Patent Publication Number: US-9419756-B2

Title: Communication apparatus, integrated circuit, and communication method

Description:
INCORPORATION BY REFERENCE 
     This application is related to the following patent which is hereby incorporated by reference in its entirely: U.S. Pat. No. 6,944,232, RECEIVING APPARATUS AND METHOD FOR DIGITAL MULTI-CARRIER TRANSMISSION, Inventors: Hisao Koga et al., filed on Feb. 19, 2004. 
     This is a continuation application of application Ser. No. 13/920,984 filed Jun. 18, 2013, which is a continuation application of application Ser. No. 13/039,522 filed Mar. 3, 2011, which is a divisional application of application Ser. No. 11/545,779 filed Oct. 11, 2006, which is based on Japanese Application No. 2005-297529 filed Oct. 12, 2005, and Japanese Application No. 2006-114191 filed Apr. 18, 2006, the entire contents of each which are incorporated by reference herein. application Ser. No. 13/039,515 filed Mar. 3, 2011 is also a divisional application of application Ser. No. 11/545,779 filed Oct. 11, 2006. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a communication apparatus, an integrated circuit and a communication method that are capable of easily detecting signals output from other communication apparatuses, which use different communication methods and are connected to a common transmission line, while avoiding interference between signals without performing relatively cumbersome modulation and other processes. 
     2. Description of Related Art 
     With the recent development of communication technology, PLC (Power Line Communication) has been gaining attention. PLC is a technology that performs multi-carrier communications among a plurality of terminal apparatuses, using power lines installed indoors as transmission lines, and utilizes an OFDM (Orthogonal Frequency Division Multiplexing) system (e.g., Japanese Patent Laid-Open Publication 2000-165304). OFDM is a modulation method for multi-carrier data transmission, by which a plurality of carriers are transmitted in a multiplex way on a frequency axis. OFDM uses an FFT (Fast Fourier Transform) or a DWT (Discrete Wavelet Transform) to narrow frequency intervals of multi-carriers and to closely space a plurality of carriers in such a way that they partially overlap and yet do not interfere with one another. OFDM thus enables broadband transmission by efficiently using a narrow frequency spectrum. 
     For multi-carrier communications, such as power line communications, a technology is proposed to suppress interference in such manner that a phase vector flattens time waveform levels to prevent occurrence significant peak. In this technology, when a time waveform has no significant peak, the phase of each sub-carrier is rotated using the phase vector of default. However, when the significant peak is detected, the phase vector is changed until a phase vector that generates no waveform peak is found, and the phase of each sub-carrier is thus rotated according to the changed phase vector (Denis J. G. Mestdagh and Paul M. P. Spruyt, “A Method to Reduce the Probability of Clipping in DMT-Based Transceivers”, IEEE Transactions on Communications, Vol. 44, No. 10, pp. 1234-1238, 1996). Such a technology for suppressing peaks is essential for reducing the design difficulty for a power amplifier for multi-carrier communications. 
     Usually, when the specifications of the same communication method are used, the specifications of communication apparatuses connected to each network are generally common even for a case where different logical networks are formed using a network key, or the like. This way, the communication apparatuses can detect (carrier sense) signals transmitted between different networks, on a physical layer level of the communication apparatuses, and it is possible to prevent interference between signals using a CSMA (Carrier Sense Multiple Access), thus enabling smooth communication even for relatively closely located different networks. 
     However, different manufacturers may use different specifications for a communication method such as a communication protocol, a modulation scheme and a frequency band. Such communication technology is highly likely to be used in an environment where a plurality of types of communication methods are mixed in the same location. For instance, users (communication apparatus users) in collective housing such as an apartment or a condominium do not necessarily use communication apparatuses (e.g., modems) of the same manufacturer. In this case, a plurality of types of communication apparatuses independently made by a plurality of manufacturers may be simultaneously connected to a common power line. 
     When the a plurality of types of communication apparatuses are connected to the common power line, a communication apparatus cannot demodulate a signal transmitted from a different communication apparatus using a different type of communication method. Therefore, such a signal is acknowledged merely as noise. Accordingly, although the plurality of types of communication apparatuses use the same frequency band, even the existence of other communication apparatuses is not acknowledged. This causes interference between signals transmitted from the plurality of types of communication apparatuses, thereby causing communication errors. In other words, the plurality of types of communication apparatuses sometimes cannot coexist on the common power line. 
     On the other hand, when each communication apparatus is set up to perform modulation, signals transmitted from other communication apparatuses can be differentiated. However, modulation processes performed to allow the plurality of types of communication apparatuses to coexist have an adverse effect of increasing the workload. 
     SUMMARY 
     An object of embodiments described in the following is to provide a communication apparatus, an integrated circuit and a communication method that are capable of easily detecting signals output from other communication apparatuses, even when a plurality of types of communication apparatuses using different communication methods are connected to a common transmission line, without performing relatively cumbersome modulation and other processes. 
     A first communication apparatus, which is described in the embodiments, is a communication apparatus is capable of connecting to a power line connected to at least a first communication apparatus and a second communication apparatus. The first communication apparatus is capable of performing a data transmission with said communication apparatus. The second communication apparatus is incapable of performing the data transmission with said communication apparatus. The communication apparatus includes a receiver, a carrier detector, a channel setting unit and a transmitter. The receiver receives a signal from the second communication apparatus. The carrier detector detects a predetermined data in the signal. The channel setting unit sets at least one of time slot and frequency band used for the first communication apparatus when the carrier detector detects the predetermined data, the time or the frequency band used for the first communication apparatus being different from a time or a frequency band used for the second communication apparatus. The transmitter performs the data transmission with the first communication apparatus in at least one of the time and the frequency band used for the first communication apparatus. 
     An integrated circuit, which is described in the embodiments, is an integrated circuit is capable of connecting to a power line connected to at least a first communication apparatus and a second communication apparatus. The first communication apparatus is capable of performing a data transmission with said integrated circuit. The second communication apparatus is incapable of performing the data transmission with said integrated circuit. The integrated circuit includes a receiver, a carrier detector, a channel setting unit and a transmitter. The receiver receives a signal from the second communication apparatus. The carrier detector detects a predetermined data in the signal. The channel setting unit sets at least one of time and frequency band used for the first communication apparatus when the carrier detector detects the predetermined data, the time or the frequency band used for the first communication apparatus being different from a time or a frequency band used for the second communication apparatus. The transmitter performs the data transmission with the first communication apparatus in at least one of the time and the frequency band used for the first communication apparatus. 
     A communication method, which is described in the embodiments, is a communication method controls data transmission that a communication apparatus performs through a power line connected to at least a first communication apparatus and a second communication apparatus. The first communication apparatus is capable of performing the data transmission with said communication apparatus. The second communication apparatus is incapable of performing the data transmission with said communication apparatus. The communication method includes: receiving a signal from the second communication apparatus; detecting a predetermined data in the signal; setting at least one of time and frequency band used for the first communication apparatus when the carrier detector detects the predetermined data, the time or the frequency band used for the first communication apparatus being different from a time or a frequency band used for the second communication apparatus; and performing the data transmission with the first communication apparatus in at least one of the time and the frequency band used for the first communication apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration view of a communication system according to a first embodiment; 
         FIG. 2( a )  is an external perspective view of a front side of a modem; 
         FIG. 2( b )  is an external perspective view of a rear side of the modem; 
         FIG. 3  is a block diagram illustrating a hardware example that constitutes the modem according to the first embodiment; 
         FIG. 4  is a functional block diagram of a PLC PHY block; 
         FIG. 5  shows a signal format of an OFDM signal; 
         FIG. 6  shows a signal spectrum of the OFDM signal; 
         FIG. 7( a )  is a time chart that employs time division; 
         FIG. 7( b )  is a time chart that employs another example of time division; 
         FIG. 7( c )  is a time chart that employs frequency and time division; 
         FIG. 8( a )  shows an example of attenuation-frequency characteristics on a power line; 
         FIG. 8( b )  shows an example of noise level-frequency characteristics on the power line; 
         FIG. 9  shows time slots corresponding to request signals transmitted during control periods; 
         FIG. 10  is a time chart illustrating exchange of control signals between modems; 
         FIG. 11  is a block diagram illustrating a hardware example that constitutes a modem according to a second embodiment; 
         FIG. 12( a )  is a time chart that employs frequency division; 
         FIG. 12( b )  is a time chart that employs frequency and time division; 
         FIG. 13  is a time chart illustrating an operation example of a plurality of modems, when different request signals are transmitted; 
         FIG. 14  is a time chart illustrating an operation example of the plurality of modems, when some communication methods are not in sync with synchronization signals; 
         FIG. 15  is a block diagram illustrating a hardware example that constitutes a modem according to a third embodiment; 
         FIG. 16  is a functional block diagram of a PLC PHY block of a sub IC; 
         FIG. 17  is a time chart illustrating an operation example of a plurality of modems according to the third embodiment; 
         FIG. 18  is a flowchart illustrating a process of detecting a request signal; 
         FIG. 19  shows time slots corresponding to request signals according to a fourth embodiment; 
         FIG. 20  is a flowchart illustrating a process of detecting a request signal according to the fourth embodiment; 
         FIG. 21  is a time chart illustrating an operation example of a plurality of modems according to a fifth embodiment; and 
         FIG. 22  is a flowchart illustrating a process of modifying a phase vector according to the fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     The first embodiment is described in the following with reference to  FIGS. 1 to 10 . 
