Abstract:
A radio communication apparatus connected to a device including a digital signal processing unit generating a clock signal, the apparatus includes an acquisition unit acquiring frequency information concerning the clock signal from the digital signal processing unit, a first measurement unit measuring a signal power in a first frequency band, a comparison unit comparing the signal power with a threshold, a first selection unit selecting, from the first frequency band, a second frequency band necessary for data communication, a bandwidth of the first frequency band whose signal power is lower than the threshold being more than a bandwidth of the second frequency band, a second selection unit selecting an optimum communication scheme from a plurality of communication schemes of the data communication according to the frequency information, and a communication unit using the optimum communication scheme to perform the data communication in the second frequency band.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2007-112255, filed Apr. 20, 2007, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to radio communication apparatus and system involved in a cognitive radio communication system in which a communication scheme is changed according to an environment. 
     2. Description of the Related Art 
     Conventionally, in order to solve a problem that receiving performance of the radio communication apparatus is decreased due to a noise generated by a digital signal processing unit included in the radio communication apparatus, for example, JP-A 2006-191315 (KOKAI) discloses a technique in which a clock frequency supplied in digital signal processing is changed such that the noise generated by the digital signal processing device does not overlap a receiving frequency of the radio communication, and JP-A H9-289527 (KOKAI) discloses a technique in which the clock frequency is temporally changed to perform FM modulation to a clock signal, thereby suppressing an interference signal generated by spread. 
     However, the noise generated by digital signal processing unit or electronic device is frequently observed in a frequency band used in the communication. Recently, frequency resources have become tight because various kinds of communication are performed. Therefore, the clock is hardly changed in the digital signal processing so as to avoid all the bands used in the communication. Additionally, from the standpoints of circuit scale and cost, it is impractical that a circuit (such as PLL and a control circuit) for changing the clock frequency of the digital signal processing is mounted on all the digital signal processing circuits in introducing the above-described conventional techniques. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with an aspect of the invention, there is provided a radio communication apparatus connected to a device including a digital signal processing unit generating a clock signal, the apparatus comprising: an acquisition unit configured to acquire frequency information concerning the clock signal from the digital signal processing unit; a first measurement unit configured to measure a signal power in a first frequency band; a comparison unit configured to compare the signal power with a threshold; a first selection unit configured to select, from the first frequency band, a second frequency band necessary for data communication, a bandwidth of the first frequency band whose signal power is lower than the threshold being more than a bandwidth of the second frequency band; a second selection unit configured to select an optimum communication scheme from a plurality of communication schemes of the data communication according to the frequency information; and a communication unit configured to use the optimum communication scheme to perform the data communication in the second frequency band. 
     In accordance with an another aspect of the invention, there is provided a radio communication system comprising a first radio communication apparatus connected to a first device including a first digital signal processing unit generating a first clock signal and a second radio communication apparatus connected to a second device including a second digital signal processing unit generating a second clock signal, 
     the first radio communication apparatus comprising: an acquisition unit configured to acquire first frequency information on the first clock signal; a first measurement unit configured to measure a first signal power in a first frequency band; a first comparison unit configured to compare the first signal power with a first threshold; a first determination unit configured to determine that data communication be started when a bandwidth of a second frequency band whose first signal power is lower than the first threshold is more than a bandwidth of a frequency band necessary for the data communication; and a first transmission unit configured to transmit a communication request signal to the second radio communication apparatus to start the data communication, 
     the second radio communication apparatus comprising: a first receiving unit configured to receive the communication request signal; and a second transmission unit configured to transmit a response signal corresponding to the communication request signal, 
     the first radio communication apparatus further comprising: a first selection unit configured to select, from the second frequency band, a third frequency band necessary for data communication; a second selection unit configured to select an optimum communication scheme from a plurality of communication schemes of the data communication according to the first frequency information; and a third transmission unit configured to transmit first information including the third frequency band and the optimum communication scheme to the second radio communication apparatus, 
