is a block diagram showing a detailed configuration example of a demodulator in the related art; 
           [0041]      FIG. 4  is a diagram showing an example of the waveforms of an I-signal and a Q-signal resulting from analog quadrature detection; 
           [0042]      FIG. 5  is a diagram showing an example of the waveform of a signal output through calculation of the mean square of the I-signal and the Q-signal in the demodulator of  FIG. 3 ; 
           [0043]      FIG. 6  is a diagram showing an example of the waveform of a signal output through calculation of the average of the signal level in the demodulator of  FIG. 3 ; 
           [0044]      FIG. 7  is a diagram for explaining an example of information latched by a shift register in the demodulator of  FIG. 3 ; 
           [0045]      FIGS. 8A and 8B  are diagrams showing an example of the waveforms of the respective signals in the demodulator of  FIG. 3 ; 
           [0046]      FIG. 9  is a diagram for explaining a system of identification of the series of a symbol by a subcarrier determiner in the demodulator of  FIG. 3 ; 
           [0047]      FIGS. 10A and 10B  are diagrams showing an example of the waveform of a signal including noise and the waveform of a moving average signal; 
           [0048]      FIGS. 11A and 11B  are diagrams showing an example of the waveform of the signal including noise and the waveform of the moving average signal; 
           [0049]      FIG. 12  is a block diagram showing a configuration example of a communication system composed of a contactless IC card and a reader/writer according to one embodiment of the present invention; 
           [0050]      FIG. 13  is a block diagram showing a detailed configuration example of a demodulator in  FIG. 12 ; 
           [0051]      FIGS. 14A to 14C  are diagrams for explaining an output result of a subcarrier correlation filter in  FIG. 13  and determination by a threshold determiner; 
           [0052]      FIGS. 15A and 15B  are diagrams for explaining the case in which noise is included in a signal transmitted from the contactless IC card; 
           [0053]      FIGS. 16A and 16B  are diagrams for explaining another example of the case in which noise is included in a signal transmitted from the contactless IC card; 
           [0054]      FIG. 17  is a flowchart for explaining demodulation processing; 
           [0055]      FIG. 18  is a block diagram showing another configuration example of the communication system composed of a contactless IC card and a reader/writer according to one embodiment of the present invention; and 
           [0056]      FIG. 19  is a block diagram showing a detailed configuration example of a demodulator in  FIG. 18 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0057]    Embodiments of the present invention will be described below with reference to the drawings. 
         [0058]    First, a related-art system will be described below. 
         [0059]      FIG. 1  is a block diagram showing a configuration example of a communication system composed of a contactless IC card and a reader/writer in a related art. In the example of  FIG. 1 , a contactless IC card  30  and a reader/writer  31  communicate with each other by the Type A communication system prescribed by ISO14443-3. 
         [0060]    When transmitting data to the reader/writer, the contactless IC card of the Type A system load-modulates a carrier (carrier wave) of 13.56 MHz by a subcarrier of 847 KHz (to be exact, 847.5 KHz) to thereby generate a signal. 
         [0061]    Specifically, in the Type A system, in 1 etu representing one-bit data, a data value “1” is represented by a symbol in which the subcarrier is superimposed only during the first-half period of this 1 etu for example. Furthermore, in 1 etu representing one-bit data, a data value “0” is represented by a symbol in which the subcarrier is superimposed only during the second-half period of this 1 etu. 
         [0062]    Moreover, the above-described three Types have frame header information and communication end information different from each other. For example, in the Type A system, at the communication start, a symbol of a data value “1” (series D) comes as the first-bit received signal (start of communication (SOC)). This serves as the frame header information. Furthermore, in the Type A system, the advent of a symbol of series F (without subcarrier modulation) indicates the communication end. 
         [0063]    The reader/writer  31  has an antenna  32  for transmitting/receiving a signal to/from the contactless IC card  30  and a semiconductor integrated circuit  33  connected to the antenna  32 . 
         [0064]    When communication between the contactless IC card  30  and the reader/writer  31  is started, a CPU  55  of the reader/writer  31  repeatedly transmits a signal for requesting response reply via the antenna  32  to the contactless IC card  30  capable of communication by the Type A system. This processing is referred to as polling. Data output from the CPU  55  is ASK-modulated via a modulator  52 , and electric waves are sent out from the antenna  32  via a transmitter  51 . 
         [0065]    If the contactless IC card  30  exists near the antenna  32 , the reader/writer  31  receives a signal returned from the contactless IC card  30  via the antenna  32 . The contactless IC card  30  returns a signal obtained by load-modulating data of a communication rate of 106 kbps to the reader/writer  31 . 
         [0066]      FIG. 2  is a diagram for explaining bit coding in the communication by the Type A system. This diagram shows bit coding in a signal transmitted from the contactless IC card  30  to the reader/writer  31 . 
         [0067]    As shown in  FIG. 2 , logic 1 and logic 0 are each represented based on the presence and absence of the subcarrier in the 1-etu interval. Logic 1 means one-bit data whose value is 1, and logic 0 means one-bit data whose value is 0. The one-data period representing one-bit data is referred to as 1 etu (Elementary Time Unit). 
         [0068]    The arrowheads represented below the respective waveforms shown in  FIG. 2  each indicate 1 etu. Each waveform diagram is so made that the abscissa indicates the time and the ordinate indicates the signal level. As shown in  FIG. 2 , the waveform in the interval with subcarrier modulation has a fine comb shape. Specifically, the signal level does not change in the interval (time) without subcarrier modulation, whereas the signal level changes with the same cycle as that of the subcarrier in the interval (time) with subcarrier modulation. 
         [0069]    In  FIG. 2 , the waveforms of 1 etu corresponding to logic 1, logic 0, communication start, and communication end are shown. Each of these four waveforms is information that should be transmitted from the contactless IC card  30  to the reader/writer  31 , and corresponds to one symbol obtained when the above-described data of the communication rate of 106 kbps is subjected to bit coding by a predetermined coding system (e.g. Manchester coding system). 
         [0070]    Logic 1 shown on the upper left side of  FIG. 2  is represented by a waveform obtained by subcarrier modulation only during the interval of first-half 50% of 1 etu. This waveform will be referred to as series D. Logic 0 shown on the upper right side of  FIG. 2  is represented by a waveform obtained by subcarrier modulation only during the interval of second-half 50% of 1 etu. This waveform will be referred to as series E. 
         [0071]    As described above, in the Type A system, at the communication start, series D comes as the first-bit received signal (start of communication (SOC)). This serves as the frame header information (lower left in  FIG. 2 ). Moreover, in the Type A system, the advent of a waveform in which subcarrier modulation is absent in the interval of 100% of 1 etu indicates the communication end. This waveform will be referred to as series F (lower right in  FIG. 2 ). 
