Abstract:
In measurement of wireless channels in an IEEE 802.11 wireless local area network (WLAN), in the case of multi-channel measurement as in North America in which eight wireless channels Ch1 through Ch8 are available, channel measurement according to a predetermined algorithm is made in which odd-numbered channels Ch1, Ch3, Ch5 and Ch7 are first measured, and if radio interference occurs in channel Ch5, channels Ch4 and Ch6 adjacent to channel Ch5 are measured. This shortens the measurement time that conventionally corresponds to measurement of the eight channels to the time of six channels, i.e., the sum of the measurement time for the odd-numbered channels and that for the channels adjacent to the interference-occurring channel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2004-365829 filed in Japan on Dec. 17, 2004 and Patent Application No. 2005-324074 filed in Japan on Nov. 8, 2005, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a wireless device and a wireless channel measurement controller, and more particularly to a technology related to a channel selection method for wireless channel measurement in the IEEE 802.11 wireless local area network (WLAN). 
     A wireless local area network (WLAN) is basically composed of a base station (access point; AP) and called stations (stations; STAs) (both the base station and the stations are also called wireless terminals). The stations and the access point that are within the same radio coverage are known as a basic service set (BSS). When such BSSs are located close to each other and operate at the same channel, which are referred to as overlapping BSSs, it is difficult to support required quality-of-service due to possible contentions between the overlapping BSSs. To avoid such problems, if channel interference occurs persistently in one or more stations and/or the access point in a particular BSS, the access point dynamically selects a new wireless link to maintain the operation of the BSS. 
     To dynamically select a channel for communication between a plurality of stations and the access point located within the coverage of a BSS, among available wireless channels, the access point first determines whether or not the channel being used in the current communication by a plurality of wireless terminals is an appropriate channel, that is, whether or not selection of a new channel better in communication state is needed, and then requests a subset of stations to make a channel signal quality measure. To achieve this process, a set of channels available to the stations is determined. Using the determined set of available channels, detected are whether or not a signal of a channel identical to one included in the set of available channels has been received from an adjacent BSS, and whether or not there is radio interference from an adjacent BSS in any of the set of available channels. The stations measure the communication states of these available channels, and report the packet error rate (PER) and the received signal strength indication (RSSI) of all the channels measured by the stations to the access point. The interference level is also measured, which is the level of interference occurring due to an effect of any other wireless device during a given time. The interference level is based on the absence of signal reception from another BSS while a station in this BSS measures a signal and reports the measurement result to the access point. Thereafter, a new channel conforming to the decision criteria of the access point is selected based on the measure of the received signal strength indication, the packet error rate and the interference level information. 
       FIG. 2  shows part of the configuration of a conventional wireless communication system (wireless device). A wireless communication system  20  of  FIG. 2  includes: a transmit/receive part  23  having a channel measurement section  22  for performing channel measurement; and a host part  21  for receiving measurement results from the channel measurement section  22 . Conventionally, measurement of wireless channels is made by a wireless communication system having such a channel measurement section  22 . 
     As a related technology, US 2002/0060995A describes a method for measuring channels to determine a new channel. 
     Also, IEEE 802.11h examines a technique of measuring the states of wireless channels to dynamically change a channel. 
     However, although measurement of wireless channels has been discussed in the literature on the channel measurement technology described above, no mention has been made of a specific measurement method (algorithm). 
       FIG. 13  is a view showing channels available in Japan as of December 2004, while  FIG. 14  is a view showing channels available in the U.S. While the number of channels available in Japan is four (5150 to 5250 [MHz]) as shown in  FIG. 13 , multiple channels (eight channels; 5150 to 5350 [MHz]) are available in North America as shown in  FIG. 14 . (Note that from May 2005 the eight channels available in North America are also available in Japan.) When the number of available wireless channels is large, the interval required until the same channel is measured next will be long if all of these wireless channels are measured in rotation, causing the problem of increasing the entire measurement time. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is providing a specific measurement architecture for shortening the measurement time of available channels. 
     Specifically, the present invention relates to a wireless device including: a channel measurement section having a function of measuring use states indicating superiority or inferiority of the communication states of two or more wireless channels; and a measuring channel determination section for determining wireless channels to be measured, wherein the measuring channel determination section has a predetermined algorithm for efficiently selecting between a state of measuring all available wireless channels and a state of measuring only some channels among all the available wireless channels. 
     The wireless device of the present invention is a wireless device having a function of measuring use states indicating superiority or inferiority of the communication states of two or more wireless channels, the device including: a channel measurement section for measuring the use states of the wireless channels; and a measuring channel determination section for determining the wireless channels to be measured and sending a measurement instruction to the channel measurement section, wherein the measuring channel determination section selects between a first state in which only some of all available wireless channels are measured and a second state in which all the available wireless channels are measured according to a predetermined algorithm, and sends the measurement instruction to the channel measurement section. 
     The wireless channel measurement controller of the present invention is the wireless device described above, wherein the wireless device constitutes a wireless network together with one or more other wireless terminals each having a channel measurement section for measuring the use states of the wireless channels. 
     In one embodiment of the wireless device of the invention, the measuring channel determination section selects the second state if having detected radio interference in measurement of the currently used wireless channel, and sends the measurement instruction to the channel measurement section. 
     In another embodiment of the wireless device of the invention, the measuring channel determination section selects the first state according to the predetermined algorithm, in which only odd- or even-numbered wireless channels among all available wireless channels are measured, and sends the measurement instruction to the channel measurement section. 
     In yet another embodiment of the wireless device of the invention, the measuring channel determination section selects the first state according to the predetermined algorithm, in which odd- or even-numbered channels including the currently used wireless channel among all available wireless channels are measured, and sends the measurement instruction to the channel measurement section, and if having detected radio interference in any of the odd- or even-numbered channels, selects the first state, in which channels adjacent to the interference-detected channel are measured, and sends the measurement instruction to the channel measurement section. 
