Patent Publication Number: US-7917110-B2

Title: Cognitive radio system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2006-126949, filed on Apr. 28, 2006; the entire contents of which are incorporate herein by reference. 
     BACKGROUND 
     1. Field 
     The present invention relates to a cognitive radio system which uses a frequency band licensed to a particular system such as a broadcasting system, only at a time and place which are not used by the system. 
     2. Related Art 
     In order to cope with tightness of frequency resources, a study for a system called “cognitive radio” as a next-generation mobile communication system has been started. The basic concept of cognitive radio technology is that a usable frequency band is scanned to detect a use-state of a wireless channel (frequency), and a usable frequency band is borrowed. Of course, in a radio system in which cognitive radio is used (hereafter, such a system is referred to as “cognitive radio system”), a wireless channel have to be shared so as not to interfere with existing radio systems such as a broadcasting system, a radar system, and a radio astronomy observing system. 
     Japanese Patent No. 3,583,962 discloses a technique of sharing a frequency wireless channel for a television broadcast as a radio system. 
     In Japanese Patent No. 3,583,962, only a television broadcast is considered as an existing licensed radio system. In a cognitive radio system, it is expected that each wireless terminal device having a function of cognitive radio detects a wireless-channel use-state. However, there are a lot of information which cannot be obtained by a wireless terminal device alone, such as wireless-channel use-states of radio astronomy and a television receiver which perform only reception. 
     In the case where wireless terminal devices of cognitive radio which are separated from each other by 100 m communicate with each other, for example, other wireless terminal devices which mutually perform transmission and reception through a distance that is very shorter than 50 m, e.g., 3 m may exist at intermediate positions which are separated from the wireless terminal devices by about 50 m. There is a possibility in which radio waves from the other wireless terminal devices cannot be detected by the wireless terminal devices of cognitive radio which are separated by 50 m. When the wireless terminal devices fail to aware the radio waves and start communication with using a frequency which is used by the other wireless terminal devices, the communication obstructs the communication between the other wireless terminal devices. Therefore, a frequency by which a wireless terminal device of cognitive radio starts communication have to be determined while the terminal device knows as correctly as possible use-states of frequencies (wireless channels) in the radio-wave arrival range. However, such use-states are hardly known by a wireless terminal device alone. 
     SUMMARY 
     According to an aspect of the invention, a cognitive radio system can obtain a wireless-channel use-state that cannot be grasped by a wireless terminal device of cognitive radio alone, and which does not interfere with an existing radio system. 
     According to an aspect of the invention, there is provided a cognitive radio system including: a state detecting device that scans a frequency band allocated to another radio system than the cognitive radio system to detect a first use-state of the frequency band; a first server including a first gathering unit that receives first information relating to the first use-state and a second gathering unit; a second server that stores a second use-state of the frequency band allocated to the other radio system, the second server configured to provide second information relating to the second use-state to the second gathering unit; and a notifying unit that notifies a terminal device in the cognitive radio system of information of an available channel based on the first information and the second information. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a diagram schematically showing the configuration of a cognitive radio system of a first embodiment of the invention; 
         FIG. 2  is a diagram schematically showing the configuration of a wireless terminal device; 
         FIG. 3  is a diagram schematically showing functions of an internal database; 
         FIG. 4  is a diagram schematically showing functions of a detecting device; 
         FIG. 5  is a sequence diagram illustrating operations of devices of a cognitive radio system of the first embodiment of the invention; 
         FIG. 6  is a sequence diagram showing an example in which the reliability of measured channel use-state information measured by the internal database is calculated, and an unused channel is recognized by using information of a channel use-state in which the reliability is high; 
         FIG. 7  is a view illustrating an example of relationships between positions of terminals including functions of a channel use-state detecting device, and the reliability; 
         FIG. 8  is a view illustrating an example of relationships between positions of terminals including functions of the channel use-state detecting device, and the reliability; 
         FIG. 9  is a sequence diagram schematically showing the configurations of terminals in a second embodiment of the invention; 
         FIG. 10  is a sequence diagram showing an example of the process and message exchange in the second embodiment; 
         FIG. 11  is a diagram schematically showing the configuration of a system in a third embodiment of the invention; 
         FIG. 12  is a diagram schematically showing the configuration of an access point; 
         FIG. 13  is a diagram schematically showing the configuration of a wireless terminal; 
         FIG. 14  is a flowchart of a process of updating database information in an access point; 
         FIG. 15  is a view showing information stored in a database of the access point; 
         FIG. 16  is a view showing a modification of the information table shown in  FIG. 15 ; 
         FIG. 17  is a sequence diagram showing an example of message exchanges which are conducted by databases for gathering wireless-channel use-state information; 
         FIG. 18  is a view showing information of use-states of frequency bands at a certain position among information held by a database, and information sources from which the information is obtained; 
         FIG. 19  is a view showing a modification of the information table shown in  FIG. 18 ; 
         FIG. 20  is a flow chart of a process for starting communication by using information stored in the database; 
         FIG. 21  is a sequence diagram showing message exchanges occurring in communication using database information; 
         FIG. 22  is a flowchart showing a communication band determination algorithm; 
         FIG. 23  is a process flow chart of the whole system for starting communication by using database information; 
         FIG. 24  is a detailed flowchart of the process of communication channel determination in  FIG. 23 ; 
         FIG. 25  is a sequence diagram relating to message exchanges occurring in communication using database information; 
         FIG. 26  is a diagram showing an example of the system configuration of a fourth embodiment of the invention; 
         FIG. 27  is a diagram schematically showing the configuration of a terminal; 
         FIG. 28  is a view showing an example of wireless-channel use-state information in a database in the terminal; 
         FIG. 29  is a view showing an example of wireless-channel use-state information in a database in a terminal; 
         FIG. 30  is a view showing an example of wireless-channel use-state information in a database in a base station; 
         FIG. 31  is a view showing an example of wireless-channel use-state information stored in a local database; 
         FIG. 32  is a flowchart in the case where a terminal notifies a base station of a wireless-channel use-state in the fourth embodiment; 
         FIG. 33  is a sequence diagram showing message exchanges occurring when a radio astronomy starts a measurement; 
         FIG. 34  is a sequence diagram showing an example of message exchanges in the case where determination that interference exists as a result of the measurement of a wireless-channel use-state by the terminal is notified; 
         FIG. 35  is a sequence diagram showing an example of message exchanges in the case where determination that interference does not exist as a result of the measurement of a wireless-channel use-state by the terminal is notified; 
         FIG. 36  is a sequence diagram showing an example of message exchanges in the case where the terminal starts communication; and 
         FIG. 37  is a sequence diagram showing an example of message exchanges in the case where the terminal starts communication. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, best embodiments of the invention will be described in detail with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a diagram schematically showing the configuration of a cognitive radio system of a first embodiment of the invention. The cognitive radio system comprises: a wireless communication system  101  including wireless terminal devices  105 ,  106  that communicate with each other; a wireless-channel use-state detecting device  102  (hereinafter, referred to as “detecting device  102 ”) which detects the use-state of a wireless channel that can be used by the wireless terminal devices  105 ,  106 ; an internal wireless-channel use-state database device  103  (hereinafter, referred to as “internal DB  103 ”); and an external wireless-channel use-state database device  104  (hereinafter, referred to as “external DB  104 ”) which stores wireless-channel use-states of frequency bands that are allocated as licensed channels to radio systems different from the wireless communication system  101  (for example, a police radio and an emergency radio which is used in a disaster situation). 
       FIG. 2  is a diagram schematically showing the configuration of the wireless terminal device  105 . The wireless terminal devices  105 ,  106  have the same configuration. The terminal  105  has a function of cognitive radio, and, in addition to a usual communication function, has the following functions. The terminal receives information which is supplied from the internal DB  103 , and which relates to a use-state of a wireless channel, and performs communication while a wireless channel (communication channel) used in the communication is set by the terminal itself. 
     Referring to  FIG. 2 , the terminal  105  comprises: an internal-DB input/output interface  1051  which is an interface with the internal DB  103 ; a communication channel setting portion  1052  which sets a communication channel on the basis of the information which is received from the internal DB  103 , and which relates to a wireless channel in an unused (not used) state; a wireless communication transmitting/receiving portion  1053  which modulates a signal to be transmitted through the set communication channel, and which demodulates a received signal; an antenna  1054  for transmission and reception; and a command producing portion  1055  which produces various commands based on instructions from a high-level layer. The internal-DB input/output interface  1051  is, for example, a wireless interface, and can communicate with the internal DB  103 . The terminal  105  further comprises a GPS receiver  1056 , and can measure its own position and notify the internal DB  103  of a result of the measurement through the internal-DB input/output interface  1051 . 
       FIG. 3  is a diagram schematically showing the functions of the internal DB  103 . The internal DB  103  can communicate with the terminal  105 , the external DB  104 , and the detecting device  102 , and comprises interfaces  1031 ,  1032 ,  1033  with the devices. The internal DB  103  further comprises: an unused-channel recognizing portion  1034  which recognizes a wireless channel in an unused state on the basis of the wireless-channel use-state information that is supplied from the detecting device  102 ; an information registering portion  1035  which organizes information received from the external DB  104  and the unused-channel recognizing portion  1034 , and which registers the organized information into a database portion  1038 ; a communication channel selecting portion  1036 ; and a command producing portion  1037  which produces various commands on the basis of instructions from the high-level layer. 
       FIG. 4  is a diagram schematically showing the functions of the detecting device  102 . The detecting device  102  is connected with the internal DB  103  by wire or wireless, and, on the basis of a request from the internal DB  103 , detects use-states of wireless channels (hereinafter, referred to as “licensed channels”) which are licensed to existing radio systems that are in the periphery of the detecting device  102 , such as a radar system and a broadcasting system. 
     Referring to  FIG. 4 , a band determining portion  1021  indicates information of a frequency band in which a use-state is to be detected, to a power measuring portion  1022 . The power measuring portion  1022  receives instructions from the band determining portion  1021 , and measures the power of a signal which is received through an antenna  1023 . A position measuring portion  1024  is a GPS receiver or the like, and measures the position of the detecting device  102 . In the case where the detecting device  102  is wirelessly connected with the internal DB  103 , a command producing portion  1025  has functions of receiving the reception power measured by the power measuring portion  1022 , and the own position information measured by the position measuring portion  1024 , and supplying them to the internal DB  103  through the antenna  1023 . A timer  1026  is used for turning ON/OFF the power measuring portion  1022  with a predetermined period, or for recording a time of the power measurement, a time period required for measurement, etc. The information which is supplied from the command producing portion  1025  to the internal DB  103  includes, in addition to the measured reception power and the position information of the detecting device  102 , information of a measured frequency band, a measured band width, a measured time, a time period required for measurement, an antenna gain of the detecting device, the direction of a beam, etc. 
     Referring to  FIG. 5 , the operations of the above-described devices will be described. 
       FIG. 5  is a sequence diagram illustrating the operations of the devices. First, a session (session  1 ) to the time when the detection result of the detecting device  102  indicating a use-state of a wireless channel is registered in the internal DB  103  will be described. 
     First, the internal DB  103  transmits a signal requesting a measurement of the use-state of the licensed channel, from the command producing portion  1037  of the internal DB  103  to the detecting device  102  (step S 501 ). 
     In response to the measurement request, the detecting device  102  detects the use-state of the licensed channel (step S 502 ), and notifies the internal DB  103  of a result of the measurement (step S 503 ). The use-state of the wireless channel includes, in addition to the reception power measured in the wireless channel to be measured and the position of the detecting device, the measured frequency band, the measured band width, the measured time, and the time period required for the measurement. The use-state may further include the antenna gain of the detecting device, the direction of a beam, etc. In the case where the measured signal has a certain degree of periodicity, the period of the measured signal, the length of a silent interval, the duty ratio, and the like may be included in the information to be notified to the internal DB  103 . In the case where the existing radio system is a radar, for example, a radar signal has a periodicity of several milliseconds. In the case where the existing radio system is a television (TV) broadcasting system, it is said that a television broadcast signal has a day-scale periodicity. Such a period also may be notified to the internal DB  103 . 
     In the internal DB  103 , from the use-state of the wireless channel which is supplied through the interface portion  1033 , the unused-channel recognizing portion  1034  recognizes whether the object wireless channel is an unused wireless channel or not (step S 504 ). A region which is orthogonal to the existing communication in the frequency, time, space, and diffusion code domains, or a wireless channel in which transmission is enabled at a power producing interference at a degree that does not largely affect the existing communication is set as an unused wireless channel. 
     The interference at a degree that does not largely affect the existing communication in term of the frequency domain means interference which, in a region that is separated by insertion of a guard band in term of the frequency domain, does not affect the reception performance of the existing communication even when a sidelobe of a frequency spectrum which has been passed through a filter overlaps with the existing communication. The interference at a degree which does not largely affect the existing communication in term of the time domain means interference which, in a region that is separated by insertion of a guard interval in term of the time, does not affect the reception performance of the existing communication even when the region exceeds a guard band to overlap with the existing communication by multipath delay spread. The interference at a degree that does not largely affect the existing communication in term of the space domain means interference which, in communication that is performed between regions geographically separated from each other, does not affect the reception performance of the existing communication even when the communication is sufficiently attenuated by a propagation loss and a signal reaches a transmitter/receiver performing the existing communication. The interference at a degree that does not largely affect the existing communication in term of the diffusion code domain means interference which, although not a code that is completely orthogonal to the diffusion code performing the existing communication, less exerts an influence due to mutual correlation and does not affect the reception performance of the existing communication. 
     Specifically, a wireless channel in which interference equal to or more than thermal noise is not measured in the frequency, time, space, and diffusion code domains is set as an unused wireless channel. The interference amount I n,f,t,c  of a terminal n in a frequency band f, a time slot t, and a diffusion code c is obtained by using following Expression (1) 
     