       FIG. 1  is a schematic configuration view of communication system  100  according to the first embodiment. As shown in  FIG. 1 , communication system  100  includes a network using power lines  2  as transmission lines. Power lines  2  include: power transmission cables of power pole  7 , which is provided outdoors; a pull-in cable connected to the power transmission cables via transformer  4 ; and an interior wiring within residence  1 . Power lines  2 , which include the power transmission cables, are connected to power distribution panel  6  via power lines  2 , which include the pull-in cable. Fiber cable  8 , which is connected to an ISP (Internet Service Provider/not shown), or the like, is connected to power distribution panel  6  via modem  10 C 3 , which functions as a communication apparatus. 
     Power lines  2 , which are connected to power distribution panel  6 , are connected to a plurality of outlets  5  installed in residence  1 . A plurality of modems using different types of communication methods are connected to outlets  5  via plugs  3  and power lines  2  (e.g., VVF cables). Power lines  2  feed commercial AC voltage (e.g., 100V, 60 Hz (or 50 Hz)) to various electric appliances, although values other than 100V, 60 Hz can also be used. For instance, an AC voltage of 120V, 60 Hz is used in the U.S. and an AC voltage of 110/220V, 50 Hz is used in China, etc. 
     As shown in  FIG. 1 , modems  10 A 1 ,  10 A 2  and  10 A 3  use communication method A; modems  10 B 1  and  10 B 2  use communication method B; and modems  10 C 1 ,  10 C 2  and  10 C 3  use communication method C. All of the modems are installed in residence  1 . Various electric appliances are connected to the respective modems via LAN cables  9 . More specifically, intercom  109  is connected to modem  10 A 1 ; and telephone with display  107  and  107  are connected to modems  10 A 2  and  10 A 3 . Television  102  is connected to modem  10 B 1 ; and server  105  is connected to modem  10 B 2 . Portable personal computer (hereinafter simply referred to as a PC)  101  is connected to modem  10 C 1 ; and television  106  is connected to modem  10 C 2 . 
     In the following description, when no particular distinction is necessary among modems  10 A 1 ,  10 A 2 ,  10 A 3 ,  10 B 1 ,  10 B 2 ,  10 C 1 ,  10 C 2  and  10 C 3 , these modems are all simply referred to as “modem  10 ”. The modem described in the present embodiment is an example of communication apparatus  10 . Any device having a communication function, other than a modem, can also be used. For instance, electric appliances having a modem function (more specifically, various electric appliances  101 ,  102 ,  103 , . . . shown in  FIG. 1 ) can also be used. 
     In the specification, power line communication used only in housings, e.g., residences and collective housings, and other structures, e.g., factories and buildings, is defined as “in-home communication”; and power line communication (including communication methods used in buildings using such power line communication) used for outdoor power transmission cables and fiber cables is defined as “access communication”. In the following, a communication system by in-home communication is simply referred to as an “in-home system”; and a communication system by access communication is simply referred to as an “access system”. In  FIG. 1 , a communication system including moderns  10 A 1 ,  10 A 2 ,  10 A 3 ,  10 B 1  and  10 B 2  belongs to the in-home system; and a communication system including modems  10 C 1 ,  10 C 2  and  10 C 3  belongs to the access system. 
       FIG. 2( a )  is an external perspective view of a front side of the modem; and  FIG. 2( b )  is an external perspective view of a rear side of the modem. Modem  10  has chassis  11  shown in  FIG. 2 . Displays  16 , such as LED (Light Emitting Diodes), are provided on the front of chassis  11 . Power connector  12 , LAN (Local Area Network) modular jack  13 , such as RJ 45 and D-sub connector  15  are provided on the rear of chassis  11 . Power lines  2 , such as a parallel cable, are connected to power connector  12 . LAN cable  9  is connected to modular jack  13 . A D-sub cable (not shown) is connected to D-sub connector  15 . 
       FIG. 3  is a block diagram illustrating a hardware example that constitutes modem  10  according to the first embodiment. As shown in  FIG. 3 , modem  10  includes circuit module  20  and switching regulator  50 . Switching regulator  50  feeds various levels of voltage (e.g., +1.2V, +3.3V, +12V) to circuit module  20 . Circuit module  20  includes main IC (Integrated Circuit)  22 , AFE IC (Analog Front End IC)  23 , band-pass filter  25 , driver IC  26 , coupler  27 , band-pass filter  29 , AMP (amplifier) IC  30 , band-pass filter  31 , ADC (AD converter) IC  32 , memory  33  and Ethernet PHY IC  12 . Power connector  12  is connected to power lines  2  via plug  3  and outlet  5 . 
     Main IC  22  includes: CPU (Central Processing Unit)  22 A, PLC MAC (Power Line Communication Media Access Control layer) block  22 C and PLC PHY (Power Line Communication Physical layer) block  22 B. CPU  22 A is equipped with a 32-bit RISC (Reduced Instruction Set Computer) processor. PLC MAC block  22 C controls a MAC layer; and PLC PHY block  22 B controls a PHY layer. AFE IC  23  includes DA converter (DAC)  23 A, variable gain amplifiers (VGAs)  23 B and  23 C, and AD converter (ADC)  23 D. Coupler  27  includes coil transformer  27 A, and coupling condensers  27 B and  27 C. 
     Circuit module  20  further includes sub IC  42 , AFE IC  43 , band-pass filter  45 , driver IC  46  and band-pass filter  49 . Sub IC  42  includes PLC MAC block  42 C and PLC PHY block  42 B. AFE IC  43  includes DA converter (DAC)  43 A, variable gain amplifiers (VGAs)  43 B and  43 C and AD converter (ADC)  43 D. 
     Main IC  22 , as with a general modem, is an electric circuit (LSI) that performs signal processing including basic control and modulation/demodulation for data communication. In other words, main IC  22  modulates received data, which are output from a communication terminal such as a PC, and outputs as a transmitted signal (data) to AFE IC  23 . Main IC  22  also demodulates transmitted data, which are input via AFE IC  23  from power lines  2 , and outputs as a received signal (data) to a communication apparatus such as a PC. Main IC  22  further outputs a predetermined communication request signal to sub IC  42  prior to the data communication, so as to check if power lines  2  can be used. 
     Driver IC  26  functions as a switch that blocks/passes transmitted and received signals between main IC  22  and power lines  2 . In other words, driver IC  26  serves as an interface between a digital signal processing circuit and the power lines; and the data communication can be controlled by switching ON/OFF driver IC  26 . Driver IC  26  can take any form of configuration, as long as it has control capabilities to allow/deny the data communication. For instance, driver IC  26  can be equipped with a switch, such as an analog switch, which enables ON/OFF control by an external signal. 
     A first signal output unit, a second signal output unit and a phase vector setting unit are provided as PLC PHY block  42 B of sub IC  42  respectively. A data communication range setting unit is provided as PLC PHY block  22 B, and band-pass filters  25  and  29 . A data communication unit is provided as PLC PHY block  22 B and AFE IC  23 . PLC PHY block  42 B is a sample of a receiver, a carrier detector, and a transmitter. 
       FIG. 4  is a functional block diagram of PLC PHY block  42 B of sub IC  42 . First, a phase setting process, which uses an inverse wavelet transform for a multi-carrier signal modulation, is described with reference to  FIG. 4 . 
     PLC PHY block  42 B, as shown in the lower section of  FIG. 4 , includes: symbol mapper  406  that maps transmitted data as serial data onto a complex coordinate plane; S/P converter  407  that converts the serial data into parallel data corresponding to respective sub-carriers of a multi-carrier; phase rotator  408  that rotates each of phases of the parallel data; inverse wavelet transformer  410  that performs multi-carrier modulation by performing inverse wavelet transform on the phase-rotated parallel data; and controller  405  that controls the phase vectors rotated by phase rotator  408 . Phase vector is a set of values that indicate phases corresponding to respective sub-carrier signals in a multi-carrier signal. The phase vector is the set of values for flattening time waveform levels to prevent occurrence significant peak. The signal phases of all the sub-carriers are randomly set, so that time waveform levels produce no peak. Accordingly, as the phase of each sub-carrier signal is randomized, the time waveform levels are flattened, thus producing no peak. 
     Symbol mapper  406  performs a first modulation in which transmitted data in the form of bit data are converted into symbol data, with a total of M−1 sub-carriers mapped onto the complex coordinate plane. S/P converter  407  converts sequentially input serial data (transmission symbols) generated through the first modulation, to be sequentially input, into parallel data corresponding to each of the sub-carriers in the multi-carrier signal. Then, phase rotator  408  rotates the phases of the input parallel data. In this case, a (2n−1) th  input (n is a positive integer) is considered as the in-phase component of the complex data, while a 2n th  input is considered as the orthogonal component (suppose 1≦n≦M/2−1) of the complex data. The numbers of sub-carriers are considered as 0˜M−1. Complex sub-carriers are made of sub-carrier pairs, and the phase of each of the sub-carriers is rotated. In this example, the maximum number of parallel data (number of sub-carriers) to be phase-rotated is M/2−1. Inverse wavelet transformer  410  performs multi-carrier modulation through the inverse wavelet transform of the phase-rotated parallel data of each sub-carrier, generating the transmitted signals in the multi-carrier. The S/P converter can be used before the symbol mapper. 
     Controller  405  supplies a signal that controls a phase vector (hereinafter simply referred to as a “vector control signal”) to phase rotator  408 , controlling settings and changes of the phase vector. In this example, controller  405  may include a random value generator. The random value generator generates a random value using, for example, a PN (Pseudo Noise) sequence and supplies the random value to phase rotator  408  as a vector control signal in order to perform phase rotation on each of its targeted sub-carriers. As such random values mentioned above, two values, i.e., 0 and π (or −1) are generated. Or, controller  405  may include a cyclic shift designator so that a vector control signal (a phase shift value) for a cyclic shift operation is generated; the vector control signal to phase rotator  405  is supplied; and phase rotation on each of the sub-carriers to be used for the communication is performed. 
