Abstract:
A method for operating a transponder, and a transponder device is disclosed. Data are wirelessly and bidirectionally transmitted between the transponder and a base station through a first interface that is based on an electromagnetic far-field coupling. According to the invention, data are additionally transmitted wirelessly and bidirectionally between the transponder and the base station through at least one second interface that is based on inductive coupling.

Description:
[0001]    This nonprovisional application claims priority to provisional application No. 60/839,420, which was filed on Aug. 23, 2006, and claims priority to German Patent Application No. DE 102006002515, which was filed in Germany on Jan. 16, 2006, and which are both herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a transponder and to a method for operating a transponder. 
         [0004]    2. Description of the Background Art 
         [0005]    Contactless identification systems, or radio frequency identification (RFID) systems typically include a base station or reader (or reader unit) and a plurality of transponders or remote sensors. The transponders and their transmitting and receiving devices customarily do not have an active transmitter for data transmission to the base station. Such non-active systems are called passive systems if they do not have their own energy supply, and semi-passive systems if they have their own energy supply. Passive transponders take the energy they require for their supply from the electromagnetic field emitted by the base station. 
         [0006]    For data transmission between the transponder and the base station, the transponder has an interface of a certain interface type, which is compatible with the corresponding interface type of the base station. In a first, rough categorization, the interface types can be divided into contacting and contactless types. 
         [0007]    The interface types in which the data transmission takes place in a contactless way are distinguished, among other characteristics, by the operating or carrier frequency used for data transmission, which is to say the frequency transmitted by the base station. Commonly used frequencies include 125 kHz (LF range), 13.56 MHz (RF range), a frequency range between 860 MHz and 960 MHz (UHF range), and a frequency range above 3 GHz (microwave range). 
         [0008]    Another distinguishing feature of different interface types is the type of coupling between the respective interfaces of the transponder and base station. In this regard, a distinction is made between what is called inductive or magnetic coupling and what is called far-field coupling, among other characteristic. In simplified terms, with inductive or near-field coupling an antenna coil of the base station and an antenna coil of the transponder form a transformer, for which reason this coupling type is also called transformer coupling. In the case of inductive coupling, a maximum separation between the transponder and the base station is limited to the region of a near field of the antennas used. The near field region is primarily determined by the operating frequency of the interface. 
         [0009]    Far-field coupling relies on the propagation of electromagnetic waves which are emitted by the antenna used. UHF and microwave systems typically rely on far-field coupling. RF and HF systems, in contrast, typically rely on inductive coupling. Fundamentals in this regard can be found in, for example, the “RFID Handbuch,” a textbook by Klaus Finkenzeller, HANSER Verlag, third edition, 2002, section 2.3, “Frequenz, Reichweite Kopplung” (Frequency, Range and Coupling), section 3.2.1, “Induktive Kopplung” (Inductive Coupling), and section 4.2.1.1, “Übergang vom Nah-zum Fernfeld bei Leiterschleifen” (Transition from Near Field to Far Field in Conductive Loops). 
         [0010]    In general, load modulation is used to transmit data from a transponder to the base station with inductive coupling; for information on this, refer to Finkenzeller, section 3.2.1.2.1, “Lastmodulation” (Load Modulation), for example. 
         [0011]    In general, backscatter coupling is used to transmit data from a transponder to the base station using UHF or microwaves in the far field of the base station. To this end, the base station emits electromagnetic carrier waves, which the transmitting and receiving device in the transponder modulates and reflects appropriately for the data to be transmitted to the base station using a modulation method. The typical modulation methods for this purpose are amplitude modulation, phase modulation and amplitude shift keying (ASK) subcarrier modulation, in which the frequency or the phase position of a subcarrier is changed; in this regard, refer once again to Finkenzeller, section 3.2.2, “elektromagnetische Backscatter-Kopplung” (Electromagnetic Backscatter Coupling). 
         [0012]    However, data transmission in the UHF range with electromagnetic far-field coupling is not possible in every country, since the regulations in some countries do not make any free frequencies available in the requisite frequency bands, for example. As a consequence, such transponders cannot be used worldwide. 
         [0013]    Moreover, on account of radiated interference or due to the presence of absorbent materials, such as water, data transmission in certain frequency ranges, in particular the UHF and microwave ranges, can be interfered with such that operation of the transponder becomes impossible. 
         [0014]    In WO 2005/109328 A1, which corresponds to U.S. Publication No. 20050237163, a transponder remote keyless applications is described, which has an active, unidirectional interface for the UHF frequency range and multiple bidirectional interfaces for the LF frequency range. However, specific functions are assigned to the UHF interface and the relevant LF interfaces, so selective operation of the various interfaces is not possible. 
       SUMMARY OF THE INVENTION 
       [0015]    It is therefore an object of the present invention to provide a method for operating a transponder, and a transponder device, which permit reliable worldwide operation of the transponder. 