       
         
           
             
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     where 
     P m,f,t,c : reception power in the frequency band f, the time slot t, and the diffusion code c, measured by a detecting device m, 
     M: number of detecting devices, 
     L m : position of a detecting device, 
     L n : position of a terminal, and 
     L p  (|L n −L m |): propagation loss with respect to distance |L n −L m |. 
     The reception power P is a value measured by the detecting device  102 , and the number of the detecting device  102  is set to 1 in the embodiment. The position of the detecting device  1102  is a value measured by the position measuring portion  1024  disposed in the detecting device  102 , the position of the terminal  105  is a value measured by the GPS receiver  1056  disposed in the terminal  105 , and the propagation loss is a constant. The unused-channel recognizing portion  1034  of the internal DB  103  obtains the interference amount on the basis of the above-mentioned values. The unused-channel recognizing portion  1034  compares the obtained interference amount with a constant threshold corresponding to the noise power of the terminal, and, if the obtained interference amount is smaller than the threshold, determines that the wireless channel is in an unused state. 
     In the internal DB  103 , the information registering portion  1035  organizes the information obtained from the detecting device  102 , and performs registration (step S 505 ). Here, the organization means a process in which, in the case where use-state information of a wireless channel having substantially same position information and the same frequency band is notified, information is overwritten with new information. 
     The internal DB  103  obtains the wireless-channel use-state from the external DB  104  (step S 506 ). The information obtained from the external DB  104  is a use-state of a wireless channel which cannot be known by the internal DB  103 , and includes a wireless-channel use-state of a frequency band which is used only in reception such as the case of a TV, a radar, and radio astronomy, and information of a frequency band the use of which is prohibited by law, and the like. 
     In the internal DB  103 , the information registering portion  1035  organizes the information obtained from the external DB  104 , and performs registration (step S 507 ). The above sequence is session  1  to the time when the detection result of the detecting device  102  indicating the use-state of the wireless channel is registered in the internal DB  103 . 
     Next, a session (session  2 ) to the time when the terminal  105  obtains information relating to the use-state of the wireless channel from the internal DB  103 , and starts communication with the terminal  106  will be described. 
     When a request for communicating with the terminal  106  is generated in the terminal  105 , the terminal  105  transmits a communication start request signal to the internal DB  103 , and notifies of its own position information (step S 508 ). The request signal is produced by the command producing portion  1055 . The position information is sent on the request signal. 
     The internal DB  103  requests the terminal  106  to send position information (step S 509 ). The terminal  106  notifies the internal DB  103  of its own position information (step S 510 ). 
     The internal DB  103  selects the unused wireless channel recognized in step S 504  as a communication wireless channel between the terminals  105 ,  106  (step S 511 ). Among wireless channels which are recognized as unused wireless channels, however, a wireless channel which is set as “inhibition of transmission” or “inhibition of use” by the external DB  104  is not selected as a communication wireless channel. The internal DB  103  determines whether, among wireless channels recognized as unused wireless channels, a wireless channel which is set as “used” by the external DB  104  can be used as a communication channel in view of position information of the terminals  105 ,  106  or not. The internal DB obtains the positions of the terminals  105 ,  106  and the distances with respect to a used apparatus, obtains interference which may be received by the used apparatus, and, if the interference is approximately at a degree smaller than thermal noise, selects as a communication channel. 
     When the terminal n is in a frequency band f, the time slot t, and the diffusion code c, an estimated interference amount I n,l,f,t,c  which may be received by a used apparatus  1  is obtained by using following Expression (2):
 
