Patent Publication Number: US-2003223483-A1

Title: DSL modem apparatus and reception method for DSL communication

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a DSL modem apparatus and reception method for DSL communication that are applicable to multi-carrier communication employing a plurality of carriers.  
       [0003] 2. Description of Related Art  
       [0004] The ADSL (Asymmetric Digital Subscriber Line) is a service that employs an existing telephone line so that both a high speed Internet connection service and ordinary telephone service can use the same line, which has rapidly become available in the recent years. For such ADSL services, ITU-T recommendations have been issued regarding ADSL modems. G.994.1 (hereafter referred to as G.hs) established in 1999 as SG15 of the ITU-T is one of the recommendations made for the ADSL standard.  
       [0005] Hereafter, signals (carriers) used for G.hs are illustrated. FIG. 8 illustrates signals used at ANNEX.C, which assumes a co-residence with a TCM-ISDN. When a signal is transmitted from ATU-C (center apparatus, e.g., at an exchange side) to ATU-R (remote apparatus, e.g., at a house), which is referred to as a downstream, carriers #12, #14, and #64 are employed. When a signal is transmitted from ATU-R to ATU-C, which is referred to as an upstream, carriers #7 and #9 are used. “#” signifies a carrier number, a multiplication of which by 4.3125 kHz becomes a real carrier frequency.  
       [0006] In addition, as a modulation method, each carrier carries same data as described below. When data “1” is placed, a topology of each carrier is inverted at 180 degrees at every 8 symbols (8/4312.5 second). When data “0” is placed, topology inversion at every 8 symbols is not performed.  
       [0007]FIG. 9 is a functional block diagram illustrating transmission and reception sides of an ADSL modem. Protocol controller  501  prepares a message to be sent in accordance with G.hs regulated protocol, converting data into a bit string of having “0s” and “1s” illustrating the message. Constellation encoder  502  calculates time for every 8 symbols, and constellation data is provided to IFFY unit  503  at the time interval of 8 symbols. For example, when transmitting “0”, constellation data same as the previous 8 symbols is provided to IFFT unit  503 . When transmitting “1”, however, constellation data with a topology inverted at 180 degrees from the previous 8 symbols are provided to IFFT unit  503 . FIGS.  10  ( a ) and ( b ) illustrate constellation data for transmitting “1”, whereas FIGS.  11  ( a ) and ( b ) illustrate constellation data for transmitting “0”.  
       [0008] The constellation data is modulated by IFFT unit  503 , and the modulated transmission data is transmitted to the phone line after a DA conversion by AFE (analog front end)  4 .  
       [0009] At the reception side, AFE  504  converts the received analog signal from the telephone line into sample data, and FFT unit  505  perform a fast Fourier transform per symbol unit on the sample data for demodulation. When FFT unit  505  outputs the data, AGC controller  506  calculates the gain control amount and gives an instruction to AFE  504  for the gain control amount.  
       [0010]FIG. 12 illustrates constellation data obtained after the AGC (automatic gain control). Clear dots within FIG. 12 are the reception points. Since operation can be normally simplified when a reception point is adjusted to be on an axis of the constellation coordinates (complex plane), a CAPC (carrier automation phase control) is performed in order to adjust the degree of the reception point so that the point is on an axis of the constellation coordinates (complex plane).  
       [0011] On X-axis of the constellation data after the CAPC, when signals mixed with “0” and “1” are received, detection signals are obtained as illustrated in FIG. 13. When a down edge is found from the detection signals, breakpoints for receiving G.hs reception is set per every 8 symbols. Data can be retrieved by determining whether the detection value is “positive” or “negative” at a point shifted 4 symbols to th e right from the breakpoint. By giving “0” for the same sign as the previous sign and “1” for the opposite of the previous sign, data retriever  9  retrieves data in sequence.  
       [0012] Upon analyzing the detection signal and when the down edge location of the detection signal is different from the location expected from the previous value, it is necessary to update the 8-symbol breakpoint. Timing regenerator 8 monitors whether the down edge location is different from the location expected from the previous value, and readjusts the breakpoint by finding a new down edge from the detection signals, when necessary.  
       [0013] The above-described CAPC control and timing regeneration are continued in order to provide a stable demodulation operation.  
