Abstract:
A communication device includes: a reception unit receiving communication signals having different frequencies, where the communication signals include logic channels, from another communication device; a measurement unit measuring line states of the received communication signals; a communication signal allocation unit allocating one or more communication signals to each of the logic channels; a margin setting unit setting a margin value for each logic channel and deciding the margin value set for the corresponding logic channel as a margin value of the communication signal; a communication speed decision unit deciding a communication speed of each of the communication signals such that an error rate of each of the communication signals is less than a prescribed value in line states degraded by the margin value of the communication signal from the measured line states; and a transmission unit notifying the other communication device of an allocation result of the communication signal and the decided communication speed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a communication device, a communication system, and a communication method, and more particularly to a communication device, a communication system, and a communication method for performing communication using signals having different frequencies. 
         [0003]    2. Description of the Background Art 
         [0004]    The xDSL (x Digital Subscriber Line) technique of performing high-speed data communication using an existing telephone line includes, for example, ADSL (Asymmetric DSL), VDSL (Very high-bit-rate DSL) and the like. 
         [0005]    The modulation scheme of xDSL includes DMT (Discrete Multi-Tone) modulation scheme in which communication is performed by dividing a transmission frequency bandwidth to be used into narrow bandwidths. For example, in full-rate ADSL (8 Mbps/12 Mbps), the transmission frequency bandwidth of about 1 MHz is divided into 256 subchannels (a bandwidth of 4 kHz). 
         [0006]    In the multi-carrier communication system using the DMT modulation scheme, training is carried out to check the state of a line between mutually connected communication devices before data communication is started. In this training, a signal-to-noise ratio (also referred to as SNR hereinafter) is observed for each divided subchannel, and the allocation of the number of bits to be allocated to a subcarrier that is a carrier wave of the subchannel is set according to the observation result. Thus, the communication speed according to the line state is automatically set (best effort method). After completion of the training, a link (connection) between the communication devices is established, and data communication is then started at the set communication speed. Usually, a dynamic change of the communication speed is not made during data communication. 
         [0007]    Furthermore, in xDSL device, the number of bits is allocated to each subcarrier such that a communication signal satisfies prescribed reception quality under the condition of SNR obtained by subtracting a prescribed margin value (also referred to as SNR margin hereinafter) from the measured SNR. In other words, the SNR margin is a margin value to prevent a transmission error. Here, SNR differs for each subchannel, and the subchannel having a lower frequency has a larger SNR under the normal use conditions of xDSL. The immunity of a communication channel against a transmission error can be adjusted by setting the SNR margin. 
         [0008]    In addition, in xDSL device, an interleaving process of transmitting transmission data in such a manner as to be dispersed in the direction of time-axis is employed in order to reduce the effect of a burst error in transmission. In xDSL device, an encoding process using a combination of this interleaving process with FEC (Forward Error Correction) and CRC (Cyclic Redundancy Check) is performed. Here, a parameter of the interleaving process includes an interleave depth which determines the degree of dispersion of transmission data in the time-axis. The immunity of a communication signal against a burst error can be adjusted by setting this interleave depth. In other words, the immunity of a communication signal against a burst error is improved with the increase in interleave depth. On the other hand, the degree of dispersion of transmission data in the time axis is increased, so that the time until which final data in a transmission data set arrives at the receiver side increases, thereby increasing the transmission delay time. It is noted that that an interleave depth sets at 1 means that an interleaving process is not performed. 
         [0009]    Here, since a general telephone line is housed in a telephone cable having lines bundled, crosstalk is caused by electromagnetic coupling between two telephone lines proximate to each other. In view of QOS (Quality of Service), in downloading data via HTTP (Hyper Text Transfer Protocol) and FTP (File Transfer Protocol), a relatively small transmission speed is acceptable, and data resending is also permissible in the event of a transmission error. However, in recent years, demands for IP telephones, TV phones, match games, video distribution using real-time transmission have been increasing. For example, in video streaming distribution, bulk data is transmitted and the communication has to be stabilized with a reduced error rate without interruption of distribution. In addition, in voice data, the transmission delay time has to be reduced. 
         [0010]    Here, G.992.1 recommended by International Telecommunication Union, Telecommunication Standardization Sector (ITU-T) (see “Asymmetric Digital Subscriber Line (ADSL) transceiver,” ITU-T Recommendations G.992.1 (referred to as Non-Patent Document 1 hereinafter)) defines that two logical data paths (also referred to as logic channels hereinafter) having different interleave depths can be used for one physical transmission path between two communication devices performing ADSL communication. In such xDSL devices, the immunity of a communication signal against a transmission error can be adjusted by setting an interleave depth. 
         [0011]    Now, the error rate is decreased with the increased SNR margin. However, the transmission speed is decreased since the number of bits allocated to a subcarrier is reduced. 
