Patent Publication Number: US-2023140667-A1

Title: Communication method and radio tag

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation application of PCT International Application No. PCT/JP2021/027112 filed on Jul. 20, 2021, designating the United States of America, which is based on and claims priority of U.S. Provisional Patent Application No. 63/060,942 filed on Aug. 4, 2020. The entire disclosures of the above-identified applications, including the specifications, drawings and claims are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present disclosure relates to communication methods and radio tags, and particularly relates to a communication method performed by an active radio tag, a communication method performed by an access point that receives radio waves from an active radio tag, and an active radio tag. 
     BACKGROUND 
     Radio tags are attached to products to track the locations of the products. Note that the radio tags are also referred to as electronic tags, integrated circuit (IC) tags, radio IC tags, contactless IC tags, or radio-frequency identification (RFID) tags. The radio tags are classified as passive radio tags which are driven using electric power from external devices and emit radio waves and active radio tags which are driven using an internal battery and emit radio waves by itself. 
     For example, Patent Literature (PTL) 1 proposes a system capable of tracking a container by using an active radio tag. PTL 1 discloses that it is possible to provide a real-time radio tracking system in a limited area with defined borders. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 2004-529049 
     SUMMARY 
     Technical Problem 
     However, frequencies available for active radio tags may vary from area to area. For example, the frequency available in Japan is 920 MHz. Meanwhile, the frequency available in China is 830 MHz, and the frequency available in Europe is 868 MHz, for example. The frequency available in the United States of America is 915 MHz. 
     This means that it is necessary to change the frequency in the case of tracking across areas where the frequencies available for radio tags are different. 
     Furthermore, the acceptable cost of radio tags that are used for tracking or the like is often limited; therefore, relatively low-price radio tags without means for receiving data are used. This means that relatively low-price radio tags are problematic in that default frequencies thereof cannot be changed. 
     The present disclosure is conceived in view of the above-described circumstances and has an object to provide a communication method in which the frequency of an active radio tag without means for receiving data can be changed, and the radio tag. 
     Solution to Problem 
     A communication method according to one aspect of the present disclosure is performed by one radio tag included in a plurality of radio tags of a communication system including an access point and the plurality of radio tags, and includes: performing carrier sensing at a predetermined time interval; transmitting first data including ID information to the access point when a signal transmitted by a radio tag different from the one radio tag to the access point is not detected during the performing of the carrier sensing and the one radio tag is not in a transmission stop state, the ID information being information for uniquely identifying the one radio tag; interpreting a command when a signal is detected during the performing of the carrier sensing and the signal is not a signal transmitted by a radio tag different from the one radio tag to the access point, the interpreting being performed under an assumption that the signal detected is a signal transmitted from the access point, the command being included in the signal transmitted from the access point; and changing a radio frequency of a signal to be transmitted by the one radio tag, when the interpreting shows that the signal includes a command instructing changing of a frequency. 
     With this, it is possible to allow the active radio tag without means for receiving data to use a carrier-sense function to interpret the command transmitted by the access point and therefore, the frequency to be used by said radio tag can be changed. 
     For example, the transmitting may include transmitting second data to the access point when the interpreting shows that the signal includes a command instructing stopping of the transmission and changing of the frequency and when the signal transmitted by the radio tag different from the one radio tag to the access point is not detected during the performing of the carrier sensing, the second data being obtained by adding, to the first data, information indicating that the command has been received. In the changing, when the interpreting shows that the signal includes the command instructing stopping of the transmission and changing of the frequency, the radio frequency of the signal to be transmitted by the one radio tag may be changed after the second data is transmitted. The communication method may further include: controlling the one radio tag to stop only execution of the transmitting until the interpreting shows that the signal transmitted from the access point and detected during the performing of the carrier sensing includes a command instructing resuming of the transmission after the second data is transmitted in the transmitting. 
     Thus, it is possible to allow the radio tag to use a carrier-sense function to interpret the command transmitted by the access point and therefore, the frequency to be used by said radio tag can be changed, and signal transmission can be stopped. 
     Furthermore, for example, the transmitting may include transmitting second data to the access point when the interpreting shows that the signal includes a command instructing stopping of the transmission and when the signal transmitted by the radio tag different from the one radio tag to the access point is not detected during the performing of the carrier sensing, the second data being obtained by adding, to the first data, information indicating that the command has been received, and the communication method may further include: controlling the one radio tag to stop only execution of the transmitting after the second data is transmitted in the transmitting. 
     Thus, it is possible to allow the radio tag to use a carrier-sense function to interpret the command transmitted by the access point and therefore, signal transmission from the radio tag can be stopped. 
     Furthermore, for example, in the changing, when the interpreting shows that the signal includes the command instructing changing of the frequency, the radio frequency of the signal to be transmitted by the one radio tag may be changed, and the controlling may include resuming the execution of the transmitting when the radio frequency of the signal to be transmitted by the one radio tag is changed in the changing. 
     Thus, it is possible to allow the radio tag to use a carrier-sense function to interpret the command transmitted by the access point and therefore, the frequency to be used by said radio tag can be changed, and signal transmission that has been stopped can be resumed. 
     Furthermore, for example, the information indicating that the command has been received is acknowledgement (ACK). 
     Furthermore, for example, the performing of the carrier sensing may include: calculating a strength of a signal received during the performing of the carrier sensing; firstly determining, in a period in which a first averaged strength exceeds a first threshold value, that the signal transmitted by the radio tag different from the one radio tag to the access point is detected, the first averaged strength being obtained by averaging, in a first cycle, the strength calculated in the calculating; and secondly determining, by assessing a pattern, that a signal different from the signal transmitted by the radio tag different from the one radio tag to the access point is detected, the pattern being defined by a total number of times a second averaged strength exceeds a second threshold value and a total number of times the second averaged strength falls below the second threshold value, the second averaged strength being obtained by averaging, in a second cycle shorter than the first cycle, the strength calculated in the calculating. 
     Thus, using a carrier-sense function, the radio tag can simultaneously perform normal carrier sensing and carrier sensing that is used to interpret the command transmitted by the access point. 
     Furthermore, for example, in the performing of the carrier sensing, first carrier sensing and second carrier sensing may be performed alternately, the first carrier sensing may include: calculating a strength of a signal received during the performing of the carrier sensing; and firstly determining, in a period in which a first averaged strength exceeds a first threshold value, that the signal transmitted by the radio tag different from the one radio tag to the access point is detected, the first averaged strength being obtained by averaging, in a first cycle, the strength calculated in the calculating, and the second carrier sensing may include: calculating a strength of a signal received during the performing of the carrier sensing; and secondly determining, by assessing a pattern, that a signal different from the signal transmitted by the radio tag different from the one radio tag to the access point is detected, the pattern being defined by a total number of times a second averaged strength exceeds a second threshold value and a total number of times the second averaged strength falls below the second threshold value, the second averaged strength being obtained by averaging, in a second cycle shorter than the first cycle, the strength calculated in the calculating. 
     Thus, using a carrier-sense function, the radio tag can alternately perform normal carrier sensing and carrier sensing that is used to interpret the command transmitted by the access point. 
     Furthermore, for example, in the interpreting, the command included in the signal transmitted from the access point may be interpreted according to a combination of the total number of times the second averaged strength exceeds the second threshold value and the total number of times the second average strength falls below the second threshold value in the pattern assessed in the secondly determining. 
     Furthermore, a communication method according to one aspect of the present disclosure is performed by an access point of a communication system including the access point and a plurality of radio tags, and includes: collecting ID information that is information for uniquely identifying each of the plurality of radio tags; broadcasting, to the plurality of radio tags at a predetermined time interval, a signal including a command instructing changing of a first frequency being currently used by the plurality of radio tags into a second frequency; and stopping the broadcasting of the signal including the command when confirming that information indicating successful reception of the command has been received from every one of the plurality of radio tags. 
     Thus, the access point can send the command to the plurality of active radio tags that are being managed and are without means for receiving data, and change the frequency that is used by said radio tags. 
     For example, the communication method may further include: recognizing that the plurality of radio tags are to move to an area where the second frequency different from the first frequency being currently used by the plurality of radio tags needs to be used, and recognizing that the plurality of radio tags have started moving to the area; and confirming that the plurality of radio tags have entered the area. In the broadcasting, when the plurality of radio tags are recognized in the recognizing as having started moving to the area, a signal including a command that instructs changing of the first frequency being currently used by the plurality of radio tags into the second frequency and instructs stopping of the transmission may be broadcast to the plurality of radio tags at the predetermined time interval, and when the plurality of radio tags are confirmed in the confirming as having entered the area, a signal including a command instructing resuming of the transmission may be further broadcast to the plurality of radio tags at the predetermined time interval. 