     In yet another embodiment of the wireless device of the invention, the wireless device is a wireless channel measurement controller constituting a wireless network together with one or more other wireless terminals each having a channel measurement section for measuring the use states of the wireless channels, and the measuring channel determination section of the wireless channel measurement controller selects between a third state in which only some of all available wireless channels are measured by at least two of the channel measurement section of the wireless channel measurement controller and the channel measurement section of the other wireless terminal and a fourth state in which all the available wireless channels are measured by any two of the channel measurement section of the wireless channel measurement controller and the channel measurement section of the other wireless terminal, according to the predetermined algorithm, and sends the measurement instruction to the channel measurement section of the wireless channel measurement controller and at least one of the channel measurement section of the other wireless terminal. 
     Alternatively, the wireless device of the present invention is a wireless device having a function of measuring use states indicating superiority or inferiority of the communication states of two or more wireless channels, the device including: a channel measurement section for measuring the use states of the wireless channels; and a measuring channel determination section for determining wireless channels to be measured and sending a measurement instruction to the channel measurement section, wherein the measuring channel determination section selects between a third state in which only some of all available wireless channels are measured by at least two of the channel measurement section and a channel measurement section of one or more other wireless terminals and a fourth state in which all the available wireless channels are measured by any two of the channel measurement section and the channel measurement section of the other wireless terminal, according to the predetermined algorithm, and sends the measurement instruction to at least one of the channel measurement sections of the wireless device itself and the other wireless terminal. 
     In one embodiment of the wireless device of the invention, the measuring channel determination section makes transition to the first or second state if the other wireless terminal becomes unconnected to the wireless network, and makes transition to the third or fourth state if the other wireless terminal becomes connected to the wireless network, according to the predetermined algorithm. 
     In another embodiment of the wireless device of the invention, the measuring channel determination section sends the measurement instruction to the channel measurement section of the other wireless terminal belonging to the same wireless network to measure different wireless channels. 
     In yet another embodiment of the wireless device of the invention, if radio interference is detected in any of the wireless channels measured by the channel measurement sections, the measuring channel determination section sends the measurement instruction to at least one of the channel measurement sections of the wireless channel measurement controller and the other wireless terminal to measure all available wireless channels. 
     In yet another embodiment of the wireless device of the invention, if radio interference is detected in any of the wireless channels measured by any of the other wireless terminal and none of the other wireless terminal is measuring a channel adjacent to the interference-detected wireless channel, the measuring channel determination section sends the measurement instruction to the other wireless terminal measuring the interference-detected wireless channel to measure the adjacent channel. 
     In yet another embodiment of the wireless device of the invention, the measuring channel determination section selects the second state according to the predetermined algorithm, stores a wireless channel second best in use state among all wireless channels as a replacement candidate channel for a currently-communicating channel, the second-best wireless channel being determined from the results of measurement made for all the wireless channels in response to the measurement instruction sent from the measuring channel determination section to the channel measurement section, selects the first state according to the predetermined algorithm in which only the currently-communicating channel and the replacement candidate channel are measured, and sends the measurement instruction to the channel measurement section. 
     In yet another embodiment of the wireless device of the invention, the measuring channel determination section selects the fourth state according to the predetermined algorithm, stores a wireless channel second best in use state among all wireless channels as a replacement candidate channel for a currently-communicating channel, the second-best wireless channel being determined from the results of measurement made for all the wireless channels in response to the measurement instruction sent from the measuring channel determination section to the channel measurement section, selects the third state according to the predetermined algorithm in which only the currently-communicating channel and the replacement candidate channel are measured, and sends the measurement instruction to the channel measurement section. 
     In yet another embodiment of the wireless device of the invention, the measuring channel determination section selects the second state if radio interference is detected in the replacement candidate channel according to the predetermined algorithm, and sends the measurement instruction to the channel measurement section to instruct the channel measurement section to measure all the available wireless channels. 
     In yet another embodiment of the wireless device of the invention, the measuring channel determination section selects the fourth state if radio interference is detected in the replacement candidate channel according to the predetermined algorithm, and sends the measurement instruction to the channel measurement section to instruct the channel measurement section to measure all the available wireless channels. 
     In yet another embodiment of the wireless device of the invention, the measuring channel determination section selects the second state periodically according to the predetermined algorithm and sends the measurement instruction to the channel measurement section, to thereby update the replacement candidate channel periodically. 
     In yet another embodiment of the wireless device of the invention, the measuring channel determination section selects the fourth state periodically according to the predetermined algorithm and sends the measurement instruction to the channel measurement section, to thereby update the replacement candidate channel periodically. 
     In yet another embodiment of the wireless device of the invention, the measuring channel determination section sends the measurement instruction to the channel measurement section of the same device so that all available wireless channels or adjacent channels be measured if radio interference is detected in a wireless channel measured by the channel measurement section and that, if an interference-source channel is detected, the first state in which the interference-source channel is not measured be selected. 
     In yet another embodiment of the wireless device of the invention, the measuring channel determination section sends the measurement instruction to the channel measurement section of the same device so that all available wireless channels or adjacent channels be measured if radio interference is detected in a wireless channel measured by the channel measurement section and that, if an interference-source channel is detected, the third state in which the interference-source channel is not measured be selected. 
     As described above, according to the present invention, the measuring channel determination section for determining channels to be measured among all available wireless channels sends a measurement instruction to the channel measurement section for measuring wireless channels to measure only some of all wireless channels determined according to a predetermined algorithm. In addition, the measuring channel determination section can select between measurement of only some of all wireless channels and measurement of all available wireless channels. With this measurement of only the minimum number of channels as required, the measurement time can be shortened, and detection of radio interference can be done efficiently. 