         [0072]    These series serve as e.g. symbols represented by the 1-etu interval of a signal transmitted from a contactless IC card. 
         [0073]    The signal received by the antenna  32  of the reader/writer  31  is supplied to an analog quadrature detector  53 , and the analog quadrature detector  53  extracts a subcarrier component from the received signal and detects the in-plane component (I-signal) and the quadrature component (Q-signal) of the subcarrier component. These I-signal and Q-signal are supplied to a demodulator  54  via a signal line d 1  and a signal lien d 2 , respectively. 
         [0074]    The analog quadrature detector  53  carries out A/D conversion of the quadrature-detected signal and carries out oversampling at a frequency of 13.56 MHz. At this time, the number of samples in the 1-etu interval is 128 (13.56 MHz÷106 KHz). Furthermore, because the subcarrier frequency in the Type A communication system is 847 KHz as described above, the number of samples in one subcarrier cycle is 16 (=13.56 MHz÷847 KHz). 
         [0075]    The demodulator  54  determines whether a subcarrier component is present or absent as described later to thereby demodulate the data transmitted from the contactless IC card  30 . The data as the demodulation result is supplied to the CPU  55 . 
         [0076]      FIG. 3  is a block diagram showing a detailed configuration example of the demodulator  54 . As shown in  FIG. 3 , the demodulator  54  includes an IQ mean square unit  61 , a timing generator  62 , a moving average unit  63 , a shift register  64 , and a subcarrier determiner  65 . 
         [0077]    As shown in  FIG. 3 , the I-signal and the Q-signal output from the analog quadrature detector  53  are supplied to the IQ mean square unit  61  and the timing generator  62  via the signal line d 1  and the signal line d 2 . Hereinafter, signals transmitted via the signal line d 1  and the signal line d 2  will be accordingly referred to as the signal d 1  and the signal d 2 , respectively. This applies also to other signals. 
         [0078]      FIG. 4  is a diagram showing an example of the waveforms of the signal d 1  (I-signal) and the signal d 2  (Q-signal). In  FIG. 4 , the waveforms of the respective signals are so shown that the abscissa indicates the time and the ordinate indicates the signal level. This example shows the waveforms of the I-signal and the Q-signal when series D, which is obtained by subcarrier modulation only during the interval of first-half 50% of 1 etu, is received. 
         [0079]    The IQ mean square unit  61  calculates the mean square of each of the I-signal and the Q-signal and outputs the calculation result as a signal d 3 .  FIG. 5  is a diagram showing an example of the waveform of the signal d 3 . In  FIG. 5 , the waveform of the signal d 3  is so shown that the abscissa indicates the time and the ordinate indicates the signal level. 
         [0080]    The timing generator  62  detects the timings of rising-up and falling-down in the waveforms of the signal d 1  and the signal d 2  to thereby generate a signal d 6  as a clock of every ½ etu and a signal d 7  as a clock of every 1 etu. Each of the clocks generated by the timing generator  62  is output in synchronization with the symbol with a waveform resulting from bit coding (series representing one bit). 
         [0081]    The signal d 3  output from the IQ mean square unit  61  is supplied to the moving average unit  63 , and the average of the signal level of the waveform in the ½-etu interval is calculated. Specifically, the ½-etu interval indicated by the arrowhead below the waveform in  FIG. 5  is slid from the left to the right in the diagram with the elapse of time, and the value of integral of the signal level of the waveform in the ½-etu interval at the time is calculated. Thereby, the averages are sequentially calculated. 
         [0082]    The moving average unit  63  calculates the average of the signal level in the above-described manner and outputs the calculation result as a signal d 4 .  FIG. 6  is a diagram showing an example of the waveform of the signal d 4 . In  FIG. 6 , the waveform of the signal d 4  is so shown that the abscissa indicates the time and the ordinate indicates the signal level. 
         [0083]    As shown in  FIG. 6 , the waveform of series D output through the processing by the moving average unit  63  has a mountain-like shape whose peak comes at the time after the elapse of ½ etu from the start position of the symbol. At the respective times indicated by arrowheads  81 - 1  to  81 - 3  in  FIG. 6 , the clock of every ½ etu is generated by the timing generator  62 . 
         [0084]    The signal d 4  output from the moving average unit  63  is supplied to the shift register  64 . The shift register  64  holds (latches) the signal level of the signal d 4  at the timing of the supply of the clock of every ½ etu.  FIG. 7  is a diagram for explaining an example of the information latched by the shift register  64 . 
         [0085]    For example, information DT 0  to information DT 3  shown in  FIG. 7  represent the signal level of the signal d 4  at the timing of the supply of the clock of every ½ etu. For example, the information DT 0  is the value of the signal level corresponding to the clock supplied at the timing of the arrowhead  81 - 1  (hereinafter, this clock will be accordingly referred to as the clock  81 - 1 , and this applies also to other clocks). The information DT 1  is the value of the signal level corresponding to a clock  81 - 2 , and the information DT 2  is the value of the signal level corresponding to a clock  81 - 3 . 
         [0086]    A signal (information) d 5  output from the shift register  64  is supplied to the subcarrier determiner  65 . The subcarrier determiner  65  determines whether or not the value of the signal level represented by the signal d 5  at the timing of the supply of the clock of every ½ etu surpasses a threshold set in advance. Specifically, if the value of the signal level represented by the signal d 5  surpasses the threshold set in advance, it is determined that this ½-etu interval is an interval in which subcarrier modulation is carried out (referred to as a subcarrier-present interval). If the value of the signal level represented by the signal d 5  does not surpass the threshold set in advance, it is determined that this ½-etu interval is an interval in which subcarrier modulation is not carried out (referred to as a subcarrier-absent interval). 
         [0087]    A further description will be made below with reference to  FIGS. 8A and 8B .  FIG. 8A  is a diagram showing the waveform of the signal d 3 . In  FIG. 8A , the waveform of series D is so shown that the abscissa indicates the time and the ordinate indicates the signal level. That is, a waveform in which the first-half ½-etu interval is a subcarrier-present interval and the second-half ½-etu interval is a subcarrier-absent interval is shown. 
         [0088]      FIG. 8B  is a diagram showing the waveform of the signal d 4 . In  FIG. 8B , the signal waveform corresponding to the waveform of series D of  FIG. 8A  is so shown that the abscissa indicates the time and the ordinate indicates the signal level. Arrowheads  81 - 1  to  81 - 3  below  FIG. 8B  each indicate the timing of the clock of the signal d 6 , and arrowheads  82 - 1  and  82 - 2  each indicate the timing of the clock of the signal d 7 . 