       [0014] However, when the ATU-C and ATU-R are far apart, there is a tendency that a stable demodulation operation is prevented because of an adverse effect on the reception due to a hybrid echo of the signal transmitted from the transmitting apparatus.  
       [0015]FIG. 14 illustrates a state where the ATU-R transmits carriers #7 and #9 on the uplink. As shown in FIG. 14, at the instant when data “1” that inverts the topology at 180 degrees is transmitted, spectrums of the signals transmitted on the uplink are diffused in wide frequency ranges. When the distance between the ATU-C and ATU-R are close (e.g., less than 3 km), it is possible to retrieve the carriers on the downlink since the signal levels of the carriers #12, #14, and #64 on the downlink are much higher than the diffusion from the uplink.  
       [0016] However, when the distance between the ATU-C and ATU-R is long, the signal levels of carriers #12, #14, and #64 on the downlink are largely attenuated, thereby preventing the detections of carriers #12, #14, and #64, since their signal levels are buried with the diffusion from the uplink. Because of this problem, it is difficult for the ATU-R to receive signals transmitted from the ATU-C.  
       SUMMARY OF THE INVENTION  
       [0017] The present invention addresses the above-described problem. The purpose of the invention is to provide a highly reliable DSL modem apparatus and reception method for DSL communication that can secure a stable demodulation operation even where the ATU-C and ATU-R are far apart.  
       [0018] This invention prevents the adverse effect on the signal reception caused by the hybrid echo of the signal transmitted from the transmitting apparatus, and provides a stable demodulation operation. This is performed by freezing certain demodulation controls relating to AGC, CAPC, timing regeneration, data retrieval, etc., only for a predetermined period when the hybrid echo of the signal transmitted to a line from the transmitting apparatus has an adverse effect on the reception signals. The hybrid echo means the wraparound of the signal transmitted to a line from the transmitting apparatus. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0019] The present invention is further described in the detailed description which follows, with reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein:  
     [0020]FIG. 1 illustrates a configuration of a communication system according to an embodiment of the present invention;  
     [0021]FIG. 2 is a functional block diagram of a transceiver illustrated in FIG. 1;  
     [0022]FIG. 3 is a functional block diagram of a processor illustrated in FIG. 2;  
     [0023]FIG. 4 is a functional block diagram of AFE and AGC controllers illustrated in FIG. 3;  
     [0024]FIG. 5 illustrates an integration filter illustrated in FIG. 4;  
     [0025]FIG. 6 is a waveform diagram of a reception signal affected by a topology inversion transmission;  
     [0026]FIG. 7 is a flowchart relating to a freeze process according to the embodiment of the present invention;  
     [0027]FIG. 8 illustrates a signal employed by G.hs;  
     [0028]FIG. 9 is a functional block diagram of parts related to G.hs in a conventional ADSL modem;  
     [0029]FIG. 10 ( a ) illustrates a constellation before 8 symbols when data “1” is transmitted;  
     [0030]FIG. 10 ( b ) illustrates a constellation after 8 symbols when data “1” is transmitted;  
     [0031]FIG. 11 ( a ) illustrates a constellation before 8 symbols when data “0” is transmitted;  
     [0032]FIG. 11 ( b ) illustrates a constellation after 8 symbols when data “0” is transmitted;  
     [0033]FIG. 12 illustrates a principle of a CAPC process;  
     [0034]FIG. 13 is a reception waveform diagram when datum “0” and “1” are mixed;  
     [0035]FIG. 14 illustrates frequency characteristics of uplink and downlink ADSL communication at a short distance;  
     [0036]FIG. 15 illustrates frequency characteristics of uplink and downlink ADSL communication at a long distance. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     [0037] The embodiments of the present invention are explained in the following, in reference to the above-described drawings.  
     [0038]FIG. 1 illustrates a diagram of a communication system at the ATU-R side according to the present invention. In the communication system as illustrated in FIG. 1, a public phone line or a similar phone line (hereafter referred to as line) is connected to ADSL communication apparatus  2  via splitter  1 . Further, communication terminal  3  is connected to ADSL communication apparatus  2 . When communication terminal  3  and telephone  4  share one line, splitter  1  is necessary. However, when telephone  4  is not used, splitter  1  is not needed. It is also possible to have a configuration where communication terminal  3  internally installs ADSL communication apparatus  2 .  
