Patent Publication Number: US-2009220018-A1

Title: Transmitter and method for digital multi-carrier transmission

Description:
This is a continuation application of application Ser. No. 10/983,010 filed Jul. 2, 2004, which is based on JP 2003-190953 filed Jul. 3, 2003, the entire contents of each of which are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a transmitter and transmission method employing a multi-carrier transmission technique, particularly a digital wavelet multi-carrier (DWMC) transmission technique utilizing real coefficient wavelet filter banks. 
     2. Description of the Related Art 
     Orthogonal frequency division multiplexing (OFDM) is frequently adopted as a conventional multi-carrier transmission method, for example, as described in U.S. Pat. No. 6,442,129. In the OFDM transmission method, discrete Fourier transform (DFT), particularly fast Fourier transform (FFT), is adopted as a modulation/demodulation method. In addition, in FFT-based OFDM, generally speaking, ramp processing is adopted, which makes the time waveform on a leading edge of a frame (preamble) smooth in order to prevent the waveform from distorting in a transmission channel or in hardware such as an amplifier. 
     Recently, wavelet-based OFDM has been proposed to replace FFT-based OFDM as a modulation/demodulation method in OFDM transmission because FET-based OFDM has basic weaknesses such as poor resistance to narrow band interference, poor resistance to internal interference, and low transmission efficiency because of the necessity of a cyclic prefix. When ramp processing is performed in wavelet-based OFDM, the length of the preamble data in the wavelet-based OFDM is longer by at least (2k−1) symbols (k is an overlapping factor) than the length of the preamble data in the FFT-based OFDM if the wavelet waveform, as it is, is used as data of the preamble. The greater the length of the preamble data, the more the redundancy of the data increases. Accordingly, the length of the preamble data is required to be as short as possible. While auto gain control (AGC) is performed in a receiver by using a wavelet waveform without ramp processing, convergence speed of the AGC becomes an issue because of the complexity of the wavelet waveform. 
     SUMMARY OF THE INVENTION 
     The present invention is made in view of the above-mentioned problems. An object of the present invention is to provide a transmitter and transmission method in the DWMC data transmitting method, which enables shortening of the length of the preamble data and improves the convergence speed of the AGO. 
     According to the invention, a preamble data generator generates preamble bit data, and outputs the preamble data. Next, a modulator modulates the preamble data, generates a plurality of subcarriers, and outputs a composite wave of the time waves of the plurality of subcarriers. Subsequently, a ramp processor performs ramp processing on the composite wave with a certain delay period from a reference position of the composite wave. 
     In this way, the invention provides a transmitter and transmitting method which enables shortening of the length of the preamble data and improves the convergence speed of the AGC. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a transmitter according to a first embodiment of the invention; 
         FIG. 2  is a waveform diagram of preamble data according to a first embodiment of the invention; 
         FIG. 3  is a diagram showing an example of ramp processing according to a first embodiment of the invention; 
         FIG. 4  is a spectrum diagram showing a relationship between subcarrier numbers and frequencies of sine waves; 
         FIG. 5  is a waveform diagram of preamble data according to a second embodiment of the invention; 
         FIG. 6  is a block diagram of an inverse wavelet transformer in a transmitter according to a third embodiment of the Invention; 
         FIG. 7  is a block diagram of a prototype filter of an inverse wavelet transformer in a transmitter according to the third embodiment of the invention; 
         FIG. 8  is a block diagram of another prototype filter of an inverse wavelet transformer in a transmitter according to the third embodiment of the invention; 
         FIG. 9  is a schematic diagram showing a relationship among symbol data of preamble data, time waveform of preamble data, and ramp processing waveform; 
         FIG. 10  is a waveform diagram showing a wavelet waveform; 
         FIG. 11  is a waveform diagram showing an example of a transmitted waveform according to the DWMC transmission method; 
         FIG. 12  is a spectrum diagram showing an example of a transmitted spectrum according to the DWMC transmission method; 
         FIG. 13  is a schematic frame diagram showing an example of a configuration of a transmitted frame according to the DWMC transmission method; 
         FIG. 14  is a block diagram of another transmitter according to the first embodiment of the invention; and 
         FIG. 15  is a block diagram of a power line communication system. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the invention will be described with reference to  FIGS. 1 through 15 . 