     In particular, according to the present invention, if radio interference is detected in measurement of odd- or even-numbered channels, only channels adjacent to the interference-detected channel are then measured. This enables more efficient channel measurement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a configuration of a BSS area including one wireless channel measurement controller in Embodiment 1 of the present invention. 
         FIG. 2  is a block diagram of a wireless communication system of a conventional wireless terminal. 
         FIG. 3  is a block diagram of a wireless communication system of a wireless channel measurement controller having an algorithm built therein in Embodiment 1 of the present invention. 
         FIG. 4  is a timing chart of measurement of only some of available channels in Embodiment 1 of the present invention. 
         FIG. 5A  is a timing chart of measurement of all available wireless channels in Embodiment 1 of the present invention, and  FIG. 5B  is a timing chart of measurement of only some of available channels in this embodiment. 
         FIG. 6  shows a configuration of a BSS area including one wireless channel measurement controller and two wireless terminals in Embodiment 2 of the present invention. 
         FIG. 7  is a block diagram of a wireless communication system of a wireless channel measurement controller having an algorithm built therein in Embodiment 2 of the present invention. 
         FIG. 8  is a block diagram of a wireless communication system of a wireless terminal in Embodiment 2 of the present invention. 
         FIG. 9  is a view showing an example of channel measurement in a BSS area including one wireless channel measurement controller and two wireless terminals in Embodiment 2 of the present invention. 
         FIG. 10  is a timing chart of channel measurement in Embodiment 2 of the present invention. 
         FIGS. 11A to 11C  are views showing examples of measurement of adjacent channels in a BSS area including one wireless channel measurement controller and two wireless terminals in Embodiment 2 of the present invention, in which  FIG. 1A  shows an example of first measurement,  FIG. 11B  shows an example of measurement of adjacent channels after occurrence of radio interference, and  FIG. 11C  shows an example of measurement of all channels after occurrence of radio interference. 
         FIG. 12  is a timing chart of channel measurement in Embodiment 2 of the present invention. 
         FIG. 13  is a view showing a frequency bandwidth within which wireless channels are available in Japan. 
         FIG. 14  is a view showing a frequency bandwidth within which wireless channels are available in North America. 
         FIG. 15  is a view showing an example of measurement of the current channel and the second best replacement candidate channel in Embodiment 3 of the present invention. 
         FIG. 16  is a flowchart showing an algorithm in Embodiment 2 of the present invention. 
         FIG. 17  is a flowchart showing an algorithm in Embodiment 3 of the present invention. 
         FIG. 18  is a flowchart showing another algorithm in Embodiment 3 of the present invention. 
         FIG. 19  is a flowchart showing yet another algorithm in Embodiment 3 of the present invention. 
         FIG. 20  is a timing chart of channel measurement in Embodiment 2 of the present invention. 
         FIG. 21  is a flowchart showing an algorithm in Embodiment 4 of the present invention. 
         FIG. 22  is a timing chart of channel measurement in Embodiment 4 of the present invention. 
         FIG. 23  is a timing chart of channel measurement in Embodiment 5 of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the wireless communication system (wireless device) and the wireless channel measurement controller of the present invention will be described with reference to the accompanying drawings. 
     Embodiment 1 
     Embodiment 1 of the present invention will be described with reference to the relevant drawings. 
       FIG. 1  is a view illustrating a basic service set (BSS) including only one wireless terminal (wireless device) A within a BSS area  10 . In this case, the wireless terminal A functions as a wireless channel measurement controller for determining wireless channels to be measured to execute channel measurement by itself. 
       FIG. 3  is a block diagram of a wireless communication system in this embodiment. The wireless communication system  30  of  FIG. 3 , which is part of a wireless channel measurement controller, includes: a host part  31  having a measuring channel determination section  32  with a predetermined algorithm built therein for determining channels to be measured; and a transmit/receive part  34  having a channel measurement section  33  for measuring the use states of two or more wireless channels. 
     The measuring channel determination section  32  requests the channel measurement section  33  to perform channel measurement by sending a measurement instruction  36 , and the channel measurement section  33  notifies the host part  31  of the result  35  of measurement made by the channel measurement section  33 . 
     In the wireless communication system  30 , switching can be made between a first state, in which the measuring channel determination section  32  instructs the channel measurement section  33  to measure only some of all available channels, and a second state, in which the measuring channel determination section  32  instructs the channel measurement section  33  to measure all available channels, according to a predetermined algorithm. By adopting the first state or the second state properly, the measurement time of all wireless channels can be shortened. 
       FIG. 4  is a timing chart showing an example of measurement in the case of adopting the first state, in which some of currently available channels are measured. The beacons in  FIG. 4  respectively indicate synchronous information output by the wireless device (wireless channel measurement controller) A in the BSS, with which a wireless device in the BSS can recognize the existence of the access point. As the frequency measurement time period, one beacon is available for only one channel. In the illustrated example, channels Ch1, Ch3, Ch5 and Ch7 are respectively measured within four beacon intervals of a measurement time  40 . 
       FIG. 5A  is a timing chart showing an example of measurement in the case of adopting the second state, in which all eight wireless channels, Ch1 through Ch8, are measured. The measurement in the second state will be selected according to a predetermined algorithm in the events of determining a currently used channel at the initial stage and selecting a candidate channel when a shift from the currently used channel is to be made in response to a request from the host. Also, according to a predetermined algorithm, transition may be made to the second state if an abnormal state of radio interference is detected in the first state, so that the channel measurement section  33  can measure all wireless channels to determine a candidate channel to which a shift is to be made. 