     [0039] ADSL communication apparatus  2  includes transceiver  11  that executes a handshake step in accordance with G.hs and various controls in accordance with the ADSL standards, and host  12  that controls entire operations including the one of transceiver  11 . At the line side of transceiver  11 , driver  15  is connected to a DA converter of AFE  13  via analog filter  14 , so that analog signal amplified by driver  15  is transmitted to the line via hybrid  16 . The analog signal transmitted from the line is received by receiver  17  via hybrid  16 , and then input into an AD converter of AFE  13  via analog filter  18 . When sampling data is output from the AD converter, AFE  13  outputs the data to transceiver  11 .  
     [0040]FIG. 2 is a functional block diagram illustrating transceiver  11 . Processor  20  has a function to execute the handshake step and initialization step prior to initiating data communication (SHOWTIME). Processor  20  also executes a process where various processes relating to a later-described demodulation (e.g., AGC, CAPC, timing regeneration, data retrieval, and other processes) are frozen in accordance with a later-described algorithm, during a handshake step. In the present embodiment, AGC, CAPC, timing regeneration, data retrieval, and other processes are employed for the illustration of freezing operation for the demodulation. The all of the above-mentioned processes can be frozen, or specific processes having especially large effects can be selected for the freezing operation.  
     [0041] The transmission side of transceiver  11  includes Reed-Solomon encoder  21  that adds a redundancy bit for checking error, interleave unit  22  that sorts data to enable a burst error correction during Reed-Solomon decoding, Trellis encoder  23  that performs data convolution from a Trellis encoding, tone ordering unit  24  that lays out a bit number for each carrier, constellation encoder  25  that converts transmission data into constellation coordinates (topology), and IFFT unit  26  that performs an Inverse Fast Fourier Transform (hereafter referred to as IFFT) on data after the constellation encoding process.  
     [0042] The reception process side of transceiver  11  includes FFT unit  27  that performs a Fast Fourier Transform (hereafter referred to as FFT) on sampling data of the received signal, constellation decoder/FEQ unit  28  that decodes data from constellation data of the FFT output signal and corrects a topology on the constellation coordinates, tone deordering unit  29  that restores data laid out to each carrier after tone ordering process at the transmission side, Viterbi decoder  30  that performs Viterbi decoding on the received data, de-interleave unit  31  that restores data being resorted by the transmission side, and Reed-Solomon decoder  32  that deletes the redundancy bit added by the transmission side. Transceiver  11  is connected to host  12  via host interface (I/F)  34 .  
     [0043]FIG. 3 is a functional block diagram of processor  20  at both transmission and reception sides, especially relating to functions to be frozen during the handshake step. Protocol controller  201  prepares a message to be sent in accordance with G.hs regulated protocol, converting data into a bit string of having “0s” and “1s” illustrating the message. Constellation encoder  202  calculates a time interval between every 8 symbols, and constellation data is provided to IFFT unit  26  at the time interval. For example, when transmitting “0”, constellation data same as the previous 8 symbols is provided to IFFF unit  26 . When transmitting “1”, however, constellation data with a topology inverted at 180 degrees from the previous 8 symbols is provided to IFFT unit  26 . Freeze processor  200  counts transmitted symbols from the beginning of the transmitted message or of the regenerated timing. When the counter reaches N, which is the timing to transmit data “1”, freeze controller sends a freeze notification to AGC controller  203 , CAPC unit  204 , timing regenerator  205 , and data retriever  206 .  
     [0044] When sample data is output from AFE  13 , the data is demodulated by FFT unit  27  that performs a fast Fourier transform per symbol unit. After the FFT output, AGC controller  203  calculates a gain control amount and gives the amount to AFE  13 . There are two situations where AFE  13  performs a gain control on the transmitted analog signal at AFE  13 , and on the received analog signal. By analyzing the FFT output, CAPC unit  204  adjusts the angle of the reception points so that they will be positioned on an axis of constellation coordinates. When the down edge location of the detection signal is different from the location expected from the previous value, timing regenerator  205  updates the 8 symbol breakpoint. Therefore, timing regenerator  205  monitors whether the down edge location of the detection signals is different from the location expected from the previous value, and readjusts the breakpoint by finding a new down edge from the detection signals, when necessary. Based on the reception breakpoint established by timing regenerator  205 , data retriever  206  determines whether the detection value is “positive” or “negative” at a point shifted 4 symbols to the right from the breakpoint, in order to retrieve data. By giving “0” for the same sign as the previous sign and “1” for the opposite sign as the previous sign, data retriever  206  retrieves data in sequence.  