         [0089]    As shown in  FIG. 8B , the subcarrier determiner  65  determines the signal level of the signal d 4  by using a threshold indicated by a line  91 . Specifically, at the timings of the arrowheads  81 - 1  and  81 - 3 , it is determined that the signal level does not surpass the threshold, and it is determined that the ½-etu intervals corresponding to the clocks of these timings are subcarrier-absent intervals. In contrast, at the timing of the arrowhead  81 - 2 , it is determined that the signal level surpasses the threshold, and it is determined that the ½-etu interval corresponding to the clock of this timing is a subcarrier-present interval. 
         [0090]    More specifically, the clock at the timing of the arrowhead  81 - 2  is regarded as the clock corresponding to the first-half ½-etu interval of this symbol, and it is determined that the first-half ½-etu interval of this symbol is a subcarrier-present interval. The clock at the timing of the arrowhead  81 - 3  is regarded as the clock corresponding to the second-half ½-etu interval of this symbol, and it is determined that the second-half ½-etu interval of this symbol is a subcarrier-absent interval. Consequently, this symbol is identified as symbol D, i.e. logic 1. 
         [0091]    The subcarrier determiner  65  identifies the series of the symbol based on the result of the determination as to whether the subcarrier is present or absent as described above and outputs the result as demodulated data. 
         [0092]      FIG. 9  is a diagram for explaining the system of identification of the series of a symbol by the subcarrier determiner  65 . In  FIG. 9 , the abscissa indicates the signal level (subcarrier component level) of the signal d 4  of the first-half ½-etu interval, and the ordinate indicates the signal level (subcarrier component level) of the signal d 4  of the second-half ½-etu interval. That is, the signal level of the subcarrier component, i.e. whether the subcarrier is present or absent, is determined about the first-half ½-etu interval and the second-half ½-etu interval separately from each other, and the determination results are each associated with the series of the symbol. 
         [0093]    For example, as shown in the lower right side of  FIG. 9 , if the first-half ½-etu interval is a subcarrier-present interval and the second-half ½-etu interval is a subcarrier-absent interval, this symbol can be identified as series D and thus the subcarrier determiner  65  outputs a bit representing a value 1 (in  FIG. 9 , logic “1”) as demodulated data. 
         [0094]    Furthermore, as shown in the upper left side of  FIG. 9 , if the first-half ½-etu interval is a subcarrier-absent interval and the second-half ½-etu interval is a subcarrier-present interval, this symbol can be identified as series E and thus the subcarrier determiner  65  outputs a bit representing a value 0 (in  FIG. 9 , logic “0”) as demodulated data. 
         [0095]    In addition, as shown in the lower left side of FIG.  9 , if the first-half ½-etu interval is a subcarrier-absent interval and the second-half ½-etu interval is also a subcarrier-absent interval, this symbol can be identified as series F and thus the subcarrier determiner  65  outputs a bit sequence or the like representing the communication end as demodulated data. 
         [0096]    Moreover, as shown in the upper right side of  FIG. 9 , if the first-half ½-etu interval is a subcarrier-present interval and the second-half ½-etu interval is also a subcarrier-present interval, this symbol is a symbol that can not exist in terms of bit coding. In this case, the subcarrier determiner  65  identifies that collision has occurred. Specifically, it is identified that two contactless IC cards simultaneously exist near the antenna  32  of the reader/writer  31  and collision has occurred because of simultaneous reception of signals transmitted from two contactless IC cards. In this case, the subcarrier determiner  65  outputs a collision flag. 
         [0097]    In this manner, the subcarrier determiner  65  can identify the series of the symbol based on the result of the determination as to whether the subcarrier is present or absent and can output the result as demodulated data. 
         [0098]    However, generally large noise is frequently included in a signal transmitted from a contactless IC card. Thus, determining whether a subcarrier signal is present or absent by the above-described related-art system involves a problem that, when a signal including large noise is subjected to the threshold determination, the signal is erroneously detected as a subcarrier signal. 
         [0099]    With reference to  FIGS. 10 and 11 , a description will be made below about a determination as to whether the subcarrier is present or absent when noise is included in a signal transmitted from the contactless IC card  30 . 
         [0100]      FIG. 10A  is a diagram showing the waveform of the signal d 3 . In  FIG. 10A , the waveform of the signal d 3  is so shown that the abscissa indicates the time and the ordinate indicates the signal level. This example shows a waveform when a signal in which the first-half ½-etu interval is a subcarrier-absent interval and the second-half ½-etu interval is also a subcarrier-absent interval (series F) is received as a signal transmitted from the contactless IC card. 
         [0101]    In the case of series F, the waveform of the signal d 3  originally has a flat shape as described above with reference to  FIG. 2 . However, in the present case, the oscillation of the signal level occurs in the second-half ½-etu interval due to the influence of the noise. If the signal d 3  like that shown in  FIG. 10A  is supplied to the moving average unit  63  and the average of the signal level of the waveform in the ½-etu interval is calculated, the signal d 4  like that shown in  FIG. 10B  is output. 
         [0102]      FIG. 10B  is a diagram showing the waveform of the signal d 4 . In  FIG. 10B , the waveform corresponding to the waveform of series F of  FIG. 10A  is so shown that the abscissa indicates the time and the ordinate indicates the signal level. 
         [0103]    If whether the subcarrier is present or absent is determined based on the waveform shown in  FIG. 10B , it is determined that the second-half ½-etu interval is a subcarrier-present interval. This is because the signal level of the second-half ½-etu interval surpasses the threshold. Thus, it is erroneously determined by the subcarrier determiner  65  that series E is received, although series F is received. 
         [0104]    In this manner, due to the influence of the noise, data corresponding to series E (logic 0) is erroneously demodulated from the signal corresponding to series F. 
         [0105]      FIG. 11A  is a diagram showing the waveform of the signal d 3 . In  FIG. 11A , the waveform of the signal d 3  is so shown that the abscissa indicates the time and the ordinate indicates the signal level. This example shows a waveform when a signal in which the first-half ½-etu interval is a subcarrier-present interval and the second-half ½-etu interval is a subcarrier-absent interval (series D) is received as a signal transmitted from the contactless IC card. 
         [0106]    In the case of series D, the waveform of the signal d 3  originally has a flat shape in the second-half ½-etu interval as described above with reference to  FIG. 2 . However, in the present case, the oscillation of the signal level occurs in the second-half ½-etu interval due to the influence of noise. If the signal d 3  like that shown in  FIG. 11A  is supplied to the moving average unit  63  and the average of the signal level of the waveform in the ½-etu interval is calculated, the signal d 4  like that shown in  FIG. 11B  is output. 