       FIG. 13  shows channels (four channels) available in Japan, while  FIG. 14  shows channels (eight channels) available in the U.S. Hatched portions  130  in  FIG. 13  and hatched portions  140  in  FIG. 14  represent radio interference caused by adjacent channels. This feature of radio communication of being susceptible to radio interference from adjacent channels is utilized. That is, every other channel is measured (odd- or even-numbered channels are measured), and if no radio interference (interference from an adjacent channel) is detected, it is recognized that the adjacent channels are unused. If all of the eight wireless channels available in North America, for example, are to be measured, a measurement time  50  corresponding to eight beacons will be necessary as shown in  FIG. 5A . However, by measuring only some of the channels as shown in  FIG. 4 , the measurement time can be shortened to the time corresponding to four beacons. 
     In the measurement in the first state shown in  FIG. 4 , if the radio interference level exceeds a predetermined level, it can be determined that there is an interference wave from the current channel or an adjacent channel. By use of this fact, reduction in the number of channels to be measured is achieved. 
     Although only odd-numbered channels were measured in  FIG. 4 , it is also possible to measure only even-numbered channels. Although one-time measurement was exemplified in  FIG. 4 , the measurement may be repeated any number of times to enhance the measurement accuracy. 
       FIG. 5B  is a timing chart showing detection and measurement of adjacent channels followed when radio interference is detected.  FIG. 5B  shows the case that interference is detected in the channel Ch5. To specify a channel that is responsible for the radio interference in the channel Ch5, channels Ch4 and Ch6 adjacent to the channel Ch5 are measured. As shown in  FIG. 5B , the channels Ch1, Ch3, Ch5 and Ch7 are measured in a measurement time  51  of four beacons and subsequently the channels Ch4 and Ch6 are measured in a measurement time  52  of two beacons. In this way, by use of a predetermined algorithm that odd-numbered channels are first measured and, if radio interference is detected by this measurement, the channels adjacent to the interference-detected channel are measured, correct radio states of wireless channels can be measured. 
     In the wireless communication system  30  described above, the predetermined algorithm built in the measuring channel determination section  32  may allow transition to the second state at any timing set by the host part  31 , such as timing at which the count set by the host part  31  is exceeded, or timing at which the time set by an internal timer in the host part  31  has expired, and instruct the channel measurement section  33  to measure all or other wireless channels. 
     Embodiment 2 
     Embodiment 2 of the present invention will be described with reference to the relevant drawings. 
       FIG. 6  is a view illustrating a BSS including three wireless devices (A, B and C) in a BSS area  60 . 
     The wireless device A constituting a wireless network in the BSS area  60  of  FIG. 6  functions as a wireless channel measurement controller, while the wireless devices B and C are wireless terminals that do not function as a wireless channel measurement controller. In this situation, in which the wireless terminals B and C exist in the BSS area in addition to the wireless channel measurement controller A, a third state can be defined, in which the measurement of only some of all available wireless channels is shared among the plurality of wireless devices. In this relation, assume that the wireless channel measurement controller A is allowed to request the wireless terminals B and C to perform channel measurement. Note that transition from the first state in the situation shown in  FIG. 1  to the third state in the situation shown in  FIG. 6  is considered to be state transition when a station is networked in a BSS having an access point. 
       FIG. 7  is a block diagram of a wireless communication system of the wireless channel measurement controller A in the BSS  60  having three wireless devices (A, B and C) shown in  FIG. 6 . In the wireless communication system  70  of  FIG. 7 , the measuring channel determination section  32  of the host part  31  requests the channel measurement section  33  of the same system to measure wireless channels, as in the wireless communication system  30  of  FIG. 3 . In addition, the wireless communication system  70  of  FIG. 7  transmits a measurement request  37  to the other wireless terminals B and C via the transmit/receive part  34 , and receives measurement results from the wireless terminals B and C that have done the measurement in response to the measurement request  37  via the transmit/receive part  34  as measurement result reception  38 . 
       FIG. 8  is a block diagram of a wireless communication system of the wireless terminal B or C in the same BSS  60  area in  FIG. 6 . The wireless communication system  80  of  FIG. 8 , essentially composed of a host part  81  and a transmit/receive part  83 , has a channel measurement section  82  in the transmit/receive part  83 , as in the wireless communication system  70  of  FIG. 7 , but has no measuring channel determination section in the host part  81 . In the wireless communication system  80 , the transmit/receive part  83  receives the measurement request  37  transmitted from the wireless communication system  70 , and notifies the host part  81  of the measurement request as measurement request reception  86 . In response to the notification, the host part  81  instructs the channel measurement section  82  to perform wireless channel measurement and receives a notification  84  of measurement results. The received notification  84  of measurement results is output from the host part  81  to the transmit/receive part  83  as measurement results  85 , which are then transmitted to the wireless communication system  70 . The measurement results  85  transmitted from the wireless communication system  80  are received by the host part  31  of the wireless communication system  70  of  FIG. 7  via the transmit/receive part  34  as the measurement result reception  38 . 
     In the case that only the wireless channel measurement controller A exists in the BSS  60 , the measuring channel determination section  32  sends an instruction on measuring channels to the channel measurement section  33 , and the channel measurement section  33  notifies the host part  31  of the channel measurement results, as described above with reference to  FIG. 3 . 
       FIG. 9  shows an example of channel measurement in which two wireless terminals (B and C) exist in the BSS  60  having one wireless channel measurement controller A shown in  FIG. 6 . In  FIG. 9 , the wireless terminal A requests the wireless terminals B and C to measure different channels, like requesting the wireless terminal B to measure the channels Ch1 and Ch3 while requesting the wireless terminal C to measure the channels Ch5 and Ch7, and receives measurement results  90  from the wireless terminals B and C, thereby achieving efficient retrieval of the channel measurement results. Note herein that no importance is especially put on the order of the channels to be measured. 
       FIG. 10  is a timing chart showing the channel measurement in  FIG. 9 . In  FIG. 10 , in a measurement time  100  of three beacons, the wireless device (wireless channel measurement controller) A first transmits a measurement instruction to the wireless devices (wireless terminals) B and C during a communication interval following issuance of the first beacon. Receiving the measurement instruction, the wireless terminal B measures the channel Ch1 during the communication interval following the first beacon, measures the channel Ch3 during a communication interval following the second beacon, and finally transmits the measurement results to the wireless channel measurement controller A during a communication interval following the third beacon. 