         [0012]    Here, in the communication device described in Non-Patent Document 1, the same SNR margin has to be set for all the subcarriers. Therefore, it is necessary to set the SNR margin for subcarriers at a larger value in accordance with the logic channel requiring the smaller error rate. Thus, the transmission speed of the other logic channel becomes smaller than necessary. Therefore, the communication device disclosed in Non-Patent Document 1 is unable to transmit data appropriately in accordance with a data type, a purpose and the like. 
       SUMMARY OF THE INVENTION 
       [0013]    An object of the present invention is to provide a communication device, and a communication system, and a communication method to allow data transmission to be performed appropriately in accordance with a data type, a purpose and the like. 
         [0014]    A communication device in accordance with an aspect of the present invention includes: a reception unit receiving communication signals having different frequencies, where the communication signals include logic channels, from another communication device; a measurement unit measuring line states of the received communication signals; a communication signal allocation unit allocating one or more of the communication signals to each of the logic channels; a margin setting unit setting a margin value for each of the logic channels and deciding the margin value set for the logic channel corresponding to the communication signal as a margin value of the communication signal; a communication speed decision unit deciding a communication speed of each of the communication signals such that an error rate of each of the communication signals is less than a prescribed value in line states degraded by the margin value of the communication signal from the measured line states; and a transmission unit notifying the other communication device of an allocation result of the communication signal and the decided communication speed. 
         [0015]    A communication device in accordance with another aspect of the present invention includes: a transmission unit transmitting communication signals having different frequencies, where the communication signals include logic channels, to another communication device; a reception unit obtaining line states of the communication signals measured by the other communication device from the other communication device; a communication signal allocation unit allocating one or more of the communication signals to each of the logic channels; a margin setting unit setting a margin value for each of the logic channels and deciding the margin value set for the logic channel corresponding to the communication signal as a margin value of the communication signal; and a communication speed decision unit deciding a communication speed of each of the communication signals such that an error rate of each of the communication signals received by the other communication device is less than a prescribed value in line states degraded by the margin value of the communication signal from the obtained line states. The transmission unit transmits the communication signals to the other communication device at the decided communication speed. 
         [0016]    Preferably, the communication signal allocation unit allocates one or more of the communication signals to each of the logic channels based on the measured line states and the set margin value of the logic channels. 
         [0017]    Preferably, the measurement unit measures a signal-to-noise ratio of the received communication signals, and the communication signal allocation unit sorts the communication signals in increasing order of the signal-to-noise ratio and allocates the sorted communication signals to the logic channels in increasing order of the margin value. 
         [0018]    A communication system in accordance with an aspect of the present invention includes a first communication device and a second communication device. The first communication device includes a transmission unit transmitting communication signals having different frequencies to the second communication device. The second communication device includes a reception unit receiving the communication signals from the first communication device, a measurement unit measuring line states of the received communication signals, a communication signal allocation unit allocating one or more of the communication signals to each of the logic channels, a margin setting unit setting a margin value for each of the logic channels and deciding the margin value set for the logic channel corresponding to the communication signal as a margin value of the communication signal, a communication speed decision unit deciding a communication speed of each of the communication signals such that an error rate of each of the communication signals is less than a prescribed value in line states degraded by the margin value of the communication signal from the measured line states, and a transmission unit notifying the first communication device of an allocation result of the communication signal and the decided communication speed. The transmission unit in the first communication device transmits the communication signals to the second communication device at the communication speed as notified. 
         [0019]    In accordance with an aspect of the present invention, a communication method in a communication system including a first communication device and a second communication device includes: a step of the first communication device transmitting communication signals having different frequencies to the second communication device; a step of the second communication device measuring line states of communication signals received from the first communication device; a step of the second communication device allocating one or more of the communication signals to each of the logic channels and setting a margin value for each of the logic channels to decide the margin value set for the logic channel corresponding to the communication signal as a margin value of the communication signal; a step of the second communication device deciding a communication speed of each of the communication signals such that an error rate of each of the communication signals is less than a prescribed value in line states degraded by the margin value of the communication signal from the measured line states; a step of the second communication device notifying the first communication device of an allocation result of the communication signal and the decided communication speed; and a transmission step of the first communication device transmitting the communication signals to the second communication device at the communication speed as notified. 
         [0020]    Preferably, in the transmission step, the first communication device sets the presence or absence of an interleaving process and an interleave depth for each of the logic channels, performs an interleaving process for the logic channels based on the setting, and transmits communication signals having different frequencies, where the communication signals include logic channels, subjected to the interleaving process to the second communication device at the communication speed as notified. 
         [0021]    In accordance with the present invention, data transmission can be performed appropriately in accordance with a data type, a purpose and the like. 
         [0022]    The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a functional block diagram showing a configuration of a communication system and a communication device in the communication system in accordance with an embodiment of the present invention. 
           [0024]      FIG. 2  is a functional block diagram showing a configuration of an input data processing unit and a control unit. 
           [0025]      FIG. 3  is a flowchart defining an operation procedure when the communication device in accordance with the embodiment of the present invention generates an allocation table for the down-link direction. 