     As described above, since phases are rotated based on the PN sequence, phase vectors having a less time correlation can be set, so that first and second signals can be differentiated with more accuracy. Particularly, using an M sequence as the PN sequence enables a setting of phase vectors having coherent auto-correlation (coherent phases), thereby achieving more accurate differentiation. Any sequence may be used to perform phase rotation as long as it has self correlation is sensitive and mutual correlation is insensitive. For example, PN sequence such as M sequence and Gold sequence may be used to perform the phase rotation. 
     Instead of rotating each of targeted sub-carriers each time, it is also possible to pre-save, in a medium such as a memory, output signals themselves from phase rotator  408  or inverse wavelet transformer  410 , and to retrieve the signal from the memory as a given data signal each time a vector control signal is generated, so as to output the generated vector control signal as a vector control signal. Or, it is also possible to retrieve given data each time a phase vector is changed, and output the given data as a vector control signal. 
     The following describes a phase re-rotation process, which uses the wavelet transform for modulating the multi-carrier signal. PLC PHY block  42 B, as indicated in the upper section of  FIG. 4 , further includes: wavelet transformer  401  that performs multi-carrier demodulation through the wavelet transform of a received signal; phase rotator  402  that rotates phases of parallel data corresponding to each of modulated sub-carriers; and P/S converter  403  that converts the parallel data corresponding to each of the phase-re-rotated sub-carriers into serial data. 
     Wavelet transformer  401  demodulates the multi-carrier signal through the wavelet transform of the received signal, and generates parallel data corresponding to each of the sub-carriers in the multi-carrier. Phase rotator  402  re-rotates the parallel data individually by rotating the phases of the input parallel data. Then, P/S converter  403  converts the input parallel data, each packet of which corresponds to each of the sub-carriers in the multi-carrier, into serial data so as to obtain the received data, Changing the order of phase rotator  402  and P/S converter  403  causes no operational difficulties. 
     Controller  405  controls settings and changes of a phase vector by supplying a vector control signal to phase rotator  402 . As with the above-described phase setting process, controller  405  includes a random value generator, which generates a random value using the PN (Pseudo Noise) sequence, for instance, and supplies the generated random value as a vector control signal to phase rotator  402 , in order to rotate each of the targeted sub-carriers. As such random values mentioned above, two values, i.e., 0 and π are generated. Or, controller  405  may include a cyclic shift designator so that a vector control signal (a phase shift value) for a cyclic shift operation is generated; the vector control signal to phase rotator  402  is supplied; and phase rotation on each of the sub-carriers to be used for the communication is performed. Accordingly, such a cyclic shift operation enables a large number of sub-carriers to be phase-rotated with relatively light workload. 
     In the first embodiment, an OFDM signal is used as a data signal or a control signal (described later).  FIG. 5  shows a signal format of an OFDM signal.  FIG. 6  shows a signal spectrum of the OFDM signal. The OFDM signal is configured the same way as a preamble signal, which is usually used for carrier detection and synchronization processes. The preamble signal includes a predetermined data. For instance, controller  405  inputs, as the predetermined data, a series of the same value for each sub-carrier (e.g., a signal in the form of 1, 1, 1, . . . for each sub-carrier) into phase rotator  408 ; rotates each of the sub-carriers by an appropriate phase vector; and generates a time signal through frequency-time transform at inverse wavelet transformer  410 . As an actual OFDM signal, a multi-tone signal with a symbol length of approximately 100 μs (e.g., 56 waves) is used for instance. 
     Although descriptions have been provided above for the case where a phase vector is rotated through the wavelet transform, other transformation methods, such as a Fourier transform, can also be used. Phase setting and re-rotation processes of PLC PHY bock  22 B are identical to those of PLC PHY block  42 B, and their descriptions are thus omitted. 
       FIG. 7( a )  is a time chart that employs time division;  FIG. 7( b )  is a time chart that employs another example of time division; and  FIG. 7( c )  is a time chart that employs frequency and time division. 
     In the first embodiment, frequency bands on power lines  2  are divided, as shown in  FIG. 7 , into control signal band BW 1  and data signal band BW 2 . Control signal band BW 1  is a band for transmitting a control signal. The control signal is for controlling communication between modems  10 , which includes a synchronization signal SS and a request signal RS, the synchronization signal SS indicating a synchronization timing for each modem  10 , and the request signal RS announcing that each modem  10  starts data communication. The request signal RS is an example of the first signal; and the synchronization signal SS is an example of the second signal. 
     Data signal band BW 2  is a band for transmitting a data signal. The data signal contains various information, such as video image, voice, and text data, which is specified in the payload of a packet. When a frequency band used for the power line communication is between 2 and 30 MHz, for instance, a frequency band of 2-3 MHz is assigned as control signal band BW 1 ; and a frequency band of 3-30 MHz is assigned as data signal band BW 2 . Although an arbitrary frequency band can be selected as control signal band BW 1 , lower frequencies allow sampling frequencies to be lowered, thereby enabling the modem to be configured with a simple circuit. 
       FIG. 8( a )  shows an example of attenuation-frequency characteristics on the power line; and  FIG. 8( b )  shows an example of noise level-frequency characteristics on the power line. As shown in  FIG. 8( a ) , signal attenuation is high in the frequency band of 2-3 MHz, resulting in a higher noise level as shown in  FIG. 8( b ) . To achieve high-speed transmission, it is preferable that the communication uses as broad frequency band as possible. However, as described above, a noise level increases concomitantly with an attenuation level in the frequency band of 2-3 MHz, and an S/N (signal-to-noise ratio) thus decreases, thereby making only a limited contribution to high-speed transmission. Therefore, the reduction of transmission speed can be kept to a minimum by allocating the frequency band of 2-3 MHz exclusively to negotiations as control signal band BW 1 . This also enables the use of a relatively higher frequency band for data transmission, thereby improving its data transmission efficiency. 
     The following describes a specific control operation performed by PLC PHY block  42 B of sub IC  42  shown in  FIG. 3 , the control operation allowing a plurality of modems  10  to coexist on the common power lines  2 . 
     In the first embodiment, two or more different types of phase vectors, which use the same specifications (e.g., a sampling frequency and symbol length) of a control signal, are used as a control signal common to a plurality of types of modems  10 . For instance, various types of phase vectors, such as a phase vector exclusively used for a synchronization signal SS and a phase vector exclusively used for a request signal RS, are used as needed, so as to control the a plurality of types of modems. 
     More specifically, PLC PHY block  42 B of sub IC  42  transmits a predetermined signal to driver IC  26 , so that driver IC  26  blocks data communication at main IC  22 . When driver IC  26  is turned OFF, PLC PHY block  42 B outputs a synchronization signal SS via AFE IC  43 , band-pass filter  45  and driver IC  46 . The synchronization signal SS is superimposed to AC power by coupler  27 , and output to power lines  2  via power connector  12 , plug  3  and outlet  5 . A synchronization signal SS is set to be output during each predetermined time period; and PLC PHY block  42 B repeatedly outputs a synchronization signal SS in each predetermined cycle. 
     As shown in  FIG. 7( a ) , PLC PHY block  42 B of modem  10 B 1  (see  FIG. 1 ), which uses communication method B, outputs a synchronization signal SS at times t1, t9, t11, t20, t30, . . . . As previously described, since two or more types of phase vectors are used, each modem  10  stores, in its predetermined memory (not shown), data (two values, i.e., 0 and π for each sub-carrier) related to phase vectors of a control signal, such as a synchronization signal SS and a request signal RS. Therefore, PLC PHY block  42 B of each modem  10  retrieves, from its memory, data related to the phase vectors, and detects a synchronization signal SS after executing the above-described phase re-rotation process at phase rotator  402  and controller  405 . By detecting a synchronization signal SS, each modem  10  sets control periods T1, T2, T3, T4, . . . , each of which defines a predetermined cycle (e.g., ms order) as one cycle. A period for transmitting a control signal, as described above, is referred to as “control period Tc”. 
       FIG. 9  shows time slots corresponding to request signals transmitted during control period Tc. PLC PHY block  42 B of each modem  10  is configured to output a request signal RS after a period corresponding to its own communication method has passed based on where a synchronization signal SS was detected. Phase rotator  408  and controller  405  execute the above-described phase setting process, so that the phase vector of the request signal RS is different from that of the synchronization signal SS. 
     As shown in  FIG. 9 , for instance, it is assumed that modem  10 B 1  outputs a synchronization signal SS between times t1 and t2. In this case, modems each of  10 A 1 ,  10 A 2  and  10 A 3 , which uses communication method A, outputs a request signal RS after the time has passed from times t1 to t2. Modems  10 B 1  and  10 B 2 , which use communication method B, output a request signal RS after the time has passed from times t1 through t3. Modems  10 C 1  and  10 C 2 , which use communication method C, output a request signal RS after the time has passed from times t1 through t4. In other words, time slots T12, T13, T14, . . . , T18, which correspond to communication methods A, B, C, . . . , are set during control period Tc. A period set for each time slot does not need to be at equal intervals. 
     Each modem  10  stores in its predetermined memory data related to the phase vector of a request signal RS. Therefore, as with the case for a synchronization signal SS, each modem  10  retrieves, from its memory, data related to the phase vector, and detects the request signal RS after executing the phase re-rotation process at phase rotator  402  and controller  405 . The request signal RS, as previously described, is set by phase rotator  408  so that its phase vector is different from that of the synchronization signal SS. Therefore, each modem  10  can differentiate the request signal RS from the synchronization signal SS based on the differences of their phase vectors. 