     Furthermore, for example, the communication method may further include: recognizing that the plurality of radio tags have started moving to an area where the second frequency different from the first frequency being currently used by the plurality of radio tags needs to be used; and confirming that the plurality of radio tags have entered the area. In the broadcasting, when the plurality of radio tags are recognized in the recognizing as having started moving to the area, a signal including a command instructing stopping of the transmission may be further broadcast to the plurality of radio tags at the predetermined time interval, and when the plurality of radio tags are confirmed in the confirming as having entered the area, a signal including a command instructing changing of the first frequency being currently used by the plurality of radio tags into the second frequency may be broadcast to the plurality of radio tags at the predetermined time interval. 
     Furthermore, a radio tag according to one aspect of the present disclosure is one radio tag included in a plurality of radio tags of a communication system including an access point and the plurality of radio tags, and includes: a carrier-sense unit that performs carrier sensing at a predetermined time interval; an output controller that transmits first data including ID information to the access point when a signal transmitted by a radio tag different from the one radio tag to the access point is not detected during the performing of the carrier sensing and the one radio tag is not in a transmission stop state, the ID information being information for uniquely identifying the one radio tag; an interpreter that interprets a command when a signal is detected during the performing of the carrier sensing and the signal is not a signal transmitted by a radio tag different from the one radio tag to the access point, the interpreting being performed under an assumption that the signal detected is a signal transmitted from the access point, the command being included in the signal transmitted from the access point; and a frequency controller that changes a radio frequency of a signal to be transmitted by the one radio tag, when the interpreter interprets that the command includes an instruction to change a frequency. 
     Note that these general or specific aspects may be implemented using a system, a method, an integrated circuit, a computer program, or a computer-readable recording medium such as a compact disc read-only memory (CD-ROM), or any combination of systems, methods, integrated circuits, computer programs, and recording media. 
     Advantageous Effects 
     With the communication method, etc., according to the present disclosure, it is possible to change the frequency of an active radio tag without means for receiving data. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other advantages and features will become apparent from the following description thereof taken in conjunction with the accompanying Drawings, by way of non-limiting examples of embodiments disclosed herein. 
         FIG.  1    is a diagram illustrating one example of the configuration of a communication system according to an embodiment. 
         FIG.  2    is a block diagram illustrating the functional configuration of an AP according to an embodiment. 
         FIG.  3    is a diagram illustrating one example of the hardware configuration of a computer that implements the functions of an AP according to an embodiment through software. 
         FIG.  4    is a diagram conceptually illustrating one example of a signal including a command that is broadcast from a signal transmitter according to an embodiment. 
         FIG.  5    is a block diagram illustrating the functional configuration of a radio tag according to an embodiment. 
         FIG.  6    is a diagram for conceptually describing execution of first carrier sensing according to an embodiment. 
         FIG.  7    is a diagram for conceptually describing execution of second carrier sensing according to an embodiment. 
         FIG.  8 A  is a diagram for describing an execution example of first carrier sensing according to an embodiment. 
         FIG.  8 B  is a diagram for describing an execution example of second carrier sensing according to an embodiment. 
         FIG.  9    is a diagram illustrating one example of a detailed function block of a control circuit included in an LSI unit illustrated in  FIG.  5   . 
         FIG.  10 A  is one example of first data according to an embodiment. 
         FIG.  10 B  is one example of second data according to an embodiment. 
         FIG.  11 A  is a diagram illustrating one example of a sequence of operation of a communication system according to a comparative example. 
         FIG.  11 B  is a diagram illustrating one example of a sequence of operation of a communication system according to an embodiment. 
         FIG.  12 A  is a diagram for describing operation of an AP and a plurality of radio tags according to CASE 1. 
         FIG.  12 B  is a diagram for describing operation of an AP and a plurality of radio tags according to CASE 1. 
         FIG.  12 C  is a diagram for describing operation of an AP and a plurality of radio tags according to CASE 1. 
         FIG.  12 D  is a diagram for describing operation of an AP and a plurality of radio tags according to CASE 1. 
         FIG.  12 E  is a diagram for describing operation of an AP and a plurality of radio tags according to CASE 1. 
         FIG.  12 F  is a diagram for describing operation of an AP and a plurality of radio tags according to CASE 1. 
         FIG.  13 A  is a diagram for describing operation of an AP and a plurality of radio tags according to CASE 3. 
         FIG.  13 B  is a diagram for describing operation of an AP and a plurality of radio tags according to CASE 3. 
         FIG.  14    is a flowchart illustrating the outline of operation of a radio tag according to an embodiment. 
         FIG.  15    is a flowchart illustrating one example of details of operation of a radio tag according to an embodiment. 
         FIG.  16    is a flowchart illustrating the outline of operation of an AP according to an embodiment. 
         FIG.  17    is a flowchart illustrating one example of details of operation of an AP according to an embodiment. 
         FIG.  18    is a flowchart illustrating one example of details of operation of an AP according to an embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the drawings. Note that each embodiment described below shows one specific example of the present disclosure. The numerical values, shapes, materials, structural elements, the arrangement and connection of the structural elements, use procedures, communication procedures, etc., shown in the following embodiment are mere examples, and are not intended to limit the present disclosure. Among the structural elements in the following embodiment, structural elements not recited in any one of the independent claims which indicate the broadest concepts of the present disclosure will be described as arbitrary structural elements. Note that the figures are not necessarily precise illustrations. In the figures, substantially identical elements are assigned the same reference signs, and overlapping description is omitted or simplified. 
     EMBODIMENT 
     Hereinafter, a radio tag and an access point (AP) according to an embodiment will be described with reference to the drawings. 
     [1 Communication System] 
       FIG.  1    is a diagram illustrating one example of the configuration of communication system  1  according to an embodiment. As illustrated in  FIG.  1   , communication system  1  includes a plurality of radio tags  10  and AP  20  capable of changing a frequency that is used by the plurality of radio tags  10  under management. For example, communication system  1  can be used for a system or the like that identifies, manages, or tracks the locations of items to which radio tags  10  are assigned. Although details of radio tags  10  will be described later, radio tags  10  are active radio tags that do not have a data reception function, but have a carrier-sense function only. Although details of AP  20  will be described later, AP  20  may be configured to be able to use a global positioning system (GPS). 
     Hereinafter, each device will be described. 
     [1.2 AP  20 ] 
     The configuration, etc., of AP  20  according to the present embodiment will be described below.  FIG.  2    is a block diagram illustrating the functional configuration of AP  20  according to the embodiment. 
     AP  20  is a wireless base station that is provided as a computer or the like and manages the plurality of radio tags  10 . AP  20  functions as an endpoint of a communication network and can be wirelessly connected to the communication network. Although AP  20  is described below as being able to use the GPS, this is not limiting; it is sufficient that an area where AP  20  is located be able to be identified via the communication network. 
     In the present embodiment, AP  20  includes information collector  201 , recognizer  202 , checker  203 , signal output controller  204 , and memory  205 , as illustrated in  FIG.  2   . 
     With reference to  FIG.  3   , one example of the hardware configuration of AP  20  according to the present embodiment will be described before describing the functional configuration of AP  20  according to the present embodiment. 
     [1.2.1 Hardware Configuration] 
       FIG.  3    is a diagram illustrating one example of the hardware configuration of computer  1000  that implements the functions of AP  20  according to the embodiment through software. 
     Computer  1000  includes input device  1001 , output device  1002 , CPU  1003 , internal storage  1004 , random access memory (RAM)  1005 , reading device  1007 , transmitting/receiving device  1008 , and bus  1009 , as illustrated in  FIG.  3   . Input device  1001 , output device  1002 , CPU  1003 , internal storage  1004 , RAM  1005 , reading device  1007 , and transmitting/receiving device  1008  are connected using bus  1009 . 
     Input device  1001 , which serves as a user interface such as an input button, a touch pad, and a touch panel display, receives user input. Note that input device  1001  may be configured to not only receive user touch input, but also receive voice control and a remote operation using a remote control or the like. 