           [0026]      FIG. 4  is a diagram showing an exemplary set value table in the communication device in accordance with the embodiment of the present invention. 
           [0027]      FIG. 5  is a diagram showing an exemplary allocation table in the communication device in accordance with the embodiment of the present invention. 
           [0028]      FIG. 6  is a graph schematically showing an operation of the communication device in accordance with the embodiment of the present invention generating an allocation table. 
           [0029]      FIG. 7  is a flowchart defining an operation procedure when the communication device in accordance with the embodiment of the present invention generates an allocation table for the up-link direction. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    In the following, an embodiment of the present invention will be described with reference to the figures. It is noted that in the figures the same or corresponding parts will be denoted with the same reference characters and thus the description will not be repeated. 
         [0031]    [Configuration and Basic Operation] 
         [0032]      FIG. 1  is a functional block diagram showing a configuration of a communication system and a communication device in the communication system in accordance with an embodiment of the present invention. 
         [0033]    Referring to  FIG. 1 , a communication system  100  includes a station-side device  1  which is a communication device and a terminal-side device  2  which is also a communication device. Station-side device  1  and terminal-side device  2  are connected to each other via a telephone line. Station-side device  1  transmits communication signals corresponding to subcarriers to terminal-side device  2  as the other party. On the other hand, terminal-side device  2  transmits communication signals corresponding to subcarriers to station-side device  1  as the other party. It is noted that the communication system may be configured to include station-side devices  1  and terminal-side devices  2  or may be configured such that one station-side device  1  communicates with terminal-side devices  2 . For example, station-side device  1  may include communication devices and each of the communication devices may perform one-to-one communicate with terminal-side devices  2 . At least one station-side device  1  may have a function as an administration device which monitors and controls other station-side devices  1  and terminal-side devices  2  in this communication system. 
         [0034]    Station-side device  1  includes a transmission unit  61 , a reception unit  62 , a storage unit  8 , a control unit  17 , an input data processing unit  10 , and an output data processing unit  32 . Transmission unit  61  includes a modulator (IFFT)  12 , a parallel/serial (P/S) converter  14 , a digital-to-analog converter (DAC)  16 , a driver unit  20 , and a hybrid circuit  22 . Reception unit  62  includes hybrid circuit  22 , a low-noise amplifier  24 , an analog-to-digital converter (ADC)  26 , a serial/parallel (S/P) converter  28 , and a demodulator (FFT)  30 . 
         [0035]    Input data processing unit  10  performs a variety of signal processing as described later for input data to be transmitted to terminal-side device  2  as the other party and allocates the input data subjected to the signal processing to subcarriers. Then, input data processing unit  10  outputs data for each subcarrier to modulator  12 . 
         [0036]    Modulator  12  digitally modulates the data for each subcarrier received from input data processing unit  10  by Inverse Fast Fourier Transform (IFFT). Then, modulator  12  outputs the digitally modulated signal to parallel/serial converter  14 . 
         [0037]    Parallel/serial converter  14  converts the parallel signal received from modulator  12  into a serial signal for output to digital-to-analog converter  16 . 
         [0038]    Digital-to-analog converter  16  converts the digital signal received from parallel/serial converter  14  into an analog signal for output to driver unit  20 . 
         [0039]    Driver unit  20  amplifies the analog signal received from digital-to-analog converter  16  to a prescribed level for output to hybrid circuit  22 . 
         [0040]    Hybrid circuit  22  transmits the analog signal received from driver unit  20  as a communication signal to terminal-side device  2  through a telephone line. Hybrid circuit  22  also outputs the analog signal, which is a communication signal received from terminal-side device  2  through a telephone line, to low-noise amplifier  24 . 
         [0041]    Low-noise amplifier  24  adjusts the analog signal received from hybrid circuit  22  to a prescribed level and thereafter outputs the adjusted analog signal to analog-to-digital converter  26 . 
         [0042]    Analog-to-digital converter  26  converts the analog signal received from low-noise amplifier  24  to a digital signal for output to serial/parallel converter  28 . 
         [0043]    Serial/parallel converter  28  converts the serial signal received from analog-to-digital converter  26  into a parallel signal for output to demodulator  30 . 
         [0044]    Demodulator  30  digitally demodulates the data received from serial/parallel converter  28  by Fast Fourier Transform (FFT). Then, demodulator  30  outputs the digitally demodulated data for each subcarrier to output data processing unit  32 . 
         [0045]    Output data processing unit  32  reconstructs original data from the data for each subcarrier received from demodulator  30  and outputs the same to the outside. Output data processing unit  32  also outputs a part or all of the reconstructed data to control unit  17  as reception data information. 
         [0046]    Control unit  17  controls each block in the communication device, such as input data processing unit  10 , modulator (IFFT)  12 , demodulator (FFT)  30  and output data processing unit  32 . 
         [0047]      FIG. 2  is a functional block diagram showing the configuration of the input data processing unit and the control unit. 