     First Embodiment 
     A first embodiment of the invention generates a digital wavelet multi-carrier (DWMC) transmission signal from a plurality of digitally modulated waves that are received from real-coefficient filter banks. Low bit rate modulation, such as quadrature phase shift keying (QPSK), quadrature amplitude modulation (QAM) or pulse amplitude modulation (PAM), may be used for modulating each carrier. 
     A data transmission method according to the DWMC transmission method will be described with reference to FIGS.  4  and  10 - 12 . 
       FIG. 10  illustrates a waveform of a wavelet, and  FIG. 11  illustrates a DWMC transmission waveform according to the invention. As shown in  FIG. 10 , each waveform  1001  of the wavelet has an impulse response, and impulse responses of each of the plurality of waveforms  1001  are transmitted in an overlapping relationship with each other. As shown in  FIG. 11 , each transmission symbol  1101  is formed by a time waveform  1102  that is a combination of impulse responses for a plurality of subcarriers. 
     In  FIG. 12 , a transmission frame is formed, for example, by several tens to several hundreds of transmission symbols according to the DWMC transmission method. This transmission frame includes a symbol for a transmission of an information data and a preamble data such as a symbol for frame synchronization and a symbol for an equalization. The DWMC transmission signal  1200  includes a plurality of subcarrier signals  1201 . 
       FIG. 13  illustrates a configuration of a DWMC transmission frame according to the invention. The DWMC transmission frame  1300  comprises one or more preamble symbols  1301  that are followed by one or more information symbols  1302 . The preamble may be used by a receiver for frame synchronization or equalization. 
     Next, a transmitter  2000  that is preferably for use in the DWMC transmission method will be described with reference to  FIGS. 1 to 3 . 
       FIG. 1  illustrates a block diagram of a transmitter according to a first embodiment of the invention. A transmitter  2000  includes a preamble data generator  10 , a symbol mapper  12 , a serial to parallel (S/P)transformer  16 , a plurality of complex data decomposers  18 , an inverse wavelet transformer  20 , and a ramp processing circuit  22 , all of which are controlled by a controller (not shown). The operation of transmitter  2000  is described below. 
     Information data may be input to preamble data generator  10 . Preamble data generator  10  generates and outputs preamble data, which is used for carrier detection, synchronization, and/or equalization in a receiver. The preamble data and information data are combined and modulated for representation by symbols. These symbols are produced by: (1) overlapping the preamble and information data and modulating the overlapped data as the actual transmitting data, (2) forming the preamble and information data in a composite configuration and modulating them together, or (3) separately and simultaneously modulating the preamble and information data prior to combining the symbols into a frame. 
     Symbol mapper  12  transforms bit data of the preamble data and information data into symbol data preferably using a low bit rate modulation method, such as QPSK, QAM, or PAM. Then, symbol mapper  12  maps the symbol data into M/2, where M is the number of subcarriers, complex coordinates and serially outputs the mapped data to S/P transformer  16 . 