     Output device  1002 , which is also used as input device  1001 , includes a touch pad, a touch panel display, or the like and notifies a user of information to be delivered. 
     Internal storage  1004  is a flash memory or the like. At least one of a program for implementing the functions of AP  20  and an application in which the functional configuration of AP  20  is used may be stored in internal storage  1004  in advance. 
     RAM  1005 , which is a random-access memory, is used to store data, etc., at the time of execution of the program or the application. 
     Reading device  1007  reads information from a recording medium such as a universal serial bus (USB) flash drive. Reading device  1007  reads the aforementioned program, application, etc., from a recording medium on which said program, application, etc., are recorded, and stores the read program, application, etc., into internal storage  1004 . 
     Transmitting/receiving device  1008  is a communication circuit for performing wired or wireless communication. For example, transmitting/receiving device  1008  may communicate with a server device or cloud storage connected to a network, download the aforementioned program, application, etc., from the server device or the cloud storage, and store the read program, application, etc., into internal storage  1004 . 
     CPU  1003 , which is a central processing unit, copies the program, application, etc., stored in internal storage  1004  onto RAM  1005 , sequentially reads commands included in said program, application, etc., from RAM  1005 , and executes the read commands. Note that the commands may be executed directly from internal storage  1004 . 
     Next, the functional configurations of AP  20  according to the present embodiment will be described. 
     [1.2.2 Information Collector  201 ] 
     Information collector  201  collects ID information that is information for uniquely identifying each of the plurality of radio tags  10 . In the present embodiment, information collector  201  collects, via the communication network, the ID information of all the radio tags that are under the management of AP  20 , and stores the ID information into memory  205 . The communication network herein is, for example, cellular wireless communication network. 
     [1.2.3 Recognizer  202 ] 
     Recognizer  202  recognizes that the plurality of radio tags  10  have started moving to an area where a second frequency different from a first frequency being currently used by the plurality of radio tags  10  needs to be used. In the present embodiment, from the location information of the plurality of radio tags  10  that are under the management of AP  20 , recognizer  202  may recognize that radio tags  10  have started moving, for example. Furthermore, recognizer  202  may obtain, via the communication network, departure-related information such as departure time obtained from the ID information of the plurality of radio tags  10  and thus recognize that the plurality of radio tags  10  have started moving. 
     Recognizer  202  may recognize that the plurality of radio tags  10  are to move to an area where the second frequency different from the first frequency being currently used by the plurality of radio tags  10  needs to be used. In the present embodiment, recognizer  202  may obtain area information of a destination for radio tags  10  that are under the management of AP  20  from the ID information of radio tags  10  via the communication network, for example. This allows recognizer  202  to recognize that the plurality of radio tags  10  are to move to an area where a frequency different from the frequency being currently used by the plurality of radio tags  10  is used. 
     [1.2.4 Checker  203 ] 
     Checker  203  confirms (recognizes) that the plurality of radio tags  10  have been placed under the management of AP  20 . Furthermore, checker  203  checks (identifies), using the GPS or the like, an area where AP  20  is located. The present embodiment assumes in the following description that AP  20  accompanies the movement of the plurality of radio tags  10 . Therefore, by checking the location of AP  20  using the GPS and confirming that the location of AP  20  is in said area, checker  203  can confirm that the plurality of radio tags  10  have entered said area. When AP  20  does not use the GPS and an area where AP  20  is located is identified via the communication network, the operation of checker  203  is substantially the same and thus, description thereof will be omitted. 
     Note that AP  20  is not limited to accompanying the movement of the plurality of radio tags  10 ; AP  20  does not need to accompany the movement of the plurality of radio tags  10 . In this case, it is sufficient that the plurality of radio tags  10  be managed by AP  20  located in each of an area where radio tags  10  are located before the movement and an area where radio tags  10  are located after the movement. 
     [1.2.5 Signal Output Controller  204 ] 
     Signal output controller  204  controls output of signals (radio waves at a specific frequency). Signal output controller  204  includes signal transmitter  2041  and signal stopper  2042 , as illustrated in  FIG.  2   , for example. 
     [1.2.5.1 Signal Transmitter  2041 ] 
     Signal transmitter  2041  broadcasts, to the plurality of radio tags  10  at a predetermined time interval, a signal including a command that instructs the plurality of radio tags  10  to change the first frequency being currently used by the plurality of radio tags  10  into the second frequency. 
     For example, assume that recognizer  202  recognizes that the plurality of radio tags  10  have started moving to said area. In this case, signal transmitter  2041  broadcasts, to the plurality of radio tags  10  at the predetermined time interval, a signal including a command that instructs the plurality of radio tags  10  to change the first frequency being currently used by the plurality of radio tags  10  into the second frequency and instructs the plurality of radio tags  10  to stop transmission. 
     Subsequently, when checker  203  confirms that the plurality of radio tags  10  have entered said area, signal transmitter  2041  broadcasts, to the plurality of radio tags  10  at the predetermined time interval, a signal including a command that instructs the plurality of radio tags  10  to resume the transmission. 
     In this manner, at a point in time when the plurality of radio tags  10  start moving (departure) to said area, signal transmitter  2041  may cause the plurality of radio tags  10  to change the frequency (radio frequency) to be used and stop transmission. 
     Note that when recognizer  202  recognizes that the plurality of radio tags  10  have started moving to said area, signal transmitter  2041  does not need to broadcast, to the plurality of radio tags  10 , the signal including the command that instructs the plurality of radio tags  10  to change the first frequency being currently used by the plurality of radio tags  10  into the second frequency. In other words, signal transmitter  2041  may broadcast, to the plurality of radio tags  10  at the predetermined time interval, the signal including the command that instructs the plurality of radio tags  10  to stop the transmission. Subsequently, when checker  203  confirms that the plurality of radio tags  10  have entered said area, signal transmitter  2041  broadcasts, to the plurality of radio tags  10  at the predetermined time interval, the signal including the command that instructs the plurality of radio tags  10  to change the first frequency being currently used by the plurality of radio tags  10  into the second frequency. 
     In this manner, signal transmitter  2041  may cause the plurality of radio tags  10  to merely stop signal transmission at a point in time when the plurality of radio tags  10  start moving (departure) to said area and then, after the plurality of radio tags  10  enter said region, cause the plurality of radio tags  10  to change the frequency (radio frequency) to be used. 
       FIG.  4    is a diagram conceptually illustrating one example of the signal including the command that is broadcast from signal transmitter  2041  according to the embodiment. The signal including the command is broadcast via an antenna of AP  20 . 
     The command that instructs the radio tags to change the frequency, the command that instructs the radio tags to stop the transmission, and the command that instructs the radio tags to resume the transmission can be included in the signal and broadcast using on-off keying (OOK) modulation in which the signal strength changes periodically, as illustrated in  FIG.  4   , for example. 
     Note that the method for including these commands into the signal is not limited to examples in the case illustrated in  FIG.  4   . In order to produce pattern signals corresponding to specific commands, on-off modulation in which the signal strength changes a predetermined number of times at fixed intervals so as to be less than or equal to a threshold value may be performed. 
     [1.2.5.2 Signal Stopper  2042 ] 
     When signal stopper  2042  confirms that information indicating successful reception of the command has been received from every one of the plurality of radio tags  10 , signal stopper  2042  stops broadcasting the signal including the command. 
     [1.2.6 Memory  205 ] 
     Memory  205 , which is one example of the recording medium, includes, for example, a rewritable non-volatile memory such as a hard disk drive or a solid-state drive. The recording medium representing memory  205  may exist in cloud storage. 
     [1.3 Radio Tag  10 ] 
     Next, the configuration, etc., of radio tag  10  according to the present embodiment will be described.  FIG.  5    is a block diagram illustrating the functional configuration of radio tag  10  according to the embodiment. 
     Radio tag  10  is an active radio tag, which is driven using an internal battery and emits radio waves by itself, and does not have a data reception function, but has a carrier-sense function only. Radio tag  10  is provided (attached) or assigned to an item such as a product. In the present embodiment, radio tag  10  is capable of obtaining a signal (interpreting a command) that instructs radio tag  10  to change the frequency (radio frequency), for example, by applying the carrier-sense function. Radio tag  10  includes battery  11 , sensor  12  that senses the state of the item, and a large scale integration (LSI) unit  13 , as illustrated in  FIG.  5   . 
     Battery  11  is included in radio tag  10  and supplies electric power to LSI unit  13 . 