 I   n,1,f,t,c   =G   n ( L   l   −L   n ) ×P   n,f,t,c   ×Lp (| L   l   −L   n |)
 
     where P n,f,t,c : transmission power which is transmitted by the terminal n in the frequency band f, the time slot t, and the diffusion code c, and 
     G n  (L l −L n ): antenna gain in the terminal n with respect to the direction (L n −L m ). 
     The internal DB  103  notifies the terminals  105 ,  106  of the communication wireless channel selected in step S 511  (steps S 512 , s 513 ), and communication between the terminals is started by using the communication channel (step S 514 ). 
     The terminal  105  previously notifies the internal DB  103  of a scheduled communication time, the interval of burst communications, the communication band, a require band width, a required bit rate, QoS (Quality of Service) information, the modulation system, etc. As a result, the internal DB  103  enables the detecting device  102  to perform setting of a communication channel in consideration of these sets of information, and unwasted detection of a channel use-state. When an expected communication time is registered in a database, for example, a waste of operation in which the detecting device  102  measures the use-state of a wireless channel that is known to be in communication is prevented from being performed. When the period of a burst communication signal is registered in the database portion  1038 , it is possible to easily recognize a temporally unused wireless channel among in recognition of an unused wireless channel on another communication. 
     When the frequency band is registered in the database portion  1038 , it is possible to recognize that interference exists in the used frequency band, without measurement in recognition of an unused wireless channel on another communication. Therefore, a waste of operation in which the detecting device  102  measures the use-state of a wireless channel that is known to be in communication is prevented from being performed. In the case where a required communication band width is registered in the database portion  1038 , when interference of a part of the band is measured, it is possible to know that the whole communication band width is used. Therefore, the use-state of a wireless channel can be easily detected. When the QoS information is registered in the database, a wireless channel which is similar in term of the frequency, time, space, and diffusion code in communication that requires a high QoS, and in which interference at a certain degree is produced is not determined as an unused wireless channel, and by contrast also a wireless channel which is similar in term of the frequency, time, space, and diffusion code in communication that is not problematic at a low QoS, and in which interference at a certain degree is produced can be determined as an unused wireless channel. 
     Furthermore, information such as a result of detection of the wireless-channel use-state, and notification of a wireless channel to be used in communication may be notified by using a fixed notification wireless channel dedicated to the present cognitive radio system, or a notified wireless channel may be designated by using a fixed control wireless channel dedicated to the present cognitive radio system. Alternatively, notification may be performed through a wire. In the case where a wireless terminal device in charging is used also as a wireless-channel use-state detecting device, for example, it is not necessary to care about the power consumption, and hence a method may be employed in which a wireless-channel use-state is frequently measured, and the use-state is notified through PLC (Power Line Communication), a wired LAN, or the like. 
     In this way, by using interference information detected by the detecting device  102 , a wireless terminal device can perform wireless communication by using an unused wireless channel, and also in consideration of a wireless-channel use-state which cannot be detected by the detecting device  102 , from the database relating to wireless. Therefore, interference with the existing radio system can be suppressed, and an unused wireless channel can be effectively used. Since a communication wireless channel is selected in the internal DB  103 , the throughput of the transmitter/receiver of the terminal can be reduced. 
     In the embodiment described above, the channel use-state detecting device  102  is shown as an independent device. Alternatively, the device may be included in the same device as the internal DB  103 , or included in the wireless terminal device  105 . Similarly, also the internal DB  103  is shown as an independent device. Alternatively, the device may be included in at least one of the wireless terminal devices  105 ,  106 . 
     (Modification of  FIG. 5 ) 
     A modification of the embodiment will be described.  FIG. 6  is a sequence diagram showing an example in which the internal DB  103  checks the positions of the terminals  105 ,  106 , thereafter calculates the reliability of measured channel use-state information, and an unused channel is recognized by using information of a channel use-state in which the reliability is high as a result of calculation. For example, the reliability is calculated by the unused-channel recognizing portion  1034  of  FIG. 3 . In  FIG. 6 , components identical with those of  FIG. 5  are denoted by the same reference numerals. 
     The internal DB  103  checks the positions of the terminals  105 ,  106 , and thereafter calculates the reliability of measured channel use-state information (step S 600 ). Hereinafter, a calculation example of the reliability of channel use-state information will be described. 
     First, the channel use-state detecting device m existing at a position L m  measures a reception power P m,f,t,c  at a time τ m  with respect to the frequency band f, the time slot t, and the diffusion code c. When the measurement is ended, the device notifies the DB of a result of the measurement. The DB  103  gathers information from an m number of channel use-state detecting devices under control, and stores the information. In the case where determination whether information is reliable or not is performed in the transmitting/receiving terminal n, the reliability of the channel use-state information is used. The information reliability R n  in the transmitting/receiving terminal n can be obtained by using following Expression (3): 
               R   n     =       ∑     m   =   0     M     ⁢     [       G   ⁡     (       L   n     -     L   m       )       ×     (     1       τ   now     -     τ   m         )     ×       L   p     ⁡     (            L   n     -     L   m            )         ]             
where G n  (L n −L m ) is the antenna gain with respect to the direction (L n −L m ). In case of an omni-antenna, the gain is 1 irrespective of the direction. Furthermore, L p (|L n −L m |) is the propagation loss with respect to the distance |L n −L m |.
 
     The propagation loss is a function of an inverse relational relationship in which, as given by the Okumura curve, Hata formula, or the like, the value is larger as the distance is shorter, and the value is smaller as the distance is longer. Here, in order to obtain reliability information, calculation is performed by using the position, the antenna gain, and the time. Alternatively, the reliability may be calculated by using only one of the items. Following Expression (4) shows an example in which the reliability is calculated by using only the term of the time. 
     
       
         
           
             
               
                 
                   
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     Following Expression (5) shows a calculation in the reliability is calculated by using only the term of the position. 
     
       
         
           
             
               
                 
                   
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     When calculation is performed by using the expression, the reliability is calculated in accordance with the position of the channel use-state detecting device. 
       FIGS. 7 and 8  show examples of the relationships between the positions of terminals and the reliability. In the examples, it is assumed that both the channel use-state detecting device and the transmitting/receiving function are mounted on the terminals. 
     Even when the same channel use-state is registered in DBs of terminals A and B, for example, reliance is placed on information of a positionally close terminal, and hence high-reliability information for the terminal A is different from that for the terminal B. In the case where the transmitter/receiver itself can measure the channel use-state, a result which is measured by itself is most reliable information. 
     In the case where the channel use-state detecting device has a configuration in which the interference signal can be accurately measured by using a high-performance device such as a high-resolution A/D converter, it is contemplated that a correction term is added as following Expression (6): 
     
       
         
           
             
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                 ] 
               
             
           
         
       
