Patent Publication Number: US-2009219909-A1

Title: Method for switching and managing frequency access in cognitive radio system, base station and subscriber station using the same

Description:
TECHNICAL FIELD 
     The present invention relates to a method of switching and managing frequency access in a cognitive radio system, and a base station (BS) and a customer premises equipment (CPE, a kind of user equipment) using the method, and more particularly, to a method of allowing a BS or a CPE to determine whether a set frequency channel (frequency access) is not available, a channel switching method for switching from a detected frequency channel, which is determined to be unavailable, to another frequency channel, and a method of allowing a BS or a CPE to efficiently manage frequency channels. 
     BACKGROUND ART 
     The present invention improves data transmission efficiency and spectral utilization efficiency by switching from a frequency channel, which is being used for communication established between a base station (BS) and a customer premises equipment (CPE), to another available frequency channel by exchanging MAC layer control messages of the BS and the CPE when the BS or the CPE determines that the communication channel in use is not available in a cognitive radio system due to access of an incumbent user thereto, multi-path fading, or other channel deterioration, thereby protecting the incumbent user&#39;s vested right for a specific frequency band and securing a better transmission channel. 
     A licensed user belonging to an existing communication system establishes communication with a priority right to use a specific frequency band. Thus, the licensed user can continue occupancy of the specific frequency band. Under such an environment, channel switching is performed in a previously allowed frequency band, and thus, a priority right to use even a channel that is to be switched is also guaranteed. Unlike the above existing communication system, the present invention relates to use of a cognitive radio technology under an open spectrum policy that does not guarantee a priority right to use. In this disclosure, for convenience, a licensed user will be referred to as an incumbent user in terms of a cognitive radio system. 
     Cognitive radio technology allows a frequency band that is not in use by an incumbent user to be used by aperiodically or periodically scanning or sensing whether the incumbent user is using the frequency band. Further, even if the frequency band is in use, whether the incumbent user accesses the frequency band is periodically sensed in order to discontinue receiving/transmitting of data and switch from a current communication channel to another frequency band that the incumbent user does not access when access of the incumbent user is sensed. Accordingly, it is possible to continue communication by re-establishing the discontinued receiving/transmitting of the data. In this connection, a cognitive radio system according to the present invention is differentiated from the existing communication system. 
     The main part of the cognitive radio technology is a channel switching process in which access of the incumbent user to a frequency band is sensed, and communication in the frequency band is discontinued and a communication channel is switched to another frequency band that the incumbent user does not access when the access is sensed. In order to solve problems with the channel switching process, the present invention provides channel switching processes for various scenarios that occur when the incumbent user accesses a frequency band. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 through 4  illustrate signaling used in a channel switching process according to an embodiment of the present invention. 
         FIG. 5  is a flowchart illustrating a channel switching method according to another embodiment of the present invention. 
         FIG. 6  illustrates diagrams for explaining scenarios for reporting access of an incumbent user in a Frequency Division Duplex (FDD) system. 
         FIG. 7  illustrates tables specifying channel switching methods available for the scenarios illustrated in  FIG. 6 . 
         FIG. 8  is a table illustrating two MAC control messages according to an embodiment of the present invention. 
         FIG. 9  is a flowchart illustrating a process in which a customer premises equipment (CPE) transmits the two messages illustrated in  FIG. 8  to a base station, according to an embodiment of the present invention. 
         FIG. 10  is a table illustrating a method of allowing a base station to determine whether a frequency channel is in use, according to an embodiment of the present invention. 
         FIG. 11  is a flowchart illustrating a channel switching method performed by a base station in a FDD system, according to an embodiment of the present invention. 
         FIG. 12  is a flowchart illustrating a channel switching method performed by a CPE in a FDD system, according to an embodiment of the present invention. 
         FIG. 13  is a flowchart illustrating a channel switching method performed by a base station in a Time Division Duplex (TDD) system, according to an embodiment of the present invention. 
         FIG. 14  is a flowchart illustrating a channel switching method performed by a CPE in a TDD system, according to an embodiment of the present invention. 
         FIG. 15  is a block diagram of channel switching apparatuses that are respectively included in a base station and a CPE of a cognitive radio system, according to an embodiment of the present invention. 
         FIG. 16  is a block diagram of channel switching apparatuses that are respectively included in a base station and a CPE of a cognitive radio system, according to another embodiment of the present invention. 
         FIG. 17  is a block diagram of channel switching apparatuses that are respectively included in a base station and a CPE of a cognitive radio system, according to another embodiment of the present invention. 
         FIG. 18  is a block diagram of channel switching apparatuses that are respectively included in a base station and a CPE of a cognitive radio system, according to another embodiment of the present invention. 
         FIG. 19  is a block diagram of a frequency-channel availability determination apparatus included in a base station of a cognitive radio system according to another embodiment of the present invention. 
         FIG. 20  is a diagram illustrating the concept of an uplink/downlink association method in a conventional FDD system. 
         FIG. 21  is a diagram illustrating the concept of a method of managing an uplink/downlink frequency channel in an OFDM-FDD system, according to an embodiment of the present invention. 
         FIG. 22  illustrates a manner in which a base station transmits channel matching information and channel grouping information to a CPE, according to an embodiment of the present invention. 
         FIG. 23  is a block diagram of an uplink/downlink frequency channel management apparatus included in a base station and an uplink/downlink frequency channel management apparatus included in a CPE, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Technical Problem 
     The present invention provides a method of determining whether a frequency channel is available in a cognitive radio system capable of efficiently sensing access of an incumbent user. 
     The present invention also provides a channel switching method for switching from a frequency channel that is sensed to be unavailable to another frequency channel that an incumbent user does not use. 
     The present invention also provides a method of efficiently managing a frequency channel in a cognitive radio system. 
     The present invention also provides a base station and a customer premises equipment (CPE) of a cognitive radio system which are capable of performing the above method. 
     Technical Solution 
     According to an aspect of the present invention, there is provided a channel switching method performed by a base station in a cognitive radio system, the method comprising detecting a currently unavailable first frequency channel from among at least one uplink/downlink frequency channel which is set as a communication channel with customer premises equipment (CPE); transmitting a channel switching request message requesting switching from the first frequency channel to a second frequency channel; and performing channel switching from the first frequency channel to the second frequency channel when receiving from the CPE a channel switching report message that reports that channel switching is to be performed in response to the channel switching request message. 
     According to another aspect of the present invention, there is provided a channel switching method performed by customer premises equipment (CPE) in a cognitive radio system, the method comprising performing channel switching from a currently unavailable first frequency to a second frequency channel, from among at least one uplink/downlink frequency channel which is set as a communication channel with a base station, when receiving a channel switching request message requesting switching from the first frequency channel to the second frequency channel; and transmitting to the base station a channel switching report message that reports that channel switching is to be performed in response to the channel switching request message. 
     According to another aspect of the present invention, there is provided a channel switching method performed by a base station in a cognitive radio system, the method comprising detecting a currently unavailable first frequency channel from among at least one uplink/downlink frequency channel which is set as a communication channel with customer premises equipment (CPE); and performing channel switching from the first frequency channel as a communication channel to a second frequency channel which is predefined with the CPE. 
     According to another aspect of the present invention, there is provided a channel switching method performed by customer premises equipment (CPE) in a cognitive radio system, the method comprising detecting a currently unavailable first frequency channel from among at least one uplink/downlink frequency channel which is set as a communication channel with a base station; and the CPE performing channel switching from the first frequency to a second frequency channel which is predefined with the base station. 
     According to another aspect of the present invention, there is provided a channel switching method performed by a base station in a cognitive radio system, the method comprising detecting a currently unavailable first frequency channel from among at least one uplink/downlink frequency channel which is set as a communication channel with customer premises equipment (CPE); transmitting a channel switching request message requesting switching from the first frequency channel to a second frequency channel, to the CPE via an outband channel; and upon receiving from the CPE a channel switching report message that reports that channel switching is to be performed in response to the channel switching request message, switching from the first frequency channel to the second frequency channel. 