     Sensor  12  senses the state of the item to which radio tag  10  is attached or provided, and transfers the sensing result to LSI unit  13 . Note that sensor  12  is not essential. 
     LSI unit  13  will be described later. 
     [1.3.1 LSI Unit  13 ] 
     LSI unit  13  includes, for example, an IC chip that is driven by battery  11 . LSI unit  13  is connected to an antenna and when radio tags  10  (hereinafter referred to as other radio tags  10 ) different from current radio tag  10  are not emitting radio waves (signals), emits radio waves (signals) and thus can transmit the radio waves (signals) to AP  20 . LSI unit  13  is provided including a CPU, memory  134 , and so on. 
     In the present embodiment, LSI unit  13  includes radio frequency (RF) unit  130 , radio field strength meter  131 , carrier-sense unit  132 , control circuit  133 , memory  134 , and data modulator  135 . 
     [1.3.1.1 RF Unit  130 ] 
     RF unit  130 , which includes TX  1301  and RX  1302 , transmits and receives signals (radio waves) via the antenna. TX  1301 , which is a transmitting circuit, converts the signals modulated by data modulator  135  into radio waves in a frequency range used by radio tag  10  at the antenna, and thus transmits the signals via the radio waves. RX  1302 , which is a receiving circuit, converts radio waves in the frequency range used by radio tag  10  that have been received by the antenna into signals (received signals), and transfers the received signals to radio field strength meter  131 . 
     [1.3.1.2 Radio Field Strength Meter  131 ] 
     Radio field strength meter  131  calculates the strength of signals received during the performing of carrier sensing. In the present embodiment, radio field strength meter  131  calculates the strength of the received signals in the frequency range used by radio tag  10  that have been transferred by RF unit  130  during the performing of carrier sensing, and thus measures the strength of the radio waves received by RF unit  130 . Note that radio field strength meter  131  calculates received signal strength indication (RSSI) as the strength of the received signals. 
     [1.3.1.3 Carrier-sense Unit  132 ] 
     Carrier-sense unit  132  performs carrier sensing at a predetermined time interval. More specifically, carrier-sense unit  132  monitors the radio field strength measured by radio field strength meter  131  during the performing of the carrier sensing, and thereby checks whether the frequency range that the current tag (radio tag  10 ) uses is occupied by the radio waves from other radio tags. Carrier-sense unit  132  transfers the check result to control circuit  133 . 
     In the present embodiment, carrier-sense unit  132  monitors the strength of the received signals calculated by radio field strength meter  131  during the performing of the carrier sensing, and senses whether other radio tags  10  are transmitting signals and whether AP  20  is transmitting signals. Note that normal carrier sensing in which whether other radio tags  10  are transmitting radio waves is sensed is referred to as first carrier sensing, and carrier sensing for pattern recognition in which whether AP  20  is transmitting signals is sensed is referred to as second carrier sensing. Carrier-sense unit  132  may perform the first carrier sensing and the second carrier sensing simultaneously during the performing of the carrier sensing that is performed at the predetermined time interval or may perform the first carrier sensing and the second carrier sensing alternately during the performing of the carrier sensing that is performed at the predetermined time interval. Note that the following describes, as an example, the case where the first carrier sensing and the second carrier sensing are performed alternately. 
       FIG.  6    is a diagram for conceptually describing execution of the first carrier sensing according to the embodiment. When other radio tags  10  are transmitting radio waves as illustrated in (a) in  FIG.  6   , radio field strength meter  131  measures a radio field strength such as that illustrated in (b) in  FIG.  6   . During a period in which the measured radio field strength exceeds a first threshold value as illustrated in (b) in  FIG.  6   , carrier-sense unit  132  detects a busy state in which other radio tags  10  are transmitting signals. 
     Note that during a period in which the radio field strength such as that illustrated in (b) in  FIG.  6    is not measured, that is, a period in which the radio field strength measured by radio field strength meter  131  is less than or equal to the threshold value, carrier-sense unit  132  detects an idle state in which other radio tags  10  are not transmitting signals. In the idle state, radio waves in the frequency range used by radio tag  10  can be transmitted using control circuit  133 , which will be described later. This means that by performing the first carrier sensing, radio tag  10  can avoid the collision of radio waves with other radio tags  10  and transmit radio waves. 
       FIG.  7    is a diagram for conceptually describing execution of the second carrier sensing according to the embodiment. When AP  20  is broadcasting a signal including a command as illustrated in (a) in  FIG.  7   , radio field strength meter  131  measures a radio field strength (not illustrated in the drawings). When there is a period in which the radio field strength measured by radio field strength meter  131  exceeds a second threshold value, carrier-sense unit  132  detects a repetitive pattern in the radio field strength (a received signal strength pattern) such as that illustrated in (b) in  FIG.  7   . Note that the repetitive pattern can be defined by the number of periods that are in a given cycle and in which the radio field strength measured by radio field strength meter  131  is less than or equal to the second threshold value as illustrated in (b) in  FIG.  7   . The second threshold value may be the same as or different from the first threshold value. 
     Next, a specific example of the execution of the first carrier sensing will be described with reference to  FIG.  8 A . 
       FIG.  8 A  is a diagram for describing an execution example of the first carrier sensing according to the embodiment. 
     Assume that during the performing of the carrier sensing, radio field strength meter  131  receives signals such as those illustrated in (a) in  FIG.  8 A  from RX  1302  and calculates a strength of the received signals (signal strength) such as that illustrated in (b) in  FIG.  8 A , for example. In this case, carrier-sense unit  132  averages the calculated strength of the received signals in a first cycle as illustrated in (c) in  FIG.  8 A , and determines, as a busy state, a period in which a first averaged strength obtained through the averaging exceeds the first threshold value, and determines other periods as an idle state. Subsequently, carrier-sense unit  132  transfers the determination result to control circuit  133  as a first carrier-sense result. 
     Next, a specific example of the execution of the second carrier sensing will be described with reference to  FIG.  8 B . 
       FIG.  8 B  is a diagram for describing an execution example of the second carrier sensing according to the embodiment. 
     Assume that during the performing of the carrier sensing, radio field strength meter  131  receives signals such as those illustrated in (a) in  FIG.  8 B  from RX  1302  and calculates a strength of the received signals (signal strength) such as that illustrated in (b) in  FIG.  8 B , for example. In this case, carrier-sense unit  132  averages the calculated strength of the received signals in a second cycle shorter than the first cycle as illustrated in (c) in  FIG.  8 B , and assesses a pattern defined by the number of times the second averaged strength obtained through the averaging exceeds the second threshold value and the number of times the second averaged strength obtained through the averaging falls below the second threshold value (the received signal strength pattern). Subsequently, carrier-sense unit  132  transfers the determination result to control circuit  133  as a second carrier-sense result. 
     [1.3.1.4 Control Circuit  133 ] 
     When carrier-sense unit  132  transfers the first carrier-sense result to control circuit  133 , control circuit  133  transfers the ID information of the current tag (radio tag  10 ) stored in memory  134  to data modulator  135 . Furthermore, when carrier-sense unit  132  transfers the second carrier-sense result to control circuit  133 , control circuit  133  stores the pattern (the received signal strength pattern) included in the second carrier-sense result into a partial area (pattern buffer  134   a ) of memory  134 . When control circuit  133  successfully interprets the command using the second carrier-sense result transferred thereto, control circuit  133  adds, to the ID information of the current tag (radio tag  10 ) stored in memory  134 , information such as acknowledgement (ACK) indicating that the command has been received, and transfers the information to data modulator  135 . Control circuit  133  performs an operation corresponding to the interpreted command. 
       FIG.  9    is a diagram illustrating one example of a detailed function block of control circuit  133  included in LSI unit  133  illustrated in  FIG.  5   . 
     Control circuit  133  includes pattern match determiner  1331 , interpreter  1332 , RF frequency controller  1333 , and RF output controller  1334 , as illustrated in  FIG.  9   . Note that control circuit  133  executes the functions of pattern match determiner  1331 , interpreter  1332 , RF frequency controller  1333 , and RF output controller  1334  using control programs stored in memory  134 . Pattern match determiner  1331 , interpreter  1332 , RF frequency controller  1333 , and RF output controller  1334  may be implemented through hardware. 