     Likewise, receiving the measurement instruction transmitted by the wireless channel measurement controller A, the wireless terminal C measures the channel Ch5 during the communication interval following the first beacon, measures the channel Ch7 during the communication interval following the second beacon, and finally transmits the measurement results to the wireless channel measurement controller A during the communication interval following the third beacon. The wireless channel measurement controller A receives the measurement results transmitted from the wireless terminals B and C during the communication interval following the third beacon. 
     As described above, when the wireless terminals B and C exist in the BSS in addition to the wireless channel measurement controller A and there are multiple available channels, as in North America, for example, in which eight channels are available, the third state can be selected according to a predetermined algorithm in which channels to be measured are allocated to the plurality of wireless terminals B and C in the BSS to achieve partial channel measurement. In this manner, a piece of measurement that would require the time of eight beacons if being done with one wireless terminal can be completed within the time of three beacons, widely shortening the measurement time. 
     Although one-time measurement is shown in  FIG. 10 , the measurement may be made repeatedly any number of times. Although only odd-numbered channels were measured in  FIG. 10 , only even-numbered channels may be measured. Although the request for measurement was made to all the wireless terminals B, C in the BSS in  FIG. 10 , it may be made to only some of the wireless terminals in the BSS. In the above description, when there were wireless terminals in the BSS area, the wireless channel measurement controller left the entire channel measurement to the wireless terminals. Alternatively, the wireless channel measurement controller itself may share the channel measurement. 
     If radio interference is detected in the third state shown in  FIGS. 9 and 10 , a fourth state may be adopted in which all available channels are measured by sharing. This state is shown in  FIGS. 11A and 11C . In  FIG. 1A , the wireless channel measurement controller A requests the wireless terminal B to measure the channels Ch1 and Ch5 while requesting the wireless terminal C to measure the channels Ch3 and Ch7. At this time, if it is found from the measurement result  110  transmitted from the wireless terminal B that radio interference has been detected in the channel Ch5, the wireless channel measurement controller A requests the wireless terminal B to measure the channels Ch2/4/1/5 among all available channels while requesting the wireless terminal C to measure the channels Ch6/8/3/7, and receives the measurement results  110  from the wireless terminals B and C. 
       FIG. 20  is a timing chart of the processing in  FIGS. 11A and 11C . In  FIG. 20 , the wireless device (wireless channel measurement controller) A first transmits a measurement instruction during the first communication interval. The wireless device (wireless terminal) B, which has received the measurement instruction, measures the channels Ch1 and Ch5 during the two consecutive communication intervals. In the illustrated example, radio interference is detected in the channel Ch5. During the third communication interval, the wireless terminal B transmits the measurement results to the wireless channel measurement controller A. Likewise, the wireless device (wireless terminal) C, which has received the measurement instruction from the wireless channel measurement controller A during the first communication interval, measures the channel Ch3 during the first communication interval and the channel Ch7 during the second communication interval, and transmits the measurement results to the wireless channel measurement controller A during the third communication interval. The wireless channel measurement controller A receives the measurement results from the wireless terminals B and C during the third communication interval, and also transmits a measurement instruction during the third communication interval to request the wireless terminal B to measure the channels Ch2/4/1/5 among all available channels and request the wireless terminal C to measure the channels Ch6/8/3/7. Receiving the measurement instruction, the wireless terminals B and C measure the respective allocated channels over the third through sixth communication intervals, and transmit the measurement results to the wireless channel measurement controller A during the next seventh communication interval. The wireless channel measurement controller A receives the measurement results from the wireless terminals B and C during the seventh communication interval. 
     As described above, the channel measurement is made according to a predetermined algorithm in which channels to be measured are allocated to a plurality of wireless terminals in the BSS (third state), and if radio interference is detected in any of the channels, all available channels are measured by sharing the measurement among the plurality of wireless terminals (fourth state). Therefore, the total measurement time can be as short as a measurement time  200  of seven beacons even in the event of detection of radio interference. After the fourth state, the third state is recovered to continue the measurement. 
     The example of measurement described above will be discussed using a flowchart of  FIG. 16 . The flowchart of  FIG. 16  will be described in two parts separately: a flowchart  16 A for the case of having only one wireless terminal in a wireless network and a flowchart  16 ABC for the case of having a plurality of wireless terminals in addition to the wireless channel measurement controller. 
     First, in the flowchart  16 A, in step S 1601 , measurement is made for all available wireless channels (second state), to determine the current channel. In step S 1602 , whether or not other devices (wireless terminals) exist in the same wireless network is determined. If other wireless terminals exist, whether or not a request for measurement is to be made to such wireless terminals is determined in step S 1603 . If a measurement request is made to such wireless terminals, the process moves to the flowchart  16 ABC. If no measurement request is made, or if no other wireless terminal exists, the process proceeds to step S 1604 , to perform measurement of odd- or even-numbered channels (first state). Once the measurement in the step S 1604  is terminated, the process proceeds to step S 1605  to determine whether or not the current channel is busy, that is, whether or not the current channel is being used for communication by another wireless network causing radio interference. If it is determined that the current channel is not busy, the process returns to the step S 1604  to measure odd- or even-numbered channels. If the current channel is determined busy, the process returns to the step S 1601  to start channel determination based on the full channel measurement. 