     the second radio communication apparatus further comprising: a second receiving unit configured to receive the first information; a second measurement configured to measure a second signal power in the third frequency band; a second comparison unit configured to compare the second signal power with a second threshold; a second determination unit configured to determine that the data communication is performed while the third frequency band and the optimum communication scheme are set to a determined frequency band and a determined communication scheme respectively, when the second signal power is lower than the second threshold in the third frequency band; and a fourth transmission unit configured to transmit second information including the determined frequency band and the determined communication scheme to the first radio communication apparatus, 
     the first radio communication apparatus further comprising: a third receiving unit configured to receive the second information; and a first start unit configured to start communication with the second radio communication apparatus by the determined frequency band and the determined communication scheme, and 
     the second radio communication apparatus further comprising a second start unit configured to start communication with the first radio communication apparatus by the determined frequency band and the determined communication scheme. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a block diagram showing a radio communication system according to an embodiment; 
         FIG. 2  shows a frequency characteristic actually measured near PC in a range of 3 GHz to 4 GHz by a spectrum analyzer; 
         FIG. 3  is a block diagram showing a configuration of PC of  FIG. 1 ; 
         FIG. 4  shows a relationship between a time and a frequency of a clock signal which is SSC used in digital signal processing; 
         FIG. 5  is a flowchart showing processing performed in each radio communication apparatus and processing between the radio communication apparatus in the radio communication system; 
         FIG. 6  is a block diagram showing a radio communication module of  FIG. 1 ; 
         FIG. 7  is a flowchart showing an operation performed by a communication scheme and communication frequency selection processing unit of  FIG. 6 ; 
         FIG. 8  is a frequency-power chart showing an example of result in Step S 701  of  FIG. 7 ; 
         FIG. 9  is a frequency-power chart showing an example of result in Step S 703  of  FIG. 7 ; 
         FIG. 10  is a frequency-power chart showing an example in the case where result in Step S 702  of  FIG. 7  is affirmative; 
         FIG. 11  is a frequency-power chart showing an example of result in Step S 708  of  FIG. 7 ; 
         FIG. 12  is a frequency-power chart showing an example of result in Step S 710  of  FIG. 7 ; 
         FIG. 13  is a frequency-power chart showing an example of result in Step S 712  of  FIG. 7 ; and 
         FIG. 14  is a frequency-power chart showing an example of result in Step S 713  of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Radio communication apparatus and system according to an exemplary embodiment of the invention will be described below with reference to the drawings. In the following embodiments, the same operation is performed in a component designated by the same numeral, and the description is omitted. 
     A radio communication system according to an embodiment will be described with reference to  FIG. 1 . 
     The radio communication system of the embodiment includes a radio communication apparatus having Personal Computer (PC)  101 , a radio communication module  102 , and an antenna  103  and a radio communication apparatus having an antenna  104 , a radio communication module  105 , and a protocol processing unit  106 . The radio communication module  105  and the protocol processing unit  106  are referred to as an access point  107 . 
     The radio communication module  102  is connected to PC  101  through an interface such as a Peripheral Component Interconnect (PCI) bus and Universal Serial Bus (USB). Alternatively, using a dedicated interface in LSI which is a component in a communication device such as the access point  107 , the radio communication module  102  may be connected to the protocol processing unit  106 . The radio communication modules  102  and  105  select a center frequency and a communication scheme suitable to the communication in response to a communication request from PC  101  or the protocol processing unit  106 , and the radio communication modules  102  and  105  perform communication through the antenna  103  and  104  using the selected center frequency and communication scheme. 
     In the configuration of  FIG. 1 , the radio communication module  102  possibly receives a noise signal generated by a clock of PC  101  through the antenna  103 . Similarly, the radio communication module  105  possibly receives a noise signal generated by a clock of the protocol processing unit  106  through the antenna  104 . 
     Referring to  FIG. 2 , a signal near PC will be described as an example of the noise generated by the digital signal processing unit or electronic device.  FIG. 2  shows the state of the signal near PC, which is actually measured in the range of 3 GHz to 4 GHz with a spectrum analyzer. As can be seen from  FIG. 2 , many signal-like noises are generated in the band. The noises such as a whisker-like signal in which the power is sharply projected at a certain frequency and a signal in which the power is projected into a trapezoidal shape over a certain frequency band are observed in  FIG. 2 . 
