Abstract:
An RFID tag includes: an antenna receiving an RF signal from a reader; an AFE (analog front end) generating voltage using the RF signal; and one or more switches interposed between the antenna and the AFE and controlling the connection between the antenna and the AFE through the switch operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority from Korean Patent Application No. 10-2010-0136352, filed on Dec. 28, 2010, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention relates to an RFID tag, and more particularly to an RFID tag capable of improving the recognition rate of the RFID and capable of recognizing a location of a certain RFID tag among a plurality of the RFID tags. 
     2. Description of the Prior Art 
     As generally known in the art, an RFID (radio frequency identification) system is the technology that can recognize an object using the radio communication, wherein a small semiconductor chip and an antenna connected to the small semiconductor chip are used to process the information of the object. Different from a barcode system recognizing only one object at a time, the RFID system has the advantage in that a number of the objects are recognized from a remote distance. The RFID system is currently used in a variety of field, such as a warehouse for storing products, the management of a special medicine, a supply network for military items, the management of the articles in a large-size market, and a traffic card. It is expected that the RFID system will be adopted in a multi-function electronic pay system for supplementing a function of a credit card in the future. 
     The RFID system includes: an RFID tag with a chip storing a variety of information and an antenna connected with the chip; and a reader supplying the current to the RFID tag and reading the information stored in the chip. 
     A passive type RFID tag does not have its own power source inside thereof, and thus the reader has to supply the power to the RFID tag using a continuous wave (CW). After receiving the power supply, the RFID tag modulates the continuous wave sent by the reader in the ASK (amplitude shift keying)/PSK (phase shift keying) manner and then transfers the modulated signal to the reader by backward-scattering. Accordingly, anybody can obtain the information stored in the RFID tag if the passive type RFID tag is supplied with the sufficient power. 
     In the meantime, when the RFID tag approaches the reader, the RFID tag performs the radio communication in response to the reader&#39;s request. Here, if a number of the RFID tags are within a range enabling the reader to perform the recognition, all of the RFID tags can be recognized instantly. However, the recognition rate, which is defined by the percentage of the RFID tags that can be recognized by the reader, cannot reach 100%, and a location of the recognized passive type RFID tag cannot be determined due to the interference between the RFID tags and the physical stacking of the RFID tags. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide an RFID tag capable of improving the recognition rate of the RFID tag and of recognizing a location of a certain RFID tag among a plurality of the RFID tags. 
     In order to accomplish this object, there is provided an RFID tag including: an antenna receiving an RF signal from a reader; an AFE (analog front end) generating voltage using the RF signal; and one or more switches interposed between the antenna and the AFE and controlling the connection between the antenna and the AFE through a switching operation. 
     In the present invention, it is possible to improve the recognition rate of the RFID tag by providing the RFID tag with a physical switch and a rechargeable battery. 
     In addition, it is possible to recognize a location of a certain RFID tag among a plurality of the RFID tags by providing the RFID tag further including an indicator indicating a state of the RFID tag. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1A  and  FIG. 1B  are block diagrams, each of which schematically illustrates a structure of an RFID tag in accordance with a first embodiment of the present invention. 
         FIG. 2A  and  FIG. 2B  are block diagrams, each of which schematically illustrates a structure of an RFID tag in accordance with a second embodiment of the present invention. 
         FIG. 3  is a flow chart sequentially illustrating an operation of the RFID tag in accordance with the present invention. 
         FIG. 4  is a block diagram schematically illustrating a structure of an RFID tag in accordance with a third embodiment of the present invention. 
         FIG. 5  is a block diagram schematically illustrating a structure of an RFID tag in accordance with a fourth embodiment of the present invention. 
         FIG. 6  is a block diagram schematically illustrating a structure of an RFID tag in accordance with a fifth embodiment of the present invention. 
         FIG. 7  is a block diagram schematically illustrating a structure of an RFID tag in accordance with a sixth embodiment of the present invention. 