         [0016]    In an embodiment, data are wirelessly and bidirectionally transmitted between the transponder and a base station through a first interface that is based on an electromagnetic far-field coupling. In addition, data can be wirelessly and bidirectionally transmitted between the transponder and the base station through a second interface that is based on inductive coupling. Data transmission can take place in alternation, i.e. as a function of the interface that is usable or available at the moment, or can take place simultaneously through both interfaces. This permits reliable operation of the transponder even when, for example, the far-field based coupling is subject to interference or is not possible in a specific country. 
         [0017]    In a further embodiment, security-related data are transmitted exclusively through the second interface. In the case of data transmission based on electromagnetic far-field coupling in the UHF or microwave frequency regions, long transmission ranges are achieved, making unauthorized interception of the data transmitted between the transponder and base station relatively easy as compared to near-field coupled systems. This is especially easy in the case of data transmitted from the base station, since these data are transmitted with a relatively high transmit power. Since interception of data transmissions based on inductive coupling is only possible within a short distance from the base station, security against interception is significantly improved thereby. 
         [0018]    In a further embodiment, an authentication operation is carried out exclusively through the second interface. An authentication operation is used, for example, to enable write and/or read access to the transponder or its memory. Customarily, a password is transmitted from the base station to the transponder for this purpose, wherein the password can be XOR combined with a random number previously transmitted by the transponder, for example. The authentication operation is also referred to as an access operation. One example of an authentication or access operation is described in the proposed standard ISO/IEC_CD 18000-6C dated Jan. 7, 2005, section 6.3.2.10.3.6. In accordance with the invention, passwords and/or random numbers used for the XOR combination are transmitted exclusively through the second interface when they are transmitted in plain text. 
         [0019]    In yet a further embodiment, a Data Encryption Standard (DES) encryption method, a Triple DES encryption method, or an Advanced Encryption Standard (AES) encryption method is performed during data transmission through the second interface. Such computationally intensive encryption methods may require a comparatively large amount of operating power for their execution. In the case of passive transponders with UHF or microwave far-field coupling, the power provided by the electromagnetic field of the base station under typical operating conditions is not adequate for providing the power required for the encryption algorithms. 
         [0020]    Further, the base station can execute an anticollision routine through the first interface. When the transponder is selected, the first interface is deactivated and data transmission is subsequently performed exclusively through the second interface. The anticollision routine can be, for example, a deterministic selection method such as a binary tree search method, or a stochastic method such as an ALOHA selection method. 
         [0021]    In a further embodiment, the transponder is supplied with operating power exclusively through the first interface and/or through the second interface, i.e., the transponder is passive. 
         [0022]    In a further embodiment, data are transmitted from the transponder to the base station through the first interface on the basis of backscatter. 
         [0023]    In a further embodiment, the first interface is operated in a frequency range from 400 MHz to 6 GHz. Preferably, the frequency is in the UHF range at 860 MHz to 960 MHz. 
         [0024]    In a further embodiment, the second interface is operated in a frequency range from 100 kHz to 150 kHz, or 10 MHz to 20 MHz. 
         [0025]    In a further embodiment, both the first and second interfaces are activated after an initialization of the transponder, and when a command is received only the interface through which the command is received remains activated. Alternatively, a field strength measurement is performed at the first and second interfaces, and the interface at which the greater field strength is present is activated. The other interface is then deactivated. In another alternative, a bit error rate is measured at the first and second interfaces, and the interface at which the lower bit error rate is measured is activated. The other interface is then deactivated. The variants described permit simple, automated interface selection based on reproducible criteria, wherein the interface that ensures optimal data transmission is always selected. 
         [0026]    The transponder includes, for example, a first interface for bidirectional, wireless data transmission between the transponder and a base station, wherein the first interface is based on electromagnetic far-field coupling. In addition, it can include a second interface for bidirectional, wireless data transmission between the transponder and the base station, wherein the second interface is based on inductive coupling. 
         [0027]    In a further embodiment, the transponder includes an interface selection unit that is designed such that it selects the second interface for data transmission in the event of security-related data to be transmitted. 
         [0028]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein: 
           [0030]      FIG. 1  illustrates an RFID system with a base station and a transponder; and 
           [0031]      FIG. 2  illustrates a timing diagram of a selection process. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 1  shows an RFID system  1  with a base station  10  and a transponder  20 . 
         [0033]    The base station  10  includes a first interface in the form of an analog front end  11 , and an antenna  12  coupled to the analog front end  11 . The first interface operates through, for example, a far-field coupling in a frequency range from 860 MHz to 960 MHz. The base station  10  additionally includes a second interface in the form of another analog front end  13  and an antenna  14  coupled to the analog front end  13 . The second interface operates through an inductive coupling in a frequency range of 13.56 MHz. 
         [0034]    The transponder  20  includes a first interface in the form of an analog front end  21  and an antenna  22  coupled to the analog front end  21 . The first interface operates, for example, through a far-field coupling in a frequency range from 860 MHz to 960 MHz. Data transmission between the base station  10  and the transponder  20  takes place through their respective first interfaces using, for example, a data transmission protocol in conformity with the proposed standard ISO/IEC_CD 18000-6C dated Jan. 7, 2005. The transponder  20  additionally includes a second interface in the form of another analog front end  23  and an antenna  24  coupled to the analog front end  23 . The second interface operates through an inductive coupling in a frequency range of 13.56 MHz. Data transmission between the base station  10  and the transponder  20  takes place through their respective second interfaces in accordance with the ISO 14443 standard. 