     The flowchart  16 ABC will then be described. If it is determined to make a measurement request to the other wireless terminals in the step S 1603 , a measurement request is transmitted from the wireless channel measurement controller to the other wireless terminals (third state) in step S 1606  in the flowchart  16 ABC. The results of the measurement by the other wireless terminals are then returned in step S 1607 . The wireless channel measurement controller that has received the measurement results determines whether or not the current channel is busy in step S 1608 . If the current channel is determined busy, the process returns to the step S 1606 . If it determined that the current channel is not busy, the process proceeds to step S 1609 . In the step S 1609 , whether or not all channels are to be measured is determined. If all channels are to be measured, the wireless channel measurement controller transmits a measurement request to the other wireless terminals to share the measurement of all channels (fourth state) in step S 1610 . In step S 1611 , the wireless channel measurement controller receives the results of the measurement done by the other terminals that have received the measurement request. After the fourth state, the process returns to the step S 1606  recovering the third state and the measurement is continued. 
     In the above description, when there were wireless terminals in the BSS area, the wireless channel measurement controller left the entire channel measurement to the wireless terminals. Alternatively, the wireless channel measurement controller itself may share the channel measurement. 
     Next, the case of measuring adjacent channels if radio interference is detected in the third state shown in  FIGS. 9 and 10  will be described with reference to  FIGS. 11A and 11B . This case is regarded as the third state because some of available channels are measured by sharing. 
     In  FIG. 11A , the wireless channel measurement controller A requests the wireless terminal B to measure the channels Ch1 and Ch5 while requesting the wireless terminal C to measure the channels Ch3 and Ch7. At this time, if it is found from the measurement results  110  from the wireless terminal B that radio interference has been detected in the channel Ch5, the wireless channel measurement controller A requests the wireless terminals B and C to measure the adjacent channels Ch4 and Ch6, respectively, and receives the measurement results  110  from the wireless terminals B and C. 
       FIG. 12  is a timing chart of the processing in  FIGS. 11A and 11B . In  FIG. 12 , the wireless device (wireless channel measurement controller) A first transmits a measurement instruction during the first communication interval. The wireless device (wireless terminal) B, which has received the measurement instruction, measures the channels Ch1 and Ch5 during the two consecutive communication intervals. In the illustrated example, radio interference is detected in the channel Ch5. During the third communication interval, the wireless terminal B transmits the measurement results to the wireless channel measurement controller A. Likewise, the wireless device (wireless terminal) C, which has received the measurement instruction from the wireless channel measurement controller A during the first communication interval, measures the channel Ch3 during the first communication interval and the channel Ch7 during the second communication interval, and transmits the measurement results to the wireless channel measurement controller A during the third communication interval. The wireless channel measurement controller A receives the measurement results from the wireless terminals B and C during the third communication interval, and also transmits a measurement instruction during the third communication interval to request the wireless terminal B to measure the channel Ch4 adjacent to the interference-detected channel Ch5 and the wireless terminal C to measure the channel Ch6 as the other adjacent channel. Receiving the measurement instruction, the wireless terminals B and C measure the respective assigned channels during the third communication interval, and transmit the measurement results to the wireless channel measurement controller A during the next fourth communication interval. The wireless channel measurement controller A receives the measurement results from the wireless terminals B and C during the fourth communication interval. 
     Thus, partial channel measurement according to a predetermined algorithm is made in which channel measurement is made by allocating channels to be measured to a plurality of wireless terminals in the BSS (third state) and, if radio interference is detected in any of the channels, channels adjacent to the interference-detected channel on both sides are measured by further allocating the adjacent channels to the wireless terminals. In this manner, the total measurement time can be as short as a measurement time  120  of four beacons even in the event of detection of radio interference. Alternatively, for improvement of the measurement accuracy of radio interference, the terminal that has detected the radio interference may be instructed to measure the adjacent channels. 
     As described above, when wireless terminals other than the wireless channel measurement controller exist in the BSS area, the wireless channel measurement controller can request the wireless terminals to perform channel measurement to shorten the measurement time. 
     The example of measurement described above will be discussed using the flowchart of  FIG. 16 . 
     If it is determined to make a measurement request to the other wireless terminals in the step S 1603  in the flowchart  16 A, a measurement request is transmitted from the wireless channel measurement controller to the other wireless terminals (third state) in the step S 1606  in the flowchart  16 ABC. The results of the measurement by the wireless terminals are then returned in the step S 1607 . The wireless channel measurement controller that has received the measurement results determines whether or not the current channel is busy in the step S 1608 . If the current channel is determined busy, the process returns to the step S 1606 . If it is determined that the current channel is not busy, the process proceeds to the step S 1609 . In the step S 1609 , whether or not all channels are to be measured is determined. If all channels are not to be measured, whether or not adjacent channels (predetermined channels) are to be measured is determined in step S 1612 . If adjacent channels are not to be measured, the process returns to the step S 1606 . Otherwise, the process proceeds to step S 1613 . In the step S 1613 , the wireless channel measurement controller transmits a measurement request to the other wireless terminals to perform measurement of adjacent channels (third in-depth state) in step S 1613 . In the step S 1611 , the wireless channel measurement controller receives the results of the measurement done by the other terminals that have received the measurement request. After the third in-depth state, the process returns to the step S 1606  recovering the third state and the measurement is continued. 
     In the above description, when there were wireless terminals in the BSS area, the wireless channel measurement controller left the entire channel measurement to the wireless terminals. Alternatively, the wireless channel measurement controller itself may share the channel measurement. 
     Embodiment 3 
     Next, a channel selection method for selecting the minimum number of channels to be measured will be described. 
     In this embodiment, in the wireless channel measurement controller having the wireless communication system  70  of  FIG. 7 , the channel measurement section  33  is instructed to measure all channels (second state), and a channel second best in measurement results is stored in the measuring channel determination section  32  as a replacement candidate channel. In the next measurement, precedence is given to the second-best replacement candidate channel (first state), rather than measuring all channels. In this way, the measurement time for all wireless channels can be shortened. 