         FIG. 8  is a block diagram schematically illustrating a structure of an RFID tag in accordance with a seventh embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, a preferred embodiment of the present invention will be described with reference to the accompanying drawings. In the following description and drawings, the same reference numerals are used to designate the same or similar components, and so repetition of the description on the same or similar components will be omitted. 
     The present invention is characterized in that a physical hardware device is added to a conventional RFID tag in order to improve the recognition rate of a passive type RFID tag and to recognize a location of the RFID tag. 
     In general, the RFID tag includes an antenna, an Analog Front End (AFE), a control module and a memory. In the present invention, however, a hardware device for performing a switch operation of the RFID tag is added to thereby realize a higher recognition rate. 
       FIG. 1A  and  FIG. 1B  illustrate block diagrams, each of which schematically illustrates a structure of a RFID tag in accordance with a first embodiment of the present invention. 
     When the RFID tag and a reader approach each other for performing the radio communication there-between, the RFID operates according to the reader&#39;s request. Here, if a number of the RFID tags are located within a communication range of the reader, the reader may recognize the RFID tags instantly. In reality, however, it is difficult to recognize 100% of the RFID tags due to the interference between the RFID tags and the physical stacking of the RFID tags, In other words, a majority of the RFID tags may send a response message in response to the reader&#39;s request, but a minority of the RFID tags may not operate as desired due to many kinds of the physical phenomena. 
     In the present invention, a switch is arranged between the antenna and the AFE, so that the RFID tag may turn the switch off in response to the reader&#39;s request to thereby remove a signal entered into the RFID tag through the antenna, so that the interference phenomenon between the RFID tags can be avoided. 
     Referring to  FIG. 1A , when the reader approaches the RFID tag, a first switch  101  and a third switch  103  are turned on, whereas a second switch  102  and a fourth switch  104  are turned off. Accordingly, a rechargeable battery  140  is charged by a voltage supplied from the AFE  110 . 
     When the normal communication between the RFID tag and the reader is completed, the second switch  102  and the fourth switch  104  are turned on, whereas the first switch  101  and the third switch  103  are turned off. Here, the voltage charged in the rechargeable battery  140  flows through the second switch  102  and the fourth switch  104  and then control the first switch  101  and the third switch  103 . 
     The above-mentioned switch control is controlled by a control signal from the control module  120 , wherein the RFID tag itself may sense the radio communication signal to thereby turn on/off the second switch  102  and the fourth switch  104  or the second switch  102  and the fourth switch  104  may be turned on/off according to the reader&#39;s request signal when the RFID tag and the reader perform the radio communication there-between. 
     If the RFID tag controls the switch operation of the second switch  102  and the fourth switch  104 , the control module  120  senses the completion of the radio communication with the reader and then turns the second switch  102  and the fourth switch  104  on. Thereby, the first switch  101  and the third switch  103  may be turned off. If the second switch  102  and the fourth switch  104  are controlled in response to the reader&#39;s request, the RFID tag, the recognition of which was completed, receives predetermined flag information included in the signal sent from the reader or receives from the reader a data tag commanding the first switch  101  and the third switch  103  to turn off, to thereby enable controlling the switch. 
     Accordingly, the RFID tag, the recognition of which was completed by finishing the communication with the reader, short-circuits the antenna by means of the switch operation for a predetermined time, so that it is possible to precisely recognize the minority of the RFID tags that could not be recognized due to the interference between the RFID tags and the physical phenomenon. 
     Referring to  FIG. 1B , switch devices S 1 , S 2 , S 3  and S 4  are the analog switches, wherein a transistor like CMOS or BJT may be used as the switch device. Particularly, it is preferable to use PMOS device (P-type Metal Oxide Semiconductor device) as the first switch  101 . 
     If the PMOS device is used as the first switch  101 , a gate terminal of the first switch  101  indicates 0V at an initial state and it is possible to perform the radio communication when the reader approaches the RFID tag, because the antenna is in a connected state. Here, the rechargeable battery  140  is charged by the voltage supplied from the AFE  110 . Then, after completing the normal communication between the RFID tag and the reader, the second switch  102  is converted from the off-state to the on-state by means of the reader&#39;s request or the RFID tag&#39;s own sensing function and the current flows from the rechargeable battery  140  to a gate terminal of the first switch  101  through the second switch  102 . 