     
     In the above expression, S m  is the interference measurement accuracy of the channel use-state detecting device, and proportional to a value such as the number of bits of the A/D converter. If the reliability is sufficiently high, it is determined that the currently existing channel use-state information is reliable. 
     In the case where the reliability is low and it is determined that the channel use-state information is not reliable, a channel measurement request is issued to the detecting device  102  so as to perform a change of the antenna gain, or the like, and again the measurement (step S 601 ). The detecting device  102  measures the channel use-state (step S 602 ), and transmits a result of the measurement to the DB  103  (step S 603 ). 
     In the case where, even when the measurement is performed, reliable information is not obtained, it is determined that the channel use-state detecting device does not exist in the vicinity of the transmitting/receiving terminal n, and transmission of the transmitting/receiving terminal n is postponed because the channel use-state cannot be detected. The determination whether the reliability is high or not may be performed by a method in which the determination is performed based on a threshold. It is preferable that the threshold is determined by systems which share the channel. In the case where cognitive wireless communication is to be performed by using a channel of a wireless apparatus which has a low resistance against interference, such as radio astronomy, the threshold is set to high. By setting the threshold in this way, in the case where cognitive wireless communication is to be performed by using a channel of a wireless apparatus which has a low resistance against interference, communication is allowed to be performed only when the channel use-state measurement information is highly reliable, whereby the influence exerted on the existing communication can be suppressed to the minimum level. In the case where cognitive wireless communication is to be performed by using a channel of a wireless apparatus which has a certain level of resistance against interference, such as digital wireless communication like a wireless LAN, the threshold is set to low. By setting the threshold in this way, in the case where cognitive wireless communication is to be performed by using a channel of a wireless apparatus which has a certain level of resistance against interference, communication is allowed to be performed even when the channel use-state measurement information is not highly reliable, whereby the frequency use efficiency can be improved. 
     In the case where it is determined that information has high reliability and determination whether the channel is an unused channel or not based on information currently existing in the DB is reliable, it is determined whether the channel which has been measured in the detecting device  102  is an unused channel in the internal DB  103  or not. In this way, an unused channel is recognized while discriminating channel use-state information which has been measured by using the reliability. 
     Therefore, an unused channel can be accurately recognized. 
     Second Embodiment 
     Next, a second-embodiment of the invention will be described. In the first embodiment described above, recognition of an unused channel is performed by the internal DB  103 . In the second embodiment, from channel use-state information supplied from the internal DB  103 , the terminals  105 ,  106  recognize an unused channel, and set a communication channel. 
       FIG. 9  is a diagram schematically showing the configurations of the terminals  105 ,  106  in the second embodiment. The terminals  105 ,  106  have the same configuration, and therefore description of the terminal  106  is omitted. In  FIG. 9 , components identical with those of  FIG. 2  are denoted by the same reference numerals, and their detailed description is omitted. The figure is different from  FIG. 2  in that an unused-channel recognizing portion  1056  is disposed. 
       FIG. 10  is a sequence diagram showing an example of the process and message exchange in the second embodiment. First, the wireless-channel use-state detecting device  102  (hereinafter, referred to as “detecting device  102 ”) receives a request of the internal DB  103  (step S 701 ), and detects the use-state of a wireless channel (step S 702 ). The detecting device  102  notifies the internal DB  103  of information relating to a measurement result of the detected wireless-channel use-state information (step S 703 ). The internal DB  103  registers the notified measurement result (step S 704 ). On the other hand, the internal DB  103  receives information relating to the wireless-channel use-state, also from the external DB  104  (step S 705 ). The internal DB  103  organizes information received from the detecting device  102  and the external DB  104 , and registers the information (step S 706 ). The processes so far performed correspond to the session (session  1 ) to the time when the detection result of the detecting device  102  indicating a use-state of a wireless channel is registered in the internal DB  103 . 
     Next, when a request for communicating by wireless with the terminal  106  is generated in the terminal  105 , the terminal  105  notifies the internal DB  103  of its own position information and identification information of the communication partner (step S 707 ). The internal DB  103  identifies the terminal  106  which is the communication partner, on the basis of the identification information notified from the terminal  105 , and requests the terminal  106  to send position information (step S 708 ) The terminal  106  which receives the request notifies the position information to the internal DB  103  (step S 709 ). 
     Thereafter, the internal DB  103  notifies the terminal  105  of information relating to the wireless-channel use-state (step S 710 ). At this time, in accordance with the position of the terminal, it may be contemplated that, as the information relating to the wireless-channel use-state, a wireless-channel use-state which is measured in the vicinity, and a state of a signal which is transmitted in the vicinity are selected and notified. If the transmission system registers a communication schedule in the database, or if the system of the measured interference can be deduced, the distance by which the transmission signal reaches at a power more than noise to provide interference is calculated from the transmission power and carrier frequency of the system. When the distance is shorter than the calculated distance, the distance is set as the vicinity. In the terminal  105 , using the information, an unused-channel recognizing portion  1057  (see  FIG. 9 ) of the terminal  105  recognizes an unused channel (step S 711 ). Thereafter, the communication channel setting portion  1052  (see  FIG. 9 ) of the terminal  105  selects a wireless channel through which communication is to be performed (step S 712 ). The selected wireless channel is sent from the communication channel setting portion  1052  to the wireless communication transmitting/receiving portion  1053 , and notified to the terminal  106  (step S 713 ). After a reply (ACK) is returned from the terminal  106  (step S 714 ), communication is started by using the selected wireless channel (step S 715 ). The processes so far is the session (session  2 ) to the time when the terminal  105  obtains information relating to the use-state of the wireless channel from the internal DB  103  to recognize an unused channel, and starts communication with the terminal  106 . 
     In the embodiment, the terminal itself recognizes an unused channel, and independently selects a communication wireless channel. Therefore, the process in the internal DB can be reduced. 
     Third Embodiment 
     Next, a third embodiment of the invention will be described. 
       FIG. 11  is a diagram schematically showing the configuration of a system in the third embodiment. In the embodiment, a television broadcast (TV broadcast), and a local area network (W-LAN) using a radio system share a frequency band for the TV broadcast. A TV broadcast antenna  201  transmits a broadcast signal to a certain area by using several frequency bands. At this time, a wireless information DB  202  in a broadcast area stores information such as the frequency band of the current broadcast, the position of a broadcast station, and the radio-wave arrival range. 
     In private houses  218 ,  228 , a signal transmitted from the TV broadcast antenna  201  is received by TV receiving antennas  213 ,  223 . The TV receiving antenna  223  is disposed outdoors. It is assumed that TVs  214 ,  224  can share information by wire or wireless through W-LAN access points (APs)  215 ,  225  in the private houses  218 ,  228 . Therefore, the TVs  214 ,  224  are wireless devices having a television receiving function which can communicate with the wireless LAN access points, and which have a function of receiving a television broadcast. The TVs  214 ,  224  and the APs  215 ,  225  can share information of frequency bands which are currently received by the TVs  214 ,  224 . The APs  215 ,  225  and the wireless information DB  202  can share information by wire or wireless. The APs  215 ,  225  comprise a wireless-channel use-state detecting device, so that the access points themselves can detect the use-state of a wireless channel. The access points function also as a home server which obtains the reception status of each TV and the status of indoor communication. 
     Wireless transmitter/receivers  216 ,  217 ,  226 ,  227  (hereinafter, referred to as STs  216 ,  217 ,  226 ,  227 ) function as a station (ST) of the W-LAN, the STs  216 ,  217  communicate with the AP  215 , and the STs  226 ,  227  communicate with the AP  225 . The STs can communicate also with themselves. The TV  214 ,  224  know the positions of the antennas  213 ,  223 , respectively, and can notify the APs  215 ,  225  of the position information, respectively. As a method of knowing the positions of the antennas  213 ,  223 , a method may be employed in which, since a TV antenna is usually fixed, the position information is registered in a TV in the initial disposition of an antenna. The STs  216 ,  217 ,  226 ,  227  have a function of knowing the own position, and can notify the APs  215 ,  225  of the position information. Similarly, the APs  215 ,  225  have a function of knowing the own position. As the function of knowing position information, for example, a position information obtaining function using a GPS may be used. 
       FIG. 12  is a diagram schematically showing the configuration of the access point  225 . The AP  215  is configured in the same manner as the AP  225 , and therefore its description is omitted. The AP  225  can communicate with the STs  226 ,  227 , the wireless information DB  202 , and the TV  224 . Interfaces  2251 ,  2252  receive signals from the wireless information DB  202  and the TV  224 , respectively. A wireless communication transmitting/receiving portion  2253  modulates a signal to be transmitted through the set communication channel, and demodulates a received signal. The AP  225  further comprises: an unused-channel recognizing portion  2254  which recognizes a wireless channel in an unused state on the basis of the wireless-channel use-state information that is supplied from the STs  226 ,  227 ; an information registering portion  2256  which organizes information received from the wireless information DB  202 , and which registers the organized information into a database portion  2255 ; a communication channel selecting portion  2257 ; and a command producing portion  2258  which produces various commands on the basis of instructions from the high-level layer. 
       FIG. 13  is a diagram showing the configuration of the ST  227 . The STs  216 ,  217 ,  226  are configured in the same manner as the ST  227 , and therefore their description is omitted. The ST  227  further comprises: a wireless communication transmitting/receiving portion  2271  which modulates a signal to be transmitted through the set communication channel, and which demodulates a received signal; and a command producing portion  2272  which produces various commands on the basis of instructions from the high-level layer. The ST further comprises; a communication channel setting portion which sets a communication channel on the basis of the information that is received from the AP  225 ; and a GPS receiver  2274  which measures its own position, and notifies it to the AP  225 . The ST further comprises an interference measuring portion  2275  which, based on a request from the AP  225 , receives a signal of a designated wireless channel, calculates the reception power, and measures the noise level. 