         [0048]    Referring to  FIG. 2 , input data processing unit  10  includes a logic channel generation unit (data input unit)  41 , error correction coding units  42 - 44 , interleaving process units  45 - 47 , and a data allocation processing unit  48 . Control unit  17  includes a subcarrier allocation (communication signal allocation)/communication speed decision unit  51 , a modulation method decision unit  52 , an SNR measurement unit  53 , an error rate measurement unit  54 , a parameter setting unit (margin setting unit)  55 , and a subcarrier sort unit  56 . 
         [0049]    Logic channel generation unit  41  receives externally input data and generates data of channels CH 1 -CH 3  from the input data for output to error correction coding units  42 - 44 . 
         [0050]    Error correction coding units  42 - 44  perform, for example, a CRC process and an FEC process for the data of logic channels CH 1 -CH 3  received from logic channel generation unit  41 , for output to interleaving process units  45 - 47 . 
         [0051]    Interleaving process units  45 - 47  perform an interleaving process for the data of logic channels CH 1 -CH 3  received from error correction coding units  42 - 44 , based on the interleave depth set for each logic channel by parameter setting unit  55 , for output to data allocation processing unit  48 . 
         [0052]    Data allocation processing unit  48  rearranges the data of logic channels CH 1 -CH 3  received from interleaving process units  45 - 47 , based on the correspondence between logic channels and subcarriers represented by an allocation table described later, for output to modulator  12 . 
         [0053]    It is noted that the operation of each block in control unit  17  will be described later. Furthermore, the configuration and basic operation of terminal-side device  2  is similar to those of station-side device  1  and therefore the detailed description will not be repeated here. 
         [0054]    Next, description will be made to the operation where the communication device in accordance with the embodiment of the present invention decides a communication signal to be allocated to a logic channel and the communication speed of a communication signal. 
         [0055]    [Operation] 
         [0056]      FIG. 3  is a flowchart defining an operation procedure when the communication device in accordance with the embodiment of the present invention generates an allocation table for the down-link direction. 
         [0057]    In the state where station-side device  1  and terminal-side device  2  are performing communication, station-side device  1  monitors the reception quality of the communication signal between station-side device  1  and terminal-side device  2 . 
         [0058]    More specifically, in station-side device  1 , error rate measurement unit  54  in control unit  17  calculates the error rate of the communication signal in the communication direction from terminal-side device  2  to station-side device  1  (also referred to as the up-link direction hereinafter) based on reception data information received from output data processing unit  32 . 
         [0059]    On the other hand, in terminal-side device  2 , error rate measurement unit  54  in control unit  17  calculates the error rate of the communication signal in the communication direction from station-side device  1  to terminal-side device  2  (also referred to as the down-link direction) based on reception data information received from output data processing unit  32 . Then, control unit  17  in terminal-side device  2  controls input data processing unit  10  and the like to incorporate the result of the error rate calculation into a communication signal for transmission to station-side device  1 . Then, in station-side device  1 , error-rate measurement unit  54  extracts the error rate of the communication signal in the down-link direction from the reception data information received from output data processing unit  32 . 
         [0060]    If the error rate of the communication signal in the down-link direction or the error rate of the communication signal in the up-link direction is a prescribed value or more (YES at S 1 ), station-side device  1  performs control to disconnect the up-link and down-link lines (S 2 ). 
         [0061]    Then, training is started in the communication system in accordance with the embodiment of the present invention. More specifically, when the up-link and down-link lines are disconnected, station-side device  1  and terminal-side device  2  execute initialization (S 3  and S 11 ). For example, control units  17  in station-side device  1  and terminal-side device  2  perform gain setting or the like for AGC (Auto Gain Control) circuits included in respective driver units  20  and low-noise amplifiers  24  by controlling modulator  12 , demodulator  30  and the like to transmit/receive an unmodulated signal. It is noted that station-side device  1  and terminal-side device  2  may be configured to execute initialization when station-side device  1  or terminal-side device  2  is powered on or when the user inputs an initialization command to station-side device  1  or terminal-side device  2 . 
         [0062]    Upon completion of the initialization, station-side device  1  generates a set value table for each of the up-link direction and the down-link direction (S 4 ). More specifically, parameter setting unit  55  in station-side device  1  sets an SNR margin, a data rate and an interleave depth for each logic channel. For example, storage unit  8  stores a set value table in which an SNR margin, a data rate and an interleave depth are defined for each logic channel. Parameter setting unit  55  selects any one of set value tables and sets an SNR margin, a data rate and an interleave depth for each logic channel. 
         [0063]      FIG. 4  is a diagram showing an exemplary set value table in the communication device in accordance with the embodiment of the present invention. 
         [0064]    Referring to  FIG. 4 , for example, logic channel CH 1  corresponds to VoIP (Voice over Internet Protocol) that is voice data. Logic channel CH 2  corresponds to video data subjected to streaming distribution. Logic channel CH 3  corresponds to normal video data. 