       FIG. 15  is a timing chart of the operation described above. In the case of having available channels as many as eight channels as in North America, all wireless channels are first measured in a measurement time  150 . Thereafter, only the current channel (channel Ch1 in this example) and the second-best channel (channel Ch8 in this example), among the eight channels, are measured in a measurement time  151 . This can shorten the measurement time to the time of two beacons. Although the measurement of the two channels Ch1 and Ch8 was made only twice in the measurement times  151  and  152  in  FIG. 15 , it can be made repeatedly any number of times to improve the measurement accuracy.  FIG. 17  shows a flowchart of this processing. 
     In  FIG. 17 , in step S 1701 , all wireless channels are measured (second state). In step S 1702 , a channel second best among all the wireless channels is selected as a replacement candidate. In step S 1703 , measurement is made for the two channels: the current channel and the replacement candidate channel (first state). In step S 1704 , whether or not the current channel is busy is determined. If it is determined that the current channel is not busy, the process returns to the step S 1703  for channel measurement. If the current channel is determined busy, the process proceeds to step S 1705  to replace the current channel with the replacement candidate channel. The process then returns to the step S 1701  for measurement of all wireless channels, to select a new replacement candidate channel. In this manner, measurement of all wireless channels is initially performed, and based on the measurement results, the wireless channel second best to the currently communicating channel in communication state (use state) is stored as the replacement candidate channel. As long as no radio interference occurs in the current channel, measurement of the two channels is repeated in the steps S 1703  and S 1704 . By using such a predetermined algorithm, the measurement time can be shortened. 
     Although the wireless channel measurement controller performed the measurement in the illustrated example, it may request a wireless terminal in the BSS to perform the measurement. 
     As an another example of processing, in the wireless channel measurement controller having the wireless communication system  70  of  FIG. 7 , the channel measurement section  33  is instructed to measure all channels, and a channel second best in the measurement results is stored in the measuring channel determination section  32  as a replacement candidate channel. In this example, all wireless channels will be measured not only when radio interference occurs in the current channel, but also when radio interference occurs in the second-best replacement candidate channel. This allows for holding of an always-updated replacement candidate channel, and thus can shorten the measurement time of all wireless channels.  FIG. 18  is a flowchart of this processing. 
     The flowchart of  FIG. 18  is substantially the same as the flowchart of  FIG. 17  in the processing in steps S 1801  through S 1804  and S 1806 , but is different therefrom in that if it is determined that the current channel is not busy in the step S 1804 , whether or not the replacement candidate channel is busy is further determined in step S 1805 . If the replacement candidate channel is determined busy in the step S 1805 , the process returns to the step S 1801  for measurement of all wireless channels to select a new replacement candidate channel. If it is determined that the replacement candidate channel is not busy in the step S 1805 , the process returns to the step S 1803  to continue the measurement of the current channel and the current replacement candidate channel. In this manner, measurement of all wireless channels is initially made, and based on the measurement results, a wireless channel second best to the currently communicating channel in communication state (use state) is stored as a replacement candidate channel. As long as no radio interference occurs in the current channel and the replacement candidate channel, the measurement of the two channels is repeated in the processing in the steps S 1803  through S 1805 . By adopting such a predetermined algorithm, the measurement time can be shortened. 
     As yet another example of processing, in the wireless channel measurement controller having the wireless communication system  70  of  FIG. 7 , the channel measurement section  33  is instructed to measure all channels, and a channel second best in the measurement results is stored in the measuring channel determination section  32  as a replacement candidate channel. In this example, the measurement of all channels is made periodically, to enable holding of an always-updated replacement candidate channel. This can shorten the measurement time of all wireless channels.  FIG. 19  is a flowchart showing this processing. 
     The flowchart of  FIG. 19  is substantially the same as the flowchart of  FIG. 17  in the processing in steps S 1901  through  1904  and S 1907 , but is different therefrom in the processing after the determination that the current channel is not busy in the step S 1904 . Specifically, after the step S 1904  of determining whether or not the current channel is busy, whether or not the replacement candidate channel is busy is determined in step S 1905 . If the replacement candidate channel is determined busy, the process returns to the step S 1901  for selection of a new replacement candidate. If it is determined that the replacement candidate channel is not busy, process proceeds to step S 1906  to decide whether or not the replacement candidate is to be re-determined. If re-determination is decided, the process returns to the step S 1901 . Otherwise, the process returns to the step S 1903  to continue the measurement of the current channel and the current replacement candidate channel. In this manner, measurement of all wireless channels is initially made, and based on the measurement results, a wireless channel second best to the currently communicating channel in communication state (use state) is stored as the replacement candidate channel. As long as no radio interference occurs in the current channel or the replacement candidate channel, the measurement of the two channels is repeated in the processing in the steps S 1903  through S 1906 . In addition, whether or not re-determination is to be made for a better replacement candidate is determined even if no radio interference occurs in the current replacement candidate channel. By adopting such a predetermined algorithm, an always-updated replacement candidate channel can be held. 
     Embodiment 4 
     A measurement method using an algorithm different from that used in Embodiment 2 will be described as Embodiment 4 with reference to the relevant drawings.  FIG. 22  is a timing diagram of this measurement method. 
     In  FIG. 22 , only the wireless device (wireless channel measurement controller) A performs measurement of the channels Ch1/3/5/7. In the illustrated example, radio interference is detected in the channel Ch7, and during the subsequent fifth through thirteenth communication intervals, all the channels Ch1 to Ch8 are measured. Once the channel Ch7 is specified as the interference source, the channels Ch1/3/5 excluding Ch7 are then measured. In this manner, if radio interference is detected in any of the channels in the first state, measurement of all available channels is made (second state). Once the interference source is specified, the specified channel is no more measured. By adopting such a predetermined algorithm, channel measurement in the first state with a reduced number of channels can be made. 
     In the timing chart described above, all the channels were measured after detection of radio interface. Alternatively, only channels adjacent to the interference-detected channel may be measured to specify an interference-source channel. 