       FIG. 14  shows a process flow of the whole system for updating DB information in the APs  215 ,  225  and information of the DB  202 . First, the APs  215 ,  225  determine the time of starting the wireless-channel use-state detection, and a wireless channel to be measured (step S 1401 ). For example, the APs  215 ,  225  determine that a terminal under control (e.g., the ST  217 ) is caused to detect the wireless-channel use-state of a certain frequency band at a frequency of once per millisecond. Furthermore, the APs  215 ,  225  determine that the ST  216  is caused to detect the wireless-channel use-state of another frequency band, and determine that the APs themselves to detect the wireless-channel use-states of the other frequency bands. In the embodiment, all object frequency regions are sequentially subjected to detection. Alternatively, all object frequency regions may be subjected to detection at one time. 
     After the time of starting the wireless-channel use-state detection and a wireless channel to be measured are determined, the wireless-channel use-state is actually measured (step S 1402 ). In the embodiment, the APs  215 ,  225  request the terminals STs under control to measure the frequency status. The STs which are requested to perform the measurement measure the wireless-channel use-state in a designated frequency region. In the measurement of the wireless-channel use-state, a signal of a designated wireless channel is received, and the reception power of the signal is calculated. If the reception power is in the noise level, it is determined that no interference exists, and, if the reception power is higher than the noise level, it is determined that interference exists, or the radio wave is used by another terminal and the TV broadcast. For example, although a reception power exists, a power that is lower than a level at which the reception power can be demodulated and decoded as a signal is defined as the noise level. When the power is lower than the noise level, it is determined that a radio wave is not used. An ST in which existence of interference has been measured notifies an AP which controls the ST, of the position information and a measurement result. Since the APs  215 ,  225  have a wireless receiving function, the APs themselves can detect the wireless-channel use-state. 
     When the wireless-channel use-state measurement is ended and the result is notified to the APs, each of the APs updates the DB information by using information relating to the wireless-channel use-state which has been measured by itself, information notified from the STs, and information from the DB  202  (step S 1403 ). 
       FIG. 15  shows information of use-states of frequency bands at certain positions, and information sources from which the information is obtained, in information stored in the DB  2255  of the AP  225 . 
     When the ST  227  existing at position Nx 03 , Ey 03  notifies the AP  225  of information relating to the wireless-channel use-state of f07 MHz band, the AP  225  searches the current DB information. If information of the same position and the same frequency band exists, the old information is deleted, and the new information is added. In the case where the AP  225  obtains from the DB  202  information relating to the broadcast, the AP obtains information indicating the wireless channel in which a TV broadcast is performed. In the example of  FIG. 15 , it is shown that a TV broadcast is performed in wireless channels of frequency bands of f01 MHz, f03 MHz, f04 MHz, and f06 MHz. Also when information of a wireless channel which is currently received is obtained from the TV  224 , similarly, information is obtained, thereafter the old information is deleted, and the new information is added. In the example of  FIG. 15 , it is shown that, in a wireless channel of the frequency band of f01 MHz, reception is performed at position Nx 01 , Ey 01  where the TV antenna  223  exists. 
     A term of validity may be disposed in information of a wireless channel use-state. In this case, when a respective term of validity expires, corresponding information is deleted from the DB, and required information is obtained from the DB  202  and the TV  224  as necessary, or the STs  226 ,  227  are caused to measure a wireless-channel use-state and notify it. 
     (Modification of  FIG. 15 ) 
       FIG. 16  shows a modification of the information table shown in  FIG. 15 . As shown in  FIG. 16 , when information is received, also the time of updating information may be simultaneously recorded. Raw data of an interference power may be recorded as they are in the use-state column of each frequency band. For example, it is indicated that, at position Nx 02 , Ey 02 , the ST  226  receives a wireless channel of f07 MHz at −61 dBm, and the information is written into the database of the AP  225  at 12:21:48.20 to perform updation. Furthermore, it is indicated that, at position Nx 03 , Ey 03 , the ST  227  receives a wireless channel of f07 MHz at −68 dBm, and the information is written into the database of the AP  225  at 12:21:49.20 to perform updation. 
       FIG. 17  is a sequence diagram showing an example of message exchanges which are conducted by the DBs for gathering wireless-channel use-state information. The database (DB  2255 ) in the AP  225  causes the STs  226 ,  227  and TV  224  which can exchange information with the AP by wire or wireless, to notify a wireless-channel use-state. In  FIG. 17 , the TV  224  is requested so that information of a received wireless channel is notified from the AP  225  to the TV  224 , at an interval of several to several tens of seconds. Similarly, it is requested so that information is notified from the AP  215  to the TV  214  (step S 1201 ). 
     Upon receiving the request, the TV  224  returns information of a wireless channel which is currently received, and position information of the receiving antenna  223 . Similarly, upon receiving the request, the TV  214  returns information of a wireless channel which is currently received, and position information of the receiving antenna  213  (step S 1202 ). Only when, without a request from the AP  225  (AP  215 ), the power is turned ON/OFF or a receiving wireless channel is changed, the TV  224  (TV  214 ) may notify the information to the AP  225  (AP  215 ). 
     Next, the AP  225  issues to the ST  226  and the ST  227  a request for notifying interference power information. Similarly, the AP  215  issues to the ST  216  and the ST  217  a request for notifying interference power information (step S 1203 ). This request includes information of a wireless channel to be measured. The ST  226  (ST  216 ) and ST  227  (ST  217 ) which receive the request check the presence or absence of an interference power in a wireless channel designated by the AP  225  (AP  215 ), and notify the AP  225  (AP  215 ) of a combination of a result of the check and information of the position where the ST currently exists (step S 1204 ). When such information is gathered and a process of  FIG. 21  described below is performed, the database in the AP  225  (AP  215 ) can know the use-states of the frequencies (wireless channels) at the positions shown in  FIG. 15 . 
     Returning to  FIG. 17 , the DB  202  relating to the TV broadcast handles reception information of a TV in the area and interference information, in addition to transmission information of the TV broadcast. For example, the DB  202  requests the TV broadcast station (antenna)  201  to send information of the wireless-channel use-state of current transmission (step S 1205 ). Upon request, the TV broadcast station  201  notifies the DB  202  of information of the wireless-channel use-state (step S 1206 ). 
     Furthermore, the DB  202  periodically issues a request for notifying information, to databases in APs in the area (steps S 1207 , S 1209 ), and receives information from the databases (steps S 1208 , S 1210 ). In this example, information in which position information is added to information during TV reception in each AP and information relating to detected interference is notified to the DB  202 . In place of the method in which the local DB  202  issues an information request and an AP that receives the request notifies information, another method may be employed in which information is notified to the local DB  202  while using updation of the contents of the DB  202  in the AP as a trigger. 
       FIG. 18  shows information of use-states of frequency bands at a certain position among information held by the DB  202 , and information sources from which the information is obtained. When information is gathered as described above, the DB  202  can know the use-states of the frequencies (wireless channels) at the positions as shown in  FIG. 18 . 
     (Modification of  FIG. 18 ) 
       FIG. 19  shows a modification of the information table shown in  FIG. 18 . As shown in  FIG. 19 , when information is received, also the time of updating information may be simultaneously recorded. Raw data of an interference power may be recorded as they are in the use-state column of each frequency band. This is identical with the table shown in  FIG. 16 . 
       FIG. 20  is a process flow chart of the whole system of the embodiment for starting communication by using information stored in the DB  202 . Hereinafter, the process flow will be described with reference to  FIG. 20 . 
     An AP (for example, the AP  225 ) which receives a communication start request from an ST (for example, the ST  227 ) first obtains information from the DB  202  in order to obtain the latest wireless-channel use-state, and, based on the information, updates the DB information (for example, the DB 2255  in the AP 225 ) (step S 2001 ). At this time, when a method in which the DB information of the AP  225  is updated each time when the state of the DB  202  connected to the AP  225  or the TV is changed is employed, the work of step S 2001  is not required. Then, the AP  225  determines a wireless channel through which the ST  227  is to perform communication (step S 2002 ). The determined wireless channel is notified to the ST  227 , and the ST  227  which receives the notification starts communication by using the designated wireless channel (step S 2003 ). 
       FIG. 21  is a sequence diagram showing an example relating to message exchanges occurring in communication using DB information. The figure shows the case where the STs  226 ,  227  communicate with each other in the ad-hoc mode. 
     First, in the case where a request for starting communication is generated in the ST  226 , the ST  226  transmits a communication start request message to the AP  225  (step S 1601 ). At this time, in the case where the ST wishes to communicate with another ST in the ad-hoc mode, the message includes information such as the identification number of the communication partner. In the case of  FIG. 21 , there is a request for communicating with the ST  227 , and hence the identification number of the ST  227  is notified to the AP  225 . It is assumed that identification information such as the identification number of STs which are under control of the AP  225  is previously known at the establishment of the network (at power ON or the like) or through periodical updation. At the same time, also position information of the ST  226  is notified to the AP  225 . When these sets of information are to be notified to the AP  225 , communication may be performed by using a predetermined wireless channel dedicated to notification, or a wireless channel for notifying the information may be previously transmitted to the AP  225  by using a predetermined wireless channel dedicated to control. Alternatively, interference may be previously measured, and then the information may be notified by using an arbitrary wireless channel in which interference does not exist. 
     The AP  225  which receives the request of the ST  226  for communicating with the ST  227  first issues a position information request in order to check the position of the ST  227 , to the ST  227  (step S 1602 ). The ST  227  notifies the AP  225  of information of the position where the ST  227  currently exists (step S 1603 ). Thereafter, the AP  225  requests the DB  202  having interference information of the area to send a wireless-channel use-state (step S 1604 ). In response to the request, the DB  202  provides latest information (step S 1605 ). At this time, the AP  225  notifies the DB  202  of position information of the ST  226 , and position information with reference to information of the ST  227 . In the DB  202 , the amount of information to be subjected to communication can be reduced by providing the wireless-channel use-state relating to the periphery of the STs  226 ,  227 . Here, the periphery indicates a range where existing communication is affected by transmitting a signal from the STs  226 ,  227 , with reference to position information and in consideration of the transmission powers and antenna directionalities of the STs  226 ,  227 . 
