BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a transmission apparatus and a peak reduction method. In particular, the present invention relates to a transmission apparatus for CDMA (Code Division Multiple Access) and a method for reducing the amplitude peak in its transmission signal. 
   2. Description of the Related Art 
     FIG. 8  is a block diagram showing an example of a configuration of a conventionally known transmission apparatus for CDMA terminal. In  FIG. 8 , data input to a transmission apparatus  100  are transmission data D 1  to D 6  and control data C 0  to C 2 . The number of actually used transmission data and the number of control data might be decreased according to the subject apparatus. Spreaders  100   a ,  100   b ,  100   c ,  100   d ,  100   e ,  100   f ,  100   g ,  100   h  and  100   i  respectively spread the input transmission data D 1 , D 3  and D 5 , control data C 1 , transmission data D 2 , D 4  and D 6 , and control data C 2  and C 0  in accordance with a spreading factor specified by an upper layer which is not illustrated. The transmission data D 1 , D 3  and d 5  and the control data C 1  are assigned to the Ich (In-phase channel), whereas the transmission data D 2 , D 4  and D 6  and the control data C 0  and C 2  are assigned to Qch (Quad-phase channel). 
   An adder  101   a  is an adder for conducting addition for Ich. The adder  101   a  adds up signals output from spreaders  100   a ,  100   b ,  100   c  and  100   d . An adder  101   b  is an adder for conducting addition for Qch. The adder  101   b  adds up signals output from spreaders  100   e ,  100   f ,  100   g ,  100   h  and  100   i.    
   A scrambling circuit  103  generates a scrambling code for conducting spreading modulation. A spreading modulation circuit  102  is supplied with a scrambling code which is an output of the scrambling circuit  103 , and the spreading modulation circuit  102  conducts spreading modulation on data respectively input from the adder  101   a  and the adder  101   b.    
   Band limiting filters  104   a  and  104   b  are filters for conducting band limiting on signals respectively of the Ich and Qch subjected to spreading modulation and output from the spreading modulation circuit  102 . In order to raise the frequency utilization efficiency in an analog baseband signal when generating a transmission signal, band limiting filters are needed, therefore, the band limiting filters  104   a  and  104   b  include LPFs (Low Pass Filters) for conducting spectral shaping on the transmission signal. In terminal stations, for example, root raised cosine filters having a roll off factor of 0.22 are used as the LPFs. 
   DA converters  106   a  and  106   b  are digital-analog converters for the Ich and Qch, respectively. The DA converters  106   a  and  106   b  convert signals output from the band limiting filters  104   a  and  104   b  to analog signals, respectively. A modulation circuit  107  is a circuit for conducting analog modulation (QPSK: Quadrature Phase Shift Keying) on the Ich signal and Qch signal converted to analog signals by the DA converters  106   a  and  106   b . An RF circuit  108  has a function of converting the analog signal modulated by the modulation circuit  107  to a carrier frequency and a function of amplifying a transmission signal. An antenna  109  has a function of radiating the transmission signal output from the RF circuit  108  as an electromagnetic wave. 
   As for amplifiers in the modulation circuit  107  and the RF circuit  108  used in the transmission apparatus  100  having such a configuration, it is necessary to use A-class amplifiers having fine linearity and wide dynamic characteristics in order to amplify an input waveform without distortion. If multiplexing (multi-coding) of signals advances, a large peak occurs in amplitude and the PAR (Peak-to-Average power ratio) representing the ratio of the peak to the average power becomes large. Therefore, it is conceivable to conduct clipping on the amplitude in order to improve the efficiency of power conversion. By doing so, however, distortion occurs in the spectrum, resulting in influence upon adjacent channels. 
   By the way, in the field of the mobile communication system represented by cellular phone, drawing up and proposal of standards are being performed by 3GPP (3rd Generation Partnership Project). Among them, specifications of a UE (User Equipment) which is a cellular phone or a terminal station, such as provisions concerning leak power of adjacent channels, are defined. When forming a transmitter in a terminal station on the basis of specifications of 3GPP, the design should satisfy the provisions. 