     [1.3.1.4.1 Pattern Match Determiner  1331 ] 
     Pattern match determiner  1331  performs matching determination on the repetitive pattern (the received signal strength pattern) included in the second carrier sensing result transferred from carrier-sense unit  132  and stored in pattern buffer  134   a  which is a partial area of memory  134 . More specifically, pattern match determiner  1331  determines whether the repetitive pattern (the received signal strength pattern) included in the second carrier-sense result matches any of a plurality of patterns that have been set in advance. 
     [1.3.1.4.2 Interpreter  1332 ] 
     When a signal is detected during the performing of the carrier sensing and said signal is not the signal transmitted from other radio tags to AP  20 , interpreter  1332  determines said detected signal as a signal transmitted from AP  20  and interprets a command included in the signal transmitted from AP  20 . More specifically, from a combination of the number of times the second averaged strength in the repetitive pattern included in the second carrier-sense result exceeds the second threshold value and the number of times the second averaged strength in the repetitive pattern included in the second carrier-sense result falls below the second threshold value, interpreter  1332  interprets the command included in the signal transmitted from AP  20 . For example, interpreter  1332  interprets the command included in the signal as a command that instructs the radio tag to stop the transmission and change the frequency or a command that instructs the radio tag to resume the transmission. Furthermore, interpreter  1332  interprets the command included in the signal as a command that instructs the radio tag to stop the transmission or a command that instructs the radio tag to change the frequency. 
     In the present embodiment, when the repetitive pattern (the received signal strength pattern) included in the second carrier-sense result matches any of the plurality of patterns that have been set in advance, interpreter  1332  interprets that the command corresponding to the matching pattern is the command included in the signal transmitted from AP  20 . 
     [1.3.1.4.3 RF Frequency Controller  1333 ] 
     When interpreter  1332  interprets that the signal transmitted from AP  20  includes a command that instructs the radio tag to change the frequency, RF frequency controller  1333  changes the radio frequency of signals to be transmitted by the current radio tag (radio tag  10 ). 
     Note that when interpreter  1332  interprets that the signal transmitted from AP  20  includes a command that instructs the radio tag to stop the transmission and change the frequency, RF frequency controller  1333  changes the radio frequency of signals to be transmitted by the current radio tag after second data obtained by adding ACK to the ID information is transmitted. 
     Note that the ACK is one example of information indicating that the command has been received, as mentioned above. 
     [1.3.1.4.4 RF Output Controller  1334 ] 
     RF output controller  1334  controls output of signals to be transmitted to AP  20  and controls stoppage of signal transmission to AP  20  or resumption (start) of the signal transmission to AP  20 , for example. 
     For example, when signals transmitted from other radio tags  10  to AP  20  are not detected during the performing of the carrier sensing and the radio tag is not instructed to stop the transmission, RF output controller  1334  transmits first data including the ID information of the current radio tag (radio tag  10 ) to AP  20 . 
       FIG.  10 A  is one example of the first data according to the embodiment. As illustrated in  FIG.  10 A , the first data according to the present embodiment is the ID information of radio tag  10  included in a data field. 
     Furthermore, for example, when interpreter  1332  interprets that the signal transmitted from AP  20  includes a command that instructs the radio tag to stop the transmission and change the frequency, and signals transmitted from other radio tags  10  to AP  20  are not detected during the performing of the carrier sensing, RF output controller  1334  may transmit, to AP  20 , the second data obtained by adding the ACK to the first data. In this case, RF output controller  1334  suspends the signal transmission to AP  20  after the second data is transmitted until interpreter  1332  interprets that the signal transmitted from AP  20  and detected during the performing of the carrier sensing includes a command that instructs the radio tag to resume the transmission. Subsequently, when RF frequency controller  1333  changes the radio frequency of signals to be transmitted by the current radio tag (radio tag  10 ), RF output controller  1334  may resume the signal transmission to AP  20 . 
       FIG.  10 B  is one example of the second data according to the embodiment. As illustrated in  FIG.  10 B , the second data according to the present embodiment includes the ACK in addition to the ID information (the first data) of radio tag  10  included in the data field. The ACK may be added to an empty region of the data field. 
     Note that, for example, when interpreter  1332  interprets that the signal transmitted from AP  20  includes a command that instructs the radio tag to stop the transmission, and signals transmitted from other radio tags  10  to AP  20  are not detected during the performing of the carrier sensing, RF output controller  1334  may transmit, to AP  20 , the second data obtained by adding the ACK to the first data. In this case, after the second data is transmitted, RF output controller  1334  stops the signal transmission to AP  20 . 
     [1.3.1.5 Memory  134 ] 
     Memory  134  is one example of the storage medium such as flash memory. Memory  134  is not limited to being disposed inside control circuit  133  as illustrated in  FIG.  5   , but may be disposed outside control circuit  133 , but inside LSI unit  13 , or may be disposed outside LSI unit  13 . 
     In the present embodiment, memory  134  stores control programs for allowing LSI unit  13  to cause radio field strength meter  131 , carrier-sense unit  132 , and data modulator  135  to function. Furthermore, memory  134  stores control programs for allowing control circuit  133  to cause pattern match determiner  1331 , interpreter  1332 , RF frequency controller  1333 , and RF output controller  1334  to function. Moreover, a partial area of memory  134  is also used as a buffer (pattern buffer  134   a ) by control circuit  133 . 
     [1.3.1.6 Data Modulator  135 ] 
     Data modulator  135  performs modulation for placing data on radio waves. More specifically, data modulator  135  performs modulation for placing, on radio waves, the ID information of current radio tag  10  transmitted from control circuit  133 . 
     [2 Operation, etc., of Communication System  1 ] 
     Next, the operation of communication system  1  including the plurality of radio tags  10  and AP  20  configured as described above will be described. 
     [2.1 Sequence of Operation of Communication System  1 ] 
     Before describing the sequence of operation of communication system  1  according to the present embodiment, the sequence of operation of a communication system according to a comparative example that includes radio tags  90 A and  90 B capable of performing normal carrier sensing (the first carrier sensing) only will be described first. 
       FIG.  11 A  is a diagram illustrating one example of the sequence of operation of the communication system according to the comparative example. 
     As illustrated in  FIG.  11 A , in the communication system according to the comparative example, each of radio tag  90 A and radio tag  90 B performs carrier sensing at a predetermined time interval. More specifically, radio tag  90 A performs normal carrier sensing (corresponding to the first carrier sensing) at the predetermined time interval, and when radio tag  90 A detects an idle state where another radio tag  90 B is not transmitting radio waves, radio tag  90 A transmits the ID information (corresponding to the first data) of radio tag  90 A to AP  91 . Similarly, radio tag  90 B performs normal carrier sensing (corresponding to the first carrier sensing) at the predetermined time interval, and when radio tag  90 B detects an idle state where another radio tag  90 A is not transmitting radio waves, radio tag  90 B transmits the ID information (corresponding to the first data) of radio tag  90 B to AP  91 . 
       FIG.  11 B  is a diagram illustrating one example of the sequence of operation of communication system  1  according to the embodiment. 
     As illustrated in  FIG.  11 B , in communication system  1  according to the present embodiment, each of radio tag  10 A and radio tag  10 B performs normal carrier sensing and carrier sensing for pattern recognition at a predetermined time interval. More specifically, radio tags  10 A and  10 B alternately perform the first carrier sensing (normal carrier sensing) and the second carrier sensing (carrier sensing for pattern recognition) at the predetermined time interval. Here, assume that AP  20  is broadcasting, for a given period at a predetermined time interval, a signal including a command that instructs the radio tags to stop the transmission and change the frequency. 
     When radio tag  10 A performs the first carrier sensing and detects an idle state where another radio tag  10 B is not transmitting radio waves, radio tag  10 A transmits the ID information (the first data) of radio tag  10 A to AP  20 . Similarly, when radio tag  10 B performs the first carrier sensing and detects an idle state where another radio tag  10 A is not transmitting radio waves, radio tag  10 B transmits the ID information (the first data) of radio tag  10 B to AP  20 . 
     Furthermore, when radio tag  10 A performs the second carrier sensing and detects a repetitive pattern (a received signal strength pattern) from the received signals, radio tag  10 A interprets a command included in said signals from the repetitive pattern (the received signal strength pattern) that has been detected. When radio tag  10 A successfully interprets the command included in said signals, radio tag  10 A transmits, to AP  20 , the second data obtained by adding the ACK to the ID information of radio tag  10 A, at the next transmission timing, that is, when radio tag  10 A performs the first carrier sensing and detects the idle state. 