     S/P transformer  16  transforms the serially received mapped data into parallel data and outputs all but two of the M parallel data streams to complex decomposers  18 . Each complex decomposer  18  decomposes the parallel data it receives into a real part, which is the in-phase component, and an imaginary part, which is the quadrature component. Each complex decomposer  18  outputs to inverse wavelet transformer  20  the in-phase component as (2n−1)th inputted data and the quadrature component as (2n)th inputted data, where 1≦n≦(M/2−1), M is a positive integer, and subcarrier number is 0 to M−1. In total inverse wavelet transformer  20  receives M subcarrier waveforms identified in  FIG. 1  as subcarriers  0  through M−1. In  FIG. 1 , the (2n−1)th and (2n)th data inputted to inverse wavelet transformer  20  correspond respectively to subcarriers  1  and  2  for n=1, subcarriers  3  and  4  for n=2, and so on. Both ends of subcarriers, namely subcarriers  0  and M−1 are not used, because these contain much direct current components. Even if used, the end subcarrier is not in orthogonal relationship with the next subcarrier. 
     Inverse wavelet transformer  20  has M real-coefficient wavelet filters that are orthogonal with respect to each other. Using these wavelet filters, inverse wavelet transformer  20  performs an inverse wavelet transform on both the real and imaginary components it receives. Ramp processing circuit  22  receives the data generated by inverse wavelet transform  20  and ramp processes this data with a delay, which may be equivalent to fraction of a symbol period, one symbol period, or several symbol periods. The ramp processing is accomplished by multiplying data representing a ramp waveform, such as shown in  FIG. 3 , by the inverse wavelet transformed data, as explained below. Thereafter, the ramp processed data is output by ramp processing circuit  22 . 
     With reference to  FIGS. 2 and 3 , the ramp processing of the inverse wavelet transform data produced by inverse wavelet transformer  20  will be explained. Particularly,  FIG. 2  illustrates a waveform of inverse wavelet-transformed Preamble data according to the first embodiment of the invention and  FIG. 3  illustrates a ramp processing signal according to the first embodiment of the invention. 
     In general, since a wavelet time-waveform  201  of the wavelet transformed data localizes and is longer than one symbol length, as shown in  FIG. 2 , waveform  20 ′ has a moderate initial amplitude rise. Therefore, in many cases, preamble slot  1  includes few effective data for preamble functions. 
     Ramp processing circuit  22  multiplies inverse wavelet-transform waveform  201 , produced by inverse wavelet transformer  20 , with ramp waveform  301  to produce a ramp processed product waveform  202 . Accordingly, the ramp processing is performed on the wavelet transformed data, which is a composite wave, with a predetermined delay from a reference position of the composite wave as shown in  FIG. 2 . Waveform  202  of the ramp processed data has zero data for the interval of preamble slot  1 . Accordingly, waveform  202  of the ramp processed data substantially has one symbol offset from the rising edge of time waveform  201  of the inverse wavelet transformed data, as shown in  FIG. 2 . 
     As illustrated in  FIG. 3 , ramp processing waveform  301  has a linearly and monotonically increasing value in preamble slot  2  that tends to smooth the amplitude product of this waveform and the waveform produced by inverse wavelet transformer  20 . The zero value of waveform  301  in preamble slot  1  effectively eliminates the first symbol of inverse wavelet-transform waveform  201  when waveforms  201  and  301  are multiplied to produce ramp processed waveform  202 . Also, the unity value of waveform  301  in preamble slots  3 - 6  effectively reproduces the symbol waveforms of waveform  201 , during preamble periods  3 - 6 , when waveforms  201  and  301  are multiplied to produce ramp processed waveform  202 . 
     Accordingly, the structure of transmitter  2000  substantially shortens the length of the preamble data because a head portion of the preamble data is reduced by one symbol, due to the one-symbol period offset of the ramp processing. Moreover, the structure of transmitter  2000  makes it possible to improve the processing speed of an automatic gain control (AGC) in the receiver because the ramp processing smooths the wavelet waveform. 
     The first embodiment has been described based on a transmitter  2000  that includes a symbol mapper  12  that performs QAM and complex decomposers  18 . However, a symbol mapper performing PAM instead of QAM can be also used in the first embodiment, as shown in  FIG. 14 . 