     A configuration of PC  101  will be described with reference to  FIG. 3 . 
     PC  101  includes external modules such as an image processing module, a Local Area Network (LAN) adaptor module, Hard Disk Drive (HDD), Basic Input Output System (BIOS), and a radio communication module. PC  101  includes CPU (for example, two CPUs  1  and  2 ), AGPIF, a bridge circuit, a memory, a PCI slot, and an ISA slot. 
     The external module is connected to each CPU and the memory through a host bus, a PCI bus, an Industrial Standard Architecture (ISA) bus, various Interface (IF) circuits, and a bridge circuit. Generally each module has an independent operating clock frequency. 
     An example of a frequency characteristic of the clock signal used in digital signal processing (for example, used in PC) will be described with reference to  FIG. 4 . 
     In the digital signal processing, generally the clock frequency is kept constant. In such cases, a strong clock noise is generated in a particular frequency. It is believed that the strong whisker-like signal observed in  FIG. 2  is generated by a clock having a constant frequency. 
     On the other hand, sometimes the clock frequency is spread by performing clock modulation in which the clock frequency is temporally moved as shown in  FIG. 4  such that the strong noise signal is not generated at a certain frequency. This is called spread spectrum clock (SSC). The features of SSC can be expressed by three parameters, i.e., a center frequency fc, a frequency modulation width fw, and a modulation frequency fm. The center frequency fc is equal to the operating frequency of the clock. The frequency modulation width fw generally has megas hertz, and the modulation frequency fm has kilos hertz. The frequency modulation width fw and the modulation frequency fm depend on a spread scheme. As shown in  FIG. 4 , the parameters in an N-order (N is a natural number) high harmonic frequency at which the clock is generated are expressed by Nfc, Nfw, and fm respectively. 
     The frequency modulation width can be expressed by fw=0 in the clock to which SSC is not performed. A thin spectrum having a peak can be observed at the center frequency fc when the clock (fw=0) to which SSC is not performed is operated at the frequency fc. A spectrum having the frequency modulation width fw can be observed at the center frequency fc in the clock (fw=0) to which SSC is performed. These spectra become interference signals for the radio communication module and possibly deteriorate receiving performance of the radio communication module. The interference signal corresponding to the thin spectrum having the peak at the frequency fc is related to the whisker-like signal shown in  FIG. 2 , and the interference signal corresponding to the spectrum having the width fw is related to the trapezoidal signal shown in  FIG. 2 . 
     Examples of the processing performed in each radio communication apparatus and the processing between the radio communication apparatus in the radio communication system of  FIG. 1  will be described with reference to  FIG. 5 . 
     The data communication can be performed between PC  101  and the radio communication module  102  through USB IF, and the data communication can be performed between the radio communication module  105  and the protocol processing unit  106  through a dedicated IF. The radio communication module  102  and the radio communication module  105  are synchronized with each other using a synchronization signal. Using a control signal channel, a control signal can be transmitted and received at a low rate between the radio communication module  102  and the radio communication module  105  while interference with another communication system present in the neighborhood is kept at a minimum level. 
     Examples of the control signal include a communication start request signal, a data communication band notification signal, a data communication scheme notification signal, and a response notification signal. Examples of the method for performing communication at a low rate while the interference with another communication system present in the neighborhood is kept at a minimum level include a method for performing communication using a narrow frequency band dedicated to the control signal channel and a method for performing communication using the large spread like UWB. 
     The radio communication module  102  transmits a clock information request signal to PC  101  (Step S 501 ). In response to the clock information request signal, PC  101  notifies the radio communication module  102  of clock information indicating fc, fw, and fm of the module performing the digital signal processing (Step S 502 ). As shown in  FIG. 3 , many modules performing the digital signal processing are present in PC  101 . A frequency generated by a quartz crystal is present in addition to the clock frequency generated in PLL (Phase Lock Loop). Therefore, there are plural sets of fc, fw, and fm of which PC  101  notifies the radio communication module  102  at Step S 502 . 