     In other words, the first switch  101  turns off because the voltage VDD of the rechargeable battery  140  is inputted to a gate terminal of the PMOS device, i.e., the first switch  101 . In the off-state, the radio communication with the reader is not available because the connection to the antenna cuts off. On the other hand, the rechargeable battery  140  provides the gate terminal of the PMOS device, i.e. the first switch  101  with the VDD voltage for a predetermined period, and then the voltage of the rechargeable battery  140  is gradually discharged to make the VDD to reach 0V. 
       FIG. 2A  and  FIG. 2B  are block diagrams, each of which illustrates a structure of an RFID tag in accordance with a second embodiment of the present invention. 
     Referring to  FIG. 2A  and  FIG. 2B , the RFID tag according to the second embodiment of the present invention operates similarly to the RFID tag according to the first embodiment. Specifically, when the reader approaches the RFID tag, the first switch  101 , the third switch  103  and the fifth switch  105  turn on, whereas the second switch  102  and the fourth switch  104  turn off. Accordingly, the rechargeable battery  140  is charged with a constant voltage. 
     Thereafter, when the normal communication is completed between the RFID tag and the reader, the second switch  102  and the fourth switch  104  turn on, whereas the first switch  101 , the third switch  103  and the fifth switch  105  turn off. The voltage charged in the rechargeable battery  140  flows through the second switch  102  and the fourth switch  104  and controls the first switch  101 , the third switch  103  and the fifth switch  105 . 
     Accordingly, the first switch  101  of the RFID tag turns off to thereby short-circuit the antenna for a predetermined period, so that it is possible to remove the interference phenomenon between the RFID tags and to precisely recognize even the minority of the RFID tags. 
     In  FIG. 2A  and  FIG. 2B , the fifth switch  105  controls the discharge rate of the rechargeable battery  140  to thereby make the relatively slow discharge rate, because the RFID tag may restart its operation to cause the interference if the rechargeable battery  140  discharges too fast. 
       FIG. 3  is a flow chart sequentially illustrating the operation of the RFID of the present invention. 
     When the RFID tag physically approaches the reader (S 310 ), the RFID tag sends the response message to the reader in response to the reader&#39;s request to thereby initiate the radio communication between the reader and the RFID tag (S 320 ). 
     Here, the first switch  101 , i.e., the PMOS device always is maintained in a connected state while it is in a natural state in which the RFID is attached to an ordinary object or a human being without any communication with the reader. In other words, it means that the first switch  101  is always maintained in the on-state while it is in the natural state as illustrated in  FIG. 1A  and  FIG. 2A . Also, the voltage level of the rechargeable battery  140  is 0V in the natural state. Because the RFID tag does not have any power source in the natural state, the control mode of the first switch  101  has ‘0’. Here, when the control mode of the first switch  101  has ‘0’, the PMOS device is used as a representative device that maintains the first switch  101  in the connected state. The PMOS device stays in a conducting state wherein the current flows between a source 
     S and a drain D when the gate has ‘0’. To the contrary, the PMOS device stays in an interrupted state when the gate G has ‘VDD’. 
     Thereafter, when the radio communication begins, the AFE  110  provides the rechargeable battery  140  with the power, to thereby make the rechargeable battery charged (S 330 ). 
     When the radio communication between the reader and the RFID tag completes normally (S 340 ), the control module  120  controls the second switch  102  to switch from the off-state to the on-state, and the rechargeable battery  140  discharges the charged voltage, so that the first switch  101  turns off (S 350 ). 
     When the discharge of the rechargeable battery  140  is completed, the gate voltage of the first switch  101  turns back to 0V to thereby convert the first switch  101  to the on-state. Accordingly, the radio communication becomes available again when the RFID tag approaches the reader. 