     Similarly, when radio tag  10 B performs the second carrier sensing and detects a repetitive pattern (a received signal strength pattern) from the received signals, radio tag  10 B interprets a command included in said signals from the repetitive pattern that has been detected. When radio tag  10 B successfully interprets the command included in said signals, radio tag  10 B transmits, to AP  20 , the second data obtained by adding the ACK to the ID information of radio tag  10 B, at the next transmission timing, that is, when radio tag  10 B performs the first carrier sensing and detects the idle state. 
     [2.2 Use Situation of Communication System  1 ] 
     Next, a use situation where communication system  1  is used in logistics will be described. A use situation where AP  20  accompanies the movement of the plurality of radio tags  10  that are under management, in other words, accompanies the movement of radio tags  10  attached to items, will be referred to as CASE 1 and described with reference to  FIG.  12 A  to  FIG.  12 F . 
     &lt;CASE 1&gt; 
       FIG.  12 A  to  FIG.  12 B  are diagrams for describing the operation of AP  20  and the plurality of radio tags  10  according to CASE 1. 
       FIG.  12 A  is a diagram illustrating an operation example of AP  20  and the plurality of radio tags  10  in a situation where communication system  1  according to the present embodiment is used in logistics within area A. 
     As illustrated in  FIG.  12 A , each of the plurality of radio tags  10  performs the first carrier sensing and the second carrier sensing alternately and transmits the first data including the ID information in a given cycle (at a predetermined time interval). Here, each of the plurality of radio tags  10  is used in logistics within area A and therefore transmits signals at the radio frequency (the first frequency) used in area A. 
     Meanwhile, AP  20  collects and stores the ID information of the plurality of radio tags  10  that are under management. Furthermore, AP  20  is connected to a cellular wireless communication network in area A. Note that although AP  20  stores the ID information, the stored ID information may be uploaded to a specific server or cloud storage if said ID information is lost when AP  20  is powered off. 
       FIG.  12 B  and  FIG.  12 C  are diagrams illustrating an operation example of AP  20  and the plurality of radio tags  10  in a situation where communication system  1  according to the embodiment departs from area A to another area B. 
     In  FIG.  12 B , AP  20  obtains the location information of the plurality of radio tags  10  under management or the departure-related information from the ID information of the plurality of radio tags  10  via the wireless communication network, and thus recognizes that the plurality of radio tags  10  have started moving. Here, AP  20  accesses a specific server or cloud storage via the wireless communication network and obtains information of an area (area B) that is a destination of radio tags  10  from the ID information of radio tags  10  under management, for example. Subsequently, as illustrated in  FIG.  12 B , AP  20  broadcasts, to the plurality of radio tags  10 , a signal including a command that instructs radio tags  10  to change the first frequency into the second frequency and stop the transmission. 
     Meanwhile, each of the plurality of radio tags  10  performs the first carrier sensing and the second carrier sensing alternately and transmits the first data including the ID information in a given cycle (at a predetermined time interval). The plurality of radio tags  10  perform the second carrier sensing, interpret a command included in said signal, and operate according to the interpreted command. As illustrated in  FIG.  12 B , the plurality of radio tags  10  transmit ACKs at the next transmission timing and stop the subsequent signal transmission according to the interpreted command. Furthermore, after transmitting the ACKs, the plurality of radio tags  10  change the radio frequency to be used from the first frequency into the second frequency according to the interpreted command. 
     In the example illustrated in  FIG.  12 C , all the plurality of radio tags  10  have changed the radio frequency from the first frequency into the second frequency and stopped the signal transmission, and AP  20  has stopped the broadcasting as a result of obtaining the ACK from every one of the plurality of radio tags  10 . Note that even if AP  20  fails to obtain the ACK from every one of the plurality of radio tags  10 , AP  20  may stop the broadcasting when obtaining no signal from other radio tags  10  even after a predetermined period of time has elapsed since the last obtainment of the ACK from one of the plurality of radio tags  10 . 
       FIG.  12 D  is a diagram illustrating an operation example of AP  20  and the plurality of radio tags  10  in a situation where communication system  1  according to the embodiment enters area B. 
     As illustrated in  FIG.  12 D , each of the plurality of radio tags  10  performs the first carrier sensing and the second carrier sensing alternately at the radio frequency (the second frequency) that is used in area B. Meanwhile, when AP  20  confirms using the GPS or the like that AP  20  itself has entered area B, AP  20  broadcasts, at the second frequency to the plurality of radio tags  10 , a signal including a command that instructs the radio tags to start transmission. Note that when AP  20  is powered off after the departure from area A and the ID information stored at the time of the departure is lost, AP  20  may access the specific server or cloud storage and obtain said ID information. 
       FIG.  12 E  and  FIG.  12 F  are diagrams illustrating an operation example of AP  20  and the plurality of radio tags  10  in communication system  1  according to the embodiment until all of the plurality of radio tags  10  that are under the management of AP  20  start transmission in area B. 
     In the example illustrated in  FIG.  12 E , each of the plurality of radio tags  10  starts (resumes) transmission of signals of the first data according to the command interpreted by performing the second carrier sensing. Meanwhile, AP  20  continues the broadcasting until AP  20  obtains the first data from all of the plurality of radio tags  10 . In contrast, in the example illustrated in  FIG.  12 F , AP  20  stops the broadcasting as a result of obtaining the first data from all of the plurality of radio tags  10 . 
     &lt;CASE 2&gt; 
     In CASE 1, AP  20  is described as recognizing, at the time of departure, that the destination of the plurality of radio tags  10  under management is area B, but this is not limiting. AP  20  does not need to be able to recognize that the destination of the plurality of radio tags  10  under management is area B; a case where AP  20  cannot recognize that said destination is area B will be described as CASE 2. In other words, CASE 2 is a use situation where AP  20  accompanies the movement of the plurality of radio tags  10  that are under management, but cannot recognize that the destination of the plurality of radio tags  10  is area B.  FIG.  12 A  to  FIG.  12 F  will be referred to again in the following description. Operation that is substantially the same as that in CASE 1 will not be described, and only features different from CASE 1 will be described. 
     In the situation illustrated in  FIG.  12 B  in CASE 2, AP  20  obtains the location information of the plurality of radio tags  10  under management or the departure-related information from the ID information of the plurality of radio tags  10  via the wireless communication network, and thus recognizes that the plurality of radio tags  10  have started moving. Note that AP  20  in CASE 2 cannot obtain information of an area (area B) that is a destination of radio tag  10 . Therefore, AP  20  broadcasts, to the plurality of radio tags  10 , a signal including a command that instructs the plurality of radio tags  10  to stop the transmission. Thus, AP  20  in CASE 2 does not broadcast a signal including a command that instructs the radio tags to change the first frequency into the second frequency. 
     Meanwhile, each of the plurality of radio tags  10  performs the first carrier sensing and the second carrier sensing alternately and transmits the first data including the ID information in a given cycle. As illustrated in  FIG.  12 B  in CASE 2, the plurality of radio tags  10  transmit the ACK at the next transmission timing and stop the subsequent signal transmission according to the interpreted command. 
     In the situation illustrated in  FIG.  12 D  in CASE 2, each of the plurality of radio tags  10  performs the first carrier sensing and the second carrier sensing alternately, not only at the first frequency, but also at a radio frequency that is used in another area B or the like. Meanwhile, when AP  20  confirms using the GPS or the like that AP  20  itself has entered area B, AP  20  broadcasts, at the second frequency to the plurality of radio tags  10 , a signal including a command that instructs the radio tags to change the first frequency into the second frequency. Thus, AP  20  in CASE 2 does not broadcast a signal including a command that instructs the radio tags to start transmission. 
     In the situation illustrated in  FIG.  12 E  in CASE 2, each of the plurality of radio tags  10  changes the radio frequency from the first frequency into the second frequency and starts (resumes) transmission of signals of the first data at the second frequency according to the command interpreted by performing the second carrier sensing. Meanwhile, AP  20  continues the broadcasting until AP  20  obtains the first data from all of the plurality of radio tags  10 . The subsequent operation is the same as that in CASE 1 and thus, description thereof will be omitted. 
     &lt;CASE 3&gt; 
     In CASE 1 and CASE 2, AP  20  is described as accompanying the movement of the plurality of radio tags  10  under management, but this is not limiting. AP  20  that manages the plurality of radio tags  10  may vary from area to area. Hereinafter, a use situation for AP  20  in the area of departure in the case where AP  20  does not accompany the movement of the plurality of radio tags  10  under management and cannot recognize that the destination of the plurality of radio tags  10  is area B will be described as CASE 3. 