     According to another aspect of the present invention, there is provided a channel switching method performed by customer premises equipment (CPE) in a cognitive radio system, the method comprising receiving a channel switching request message requesting switching from a first frequency channel, which is a currently set communication channel, to a second frequency channel from a base station via an outband channel; switching from the first frequency channel to the second frequency channel based on the channel switching request message; and transmitting to the base station a channel switching report message that reports that channel switching is to be performed in response to the channel switching request message. 
     According to another aspect of the present invention, there is provided a channel switching method performed by a base station in a cognitive radio system, the method comprising detecting a currently unavailable first frequency channel from among at least one uplink/downlink frequency channel which is set as a communication channel with customer premises equipment (CPE); performing explicit signaling in which a channel switching request message requesting switching from the first frequency channel to a second frequency channel is transmitted to the CPE, and waiting for a channel switching report message that reports that channel switching is to be performed in response to the channel switching request message to be received from the CPE; upon receiving the channel switching report message from the CPE, performing channel switching from the first frequency channel to the second frequency channel, using explicit signaling; and when the channel switching report message is not received from the CPE, performing channel switching from the first frequency channel to a third frequency channel which is predefined with the CPE, using implicit signaling. 
     According to another aspect of the present invention, there is provided a channel switching method performed by customer premises equipment (CPE) in a cognitive radio system, the method comprising performing channel switching using explicit signaling, in which, when receiving a channel switching request message requesting switching from a currently unavailable first frequency channel to a second frequency channel from among at least one uplink/downlink frequency channel which is set as a communication channel with a base station, a channel switching report message reporting which channel switching is to be performed in response to the channel switching request message is transmitted to the base station and the first frequency channel is switched to the second frequency channel; and performing channel switching using implicit signaling, in which the currently unavailable first frequency channel is detected based on whether messages that are to be received, including the channel switching request message, are received and the result of a channel search performed by the CPE; and the detected first frequency channel is switched to a third frequency channel which is predefined with the base station. 
     According to another aspect of the present invention, there is provided a method of allowing a base station to determine whether a frequency channel for communication with customer premises equipment (CPE) is available in a cognitive radio system, the method comprising determining whether a channel search report message, which is transmitted from the CPE in response to a channel search request from the base station, and an automatic report message, which is periodically transmitted from the CPE to the base station, are received from the CPE; and determining whether an uplink/downlink frequency channel is available for the CPE, based on whether the channel search report message is received and whether the automatic report message is received. 
     According to another aspect of the present invention, there is provided a channel switching apparatus included in a base station of a cognitive radio system, the apparatus comprising a detecting unit detecting a currently unavailable first frequency channel from among at least one uplink/downlink frequency channel which is set as a communication channel with customer premises equipment (CPE); a channel switching request message transmitting unit transmitting a channel switching request message requesting switching from the first frequency channel to a second frequency channel, to the CPE; and a channel switching unit switching from the first frequency channel to the second frequency channel when receiving from the CPE a channel switching report message that reports that channel switching is to be performed in response to the channel switching request message. 
     According to another aspect of the present invention, there is provided a channel switching apparatus included in customer premises equipment (CPE) of a cognitive radio system, the apparatus comprising a channel switching unit switching from a currently unavailable first frequency channel to a second frequency channel from among at least one uplink/downlink frequency channel which is set as a communication channel with a base station, when receiving a channel switching request message requesting switching from the first frequency channel to the second frequency channel; and a channel switching report message transmitting unit transmitting to the base station a channel switching report message that reports that channel switching is to be performed in response to the channel switching request message. 
     According to another aspect of the present invention, there is provided a channel switching apparatus included in a base station of a cognitive radio system, the apparatus comprising a detecting unit detecting a currently unavailable first frequency channel from among at least one uplink/downlink frequency channel which is set as a communication channel with customer premises equipment (CPE); and a channel switching unit switching from the first frequency channel as a communication channel to a second frequency channel which is predefined with the CPE. 
     According to another aspect of the present invention, there is provided a channel switching apparatus included in customer premises equipment (CPE) of a cognitive radio system, the apparatus comprising a detecting unit detecting a currently unavailable first frequency channel from among at least one uplink/downlink frequency channel which is set as a communication channel with a base station; and a channel switching unit switching from the first frequency channel as a communication channel to a second frequency channel which is predefined with the base station. 
     According to another aspect of the present invention, there is provided a channel switching apparatus included in a base station of a cognitive radio system, the apparatus comprising a detecting unit detecting a currently unavailable first frequency channel from among at least one uplink/downlink frequency channel which is set as a communication channel with customer premises equipment (CPE); a channel switching request message transmitting unit transmitting a channel switching request message requesting switching from the first frequency channel to a second frequency channel, to the CPE via an outband channel; and a channel switching unit switching from the first frequency channel to the second frequency channel, when receiving from the CPE a channel switching report message that reports that channel switching is to be performed in response to the channel switching request message. 
     According to another aspect of the present invention, there is provided a channel switching apparatus included in customer premises equipment (CPE) of a cognitive radio system, the apparatus comprising a channel switching request message receiving unit receiving a channel switching request message requesting switching from a first frequency channel, which is a currently set communication channel, to a second frequency channel, from the base station via an outband channel; a channel switching unit switching the first frequency channel as a communication channel to the second frequency channel based on the channel switching request message; and a channel switching report message transmitting unit transmitting to the base station a channel switching report message that reports that channel switching is to be performed in response to the channel switching request message. 
     According to another aspect of the present invention, there is provided a channel switching apparatus included in a base station of a cognitive radio system, the apparatus comprising a detecting unit detecting a currently unavailable first frequency channel from among at least one uplink/downlink frequency channel which is set as a communication channel with customer premises equipment (CPE); an explicit signaling unit transmitting a channel switching request message requesting switching from the first frequency channel to a second frequency channel to the CPE, and waiting to receive from the CPE a channel switching report message that reports that channel switching is to be performed in response to the channel switching request message; a channel switching unit switching from first frequency channel to the second frequency channel by using explicit signaling, when receiving the channel switching report message from the CPE; and a channel switching unit switching from the first frequency channel to a third frequency channel, which is predefined with the CPE, by using implicit signaling, when the channel switching unit does not receive the channel switching report message from the CPE. 
     According to another aspect of the present invention, there is provided a channel switching apparatus included in customer premises equipment (CPE) of a cognitive radio system, the apparatus comprising a channel switching unit transmitting to a base station a channel switching report message that reports that channel switching is to be performed in response to a channel switching request message, and switching from a first frequency channel to a second frequency channel by using explicit signaling, when receiving the channel switching request message requesting switching the first frequency channel, which is currently unavailable, to the second frequency channel from among at least one uplink/downlink frequency channel which is set as a communication channel with the base station; and a channel switching unit detecting the currently unavailable frequency channel, based on whether messages that are to be received, including the channel switching request message, are received and the result of a channel search performed by the CPE, and switching the detected first frequency channel to a third frequency channel which is predefined with the base station. 
     According to another aspect of the present invention, there is provided an apparatus which is included in a base station of a cognitive radio system and determines whether a frequency channel is available for communication with customer premises equipment (CPE), the apparatus comprising message reception determination unit determining whether a channel search report message, which is transmitted from the CPE in response to a channel search request from the base station, and an automatic report message, which is periodically transmitted from the CPE to the base station, are received from the CPE; and an availability determination unit determining whether an uplink/downlink frequency channel is available between the base station and the CPE, based on whether the channel search report message is received and whether the automatic report message is received. 
     According to another aspect of the present invention, there is provided a method of allowing a base station to manage an uplink/downlink frequency channel for use in communication with each customer premises equipment (CPE) in a cognitive radio system, the method comprising generating and managing channel matching information regarding a pair of a uplink frequency channel and a downlink frequency channel which are used by the same CPE; and transmitting the generated channel matching information to the CPEs. 