   In the standards of 3GPP, multiplexing of transmission codes at a terminal station is allowed, however, if the multiplex number of codes increases, a large peak occurs in the signal amplitude in some combination of the transmission data sequence. It is known that the transmission data sequence that generates the peak occurs with some probability and the peak pattern have a relation to the transmission data sequence. 
   On the other hand, when generating a transmission signal in wireless communication, a filter is used to conduct band limiting on an analog baseband signal. As the band limiting filter, an LPF (low pass filter) for conducting spectrum shaping on the transmission signal is used. 
   Typically, the band limiting filter of this kind is implemented as a digital circuit using an FIR (Finite Impulse Response) filter in order to obtain linear phase characteristics. The FIR filter outputs a waveform of an impulse response as shown in, for example,  FIG. 9 . Each of output waveforms of the DA converters  106   a  and  106   b  is a waveform formed by impulse response waveforms of individual transmission data overlapping along the time axis. An example thereof is shown in  FIG. 10 . In order to facilitate illustration,  FIG. 10  shows not a waveform at the time of a multi-code, but a waveform at the time of a single code. If impulse response waveforms each having a waveform as shown in  FIG. 9  overlap at a peak such as a point P, a point Q and a point R, a large peak which exceeds range of planned target R, such as a peak at a point S or a point T shown in  FIG. 10 , occurs sometimes. As already described, such a peak occurs with some probability. 
   When designing the transmission circuit of a terminal station in the mobile communication system, therefore, the number of bits in the DA converter  106   a  and  106   b  are determined with due regard to the characteristics of the filter so as to be capable of representing the entire waveform even if a peak exceeding the range of planned target occurs as represented by the point S and T in  FIG. 10 . For conducting the design with such a number of bits, however, a wide range is demanded for the frequency handled by the DA converters  106   a  and  106   b . Furthermore, it also becomes important to take the CCDF (Complementary Cumulative Distribution Function) and the PAR into consideration in representing the amplitude of the transmission signal. 
   In other words, if the bit width of the DA converters  106   a  and  106   b  is determined so as to represent all amplitudes, the ratio of the peak to the average power becomes large and consequently it becomes necessary to take the influence of the quantization noise as well into consideration. In addition, there is also a problem that the circuit scale becomes large. 
   If there is a large peak in the transmission signal input to an amplifier in the succeeding modulation circuit  107  or RF circuit  108 , a nonlinear region is used and consequently there is a fear that leak power to adjacent channels will be aggravated. For preventing this, it is demanded to use an A-class amplifier having a fine linearity and a wide range as described above. However, it leads to an increased cost of the apparatus. 
   In the circuit configuration of the transmitter in the conventional terminal station, the code multiplex number is comparatively small and consequently the circuit is designed so as to be able to represent all amplitudes without being much conscious of peaks. Recently, however, an increase of quantity of data transmitted to the base station on an uplink is expected. Furthermore, if multi-coding is adopted in the terminal station as well, it is considered that the amplitude peak in the transmission signal as described above will become larger. Therefore, means for suppressing the peaks are demanded. 
   Some conventional apparatuses suppressing the amplitude peaks in the transmission signal are known. For example, in a peak reduction apparatus described in JP-A-10-271072 (FIG. 1), a code sequence having a predetermined length and having a pattern which increases in amplitude when a transmission code sequence is limited in band by a transmission filter is set in a register. The peak reduction apparatus includes a comparison unit for comparing the code sequence with a transmission code sequence shifted into a shift register, and a code amplitude reduction unit supplied with a comparison coincidence signal from the comparison unit to reduce amplitude of at least one code disposed in the center of a predetermined length of the transmission code sequence. According to the peak reduction apparatus described in JP-A-10-271072, spread of the envelope of the signal limited in band by the transmission filter is reduced. Therefore, spread of the band caused by nonlinear distortion in a transmission amplifier unit can be suppressed. 