       FIG. 14  illustrates a block diagram of another transmitter according to the first embodiment of the invention. In  FIG. 14 , transmitter  2000  includes a symbol mapper  210  that modulates the preamble data provided by preamble data generator  10  using PAM. Symbol mapper  210  performs almost the same operation as the combined structures of symbol mapper  12  and complex decomposers  19  in  FIG. 1 , by treating the (2n−1)th and (2n)th subcarriers outputted by S/P transformer  16  as the in-phase and quadrature components, respectively, in inverse wavelet transformer  20 . 
     In the above description of the first embodiment, the symbol interval offset is set for one symbol interval from the rising edge of the time waveform. However, the offset is changeable and can be set for several symbol intervals, as necessary. In addition, the period of the ramp processing is set for one symbol interval in the first embodiment. However, the period is changeable and can be set for several symbol intervals as necessary. 
     Furthermore, a curved waveform such as a raised cosine curve can be employed as the ramp waveform instead of the linear waveform illustrated by  FIG. 3 . The curved waveform makes it possible to set the period of the ramp processing to less than one symbol interval because the curved waveform will prevent the preamble data from increasing transmission side lobes when the curved waveform and the preamble data are multiplied together by ramp processing circuit  22 . 
     Second Embodiment 
     A transmitter of the second embodiment basically has the same configuration as the transmitter of the first embodiment. However, the ramp processing is different from that employed in the transmitter of the first embodiment. This difference will be described in detail with reference to  FIGS. 1 ,  3 - 5 , and  16 . 
     In the present embodiment, preamble data generator  10  normally outputs serial data having values of “O” until instructed by the controller to output preamble data. When the instruction is received to output preamble data, preamble data generator  10  serially generates a value, such as “1,” over several symbol periods so that each subcarrier produced by S/P transformer  16  contains a series of this value as its preamble data. 
       FIG. 9  illustrates a relationship among symbol data  901  of the Preamble data, a time waveform  902  of the preamble data, and a ramp processing waveform  903  used for generating a DWMC transmission signal according to the Invention. In  FIG. 9 , the same symbol data value of “1” is illustrated as being output by preamble data generator  10  for a period of time beginning with the fourth preamble period so as to provide each subcarrier produced by S/P transformer  16  with a series of 1 values in its preamble data. 
     In the present embodiment, inverse wavelet-transformer  20  includes a wavelet filter  904  having a four-symbol interval length. Accordingly, a time waveform value of the preamble data for preamble slot  1  is generated from the first four preamble symbol data values (0, 0, 0, 1), which are inputted to wavelet filter  904 . Next, a time waveform value of the preamble symbol data for preamble slot  2  is generated from the next group of four preamble symbol data values (0, 0, 1, 1) in the series of symbol data, which is inputted to wavelet filter  904 . According to this waveform generation process, each subsequent group of four data values includes a fourth data value in the sequence of preamble symbol data and the three preceding data values of the preamble symbol data, which were included in the previous group of four data values. By repeatedly producing waveform values in this way, a time waveform  902  of the preamble data can be obtained, as shown in  FIG. 9 . 
       FIG. 4  illustrates the spectrum of a DWMC multi-carrier transmission signal, and the relationship between subcarrier numbers and frequencies of sine waves. For the purpose of simplifying the explanation, it is assumed that there are eight subcarriers in the present embodiment, as shown in  FIG. 4 . The output of the transmitter  2000  is a composite of three sine waves that have frequencies of f 1 , f 2 , and f 3 , respectively, as shown by the solid heavy lines in  FIG. 4 . The three sine waves have phases, φ 1 , φ 2 , and φ 3 , respectively. Each of the phases, φ 1 , φ 2 , and φ 3 , can take any value ranging from −π to π. 