     According to another aspect of the present invention, there is provided a method of allowing customer premises equipment (CPE) to manage an uplink/downlink frequency channel for use in communication in a cognitive radio system, the method comprising receiving channel matching information regarding a pair of a uplink frequency channel and a downlink frequency channel, which are used by the same CPE, from a base station; and performing processing by obtaining information regarding a channel to the CPE or by performing channel switching. 
     According to another aspect of the present invention, there is provided an apparatus which is included in a base station of a cognitive radio system and manages a uplink/downlink frequency channel, the apparatus comprising a management unit generating and managing channel matching information regarding a pair of a uplink frequency channel and a downlink frequency channel which are used by the same customer premises equipment (CPE); and a transmitting unit transmitting the generated channel matching information to the CPEs. 
     According to another aspect of the present invention, there is provided an apparatus which is included in customer premises equipment (CPE) of a cognitive radio system and manages a uplink/downlink frequency channel, the apparatus comprising a receiving unit receiving channel matching information regarding a pair of uplink frequency channel and a downlink frequency channel, which are used by the same CPE, from a base station; and a processor obtaining information regarding a channel allocated to the CPE or performing channel switching, based on the received channel matching information. 
     ADVANTAGEOUS EFFECTS 
     According to the present invention, when a frequency channel used in a cognitive radio system is unavailable due to access of an incumbent user to the frequency channel or due to a propagation environment of the frequency channel, the frequency channel can be efficiently switched to a new frequency channel, thereby securing a reliable channel environment of the cognitive radio system while guaranteeing the incumbent user&#39;s right to use the frequency channel. 
     Also, according to the present invention, it is possible to efficiently determine whether a frequency channel used in a cognitive radio system is unavailable due to access of an incumbent user to the frequency channel or due to a propagation environment of the frequency channel. 
     Also, according to the present invention, it is possible to efficiently perform channel management for customer premises equipments (CPEs) in a cell in an FDD system through channel matching and channel grouping, and reduce overhead of an MAP message. 
     MODE FOR THE INVENTION 
     Hereinafter, exemplary embodiments of the present invention will be described in greater detail with reference to the accompanying drawings. 
     Conventionally, a fixed communication system is established between a base station and customer premises equipment by using a permitted frequency and a fixed channel bandwidth. However, since frequency resources may be limited according to time and place, there is a need to effectively supply idle frequency resources to a location where a demand occurs. In particular, since permitted frequency resources are not always used in a low-populated region or in a low frequency communication, e.g., a public safety channel, wasting of frequency resources may be more serious. A cognitive radio system, which is designed in order to solve these problems, is a communication system whereby frequency resources are effectively used by searching for a frequency band available at a desired time period and at a desired place and allocating it to a proper person. The present invention introduces a method of satisfactorily performing the channel switching process that is the main part of the cognitive radio technology. 
     To help understand the present invention, a general idea of the present invention will be described with respect to the IEEE 802.22 WRAN (Wireless LAN) system which is an OFDM/FDD (Orthogonal Frequency Division Multiplexing/Frequency Division Duplex) system or an OFDM/TDD (Time Division Duplex) system. The IEEE 802.22 WRAN system is capable of searching broadcast very high frequency (VHF)/ultra high frequency (UHF) bands for an idle broadcast channel bandwidth and providing a service to a cognitive mobile user by using the idle broadcast channel bandwidth. 
       FIGS. 1 through 4  illustrate signaling used in a channel switching process according to an embodiment of the present invention. In detail,  FIGS. 1 through 4  illustrate the flow of a medium access control (MAC) control message exchanged between a base station and customer premises equipment (CPE) in an OFDM/FDD-based cognitive radio system, according transmission time of each packet.  FIG. 1  illustrates the flow of a MAC control message used in a channel switching process using explicit signaling.  FIG. 2  illustrates the flow of a MAC control message used in a channel switching process using short implicit signaling.  FIG. 3  illustrates the flow of a MAC control message used in a channel switching process using outband signaling.  FIG. 4  illustrates the flow of a MAC control message used in a channel switching process using implicit signaling. 
     Referring to  FIG. 1 , the base station transmits information to the CPE by using a channel  1 (CH 1 ), which is a downlink frequency channel, and the CPE transmits information to the base station by using a channel  3 (CH 3 ) which is an uplink frequency channel. In the case of the OFDM/TDD-based cognitive radio system, the channels  1  and  3  (CH 1  and CH 3 ) are the same frequency channel. 
     More specifically,  FIG. 1  illustrates a case where the base station cannot receive a message from the CPE, due to access of an incumbent user to the channel  1 (CH 1 ), which is a downlink frequency channel, or due to deterioration of the channel  1 (CH 1 ). That is, the base station and the CPE must switch from the channel  1 (CH 1 ), which is a downlink frequency channel, to another downlink frequency channel, i.e., a channel  2 (CH 2 ). 
     The channel switching process using explicit signaling according to an embodiment of the present invention will be described with reference to  FIG. 1 .  FIG. 1  illustrates a channel switching process in which the base station cannot receive a BLM_REP message, indicated with an arrow  1 , which is to be received from the CPE, due to a problem with the channel  1 (CH 1 ) which is a downlink frequency channel. That is, a BLM_REQ message is not transmitted to the CPE due to the problem with the channel  1 (CH 1 ), which is a downlink frequency channel, and thus, the CPE does not transmit the BLM_REP message, indicated with the arrow  1 , to the base station. Here, the BLM_REP message is a channel search report message containing the result of a channel search performed by the CPE, in response to the BLM_REQ message that is a channel search request message containing a request, from the base station, which requests the CPE to perform a channel search (spectrum sensing). 
     In  FIG. 1 , when the base station does not receives the BLM_REP message, which is to be received, from the CPE via a first frame, the base station transmits a CHS_REQ (channel switching request) message, indicated with an arrow  3 , which is a message corresponding to explicit signaling, to the CPE. Here, the CHS_REQ message is a channel switching request message containing information regarding a frequency channel before/after channel switching and the base station&#39;s instruction for the CPE to perform channel switching. That is, when the base station does not receive the BLM_REP message from the CPE via the first frame, the base station transmits the CHS_REQ message, indicated with the arrow  3 , which is a message corresponding to explicit signaling, to the CPE in order that the CPE switches from the channel  1 (CH 1 ) to the channel  2 (CH 2 ). That is, the CHS-REQ message is a MAC control message that contains an explicit message indicating a channel to be switched. 
     Upon receiving the CHS-REQ message indicated with the arrow  3 , the CPE switches to the channel indicated in the CHS-REQ message, and transmits a CHS_REP message, indicated with an arrow  5 , which is a channel switching report message that reports channel switching, to the base station. Then, as soon as receiving the CHS_REP message, the base station can secure a new communication channel via the channel  2 (CH 2 ). 
     A channel switching process using short implicit signaling according to an embodiment of the present invention will be described with reference to  FIG. 2 . Similar to  FIG. 1 ,  FIG. 2  illustrates a channel switching process in which the base station cannot receive a BLM_REP message, indicated with an arrow  1 , which is to be received, from a CPE, due to a problem with a channel  1 (CH 1 ) which is a downlink frequency channel. 
     When the base station does not receive the BLM_REP message, indicated with the arrow  1 , which is to be received, from the CPE, the base station transmits a CHS_REQ message, indicated with an arrow  2 , to the CPE. The CHS_REQ message contains an instruction, from the base station, which has been described above with respect to explicit signaling with reference to  FIG. 1 . 