       FIG.  13 A  and  FIG.  13 B  are diagrams for describing the operation of AP  20  and the plurality of radio tags  10  according to CASE 3.  FIG.  13 A  illustrates an operation example of AP  20  and the plurality of radio tags  10  in a situation where communication system  1  according to the embodiment is used in logistics within area A. Note that the operation example illustrated in  FIG.  13 A  is as described with reference to  FIG.  12 A  and thus, description thereof will be omitted. 
       FIG.  13 B  illustrates an operation example of AP  20  and the plurality of radio tags  10  in a situation where communication system  1  according to the embodiment prepares to depart from area A. 
     In  FIG.  13 B , AP  20  obtains an instruction indicating that the plurality of radio tags  10  are departing from area A, and thus broadcasts a signal including a command that instructs the radio tags to stop transmission. Thus, AP  20  in CASE 3 does not broadcast a signal including a command that instructs the radio tags to change the first frequency into the second frequency. Meanwhile, each of the plurality of radio tags  10  performs the first carrier sensing and the second carrier sensing alternately and transmits the first data including the ID information in a given cycle. In  FIG.  13 B , the plurality of radio tags  10  transmit the ACK at the next transmission timing and stop the subsequent signal transmission according to the interpreted command. The subsequent operation is substantially the same as that in CASE 2 except for the situation illustrated in  FIG.  12 D  and thus, description thereof will be omitted. 
     Note that in the situation illustrated in  FIG.  12 D  in CASE 3, each of the plurality of radio tags  10  performs only the first carrier sensing and the second carrier sensing alternately, not only at the first frequency, but also at a radio frequency that is used in another area. Meanwhile, when AP  20  in area B obtains an instruction indicating that the plurality of radio tags  10  have entered area B, AP  20  in area B accesses a specific server or cloud storage and obtains the ID information. Subsequently, AP  20  in area B broadcasts, at the second frequency to the plurality of radio tags  10 , a signal including a command that instructs the radio tags to change the first frequency into the second frequency. Thus, AP  20  in area B in CASE 3 does not broadcast a signal including a command that instructs the radio tags to start transmission. 
     [2.3 Operation of Radio Tag  10 ] 
     Next, the operation of radio tag  10  according to the present embodiment will be described. 
       FIG.  14    is a flowchart illustrating the outline of operation of radio tag  10  according to the embodiment. 
     First, radio tag  10  performs carrier sensing at a predetermined time interval (S 10 ). 
     Next, when a signal transmitted from other radio tags  10  to AP  20  is not detected during the performing of the carrier sensing and radio tag  10  is not in a transmission stop state, radio tag  10  transmits the first data including the ID information of the current radio tag (radio tag  10 ) to AP  20  (S 11 ). 
     Next, a signal is detected during the performing of the carrier sensing and said signal is not the signal transmitted from other radio tags to AP  20 , radio tag  10  determines said detected signal as a signal transmitted from AP  20  and interprets a command included in the signal transmitted from AP  20  (S 12 ). 
     Next, when said signal is interpreted as including a command that instructs the radio tag to change the frequency, radio tag  10  changes the radio frequency of signals to be transmitted by the current radio tag (radio tag  10 ) (S 13 ). 
       FIG.  15    is a flowchart illustrating one example of details of the operation of radio tag  10  according to the embodiment. 
     First, radio tag  10  performs carrier sensing periodically (at a predetermined time interval) (S 101 ). For example, radio tag  10  periodically performs the first carrier sensing and the second carrier sensing in an alternate manner, but may perform the first carrier sensing and the second carrier sensing at the same time. 
     Next, radio tag  10  checks whether no signal from other radio tags  10  has been detected during the performing of the carrier sensing and an instruction to stop transmission has not been received (S 102 ). 
     When a signal from other radio tags  10  has been detected or a stop signal has been received in Step S 102  (Yes in Step S 102 ), radio tag  10  checks whether it is not possible to properly receive the detected signal (S 103 ). 
     When it is possible to properly receive the detected signal in Step S 103  (Yes in Step S 103 ), radio tag  10  checks whether the signal from another radio tag  10  has been detected (S 104 ). Note that a situation where radio tag  10  cannot properly receive the detected signal (No in Step S 103 ) is a situation where the signals from other radio tags  10  overlap each other or a situation where the signal from other radio tags  10  and the signal from AP  20  overlap each other. 
     When the signal from other radio tags  10  has not been detected in Step S 104  (No in Step S 104 ), radio tag  10  determines that the signal from AP  20  has been detected (Yes in S 105 ) and interprets a command included in the signal from AP  20  (S 106 ). When radio tag  10  interprets the command included in the signal from AP  20 , it is sufficient that radio tag  10  change, for example, the state of command reception (ACK) held in the internal memory (memory  134 ) into a state indicating that the command from the AP has been received. Examples of the command included in the signal from AP  20  include a command that instructs the current radio tag (radio tag  10 ) to stop signal transmission (disable the transmission) and a command that instructs the current radio tag (radio tag  10 ) to change the radio frequency being currently used by the radio tag into a specific radio frequency. 
     Note that the process in Step S 105  may be skipped. When the result in Step S 103  is No, when the result in Step S 104  is Yes, and when the result in Step S 105  is No, the processing returns to Step S 101 . 
     When no signal from other radio tags  10  has been detected and the stop signal has not been received in Step S 102  (No in Step S 102 ), radio tag  10  checks whether the command has been interpreted in the last execution of the carrier sensing (the second carrier sensing) (S 107 ). 
     When it is determined in S 107  that the command has not been interpreted in the last execution of the second carrier sensing (No in Step S 107 ), radio tag  10  transmits the ID information to AP  20  (S 108 ). 
     When it is determined in Step S 107  that the command has been interpreted in the last execution of the second carrier sensing (Yes in Step S 107 ), radio tag  10  adds the ACK to the ID information and transmits the ID information with the ACK to AP  20  (S 109 ). 
     Next, radio tag  10  performs an operation corresponding to the command interpreted in the last execution of the second carrier sensing (S 110 ). After performing the operation, radio tag  10  returns to Step S 101 . 
     In Step S 110 , for example, when the command included in the signal from AP  20  is a command that instructs the current radio tag (radio tag  10 ) to stop signal transmission (disable transmission), radio tag  10  stops the subsequent signal transmission after transmitting the ACK to AP  20 . Subsequently, it is sufficient that radio tag  10  change, for example, the state of transmission availability (TxStop) held in the internal memory (memory  134 ) into a state indicating that the signal transmission from the current radio tag (radio tag  10 ) has been stopped. 
     Furthermore, in Step S 110 , for example, when the command included in the signal from AP  20  is a command that instructs the current radio tag (radio tag  10 ) to change the radio frequency being currently used by the radio tag into a specific radio frequency, radio tag  10  changes the radio frequency being currently used by the radio tag into the specific radio frequency after transmitting the ACK to AP  20 . Subsequently, it is sufficient that radio tag  10  change, for example, the state of frequency selection (Freq) held in the internal memory (memory  134 ) into a state indicating that the specific radio frequency has been selected. 
     [2.4 Operation of AP  20 ] 
     Next, the operation of AP  20  according to the present embodiment will be described. 
       FIG.  16    is a flowchart illustrating the outline of operation of AP  20  according to the embodiment. 
     First, AP  20  collects the ID information of each of the plurality of radio tags  10  (S 20 ). More specifically, AP  20  collects ID information that is information for uniquely identifying each of the plurality of radio tags  10  under management. 
     Next, AP  20  broadcasts, to the plurality of radio tags  10  at a predetermined time interval, a signal including a command that instructs the plurality of radio tags  10  to change the first frequency being currently used by the plurality of radio tags  10  into the second frequency (S 21 ). 
     Next, when AP  20  confirms that all of the plurality of radio tags  10  have received said command, AP  20  stops broadcasting the signal including said command (S 22 ). 
       FIG.  17    and  FIG.  18    are flowcharts each illustrating one example of details of the operation of AP  20  according to the embodiment. With reference to  FIG.  17    and  FIG.  18   , the following description will be given assuming that AP  20  and the plurality of radio tags  10  that are under the management of AP  20  are used in logistics, and radio tags  10  are moving from area A to area B where available radio frequencies are different from each other.  FIG.  17    shows the operation of AP  20  in area A, and  FIG.  18    shows the operation of AP  20  in area B. 