   In a peak factor reduction apparatus described in JP-A-2003-124824 (FIGS. 1 and 3), a correction signal which concentrates energy only immediately in the vicinity of a peak pulse is generated. Elimination of the peak pulse is conducted on the basis of the correction signal. As a result, the influence on the degradation of the signal quality can be held down to a slight value. This apparatus inputs an input signal to a reference filter and predicts what kind of a peak occurs when band limiting is conducted. Only a part for which an output of the reference filter has exceeded a preset value is extracted by an amplitude control unit to form a peak pulse. Subsequently, at a point in time in which the peak pulse becomes the maximum, an impulse signal having amplitude proportionate to the peak pulse is generated. The input signal is delayed by a delay unit and aligned in timing with the impulse signal. The impulse signal is subtracted in signal from an output of the delay unit by an adder, and a resultant signal is output. The output signal is finally limited in band by a band limiting filter. On the basis of the superposition theorem in the linear circuit, the peak amplitude generated by the input signal and the impulse response amplitude generated by the impulse signal coincide with each other in position and amplitude, and they are opposite in phase. Therefore, an amplitude component that has exceeded the peak is suppressed, and the peak factor can be held down to a preset value. 
   In JP-A-2002-164799 (FIG. 2), a transmission power control method for communication apparatus capable of suppressing the occurrence of a peak factor at the time of user multiplexing is disclosed. In this method, transmission data before being input to a band limiting filter is branched, then one of the branched transmission data is passed through a peak detection filter having the same configuration as a band limiting filter to obtain a correction value for suppressing a power peak of the transmission data. The other of the branched transmission data is delayed by a time for obtaining the correction value, then corrected by the correction value, and then input to the band limiting filter. A maximum value of a power peak of transmission data passed through the peak detection filter is obtained every sampling time. The maximum value is compared with a peak suppression threshold to obtain a correction value. 
   By the way, it is desirable that an increase of signal delay caused by a circuit added to hold down the amplitude peak of the transmission signal is as small as possible. In the conventionally known techniques, however, a considerably long delay is caused by the added circuit. 
   In other words, in the peak reduction apparatus described in JP-A-10-271072, a delay time for shifting the transmission code sequence in the shift register is added before holding down the amplitude peak of the transmission signal in the code amplitude reduction unit. 
   In the peak factor reduction apparatus described in JP-A-2003-124824, the input signal is delayed by the delay unit to align the input signal in timing with the impulse signal, resulting in an increased delay time. 
   In the transmission power control method described in JP-A-2002-164799, the delay is increased by the time for obtaining the correction value. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to avoid causing an excessive peak in frequency characteristics while conducting signal processing for generating a transmission signal quickly. 
   A transmission apparatus according to the present invention includes a pattern detection unit for making a decision whether a specific pattern which causes a peak exceeding a predetermined amplitude range to be generated in frequency characteristics of a transmission signal is included in a transmission code sequence for forming the transmission signal, and a filter unit for selecting filter coefficients which prescribe a band limiting factor for the frequency characteristics of the transmission signal, on the basis of a result of the decision, and conducting filtering on the transmission code sequence by using the selected filter coefficients. 
   A peak reduction method according to the present invention is a method in which a transmission apparatus for conducting wireless communication executes the steps of making a decision whether a specific pattern which causes a peak exceeding a predetermined amplitude range to be generated in frequency characteristics of a transmission signal is included in a transmission code sequence for forming the transmission signal, and selecting filter coefficients which prescribe a band limiting factor for the frequency characteristics of the transmission signal, on the basis of a result of the decision, and conducting filtering on the transmission code sequence by using the selected filter coefficients. 
   According to the present invention, as regards patterns of a transmission code sequence, a specific pattern which causes an excessive peak in frequency characteristics of the transmission signal is previously grasped. When the specific pattern is detected in the actually processed transmission code sequence, operation is conducted to execute switching to filter coefficients which can reduce the excessive peak. As a result, the frequency characteristics of the transmission signal can be adjusted suitably without causing a remarkable delay in the transmission signal generation processing. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing a configuration of a transmission apparatus according to an embodiment of the present invention; 
       FIG. 2  is a block diagram showing a configuration of a band limiting filter according to a first embodiment of the present invention; 
       FIG. 3  is a block diagram showing a configuration of a coefficient unit having two coefficients according to an embodiment of the present invention; 
       FIG. 4  is a diagram showing examples of coefficients in the coefficient unit; 
       FIG. 5  is a block diagram showing a configuration of a pattern comparison circuit according to an embodiment of the present invention; 
       FIG. 6  is a diagram showing an example of an output waveform of a DA converter according to an embodiment of the present invention; 
       FIG. 7  is a block diagram showing a configuration of a band limiting filter according to a second embodiment of the present invention; 
       FIG. 8  is a block diagram showing an example of a configuration of a conventional transmission apparatus for CDMA terminal; 
       FIG. 9  is a diagram showing a waveform of an impulse response; and 
       FIG. 10  is a diagram showing an example of an output waveform of a conventional DA converter. 