     Referring now to  FIG. 1 , the operation of transmitter  2000  in accordance with the second embodiment of the invention will be described in greater detail. First, symbol mapper  12  transforms a bit value of the preamble data into symbol data by using QAM and maps the symbol data into M/2 complex coordinates. The mapped complex data of “exp(jφn)” can be obtained by the operation of the symbol mapper  12 . Next, S/P transformer  16  transforms the serially inputted mapped complex data into parallel data and outputs M−2 of the parallel complex data streams to complex decomposers  18 . Each complex decomposer  18  decomposes the parallel data it receives into a real part (cos(φn)) and an imaginary part (sin(φ)). Subsequently, each complex decomposer  18  allots “cos(φn)” and “sin(φn)” to the (2n−1)th and (2n)th subcarrier inputs, respectively, provided to inverse wavelet transformer  20 . The output from inverse wavelet transformer  20  is the composite wave of the sine waves of “cos(2πfn·t+φn)”, where “fn” is a frequency of the n-th sine wave and “φn” is a phase of the n-th sine wave. 
       FIG. 5  illustrates a waveform of preamble data according to a second embodiment of the invention. A waveform  501  is produced from preamble data comprising the same data value, such as “1”, output by preamble data generator  10  for a series of sequential symbol periods. The wavelet filter length used to produce waveform  501  is determined from the expression X=2kN, where N is the symbol length and, generally, N=M, when k (overlapping factor) is equal to “2”. In  FIG. 5 , the real composite wave  501  is a proper sine waveform in preamble No. 4 (i.e. it corresponds closely to a true sine wave), the ramp processing is carried out from the preamble No. 4 using the ramp form of  FIG. 3  (i.e., using ramp form  903  of  FIG. 9 ). In other words, waveform  502  illustrates composite waveform  501  after it has been ramp processed by ramp processing circuit  22 . The ramp processing effectively multiplies composite waveform  501  by time waveform  903  of the ramp processing data of  FIG. 9 . 
     The above-described structure of the second embodiment makes it possible to substantially shorten the preamble length, since the frame-head portion of the preamble data can be substantially deleted. It is preferable to substantially delete (X−1) symbol length as the deleted frame-head position. Where, “X” means the filter length determined by the expression X=2kN. Furthermore, the above-mentioned configuration makes it Possible to improve the accuracy of the modulation relative to that of the transmitter described in the first embodiment, since almost the entire range of the preamble data is a composite waveform consisting of proper sine waves. Furthermore, because initial phases mapped on the complex coordinates by the symbol mapper can be voluntarily provided to each of the (2n−1)th and (2n)th subcarriers, the above-mentioned configuration makes it possible to reduce the instantaneous power consumption peak when the phases of the subcarriers are set to eliminate an overlap with each other. 
     Alternative ways of inserting preamble symbols  1301  into the head portion of frame  1300 , illustrated by  FIG. 13 , may be used as well. For example, a time waveform of the preamble data without the frame-head portion may be preliminarily created and stored in a memory (not shown), assuming the same preamble data waveform can always be used. When a need arises, the stored waveform can be inserted into the rising edge of the time waveform of the information data to produce the composite time waveform of the frame. Also, the number of symbols deleted from the preamble data by the ramp processing may be regulated in accordance with the number of “0” values serially occurring in the preamble data before the series of “1” values is begun. 
     A symbol mapper  210  performing PAM instead of QAM can be also used in the present embodiment, like the first embodiment. 
     In addition, as described previously for the present embodiment, preamble data generator  10  generates the preamble data by outputting the same data value (for example “1”) to each subcarrier for a sequence of serial symbol intervals. As the sequence of serial symbol intervals is lengthened, the present embodiment becomes more effective. Alternatively, the period of ramp processing can be set for less than one symbol interval. 
     Third Embodiment 
     A transmitter of the third embodiment has basically the same configuration as the transmitter of the first embodiment. However, the configuration of the inverse wavelet transformer  20  will be described in greater detail here, with reference to  FIGS. 6-8 . 