     Likewise, the CPE does not receive the CHS_REQ message or a MAP message, which is to be received, from the base station via a second frame, due to a problem with the channel  1 (CH 1 ), which is a downlink frequency channel, and thus cannot transmit a signal to the base station in response to the CHS_REQ message or the MAP message. That is, the base station transmits the MAP message in a normal communication channel state and transmits the CHS_REQ message when channel switching is required due to access of a licensed user. Thus, an instruction that the CPE is to receive from the base station is the MAP message or the CHS_REQ message. In particular, in an embodiment of  FIG. 2 , an instruction from the base station is the CHS_REQ message indicated with the arrow  2 , and thus, the CPE cannot transmit a CHS_REP message which is a response message to the CHS_REQ message. Accordingly, the base station does not receive any response from the CPE and retransmits the CHS_REQ message, indicated with the arrow  2 , via a third frame. 
     In this case, both the base station and the CPE cannot receive messages that are to be received. That is, a propagation environment that deteriorates a channel environment (e.g., access of an incumbent user) influences both uplink and downlink frequency channels. 
     Under such a condition, upon not receiving a message, which is to be received, from the CPE a predetermined number of times, the base station switches from the channel  1 (CH 1 ), which is a current downlink frequency channel, to a channel  2 (CH 2 ) which is predefined with the CPE. Likewise, the CPE does not receive an instruction from the base station (the CHS_REQ message or the MAP message), which is to be received a predetermined number of times, and thus switches from the channel  1  (CH 1 ), which is a current downlink frequency channel, to the channel  2 (CH 2 ) which is predefined with the base station. 
     Specifically, the base station transmits the CHS_REQ message to the CPE the predetermined number of times. Upon not receiving a response (the CHS_REP message) from the CPE, the base station assumes that there is a problem with a communication line between the base station and the CPE, and switches from the current channel  1 (CH 1 ) to the predefined channel  2 (CH 2 ). Similarly, the CPE does not receive the message (the CHS_REQ message or the MAP message), which is to be received the predetermined number of times, from the base station and thus assumes that the communication line has a problem and switches to the predefined channel  2 (CH 2 ) for rendezvous with the base station. A channel switching process in which whether channel switching is required is determined by checking whether a message, which is to be received, is received, without explicit signaling that has been described above with reference to  FIG. 1 , is referred to as the channel switching process using implicit signaling. 
     Implicit signaling is divided into short implicit signaling and implicit signaling, according to the amount of time to wait for the opposite party&#39;s response ( FIG. 2  illustrates short implicit signaling and  FIG. 4  illustrates implicit signaling). In an implicit communication method as illustrated in  FIG. 4 , the amount of time that the base station requires to wait for a response from the CPE is greater than a subsequent quiet period (QP). Here, in the cognitive radio technology, the QP denotes a duration for which the base station or the CPE discontinues data transmission and listens whether an incumbent user establishes communication in order to scan or sense whether a licensed user accesses a channel in use. In the case of a TDD system, both the base station and the CPE must discontinue data transmission. 
     A channel switching process using outband signaling according to an embodiment of the present invention will be described with reference to  FIG. 3 . Here, outband denotes a frequency channel not in use by both a base station and a CPE. 
     Similarly to  FIG. 1 ,  FIG. 3  also illustrates a channel switching process in which the base station cannot receive a BLM_REP message, indicated with an arrow  1 , which is to be received from the CPE, due to a problem with a channel  1 (CH 1 ) which is a downlink frequency channel. 
     Upon not receiving the BLM_REP message, indicated with the arrow  1 , which is to be received from the CPE, the base station transmits a CHS_REQ message, indicated with an arrow  2 , which is the base station&#39;s instruction described above with respect to explicit signaling of  FIG. 1 , to the CPE, and transmits to the CPE a CHS_REQ message or a MAP message, indicated with an arrow  4 , via a channel  2 (CH 2 ), which is an outband, using a subsequent frame. 
     In this case, when the CPE does not receive the base station&#39;s response (the CHS_REQ message or the MAP message indicated with the arrow  2 ) to the BLM_REP message, indicted with the arrow  1 , via the channel  1 (CH 1 ) (a current downlink frequency channel) a predetermined number of times, the CPE switches to channel  2 (CH 2 ) in order to receive a MAC control message (the CHS_REQ message or the MAP message indicated with the arrow  4 ) via the channel  2 (CH 2 ). 
     In this case, the base station does not discontinue access to the channel  1 (CH 1 ). The base station permits access of the CPE to the channel  2 (CH 2 ), upon receiving from the CPE a response (the CHS_REP message or the BLM_REP message indicated with an arrow  5 ) to the CHS_REQ message or the MAP message, indicated with the arrow  4 , which is transmitted via the channel  2 (CH 2 ). If the base station does not receive from the CPE a response, indicated with the arrow  5 , to the CHS_REQ message or the MAP message, indicated with the arrow  4 , which is transmitted via the channel  2 (CH 2 ), the base station considers switching to another channel (not shown). 
       FIG. 3  illustrates that the base station and the CPE continuously use the channel  2 (CH 2 ) continuously and for a channel switching time period. 
     A channel switching process using implicit signaling according to an embodiment of the present invention will be described with reference to  FIG. 4 . In the cognitive radio system, the base station obtains a large amount of information regarding a channel environment after a QP, and thus delays determining whether channel switching is required until a subsequent QP, unlike in the embodiment of  FIG. 2 . If the base station does not receive a BLM_REP message, which is the CPE&#39;s response within the QP, the base station switches to a predefined channel  2 (CH 2 ) and establishes communication with the CPE via the channel  2 (CH 2 ), similar to the short implicit communication method illustrated in  FIG. 2 . Likewise, when the CPE does not receive from the base station the CHS_REQ message which is the base station&#39;s response or the MAP message containing information regarding a frequency channel allocated to the CPE, the CPE switches to the predefined channel  2 (CH 2 ) and establishes communication with the base station via the channel  2 (CH 2 ). 
       FIG. 5  is a flowchart illustrating a channel switching method, according to another embodiment of the present invention, in which of the channel switching processes of  FIGS. 1 through 4  are sequentially performed. 
     First, a base station determines whether channel switching is required, based on a CPE&#39;s report and/or an incumbent user&#39;s report. When it is determined that channel switching is required, the base station transmits to the CPE a channel switching request CHS_REQ message indicating a frequency channel to be moved from a current frequency channel. Here, a method of transmitting the CHS_REQ message via the current downlink frequency channel is explicit signaling, and a method of transmitting the CHS_REQ message via an outband is outband signaling. 
     When communication is not established after performing explicit signaling, implicit signaling is performed. That is, the base station and the CPE perform channel switching rapidly when explicit signaling succeeds, and implicit signaling when explicit signaling fails. That is, the base station and the CPE perform implicit signaling in which whether channel switching is required is determined according to whether messages, which are to be respectively received by the base station and the CPE, are actually received. 
     Although not included in the above description, outband signaling may be used in place of explicit signaling or implicit signaling, as illustrated in  FIG. 5 . In this case, the base station determines whether an outband is available, and transmits the CHS_REQ message or a MAP message to the CPE via the outband when the outband is available. 
     As described above, the technical purpose of the present invention is to satisfactorily perform channel switching by performing the above signaling methods sequentially or complementarily in the cognitive radio system. 
       FIG. 6  illustrates diagrams for explaining scenarios for reporting access of an incumbent user in the FDD system. 
     For example, when a signal from an incumbent user is detected in a large region and both a base station and a CPE can thus recognize access of the incumbent user (CASE  0 ), the base station and the CPE rapidly perform channel switching to a predefined specific frequency band. In this case, the above signaling need not be performed since both the base station and the CPE recognized the presence of the incumbent user. 
     In CASES  1  through  4 , only one of the base station and the CPE recognizes the access of the incumbent user. CASES  1  through  4  are categorized according to whether the base station or the CPE recognizes the access of the incumbent user and whether the incumbent user accesses an uplink frequency channel or a downlink frequency channel. 
     As illustrated in  FIG. 4 , in CASE  1 , the base station detects access of an incumbent user, and the incumbent user accesses the uplink frequency channel. Similarly, in CASE  2 , the base station detects access of the incumbent user to the downlink frequency channel. In CASE  3 , the CPE detects access of the incumbent user to the uplink frequency channel. In CASE  4 , the CPE detects access of the incumbent user to the downlink frequency channel. 