         [0107]      FIG. 11B  is a diagram showing the waveform of the signal d 4 . In  FIG. 11B , the waveform corresponding to the waveform of series D of  FIG. 11A  is so shown that the abscissa indicates the time and the ordinate indicates the signal level. 
         [0108]    If whether the subcarrier is present or absent is determined based on the waveform shown in  FIG. 11B , it is determined that the second-half ½-etu interval is a subcarrier-present interval. This is because the signal level of the second-half ½-etu interval surpasses the threshold. Thus, it is erroneously determined by the subcarrier determiner  65  that collision has occurred, although series D is received. 
         [0109]    In this manner, due to the influence of the noise, the occurrence of collision is erroneously detected from the signal corresponding to series D. 
         [0110]    To address this problem, in an embodiment of the present invention, the correlation value with respect to the subcarrier is taken into consideration for the signal used for determining whether the subcarrier is present or absent, in order to prevent noise from being erroneously regarded as the subcarrier. 
         [0111]      FIG. 12  is a block diagram showing a configuration example of a communication system composed of a contactless IC card and a reader/writer according to one embodiment of the present invention. In the example of  FIG. 12 , a contactless IC card  110  and a reader/writer  111  communicate with each other by the Type A communication system prescribed by ISO14443-3. 
         [0112]    The reader/writer  111  has an antenna  112  for transmitting/receiving a signal to/from the contactless IC card  110  and a semiconductor integrated circuit  113  connected to the antenna  112 . 
         [0113]    When communication between the contactless IC card  110  and the reader/writer  111  is started, a CPU  135  of the reader/writer  111  repeatedly transmits a signal for requesting response reply via the antenna  112  to the contactless IC card  110  capable of communication by the Type A system. This processing is referred to as polling. Data output from the CPU  135  is ASK-modulated via a modulator  132 , and electric waves are sent out from the antenna  112  via a transmitter  131 . 
         [0114]    If the contactless IC card  110  exists near the antenna  112 , the reader/writer  111  receives a signal returned from the contactless IC card  110  via the antenna  112 . The contactless IC card  110  returns e.g. a signal obtained by load-modulating data of a communication rate of 106 kbps to the reader/writer  111 . 
         [0115]    The signal received by the antenna  112  of the reader/writer  111  is supplied to an analog quadrature detector  133 , and the analog quadrature detector  133  extracts a subcarrier component from the received signal and detects the in-plane component (I-signal) and the quadrature component (Q-signal) of the subcarrier component. These I-signal and Q-signal are supplied to a demodulator  134  via a signal line d 11  and a signal line d 12 , respectively. 
         [0116]    The analog quadrature detector  133  carries out A/D conversion of the quadrature-detected signal and carries out oversampling at a frequency of 13.56 MHz. At this time, for example, the number of samples in the 1-etu interval is 128 (13.56 MHz÷106 KHz). Furthermore, because the subcarrier frequency in the Type A communication system is 847 KHz as described above, the number of samples in one subcarrier cycle is 16 (=13.56 MHz÷847 KHz). 
         [0117]    The demodulator  134  determines whether a subcarrier component is present or absent as described later to thereby demodulate the data transmitted from the contactless IC card  110 . The data as the demodulation result is supplied to the CPU  135 . 
         [0118]      FIG. 13  is a block diagram showing a detailed configuration example of the demodulator  134  in  FIG. 12 . As shown in  FIG. 13 , the demodulator  134  includes a moving average unit  151 , a subcarrier correlation filter  152 , an IQ mean square unit  153 , a synchronization processor  154 , and a threshold determiner  155 . 
         [0119]    As shown in  FIG. 13 , the I-signal and the Q-signal output from the analog quadrature detector  133  are supplied to the moving average unit  151  and the subcarrier correlation filter  152  via the signal line d 11  and the signal line d 12 . 
         [0120]    The moving average unit  151  calculates the average of the signal level of the waveform in the ⅛-etu interval (one subcarrier cycle) about the signal d 11  and the signal d 12 . For example, the value of integral of the signal level of the waveform in the ⅛-etu interval at the time is calculated, and thereby the averages are sequentially calculated. The signal d 11  and the signal d 12  are generally output as a rectangular wave. However, the waveforms of a signal d 16  and a signal d 17  output from the moving average unit  151  have a shape close to a saw-tooth wave. 
         [0121]    The signal d 16  and the signal d 17  obtained through the processing by the moving average unit  151  have almost the same waveforms as those obtained when the signal d 11  and the signal d 12  are made to pass through a low-pass filter to cut high-frequency noise. Due to this scheme, for example even if the signal d 11  and the signal d 12  are deformed to a waveform that makes it hard to determine one cycle due to the influence of the reception environment and so forth, the interval of one cycle of the subcarrier can be easily identified by detecting the peaks of the waveforms of the signal d 16  and the signal d 17 . That is, the synchronization processor  154  generates an enable based on the signal d 16  and the signal d 17  output from the moving average unit  151 . This allows generation of a more accurate enable. 
         [0122]    The moving average unit  151  may be replaced by a low-pass filter. Furthermore, for example if the possibility of the deformation of the waveforms of the signal d 11  and the signal d 12  is sufficiently low, it is also possible that the moving average unit  151  is not provided. 
         [0123]    The subcarrier correlation filter  152  is a filter having filter coefficients corresponding to the waveform of the subcarrier. 
         [0124]    As described above, the number of samples (clocks) in a 1-etu interval is 128 and the number of samples in one subcarrier cycle is 16. Therefore, the 1-etu interval is equivalent to eight subcarrier cycles. For example, the code bit sequence obtained when a signal modulated by the subcarrier is binarized during two subcarrier cycles is given as (1, 1, 1, 1, 1, 1, 1, 1, −1, −1, −1, −1, −1, −1, −1, −1, 1, 1, 1, 1, 1, 1, 1, 1, −1, −1, −1, −1, −1, −1, −1, −1). 
         [0125]    In this code bit sequence, the sign is reversed every eight samples. Thus, the cycle of this code bit sequence is 16 samples. That is, this code bit sequence corresponds to the shape of a rectangular wave in which the signal level oscillations in such a manner as to change to the higher level and the lower level alternately with the same cycle as that of the subcarrier. This code bit sequence is used as filter coefficients of the ¼-etu interval length. Two subcarrier cycles are equivalent to the ¼-etu interval. 