     [0045]FIG. 4 illustrates configurations of AFE  13  and AGC controller  203 . AFE  13  includes gain controller  101  that performs a gain control on a reception analog signal received from the line or a transmission analog signal output to the line, AD converter  102   a  that performs a sampling by synchronizing the received analog signal with a sampling clock, and DA converter  102   b  that converts the digital transmission analog signal into an analog signal. AGC controller  203  includes buffer  106  that stores the FFT output, maximum value retriever  107  that retrieves a maximum value from the FFT output (stored by buffer  106 ), integration filter  108  that performs a predetermined integral calculation on the maximum value retrieve d by maximum value retriever  107 , and gain control amount determiner  109  that determines the gain control amount by gain controller  101 , from the output of integration filter  108 .  
     [0046]FIG. 5 is a block diagram illustrating a configuration of integration filter  108 . Integration filter  108  multiplies the maximum carrier energy amount (retrieved by maximum value retriever  107 ) by 0.1 using multiplier  301 , which is referred to as value A, and outputs value A to adder  302 . Then, integration filer  108  multiplies value B (stored in inner register  303 ) by 0.9 using multiplier  304 , which is referred to as value B′, and outputs value B′ to adder  302 . Then, integration filter  108  uses adder  302  to add value A (input from multiplier  301 ) and value B′ (input from multiplier  304 ), which is referred to as value B, and outputs value B to inner register  303 . This value B is stored within inner register  303 .  
     [0047] Since the above described predetermined integral calculation is performed on the maximum carrier energy amount that is retrieved by maximum retriever  107 , even when the maximum carrier energy amount is suddenly decreased afterwards, it is possible to prevent a situation where value B is largely affected because of the lowered energy amount.  
     [0048] Hereafter, an illustration is given for a process that eliminates the effect on the downlink after the topology inversion transmission that inverts the topology at 180 degrees on the uplink. In this illustration the ATU-R is used; however, it is possible to achieve the same result at the ATU-C.  
     [0049] The 180 degree topology inversion for transmitting data “1” to uplink according to G.hs is performed at a maximum rate of 1 symbol per 8 symbols. Therefore, the downlink is affected by the topology inversion transmission at the maximum rate of once every 8 symbols. In addition, the timing is also limited to the number of symbols after the topology inversion transmission.  
     [0050]FIG. 6 illustrates a waveform of the plotted X-axis of the constellation after the CAPC process. As illustrated in the upward arrows in FIG. 6, hybrid echo interferences can be seen at points shifted about 4 symbols from each edge of a wave.  
     [0051] In the example shown in FIG. 6, the hybrid echo interference on the downlink appears at timing close to the 4 th  symbol after the ATU-R (transmitting terminal) performs the topology inversion transmission on the uplink. Since it depends on a system at which symbol the hybrid echo interference would appear after the topology inversion transmission, this embodiment uses N as the symbol. In real situation, it is desirable to fix constant N at the time of the product development/experiment. Additionally, the number of symbols to be frozen can be arbitrarily set as long as it does not have a substantial effect for the real demodulation.  
     [0052] Therefore, in the present embodiment, when the topology inversion transmission is performed, AGC, CAPC, and timing regeneration are frozen after N symbols are counted. When there is a need to retrieve data at this time, data at 1 previous symbol is given. This way, it is possible to eliminate the wraparound effect during the demodulation.  
     [0053] The following provides a specific illustration of the operation relating to the transmission and reception processes of a flowchart of FIG. 7, according to the embodiment.  