     Examples of the method for collecting fc, fw, and fm include a method in which each module notifies Operating System (OS) of frequency information through a driver to transmit the frequency information to the radio communication module connected through USB IF and a method in which BIOS collectively manages fc, fw, and fm of each module in starting up PC and transmits information on fc, fw, and fm to the radio communication module in response to the request from the radio communication module. In the module such as CPU in which the operating frequency is changed according to a processing amount, heat, and remaining battery power, even if the request is not made from the radio communication module  102 , the side of PC  101  may become a master to notify the radio communication module  102  of fc, fw, and fm each time the operating frequency is changed. 
     On the other hand, the other radio communication apparatus performs the same processing (Steps S 503  and S 504 ). An example different from Steps S 501  and S 502  will be described below. When the radio communication module  105  notifies the protocol processing unit  106  of the clock information request signal (Step S 503 ), and when the protocol processing unit  106  notifies the radio communication module  105  of only the center frequency fc of the operating frequency (Step S 504 ), the radio communication module performs sensing to estimate fw and fm while the data communication is not performed. The estimation of fw and fm can be performed by power analysis having frequency resolution finer than that of fw in a period shorter than 1/(2×fm). These pieces of processing may be performed in Steps S 501  and S 502 . 
     The radio communication module may possess information on fc, fw, and fm concerning the clock of the digital signal processing unit included in the radio communication module of itself. Thus, the radio communication module possesses the information on fc, fw, and fm concerning each of the clock group which the modules use for the digital signal processing. 
     When the radio communication apparatus including PC  101  performs communication with the radio communication apparatus including the access point  107 , PC  101  notifies the radio communication module  102  of the communication request signal (Step S 505 ). The radio communication module  102  receiving the communication request performs sensing in the frequency band (main band) which becomes a communication candidate (Step S 506 ). At this point, for example, the main band is 1 GHz, and the sensing has the frequency resolution of 1 MHz. In the sensing, receiving signal power is measured in each frequency to investigate the clock noise generated by the clock. Using the sensing result and the clock information, the radio communication module  102  determines whether the communication is enabled or disabled (Step S 507 ). The determination will be described later with reference to  FIG. 7 . It is assumed that the radio communication module  102  determines that the communication is enabled. When the radio communication module  102  determines that the communication is disabled, the radio communication module  102  notifies the other radio communication apparatus of a communication disable notification signal, and the radio communication module  102  performs the processing such that the communication is performed in a different frequency band. 
     When the radio communication module  102  determines that the communication is enabled in Step S 507 , the radio communication module  102  notifies the radio communication module  105  of the communication request signal using the control signal channel (Step S 508 ). The radio communication module  105  receives the communication request signal, and the radio communication module  105  notifies the protocol processing unit  106  that the radio communication module  105  has received the communication request signal from PC  101  (Step S 509 ). The radio communication module  105  notifies the radio communication module  102  that the radio communication module  105  has received the communication request signal by sending a communication request response signal to the radio communication module  102  through the control signal channel (Step S 510 ). The radio communication module  102  receives the communication request response signal to perform processing for selecting the communication band and communication scheme candidates (Step S 511 ). The communication band and communication scheme candidate selection processing will be described later with reference to  FIG. 7 . As used herein, the communication band shall mean a frequency band in which data signal communication is performed, and the communication scheme shall mean a communication scheme for performing the data signal communication. In a communication frequency band in which the data signal communication is performed, it is necessary to enhance a transmission rate unlike the control signal communication. Therefore, because generally the communication with the high power is required in the wide band, there is a high risk of the interference with other systems. Accordingly, in the communication frequency band, it is necessary that an available band be selected more carefully than the control signal channel. In the communication scheme, single carrier communication and multi-carrier communication are distinguished from each other, and the modulation scheme (such as QPSK and 16QAM) is selected. As used herein, the multi-carrier communication shall mean a communication scheme in which carriers are arranged at frequencies such as a communication scheme in which an orthogonal frequency is used like OFDM and a scheme in which the frequency is multiplexed in the single carrier communication. When the communication performed between the radio communication module  102  and the radio communication module  105  is Frequency Division Duplex (FDD), the communication band and communication scheme candidates are selected. 