         [0065]    The set value table is a table representing the correspondence between logic channels and parameters, which are a data rate, an SNR margin and an interleave depth, for use in the communication between station-side device  1  and terminal-side device  2 . The set value table for the up-link direction represents the correspondence between a logic channel and each parameter for use in communication in the up-link direction, and the set value table for the down-link direction represents the correspondence between a logic channel and each parameter for use in communication in the down-link direction. 
         [0066]    Parameter setting unit  55  sets the data rate of logic channel CH 1  at 1 Mbps, sets the SNR margin at 10 dB and sets the interleave depth at 1. Parameter setting unit  55  sets the data rate of logic channel CH 2  at 20 Mbps, sets the SNR margin at 1 dB and sets the interleave depth at 8. Parameter setting unit  55  sets the data rate of logic channel CH 3  at 10 Mbps, sets the SNR margin at 10 dB and sets the interleave depth at 4. 
         [0067]    In this manner, for logic channel CH 1  corresponding to VoIP that is voice data, the interleave depth is set at 1 in order to shorten the transmission delay time. On the other hand, since the immunity against a burst error is reduced, the SNR margin is set at relatively high 10 dB in order to enhance the immunity against a transmission error. For logic channel CH 2  corresponding to video data subjected to streaming distribution, the SNR margin is set at relatively low 1 dB to increase the transmission speed, and on the other hand, the interleave depth is set at 8 to enhance the immunity against a burst error. 
         [0068]    Referring to  FIG. 3  again, station-side device  1  transmits the generated set value table for the down-link direction to terminal-side device  2 . More specifically, parameter setting unit  55  of control unit  17  in station-side device  1  outputs the generated set value table for the down-link direction to input data processing unit  10 . The set value table for the down-link direction allows signal processing such as error correction coding and allocation to a subcarrier to be performed in input data processing unit  10  and is transmitted to terminal-side device  2  through modulator  12 , P/S converter  14 , digital-to-analog converter  16 , driver unit  20 , and hybrid circuit  22  (S 5 ). 
         [0069]    In addition, station-side device  1  transmits, for example, PN (Pseudo Noise) sequence signal (also referred to as a test signal hereinafter) as a communication signal to terminal-side device  2 . Similarly to the set value table, the test signal is transmitted to terminal-side device  2  through modulator  12 , P/S converter  14 , digital-to-analog converter  16 , driver unit  20 , and hybrid circuit  22  (S 6 ). 
         [0070]    Terminal-side device  2  measures the SNR of the test signal received from station-side device  1 . More specifically, in terminal-side device  2 , demodulator  30  receives from serial/parallel converter  28  and digitally demodulates the data corresponding to the test signal and outputs constellation to control unit  17 . SNR measurement unit  53  in control unit  17  measures the signal-to-noise ratio of the test signal based on the constellation received from demodulator  30  (S 12 ). Here, constellation means the symbol arrangement in IQ coordinate plane formed of in-phase (I phase) component and quadrature (Q phase) component of the modulated signal. 
         [0071]    Terminal-side device  2  decides one or more subcarriers to be allocated to each of logic channels based on the SNR measurement result of the test signal and the set value table for the down-link direction received from station-side device  1  and also decides the communication speed of each of communication signals transmitted from station-side device  1 . For example, terminal-side device  2  generates an allocation table for the down-link direction which represents the correspondence between logic channels used by station-side device  1  to transmit communication signals, subcarriers and the number of bits allocated to each subcarrier and transmits the generated table to station-side device  1  (S 13 -S 15 ). 
         [0072]    More specifically, subcarrier sort unit  56  in terminal-side device  2  sorts subcarriers, for example, in increasing order of SNR based on the SNR measurement result of the test signal and outputs the sort result to subcarrier allocation/communication speed decision unit  51  (S 13 ). Subcarrier sort unit  56  also outputs the SNR measurement result of the test signal to subcarrier allocation/communication speed decision unit  51 . 
         [0073]    Subcarrier allocation/communication speed decision unit  51  extracts the set value table for the down-link direction transmitted by station-side device  1  from the reception data information received from output data processing unit  32 . Then, subcarrier allocation/communication speed decision unit  51  decides a subcarrier to be allocated to each of logic channels, based on the sort result received from subcarrier sort unit  56 , the SNR measurement result of the test signal, and the SNR margin and data rate for each logic channel represented by the extracted set value table for the down-link direction, and decides the number of bits to be allocated to each subcarrier, that is, the communication speed for each communication signal. Subcarrier allocation/communication speed decision unit  51  generates an allocation table for the down-link direction, which represents a subcarrier allocated to each of logic channels and the number of bits allocated to each subcarrier, and outputs the generated allocation table to logic channel generation unit  41  and modulation method decision unit  52  (S 14 ). 
         [0074]    The allocation table for the down-link direction is transmitted to station-side device  1  through input data processing unit  10 , modulator  12 , P/S converter  14 , digital-to-analog converter  16 , driver unit  20 , and hybrid circuit  22  (S 15 ). 