     [0054] As described above, protocol controller  210  prepares a transmitting message according to the protocol set by G.hs, and transmits the message to constellation encoder  202  by converting the transmission message into a bit string having “0s” and “1s”. Constellation encoder  202  counts time Tm for 8 symbols and provides IFFT unit  26  with the constellation data every Tm. The topology of the constellation data is generated according to “0” or “1” contained within the transmission message. When transmitting data “1”, the topology transmitted before the previous 8 symbols is inverted at 180 degrees.  
     [0055] Freeze processor  200  increments the number count of transmission symbols every time constellation encoder  202  provides constellation data to IFFT unit  26  (step T 1 ). Then, constellation data  202  performs the topology inversion transmission at a rate of 1 symbol per 8 symbols. By recognizing the symbol number at the topology inversion transmission, it is checked whether the transmission symbol number reaches N (step T 2 ). Before the transmission symbol number reaches N, the freeze notification process to AGC controller  203  or the like is not performed (step T 3 ), and symbol transmission process is performed (step T 4 ). When 1 symbol is transmitted, the counter of the transmission symbol number is incremented (step T 5 ), and the control moves to the next symbol transmission process.  
     [0056] When it is determined that the transmission symbol number reaches N at step T 2 , freeze processor  200  sends a freeze notification to AGC controller  203  or the like.  
     [0057] The reception process is also executed during the transmission process. The reception process constantly monitors whether the transmission process has issued a freeze notification (step R 1 ). When there is no freeze notification, AGC controller  203  performs a later-described gain control (step R 2 ). Further, CAPC unit  204  performs a CAPC process on the constellation data (step R 3 ). Timing regenerator  205  also monitors the need for timing regeneration. When there is a timing deviation greater than a predetermined value, a new breakpoint (timing) is reestablished in order to group 8 symbols (step R 4 ).  
     [0058] The following provides an illustration of the AGC as an example of the demodulation related process. AGC controller  203  performs a gain control by utilizing the frequency characteristics of REVERB signals that are transmitted in accordance with ITU-T recommended G.992.1 (G.DMT) or G.992.2 (G.lite).  
     [0059] G.992.1 sets an initialization sequence in which the ATU-C and ATU-R exchanges REVERB signals (C, R-REVERB 1 - 3 ) three times.  
     [0060] Upon transmitting the third REVERB signal (C-REVERB 3 ), ATU-C transmits a SEGUE signal (C-SEGUE 1 ) indicating that subsequent data follows. Then, ATU-C transmits C-RATES 1  that sets the transmission speed and C-MSG 1  that sets additive information such as noise margin. Further, the ATU-C transits a C-MEDLEY that sets a transmission speed and bit number of data to be placed with each carrier (multi-carrier).  
     [0061] Since an important control signal transmission follows immediately after the third REVERB signal, it is preferable to complete the gain control before the third REVERB signal. In this embodiment, the gain control is performed when the first REVERB signal (C-REVERB 1 ) is received, so as to perform the gain control to REVERB signals that appear after C-REVERB 1  (first exchanged REVERB signal).  
     [0062] A REVERB signal has frequency characteristics of a plurality of carriers having the same energy amount signals in a frequency sequence of 4.3125 kHz up to 1,104 kHz. However, even when all carriers are transmitted with the gain control in order to have a constant energy amount at the transmission side, each carrier energy amount received at the reception side can become attenuated because of factors such as line conditions.  
     [0063] In the present embodiment, the above-described frequency characteristics of the REVERB signal are employed to appropriately perform a gain control for communication using a multi-carrier method.  
     [0064] In other words, when REVERB signals are being received at the ATU-R, an FFT output of the received REVERB signals is stored in buffer  106 . In particular, when FFT unit  27  performs an FFT on 1 symbol data, signal values of all carriers can be obtained as a form of constellation for each carrier (as coordinate values in complex plane coordinates) in one operation. Accordingly, carrier signal value (energy value) for each symbol is shown as 1 coordinate point on the R-I (Real-Imaginary) plane, and such (R, I) coordinates corresponding to each carrier are stored in buffer  106 .  
     [0065] When (R, I) coordinate information corresponding to the sampling data for each symbol is stored in buffer  106 , maximum value retriever  107  retrieves a carrier energy amount having the maximum value, based on the (R, I) coordinates, among energy amounts for multiple carriers within a REVERB signal. The distance from the origin point to (R, I) coordinates for each sampling data on the R-I planes are equivalent to the energy amount for each carrier. Therefore, by comparing the distance from the origin point to (R, I) coordinates for each sampling data, maximum value retriever  107  can retrieve the carrier energy amount having the maximum value.  