     On the other hand, the RFID tag that includes a switch for the purpose of switching on/off the antenna connection is capable of switching on/off the antenna even when the rechargeable battery is removed. Also, it is possible to improve the recognition rate. Here, the RFID tag makes use of the VDD power source supplied from the AFE in order to turn the switch off. Accordingly, an off-period of the antenna connection is shorter in the RFID tag without a separate rechargeable battery than in the RFID tag with the rechargeable battery. 
       FIG. 4  is a block diagram of the structure of a RFID tag in accordance with a third embodiment of the present invention. 
     Generally, it is uneasy to recognize the location of the RFID tag by means of only one reader. In particular, it is uneasy for a user to find out a desired RFID tag if there are a hundreds of the RFID tags. Thus, the embodiment of the present invention intends to realize the recognition of the location by adding a separate indicator  450  to the RFID tag. 
     First of all, when the reader and the RFID tag approach to each other for performing the radio communication, the RFID tag operates according to the reader&#39;s request. The RFID tag is used for identifying and searching for an object (a human being) because it is attached to the object, but it is uneasy to search for a desired RFID tag or a malfunctioning RFID tag if the reader operates at the location where a number of the RFID tags are presents. 
     The RFID tag responding to the reader&#39;s request charges a rechargeable battery  440  with the output voltage VDD of an AFE  410 . The voltage in the rechargeable battery  440  causes the indicator  450  to operate through a first switch  401 . 
     Specifically, when the reader and the RFID tag approach each other, the first switch  401  turns off by means of the control signal from the control module  420  and the rechargeable battery  440  is charged with a constant voltage. When the normal communication between the RFID tag and the reader is completed, a control module  420  controls the first switch  401  to turn on, to thereby operate the indicator  450  using the voltage of the rechargeable battery  440 . 
     Here, the control of the first switch  401  in the control module  420  may be performed in such a way that the first switch turns on/off according to the reader&#39;s request or the first switch  401  is controlled according to whether an ID stored in the RFID tag is coincident with an ID of the RFID or according to a tag indicator that is required by the reader. 
     According to the circumstances, the process can be performed in the RFID tag itself wherein the first switch  401  may include the PMOS device and wherein the first switch  401  may turn on to thereby operate the indicator  450  when the power supply from the reader is interrupted. 
     The rechargeable battery  440  is charged with the voltage while the radio communication with the reader is performed, whereas a substitute power source may be used when the RF power supply from the reader is unavailable. Any sensible means, such as an electronic paper (e-paper), an LED, a speaker etc., may be used as the indicator  450 . 
       FIG. 5  is a block diagram schematically illustrating the structure of the RFID tag according to a fourth embodiment of the present invention. 
     Referring to  FIG. 5 , when the reader and the RFID tag approach to each other, the first switch  401  turns off; a second switch  402  turns on; and the rechargeable battery  440  is charged with a constant voltage. Thereafter, the normal communication between the RFID tag and the reader is completed, the first switch  401  turns on; the second switch  402  turns off; and the voltage in the rechargeable battery  440  operates the indicator  450 . 
       FIG. 6  is a block diagram schematically illustrating the structure of an RFID tag according to a fifth embodiment of the present invention. 
     When the reader and the RFID tag approach to each other, a first switch  601  and a third switch  603  turn on, whereas a second switch  602  and a fourth switch  604  are turned off by the control module  620 . Also, a rechargeable battery  640  is charged with a constant voltage. When the normal communication between the RFID tag and the reader is completed, by means of the control module  620 , the first switch  601  and the third switch  603  are turned off, whereas the second switch  602  and the fourth switch  604  are turned on. The voltage charged in the rechargeable battery  640  operates the indicator  650  through a fifth switch  605 . 