         [0075]      FIG. 5  is a diagram showing an exemplary allocation table in the communication device in accordance with the embodiment of the present invention.  FIG. 6  is a graph schematically showing an operation of the communication device in accordance with the embodiment of the present invention generating an allocation table. It is noted that although the allocation table shown in  FIG. 5  includes SNR margin and data rate for each logic channel in order to facilitate understanding, they may not be included in the allocation table. Furthermore, for the sake of brevity, a subcarrier number refers to a number provided after subcarrier sort unit  56  performs sorting, unless otherwise specified. 
         [0076]    Referring to  FIGS. 5 and 6 , subcarriers are arranged in increasing order of SNR, starting from number one. 
         [0077]    Subcarrier allocation/communication speed decision unit  51  allocates the subcarriers sorted in increasing order of SNR to logic channels in order of increasing SNR margins. It is noted that subcarrier allocation/communication speed decision unit  51  sorts subcarriers with the same SNR in increasing order of frequency. Subcarrier allocation/communication speed decision unit  51  allocates as many subcarriers as required to realize the data rate of a logic channel to the logic channel. 
         [0078]    More specifically, first, subcarrier allocation/communication speed decision unit  51  decides the order of logic channels to which sorted subcarriers are allocated. For example, subcarrier allocation/communication speed decision unit  51  puts logic channel CH 2  having the smallest SNR margin of 1 dB in the first place, of logic channels CH 1 -CH 3 . Then, subcarrier allocation/communication speed decision unit  51  puts logic channel CH 1  having the smaller interleave depth in the second place, of logic channels CH 1  and CH 3  having the same SNR margin of 10 dB, and then puts logic channel CH 3  in the third place. 
         [0079]    Next, subcarrier allocation/communication speed decision unit  51  decides the subcarrier to be allocated to logic channel CH 2  and the communication speed of the communication signal corresponding to the subcarrier allocated to logic channel CH 2 , based on the SNR margin of logic channel CH 2  to which the sorted subcarrier is allocated in the first place. 
         [0080]    Subcarrier allocation/communication speed decision unit  51  decides, as the communication speed of the communication signal from the other party, the communication speed lower than the communication speed at which a communication signal can satisfy prescribed reception quality under the condition of the SNR measurement result of the test signal, that is, the SNR of the communication signal measured by SNR measurement unit  53 . In other words, subcarrier allocation/communication speed decision unit  51  decides, as the communication speed of the communication signal from the other party, the communication speed at which the error rate of the communication signal is less than a prescribed value under the condition of the SNR degraded by the SNR margin from the measured SNR of the communication signal. 
         [0081]    Specifically, for example if the SNR of the communication signal in subcarrier  1  is 10 dB, subcarrier allocation/communication speed decision unit  51  determines that three bits can be allocated to subcarrier  1 , where the error rate of the communication signal in subcarrier  1  is less than 10 −7  under the condition of the SNR of 9 dB degraded by 1 dB of the SNR margin of logic channel CH 2  from 10 dB. 
         [0082]    Then, subcarrier allocation/communication speed decision unit  51  allocates subcarrier  1 -subcarrier  30  required to secure the data rate 20 Mbps to logic channel CH 2 , based on the number of bits that can be allocated to each subcarrier as calculated in this manner. In addition, subcarrier allocation/communication speed decision unit  51  assumes the number of allocated bits of subcarrier  1 -subcarrier  30  allocated to logic channel CH 2  as the number of bits calculated based on the SNR margin of logic channel CH 2 . 
         [0083]    Next, subcarrier allocation/communication speed decision unit  51  decides a subcarrier to be allocated to logic channel CH 1 , of unallocated subcarriers, and the communication speed of the communication signal corresponding to the subcarrier allocated to logic channel CH 1 , based on the SNR margin of logic channel CH 1  to which the sorted subcarrier is allocated in the second place. 
         [0084]    Specifically, if the SNR of the communication signal in subcarrier  31  is 19 dB, subcarrier allocation/communication speed decision unit  51  determines that three bits can be allocated to subcarrier  31 , where the error rate of the communication signal in subcarrier  31  is less than 10 −7  under the condition of the SNR of 9 dB which is degraded by 10 dB of the SNR margin of logic channel CH 2  from 19 dB. 
         [0085]    Then, subcarrier allocation/communication speed decision unit  51  allocates subcarrier  31 -subcarrier  40  required to secure the data rate 1 Mbps to logic channel CH 1 , based on the number of bits that can be allocated to each subcarrier as calculated in this manner. In addition, subcarrier allocation/communication speed decision unit  51  assumes the number of bits allocated to subcarrier  31 -subcarrier  40  allocated to logic channel CH 1  as the number of bits calculated based on the SNR margin of logic channel CH 1 . 