   


   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A transmission apparatus according to an embodiment of the present invention includes a pattern comparison circuit for detecting a specific pattern included in a transmission code sequence and outputting a result of detection, and a band limiting filter for conducting predetermined band limiting on the transmission code sequence, outputting a resultant code sequence as a transmission signal, and changing filter coefficients used for the band limiting on the basis of the detection result of the specific pattern. 
   The pattern comparison circuit records a specific pattern that can become an excessive peak and that is included in the transmission code sequence, in a memory, and makes a decision whether there is the specific pattern by comparing the specific pattern with a transmission code sequence that becomes a subject of processing at the current point in time. The band limiting filter adjusts a band limiting factor of filtering by selecting filter coefficients on the basis of a result of the decision made by the pattern comparison circuit. According to the present embodiment, the pattern comparison of the code sequence is conducted concurrently with and in parallel to the original operation of the band limiting filter. Therefore, the processing delay caused by the addition of the pattern comparison circuit can be made short to the utmost. 
   First Embodiment 
     FIG. 1  is a block diagram showing a configuration of a transmission apparatus according to an embodiment of the present invention. As for spreaders  10   a ,  10   b ,  10   c ,  10   d ,  10   e ,  10   f ,  10   g ,  10   h  and  10   i , adders  11   a  and  11   b , a spreading modulation circuit  12 , a scrambling circuit  13 , DA converters  16   a  and  16   b , a modulation circuit  17 , an RF circuit  18  and an antenna  19  shown in  FIG. 1 , components similar to the spreaders  100   a ,  100   b ,  100   c ,  100   d ,  100   e ,  100   f ,  100   g ,  100   h  and  100   i , the adders  101   a  and  101   b , the spreading modulation circuit  102 , the scrambling circuit  103 , the DA converters  106   a  and  106   b , the modulation circuit  107 , the RF circuit  108  and the antenna  109  included in the conventional transmission apparatus as shown in  FIG. 8  can be used, respectively. Description of those components will be omitted. Hereafter, the band limiting filters  14   a  and  14   b  and the pattern comparison circuits  15   a  and  15   b  according to the present embodiment will be mainly described. 
   The band limiting filters  14   a  and  14   b  are filters for conducting band limiting on the spreading modulation signals for Ich and Qch subjected to the spreading modulation in the spreading modulation circuit  12 . When the corresponding pattern comparison circuits  15   a  and  15   b  have detected the specific pattern, the band limiting filters  14   a  and  14   b  change the frequency characteristics by altering their own filter coefficients. 
   Each of the pattern comparison circuits  15   a  and  15   b  previously store a specific pattern of transmission data that causes an excessive peak in the transmission signal. Transmission data output from the spreading modulation circuit  12  are successively input to the pattern comparison circuits  15   a  and  15   b . Each of the pattern comparison circuits  15   a  and  15   b  makes a decision whether the transmission data coincides with the specific pattern. 
   Details of the band limiting filters  14   a  and  14   b  will now be described.  FIG. 2  is a block diagram showing a configuration of each of the band limiting filters  14   a  and  14   b  according to a first embodiment. In  FIG. 2 , latch circuits  21 ,  22 , . . . ,  2   k ,  2   k+ 1   , . . . ,  2   n  are circuits for successively latching the transmission data output from the spreading modulation circuit  12 . The latch circuits may include shift registers. Coefficient units  31 ,  32 , . . . ,  3   ka ,  3   k + 1   a , . . . ,  3   n  include multipliers, respectively. The coefficient units  31 ,  32 , . . . ,  3   ka ,  3   k + 1   a , . . . ,  3   n  multiply outputs of the coefficient units  31 ,  32 , . . . ,  3   ka ,  3   k + 1   a , . . . ,  3   n  by preset coefficients, respectively. The coefficients retained by the coefficient units  31  to  3   n  correspond to coefficients of the FIR filter. In the illustrated coefficient units  3   ka  and  3   k + 1   a , a plurality of coefficients are set. They are switched according to an output of the pattern comparison circuit  15   a . An adder  40  adds up signals output from the coefficient units  31 ,  32 , . . . ,  3   ka ,  3   k + 1   a , . . . ,  3   n , and supplies a result of the addition to the DA converter  16   a  or  16   b  as an output signal of the band limiting filter  14   a  or  14   b.    