     In the case of the FDD system, the above channel switching process is limitedly used according to such a scenario related to access of the incumbent user, and thus, access of a licensed user is categorized using a total of five scenarios as illustrated in  FIG. 6 . 
       FIG. 7  illustrates tables specifying channel switching methods available for each of the scenarios categorized in  FIG. 6 . The upper table shows channel switching methods when the FDD system is used, and the lower table shows channel switching methods when the TDD system is used. First, the upper table will be described. 
     In CASE  1 , all implicit signaling, short implicit signaling, and explicit signaling are available. Here, outband signaling is available for all CASES  1  through  4 , and is thus not included in the tables. 
     However, in CASE  2 , only implicit signaling and short implicit signaling are available, and explicit signaling may be available in some cases. 
     Explicit signaling is not available when a message (the CHS_REQ message), for explicit signaling, from the base station is interfered with by access of the incumbent user to the downlink frequency channel, thus preventing the CPE from receiving the CHS_REQ message or the content of the CHS_REQ message from being restored. 
     However, in CASE  2  where only the base station senses the access of the incumbent user, the access of the incumbent user does not influence the CPE, and thus, the power level of a signal transmitted from the base station to the CPE is stronger than that of a signal transmitted from the incumbent user to the CPE. Accordingly, explicit signaling may succeed or fail depending on the power level of signal transmitted to the CPE. 
     In CASE  3 , all signaling methods are available. CASE  4  is similar to CASE  2  but it is very probable that the CPE cannot receive a message (the CHS_REQ message) from the base station since the power level of a signal transmitted from the base station to the CPE is similar to that of a signal transmitted from the incumbent user to the CPE. That is, in CASE  4 , all signaling methods other than implicit signaling, are likely to fail. 
     The lower table of  FIG. 7  shows channel switching methods when the TDD system is used. In the TDD system, an uplink frequency channel and a downlink frequency channel are the same, and there are also problems with the downlink frequency channel that plays a leading role in the FDD system. Thus, the portions related to the downlink frequency channel that are disclosed in the upper table regarding the FDD system, are also applied to the lower table regarding the TDD system. 
       FIG. 8  is a table illustrating two types of MAC control messages according to an embodiment of the present invention. That is,  FIG. 8  illustrates two types of MAC control messages designed to solve the problems described with reference to  FIG. 7 . 
     First, a BLM_RSP (bulk response) message is automatically and periodically reported by a CPE without an instruction from a base station. In this disclosure, the BLM_RSP message will be referred to as an automatic report message for convenience of explanation. The BLM_RSP message is automatically reported to the base station via a specific part (a system slot or an Urgent Coexistence Slot (UCS) slot) of an uplink frame defined in the cognitive radio system, or via a traffic channel allocated to the CPE by the base station. 
     The BLM_REP (bulk report) message is reported only via a specific burst (US burst) on an uplink frame allocated by the base station, in response to a BLM_REQ (bulk request) message which is a channel search request message from the base station. The BLM_REP message cannot be reported to the base station without the BLM_REQ message. 
       FIG. 9  is a flowchart illustrating a process in which a CPE transmits the two types of messages (the BLM_REP message and the BLM_RSP message), illustrated in  FIG. 8 , to a base station. 
     The CPE periodically performs spectrum sensing so as to detect access of an incumbent user, and transmits a BLM_RSP message to a system slot or a UCS slot. Also, the CPE waits for an uplink MAP (UL-MAP) message, and transmits a BLM_REP message containing the result of spectrum sensing to an uplink burst assigned to the uplink MAP message. 
     Next, the CPE waits for an instruction from the base station. The instruction from the base station is a CHS_REQ message when the base station determines to perform channel switching depending on whether the two messages are received, and the MAP message regarding a current uplink/downlink frequency channel when the base station determines not to perform channel switching. 
       FIG. 10  is a table illustrating a method of allowing a base station to determine whether a frequency channel is in use, according to an embodiment of the present invention. That is,  FIG. 10  illustrates a method in which the base station determines whether a current uplink/downlink frequency channel is available, using two messages (the BLM_REP message and the BLM_RSP message) reported from a CPE according to the process illustrated in  FIG. 9 . 
     First, the second column of the table of  FIG. 10  will be described. The base station determines that both the uplink and downlink frequency channels operate normally when the BLM_REP message is normally received, regardless of whether the BLM_RSP message is received. This is because the base station can receive the BLM_REP message only when both the uplink and downlink frequency channels normally operate. In this case, communication is continuously established with the current uplink and downlink frequency channels without performing channel switching. 
     Next, the third column of the table of  FIG. 10  will be described. The third row shows a case where only the BLM_RSP message of received messages is readable. In this case, the base station determines there is a problem with the downlink frequency channel. That is, it is determined that the uplink frequency channel has no problem since the BLM_RSP message is normally received, and a BLM_REQP message causing the BLM_REP message to be transmitted is not properly transmitted to the CPE via the downlink frequency channel since the BLM_REP message is not received. In this case, the base station transmits a CHS_REQ message to the CPE in order to switch the downlink frequency channel. 
     Next, the fourth column of the  FIG. 10  will be described. Upon not receiving both the BLM_REP message and the BLM_RSP message, the base station determines that the uplink frequency channel has a problem, but cannot determine whether the downlink frequency channel also has a problem. In this case, the base station transmits a request for channel switching for the uplink frequency channel to the CPE. However, nevertheless, when communication cannot be re-established, the base station determines that the downlink frequency channel also has a problem and performs channel switching on the downlink frequency channel, using the above signaling method. 
     Although each message to be transmitted to a system slot contains only specific CDMA code, the base station has previously obtained information regarding the code and can decipher the BLM_RSP message in most cases. That is, the base station can generally, normally receive the BLM_RSP message, and therefore, the case corresponding to the fourth column of the table of  FIG. 10  hardly occurs. Accordingly, the base station can determine whether each of the uplink and downlink frequency channels has a problem, based on whether the BLM_REP message and the BLM_RSP message are received. 
       FIG. 11  is a flowchart illustrating a channel switching method performed by a base station in the FDD system, according to an embodiment of the present invention. 
     First, the base station performs a channel search by using spectrum sensing, and analyzes the result of the channel search so as to determine whether an incumbent user accesses an uplink frequency channel or a downlink frequency channel. If it is determined that the incumbent user accesses the downlink frequency channel, a T1 timer operates. If it is determined that the incumbent user accesses the uplink frequency channel, a T2 timer operates. If it is determined that the incumbent user does not access both the uplink and downlink frequency channels, a T3 timer operates. 
     After operating the above timer, the base station waits for messages to be reported from the CPE. Here, the messages are the BLM_REP message and the BLM_RSP message. 
     When the result of channel search shows that the incumbent user does not access the uplink and downlink frequency channels and the base station receives the BLM_REP message, the base station determines that channel switching is not required. Therefore, the base station continues communication with the CPE via the current uplink/downlink frequency channel. 
     When the base station receives the BLM_REP message and determines that the incumbent user accesses the downlink frequency channel, based on the result of channel search of the CPE and the result of channel search of the base station that are contained in the BLM_REP message, the base station transmits a CHS_REQ message containing a request to switch from the downlink frequency channel to the CPE. Similarly, upon receiving the BLM_RSP message and not the BLM_REP message, the base station transmits the CHS_REQ message to the CPE. Then, the base station waits for a CHS_REP message as a response to the CHS_REQ message until a timeout of the T1 timer or the T3 timer. If receiving the CHS_REP message before the timeout of the T1 timer or the T3 timer, the base station switches the downlink frequency channel and performs an initialization process which is a follow-up process to channel switching. If the base station does not receive the CHS_REP message until the timeout of the T1 timer or the T3 timer, the base station determines that the downlink frequency channel has a problem and switches from the downlink frequency channel to another frequency channel that is predefined with the CPE, using implicit signaling described above with reference to  FIG. 2  or  4 . 