       FIG. 6  illustrates a block diagram of an inverse wavelet transformer in a transmitter according to a third embodiment of the invention. Inverse wavelet transformer  20  includes a fast discrete cosine transformer (type  4 ) 40 , a prototype filter  42 , M up-samplers  44 , and M−1 delays  46 . Up-samplers  44  multiply the sampling rate of the transmitted waveform by M, and delays  46  delay the transmitted waveform. 
       FIG. 7  illustrates a block diagram of a prototype filter  42  of the inverse wavelet transformer according to the third embodiment of the invention. Prototype filter  42  is a polyphase filter that includes multipliers  62 , which hold prototype filter coefficients, and adders  64 . A general configuration of a polyphase filter is described in “Signal Processing With Lapped Transform” by Henrique S. Malvar. The order of the prototype filter  42  is 2M in the present embodiment. 
     An operation of the transmitter that has an above-mentioned configuration will now be described. The parallel data outputted from S/P transformer  16  are received by fast discrete cosine transformer (DCT)  40  in inverse wavelet transformer  20 . Fast DCT  40  performs a DCT transform on the received data and outputs the DCT trans formed data to prototype filter  42 . Prototype filter  42  filters the DCT transformed data and produces outputs of filtered data. Each up-sampler  44  performs an up-sampling on a respective one of the filtered data outputs and outputs up-sampling data. Finally, the up-sampling data are combined, with the cooperation of delays  46 , to transform the parallel data into serial data and the serial data are outputted as transmitting data. In the present embodiment, the modulation is performed though the cooperation of prototype filter  42  and fast DCT  40 . 
       FIG. 8  illustrates a block diagram of another prototype filter of the inverse wavelet transformer according to the third embodiment of the invention. Prototype filter  80  has basically the same configuration as prototype filter  42  but includes delays  60  for delaying received data by one symbol period. The one symbol period is equal to M sampling periods in the present embodiment. Prototype filter  80  does not process preamble data but, instead, processes information data. Since the configuration of prototype filter  42  has nearly the same configuration as filter  80 , a single device similar to that of prototype filter  80  having switchable bypass circuits around delays  60  may be used to process both the preamble data and information data, thereby reducing the amount of circuitry relative to a transmitter having both prototype filters  42  and  80 . Prototype filter  42  has no delay devices  60 , as does prototype filter  80 . Therefore, prototype filter  42  induces less latency in the outputted data than does prototype filter  80 . Furthermore, while the order of the prototype filter  42  is 2M (this means k=1) in the present embodiment, the time latency can be reduced when the order of the prototype filter  42  is 2kM. 
     Although a fast DCT is used in the inverse wavelet transformer of the third embodiment, the same processing will be achieved when a fast discrete sine transformer (DST) is used instead of the fast DCT. The fast DST and fast DCT have basically the same configuration, though a filter coefficient differs between the two. The first through third embodiments should not be construed as limiting, but rather merely illustrating, the invention. The DWMC wavelet waveform transmitter described herein can be used in many applications where a general digital multi-carrier transmitter is appropriate, such as situations requiring a waveform localized in the time and frequency domains. 
     For each of the first to third embodiments, it is preferable to perform the ramp processing for one symbol interval or more so as to suppress distortion in the wavelet waveform and prevent the increase of side lobes in the amplitude spectrum of the wavelet waveform. 
     While the invention may be applied to a wide variety of communication apparatuses for transmitting and receiving signals, it is especially suitable for power line communication (PLC) systems that may communicate information across a poor transmission path. Deregulation is in progress to allow the use of the band from 2 MHz to 30 MHz for PLC. However, other existing systems (e.g., amateur radios and shortwave broadcasts) use the same band. Since no interference with other existing systems is allowed, ideally, no PLC signals should be transmitted to the portions of the band used by other existing systems. 
     Normally, a notch filter is used to reduce the amplitude of signals communicated in the portions of the band used by existing systems. A notch filter providing 30 dB of attenuation is used in HOMEPLUG 1.0 released by HOMEPLUG, which is an alliance of PLC businesses in the United States. Thus, a possible target for the suppression of interference to other existing systems is 30 dB or more. 