   The coefficient units  3   ka  and  3   k + 1   a  each having a plurality of coefficients will now be described.  FIG. 3  is a block diagram showing a configuration of a coefficient unit having two coefficients. An illustrated coefficient unit  3   ja  corresponds to each of the coefficient units  3   ka  and  3   k + 1   a  shown in  FIG. 2 . The coefficient unit  3   ja  includes switches  41  and  42  and coefficient units  43  and  44 . The switches  41  and  42  function to select the coefficient unit  43  (a x ) when the specific pattern is not detected by the corresponding pattern comparison circuits  15   a  and  15   b , and select the other coefficient unit  44  (a y ) when the specific pattern is detected. When selected, each of the coefficient units  43  and  44  multiplies a signal output from a latch circuit  2   j  by its coefficient (a x  or a y ), and outputs a result of the multiplication toward the adder  40 . 
   Each of the band limiting filters  14   a  and  14   b  as heretofore described forms an FIR filter having frequency characteristics determined by the coefficients in the coefficient units  31 ,  32 , . . . ,  3   ka ,  3   k + 1   a , . . . ,  3   n , and changes frequency characteristics according to whether the pattern comparison circuit  15   a  or  15   b  has detected the specific pattern. When the pattern comparison circuit  15   a  or  15   b  has detected the specific pattern, the corresponding band limiting filter  14   a  or  14   b  changes the frequency characteristics so as to decrease the peak value of its output signal. 
   An example of coefficients set in the coefficient units  31 ,  32 , . . . ,  3   ka ,  3   k + 1   a , . . . ,  3   n  is shown in  FIG. 4 . A “Numerical value A” and a “numerical value B” are set in each of illustrated coefficients a 1  to a 30 . In “a 17 ” and “a 18 ” among the illustrated coefficients, different values are set as the “numerical value A” and the “numerical value B.” These coefficients “a 17 ” and “a 18 ” correspond to the coefficient units  3   ka  and  3   k + 1   a  which switch a plurality of coefficients according to the detection result of the pattern comparison circuits  15   a  and  15   b . The coefficient units  31  to  3   n  having coefficients set as shown in  FIG. 4  use the “numerical value A” when the specific pattern is not detected by the corresponding pattern comparison circuit  15   a  or  15   b , and use the “numerical value B” when the specific pattern is detected by the corresponding pattern comparison circuit  15   a  or  15   b . In this way, each of the band limiting filter  14   a  and  14   b  changes frequency characteristics by changing the values of the coefficients “a 17 ” and “a 18 ” on the basis of the detection result of the specific pattern. 
   The pattern comparison circuits  15   a  and  15   b  will now be described.  FIG. 5  is a block diagram showing a configuration of a pattern comparison circuit according to an embodiment of the present invention. In  FIG. 5 , each of the pattern comparison circuits  15   a  and  15   b  includes latch circuits  51 ,  52 , . . . ,  5   m − 1 ,  5   m , exclusive OR circuits  61 ,  62 , . . . ,  6   m− 1   ,  6   m , a storage circuit  70 , a logical product circuit  80 , and a timing adjustment circuit  81 . The latch circuits  51 ,  52 , . . . ,  5   m  − 1 ,  5   m  are circuits for successively latching transmission data output from the spreading modulation circuit  12 . The latch circuits  51 ,  52 , . . . ,  5   m  − 1 ,  5   m  may be formed of shift registers. The storage circuit  70  is a circuit for storing the specific pattern. The storage circuit  70  stores data P 1 , P 2 , . . . , Pm− 1 , Pm that constitute the specific pattern. The storage circuit  70  includes ROMs or RAMs. A sequence that generates a predetermined peak is previously calculated by using a theoretical value or simulation, and it is stored as the illustrated data P 1  to Pm. In  FIG. 5 , an example in which one specific pattern is stored in the storage circuit  70  is shown. However, a plurality of specific patterns may also be stored. If a plurality of specific patterns are stored, each of the pattern comparison circuits  15   a  and  15   b  compares each of the specific patterns with the transmission data. 