     When the base station does not receive both the BLM_REP message and the BLM_REP message, the base station first transmits the CHS_REQ message requesting switching of the uplink frequency channel to the CPE, and determines whether it receives the CHS_REP message which is a response message to the CHS_REQ message from the CPE before a timeout of the T2 timer. When receiving the CHS_REP message from the CPE before the timeout of the T2 timer, the base station switches the uplink frequency channel and performs the initialization process which is a follow-up process to channel switching. If the base station does not receive the CHS_REP message until the timeout of the T2 timer, the base station performs channel switching on the uplink frequency channel, using short implicit signaling described above with reference to  FIG. 2 . Thereafter, when communication is not re-established with the CPE before the timeout of the T3 timer, channel switching is performed on the uplink/downlink frequency channel, using implicit signaling described above with reference to  FIG. 4 . 
       FIG. 12  is a flowchart illustrating a channel switching method performed by a CPE in the FDD system according to an embodiment of the present invention. 
     First, the CPE performs a channel search by using spectrum sensing, and reports the result of channel search to the base station. Then, the CPE analyzes the result of the channel search in order to determine whether an incumbent user accesses an uplink frequency channel or a downlink frequency channel. If it is determined that the incumbent user accesses the downlink frequency channel, a T4 timer operates. If it is determined that the incumbent user accesses the uplink frequency channel, a T5 timer operates. If it is determined that the incumbent user does not access both the uplink and downlink frequency channels, a T6 timer operates. 
     After operating the above timer, the CPE waits for instruction messages from the base station. Here, the instruction messages from the base station are the above MAP message and the CHS_REQ message. 
     If the result of channel search shows that the incumbent user does not access the uplink frequency channel, the CPE determines whether the instruction messages are received from the base station until a timeout of the T4 timer or the T6 timer. When the CPE does not receive the instruction messages from the base station until the timeout of the T4 timer or the T6 timer base station, the CPE performs channel switching on the downlink frequency channel using implicit signaling described above with reference to  FIG. 2  or  4 . 
     If the result of the channel search shows that the incumbent user accesses the uplink frequency channel, the CPE determines whether it receives an instruction message from the base station until a timeout of the T5 timer. If the CPE does not receive the instruction message from the base station until the timeout of the T5 timer, the CPE performs channel switching on the uplink frequency channel, using short implicit signaling described above with reference to  FIG. 2 . Then, the base station determines whether communication is re-established with the base station until a timeout of the T6 timer, and performs channel switching on the uplink/downlink frequency channel by using implicit signaling described above with reference to  FIG. 2  or  4  when the communication is not re-established. 
     Upon receiving the MAP message, the CPE continues the communication by using a current uplink/downlink frequency channel. 
     Upon receiving the CHS_REQ message, the CPE performs channel switching on the uplink frequency channel or the downlink frequency channel according to an instruction contained in the CHS_REQ message, and waits for the MAP message from the base station. Upon receiving the MAP message, the CPE performs the initialization process which is a follow-up process to channel switching. 
       FIG. 13  is a flowchart illustrating a channel switching method performed by a base station in the TDD system, according to an embodiment of the present invention. 
     First, the base station performs a channel search by using spectrum sensing, and analyzes the result of the channel search in order to determine whether an incumbent user accesses an uplink/downlink frequency channel. If the result of the channel search reveals that the incumbent user accesses the uplink/downlink frequency channel, a T1 timer operates, and if not so, a T2 timer operates. 
     After operating the above timer, the base station waits for a message to be reported from a CPE. Here, the message is the above BLM_REP message. 
     The base station transmits the CHS_REQ message requesting switching of the uplink/downlink frequency channel to the CPE, when the result of the base station reveals that the incumbent user does not access the uplink/downlink frequency channel and the base station does not receive the BLM_REP message, or when the result of the channel search reveals that the incumbent user accesses the uplink/downlink frequency channel. Thereafter, when receiving the CHS_REP message until a timeout of the T1 timer or the T2 timer, the base station switches the uplink/downlink frequency channel and performs an initialization process which is a follow-up process to channel switching. When the base station does not receive the CHS_REP message until the timeout of the T1 timer or the T2 timer, the base station performs channel switching on the uplink/downlink frequency channel using implicit signaling described above with reference to  FIG. 2  or  4 . 
     When the result of channel search reveals that the incumbent user does not access the uplink/downlink frequency channel and the base station receives the BLM_REP message, the base station continues communication with the CPE via a current uplink/downlink frequency channel. 
       FIG. 14  is a flowchart illustrating a channel switching method performed by a CPE in the TDD system, according to an embodiment of the present invention. 
     First, the CPE performs a channel search by using spectrum sensing and reports the result of channel search to the base station. Then, the CPE analyzes the result of the channel search in order to determine whether an incumbent user accesses an uplink/downlink frequency channel. If it is determined that the incumbent user accesses the uplink/downlink frequency channel, a T3 timer operates, and if not so, a T4 timer operates. 
     After operating the above timer, the CPE waits for instruction messages from the base station. Here, the instruction messages are the above MAP message and the CHS_REQ message. 
     The CPE waits to receive the CHS_REQ message until a timeout of the T3 timer or the T4 timer, when the result of the channel search reveals that an incumbent user accesses an uplink/downlink frequency channel or when the result of the channel search reveals that the incumbent user does not access the uplink/downlink frequency channel and the CPE does not receive the MAP message. When the CPE does not receive the CHS_REQ message timeout of the T3 timer or the T4 timer, the CPE performs channel switching on the uplink/downlink frequency channel using implicit signaling described above with reference to  FIG. 2  or  4 . When receiving the CHS_REQ message before the timeout of the T3 timer or the T4 timer, the CPE performs channel switching on the uplink/downlink frequency channel in response to the CHS_REQ message, and performs the initialization process which is a follow-up process to channel switching. 
     When the result of the channel search reveals that the incumbent user does not access the uplink/downlink frequency channel and the CPE receives the MAP message, the CPE continues communication with the base station via a current uplink/downlink frequency channel. 
       FIG. 15  is a block diagram of channel switching apparatuses  1500  and  1550  that are respectively included in a base station and a CPE of a cognitive radio system, according to an embodiment of the present invention. 
     In an embodiment of the present invention, referring to  FIG. 15 , the channel switching apparatus  1500 , which is included in the base station for channel switching using explicit signaling, includes a detecting unit  1510 , a channel switching request message transmitting unit  1520 , and a channel switching unit  1530 . 
     The detecting unit  1510  detects a currently unavailable first frequency channel from among at least one uplink/downlink frequency channel that is set as a communication channel together with the CPE. Here, the detecting unit  1510  detects the first frequency channel, based on at least one of whether a message that is to be received is received and the result of a channel search of the base station. In an embodiment of the present invention, the message that is to be received includes a channel search report message that the CPE transmits in response to a channel search request from the base station, and an automatic report message that the CPE periodically transmits to the base station. The detecting unit  1510  detects the frequency channel, based on whether the channel search report message is received, the result of channel search of the CPE, which is contained in the received channel search report message, and whether the automatic report message is received. 
     The channel switching request message transmitting unit  1520  transmits a channel switching request message requesting switching from the first frequency channel to a second frequency channel to the CPE. 
     Upon receiving from the CPE a channel switching report message reporting that channel switching is to be performed in response to the channel switching request message, the channel switching unit  1530  switches from the first frequency channel to the second frequency channel. 
     In an embodiment of the present invention, referring to  FIG. 15 , the channel switching apparatus  1550 , which is included in the CPE for channel switching using explicit signaling, includes a channel switching unit  1560 , a channel switching report message transmitting unit  1570 , a channel search report message transmitting unit  1580 , and an automatic report message transmitting unit  1590 . 
     Upon receiving a channel switching request message that requests switching from a currently unavailable first frequency channel to a second frequency channel from among at least one uplink/downlink frequency channel that is set as a communication channel with the base station, the channel switching unit  1560  switches from the first frequency channel to the second frequency channel. 