     When the same phase vector is used for a synchronization signal SS and a request signal RS, and when a carrier detection is performed using signals output from wavelet transformer  401 , for instance, using correlations between carriers and a distribution of correlation values in a frequency domain, both signals become receivable, thereby making it impossible to tell whether the synchronization signal SS or the request signal RS has been transmitted. The power line communication apparatus, however, operates controller  405  to perform a carrier detection using the phase vector used for the synchronization signal SS, as well as performing a carrier detection using the phase vector used for the request signal RS. In this manner, two different phase vectors are used for two different signals, and it has thus become impossible to simultaneously perform carrier detections for a plurality of signals in a frequency domain. This enables differentiation between the synchronization signal SS and the request signal RS, which allows each modem  10  to acknowledge what a control signal signifies. 
     Each modem  10  stores, in its predetermined modem (not shown in the figure), data related to a correlation between a time slot and a communication method. Based on the correlation, it is possible to detect in which time slot during one control period Tc a request signal RS is output, and thus to know the number of communication methods (namely, the number of types of communication methods) of modems that have announced initiation of data transmission. 
     As described above, since each request signal RS is output in its corresponding time slot T12, T13, . . . , T18, interference between request signals RS can be prevented. As a result, each modem  10  can reliably detect request signals RS output from other modems  10 . When a correlation between a time slot and a communication method is predetermined, the order of outputting a request signal RS is not limited to A→B→C→ . . . , but can be changed as needed. Time slots T12, T13, . . . , T18 do not need to be at equal intervals. 
     In addition, when a control signal is output to each of the time slots during control period Tc, any functional signification is possible for each slot. For instance, it is possible to use a specific time slot during control period Tc (e.g., time slot T18) as a special time slot for allowing a plurality of modems to coexist by employing frequency division. 
     The following describes an example of a specific operation performed by modem  10  according to the first embodiment with reference to  FIGS. 1, 3, 7 ( a ),  9  and  10 .  FIG. 10  shows a timing chart illustrating exchange of control signals between modems  10 . In this example, modem  10 B 1 , which uses communication method B, outputs synchronization signals. Descriptions are provided for transmission of control signals from modems  10 A 1 ,  10 B 1  and  10 C 1  only, to facilitate understanding of the embodiment. 
     As shown in  FIGS. 7( a ) ,  9  and  10 , modem  10 B 1  outputs, to power lines  2 , synchronization signals SS at time t1. PLC PHY block  42 B of each modem  10  monitors the status of all the time slots, i.e., T12, T13, . . . , T18 during control period Tc; therefore, other modems  10 A 1  and  10 C 1  detect the synchronization signals SS output from modem  10 B 1 . Here, it is assumed that the signal of a video image captured by intercom  109  (see  FIG. 1 ) is transmitted to modem  10 A 1  via LAN cable  9 . Modem  10 A 1  outputs, to power lines  2 , request signals RS at time t2, so as to output the received signal of the video image to display telephone  103  (see  FIG. 1 ) via modem  10 A 2 . Other modems  10 B 1  and  10 C 1  detect the request signals RS output from modem  10 A 1 . The request signal RS and the synchronization signal SS transmitted to modem  10 A 2  are not describes in  FIG. 10 . 
     Modems  10 B 1  and  10 B 2 , which use communication method B, and modems  10 C 1 ,  10 C 2  and  10 C 3 , which use communication method C, do not perform data communication between times t3 and t9, and therefore output no request signal RS as shown in  FIGS. 7( a )  and  9 . Since modem  10 A 1  monitors for a request signal RS in time slots T12, T13, . . . , T18, and detects no request signal RS, modem  10 A 1  performs data communication using the following entire control period Tc (T2). 
     When modem  10 B 1  outputs to power lines  2  synchronization signals SS at time t9, main IC  22  of modem  10 A 1  (see  FIG. 3 ) outputs a communication request signal to sub IC  42  (see  FIG. 3 ). Upon receiving the communication request signal, sub IC  42  transmits a predetermined signal to driver IC  26 , and allows transmitted and received signals to pass. In this state, modem  10 A 1 , as shown in  FIG. 10 , transmits to modem  10 A 2  a data signal DS of the video signal, which has been received from intercom  109 . 
     Upon receiving the data signal DS, modem  10 A 2  transmits an ACK (acknowledgement reply) to modem  10 A 1 . Upon receiving the ACK, modem  10 A 1  transmits a following data signal DS. Modem  10 A 2  transmits the received data signal DS to telephone  103  via LAN cable  9 . As a result, the video image captured by intercom  109  is displayed on the telephone  103  display. As previously described, since data communication is performed in data signal band BW 2 , data communication using in-home communication method A is, as shown in  FIG. 7( a ) , performed in the frequency band of 3-30 MHz during control period Tc (T2). 
     At time t9, it is assumed that the user operates TV  102  (see  FIG. 1 ) to replay motion data, which are stored in server  105  (see  FIG. 1 ). TV  102  then transmits a signal of requesting the motion data to modem  10 B 1  via LAN cable  9 . Upon receiving the signal, modem  10 B 1 , as shown in  FIG. 7( a ) , outputs at time t10 a request signal RS to power lines  2 . During control period Tc (T2) between times t9 and t10, other modems  10  output no request signal RS. As a result, modem  10 B 1  detects no request signal RS from other modems  10 , and therefore performs data communication using the following entire control period Tc (T3). At time t11, modem  10 B 1  outputs a synchronization signal SS, and then transmits a signal of requesting the motion data to server  105  via modem  10 B 2 . Upon receiving the request signal, server  105  transmits a data signal DS of a video signal to modem  10 B 1 , after which the motion picture stored in server  105  is displayed on TV  102 . In other words, data communication using in-home communication method B is performed, as shown in  FIG. 7( a ) , in the frequency band of 3-30 MHz during control period Tc (T3), as with the case of communication method A. 
     Next, it is assumed that PC  101  (see  FIG. 1 ) transmits to an ISP (not shown) a signal of requesting, for instance, HTML (Hyper Text Markup Language) data. Upon receiving the request signal from PC  101 , and detecting a synchronization signal SS output at time t11, modem  10 C 1  outputs a request signal RS to power lines  2  at time t14. Since other modems  10  output no request signal RS, modem  10 C 1  performs data communication using the entire following control period Tc (T4). After modem  10 C 1  transmits a request signal to modem  10 C 3 , modem  10 C 3  requests a Web (World Wide Web) server (not shown) of the ISP to send the HTML data via fiber cable  8  (see  FIG. 1 ). Upon receiving the HTML data, modem  10 C 3  sends the HTML data to PC  101  via modem  10 C 1 , after which the HTML data are displayed on PC  101 . In other words, data communication using access communication method C is performed, as shown in  FIG. 7( a ) , in the frequency band of 3-30 MHz during control period Tc (T4), as with the case of communication methods A and B. 
     At time t20, modem  10 B 1  outputs a synchronization signal SS. During control period Tc (T4), however, none of modems  10  outputs a request signal RS. Therefore, no data communication is performed during control period Tc from time t30. Modem  10 B 1  outputs a synchronization signal SS during each control period Tc. When any modem  10  outputs a request signal RS, one of the modems  10 B 1  performs data communication using the following control period Tc. 
     As described above, in the first embodiment, different phase vectors are used for a synchronization signal SS and a request signal RS. Therefore, each modem  10  can easily detect a request signal RS output from another modem  10  based on a synchronization signal SS without performing relatively cumbersome modulation and other processes. This allows a plurality of types of modems  10  using different communication methods on the common power lines  2  to easily coexist. Particularly, for power line communication that has a great amount of co-relational noise on the time axis, each communication apparatus can perform data communication while avoiding interference between signals. 
     In the above-described first embodiment, descriptions have been provided for the case where the number of time slots is 8 as shown in  FIG. 9 . However, the number does not need to be 8, and can be arbitrary as long as it is 2 or more. Also, descriptions have been provided for the case where each time slot is pre-allocated to its corresponding communication method. However, a corresponding correlation does not need to be predetermined. When a modem is newly installed to the network, for instance, it is possible to monitor the output status of a request signal RS; and, when a vacant time slot is detected (e.g., when a time slot in which no request signal RS is output during a predetermined period is detected), the detected time slot can be used. 
     In the above-described first embodiment, a case has been described where data communication is performed using one communication method during one control period Tc. However, data communication can also be performed using a plurality of communication methods during one control period Tc. 
     Descriptions are provided, with reference to  FIG. 7( b ) , for the case where data communication is performed by employing timed division, using a plurality of communication methods during one control period Tc. Operations between times t1 and t11 in  FIG. 7( b )  are identical to those described in  FIG. 7( a ) , and their descriptions are thus omitted. Modem  10 A 1  outputs a request signal RS at time t12; and, modem  10 B 1  outputs a request signal RS at time t13. Each modem  10  detects, from the request signal RS detected during one control period Tc, the number of communication methods of modems  10  that perform data communication. More specifically, modems  10 A 1  and  10 B 1  detect the request signal RS in time slot T12 corresponding to communication method A (see  FIG. 9 ), and the request signal RS in time slot T13 corresponding to communication method B. On the other hand, modems  10 A 1  and  10 B 1  detect no request signal RS in other time slots T14, T15, . . . , T18. As a result, modems  10 A 1  and  10 B 1  detect that the number of communication methods is two, i.e., communication methods A and B. 
     PLC PHY  22 B of each modem  10  divides, based on the number of communication methods, time domains during control period Tc for data communication. In this example, the order of the divided time domains is set as communication methods A→B. Accordingly, PLC PHY  22 B of modem  10 A 1  sets its time domain so that its data communication is performed between times t20 and t21. On the other hand, PLC PHY  22 B of modem  10 B 1  sets its time domain so that its data communication is performed between times t21 and t30. As a result, data communication using communication method A and data communication using communication method B are performed based on time division during control period Tc (T4) as shown in  FIG. 7( b ) . 