         [0086]    Next, subcarrier allocation/communication speed decision unit  51  decides a subcarrier to be allocated to logic channel CH 3 , of unallocated subcarriers, and the communication speed of the communication signal corresponding to the subcarrier allocated to logic channel CH 3 , based on the SNR margin of logic channel CH 3  to which the sorted subcarrier is allocated in the third place. 
         [0087]    Specifically, for example if the SNR of the communication signal in subcarrier  41  is 22 dB, subcarrier allocation/communication speed decision unit  51  determines that six bits can be allocated to subcarrier  41 , where the error rate of the communication signal in subcarrier  41  is less than 10 −7  under the condition of the SNR of 12 dB degraded by 10 dB of the SNR margin of logic channel CH 3  from 22 dB. 
         [0088]    Then, subcarrier allocation/communication speed decision unit  51  allocates subcarrier  41 -subcarrier  46  required to secure the data rate 10 Mbps to logic channel CH 3 , based on the calculated number of bits that can be allocated to each subcarrier. In addition, subcarrier allocation/communication speed decision unit  51  assumes the number of bits allocated to subcarrier  41 -subcarrier  46  allocated to logic channel CH 3  as the number of bits calculated based on the SNR margin of logic channel CH 3 . 
         [0089]    Referring to  FIG. 3  again, station-side device  1  decides, for example, the modulation method of each of communication signals based on the communication speed of communication signals corresponding to subcarriers as represented by the allocation table for the down-link direction received from terminal-side device  2 . More specifically, modulation method decision unit  52  of control unit  17  in station-side device  1  extracts the allocation table for the down-link direction transmitted by terminal-side device  2 , from the reception data information received from output data processing unit  32 . Then, modulation method decision unit  52  decides the modulation method of each of communication signals corresponding to subcarriers based on the extracted allocation table for the down-link direction and notifies modulator  12  of the decided modulation method. Modulator  12  modulates data of each subcarrier in the modulation method indicated by the notification from control unit  17  (S 7 ). 
         [0090]    Modulation method decision unit  52  in terminal-side device  2  recognizes the modulation method of the communication signal transmitted from station-side device  1  based on the allocation table for the down-link direction received from subcarrier allocation/communication speed decision unit  51  and notifies demodulator  30  of the modulation method. Demodulator  30  demodulates the communication signal of each subcarrier in the modulation method indicated by the notification from control unit  17  (S 16 ). 
         [0091]    More specifically, for example, since the number of bits allocated to subcarrier  1  is three, modulation method decision unit  52  decides on 8-PSK (Phase Shift Keying) having a small symbol rate corresponding to three as the modulation method for the communication signal of subcarrier  1 . Furthermore, since the number of bits allocated to subcarrier  41  is six, modulation method decision unit  52  decides on 64 QAM (Quadrature Amplitude Modulation) having a large symbol rate corresponding to six bits as a modulation method for the communication signal of subcarrier  41 . 
         [0092]      FIG. 7  is a flowchart defining an operation procedure when the communication device in accordance with the embodiment of the present invention generates the allocation table for the up-link direction. 
         [0093]    Referring to  FIG. 7 , step S 21 -step S 24  and step S 31  are similar to step S 1 -step S 4  and step S 11  in the flowchart shown in  FIG. 3 . 
         [0094]    Station-side device  1  transmits the generated set value table for the up-link direction to terminal-side device  2  (S 25 ). Parameter setting unit  55  of control unit  17  in station-side device  1  outputs the generated set value table for the up-link direction to subcarrier allocation/communication speed decision unit  51 . 
         [0095]    Furthermore, terminal-side device  2  transmits a test signal to station-side device  1  (S 32 ). 
         [0096]    Station-side device  1  measures the SNR of the test signal received from terminal-side device  2  (S 26 ). 
         [0097]    Station-side device  1  decides one or more subcarriers to be allocated to each of logic channels and also decides the communication speed of each of communication signals transmitted from terminal-side device  2 , based on the SNR measurement result of the test signal and the set value table for the up-link direction generated by itself. For example, station-side device  1  generates an allocation table for the up-link direction, which represents the correspondence between logic channels used by terminal-side device  2  to transmit communication signals, subcarriers and the number of bits allocated to each subcarrier, and transmits the generated allocation table to terminal-side device  2  (S 27 -S 29 ). 
         [0098]    Terminal-side device  2  decides, for example, the modulation method of each of communication signals based on the communication speed of communication signals corresponding to subcarriers as represented by the allocation table for the up-link direction received from station-side device  1 . Terminal-side device  2  modulates data of each subcarrier in the modulation method indicated by the notification from control unit  17  (S 33 ). 
         [0099]    Modulation method decision unit  52  in station-side device  1  recognizes the modulation method for the communication signal transmitted from terminal-side device  2  based on the allocation table for the up-link direction received from subcarrier allocation/communication speed decision unit  51  and notifies demodulator  30  of the modulation method. Demodulator  30  demodulates the communication signal of each subcarrier in the modulation method indicated by the notification from control unit  17  (S 30 ). 