         [0035]    Data transmission from the transponder  20  to the base station  10  through the first interface  21  and  22  takes place on the basis of backscatter. Data transmission from the transponder  20  to the base station  10  through the second interface  23  and  24  takes place on the basis of load modulation. 
         [0036]    The analog front ends  11 ,  13 ,  21  and  23  each include circuit components (not shown), which serve to drive the applicable antennas  12 ,  14 ,  22  and  24  and to process signals received by the applicable antennas  12 ,  14 ,  22  and  24 . 
         [0037]    The transponder  20  further includes an interface selection unit  25  coupled to the first interface  21  and  22 , and to the second interface  23  and  24 . The selection unit  25  is designed such that it selects the second interface  23  and  24  for data transmission in the event of security-related data to be transmitted. In addition, the interface selection unit  25  serves to process the signals received from, and those to be transmitted to, the first interface or the first analog front end  21  and the signals received from, and those to be transmitted to, the second interface or second analog front end  23 . To this end, the interface selection unit  25  includes digital circuits that are not shown, for instance logic gates, counters, timers, etc. 
         [0038]    The interface selection unit  25  is coupled to a data processing unit  26 , which can be implemented as a state machine or as a microprocessor, for example. The data processing unit  26  serves to control the function of the transponder  20 , and thus implements the application layer in the ISO layer model. For example, the commands transmitted by the base station  10  are processed in the data processing unit  26 . 
         [0039]    The data processing unit  26  is coupled to an electrically erasable memory  27  that serves to dynamically store transponder application-related data. 
         [0040]      FIG. 2  schematically shows a timing diagram of a selection method and a subsequent authentication, where signals SS 1  at the first interface  21  and  22  of the transponder  20  and signals SS 2  at the second interface  23  and  24  of the transponder  20  are shown. 
         [0041]    The selection method is a conventional slotted ALOHA method. This method is described in detail in the proposed standard ISO/IEC_CD 18000-6C dated Jan. 7, 2005, section 6.3.2 ff., and in particular in FIG. 19 there, for example. To select, the base station  10  first transmits what is called a query command to the first interface  21  and  22  of the transponder  20  in the UHF range during a time interval or first slot SA 1 . The transponder  20  then switches a state of its internal state machine to an “arbitrate” state. Within the context of the selection method as described in FIG. 19 of the proposed standard, data are now transmitted bidirectionally between the transponder  20  and the base station  10  in the first slot SA 1  through the first interface  21  and  22  until the transponder  20  assumes an “open” state. 
         [0042]    In order to carry out certain security-related operations with the transponder  20 , the base station  10  should transmit a password PW to the transponder  20  within the scope of an authentication operation. If the password PW is correctly transmitted, the transponder  20  switches to a “secure” state. However, the password PW is not transmitted through the first UHF interface  21  and  22 , but rather through the near-field coupled HF interface  23  and  24 , which is significantly more secure from interception. The transponder  20  now deactivates its first interface  21  and  22 , and subsequently communicates with the base station  10  in a time interval DT solely through the second interface  23  and  24 . 
         [0043]    The base station  10  can now continue the selection process or the selection of additional transponders (not shown) through their first interface  11  and  12  during a time interval SA 2  in subsequent slots, while communicating in an overlapping manner with the transponder  20  through its second interface  13  and  14 , i.e. reading out a memory area, for example. 
         [0044]    It is possible for powerful encryption algorithms, such as a DES encryption method, a triple DES encryption method, or an AES encryption method, to be performed during the data transmission through the second interface  23  and  24 . This produces an additional drastic reduction in the danger of interception. 
         [0045]    The transponder  20  is passive, i.e. it is supplied with operating power exclusively through the first interface  21  and  22  and/or through the second interface  23  and  24 . 
         [0046]    When the requirements for security from interception are low, the interfaces of the transponder  20  can also be operated with equal privileges, in other words, all functions are accessible through both interfaces, i.e., an authentication is also possible through the first interface  21  and  22 , for example. 
         [0047]    The interface selection can take place in accordance with a variety of criteria here. For example, it is possible for both the first and second interfaces to be activated following initialization of the transponder. However, when a command is received, only the interface through which the command is received remains activated. 
         [0048]    Alternatively, a field strength measurement can be performed at the first and second interfaces, for example. Then, the interface at which the greater field strength is present is activated, while the other interface is deactivated. The inclusion of a bit error rate as a selection criterion is also possible. 
         [0049]    The use of interfaces with equal privileges permits worldwide operation of the transponder  20 , even in countries where no UHF data transmission is possible, especially in Asia. 
         [0050]    Of course, additional interfaces of another type can also be provided in the transponder  20  along with the two interfaces  21  and  22 , and  23  and  24 , shown. Moreover, it is possible for a separate base unit having only one type-specific interface to be provided for each interface type, i.e., it is possible for the transponder  20  to be operated with base stations having only one of the two interface types. 
         [0051]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.