     The radio communication module  102  notifies the radio communication module  105  of the communication band and communication scheme candidates selected by the radio communication module  102  through the control channel (Step S 512 ). Similarly to the radio communication module  102 , the radio communication module  105  selects the communication band and communication scheme candidates (Step S 513 ). The radio communication module  105  compares the communication band and communication scheme candidates selected by the radio communication module  105  with the communication band and communication scheme candidates selected by the radio communication module  102 , and the radio communication module  105  makes a determination of the communication band and communication scheme (Step S 514 ). In the case of Time Division Duplex (TDD), the radio communication module  105  performs only the sensing to investigate whether or not the communication band is available in Step S 513 , and the radio communication module  105  makes the determination of the communication band in Step S 514  when the communication band is available. 
     The radio communication module  105  notifies radio communication module  102  of the determined communication band and communication scheme through the control channel (Step S 515 ), and the data communication between the radio communication apparatus is started using the determined communication band and communication scheme (Step S 516 ). 
     The radio communication modules  102  and  105  of  FIG. 1  will be described with reference to  FIG. 6 . In  FIG. 6 , the radio communication modules  102  and  105  are connected to the host using USB IF  601 . PCI IF is used in the case where the radio communication modules  102  and  105  are connected to the host through the PCI bus. The dedicated IF may be used in the case where radio communication module  105  is directly connected to upper layer processing. 
     The radio communication modules  102  and  105  have the configurations shown in  FIG. 6 . 
     USB IF  601  performs communication with an MAC processing unit  602 . In addition to the transmitted and received data signals, contents of the communication include the information on fc, fw, and fm concerning the clock of the digital signal processing unit used on the PC side and control signals such as the communication start request signal, and a data rate specification signal. 
     The MAC processing unit  602  adjusts the data rate. The MAC processing unit  602  supplies information to a communication scheme and communication frequency selection processing unit  619  and a sensing frequency selection processing unit  621 . The MAC processing unit  602  stores the results transmitted from the communication scheme and communication frequency selection processing unit  619  and sensing frequency selection processing unit  621 , and the MAC processing unit  602  stores fc, fw, and fm transmitted from the PC side. The MAC processing unit  602  also performs data error check and re-transmission control. 
     The communication data supplied from the MAC processing unit  602  is delivered from the antenna  622  through an error correction coding processing unit  603 , a modulation processing unit  604 , a mapping processing unit  605 , a frequency conversion processing unit  606 , an up-sample processing unit  607 , Low-Pass Filter (LPF)  608 , and an intermediate frequency and radio frequency (IF/RF) circuit  609 . 
     The error correction coding processing unit  603  performs interleaving after coding processing, thereby randomizing data arrangement. 
     The modulation processing unit  604  modulates the data randomized by the error correction coding processing unit  603 . 
     The mapping processing unit  605  performs mapping of data and a pilot signal according to a predetermined format. In the case where the multi-carrier communication is performed, the mapping processing unit  605  determines which frequency is allocated to the modulated data. 
     The frequency conversion processing unit  606  switches the carrier frequency to an arbitrary frequency in the main band according to an instruction of the communication scheme and communication frequency selection processing unit  619 . In the case where the multi-carrier communication with OFDM is selected, the frequency conversion processing unit  606  performs Inversed Discrete Fourier Transform (IDFT) processing and guard interval addition processing. 
     Because the up-sample processing unit  607 , LPF  608 , and the IF/RF circuit  609  are operated in the same manner as the conventional radio communication apparatus, the description is omitted. 
     In the signal received from the antenna  622 , the MAC processing unit  602  is notified of the receiving data through an intermediate frequency and radio frequency circuit  610 , LPF  611 , a synchronization processing unit  612 , a down-sample processing unit  613 , a frequency conversion processing unit  614 , a demapping processing unit  615 , a channel response estimation processing unit  617 , a demodulation processing unit  616 , and an error correction decoding processing unit  618 . 