         [0126]    The subcarrier correlation filter  152  calculates the correlation value between the I-signal and the Q-signal and the subcarrier of two cycles by using the above-described filter coefficients of the ¼-etu interval length. Specifically, the subcarrier correlation filter  152  calculates each of the inner products of the filter coefficients of the ¼-etu interval length and the I-signal and the Q-signal of 32 samples, and defines the absolute values of the calculation results as the I-component subcarrier correlation value and the Q-component subcarrier correlation value. Specifically, if the subcarrier components on the I-signal and the Q-signal are binarized equally to the above-described filter coefficients of the ¼-etu interval length, the correlation value between the I-signal and the Q-signal in this ¼-etu interval and the subcarrier is high. 
         [0127]    The I-component subcarrier correlation value and the Q-component subcarrier correlation value are output as a signal d 13  and a signal d 14 , respectively, and supplied to the IQ mean square unit  153 . 
         [0128]    The IQ mean square unit  153  calculates the mean square of each of the signal d 13  and the signal d 14  and outputs the calculation result as a signal d 15 . 
         [0129]    The synchronization processor  154  detects the frame head (communication start) described above with reference to  FIG. 2  based on the signal d 15 , the signal d 16 , and the signal d 17 . As described above, in the Type A system, a data value “1” obtained by superimposing the subcarrier only during the first-half period of 1 etu comes as the first-bit received signal (start of communication). This serves as the frame header information. 
         [0130]    For example, when the level of the signal d 15  surpasses the threshold set in advance in the ½-etu interval (64 samples) from the start of the oscillation of the signal d 16  and the signal d 17 , the synchronization processor  154  detects this etu as the frame head (communication start). When detecting the frame head (communication start), the synchronization processor  154  sets and outputs a frame detection flag. 
         [0131]    Furthermore, the synchronization processor  154  generates e.g. a clock of every one subcarrier cycle by detecting the peaks of the waveforms of the signal d 16  and the signal d 17 . The synchronization processor  154  has a configuration having e.g. a digital phase-locked loop (PLL) circuit and so forth, and generates a pulse of a clock in linkage with the peaks of the waveforms of the signal d 16  and the signal d 17 . 
         [0132]    The synchronization processor  154  can generate an enable of every ½ etu by outputting a pulse of an enable every four subcarrier cycles by the above-described clock, for example. Furthermore, the synchronization processor  154  can generate an enable of every 1 etu by outputting a pulse of an enable every eight subcarrier cycles by the above-described clock, for example. 
         [0133]    Each of the enable of every ½ etu and the enable of every 1 etu is output in synchronization with the symbol with a waveform resulting from bit coding (series representing one bit). 
         [0134]    Specifically, the synchronization processor  154  can detect the timing of the first cycle of the subcarrier in one etu based on the waveforms of the signal d 16  and the signal d 17  in the etu detected as the frame head as described above. Furthermore, the synchronization processor  154  can generate a signal d 19  as the clock (enable) of every ½ etu and a signal d 20  as the clock (enable) of every 1 etu in synchronization with the detected timing. 
         [0135]    As just described, the synchronization processor  154  generates the signal d 19  as the enable of every ½ etu and the signal d 20  as the enable of every 1 etu by using not only the signal d 15  but also the signal d 16  and the signal d 17 . 
         [0136]    The setting is so made that the mean square value (signal d 15 ) of the output value of the subcarrier correlation filter  152  at the timing when the oscillation direction of the rectangular waveform of the signal d 11  corresponds with the upward arrowheads and the downward arrowheads surpasses the threshold. In the above-described example, the setting is so made that the mean square value surpasses the threshold at the timing when the oscillation direction of the waveform of two cycles of the subcarrier corresponds with the upward arrowheads and the downward arrowheads. Therefore, in the signal d 15 , which is obtained by taking into consideration the correlation value with respect to the subcarrier, the oscillation of the first cycle of the subcarrier is small. Thus, it is difficult to accurately detect the peak of the first cycle of the subcarrier from the signal d 15 . 
         [0137]    Therefore, the synchronization processor  154  generates the signal d 19  as the clock of every ½ etu and the signal d 20  as the clock of every 1 etu by using not only the signal d 15  but also the signal d 16  and the signal d 17 . 
         [0138]    The signal d 15  output from the IQ mean square unit  153  is supplied also to the threshold determiner  155 . The threshold determiner  155  compares the level of the signal d 15  with the threshold set in advance based on the timing specified by the signal d 19 , to thereby determine whether or not subcarrier modulation is carried out every ½-etu interval. 
         [0139]    Here, for example, suppose that the value of the level of the signal d 15  is held by a shift register or the like provided inside the threshold determiner  155  and the maximum value in the ½-etu interval is sequentially updated. Furthermore, the threshold determiner  155  compares the maximum value of the level of the signal d 15  of each ½-etu interval with the threshold set in advance, to thereby determine whether or not subcarrier modulation is carried out. 
         [0140]    In addition, the threshold determiner  155  identifies the series of the symbol represented by this etu based on the timing specified by the signal d 20 . Specifically, the threshold determiner  155  identifies the series of the symbol represented by this etu by the system of identification of the series of a symbol, described above with reference to  FIG. 9 . 
         [0141]    For example, if the first-half ½-etu interval is a subcarrier-present interval and the second-half ½-etu interval is a subcarrier-absent interval, the symbol can be identified as series D and thus the threshold determiner  155  outputs a bit representing a value 1 as demodulated data. 
         [0142]    If the first-half ½-etu interval is a subcarrier-absent interval and the second-half ½-etu interval is a subcarrier-present interval, the symbol can be identified as series E and thus the threshold determiner  155  outputs a bit representing a value 0 as demodulated data. 
         [0143]    If the first-half ½-etu interval is a subcarrier-absent interval and the second-half ½-etu interval is also a subcarrier-absent interval, the symbol can be identified as series F and thus the threshold determiner  155  outputs a bit sequence or the like representing the communication end as demodulated data. 
         [0144]    If the first-half ½-etu interval is a subcarrier-present interval and the second-half ½-etu interval is also a subcarrier-present interval, this symbol is a symbol that can not exist in terms of bit coding and thus the threshold determiner  155  identifies that collision has occurred. Specifically, it is identified that two contactless IC cards simultaneously exist near the antenna  112  of the reader/writer  111  and collision has occurred because of simultaneous reception of signals transmitted from two contactless IC cards. In this case, the threshold determiner  155  outputs a collision flag. 
         [0145]    In this manner, the threshold determiner  155  can identify the series of the symbol based on the result of the determination as to whether the subcarrier is present or absent and can output the demodulated data of the result as a signal d 21 . 
         [0146]      FIGS. 14A to 14C  are diagrams for explaining the output result of the subcarrier correlation filter  152  and the determination by the threshold determiner  155 . 