     The following describes, with reference to  FIG. 7( c ) , a case where data communication is performed by employing frequency division, using a plurality of communication methods during one control period Tc. In  FIG. 7( c ) , operations between times t1 and t11 are identical to those described in  FIG. 7( a ) , and their descriptions are thus omitted. Modem  10 B 1  outputs a request signal RS at time t13; and modem  10 C 1  outputs a request signal RS at time t14. On the other hand, during control period Tc (T4), other modems  10  output no request signal RS. As a result, modems  10 B 1  and  10 C 1  detect that the number of communication methods is two, i.e., communication methods B and C. 
     PLC PHY  22 B of each modem  10  divides, based on the number of communication methods, frequency domains during control period Tc for data communication. In this example, the in-home system is set in a high frequency band within data communication band BW 2 ; and the access system is set in a low frequency band within data communication band BW 2 . As a result, PLC PHY  22 B of modem  10 B 1  sets its frequency domain so that its data communication is performed in the high frequency band within data communication band BW 2  via band-pass filters  25  and  29 . PLC PHY  22 B of modem  10 C 1 , on the other hand, sets its frequency domain so that its data communication is performed in the low frequency band within data communication band BW 2  via band-pass filters  25  and  29 . As a result, data communication through communication method B and data communication through communication method C are performed based on frequency division during control period Tc (T4) as shown in  FIG. 7( c ) . As for a system such as the access system having a long transmission line, components in a high frequency band have relatively high attenuation. Therefore, the entire frequency spectrum can be more efficiently used by allocating the access system to a low frequency band. 
     As previously described, at least one of a time domain and a frequency domain for data communication is set based on the number of communication methods, and data communication is performed using the set domain. Therefore, each modem  10  can perform data communication while avoiding interference between data signals. 
     Second Embodiment 
     The second embodiment is described in the following with reference to  FIGS. 1, 2, and 11 through 14 . 
     Communication system  100  according to the second embodiment is identical to that described in the first embodiment, and its descriptions are thus omitted. The communication apparatus according to the second embodiment is the same modem  10  described in the first embodiment, and its description are thus omitted. 
       FIG. 11  is a block diagram illustrating a hardware example that constitutes modem  10  according to the second embodiment. Modem  10 , as shown in  FIG. 11 , lacks sub IC  42 , which is described in  FIG. 3 . Modem  10 , as shown in  FIG. 11 , further lacks AFE IC  43 , band-pass filters  45  and  49 , and driver IC  46  (hereinafter these are referred to as “AFE circuit” that have been described in  FIG. 3 ). In other words, modem  10  has the same components as described in the first embodiment except for the deleted sub IC  42  and AFE circuit, and its descriptions are thus omitted. Main IC  22  of  FIG. 11  also has the function of sub IC  42  of  FIG. 3 . Therefore, PLC PHY block  22 B of main IC  22  has the respective components described in  FIG. 4 , and its descriptions are thus omitted. 
     The following describes an example of a specific operation of modem  10  according to the second embodiment with reference to  FIGS. 11 and 12 .  FIG. 12( a )  is a time chart that employs frequency division; and  FIG. 12( b )  is a time chart that employs frequency and time divisions. 
     First, descriptions are provided for an operation example shown in  FIG. 12( a ) . In this example, the operation is different from that described in the first embodiment. The same frequency band is used as shared frequency band BW 1 , BW 21 , BW 2  for both transmitting a control signal and performing data communication. When the frequency band for performing power line communication is set between 2 and 30 MHz, for instance, the shared frequency band BW 1 , BW 2  is set between 2 and 30 MHz. The shared frequency band BW 1 , BW 2  can be changed to different from the frequency band for use. 
     At time t41, PLC PHY block  22 B of modem  10 B 1  outputs a synchronization signal SS to power lines  2  via band-pass filter  25 , the synchronization signal SS being set in the shared frequency band BW 1 , BW 2 . At time t42, PLC PHY block  22 B of modem  10 A 1  outputs a request signal RS using band-pass filter  25 , as with the synchronization signal SS, the request signal RS being set in the shared frequency band BW 1 , BW 2 . At time t43, PLC PHY block  22 B of modem  10 B 1 , as with modem  10 A 1 , outputs a request signal RS, which is set in the shared frequency band BW 1 , BW 2 . 
     In the second embodiment, as with the first embodiment, a period between two adjacent synchronization signals SS is set as one cycle. As shown in  FIG. 12 , however, one cycle is divided into control period Tc (T21) and its following data period Td. In other words, a control signal and a data signal are time-divided, unlike the first embodiment. Further, as shown in the example shown in  FIG. 12( a ) , data period Td is time-divided into a plurality of data periods T22, T23, T24, . . . . 
     More specifically, modem  10 A 1  performs data communication between times t49 and t50 in the shared frequency band BW 1 , BW 2  during the first data period T22; and modem  10 B 1  performs data communication between times t50 and t51 in the shared frequency band BW 1 , BW 2 . Modem  10 A 1  performs data communication between times t51 and t52 during the second data period T23; and modem  10 B 1  performs data communication between times t52 and t53. Modem  10 A 1  performs data communication between times t53 and t54 during the third data period T24; and modem  10 B 1  performs data communication between times t54 and t55. 
     As described above, in the second embodiment, the same frequency band is used for transmitting a control signal and for performing data communication. Therefore, as described in  FIG. 3  of the first embodiment, sub IC  42  and AFE circuits can be omitted. This configuration makes it possible to avoid a large-scale circuit modification so that a plurality of modems  10  can coexist on the common power lines  2 . 
     Although time division has been described in the above-described second embodiment, frequency division can also be employed. Time division and frequency division can also be combined. A case where both time and frequency division are combined is described in the following with reference to  FIG. 12( b ) . 
     For instance, when each modem  10  detects a request signal RS from only the in-home system during control period Tc, data communication is performed using time division between different communication methods as with  FIG. 12( a ) . Next, as shown in  FIG. 12( b ) , when each modem  10  detects communication methods A, B and C, namely, request signals RS from both in-home and access systems, in-home communication methods A and B perform data communication by employing time division; and access communication method C performs data communication by employing frequency division. In this case, modems  10 A 1  and  10 B 1  using the in-home system perform data communication by narrowing the frequency band of 2-30 MHz used for transmitting control signals to, for instance, the frequency band of 3-30 MHz so that data communication can be achieved in that narrowed frequency band. On the other hand, modem  10 C 1  using the access system performs data communication in the vacant frequency band of 2-3 MHz. In this case, since different frequency bands are used for transmitting control signals and data signals DS, each modem  10  may have the hardware configuration described in  FIG. 3 . 
     In addition,  FIG. 12( b )  is a mere example of a combination of time division and frequency division, and a different combination can also be used. For instance, when there are a plurality of communication methods using the access system, data communication can be performed by using time division among the communication methods using the access system. It is also possible to use time division as a multiple-access method for the in-home and access systems, while using frequency division within each of the in-home and access systems. Further, it is possible to determine whether to use time division or frequency division as its communication method on the basis of which time slot is to be used. 
     Further, in the above-described second embodiment, descriptions have been provided for the case where control signals are all transmitted in the same frequency band. However, it is also possible to use different frequency bands for transmitting different control signals.  FIG. 13  is a time chart illustrating an operation example of a plurality of modems  10 , when different request signals are transmitted. In this case, a control signal using the in-home system uses the frequency band of 2-30 MHz; and a control signal using the access system uses the frequency band of 2-3 MHz. In-home data communication uses the frequency band of 3-30 MHz, which is different from the band used for transmitting control signals. Access data communication, on the other hand, uses the frequency band of 2-3 MHz, which is the same as the band used for transmitting control signals. This way (for the purpose of reducing the circuit size, for instance), a communication method using a narrow frequency band only can prevent the circuit size from being large. 
     In the first and second embodiments described above, a case has been described where all the communication methods are in sync with synchronization signals SS. However, it is also possible not to synchronize some communication methods.  FIG. 14  is a time chart illustrating an operation example of a plurality of modems  10 , when some communication methods are not in sync with synchronization signals. 
     In the  FIG. 14  example, it is necessary to transmit/receive a request signal RS not in sync with a synchronization signal SS. Other communication methods need to detect a carrier of a request signal RS of communication method C, the request signal RS being transmitted/received asynchronous with a synchronization signal SS. When the carrier is detected, it is necessary to narrow the frequency band used for the synchronization signal SS and the request signal RS so that both signals do not interfere with communication method C. A communication method in sync with the synchronization signal SS can recognize which communication method uses power lines  2  in what form in each time slot. 
     It is possible to recognize communication methods asynchronous with each other by receiving asynchronous request signals. However, considering the condition of the transmission line as described in  FIG. 8( b ) , there may be a case where it is impossible to tell whether a request signal RS, which is in a broad band for a communication method (which can be in a receiving mode), appears to be concentrated in a lower frequency band, affected by the characteristics of the transmission line, or the request signal RS is originally set in the lower frequency band only. To prevent this, the phase vector of a request signal RS for synchronous coexistence and the phase vector of a request signal RS for asynchronous coexistence are set differently, so that it becomes possible to recognize whether it is the request signal RS in a broad band or the request signal RS in an originally narrow band. It is still impossible to recognize, through an asynchronous communication method, a communication method in sync with a synchronization signal SS. However, asynchronous communication methods can coexist by employing a coexistent method using frequency division even when a synchronous communication method can not be recognized. 
     Affected by the transmission lines as power lines  2 , even when the request signal RS in the broad band and the request signal RS in the narrow band cannot be differentiated, it has been described that both signals can be differentiated by using different phase vectors. However, it is possible to differentiate both signals by determining whether or not request signals RS are detected synchronously with respect to synchronous and asynchronous types. 