     The frequency conversion processing unit  614  converts a data communication carrier present in the main band into a base band signal according to an instruction of the communication scheme and communication frequency selection processing unit  619 . In the case where the multi-carrier communication is being performed with OFDM, the frequency conversion processing unit  614  performs a guard interval removal processing and Discrete Fourier Transform (DFT) processing. 
     The demapping processing unit  615  divides the data and the pilot signal according to a predetermined frame format. 
     The error correction decoding processing unit  618  performs reversal processing of the interleaving performed in the transmission system before error correction decoding is performed, thereby returning the data arrangement to the original order. 
     The communication scheme and communication frequency selection processing unit  619  sets parameters in the pieces of processing from the error correction coding processing unit  603  to the error correction decoding processing unit  618  in the transmission system and receiving system. Each unit is operated according to the setting parameter. Examples of the setting parameter include a kind of error correction code, a coding ratio, and a interleaving length for the error correction coding processing unit  603  and error correction decoding processing unit  618 , a modulation scheme for the modulation processing unit  604 , demodulation processing unit  616 , and channel response estimation processing unit  617 , a carrier frequency, distinction between the single carrier and the multi-carrier, and the number of sub-carriers for the mapping processing unit  605 , frequency conversion processing units  606  and  614 , demapping processing unit  615 , and synchronization processing unit  612 , a symbol rate for the up-sample processing unit  607  and down-sample processing unit  613 , a base band signal bandwidth for LPFs  608  and  611 , and the center frequency of the main band for the IF/RF circuits  609  and  610 . 
     The communication scheme and communication frequency selection processing unit  619  selects the data rate used in the data communication requested from PC, and the information on fc, fw, and fm received from PC. The communication scheme and communication frequency selection processing unit  619  selects the candidates of the kind of error correction code, the coding ratio, the interleaving length, the modulation scheme, the carrier frequency, the distinction between the single carrier and the multi-carrier, the number of sub-carriers, the symbol rate, the base band signal bandwidth, and the center frequency of the main band according to the result of the sensing processing. Using the communication scheme and communication frequency candidates from the other side of the communication, the communication scheme and communication frequency selection processing unit  619  determines these parameters to set the parameters for the pieces of processing. The method for determining the parameter will be described later with reference to  FIG. 7 . 
     The sensing processing unit  620  measures the power in each frequency from the signal in the main band. The sensing processing unit  620  measures the power in each frequency while dividing the main band into fine grids according to an instruction of the sensing frequency selection processing unit  621 , and the sensing processing unit  620  sends back the collected power data to the sensing frequency selection processing unit  621 . 
     The sensing frequency selection processing unit  621  controls sensing timing of the sensing processing unit  620  in cooperation with the MAC processing unit  602 , and the sensing frequency selection processing unit  621  stores the sensing result. 
     The selection of the communication scheme and communication frequency, performed by the communication scheme and communication frequency selection processing unit  619  of  FIG. 6 , will be described below with reference to  FIGS. 7 to 14 . The selected communication scheme and communication frequency are the data transfer channel (channel through which the communication is started in Step S 516 ). It is assumed that the communication request in Step S 508 , the communication request response in Step S 510 , the communication band and communication scheme candidate notification in Step S 512 , and the communication band and communication scheme notification in Step S 515  are already determined. It is also assumed that the parameters of fc, fm, and fw of each digital signal processing module are received from PC  101 . The selected communication frequency is the parameter set to the frequency conversion processing units  606  and  614 . The selected communication scheme is the distinction between the single carrier and the multi-carrier, and the selected communication scheme is set to the mapping processing unit  605 , frequency conversion processing units  606  and  614 , and demapping processing unit  615 . 
     The determination whether the communication is enabled or disabled and the method for determining the frequency band candidate in which the communication is performed will be described. When having received the communication request from PC  101 , the radio communication module  102  starts the sensing in the main band to know the usage of the frequency (Step S 701 ). As a result of the sensing, the radio communication module  102  determines that the frequency band whose power is lower than a threshold Pth is the available band. 