         [0147]      FIG. 14A  is a diagram showing the waveform of the signal d 11 . In  FIG. 14A , the waveform of the signal d 11  is so shown that the abscissa indicates the time and the ordinate indicates the signal level. This example shows a waveform when a signal in which the first-half ½-etu interval is a subcarrier-present interval and the second-half ½-etu interval is a subcarrier-absent interval (series D) is received as a signal transmitted from a contactless IC card. 
         [0148]    Furthermore,  FIG. 14A  virtually shows the filter coefficients of the subcarrier correlation filter  152 , through which the signal d 11  is made to pass, by arrowheads represented below the waveform in  FIG. 14A . Specifically, the upward arrowhead indicates a filter coefficient “1,” and the downward arrowhead indicates a filter coefficient “−1.” In the diagram, four upward arrowheads and four downward arrowheads are represented per one cycle of the subcarrier for convenience. However, in practice, eight upward arrowheads and eight downward arrowheads are necessary per one cycle of the subcarrier. 
         [0149]    The output value of the subcarrier correlation filter  152  becomes the maximum at the timing when the oscillation direction of the rectangular waveform of the signal d 11  corresponds with the upward arrowheads and the downward arrowheads. 
         [0150]      FIG. 14B  is a diagram for explaining an example of the waveforms of the signal d 13  and the signal d 15 . In  FIG. 14B , the waveform of the signal d 13  output from the subcarrier correlation filter  152  and the waveform of the signal d 15  output from the IQ mean square unit  153  are so shown that the abscissa indicates the time and the ordinate indicates the signal level. In this diagram, the waveform of the signal d 13  and the waveform of the signal d 15  are so shown as to be juxtaposed in the vertical direction in the diagram, for easy understanding of the description. However, two waveforms have signal levels that are originally represented on different ordinates. 
         [0151]    In the waveform of the signal d 13 , the signal level, which is at zero at the time on the leftmost side in the diagram, oscillates in such a manner as to take a positive or negative value with the elapse of time. In the waveform of the signal d 15 , the signal level, which is at zero at the time on the leftmost side in the diagram, oscillates in such a manner as to take a positive value with the elapse of time. 
         [0152]    The output values of the subcarrier correlation filter  152  about the ¼-etu intervals shown as Interval  1  to Interval  3 , respectively, in  FIG. 14A  are output as the waveforms at the positions of the ellipses indicated by the arrowheads in  FIG. 14B . 
         [0153]    The signal levels at the positions indicated by the black circles as the peaks of the waveform of the signal d 15  in  FIG. 14B  are the maximum value in the first-half ½-etu interval and the maximum value in the second-half ½-etu interval, respectively. 
         [0154]      FIG. 14C  is a diagram in which the maximum values of the level of the signal d 15  are plotted in the first-half ½-etu interval and the second-half ½-etu interval. Specifically, the black circles in the diagram represent the plotted maximum values. As described above, the threshold determiner  155  compares the maximum value of the level of the signal d 15  in the ½-etu interval with the threshold set in advance based on the timing specified by the signal d 19 , to thereby determine whether or not subcarrier modulation is carried out every ½-etu interval. 
         [0155]    In the present case, the maximum value in the first-half ½-etu interval surpasses the threshold and thus it turns out that subcarrier modulation is carried out. Furthermore, the maximum value in the second-half ½-etu interval does not surpass the threshold and thus it turns out that subcarrier modulation is not carried out. The vertical position of the straight line along the horizontal direction in the diagram indicates the threshold. 
         [0156]    That is, the setting is so made that the mean square value (signal d 15 ) of the output value of the subcarrier correlation filter  152  at the timing when the oscillation direction of the rectangular waveform of the signal d 11  corresponds with the upward arrowheads and the downward arrowheads surpasses the threshold. 
         [0157]    Although the signal d 11  is shown in  FIG. 14A  and the signal d 13  is shown in  FIG. 14B , the signal d 12  and the signal d 14  are also used to generate the signal d 15  in the actual processing. 
         [0158]      FIGS. 15A and 15B  are diagrams for explaining the case in which noise is included in a signal transmitted from a contactless IC card. 
         [0159]      FIG. 15A  is a diagram showing the waveform of the signal d 11 . In  FIG. 15A , the waveform of the signal d 11  is so shown that the abscissa indicates the time and the ordinate indicates the signal level. This example shows a waveform when a signal in which the first-half ½-etu interval is a subcarrier-absent interval and the second-half ½-etu interval is also a subcarrier-absent interval (series F) is received as a signal transmitted from a contactless IC card. However, in this example, the waveform of the second-half ½-etu interval, which is originally flat, includes oscillation due to noise. 
         [0160]    Specifically, if the signal d 11  is modulated by the subcarrier, a rectangular waveform including oscillation at a certain cycle is observed. However, the waveform of the second-half ½-etu interval of  FIG. 15A  includes irregular oscillation. 
         [0161]      FIG. 15B  is a diagram showing the waveform of the signal d 15 . In  FIG. 15B , the waveform of the signal d 15  corresponding to the signal d 11  of  FIG. 15A  is so shown that the abscissa indicates the time and the ordinate indicates the signal level. 
         [0162]    As shown in  FIG. 15B , in the signal d 15 , which is output through the processing by the IQ mean square unit  153  for the output value of the subcarrier correlation filter  152 , the waveform of the second-half ½-etu interval, which is originally flat, includes oscillation due to noise. However, because the subcarrier correlation filter  152  has the above-described filter coefficients, the correlation value with respect to the oscillation of the signal d 11  in the second-half ½-etu interval due to the noise is sufficiently small. Specifically, the oscillation direction of the waveform represented by upward arrowheads and downward arrowheads like those shown in  FIG. 14A  is greatly different from the direction of the oscillation due to the noise in  FIG. 15A . Thus, the correlation between this noise and the subcarrier is low, so that a low correlation value is output. 
         [0163]    As shown in  FIG. 15B , the maximum value of the level of the signal d 15  surpasses the threshold in neither the first-half ½ etu nor the second-half ½ etu. This allows the threshold determiner  155  to identity that, in this etu, subcarrier modulation is carried out in neither the first-half ½-etu interval nor the second-half ½-etu interval and thus demodulate series F from the signal d 11  of  FIG. 15A . 
         [0164]    Although the signal d 11  is shown in  FIG. 15A , naturally the signal d 12  is also used to generate the signal d 15  in the actual processing. 
         [0165]      FIGS. 16A and 16B  are diagrams for explaining another example of the case in which noise is included in a signal transmitted from a contactless IC card. 
         [0166]      FIG. 16A  is a diagram showing the waveform of the signal d 11 . In  FIG. 16A , the waveform of the signal d 11  is so shown that the abscissa indicates the time and the ordinate indicates the signal level. This example shows a waveform when a signal in which the first-half ½-etu interval is a subcarrier-present interval and the second-half ½-etu interval is a subcarrier-absent interval (series D) is received as a signal transmitted from a contactless IC card. However, in this example, the waveform of the second-half ½-etu interval, which is originally flat, includes oscillation due to noise. 