     The channel switching report message transmitting unit  1570  transmits a channel switching report message reporting that channel switching is to be performed in response to the channel switching request message, to the base station. 
     Upon receiving a channel search request message requesting a channel search from the base station, the channel search report message transmitting unit  1580  transmits a channel search report message containing the result of channel search of the CPE to the base station. The automatic report message transmitting unit  1590  periodically transmits an automatic report message to the base station. 
       FIG. 16  is a block diagram of channel switching apparatuses  1600  and  1650  that are respectively included in a base station and a CPE of a cognitive radio system, according to another embodiment of the present invention. 
     In an embodiment of the present invention, referring to  FIG. 16 , the channel switching apparatus  1600 , which is included in the base station for channel switching using implicit signaling, includes a detecting unit  1610  and a channel switching unit  1620 . 
     The detecting unit  1610  detects a currently unavailable frequency channel from among at least one uplink/downlink frequency channel that is set as a communication channel together with the CPE. 
     If the uplink/downlink frequency channel between the base station and the CPE adopts the FDD system, a message that is to be received includes a channel search report message that the CPE transmits in response to a channel search request from the base station, and an automatic report message that the CPE periodically transmits to the base station. The detecting unit  1610  detects the first frequency channel, based on whether the channel search report message is received, the result of a channel search of the CPE, which is contained in the received channel search report message, and whether the automatic report message is received. If the uplink/downlink frequency channel between the base station and the CPE adopts the TDD system, the message that is to be received includes a channel search report message that the CPE transmits in response to a channel search request from the base station. The detecting unit  1610  detects the first frequency channel, based on whether the channel search report message is received and the result of the channel search of the CPE, which is contained in the received channel search report message. 
     The channel switching unit  1620  switches from the first frequency channel as a communication channel to a second frequency channel that is predefined together with the CPE. 
     The above operations of the detecting unit  1610  and the channel switching unit  1620  are related to channel switching using short implicit signaling. For channel switching using implicit signaling, the detecting unit  1610  delays determination regarding detection of the first frequency channel until a QP used for channel search, and detects the first frequency channel based on the result of the channel search, which is obtained after the QP. Similarly, the channel switching unit  1620  performs channel switching on the first frequency channel that has been finally detected after the QP. 
     In an embodiment of the present invention, referring to  FIG. 16 , the channel switching apparatus  1650 , which is included in the CPE for channel switching using implicit signaling, includes a detecting unit  1660  and a channel switching unit  1670 . 
     The detecting unit  1660  detects a currently unavailable first frequency channel from among at least one uplink/downlink frequency channel that is set as a communication channel with the base station. The detecting unit  1660  may detect the first frequency channel, based on at least one of whether a message that is to be received is received and the result of the channel search of the CPE. Here, the message that is to be received, includes a MAP message containing information regarding a frequency channel allocated to the CPE, and a channel switching request message requesting channel switching. 
     The channel switching unit  1670  switches from the first frequency channel as a communication channel to a second frequency channel that is predefined with the base station. 
     The above operations of the detecting unit  1660  and the channel switching unit  1670  are related to channel switching using short implicit signaling. For channel switching using implicit signaling, the detecting unit  1660  delays determination regarding detection of the first frequency channel until a QP for a channel search, and detects the first frequency channel based on the result of the channel search which is obtained after the QP. Similarly, the channel switching unit  1670  performs channel switching on the first frequency channel that is finally detected after the QP. 
       FIG. 17  is a block diagram of channel switching apparatuses  1700  and  1750  that are respectively included in a base station and a CPE of a cognitive radio system, according to another embodiment of the present invention. 
     In an embodiment of the present invention, referring to  FIG. 17 , the channel switching apparatus  1700 , which is included in the base station for channel switching using outband signaling, includes a detecting unit  1710 , a channel switching request message transmitting unit  1720 , and a channel switching unit  1730 . 
     The detecting unit  1710  detects a currently unavailable first frequency channel from among at least one uplink/downlink frequency channel that is set as a communication channel with the CPE. The detecting unit  1710  may detect the first frequency channel, based on at least one of whether messages that are to be received are received and the result of the channel search from the base station. Here, if a channel search report message that the CPE transmits in response to a channel search request from the base station, and an automatic report message that the CPE periodically transmits to the base station are used as the messages that are to be received, the detecting unit  1710  detects the first frequency channel, based on whether the channel search report message is received, the result of channel searching received from the CPE, which is contained in the received channel search report message, and whether the automatic report message is received. 
     The channel switching request message transmitting unit  1720  transmits a channel switching request message requesting switching from the first frequency channel to the second frequency channel, to the CPE via an outband channel. Upon receiving from the CPE a channel switching report message reporting that channel switching is to be performed in response to the channel switching request message, the channel switching unit  1730  switches from the first frequency channel to the second frequency channel. 
     In an embodiment of the present invention, referring to  FIG. 17 , the channel switching apparatus  1750 , which is included in the CPE for channel switching using outband signaling, includes a channel switching request message receiving unit  1760 , a channel switching unit  1770 , a channel switching report message transmitting unit  1780 , a channel search report message transmitting unit  1785 , and an automatic report message transmitting unit  1790 . 
     The channel switching request message receiving unit  1760  receives a channel switching request message requesting switching from the first frequency channel, which is a currently set communication channel, to the second frequency channel, from the base station via the outband channel. 
     The channel switching unit  1770  switches from the first frequency channel as a communication channel to the second frequency channel, based on the channel switching request message. The channel switching report message transmitting unit  1780  transmits a channel switching report message reporting that channel switching is to be performed in response to the channel switching request message, to the base station. 
     Upon receiving a channel search request message requesting a channel search from the base station, the channel search report message transmitting unit  1785  transmits a channel search report message containing the result of channel searching from the CPE to the base station. The automatic report message transmitting unit  1790  periodically transmits an automatic report message to the base station. 
       FIG. 18  is a block diagram of channel switching apparatuses  1800  and  1850  that are respectively included in a base station and a CPE of a cognitive radio system, according to another embodiment of the present invention. 
     In an embodiment of the present invention, referring to  FIG. 18 , the channel switching apparatus  1800 , which is included in the base station for channel switching using sequential signaling described above with reference to  FIG. 5 , includes a detecting unit  1805 , an explicit signaling unit  1810 , a channel switching unit  1815  using explicit signaling, a channel switching unit  1820  using implicit signaling, a determination unit  1825 , a channel switching request message transmitting unit  1830 , and a channel switching unit  1835  using outband signaling. 
     The detecting unit  1805  detects a currently unavailable first frequency channel from among at least one uplink/downlink frequency channel that is set as a communication channel with the base station. 
     The explicit signaling unit  1810  transmits to the CPE a channel switching request message requesting switching from the first frequency channel to a second frequency channel, and waits to receive from the CPE a channel switching report message reporting that channel switching is to be performed in response to the channel switching request message. 
     Upon receiving the channel switching report message from the CPE, the channel switching unit  1815  using explicit signaling switches from the first frequency channel to the second frequency channel. 
     Upon not receiving the channel switching report message from the CPE, the channel switching unit  1820  using implicit signaling switches from the first frequency channel to a third frequency channel that is predefined with the CPE. 
     As illustrated in  FIG. 18 , the channel switching apparatus  1800  included in the base station further includes the determination unit  1825 , the channel switching request message transmitting unit  1830 , and the channel switching unit  1835  using outband signaling. In this case, upon not receiving the channel switching report message from the CPE, the determination unit  1825  determines whether an outband channel is available for the base station. 
     If it is determined that the outband channel is available, the channel switching request message transmitting unit  1830  transmits the channel switching request message to the CPE via the outband channel. 
     Upon receiving from the CPE a channel switching report message, which is a response to the channel switching request message transmitted via the outband, the channel switching unit  1835  using outband signaling switches from the first frequency channel to the second frequency channel. 