     As a result of the above process, a DB  225 A in the AP  225  obtains information such as shown in  FIG. 15 . These sets of information are classified into several attributes. For example, information can be divided and organized into attributes such as a wireless channel which is received by the TV  224 , that which is transmitted by the TV broadcast station  201 , that which is used in communication by another terminal, that which is used by an unknown system, and in which interference exists, and that in which interference does not exist. 
     Returning to  FIG. 21 , the AP  225  determines the communication band by a method shown in  FIG. 22  described below, by using the position information of the ST  226 , the position information of the ST  227 , and the channel use-state obtained from the DB  202 , and notifies the STs  226 ,  227  of the communication channel (step S 1606 ). The STs  226 ,  227  perform a desired communication by using the notified communication band (wireless channel) (step S 1607 ). For example, the ST  226  transmits a preamble by using the notified wireless channel, and the ST  227  receives the preamble transmitted by the ST  226  and returns an ACK. Upon receiving the ACK, the ST  226  starts transmittance of data to be transmitted. In the AP  225 , at this time, the phenomenon that the STs  226 ,  227  are communicating with each other is registered in the DB. The AP  225  knows the wireless channel which is being used in communication. When a request for performing communication is newly generated, therefore, it is possible to prevent the communication between the STs  226 ,  227  from being disturbed. 
     When the communication is ended (step S 1608 ), the STs  226 ,  227  notify the AP  225  of the end of communication (step S 1609 ). In the AP  225 , the information indication that the STs  226 ,  227  are communicating with each other is deleted from the DB, and the wireless channel is opened to other communication. 
       FIG. 22  is a flowchart showing an example of a communication band determination algorithm. First, the AP  225  obtains a communication start request of the ST  226 , information of the communication partner, and position information of the ST  226  (step S 1701 ). Next, the ST  227  is requested to send position information of the ST  227 , and notify it to the AP  225 . Furthermore, information relating to the wireless-channel use-state is obtained from the DB  202  and the TV  224  (step S 1702 ). At this time, information relating to the wireless-channel use-state is periodically received from the STs  226 ,  227 . Therefore, it is assumed that, as the example shown in  FIG. 15 , the AP  225  already knows the wireless-channel use-states of the STs  226 ,  227 . 
     Next, the AP  225  checks and determines whether an unused wireless channel exists at the positions of the ST  226  issuing the communication request, and the ST  227  which is the communication partner or not (step S 1703 ). The criterion by which it is determined whether an unused wireless channel exists or not is as follows. The determination is conducted depending on whether, when the wireless-channel use-state is to be detected, a power which is higher than a threshold is measured at the position and in the wireless channel or not. If a common unused wireless channel exists (the result of the determination in step S 1703  is Yes), the AP  225  determines that the unused wireless channel is a wireless channel to be used in communication in the STs  226 ,  227 , and notifies the determination to the STs  226 ,  227 , thereby causing the STs  226 ,  227  to start communication (step S 1704 ). In case of the DB information such as the example shown in  FIG. 15 , a common unused wireless channel does not exist in the STs  226 ,  227 , and hence the determination in step S 1703  is No. 
     If a common unused wireless channel does not exist in STs to perform communication (the determination in step S 1703  is No), the AP  225  checks whether an unused wireless channel exists in one of the STs and a wireless channel that has not been measured exists in the other ST or not (step S 1705 ). In the example shown in  FIG. 15 , f02 MHz is an unused wireless channel for the ST  226 , and an unmeasured wireless channel for the ST  227 . Similarly, f07 MHz is an unused wireless channel for the ST  227 , and an unmeasured wireless channel for the ST  226 . If such a wireless channel exists (the result of the determination in step S 1705  is Yes), an interference measurement is performed on the unmeasured ST (step S 1706 ). In the example of  FIG. 15 , namely, a request for measuring the wireless channel of f07 MHz is issued to the ST  226 , and the wireless-channel use-state DB is updated by using a result of the measurement. Similarly, a request for measuring the wireless channel of f02 MHz is issued to the ST  227 , and the wireless-channel use-state DB is updated by using a result of the measurement. After, information of the wireless-channel use-state DB is updated, the process is restarted from the obtaining of position information in the ST  227  (step S 1701 ). 
     If the result of the determination in step S 1705  is No, it is checked whether an unmeasured wireless channel exists in both the STs  226 ,  227  or not (S 1707 ). If the result of the determination in step S 1707  is Yes, the measurement request is issued (step S 1708 ). 
     If a common unused wireless channel does not exist in the STs  226 ,  227  and also an unmeasured wireless channel does not exist (the result of the determination in step S 1707  is No), it is determined whether an apparatus using a wireless channel which is not an unused wireless channel exists in the periphery of apparatuses performing communication (the STs  226 ,  227 ) or not (step S 1709 ). In an example in which it is determined whether the position where another apparatus exists is in “periphery” or not, the determination is conducted depending on whether, when a transmission signal is transmitted, the reception performance of an apparatus which performs communication and broadcast reception through the same wireless channel is affected in consideration of the transmission powers and antenna directionalities or not. For example, the case where the ST  226  having an omnidirectional antenna performs transmission at a frequency band of f01 MHz by using a certain transmission power will be considered. The distance between the TV  224  and ST  226  which are receiving in a wireless channel of f01 MHz is calculated, and the propagation loss of a signal transmitted from the ST  226  is calculated. If the distance is a distance at which a signal power higher than the reception tolerance of the TV  224  is expected to reach the TV  224 , it is determined that the TV  224  is in the periphery of the ST  227 . Similarly, determination whether, with respect to communication performed at position Nx 04 , Ey 04  and in the frequency band of f05 MHz, the position where the communication is performed in the periphery of the STs  226 ,  227  or not is conducted in the following manner. The propagation distance in which the transmission powers and antenna directionalities of the STs  226 ,  227  are considered is calculated. From the distances between the STs  226 ,  227  and the position Nx 04 , Ey 04 , a signal power which reaches the position Nx 04 , Ey 04  is calculated. Then, the reception tolerance of the communication is compared with the calculated signal power which reaches the position Nx 04 , Ey 04 . If the reception tolerance of the communication is larger, it is determined that the position is not in the periphery, and, if the reaching signal power is larger, it is determined that the position is in the periphery. In the case where determination whether an apparatus is in the periphery or not is conducted by using such a criterion, transmission powers, antenna directionalities, reception tolerances and the like of communication system apparatuses which can be set to be under control must be previously registered in the AP  225 . 
     If it is determined in step S 1709  that an apparatus using the wireless channel does not exist in the periphery (the result of the determination is Yes), the wireless-channel use-state is measured in the place by way of precaution in the STs  226 ,  227  (step S 1710 ). This is performed because, even in the case where an apparatus using the wireless channel does not exist in the periphery, when an apparatus which is not in the periphery has a large transmission power, the power causes interference to disable communication. In the example of  FIG. 15 , also in the case where it is determined that communication performed at position Nx 04 , Ey 04  and in the frequency band of f05 MHz is not in the periphery, a measurement is performed in the frequency band of f05 MHz, and, if a signal power higher than the reception tolerance is measured in the STs  226 ,  227  (the result of the determination in step S 1711  is No), it is determined that the wireless channel is not an unused wireless channel. If it is determined in step S 1709  or S 1711  that an unused wireless channel does not exist, it is determined that wireless channels are busy and communication is disabled, and, after a predetermined time period elapses (step S 1712 ), position information of a communication terminal is again obtained (step S 1701 ). If it is determined in step S 1711  that the wireless channel is an unused wireless channel (the result of the determination in step S 1711  is Yes), it is determined that an apparatus using the wireless channel does not exist in the periphery, and instructions for performing communication by using the wireless channel are issued to the STs  226 ,  227 . As a result, the STs  226 ,  227  start communication (step S 1713 ). 
     As described above, in the system, an ST of a wireless LAN is caused to detect a wireless-channel use-state, reception information of a TV and information in a local server are obtained, and an AP designates a wireless channel used for an ST, whereby communications between ST-AP and ST-ST are enabled by using a time zone in which a TV broadcast is not performed, a frequency band and place in which a peripheral TV does not perform reception, and a frequency band and place in which other STs do not perform reception. Therefore, the frequency use efficiency is remarkably improved as compared with a method in which a frequency is allocated to each system. 
     Modification of the Third Embodiment 
     Hereinafter, a modification of the third embodiment will be described. 
       FIG. 23  is a process flow chart of the whole system for starting communication by using DB information. An AP which receives a communication start request from an ST checks position information of an apparatus which performs communication (step S 2301 ). Thereafter, the reliability is calculated by using information registered in the DB of the AP (step S 2302 ). If it is determined that current information is reliable (step S 2303 ), unused channels in an apparatus which performs communication are determined by using the information (step S 2304 ), and a communication channel is selected from the unused channels (step S 2305 ). If a communication channel exists (step S 2310 ), this is notified to the ST. The ST which receives the notification starts communication by using the notified channel (step S 2306 ). 
     If reliable information does not exist (the determination in step S 2303  is No), or if an unused channel exists but it is determined that a communication channel does not exist (the determination in step S 2310  is No), it is determined whether the time is out or not (step S 2307 ). If it is determined that the time is not out (the determination in step S 2307  is No), the channel use-state detection is measured (step S 2308 ), and the process is restarted from the checking of position information (step S 2301 ). If it is determined that the time is out (the determination in step S 2307  is Yes), it is determined that communication is disabled (step S 2309 ), and this is notified to the user or the high-level layer. As an index of the time-out, the number or time of remeasurements may be used. 