   The exclusive OR circuits  61 ,  62 , . . . ,  6   m  − 1 ,  6   m  perform exclusive OR-ing function on the output data of the latch circuits  51 ,  52 , . . . ,  5   m  − 1 ,  5   m  and the data P 1 , P 2 , . . . , Pm− 1 , Pm of the specific pattern, respectively. In other words, the exclusive OR circuits  61 ,  62 , . . . ,  6   m  − 1 ,  6   m  makes a decision whether they coincide with each other. The logical product circuit  80  obtains a logical product of outputs of the exclusive OR circuits  61  to  6   m . On the basis of a result of the logical product, it is found whether the output data of the latch circuits  51  to  5   m  coincide with the data P 1  to Pm of the specific pattern, i.e., whether the specific pattern is detected. The timing adjustment circuit  81  provides an output signal of the logical product circuit  80  with suitable timing by using delay processing, and outputs the output signal to the band limiting filter ( 14   a  or  14   b ). As for the latch circuits  51 ,  52 , . . . ,  5   m  − 1 ,  5   m  shown in  FIG. 5 , a part of the latch circuits  21 ,  22 , . . . ,  2   k ,  2   k + 1 , . . . ,  2   n  in the band limiting filter  14   a  or  14   b  shown in  FIG. 2  can also be used. 
   As heretofore described, each of the pattern comparison circuits  15   a  and  15   b  detects whether the transmission data output from the spreading modulation circuit  12  coincides with the specific pattern. Upon receiving the result of the detection, the band limiting filter  14   a  or  14   b  changes the frequency characteristics so as to reduce the predetermined peak value in the transmission signal when the specific pattern is included in the transmission signal. 
   An example of a waveform output from the band limiting filter  14   a  or  14   b  will now be described.  FIG. 6  is a diagram showing an example of an output waveform of the DA converter  16   a  or  16   b . By the way,  FIG. 6  shows a waveform example of not a multi-code output but a single code output for the purpose of description. It is appreciated that in the waveform shown in  FIG. 6  peaks such as the points S and T in the conventional output waveform shown in  FIG. 10  are reduced and the waveform falls in a desired range R (index) in the illustrated period of time. 
   Second Embodiment 
     FIG. 7  is a block diagram showing a configuration of each of the band limiting filters  14   a  and  14   b  according to a second embodiment of the present invention. Each of the band limiting filters  14   a  and  14   b  shown in  FIG. 7  has nearly the same configuration as that of the band limiting filter shown in  FIG. 2 . However, each of the band limiting filter  14   a  and  14   b  differs in that each of coefficient units  31   a ,  32   a , . . . ,  3   ka ,  3   k + 1   a , . . . ,  3   na  has two coefficients and the two coefficients are switched according to the output of the pattern comparison circuit  15   a  or  15   b . Thus, in the band limiting filters  14   a  and  14   b  of the present embodiment, all of the coefficient units  31   a  to  3   na  are provided with coefficient choices. Therefore, the degree of freedom at the time when adjusting the amplitude of the output signal can be increased. 
   By the way, the transmission apparatus of the embodiments can be included in portable communication terminals such as cellular phones.
There is a problem that a delay is caused in signal processing by exercising control so as to cause amplitude peaks in a transmission signal to fall in a prescribed range. A transmission apparatus for conducting wireless communication makes a decision whether a specific pattern which causes a peak exceeding a predetermined amplitude range to be generated in frequency characteristics of a transmission signal is included in a transmission code sequence for forming the transmission signal. And the transmission apparatus selects filter coefficients which prescribe a band limiting factor for the frequency characteristics of the transmission signal, on the basis of a result of the decision, and conducts filtering on the transmission code sequence by using the selected filter coefficients.