     First, as shown in  FIG.  17   , AP  20  recognizes (confirms) using the GPS or the like that AP  20  is located in area A (S 201 ). 
     Next, AP  20  collects the entire ID information of radio tags  10  that are under the management (S 202 ). For example, AP  20  is connected to a cellular wireless communication network in area A and thus collects and stores the ID information of the plurality of radio tags  10  that are under management. 
     Next, while being located in area A, AP  20  recognizes that radio tags  10  are to move to area B where the frequency (the radio frequency) to be used by radio tags  10  to transmit signals is different (S 203 ). For example, by accessing a specific server or cloud storage via the wireless communication network and obtaining information of area B, which is a destination of radio tags  10 , from the ID information of radio tags  10  that are under management, AP  20  recognizes that radio tags  10  under management are to move to area B. 
     Next, AP  20  transmits a signal including a command that instructs radio tags  10  under management to change the frequency and stop the transmission (S 204 ). In the present embodiment, AP  20  broadcasts, to the plurality of radio tags  10 , a signal including a command that instructs the radio tags to change the first frequency into the second frequency and stop the transmission, and thus transmits the signal including said command to radio tag  10  that is under management. Note that AP  20  may transmit a signal not including a command that instructs the radio tags to change the frequency, but including a command that instructs the radio tags to stop the transmission. 
     Next, when AP  20  confirms that every radio tag  10  has received the command (Yes in S 205 ), AP  20  stops transmitting the command to the radio tags that are under management (S 206 ). In the present embodiment, as a result of obtaining the ACK from every one of the plurality of radio tags  10  that are under management, AP  20  stops the broadcasting and thus stops transmitting the command to the radio tags that are under management. Note that when AP  20  fails to confirm in Step S 205  that every radio tag  10  has received the command (No in S 205 ), AP  20  performs Step S 205  again and checks whether every radio tag  10  has received the command. 
     Next, the operation of AP  20  will be described with reference to  FIG.  18   . 
     First, as shown in  FIG.  18   , AP  20  recognizes (confirms) using the GPS or the like that AP  20  is located in area B (S 301 ). When AP  20  moves from area A to area B together with radio tags  10  that are under management, AP  20  can recognize using the GPS or the like that AP  20  is located in area B and thus can recognize that AP  20  has entered area B. Note that when AP  20  does not move together with radio tags  10  that are under management, it is sufficient that by receiving input from a user, etc., of AP  20 , AP  20  recognize (confirm) that radio tags  10  under management have entered area B. Furthermore, it is sufficient that AP  20  obtain, via the wireless communication network, the ID information of radio tags  10  under management that have been uploaded by AP  20  located in area A to the specific server or cloud storage. 
     Next, AP  20  transmits a signal including a command that instructs radio tags  10  under management to resume the transmission (S 302 ). In the present embodiment, AP  20  broadcasts, at the frequency for area B to the plurality of radio tags  10  that are under management, a signal including a command that instructs the radio tags to start the transmission. Note that when AP  20  transmits a signal not including a command that instructs the radio tags to change the frequency, but including a command that instructs the radio tags to stop the transmission in  FIG.  17   , it is sufficient that AP  20  transmit a signal including a command that instructs the radio tags to change the frequency instead of the command that instructs the radio tags to resume the transmission. 
     Next, when AP  20  confirms that every radio tag  10  has received the command (Yes in S 303 ), AP  20  stops transmitting the command to the radio tags that are under management (S 304 ). In the present embodiment, as a result of obtaining the ACK from every one of the plurality of radio tags  10  that are under management, AP  20  stops the broadcasting and thus stops transmitting the command to the radio tags that are under management. Note that when AP  20  fails to confirm in Step S 303  that every radio tag  10  has received the command (No in S 303 ), AP  20  performs Step S 303  again and checks whether every radio tag  10  has received the command. 
     [3 Advantageous Effects, Etc.] 
     As described above, radio tag  10  according to the present embodiment receives the command from the access point using the carrier-sense function before moving to an area where the frequency to be used is different, changes the frequency settings, and then stops the transmission to the access point. The operation of receiving commands continues with a new frequency, and when a transmission resumption instruction is received from the access point, the transmission to the access point resumes. 
     In this manner, according to the present embodiment, by merely changing control programs stored in a memory of an active radio tag without means for receiving data, it is possible to change the frequency of said radio tag using the carrier-sense function thereof. Furthermore, according to the present embodiment, using the carrier-sense function, the radio tag can simultaneously or alternately normal carrier sensing and carrier sensing that is used to interpret the command transmitted by the access point. 
     More specifically, the access point according to the present embodiment manages an active radio tag without means for receiving data and recognizes an area where said radio tag is currently located, and thus can transmit a command to said radio tag and cause a change in the frequency being used by said radio tag. Meanwhile, the radio tag according to the present embodiment, which is an active radio tag without means for receiving data, can nevertheless interpret, using the carrier-sense function, a command transmitted by the access point and thus can change the frequency being used by said radio tag. 
     The radio tag and the AP according to an aspect of the present disclosure have been described thus far based on the embodiment, etc., but the present disclosure is not limited to this embodiment. For example, other embodiments that can be realized by arbitrarily combining structural elements described in the present specification or by removing some structural elements may be embodiments of the present disclosure. Furthermore, variations obtainable through various changes to the above-described embodiment that can be conceived by a person having ordinary skill in the art without departing from the essence of the present disclosure, that is, the meaning of the recitations in the claims are included in the present disclosure. 
     Furthermore, the embodiments described below may also be included in one or more aspects of the present disclosure. 
     (1) Some of the structural elements included in the above-described AP may be a computer system configured of a microprocessor, a read only memory (ROM), a random access memory (RAM), a hard disk unit, a display unit, a keyboard, and a mouse, for example. A computer program is stored in the RAM or the hard disk unit. The microprocessor achieves its function by way of the microprocessor operating according to the computer program. Here, the computer program is configured of a combination of command codes indicating instructions to the computer in order to achieve a predetermined function. 
     (2) Some of the structural elements included in the above-described AP may be configured from a single system Large Scale Integration (LSI). A system LSI is a super-multifunction LSI manufactured with a plurality of components integrated on a single chip, and is specifically a computer system configured of a microprocessor, ROM, and RAM, for example. A computer program is stored in the RAM. The system LSI achieves its function by way of the microprocessor operating according to the computer program. 
     (3) Some of the structural elements included in the above-described AP may be configured from a standalone module or an IC card that can be inserted into and removed from the device. The IC card or the module is a computer system made up of a microprocessor, ROM, RAM, and so on. The IC card or the module may include the aforementioned super multifunctional LSI. The IC card or the module achieves its functions by way of the microprocessor operating according to the computer program. The IC card and the module may be tamperproof. 
     (4) Furthermore, some of the structural elements included in the above-described AP may be the aforementioned computer program or a digital signal recorded on a computer-readable recording medium, such as a flexible disk, a hard disk, a compact disc read-only memory (CD-ROM), a magneto-optical disc (MO), a digital versatile disc (DVD), DVD-ROM, DVD-RAM, a Blu-ray (registered trademark) disc (BD), or a semiconductor memory, for example. The present disclosure may also be the digital signal recorded on these recoding media. 
     Furthermore, in some of the structural elements included in the above-described AP, the computer program or the digital signal may be transmitted via an electrical communication line, a wireless or wired communication line, a network represented by the Internet, data broadcasting, or the like. 
     (5) The present disclosure may be the above-described methods. Furthermore, the present disclosure may be a computer program for implementing these methods using a computer or may be a digital signal of the computer program. 
     (6) Furthermore, the present disclosure may be a computer system including a microprocessor and a memory. The memory may have the computer program stored therein, and the microprocessor may operate according to the computer program. 
     (7) Moreover, by transferring the recording medium having the program or the digital signal recorded thereon or by transferring the program or the digital signal via the network or the like, the present disclosure may be implemented by a different independent computer system. 
     (8) The above embodiment and the above variations may be combined with each other. 
     Although only an exemplary embodiment of the present disclosure has been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiment without materially departing from the novel teachings and advantages of the present disclosure. Accordingly, all such modifications are intended to be included within the scope of the present disclosure. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure can be used for an active radio tag without means for receiving data and an access point that manages the active radio tag, for example.