     In an embodiment of the present invention, referring to  FIG. 18 , the channel switching apparatus  1850 , which is included in the CPE for channel switching using sequential signaling described above with reference to  FIG. 5 , includes a channel switching unit  1855  using explicit signaling, a channel switching unit  1860  using implicit signaling, a determination unit  1865 , and a channel switching unit  1870  using outband signaling. 
     Upon receiving a channel switching request message requesting switching from a currently unavailable first frequency channel to a second frequency channel, from among at least one uplink/downlink frequency channel that is set as a communication channel with the base station, the channel switching unit  1855  that uses explicit signaling transmits a channel switching report message reporting that channel switching is to be performed in response to the channel switching request message, to the base station, and switches from the first frequency channel to the second frequency channel. 
     The channel switching unit  1860  that uses implicit signaling detects the unavailable first frequency channel based on whether messages that are to be received (including the channel switching request message) are received and the result of a channel search of the CPE, and switches the detected first frequency channel to a third frequency channel that is predefined with the base station. 
     As illustrated in  FIG. 18 , the channel switching apparatus  1850  included in the CPE further includes a determination unit  1865 , and a channel switching unit  1870  using outband signaling. In this case, if the determination unit  1865  does not receive the messages that are to be received, the determination unit  1865  determines whether the channel switching request message is received via the outband channel. If it is determined that the channel switching request message is received via the outband channel, the channel switching unit  1870  that uses outband signaling switches from the first frequency channel to the second frequency channel and transmits a channel switching report message that is a response to the channel switching request message. 
       FIG. 19  is a block diagram of a frequency channel availability determination apparatus included in a base station of a cognitive radio system, according to an embodiment of the present invention. Referring to  FIG. 19 , the frequency channel availability determination apparatus includes a message reception determination unit  1900  and an availability determination unit  1910 . 
     The message reception determination unit  1900  determines whether a channel search report message (a message that a CPE transmits in response to a channel search request from the base station) and an automatic report message (a message that the CPE periodically transmits to the base station) are received from the CPE. 
     The message reception determination unit  1910  determines whether an uplink/downlink frequency channel is available between the base station and the CPE, based on whether the channel search report message is received and whether the automatic report message is received. Here, the message reception determination unit  1910  determines that the uplink and downlink frequency channels are available when it is determined that the channel search report message is received, and that only the downlink frequency channel is not available when it is determined that only the automatic report message is received. Also, when it is determined that both the channel search report message and the automatic report message are not received, the message reception determination unit  1910  first determines that the uplink frequency channel is not available, and later detects whether there is also a problem with the downlink frequency channel by determining whether communication is re-established, using channel switching. 
       FIG. 20  is a diagram illustrating the concept of association of uplink and downlink frequency channels in a conventional FDD system. The OFDM/FDD system uses the uplink frequency channel and the downlink frequency channel that are different frequency channels. The relationship between the uplink frequency channel and the downlink frequency channel is specified in map (UL-MAP or DL-MAP) messages of the uplink and downlink frequency channel. In order to include the relationship into the MAP message, an identifier of the uplink frequency channel and an identifier of the downlink frequency channel must be included into the MAP message. Use of identification fields that are comparatively long not only causes waste of resources but also prevents frequency channel information from being systematically managed. 
       FIG. 21  is a diagram illustrating the concept of a method of managing uplink and downlink frequency channels in an OFDM.FDD system, according to an embodiment of the present invention. In detail, in an embodiment of the present invention, channel matching and channel grouping are used in a method of managing uplink and downlink frequency channels in OFDM.FDD system. In channel matching, matching information of frequency channels that are to be respectively used in an uplink and a downlink is set and provided to a base station and a CPE beforehand. For example, information about the uplink channel corresponding to channel number  1  and information about the downlink channel corresponding to channel number  1  may have been previously provided to a base station and a CPE. In this way, it is possible to allow each of the base station and the CPE to detect a channel matching its channel by simply using the channel number  1 . Further, another CPE or another base station that does not use the channel to which the number  1  is assigned, can also detect the identifiers of channels corresponding to the number  1 , which are respectively used in the uplink and the downlink. In other words, channel matching is performed in order to primarily manage and allow movement of the matched uplink and downlink frequency channels as a group. Further, it is possible to control two channels by using an identifier. Thus, it is possible to reduce the amount of information to be included into the MAP message as illustrated in  FIG. 21 , thereby generating the MAP message that reduces overhead. 
     In channel grouping, CPEs having the same channel matching information are managed together. In this case, CPEs belonging to the same group are grouped, based on the same channel information, geographical information, etc. When an incumbent user accesses a frequency channel belonging to a group, grouping of CPEs allows channel switching to be easily performed on all CPEs belonging to the group. 
       FIG. 22  illustrates a manner in which a base station transmits channel matching information and channel grouping information to a CPE, according to an embodiment of the present invention. The channel matching information and the channel grouping information are included in a frame prefix in a frame control header (FCH), as illustrated in  FIG. 22 . 
     A frame of a conventional system, such as a IEEE 802.16x system, is sequentially comprised of a preamble that facilitates signal detection, a FCH, a DL-MAP message, and a UL-MAP message. The CPE first detects a signal transmitted from a base station. The DL-MAP message that is obtained by detecting a base station that transmits a signal with the most appropriate power level, includes a base station identification (ID) field. The CPE detects a base station in order to receive a service therefrom, based on the base station ID field. Then, the CPE transmits information that is to be transmitted via an unlink, to an uplink channel defined in an uplink channel ID field contained in a UL-MAP message received from the detected base station. 
     According to the present invention, the channel matching information is included in the FCH, and is transmitted whenever channel switching occurs. Accordingly, it is possible to reduce the size of the MAP message more significantly than in a conventional method of recording channel information in the DL/UL-MAP message transmitted for each frame. Instead, according to the present invention, the channel matching information and the channel grouping information are transmitted to the FCH. That is, the channel matching information and the channel grouping information are transmitted only when a channel situation changes, thereby reducing the overall frame overhead. 
       FIG. 23  is a block diagram of an uplink/downlink frequency channel management apparatus  2300  included in a base station and an uplink/downlink frequency channel management apparatus  2350  included in a CPE, according to an embodiment of the present invention. 
     Referring to  FIG. 23 , the uplink/downlink frequency channel management apparatus  2300  included in the base station includes a management unit  2310  and a transmitting unit  2320 . The uplink/downlink frequency channel management apparatus  2350  included in the CPE includes a receiving unit  2360  and a processor  2370 . 
     The management unit  2310  groups an uplink frequency channel and a downlink frequency channel used by the same CPE as a pair, and generates and manages channel matching information regarding the pair. 
     The transmitting unit  2320  transmits the generated channel matching information to CPEs. 
     When the above channel grouping is adopted, the management unit  2310  divides CPEs into several groups according to an uplink/downlink frequency channel, and generates and manages channel group information that includes channel matching information and a CPE identifier of each of the groups. The transmitting unit  2320  transmits the generated channel group information to the CPEs. 
     The receiving unit  2360  receives the channel matching information or the channel group information from the base station. The processor  2370  performs an operation of a CPE that desires to obtain channel information in a cognitive radio system, based on the received channel matching information or the channel group information. An example of the operation of the CPE includes channel switching. 
     The present invention can be embodied as computer readable code in a computer readable medium. The computer readable medium may be any recording apparatus capable of storing data that is read by a computer system, e.g., a read-only memory (ROM), a random access memory (RAM), a compact disc (CD)-ROM, a magnetic tape, a floppy disk, an optical data storage device, and so on. Also, the computer readable medium may be a carrier wave that transmits data via the Internet, for example. The computer readable medium can be distributed among computer systems that are interconnected through a network, and the present invention may be stored and implemented as computer readable code in the distributed system. Also, a functional program, code, and code segments required to perform the present invention can be easily derived by programmers skilled in the technical field to which the invention pertains. 
     While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.