     In  FIG. 22 , only one-time measurement is shown. Alternatively, to enhance the measurement accuracy, the measurement may be made repeatedly a plurality of times.  FIG. 21  is a flowchart showing this processing. In the flowchart of  FIG. 21 , the steps S 1601  through S 1605  are substantially the same as those in the flowchart of  FIG. 16 . After the measurement of all channels in the step S 1601 , if a given channel is specified as a radio interference source in step S 2101 , whether or not the interference-source channel should be deleted from the channels to be measured is determined in step S 2102 . If deletion is determined, the interference-source channel is deleted from the channels to be measured in step S 2103  and then whether or not there is left any channel to be measured is determined in step S 2104 . If there is left, the process returns to the step S 1602 . Otherwise, the channels to be measured are reset to include any deleted channel and the process returns to the step S 1602 . 
     Embodiment 5 
     A measurement method using an algorithm different from that used in Embodiment 2 in the case of existence of wireless terminals will described as Embodiment 5 with reference to the relevant drawing.  FIG. 23  is a timing diagram of this measurement method. 
     In  FIG. 23 , the wireless device (wireless channel measurement controller) A first transmits a measurement instruction during the first communication interval. The wireless device (wireless terminal) B, which has received the measurement instruction, measures the channels Ch1 and Ch5 during the two consecutive communication intervals. During the third communication interval, the wireless terminal B transmits the measurement results to the wireless channel measurement controller A. Likewise, the wireless device (wireless terminal) C, which has received the measurement instruction from the wireless channel measurement controller A during the first communication interval, measures the channel Ch3 during the first communication interval and the channel Ch7 during the second communication interval. In the illustrated example, radio interference is detected in the channel Ch7. During the third communication interval, the wireless terminal C transmits the measurement results to the wireless channel measurement controller A. The wireless channel measurement controller A receives the measurement results from the wireless terminals B and C during the third communication interval, and also transmits a measurement instruction to request the wireless terminal B to measure the channel Ch6 adjacent to the interference-detected channel Ch7 and the wireless terminal C to measure the channels Ch8 as the other adjacent channel. Receiving the measurement instruction, the wireless terminals B and C measure the respective assigned channels during the third communication interval, and transmits the measurement results to the wireless channel measurement controller A during the next fourth communication interval. The wireless channel measurement controller A receives the measurement results from the wireless terminals B and C during the fourth communication interval. 
     Once the channel Ch7 is determined as an interference-source channel as a result of the measurement, the interference-source channel is deleted from the channels to be measured for the subsequent channel measurement. Specifically, the wireless device (wireless channel measurement controller) A transmits a measurement instruction during the fourth communication interval. The wireless device (wireless terminal) B measures the channels Ch1 and Ch5 during the two consecutive communication intervals, and then transmits the measurement results to the wireless channel measurement controller A during the sixth communication interval. The wireless device (wireless terminal) C measures the channel Ch3 during the fourth communication interval in response to the measurement instruction from the wireless channel measurement controller A, and then transmits the measurement results to the wireless channel measurement controller A during the subsequent fifth communication interval. 
     As described above, if radio interference is detected in any channel in the third state, measurement of channels adjacent to the interference-detected channel is made by sharing. Once the interference-source channel is specified, the specified channel is no more measured. By adopting such a predetermined algorithm, channel measurement in the third state with a reduced number of channels can be made. 
     In the timing chart of  FIG. 23 , only the adjacent channels are measured after detection of radio interference. Alternatively, all the channels may be measured after detection of radio interference. Also, to enhance the measurement accuracy, the terminal that has detected the radio interference may be instructed to measure the adjacent channels. 
     In this embodiment, only the wireless terminals perform the channel measurement. Alternatively, the wireless channel measurement controller may also share the measurement. The above processing is shown in the flowchart of  FIG. 21 . 
     In the flowchart of  FIG. 21 , the steps S 1601  through S 1613  are substantially the same as those in the flowchart of  FIG. 16 . If measurement of all channels is determined in the step S 1609  after radio interference is detected in the current channel in the step S 1608 , the wireless channel measurement controller transmits a measurement request to the other wireless terminals to share the measurement of all channels in the step S 1610 , and receives the measurement results in the step S 1611 . If all channels are not to be measured in the step S 1609 , whether or not adjacent channels are to be measured is determined in the step S 1612 . If adjacent channels are to be measured, the wireless channel measurement controller transmits a measurement request to the other wireless terminals to perform measurement of adjacent channels in the step S 1613 , and receives the measurement results in the step S 1611 . 
     After the interference-source channel is specified from the received results, whether or not the interference-source channel should be deleted from the channels to be measured is determined in step S 1620 . If deletion is determined, the interference-source channel is deleted from the channels to be measured in step S 2202 , and then whether or not there is left any channel to be measured in step S 2203 . If there is left, the process returns to the step S 1606  to recover the third state. Otherwise, the channels to be measured are reset to include any deleted channel and the process returns to the step S 1606  to recover the third state. 
     As described above, if radio interference is detected in any channel in the third state, measurement of channels adjacent to the interference-detected channel is made by sharing. Once the interference-source channel is specified, the specified channel is no more measured. By adopting such a predetermined algorithm, channel measurement in the third state with a reduced number of channels can be done. 
     In the timing chart of  FIG. 23 , only the adjacent channels were measured after detection of radio interference. Alternatively, all the channels may be measured after detection of radio interference. Also, in  FIG. 23 , only one-time measurement is shown. To enhance the measurement accuracy, naturally, the measurement may be made repeatedly a plurality of times. To further enhance the measurement accuracy, the terminal that has detected the radio interference may be instructed to measure the adjacent channels. 
     In this embodiment, only the wireless terminals performed the channel measurement. Alternatively, the wireless channel measurement controller may also share the measurement. 
     While the present invention has been described in preferred embodiments, it will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than that specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope of the invention.