       FIG. 8  shows an example of the sensing result. Referring to  FIG. 8 , because only the power lower than a threshold Pth is detected in other portions in the main band although the signal having the peak is present near the center of the main band, the available band occupies the large portion of the main band. 
     The radio communication module  102  determines whether or not the communication bandwidth necessary for the data communication computed by the communication amount requested from PC  101  can be ensured from the result of the sensing in Step S 701  (Step S 702 ). When the bandwidth necessary for the data communication is smaller than the available band (NO in Step S 702 ), the radio communication module  102  determines that the communication frequency is the candidate such that the communication is performed using the available band as shown in  FIG. 9  (Step S 703 ). At this point, a priority to the frequency band N×fc delivered from PC  101  is decreased in selecting the frequency band used in the data communication. The priority represents a degree for selecting a natural number times a center frequency of a clock frequency. As to the communication scheme, the radio communication module  102  determines that an arbitrary communication scheme is the candidate (Step S 704 ). 
     On the other hand, as shown in  FIG. 10 , when the bandwidth necessary for the data communication is larger than the available band (YES in Step S 702 ), the radio communication module  102  distinguishes between the clock noise signal and other interference signals based on the data fc received from PC  101 . 
     When the data concerning the three clock noises (fc 1 , fw 1 , and fm 1 ), (fc 2 , fw 2 , and fm 2 ), and (fc 3 , fw 3 , and fm 3 ) is received from PC  101  as shown in  FIG. 10 , a signal  1001  present at a frequency N×fc 1 , a signal  1002  present at a frequency M×fc 2 , and a signal  1003  present at a frequency fc 3  are quite likely to be the clock noise (N and M are natural numbers). Because of no pieces of information on a signal  1004 , the signal is possibly the interference signal from another system. In the frequency band in which only the clock noise is present while the interference signal cannot be detected from another system, it is believed that a possibility of affecting another system is sufficiently low. Therefore, it is believed that the frequency band in which only the clock noise is present can be utilized in the data communication. The radio communication module  102  compares the necessary communication bandwidth with the available band of the case in which the band in which the clock noise is present is used as the available band, i.e., the case in which the signal  1001  present at the frequency N×fc 1 , the signal  1002  present at the frequency M×fc 2 , and the signal  1003  present at the frequency fc 3  are not detected (Step S 705 ). When the available band is smaller than the necessary communication band even if the band in which the clock noise is present is set to the available band (NO in Step S 705 ), or when the power of the clock noise cannot ensure SIR necessary to perform communication in the frequency band even if the available band is present (NO in Step S 706 ), the radio communication module  102  determines that the necessary communication cannot be performed and notifies PC  101  that the necessary communication cannot be performed (Step S 707 ), and the radio communication module  102  waits for the new communication request. In Step S 706 , the radio communication module  102  compares the power of the clock noise with a threshold. The threshold differs from the threshold Pth in Steps S 701  and S 702 , and the threshold is determined by a distance between the radio communication apparatus, the transmission power of the radio communication apparatus of the other side, and throughput required between the radio communication apparatus. 
     In the case where the band in which the clock noise is present is set to the available band, when the necessary communication band is smaller than the available band (YES in Step S 705 ), and when the power of the clock noise can ensure SIR necessary to perform communication in the frequency band (YES in Step S 706 ), the radio communication module  102  determines that the necessary communication is enabled, and the radio communication module  102  determines the communication frequency candidate such that the data communication is performed in the frequency band in which the clock noise is present (Step S 708 ).  FIG. 11  shows the case in which the data communication is performed at fc 3 . 