     The RFID tag described with reference to  FIG. 6  is characterized in that a hardware device is added in order to attain a higher recognition rate and the recognition of the location. Specifically, when the reader and the RFID tag approach to each other for performing the radio communication, the RFID tag operates according to the reader&#39;s request. Here, although the reader can instantly recognize hundreds of the RFID tags, it is difficult to recognize 100% of the RFID tags due to the interference between the tags or to the physical stacking of the tags. In other words, a majority of the RFID tags can send the response message in response to the reader&#39;s request, whereas a minority of the RFID tags may not work normally. Here, the RFID tag that has reacted to the reader&#39;s request turns off the first switch  601  between the antenna and the AFE  610  to thereby cut off a signal from the antenna, and thus it is possible to remove the interference phenomenon. Accordingly, the reader can recognize the minority of the RFID tag, which was not normally recognized due to the interference phenomenon. 
     Also, when the normal communication between the RFID tag and the reader is completed at the same time as the above-described operation, the fifth switch  605  turns on to thereby enable the operation of the indicator  650  using the voltage stored in the rechargeable battery  640 . 
       FIG. 7  is a block diagram schematically illustrating the structure of an RFID tag according to a sixth embodiment of the present invention. 
     Referring to  FIG. 7 , when the reader and the RFID tag approach to each other, a first switch  701 , a third switch  703  and a fifth switch  705  turn on; a second switch  702 , a fourth switch  704  and a sixth switch  706  turn off; and a rechargeable battery  740  is charged with a constant voltage supplied from an AFE  710 . 
     Thereafter, when the normal communication between the RFID tag and the reader is completed, the second switch  702 , the fourth switch  704  and the sixth switch  706  turn on; the first switch  701 , the third switch  703  and the fifth switch  705  turn off; and the voltage charged in the rechargeable battery  740  flows through the second switch  702  and then controls the first switch  701 . Accordingly, the RFID tag short-circuits the antenna for a while, so that the interference phenomenon between the RFID tags may be removed and the reader may precisely recognize the minority of the RFID tags. 
     Also, when the normal communication between the RFID tag and the reader is completed at the same time as the above-described operation, the sixth switch  706  turns on to thereby operate the indicator  750  using the voltage stored in the rechargeable battery  740 . 
       FIG. 8  is a block diagram schematically illustrating the structure of an RFID tag according to a seventh embodiment of the present invention. 
     The operation of the RFID tag illustrated in  FIG. 8  is the same as that of the RFID tag illustrated in  FIG. 7 , but the embodiment illustrated in  FIG. 8  adopts an electronic paper  850 , instead of the indicator  750  illustrated in  FIG. 7 . 
     Also, the sixth switch  706  illustrated in  FIG. 7  is represented as four switches S 81  and S 82  illustrated in  FIG. 8 . Specifically, the operation of the switches S 81  and S 82  are controlled by a control module  820 . When the control module  820  is in a high state, the switch S 81  turns on, whereas the switch S 82  turns off. To the contrary, when the control module  820  is in a low state, the switch S 81  turns off, whereas the switch S 82  turns on. 
     As such, the on/off operation of the four switches S 81  and S 82  causes the electronic paper  850  to display color die to the charge transfer. Accordingly, with the discernment of this color, it enables the user&#39;s naked eye to determine whether or not the RFID is in a state of a malfunction or whether or not the reader normally reads the RFID tag. 
     As described herein before, the RFID tag can be embodied as a one-piece chip excluding the indicators  750  and  850 ; otherwise, the RFID tag can be embodied as a one-piece chip excluding the batteries  740  and  840  and the indicators  750  and  850 . Also, the rechargeable batteries  750  and  850 , each of which is a capacitor, can be charged instantly and can be embodied as film capacitors. 
     The above described RFID tag including the indicator can recognize the location of the RFID tag by means of the human being&#39;s sense even when the rechargeable battery is removed. The location recognition of the RFID tag is dependent on a characteristic of the indicator. However, because the indicator like the electronic paper operates using the voltage, the electronic paper may react to thereby display a certain sign (e.g., the number, O/X sign, a certain symbol and a certain color) when the VDD voltage is once applied to the electronic paper. 
     Accordingly, in the RFID tag which has the switch for connecting the antenna or the switch for connecting the indicator may control the switch to make the antenna connected or to indicate an operation state of the RFID tag even if it does not have the separate rechargeable battery. 
     Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.