       FIG. 24  is a detailed flowchart of the process of the communication channel determination (step S 2305 ) in  FIG. 23 . In the communication channel determination process, one channel is selected from candidates of a channel which performs communication, and it is judged whether the channel can be determined as a communication channel. First, one cannel is selected (step S 2401 ), and it is determined whether the channel is a channel in which radio transmission is inhibited by the law of the country, or that which is used by an apparatus that is very susceptible to interference, such as radio astronomy, and in which transmission is inhibited (step S 2402 ). If the channel is a channel in which transmission is inhibited, it is immediately determined that the channel is not used as a communication channel (step S 2405 ), and the process is transferred to the next candidate channel (step S 2406 ). By contrast, if the channel is not a channel in which transmission is inhibited, it is determined whether the channel is an unused channel in an apparatus which performs communication or not (step S 2403 ). Here, it is determined whether the channel is an unused channel in the STs  226 ,  227  or not. In the example of  FIG. 16 , it is determined in S 404  that the channel of f07 MHz is an unused channel in the STs  226 ,  227 , and hence the condition of step S 2403  is Yes. In this case, the channel of f07 MHz is set as a communication channel (step S 2408 ). If a common channel does not exist and the determination of step S 2403  is No, it is further determined whether, for the channel, communication that is used on the transmission side but not used on the reception side exists or not (step S 2404 ). In this example, it is determined whether, for a TV broadcast, the channel is a channel in which the broadcast station  201  performs transmission and the TVs  214 ,  224  connected with the TV receiving antennas  213 ,  223  existing in the periphery do not perform reception or not. 
     The determination of “periphery” is conducted by using above-described Expression (2) depending on whether transmission signals of the STs  226 ,  227  interfere with the TV receiving antennas  213 ,  223  or not. If the position, antenna directionality, and transmission power by which interference is suppressed to the noise level or lower are attained, the determination of S 1004  is Yes. In this case, the channel is set as a communication channel (step S 2408 ). If interference cannot be suppressed to the noise level or lower, it is determined that the channel cannot be set as a communication channel (step S 2405 ), and the process is transferred to the next candidate channel (step S 2406 ). The determination is conducted on all channels (step S 2406 ). If a communication channel is not determined, it is determined that a communication channel does not exist (step S 2407 ). 
       FIG. 25  is a sequence diagram relating to message exchanges occurring in communication using DB information. The figure shows the case where the STs  226 ,  227  communicate with each other in the ad-hoc mode. In the case where a request for starting communication is generated in the ST  226 , the ST  226  transmits a communication start request message to the AP  225  (step S 2501 ). At this time, in the case where the ST wishes to communicate with another ST in the ad-hoc mode, the message includes information such as the identification number of the communication partner. In the case of  FIG. 25 , there is a request for communicating with the ST  227 , and hence the identification number of the ST  227  is notified to the AP  225 . It is assumed that identification information such as the identification number of STs which are under control of the AP  225  is previously known at the establishment of the network (at power ON or the like) or through periodical updation. At the same time, also position information of the ST  226  is notified to the AP  225 . When these sets of information are to be notified to the AP  225 , communication may be performed by using a predetermined channel dedicated to notification, or a channel for notifying the information may be previously transmitted to the AP  225  by using a predetermined channel dedicated to control. Alternatively, interference may be previously measured, and then the information may be notified by using an arbitrary channel in which interference does not exist. 
     The AP  225  which receives the request of the ST  226  for communicating with the ST  227  first issues a position information request in order to check the position of the ST  227 , to the ST  227  (step S 2502 ). The ST  227  notifies the AP  225  of information of the position (position information) where the ST  227  currently exists (step S 2503 ). Thereafter, the AP  225  requests the DB  202  having interference information of the area to send a channel use-state. In response to the request, the DB  202  provides latest information. 
     The AP  225  calculates the reliability of information by using position information of the ST  226 , position information of the ST  227 , information obtained from the DB  202 , information of the DB in the AP  225 , and the like (step S 2504 ). An example of calculation of the reliability of information of the channel of f07 MHz in the ST  226  will be p described with reference to  FIG. 16 . Information which is in the AP  225 , and which relates to the channel of f07 MHz is information ( 16 A) measured in the ST  226  and information ( 16 B) measured in the ST  227 . By using these sets of information, the reliability of information in the channel of f07 MHz in the ST  226  is calculated. When the current time is set to 12:21:52.20 and the distance between the STs  226 ,  227  is set to 5 m, following Expression (7) is obtained. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     This is the position of the ST  226 . As a result, when the reliability is higher than a threshold, an unused channel is recognized by using these sets of information (step S 2505 ), and, when the reliability is lower than the threshold, a process for enhancing the reliability is performed. In order to enhance the reliability, in this case, the ST  226  may again measure the channel use-state. Alternatively, in the case such as that the ST  226  is an ST which cannot measure a channel use-state, a channel use-state measuring apparatus which is situated as close as possible may be caused to measure a channel use-state. 
     It is assumed that determination on an unused channel is conducted by using a reception power of −50 dBm in the example of  FIG. 16 . For the ST  226 , the channels of f02 MHz and f07 MHz are unused channels, and, for the ST  227 , the channel of f07 MHz is an unused channel. 
     Thereafter, the AP  225  selects and determines the communication band (communication channel) by a method described with reference to  FIG. 24  by using position information of the ST  226 , position information of the ST  227 , information obtained from the DB  202  (step S 2506 ). 
     When the communication band is determined, it is notified to the STs  226 ,  227  (steps S 2507 , S 2508 ). The STs  226 ,  227  perform a desired communication by using the notified communication band (step S 2509 ). For example, the ST  226  transmits a preamble by using the notified channel, and the ST  227  receives the preamble transmitted by the ST  226  and returns an ACK. Upon receiving the ACK, the ST  226  starts transmittance of data to be transmitted. In the AP  225 , at this time, the phenomenon that the STs  226 ,  227  are communicating with each other is registered in the DB. The AP  225  knows the channel which is being used in communication. When a request for performing communication is newly generated, therefore, it is possible to prevent the communication between the STs  226 ,  227  from being disturbed. 
     When the communication is ended (step S 2510 ), the STs  226 ,  227  notify the AP  225  of the end of communication (steps S 2511 , S 2512 ). In the AP  225 , the information indicating that the STs  226 ,  227  are communicating with each other is deleted from the DB, and the channel is opened to other communication. 
     As described above, an ST of a wireless LAN is caused to detect a channel use-state, an unused channel is determined by using a reliability, reception information of a TV and information in a local server are obtained to determine a communication band, and an AP designates a channel used for an ST, whereby communications between ST-AP and ST-ST are enabled by using a time zone in which a TV broadcast is not performed, a frequency band and place in which a peripheral TV does not perform reception, and a frequency band and place in which other STs do not perform reception. Therefore, the frequency use efficiency is remarkably improved as compared with a method in which a frequency is allocated to each system. 
     Fourth Embodiment 
     Next, a fourth embodiment of the invention will be described. The embodiment is an example in the case where a cellular radio system performs communication by using a frequency band of a radar or radio astronomy. 
       FIG. 26  is a diagram showing an example of the system configuration of the embodiment. A radio astronomy receiver  501  and a local DB  502  can exchange information about the current wireless-channel use-state by wire or wireless. Each of base stations  506 ,  507  has a database (DB) for a wireless-channel use-state, and the base stations can exchange information through the local DB  502 . Terminals  504 ,  505  are under control of the base station  506 , and a terminal  508  is under control of the base station  507 . Each of the terminals also has a wireless-channel use-state database. The terminals communicate by wireless with the respective base stations which control the terminals, and the base stations communicate with each other by wire. When two of the terminals communicate with each other, the communication is performed through one of the base stations. It is assumed that a radar  503  transmits a signal to the periphery, and gives interference to the positions of the base station  506  and the terminal  504 , or in a wireless channel. 
     The base station  506  is configured in a substantially same manner as the AP  225  in above-described third embodiment except that the TV input IF is not disposed. 
       FIG. 27  is a diagram schematically showing the configuration of the terminal  504 . The terminal  504  of  FIG. 27  comprises: an interference measuring portion  5041  which scans wireless channels of a usable frequency band to measure interference; an unused-channel recognizing portion  5042  which recognizes an unused wireless channel on the basis of a result of the scan of wireless channels in the interference measuring portion  5041 ; an information registering portion  5043  which registers information of the unused channel recognized by the unused-channel recognizing portion  5042 , into a database  5044 ; a communication channel setting portion  5047  which sets a communication channel on the basis of information received from the database  5044  and relating to a wireless channel in an unused (not used) state; a wireless communication transmitting/receiving portion  5046  which modulates a signal to be transmitted through the set communication channel, and which demodulates a received signal; and a command producing portion  5045  which produces various commands on the basis of instructions from a high-level layer. 
       FIG. 28  is a view showing an example of wireless-channel use-state information in the database  5044  which stores the wireless-channel use-state of the terminal  504 . The terminal  504  measures interference at position Nx 01 , Ey 01  in f01 MHz, f02 MHz, f03 MHz, and f04 MHz that are candidates of a wireless channel in which communication is to be performed, and records a result of the measurement into the DB. In the example of  FIG. 28 , interference is not measured in f01 MHz, f02 MHz, and f03 MHz, and interference is measured in f04 MHz. The use-state of radio astronomy is notified from the base station  506 . 
       FIG. 29  is a view showing an example of wireless-channel use-state information stored in the wireless-channel use-state DB of the terminal  505 . In the same manner as the terminal  504 , interference is measured at position Nx 02 , Ey 02  in f01 MHz, f02 MHz, f03 MHz, and f04 MHz, and a result of the measurement is recorded into the DB. Here, interference is not measured in all of the measured wireless channels. The use-state of radio astronomy is notified from the base station  506 . 