     [0066] Integration filter  108  performs a predetermined integration calculation on the carrier energy amount of the maximum value, which is retrieved by maximum value retriever  107 . Gain control indicator  109  compares value B (obtained from the integral calculation) with the target range. When value B is greater than the upper limit value of the target range, a gain control is instructed for gain controller  101  to decrease the energy amount of the received analog signal. For example, all carrier energy amounts from the future input signals are decreased by 1 db. Conversely, when value B is smaller than the lower limit value of the target range, gain control is instructed for gain controller  101  to raise the energy amount of the receiving analog signals (e.g., raising all carrier energy amounts from the future input signals by 1 db). In addition, when value B is within the target range, no gain control is instructed.  
     [0067] With the above-described gain control, it is possible to perform a gain control according to the signal degradation, which is cause by line conditions or the like, and securely avoid a situation where some carriers overflow. Therefore, even when multi-carriers are used for communication, it is possible to appropriately perform a gain control so as to prevent an overflow.  
     [0068] As described above, steps R 2 ,  3 , and  4  are performed upon every symbol reception during the reception process. By counting reception symbols (step R 5 ), whether it is time for a data retrieval is determined (step R 6 ). In this example, a data retrieval timing is set at every 4 th  reception symbol from the breakpoint of 8 reception symbols. When it is determined not to be the timing for a data retrieval at step R 6 , it is confirmed that the transmission process has not completed (step R 7 ). Then, the control returns to step R 1 .  
     [0069] When a freeze notification is detected at step R 1 , processes for the AGC, CAPC, and timing regeneration are stopped. In particular, steps R 2 - 4  are skipped and the control moves from step R 1  to R 5  to increment the count number for the reception symbols. When it is the timing for a data retrieval (step R 6 ), the control moves to step R 8 . When it is not the timing for a data retrieval, the control moves to step R 7 .  
     [0070] Accordingly, although the arrowed effects of FIG. 6 appear at the N th  symbol after performing a topology inversion transmission, the AGC, CAPC, and timing regeneration processes are stopped by the freeze notification, thereby preventing the demodulation of data affected by the topology inversion transmission.  
     [0071] When it is determined to be the data retrieval timing at step R 6 , it is checked whether the current data retrieval timing is at the N th  symbol after the topology inversion transmission (step R 8 ). When it is not at the N th  symbol after the topology inversion transmission, the control moves to step R 9  where data retrieval is performed, since the reception data is not affected by the topology inversion transmission. However, when the current data retrieval timing is at the N th  symbol after the topology inversion transmission, the control moves to step R 10  where the retrieval data of 1 previous symbol is used as the current retrieval data.  
     [0072] Accordingly, when the reception data is affected by the topology inversion transmission, the reception data is replaced with an unaffected proximity data for demodulation.  
     [0073] In the above illustration, N symbol is used as a parameter necessary for the freeze process, in order to eliminate the need for repetitively specifying the hybrid echo period caused by the transmission signals, thereby simplifying the system design. However, this invention is not limited to the method of setting an N symbol to control freeze timing, but can be applied to other methods as long as hybrid echo generated periods can be specified.  
     [0074] In the above embodiment, illustrations are given in a premise of communications in accordance with ADSL standard protocols (especially, G.hs). However, this invention can be applied to other protocols when similar problems need to be addressed.  
     [0075] Additionally, it is especially useful to perform the above-described freezing control during the handshake step in accordance with G.hs, since it is possible to predict the effects of topology inversion transmission and freezing about 1 symbol period does not have much effect. However, this invention also have a configuration where the above-described freezing process is appropriately performed in order to eliminate the effects of the topology inversion transmission as an arbitral process after performing the handshake step of G.hs.  
     [0076] It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to exemplary embodiments, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular structures, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.  
     [0077] The present invention is not limited to the above-described embodiments, and various variations and modifications may be possible without departing from the scope of the present invention.  
     [0078] This application is based on the Japanese Patent Application No. 2002-160495 filed on May 31, 2002, entire content of which is expressly incorporated by reference herein.