     In the above-described first and second embodiments, a synchronization signal SS can be generated in any form, as long as it is repeatedly output during a predetermined period. For instance, commercial alternating current voltage AC (or current) on power lines  2  can be used to generate a synchronization signal SS. In this case, for instance, a zero cross of the commercial alternating current voltage AC is detected, and a synchronization signal SS (e.g., a pulse waveform made of rectangular waves) is generated using a point where the zero cross is detected as a reference time. When the commercial alternating current voltage AC is 100V, 60 Hz, for instance, a synchronization signal SS is generated with 60 Hz as a reference frequency. In this case, a zero cross circuit, which includes a comparator or the like, and is connected (directly or indirectly) to power lines  2 , can be installed in modem  10  shown in  FIG. 3 or 11 . Average of plurality of reference times representing the zero cross may be used for the reference time. The stable reference time can be set even if the zero cross fluctuates. 
     In the above-described first and second embodiments, descriptions have been provided for the case where modem  10 B 1 , which uses communication method B, outputs a synchronization signal SS. However, it is also possible that modems  10 , which use other communication methods A and C, output a synchronization signal SS as long as at least one modem  10  outputs a synchronization signal SS. Modem  10 , which outputs the synchronization signal SS, can be set in either a fixed or variable mode; further, when the variable mode is selected, its setting can be made either manually or automatically. 
     For fixed setting, for instance, modem  10  using a specific communication method can be set as a default to output a synchronization signal SS. For manual variable setting, the user can provide in model  10  an interface (e.g., a switch) that can control whether or not to output a synchronization signal SS. For automatic variable setting, on the other hand, modem  10  searches for (listens to) a synchronization signal SS (or a request signal) during at least one control period Tc. When a synchronization signal SS is detected, modem  10  itself does not output a synchronization signal SS. On the other hand, when a synchronization signal SS is not detected, modem  10  outputs a synchronization signal SS. This way, priority is given to a synchronization signal SS transmitted from modem  10  that has already performed power line communication on power lines  2 . Accordingly, even when the modem  10  is disconnected from power lines  2 , one of the other modems  10  automatically outputs a synchronization signal SS. 
     In the above-described first and second embodiments, descriptions have been provided for the case where the phase vectors of a synchronization signal SS and a request signal RS are different, but the phase vectors of request signals RS are all identical. However, it is also possible to set different phase vectors for request signals RS depending on each of different communication methods. For instance, when sending a signal of transmission completion (a completion signal), a new different phase vector can be used for the completion signal. This can build a more flexible environment where modems  10  can coexist. In other words, each modem  10  can identify each other even when request signals RS are randomly output (namely, regardless of time slots). This reduces time required for outputting a request signal RS (namely, control period Tc), and improves communication efficiency of the request signal RS. 
     Third Embodiment 
     The third embodiment is described in the following with reference to  FIGS. 15 through 17 . 
     Communication system  100  according to the third embodiment is identical to that described in the first embodiment, and its descriptions are thus omitted. As shown in  FIG. 2 , the communication apparatus according to the third embodiment is identical to modem  10  according to the first embodiment, and its descriptions are thus omitted. 
       FIG. 15  is a block diagram illustrating a hardware example that constitutes modem  10  according to the third embodiment. In the circuit configuration shown in  FIG. 15 , zero cross circuit  63  is provided in modem  10  described in  FIG. 3 . The circuit configuration shown in  FIG. 15  is identical to that described in  FIG. 3  except for zero cross circuit  63 , and PLC PHY block  42 D (described later) of sub IC  42 . Therefore, the same components are assigned the same numbers, and their descriptions are thus omitted. 
     Zero cross circuit  63  includes bridge connection diode  63   a , resistors  63   b  and  63   c . DC power  63   e  and comparator  63   d . Bridge connection diode  63   a  is connected to resistor  63   b ; and the connected resistor  63   b  is connected in series to another resistor  63   c . These two resistors  63   b  and  63   c  are connected parallel to an input terminal on one end, which is provided in comparator  63   d . A plus side of DC power  63   e  is connected to an input terminal on the other end, which is provided in comparator  63   d . PLC MAC block  42 C of sub IC  42  is connected to an output terminal, which is provided in comparator  63   d.    
       FIG. 16  is a functional block diagram of PLC PHY block  42 D of sub IC  42 . PLC PHY block  42 D performs FFT (Fast Fourier Transform) as time-frequency transform. In other words, PLC PHY block  42 D includes FFT transformer  411  and IFFT (Inverse Fourier Transform) transformer  420  instead of wavelet transformer  401  and inverse wavelet transformer  410  as described in  FIG. 4 . In the functional block described in  FIG. 16 , the components common to those of  FIG. 4  are assigned the same numbers, and their descriptions are thus omitted. Time-frequency transform does not need to be FFT transform, but can also be wavelet transform described in the first and second embodiments. 
     The following describes an example of a specific operation of modem  10  according to the third embodiment with reference to  FIGS. 15 through 17 .  FIG. 17  is a time chart illustrating an operation example of a plurality of modems  10  according to the third embodiment. The operation shown in  FIG. 17  is different from that shown in  FIG. 14  only in that synchronization is executed in accordance with commercial alternating current voltage AC, and request signals RS have different phase vectors. In  FIG. 17 , the operations common to those shown in  FIG. 14  are assigned the same numbers, and their descriptions are thus omitted. Commercial alternating current voltage AC shown in  FIG. 17  indicates “voltage” on the vertical scale, for the sake of easy understanding. The following describes a case where commercial alternating current voltage AC is indicated in the time chart, as shown in  FIG. 17 . Further, in  FIG. 17 , 60 Hz is indicated as commercial alternating current voltage AC, but other voltage values, for instance, 50 Hz, can also be used. 
     In this example, each modem  10 A 1 ,  10 A 2 ,  10 B 1 ,  10 B 2 , . . . has its predetermined phase vector set differently, depending on a frequency band used for a request signal RS. Communication methods A and B use the entire frequency band of 2-30 MHz (of 2-30 MHz). Communication method C uses the frequency band of 2-16 MHz (of 2-30 MHz). Arbitrary frequency band can be used for transmitting a request signal RS. 
     Each modem  10  is designed to transmit a request signal RS and perform data communication using as a reference point: a zero cross point (voltage is 0 VAC) of commercial alternating current voltage AC in zero cross circuit  63 . In this case, 2AC cycle is considered as one cycle from the zero cross of the commercial alternating current voltage AC; and time slots for outputting a request signal RS are set, starting at the zero cross, in the order of communication methods A, B and C. 
     At time t42, zero cross circuit  63  of modem  10 A 1  detects the zero cross ZC of the commercial alternating current voltage AC. When the zero cross ZC is detected, controller  405  of PLC PHY block  42 D of modem  10 A 1  retrieves data related to a phase vector from memory  33 . The data related to the phase vector indicates phase vector PV1. More specifically, PV1 includes rotation degree coefficients which are made of two values, i.e., 0 and π, corresponding to each sub-carrier, or phase shift values to cyclically shift the sub-carriers with these coefficients. Phase rotator  408  of PLC PHY block  42 D rotates the phase vector of each of the sub-carriers constituting a multi-carrier signal, by phase vector PV1. IFFT transformer  420  of PLC PHY block  42 D performs IFFT transform on the phase-rotated multi-carrier signal in order to generate a request signal RS. IFFT transformer  420  outputs the generated request signal RS to power lines  2  via AFE IC  43 , band-pass filter  45 , driver IC  46 , coupler  27 , power connector  12  and plug  3 . 
     As with modem  10 A 1 , modem  10 B 1  detects zero cross ZC in zero cross circuit  63  at time t42. When zero cross ZC is detected, controller  405  of PLC PHY block  42 D of modem  10 B 1  retrieves data related to a phase vector from memory  33 . Since communication methods A and B use the same frequency band for transmitting a request signal RS, the data related to the retrieved phase vector indicates phase vector PV1 as with modem  10 A 1 . Phase rotator  408  of PLC PHY block  42 D rotates, based on the retrieved phase-vector-related information, the phase vector of each sub-carrier constituting a multi-carrier signal, by phase vector PV1 as with modem  10 A 1 . IFFT transformer  420  of PLC PHY block  42 D performs IFFT transform on the phase-rotated multi-carrier signal in order to generate a request signal RS. At time t43, IFFT transformer  420  outputs the generated request signal RS to power lines  2 , using the detected zero cross as a reference point, in the time slot set for communication method B. 
     As with modem  10 A 1 , modem  10 C 1  detects a zero cross ZC in zero cross circuit  63  at time t42. Upon detecting the zero cross ZC, controller  405  of PLC PHY block  42 D of modem  10 C 1  retrieves, from memory  33 , data related to a phase vector indicating phase vector PV2, which is different from phase vector PV1, since communication method C uses a frequency band different from communication methods A and B for transmitting a request signal RS. Phase rotator  408  of PLC PHY block  42 D rotates the phase of each sub-carrier constituting a multi-carrier signal, by phase vector PV2, based on the data related to the retrieved phase vector, unlike modems  10 A 1  and  10 B 1 . IFFT transformer  420  of PLC PHY block  42 D performs IFFT transform on the phase-rotated multi-carrier signal in order to generate a request signal RS. At time t44, IFFT transformer  420  outputs the generated request signal RS to power lines  2 , using the detected zero cross as a reference point, in the time slot set for communication method C. 