       FIG. 30  is a view showing an example of wireless-channel use-state information stored in the wireless-channel use-state DB of the base station  506 . In this example, the measurement results notified from the terminals  504 ,  505 , and a use-state of the radio astronomy  501  notified from the DB  502  are recorded. 
       FIG. 31  shows an example of the wireless-channel use-state information stored in the local DB  502 . In the example, when the radio astronomy  501  starts the measurement, the frequency band and the use-state are notified to the DB  502 . Also information notified from the connected base stations  506 ,  507  is recorded. 
       FIG. 32  is a flowchart in the case where a terminal notifies a base station of a wireless-channel use-state. It is assumed that the terminal periodically measures the wireless-channel use-state. When a fixed time period (for example, 1.0 second) has elapsed, the terminal measures the wireless-channel use-state. First, the wireless-channel use-state of a region which is determined as an unused region in the previous measurement is measured (step S 3201 ). At this time, if it is determined that interference exists in the measured wireless channel (the result of the determination in step S 3202  is Yes), the DB in the terminal is updated, and the base station is notified that the wireless-channel use-state is changed (step S 3203 ). The notified base station updates information of its own DB. The determination whether interference exists or not is performed depending on whether the reception power of the measured wireless channel is equal to lower than a threshold or not. The threshold is a preset value in the system, and, for example, a power of −60 dBm. If the reception power in the measured wireless channel is higher than the threshold, it is determined that a signal transmitted from any apparatus exists as interference in the wireless channel in addition to thermal noise. 
     After the measurement for the unused region is ended, a wireless channel in which it is determined in the previous measurement that interference exists is measured (step S 3204 ) If it is determined that interference does not exist in the measured wireless channel (the result of the determination in step S 3205  is Yes), the DB in the terminal is updated, and the base station is notified that the wireless-channel use-state is changed (step S 3206 ). Similarly, the notified base station updates information of its own DB. 
       FIG. 33  is a sequence diagram showing message exchanges occurring when the radio astronomy  501  starts the measurement. Hereinafter, message exchanges will be described with reference to  FIG. 33 . 
     Immediately before the radio astronomy  501  starts the measurement, the radio astronomy  501  notifies the DB  502  of the start of measurement, and position information of the radio astronomy  501  (step S 3301 ). When the DB  502  receives a message that the radio astronomy  501  starts the measurement, the DB notifies the base stations  506 ,  507  of the area under control of the DB  502 , of the information at the highest priority (steps S 3302 , S 3303 ). 
     The base stations  506 ,  507  notify the terminals  505 ,  504 ,  508  under control of the stations of the information (steps S 3304 , S 3305 , S 3306 ). In a terminal receiving the notification that the radio astronomy starts the measurement, even when interference does not exist in the wireless channel, the wireless channel is registered as an unusable wireless channel. When the radio astronomy stops the measurement, a message that the wireless channel is opened is notified to the terminals, and the wireless channel is again allowed to be used. 
       FIG. 34  is a sequence diagram showing an example of message exchanges in the case where determination that interference exists as a result of the measurement of a wireless-channel use-state by the terminal  504  is notified. Hereinafter, message exchanges will be described with reference to  FIG. 34 . 
     In the case where the terminal  504  determines in the present measurement that, in a wireless channel which is determined as an unused wireless channel in the previous measurement (step S 3401 ), interference exists (step S 3402 ), the terminal  504  notifies the base station  506  which controls the terminal  504 , of information of the position where the terminal  504  exists, and that of interference of the measured wireless channel (step S 3403 ). The base station  506  to which the information is notified determines that the wireless-channel use-state in a terminal under control is changed, and notifies the information to the local wireless-channel use-state DB  502  (step S 3404 ). The notification of information is performed by wireless. The terminal  504  performs notification on the base station  506  by using a predetermined notification wireless channel. The notification wireless channel is used only in notification of wireless-channel use-state information. For other user data, from a result of recognition, communication is performed by using a wireless channel other than the notification wireless channel. The notification wireless channel is a dedicated wireless channel allocated to the cognitive system, and cannot be used by another system. 
       FIG. 35  is a sequence diagram showing an example of message exchanges in the case where determination that interference does not exist as a result of the measurement of a wireless-channel use-state by the terminal  504  is notified. Hereinafter, message exchanges will be described with reference to  FIG. 35 . 
     In the case where the terminal  504  determines in the present measurement that, in a wireless channel which is determined as a wireless channel in which interference exists in the previous measurement (step S 3501 ), interference does not exist (step S 3502 ), the terminal  504  notifies the base station  506  which controls the terminal  504 , of information of the position where the terminal  504  exists, and that of an unused wireless channel of the measured wireless channels (step S 3503 ). The base station  506  to which the information is notified determines that the wireless-channel use-state in a terminal under control is changed, and notifies the information to the local wireless-channel use-state DB  502  (step S 3504 ). 
       FIG. 36  is a sequence diagram showing an example of message exchanges in the case where the terminal  504  starts communication. Hereinafter, the message exchange will be described with reference to  FIG. 36 . 
     The terminal  504  transmits a message requesting communication with the terminal  508  to the base station  506  which controls the terminal  504  (step S 3601 ). The message is transmitted by using a fixed control wireless channel. At this time, simultaneously with a communication start request, also identification information of the communication partner, position information of the terminal  504 , and information of the wireless channel to be used are notified. 
     The base station  506  notifies the base station  507  which controls the terminal  508 , of the communication start request and the identification information of the communication partner (step S 3602 ). The base station  507  notifies the terminal  508  of the communication start request and the wireless channel to be used (step S 3603 ). At this time, the to-be-used wireless channel which is notified from the base station  507  to the terminal  508  is different from that of the terminal  504  and the base station  506 . When the pairs are spatially sufficiently separated from each other, the same wireless channel may be used. When the to-be-used wireless channel which is notified from the base station  507  to the terminal  508  is usable, the terminal returns an ACK. The base station  507  which receives the ACK notification from the terminal  508  returns an ACK to the base station  506  (step S 3605 ), and the base station  506  returns an ACK to the terminal  504  (step S 3606 ). After these procedures, the terminal  504  communicates with the terminal  508  by way of the base stations  506 ,  507  (step S 3607 ). 
       FIG. 37  is a sequence diagram showing an example of message exchanges in the case where the terminal  505  starts communication. Hereinafter, the message exchange will be described with reference to  FIG. 37 . 
     The terminal  505  transmits a message requesting communication with the terminal  508  to the base station  506  which controls the terminal  505  (step S 3701 ). The message is transmitted by using the fixed control wireless channel. At this time, simultaneously with the communication start request, also identification information of the communication partner, position information of the terminal  505 , and information of the wireless channel to be used are notified. The base station  506  notifies the base station  507  which controls the terminal  508 , of the communication start request and the identification information of the communication partner (step S 3702 ). The base station  507  notifies the terminal  508  of the communication start request and the wireless channel to be used (step S 3703 ). If the to-be-used wireless channel which is notified from the base station  507  to the terminal  505  can be used, an ACK is returned (step S 3704 ) The base station  507  which receives the notification of the ACK from the terminal  508  returns the ACK to the base station  506  (step S 3705 ). If the base station  506  determines that the to-be-used wireless channel which is notified from the terminal  505  cannot be used because of a reason such as that inference exists, the base station  506  notifies the terminal  505  that the designated wireless channel cannot be used, in parallel with the message exchange with the base station  507  (step S 3706 ). At this time, also the reason of this is simultaneously notified. It is assumed that a signal of the radar  503  which cannot be measured as interference in the terminal  505  functions as interference to the base station  506  and is not determined as an unused wireless channel. The determination whether interference exists in the wireless channel designated by the base station  506  or not may be conducted by using a result of an interference measurement performed by the terminal  504  in the vicinity of the base station  506  on the basis of position information, or alternatively by using result of an interference measurement performed by the base station  506  itself. For the reason of the unusable state, for example, triangulation may be performed by using measurement information of the terminal  504  and estimated information of the arrival direction of interference, and measurement information of the base station  506  and estimated information of the arrival direction of interference, the position of the radar  503  which is an interference source may be identified, and the position may be notified to the terminal  505 . In this case, the terminal  505  and the base station  506  perform communication while the beam directions of transmission signals are directed to each other, or the null direction is directed to the direction of the radar  503 , whereby interference by the radar  503  can be avoided from occurring. By such a method, the terminal  505  again notifies the base station  506  of the to-be-used wireless channel (step S 3707 ), and the base station  506  checks that the wireless channel designated by the terminal  505  is a usable wireless channel, and the ACK is returned from the base station  507 , and thereafter returns an ACK to the terminal  505  (step S 3708 ). After these procedures, the terminal  505  communicates with the terminal  508  by way of the base stations  506 ,  507  (step S 3709 ). 
     As described above, terminals and base stations of a cellular radio system measure a wireless-channel use-state to obtain use-states of radio astronomy and a radar, and information of a local server, whereby a time zone, frequency band, and place where interference does not exist are recognized, and communication can be performed by using these wireless channels. Therefore, the frequency use efficiency is remarkably improved as compared with a method in which a frequency is allocated to each system. 
     Although the embodiments of the invention have been described, the invention is not restricted to the embodiments as they are, and, in an implementation phase, may be embodied while modifying components without departing from the spirit of the invention. Furthermore, various inventions can be formed by adequately combining plural components disclosed in the embodiments. For example, several components may be deleted from all components shown in the embodiments, and components extending over different embodiments may be adequately combined with one another.