     The method for determining the communication scheme in the case in which the communication is performed in the frequency band where the clock noise is present will be described below. It is assumed that the data communication is performed at the frequency fc 3  as shown in  FIG. 11 . At this point, it is assumed that fw 3  is the frequency modulation width fw corresponding to fc 3 . The frequency modulation width fw corresponding to the N×fc 1  becomes N×fw 1 . When fw 3  is smaller than a threshold Fth 1  (NO in Step S 709 ), a multi-carrier communication scheme in which null is inserted into fc 3  and fw 3  as shown in  FIG. 12  is set to the communication scheme candidate (Step S 710 ). The position where the null sub-carrier is inserted is the frequency corresponding to the frequency fc 3 , and the number of null sub-carriers is determined according to the width of the bandwidth fw 3 . That is, the frequency width into which the null sub-carrier is inserted is formed larger than fw 3 . At this point, the threshold Fth 1  is determined in proportion to the communication frequency bandwidth. For example, a half of the communication frequency bandwidth is set to the threshold Fth 1 . The threshold Fth 1  is set smaller than the necessary communication frequency bandwidth. 
     When fw 3  is larger than the threshold Fth 1  (YES in Step S 709 ), the radio communication module  102  makes a determination of fm (Step S 711 ). At this point, the modulation frequency fm corresponding to fc 3  is set to fm 3 . The modulation frequency fm corresponding to N×fc 1  does not become N×fm 1 , but become fm 1 . When fm 3  is smaller than the threshold Fth 2  (NO in Step S 711 ), the radio communication module  102  determines that the multi-carrier communication scheme is the communication scheme candidate as shown in  FIG. 13  (Step S 712 ). At this point, the null sub-carrier is not set unlike Step S 710 . When fm 3  is larger than the threshold Fth 2  (YES in Step S 711 ), the radio communication module  102  determines that the single carrier communication scheme is the communication scheme candidates as shown in  FIG. 14  (Step S 713 ). The threshold Fth 2  is set larger when the symbol length is increased, and the threshold Fth 2  is set smaller when the symbol length is decreased. The symbol length is proportional to an inverse number of the communication frequency bandwidth. That is, the threshold Fth 2  is determined in proportion to the necessary communication frequency. 
     Alternatively, in the case where the clock noise is generated, the decrease in error rate may be prevented by changing the error correction code to a code having higher correction performance, by lowering the coding ratio, or by changing the modulation scheme to a modulation scheme having the lower error rate according to the number of clock noises fc present in the data communication band. A probability of continuously generating the errors may be lowered to prevent a decrease in packet error rate by increasing or decreasing the interleaving length in proportion to fm. 
     Even if the sufficiently available frequency band is present like Step S 703 , the communication is performed in the frequency band avoiding the registered operating clock frequency fc. Therefore, when an operation of a certain digital signal processing module is started to generate the interference during the communication, a time necessary to change the communication frequency can be shortened to improve the throughput of the communication. 
     In the case where the data communication is performed in the frequency band where the clock noise is present while fw is smaller than the threshold Fth 1  like Step S 710 , the multi-carrier communication into which null is previously inserted is performed in consideration of the registered operating clock frequency fc and frequency modulation width fm of SSC. Therefore, the sub-carrier affected by the interference is not used, so that the error rate can be enhanced to improve the throughput of the communication. 
     Like Step S 712 , in the case where the data communication is performed in the frequency band where the clock noise is present, when fw is larger than the threshold Fth 1 , and when fm is smaller than the threshold Fth 2 , because only a part of the sub-carriers is affected by the interference by performing the multi-carrier communication, the error can be corrected by the error correction code. Therefore, the packet error rate can be lowered to improve the throughput of the communication. 
     Like Step S 713 , in the case where the data communication is performed in the frequency band where the clock noise is present, when fw is larger than the threshold Fth 1 , and when fm is larger than the threshold Fth 2 , because only a part of the symbol length is affected by the interference by performing the single-carrier communication, the error can be corrected by the error correction code. Therefore, the packet error rate can be lowered to improve the throughput of the communication. 
     Thus, in the embodiment, the communication frequency, the communication scheme, and the signal power are determined according to the characteristic of the noise generated in the digital signal processing unit. Therefore, the communication scheme robust to the interference can be selected, so that the error rate of the radio communication can be lowered to improve the throughput of the communication in the radio communication system. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.