         [0100]    Now, in the communication device described in Non-Patent Document 1, the same SNR margin has to be set for all the subcarriers, and therefore data transmission cannot be performed appropriately in accordance with a data type, a purpose and the like. However, in the communication device in accordance with the embodiment of the present invention, parameter setting unit  55  sets the SNR margin for each logic channel. Then, subcarrier allocation/communication speed decision unit  51  decides, as the communication speed of the communication signal from the other party, the communication speed at which the error rate of the communication signal is less than a prescribed value under the condition of the SNR degraded by the SNR margin from the measured SNR of the communication signal. 
         [0101]    Therefore, in the communication device in accordance with the embodiment of the present invention, the SNR margin can be set in accordance with a data type, a purpose and the like corresponding to a logic channel, and data transmission can be performed appropriately. 
         [0102]    [Modification] 
         [0103]    The present invention is not limited to the foregoing embodiment and includes, for example, the following modifications. 
         [0104]    (1) Allocation of Subcarrier 
         [0105]    In the communication device in accordance with the embodiment of the present invention, subcarrier allocation/communication speed decision unit  51  allocates as many subcarriers as required to secure a data rate of a logic channel to the logic channel, based on the calculated number of bits that can be allocated to each subcarrier. However, the present invention is not limited thereto. For example, if the ratio of subcarriers allocated to each of logic channels CH 1 -CH 3  is predetermined, subcarrier allocation/communication speed decision unit  51  can allocate a subcarrier to a logic channel without using the SNR measurement result of the test signal and SNR margin. Specifically, if the ratio of subcarriers allocated to each of logic channels CH 1 -CH 3  is 1:1:1, subcarrier allocation/communication speed decision unit  51  may allocate subcarrier  1  to logic channel CH 1 , allocates subcarrier  2  to logic channel CH 2 , allocates subcarrier  3  to logic channel CH 3 , and sequentially allocates subcarriers following subcarrier  4  to logic channels CH 1 -CH 3 , similarly. 
         [0106]    (2) Setting of Logic Channel 
         [0107]    Parameter setting unit  55  may use any one of logic channels used in communication between station-side device  1  and terminal-side device  2  as a logic channel having the smallest error rate and the highest immunity against a transmission error. For example, parameter setting unit  55  sets the SNR margin of logic channel CH 1  to be the largest and sets the interleave depth to be the largest, among all the logic channels. Then, subcarrier allocation/communication speed decision unit  51  at least allocates the subcarrier having the largest SNR among subcarriers to logic channel CH 1 . 
         [0108]    As described above, if the error rate of a communication signal in the down-link direction or the error rate of a communication signal in the up-link direction is a prescribed value or more (YES at S 1 ), control unit  17  in the communication device in accordance with the embodiment of the present invention disconnects the up-link and down-link lines and performs retraining (S 2 ). However, when of all the logic channels, logic channel CH 1  is used as a logic channel having the smallest error rate and the highest immunity against a transmission error in this way, control unit  17  may not perform line disconnection and retraining for the subcarrier allocated to logic channel CH 1  and may maintain communication. Because of such a configuration, data transmission which strongly requires stabilization of communication can be performed properly. 
         [0109]    (3) Generation and Transmission of Bit Table In the communication system in accordance with the embodiment of the present invention, when an allocation table for the down-link direction is generated, terminal-side device  2  generates an allocation table for the down-link direction based on the SNR measurement result of the test signal in the down-link direction and the set value table for the down-link direction received from station-side device  1 . However, the present invention is not limited thereto. Terminal-side device  2  may transmit the SNR measurement result of the test signal in the down-link direction to station-side device  1 . Then, station-side device  1  may generate an allocation table for the down-link direction based on the SNR measurement result of the test signal in the down-link direction and the set value table for the down-link direction and transmit the generated allocation table to terminal-side device  2 . Then, terminal-side device  2  may recognize the modulation method of the communication signal transmitted from station-side device  1  based on the allocation table for the down-link direction received from station-side device  1 . 
         [0110]    On the other hand, when an allocation table for the up-link direction is generated, station-side device  1  may transmit the SNR measurement result of the test signal in the up-link direction and the set value table for the up-link direction to terminal-side device  2 . Then, terminal-side device  2  may generate an allocation table for the up-link direction based on the SNR measurement result of the test signal in the up-link direction and transmit the generated allocation table to station-side device  1 . Then, station-side device  1  may recognize the modulation method of the communication signal transmitted from terminal-side device  2  based on the allocation table for the down-link direction received from terminal-side device  2 . 
         [0111]    (4) SNR 
         [0112]    In the communication device in accordance with the embodiment of the present invention, subcarrier allocation/communication speed decision unit  51  decides, as the communication speed of the communication signal transmitted by the other party, the communication speed at which the error rate of the communication signal is less than a prescribed value under the condition of the SNR degraded by the SNR margin from the SNR measurement result of the communication signal. However, the present invention is not limited to SNR, and any index that represents the state of line of a communication signal may be used in place of SNR. 
         [0113]    Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.