     With a DWMC transmission method, a filter bank is used to limit the band of each subcarrier, so as to suppress subcarrier signals that overlap the portion of the band used by existing systems. Therefore, the DWMC transmission method can achieve a similar attenuation of undesirable signals to that achieved by a conventional notch filter. The deeper the attenuation provided by a filter of DWMC transmitter  2000 , the greater the filter length of each of the M filters of the filter bank and the greater the delay attributable to the filters, since filter delay is a trade-off for the attenuation depth. However it is possible to form attenuation notches of 30 dB or more and suppress the filter delay by limiting the filter length of a PLC filter bank to 4N, using transmitter  2000 . 
       FIG. 15  illustrates a block diagram of a power line communication system according to the invention. As shown in  FIG. 15 , a PLC system in a building  750  includes a power line  801 ; a conventional network  802 , such as a telephone network, an optical network, or a cable television (CATV) network; a communication apparatus  800  including both transmitter  2000 , as described in the first to third embodiments, and a receiver (not shown); an audio visual (AV) apparatus  810 , such as a television set, a video device, a digital video disk (DVD) recorder or player, or a DV camera; a telecommunication apparatus  820 , such as a router, an asymmetric digital subscriber line (ADSL), a very high bit-rate digital subscriber line (VDSL), a media converter, or a telephone; a documentation apparatus  830 , such as a printer, a facsimile, or a scanner; a security apparatus  840 , such as a camera or an interphone; a computer  850 ; and a home electrical apparatus  860 , such as an air conditioner, a refrigerator, a washing machine, or a microwave oven. 
     An operation of the PLC system will now be described. Devices  810 - 860  form a network in cooperation with power line  801  and perform bi-directional communication using communication apparatuses  800 . For Internet communication, a connection may be made to the Internet via a home gateway provided in the building  750  through power line  801 . Alternatively, a connection may be made via telecommunication apparatus  820  to communicate over conventional network  802 . Additionally, a connection may be made on a wireless basis from a telecommunication apparatus  820  having a radio function. Since communication apparatus  800  performs modulation and demodulation processes using filter banks involving M filters, which are orthogonal with respect to each other, the interference with the other existing systems can be suppressed by disabling subcarriers that overlap the band used by the other existing systems. Further, since the filter length can be limited to 4N, delays attributable to the filters can be reduced while achieving an attenuation notch depth of 30 dB or more. Also, the effect of narrow band interferences from the other existing systems can be reduced. 
     Furthermore, when a notch is to be generated in a certain band, transmitter  2000  may effectively accomplish this by disabling any subcarrier that overlaps the band. It is therefore possible to comply with regulations in various countries easily and with flexibility. Even when there is a regulation change after the present system is put in use, the change can be accommodated with flexibility, for example by upgrading the firmware of transmitter  2000 . 
     In addition, the configurations of the first to third embodiments can be combined with each other as needed. 
     Furthermore, an IC (integrated circuit) chip is used as the preamble data generator  10 , the symbol mappers  12  and  112 , the serial to parallel (S/P) transformer  16 , the complex data decomposer  18 , the inverse wavelet transformer  20 , and the ramp processing circuit  22  of the transmitter  2000 . A FPGA (field programmable gate array) or an ASIC (application specific integrated circuit) is preferably used as the IC chip. Furthermore, it may be possible to use a plurality of IC chips for the functional blocks such as the preamble data generator  10 , the symbol mappers  12  and  112 , the serial to parallel (S/P) transformer  16 , the complex data decomposer  18 , the inverse wavelet transformer  20 , and the ramp processing circuit  22  of the transmitter  2000 . 
     This application is based upon Japanese Patent Application NO. 2003-190953 filed on Jul. 3, 2003, the entire technical contents of which are incorporated herein by reference in its entirety.