     The following describes a process of detecting a request signal RS performed by modem  10  with reference to  FIGS. 16 to 18 .  FIG. 18  is a flowchart illustrating a process of detecting a request signal RS. FFT transformer  411  of PLC PHY block  42 D of modem  10  performs FFT transform on a received signal (step S 11 ). Controller  405  of PLC PHY block  42 D retrieves, from memory  33 , data related to phase vector PV1. Phase rotator  402  of PLC PHY block  42 D rotates the phase of each sub-carrier by referring to the data related to phase vector PV1 and multiplying the FFT-transformed received signal by phase vector PV1 (step S 12 ). 
     Controller  405  of PLC PHY block  42 D makes a quadrant determination on the phase rotated sub-carriers (step S 13 ) as specifically described in the following. In this example, it is assumed that 512 sub-carriers are used, and phase vectors on the transmitting and receiving sides are a plurality of coefficients, which indicate rotation degrees (e.g., π, 0, π, π, . . . , 0) corresponding to sub-carrier numbers 1, 2, 3, 4, . . . , 512. 
     A request signal RS includes known transmitted data as known data, such as a preamble. The transmitted data correspond to sub-carrier numbers 1, 2, 3, 4, . . . , 512. Although known transmitted data can be arbitrary, all of the data are set as “1” in this example. “1” represents (1, 0) on the complex coordinate plane. Accordingly, the known data are in the form of 1, 1, 1, 1, . . . , 1, which correspond to sub-carrier numbers 1, 2, 3, 4, . . . , 512. Phase rotator  408  on the transmitting side multiplies the known data 1, 1, 1, 1, . . . , 1 by the phase vectors (π, 0, π, π, . . . 0), and outputs request signals RS having −1, 1, −1, −1, . . . 1 as transmitted data to power lines  2 . 
     Phase rotator  402  on the receiving side respectively multiplies transmitted data −1, 1, −1, −1, . . . , 1 by coefficients (π, 0, π, π, . . . , 0), each of the transmitted data being included in each sub-carrier of the transmitted request signal RS. As a result, known data in the form of transmitted data 1, 1, 1, 1, . . . , 1, are re-rotated. Controller  405  determines whether the transmitted data indicated by the phase-rotated sub-carriers are known data such as a preamble. In this case, controller  405  sums up the transmitted data, and compares with predetermined threshold Th1. For instance, when threshold Th1 is “258” and the transmitted data are presumably correct, integration value SUM is “512 (=1+1+1+1+ . . . +1)”. Therefore, controller  405  determines that integration value SUM has exceeded threshold Th1 (step S 13 : YES). Upon determining that integration value SUM has exceeded threshold Th1, controller  405  determines that a carrier with phase vector PV1 has been detected (step S 14 ), and terminates the process. In other words, the received signal is a multi-carrier signal whose phase vector is PV1. On the other hand, when integration value SUM has not exceeded threshold Th1, controller  405  determines that integration value SUM has not exceeded threshold Th1 (step S 13 : NO). 
     Upon determining that integration value SUM has not exceeded threshold Th1, controller  405  retrieves, from memory  33 , data related to phase vector PV2. Phase rotator  402  of PLC PHY block  42 D multiplies the FFT-transformed received signal by phase vector PV2 and rotates the phase of each sub-carrier (step S 15 ). Controller  405  of PLC PHY block  42 D makes a guardant determination on the phase-rotated sub-carriers (step S 16 ) as with step  13 . Upon determining that integration value SUM has exceeded threshold Th2 (step S 16 : Yes), controller  405  determines that a carrier with phase vector PV2 has been detected (step S 18 ), thereby terminating the process. In other words, the received signal is a multi-carrier signal whose phase vector is PV2. The guardant determination is described in detail later. 
     On the other hand, upon determining that integration value SUM has not exceeded threshold Th2 (step S 16 : No), controller  405  determines that the received signal has neither phase vector PV1 nor PV2 (that it, the signal is a multi-carrier signal whose phase vector is other than PV1 and PV2, or is noise) (step S 17 ), and determines that no carrier with phase vectors PV1 and PV2 has been detected (step S 18 ), thereby terminating the process. It is also possible to perform steps  15  and  16  before steps  12  and  13  in  FIG. 18 . The phase vector does not need to be two types, i.e., PV1 and PV2, but can be three types or more. 
     Here, it is assumed, for instance, that the transmission status of the power line has been deteriorated and a gain in the frequency band of 16-30 MHz has become lower. In this case, request signals RS output from modems  10 A 1  and  10 B 1  suffer a higher S/N ratio of sub-carriers, which are transmitted in the frequency band at or higher than 16 MHz. This makes it difficult to differentiate request signals RS output from modems  10 A 1  and  10 B 1  from request signals RS output from modem  10 C 1 . However, since different phase vectors are set for modems  10 A 1 ,  10 B 1  and  10 C 1 , request signals RS can be smoothly differentiated from each other when each modem  10  performs the above-described process of detecting a request signal RS. 
     As described above, in the third embodiment, different phase vectors are used in accordance with frequency bands used for a request signal RS. As a result, it becomes possible to differentiate request signals RS even when the transmission status of the power line is deteriorated. 
     Fourth Embodiment 
     Communication system  100  according to the fourth embodiment is identical to that described in the first embodiment, and its descriptions are thus omitted. The communication apparatus according to the fourth embodiment is identical to modem  10  according to the first embodiment as shown in  FIG. 2 , and its descriptions are thus omitted. The circuit configuration of modem  10  according to the fourth embodiment is identical to that of  FIGS. 15 and 16 , and its descriptions are thus omitted. 
     The following describes an example of a specific operation of modem  10  according to the fourth embodiment with reference to  FIGS. 19 and 20 .  FIG. 19  shows time slots corresponding to request signals according to the fourth embodiment; and  FIG. 20  is a flowchart illustrating a process of detecting a request signal according to the fourth embodiment.  FIG. 19  has extended control period Tc shown in  FIG. 17 . In the fourth embodiment, which differs from the third embodiment, different phase vectors are set for respective time slots T11, T12, . . . , T17. It is also possible that different phase vectors are used for different frequency bands for use and for different time slots. The number of time slots is arbitrary as long as it is two or more. 
     Detailed descriptions are provided in the following. It is assumed that various electric appliances (not shown) are respectively connected to outlets  5 , to which modems  10 A 1  and  10 B 1  are connected. In this case, affected by the electric appliances (e.g., impedance variation), commercial alternating current voltage AC2 at outlets  5 , to which modems  10 A 1  and  10 B 1  are connected, incurs a time-lag from commercial alternating current voltage AC1 at outlets  5 , to which other modems  10 C 1 , . . . are connected.  FIG. 19( a )  shows a waveform of commercial alternating current voltage AC1 at the outlets, to which other modems  10 C 1  . . . are connected, while  FIG. 19 ( b )  shows a waveform of commercial alternating current voltage AC2 at the outlets, to which modems  10 A 1  and  10 B 1  are connected. Commercial alternating current voltage AC2, as shown in  FIGS. 19( a ) and ( b ) , is delayed by time TD compared to commercial alternating current voltage AC1. 
     In this case, when modem  10 A 1  outputs a request signal RSa, zero cross circuit  63  detects a zero cross ZC of commercial alternating current voltage AC2. Commercial alternating current voltage AC2 is delayed only by time TD compared to commercial alternating current voltage AC1. Therefore, modem  10 A 1  outputs a request signal RSa at time t421, which is delayed only by time TD from time t42. 
     When modem  10 B 1  outputs a request signal RSb, zero cross circuit  63  detects at time t421 a zero cross ZC of commercial alternating current voltage AC2 as with modem  10 A 1 . Upon detecting zero cross ZC, modem  10 B 1  outputs a request signal RSb at time t431, which is delayed only by time TD from time t43. 
     At this stage, modem  10 C 1  has performed a process of detecting a request signal RS as shown in  FIG. 20 , and detects the request signals RSa and RSb. The following describes a carrier detection process in time slot T12 with reference to  FIG. 20 . 
     FFT transformer  411  of PLC PHY block  42 D of modem  10 C 1  performs FFT transform on a received signal (step S 21 ). Next, PLC PHY block  42 D retrieves, from memory  33 , data related to a phase vector as slot data corresponding to time slot T12. Memory  33  stores data related to different phase vectors corresponding to time slots T11, T12, T13, . . . . In this example, phase vector PV1 is set for communication method A; and phase vector PV2 is set for communication method B. Memory  33  stores the data related to phase vectors PV1 and PV2 corresponding to time slots T11 and T12, respectively. 
     PLC PHY block  42 D outputs the current slot data in zero cross circuit  63  (step S 22 ). More specifically, modem  10 C 1  recognizes, from commercial alternating current voltage AC1 in zero cross circuit  63 , that a zero cross ZC is at time t42. Each modem  10  includes a counter (not shown) and stores data indicating the time durations of the time slots. Therefore, each modem  10  can specify how many time slots exist between the current time slot and the zero cross ZC by both the elapsed time from zero cross ZC and time width of the time slot. 
     At time t43, for instance, PLC PHY block  42 D of modem  10 C 1  recognizes that an elapsed time from the zero cross ZC is a time duration per time slot, and determines that the current time slot is “T12”. As a result, controller  405  of PLC PHY block  42 D retrieves, from memory  33 , the data related to phase vector PV2 corresponding to time slot T12. 
     Then, phase rotator  402  of PLC PHY block  42 D multiplies the FFT-transformed received signal by phase vector PV2, so as to rotate the phase of each sub-carrier (step S 23 ). Phase rotator  405  of PLC PHY block  42 D makes a quadrant determination on each of the phase-rotated sub-carriers (step S 24 ) as with steps  13  and  15  described in  FIG. 18 . Steps S 25  and S 26  are identical to steps S 14  (or S 17 ) and S 18 , and their descriptions are thus omitted. 