         [0167]    Specifically, if the signal d 11  is modulated by the subcarrier, a rectangular waveform including oscillation at a certain cycle is observed. However, the waveform of the second-half ½-etu interval of  FIG. 16A  includes irregular oscillation. 
         [0168]      FIG. 16B  is a diagram showing the waveform of the signal d 15 . In  FIG. 16B , the waveform of the signal d 15  corresponding to the signal d 11  of  FIG. 16A  is so shown that the abscissa indicates the time and the ordinate indicates the signal level. 
         [0169]    As shown in  FIG. 16B , in the signal d 15 , which is output through the processing by the IQ mean square unit  153  for the output value of the subcarrier correlation filter  152 , the waveform of the second-half ½-etu interval, which is originally flat, includes oscillation due to noise. However, because the subcarrier correlation filter  152  has the above-described filter coefficients, the correlation value with respect to the oscillation of the signal d 11  in the second-half ½-etu interval due to the noise is sufficiently small. Specifically, the oscillation direction of the waveform represented by upward arrowheads and downward arrowheads like those shown in  FIG. 14A  is greatly different from the direction of the oscillation due to the noise in  FIG. 16A . Thus, the correlation between this noise and the subcarrier is low, so that a low correlation value is output. 
         [0170]    As shown in  FIG. 16B , the maximum value of the level of the signal d 15  surpasses the threshold in first-half ½ etu but does not surpass the threshold in second-half ½ etu. This allows the threshold determiner  155  to identity that, in this etu, subcarrier modulation is carried out in the first-half ½-etu interval and is not carried out in the second-half ½-etu interval and thus demodulate series D from the signal d 11  of  FIG. 16A . 
         [0171]    Although the signal d 11  is shown in  FIG. 16A , naturally the signal d 12  is also used to generate the signal d 15  in the actual processing. 
         [0172]    As just described, in the embodiment of the present invention, the correlation value with respect to the subcarrier is taken into consideration for the signal used for determining whether the subcarrier is present or absent. Therefore, the problem that noise is erroneously regarded as the subcarrier is absent. Due to this feature, for example even if a waveform including noise is received as described above with reference to  FIGS. 15A and 15B  and  FIGS. 16A and 16B , the series of the symbol that should be demodulated originally can be demodulated. 
         [0173]    Furthermore, in the embodiment of the present invention, signals obtained without taking into consideration the correlation value with respect to the subcarrier (signal d 16  and signal d 17 ) are used as the signals used for generation of the enable. Thus, generation of a more accurate enable is possible. This is because, in the embodiment of the present invention, the oscillation of the first cycle of the subcarrier can be detected in the generation of the enable and each enable can be output in synchronization with the symbol (series representing one bit). 
         [0174]    As described above, in the signal obtained by taking into consideration the correlation value with respect to the subcarrier, the oscillation of the first cycle of the subcarrier is small. Thus, it is difficult to accurately detect the peak of the first cycle of the subcarrier from the signal obtained by taking into consideration the correlation value with respect to the subcarrier. 
         [0175]    As a solution thereto, in the embodiment of the present invention, the enable is generated by detecting the peaks of the signal d 16  and the signal d 17 , which are obtained without taking into consideration the correlation value with respect to the subcarrier. Therefore, the oscillation of the first cycle of the subcarrier can be detected and each enable can be output in synchronization with the symbol. 
         [0176]    Next, with reference to a flowchart of  FIG. 17 , an example of the demodulation processing by the demodulator  134  according to the embodiment of the present invention will be described below. 
         [0177]    In a step S 21 , the moving average unit  151  outputs the moving average signals of the detected signals. 
         [0178]    At this time, about the I-signal and the Q-signal as the detected signals output from the analog quadrature detector  133 , the average of the signal level of the waveform in the ⅛-etu interval (one subcarrier cycle) is calculated. Subsequently, the signal d 16  and the signal d 17  are output as the moving average signals output from the moving average unit  151 . 
         [0179]    In a step S 22 , the subcarrier correlation filter  152  outputs the correlation value between the detected signals and the subcarrier. 
         [0180]    At this time, the above-described code bit sequence is used as the filter coefficients of the ¼-etu interval length, and the subcarrier correlation filter  152  calculates the correlation value between the I-signal and the Q-signal and the subcarrier of two cycles. Specifically, the subcarrier correlation filter  152  calculates each of the inner products of the filter coefficients of the ¼-etu interval length and the I-signal and the Q-signal of 32 samples, and defines the absolute values of the calculation results as the I-component subcarrier correlation value and the Q-component subcarrier correlation value. The I-component subcarrier correlation value and the Q-component subcarrier correlation value are output as the signal d 13  and the signal d 14 , respectively. 
         [0181]    In a step S 23 , the IQ mean square unit  153  calculates the mean square of the subcarrier correlation values (each of the signal d 13  and the signal d 14 ) output through the processing of the step S 22  and outputs the calculated mean square as the signal d 15 . 
         [0182]    In a step S 24 , the synchronization processor  154  generates an enable synchronized with the symbol based on the mean square value (signal d 15 ) output through the processing of the step S 23  and the moving average signals (signal d 16  and signal d 17 ) output through the processing of the step S 21 . 
         [0183]    At this time, the frame head (communication start) is detected based on the oscillation of the signal d 16  and the signal d 17  and the level of the signal d 15 . Furthermore, by detecting the peaks of the waveforms of the signal d 16  and the signal d 17 , the signal d 19  as a clock of every ½ etu and the signal d 20  as a clock of every 1 etu are generated. 
         [0184]    In a step S 25 , the threshold determiner  155  makes a threshold determination for the maximum value of the mean square value (signal d 15 ) output through the processing of the step S 23  every ½ etu based on the enables (signal d 19  and signal d 20 ) generated by the processing of the step S 24 . 
         [0185]    At this time, the maximum value of the level of the signal d 15  output from the IQ mean square unit  153  as the mean square of the subcarrier correlation values is compared with the threshold set in advance, and thereby whether or not subcarrier modulation is carried out is determined every ½-etu interval. 
         [0186]    In a step S 26 , the threshold determiner  155  identifies the series of the symbol represented by this etu based on the determination result by the processing of the step S 25 . 
         [0187]    At this time, the threshold determiner  155  identifies the series of the symbol represented by this etu by the system of identification of the series of a symbol, described above with reference to  FIG. 9  for example. 
         [0188]    In a step S 27 , the threshold determiner  155  outputs data corresponding to the series of the symbol identified by the processing of the step S 26  as the demodulation result. 