     In time slot T12, the phase vectors of the two request signals RSa and RSb are output as shown in  FIG. 19( a ) . As described above, however, modem  10 C 1  rotates the phases of the sub-carriers by phase vector PV2, and thus only detects the request signal RSb. 
     As described above, in the fourth embodiment, each modem  10  rotates the phases of the sub-carriers of the request signal RS output in the time slot by the phase vector corresponding to the time slot. This enables a reliable detection of request signals RS output in each time slot, even when there is a time difference between alternating current voltages ACs. 
     In the above-described fourth embodiment, descriptions have been provided for the case where different phase vectors are set for time slots T11, T12, . . . , T17. However, it is not necessary to set different phase vectors for respective time slots. Phase vectors can be reliably differentiated when phase vectors having different rotation degrees (e.g., PV1 and PV2) are set at least for adjacent time slots (e.g., T11 and T12). 
     Fifth Embodiment 
     Communication system  100  according to the fifth embodiment is identical to that described in the first embodiment, and its descriptions are thus omitted. The communication apparatus according to the fifth embodiment is modem  10  described in the first embodiment, and its descriptions are thus omitted. The circuit configuration of modem  10  according to the fifth embodiment is identical to that of  FIGS. 15 and 16 , and its descriptions are thus omitted. 
     The following describes an example of a specific operation of modem  10  according to the fifth embodiment with reference to  FIGS. 21 and 22 .  FIG. 21  is a time chart illustrating an operation example of a plurality of modems  10  according to the fifth embodiment.  FIG. 22  is a flowchart illustrating a process of modifying a phase vector according to the fifth embodiment. The process of detecting a request signal RS is identical to that described with reference to  FIG. 20  in the fourth embodiment. 
     The following describes a phase vector modification process performed by modem  10 A 1 . Modem  10 A 1  searches for a request signal RS during control period Tc (step S 31 ). For instance, it is assumed that controller  405  (see  FIG. 16 ) of PLC PHY block  42 D of modem  10 A 1  detects a zero cross ZC in zero cross circuit  63  (see  FIG. 15 ) at time t81 shown in  FIG. 21 . Controller  405  determines whether or not the request signal RS is output between times t81 and t82. The carrier detection method is identical to that described in  FIG. 18 , and its descriptions are thus omitted. 
     In the fifth embodiment, each time slot during control period Tc is allocated to communication methods in the order of “C”, “A” and “B”. When data communication is performed through communication methods A, B, . . . , data period Td is time-divided into communication methods A, B, . . . . When data communication is performed through communication methods A, B, . . . and C, a frequency band of 16-30 MHz is allocated to communication methods A, B, . . . , and a frequency band of 2-16 MHz is allocated to communication method C, thus dividing the frequency band used for power line communication. Memory  33  of each modem  10  stores data including these time slot allocations and which multiple-access scheme is employed when which request signal RS is output. 
     Modem  10 A 1  determines whether or not a desired channel has a vacancy (step S 32 ). A channel only needs to be at least one of time and frequency bands, and a frequency band is used in this example. When modem  10 A 1  wishes to use the frequency band of 2-30 MHz and when no request signal RS is output between times t81 and t82, controller  405  of PLC PHY block  42 D of modem  10 A 1  determines that the desired channel has a vacancy (step S 32 :Yes), since communication method C does not perform data communication during the following data period Td (between times t84 and t86), and terminates the process. 
     Accordingly, modem  10 A 1  performs data communication using the frequency band of 2-30 MHz without performing a phase vector modification process at time t84. In this case, since modem  10 B 1  outputs a request signal RS at time t83, modem  10 A 1  detects the request signal RS output from modem  10 B 1 ; and modems  10 A 1  and  10 B 1  alternately perform data communication during data period Td. 
     Further, in  FIG. 21 , the time durations of control period Tc and data period Td are equal to two cycles of commercial alternating current voltage AC. However, this is arbitrary as long as it is over ⅙ cycle of commercial alternating current voltage AC. Particularly, it is preferable that ½ cycle be used for a single-phase; and ⅙ or more cycle be used for three-phases. This is because it eliminates the need to determine whether commercial alternating current voltage AC is increased or decreased even when the waveform of the commercial alternating current voltage AC is inverted by an inverted insertion direction of a pair of plug terminals. 
     Time durations do not need to be equally divided for the data division of data communication. For instance, one of the time durations can be longer than the others. Although, in  FIG. 21 , data communication are performed three times for one communication method during one data period Td, the number of performing data communication is arbitrary. 
     At time t86, modem  10 A 1  starts the process described in  FIG. 22 , and again searches for a request signal RS (step S 31 ). At the same time, modem  10 A 1  determines whether or not a desired channel (frequency band) has a vacancy (step S 32 ). Controller  405  of PLC PHY block  42 D of modem  10 A 1  determines whether or not a request signal RS is output between time t86 and t87. As shown in  FIG. 21 , since modem  10 C 1  outputs a request signal RS, controller  405  determines that the desired channel has no vacancy since communication method C performs data communication during the following data period Td between times t84 and t86 (step S 32 : No). 
     Controller  405  of PLC PHY block  42 D of modem  10 A 1  modifies the phase vector corresponding to the channel (frequency band) (step S 32 ). In this example, memory  33  stores the data related to phase vector PV1, which corresponds to the frequency band of 2-30 MHz, and the data related to phase vector PV2, which corresponds to the frequency band of 16-30 MHz. Further, phase vector PV1 is set for modem  10 A 1  as a phase vector between times t81 and t87. 
     Communication method C performs data communication (since the frequency band of 2-16 MHz cannot be used) during the following data period Td (between times t86 and t89), controller  405  of PLC PHY block  42 D of modem  10 A 1  retrieves, from memory  33 , the data related to the phase vector corresponding to the frequency band of 16-30 MHz. In other words, controller  405  retrieves, from memory  33 , the data related to phase vector PV2; and phase rotator  408  of PLC PHY block  42 D of modem  10 A 1  modifies the phase vector to PV2 (step S 32 ). The phase vector modification process has been described in detail in the forth embodiment, and its descriptions are thus omitted. 
     Upon changing the phase vector, I FFT transformer  420  of PLC PHY block  42 D of modem  10 A 1  performs IFFT transform on the sub-carriers whose phase vectors are rotated using PV2, so as to generate a transmitted signal. PLC PHY block  42 D of modem  10 A 1  shuts off the frequency band of 2-16 MHz from the transmitted signal by controlling band-pass filter  45 . The transmitted signal in the frequency band of 16-30 MHz is output as a request signal RS to power lines  2  via driver IC  46 , coupler  27 , power connector  12  and plug  3 . Modem  10 A 1  outputs the request signal RS between times t87 and time t88 (step S 33 ) and terminates the process. Modem  10 B 1  performs the same process, whose descriptions are thus omitted. Accordingly, during data period Td starting at time t89, modem  10 C 1  performs data communication in the frequency band of 2-16 MHz; and modems  10 A 1  and  10 B 1  perform data communication in the frequency band of 16-30 MHz. 
     Since modem  10 A 1  modifies a phase vector according to a frequency band for a request signal RS, other modems  10 B 1 ,  10 C 1 , . . . can easily specify the frequency band used for the request signal RS even when the status of the transmission line is deteriorated. The same effects can be obtained when any other modem  10  differentiates the request signal RS. 
     As described above, in the fifth embodiment, a phase vector is modified according to a frequency band used for a request signal RS. Therefore, the frequency band used for the request signal RS can be smoothly specified despite changes of the transmission line status. As a result, a phase vector can be smoothly recognized even when the condition of the transmission line is deteriorated. 
     In the above-described third to fifth embodiments, descriptions have been provided for the case where a request signal RS is output at a timing relative to a zero cross as a reference point. However, such a timing does not need to be referenced to a zero cross. For instance, a timing can be arbitrary referenced as long as it is where commercial alternating current voltage AC reaches a predetermined voltage value (e.g., 10V) and it starts at the detected time point. 
     In the above-described first to fifth embodiments, descriptions have been provided for a power line as an example of a transmission line that performs transmission of a control signal and data communication. However, a line other than a power line can also be used. For instance, both wireless and wired cables can also be used as transmission lines. For a wired transmission line, for instance, various cables such as a coaxial cable, a telephone line and a speaker line can be used. 
     In the above-described first to fifth embodiments, a phase vector modification has been referred to as “rotating the phase of a sub-carrier”. This is same as rotating a signal point on the complex coordinate plane. In addition, “phase vector” defined in the specification is a set of values indicating a rotation degree by which the signal point of each sub-carrier is rotated on the complex coordinate plane, each sub-carrier constituting a multi-carrier signal such as an OFDM signal. “Phase vector” is therefore a combination of values for equalizing time waveforms of the multi-carrier signal (suppressing a peak on the time axis). A phase vector has two types, i.e., a fixed value, which is a combination of predetermined values, and a variable value, which is a combination of varied values according to predetermined conditions. Such predetermined conditions include a cyclic shift and a random value. In addition, a phase vector is also referred to as a “carrier phase”. In this case, a fixed value is referred to as a “deterministic carrier phase”; and a variable value is referred to as a “random carrier phase”. The above-described request signal RS is also referred to as a CDFC (Commonly Distributed Coordination Function) signal. 
     The above-described first through fifth embodiments are individually described. However, these embodiments can also be combined as needed. 
     The communication apparatus and the communication method according to the present invention are useful for power line communication particularly in collective housings such as an apartment and a condominium because of its abilities to communicate while avoiding interference between signals when a plurality of communication apparatuses using different communication methods are connected to a common transmission line. 
     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. 
     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. 
     This application is based on the Japanese Patent Application Nos. 2005-297529 filed on Oct. 12, 2005, and 2006-114191 filed on Apr. 18, 2006, entire contents of which are expressly incorporated by reference herein.