         [0189]    The demodulation processing is executed in this manner. 
         [0190]    In the example described above with reference to  FIG. 12 , a signal received by the antenna  112  of the reader/writer  111  is supplied to the analog quadrature detector  133 . However, a signal received by the antenna may be subjected to analog envelope detection. 
         [0191]      FIG. 18  is a block diagram showing another configuration example of the communication system composed of a contactless IC card and a reader/writer according to one embodiment of the present invention. In the example of  FIG. 18 , a contactless IC card  210  and a reader/writer  211  communicate with each other by the Type A communication system prescribed by ISO14443-3. 
         [0192]    In  FIG. 18 , the contactless IC card  210  is similar to the contactless IC card  110  in  FIG. 12 . The reader/writer  211  corresponds to the reader/writer  111  in  FIG. 12 , but the internal configuration thereof is different from that in  FIG. 12 . 
         [0193]    Specifically, instead of the analog quadrature detector  133  provided in the reader/writer  111  in the example of  FIG. 12 , an analog envelope detector  233  is provided in the reader/writer  211  in the example of  FIG. 18 . 
         [0194]    In the example of  FIG. 18 , a signal received by an antenna  212  of the reader/writer  211  is supplied to the analog envelope detector  233 . Subsequently, the analog envelope detector  233  carries out envelope detection for the received signal and extracts a subcarrier component from the received signal. The detected signal is supplied to a demodulator  234  via a signal line d 41 . 
         [0195]    The analog envelope detector  233  carries out A/D conversion of the envelope-detected signal and carries out oversampling at a frequency of 13.56 MHz. At this time, for example, the number of samples in the 1-etu interval is 128 (13.56 MHz÷106 KHz). Furthermore, because the subcarrier frequency in the Type A communication system is 847 KHz as described above, the number of samples in one subcarrier cycle is 16 (=13.56 MHz÷847 KHz). 
         [0196]    In the example of  FIG. 18 , due to the provision of the analog envelope detector  233 , the configuration of the demodulator  234  is different from that of the demodulator  134  in  FIG. 12  as described later. 
         [0197]    The other configuration in  FIG. 18  is the same as that in  FIG. 12  and therefore detailed description thereof is omitted. 
         [0198]      FIG. 19  is a block diagram showing a detailed configuration example of the demodulator  234  in  FIG. 18 . As shown in  FIG. 19 , the demodulator  234  includes a moving average unit  251 , a subcarrier correlation filter  252 , an absolute value calculator  253 , a synchronization processor  254 , and a threshold determiner  255 . 
         [0199]    As shown in  FIG. 19 , the detected signal output from the analog envelope detector  233  is supplied to the moving average unit  251  and the subcarrier correlation filter  252  via the signal line d 41 . 
         [0200]    The moving average unit  251  calculates the average of the signal level of the waveform in the ⅛-etu interval (one subcarrier cycle) about the signal d 41 . For example, the value of integral of the signal level of the waveform in the ⅛-etu interval at the time is calculated, and thereby the averages are sequentially calculated. The signal d 41  is generally output as a rectangular wave. However, the waveform of a signal d 46  output from the moving average unit  251  has a shape close to a saw-tooth wave. 
         [0201]    The signal d 46  obtained through the processing by the moving average unit  251  has almost the same waveform as that obtained when the signal d 41  is made to pass through a low-pass filter to cut high-frequency noise. Due to this scheme, for example even if the signal d 41  is deformed to a waveform that makes it hard to determine one cycle due to the influence of the reception environment and so forth, the interval of one cycle of the subcarrier can be easily identified by detecting the peak of the waveform of the signal d 46 . That is, the synchronization processor  254  generates an enable based on the signal d 46  output from the moving average unit  251 . This allows generation of a more accurate enable. 
         [0202]    The subcarrier correlation filter  252  is a filter having filter coefficients corresponding to the waveform of the subcarrier. These filter coefficients are similar to those of the subcarrier correlation filter  152  in  FIG. 13  and therefore detailed description thereof is omitted. 
         [0203]    The subcarrier correlation value is output as a signal d 43  and supplied to the absolute value calculator  253 . 
         [0204]    The absolute value calculator  253  calculates the absolute value of the signal d 43  and outputs it as a signal d 45 . 
         [0205]    The synchronization processor  254  detects the frame head (communication start) described above with reference to  FIG. 2  based on the signal d 45  and the signal d 46 . 
         [0206]    For example, when the level of the signal d 45  surpasses the threshold set in advance while the signal d 46  oscillates, the synchronization processor  254  detects this etu as the frame head (communication start). When detecting the frame head (communication start), the synchronization processor  254  sets and outputs a frame detection flag. 
         [0207]    Furthermore, the synchronization processor  254  detects the peak of the waveform of the signal d 46  to thereby generate a signal d 49  as an enable of every ½ etu and a signal d 50  as an enable of every 1 etu. Each of the enables (clocks) generated by the synchronization processor  254  is output in synchronization with the symbol with a waveform resulting from bit coding (series representing one bit). Specifically, as described above, the timing of the first cycle of the subcarrier can be detected in the etu detected as the frame head, and the enable of every ½ etu and the enable of every 1 etu can be generated by generating a clock in synchronization with the detected timing. 
         [0208]    The signal d 45  output from the absolute value calculator  253  is supplied also to the threshold determiner  255 . The threshold determiner  255  compares the maximum value of the level of the signal d 45  with the threshold set in advance based on the timing specified by the signal d 49 , to thereby determine whether or not subcarrier modulation is carried out every ½-etu interval. 
         [0209]    The threshold determiner  255  identifies the series of the symbol represented by this etu based on the timing specified by the signal d 50 . Specifically, the threshold determiner  255  identifies the series of the symbol represented by this etu by the system of identification of the series of a symbol, described above with reference to  FIG. 9 . 
         [0210]    In this manner, the threshold determiner  255  can identify the series of the symbol based on the result of the determination as to whether the subcarrier is present or absent and can output the demodulated data of the result as a signal d 51 . 
         [0211]    As just described, the concept of the present invention can be applied also to the case in which a signal received by the antenna is subjected to analog envelope detection. 
         [0212]    The series of processing described above in the present specification encompasses processing that is executed in a time-series manner in the described order of course, and encompasses also processing that is executed in parallel or individually even when it is not necessarily executed in a time-series manner. 
         [0213]    Embodiments of the present invention are not limited to the above-described embodiment, but various changes can be made without departing from the gist of the present invention. 
         [0214]    The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2009-283761 filed in the Japan Patent Office on Dec. 15, 2009, the entire content of which is hereby incorporated by reference.