Communication between electromagnetic transponders

The present invention relates to systems using electromagnetic transponders, that is to say generally mobile transceivers capable of being interrogated in a contactless and wireless, by a generally fixed unit, said reading terminal and / or writing. The invention relates more particularly transponders no independent power supply, for example of the contactless card or electronic tag. These transponders extract the power supply required by the electronic circuits comprise the high frequency field radiated by an antenna of the terminal reading and writing. The invention applies to such transponders, whether it is of read-only transponders, that is to say adapted to operate with a terminal simply by reading data from the transponder or transponders to read -Writing that contain data that can be modified by the terminal.

1 shows very schematically and functionally, a classic example of data exchange between a terminal 1 read-write (STA) and a transponder 10 (CAR).

The terminal 1 is essentially formed of an oscillating circuit formed of an inductance L1 in series with a capacitor C1 and a resistor R1 between a 2p terminal of an amplifier output or antenna coupler 3 and a terminal 2m a reference potential (usually ground). Amplifier 3 receives a signal Tx high frequency transmission from a modulator 4 (MOD). The modulator receives a reference frequency, for example a quartz oscillator 5 and, if necessary, a DATA signal data to be transmitted. In the absence of data transmission from terminal 1 to the transponder 10, the signal Tx is only used as an energy source to activate the transponder 10 if the latter enters the field. Data to be transmitted generally originate from a digital system, for example, a microprocessor 6 (.mu.P).

The connection point of capacitor C1 and inductor L1 is, in the example shown in Figure 1, a terminal for sampling a data signal Rx received from a transponder 10 to a demodulator (DEM) . An output of the demodulator communicates (if necessary via a decoder 8 (DEC)) data received from transponder 10 to microprocessor 6 of terminal 1. Demodulator 7 receives, generally from oscillator 5, a signal clock or reference for a phase demodulation. Where appropriate, the demodulation is performed from a signal tapped between the capacitor C1 and resistor R1, and not across the inductor L1. The microprocessor 6 communicates (BUS) with different input / output circuits (keyboard, screen, means of transmission to a server, etc.) and / or treatment. The circuits of the terminal read / write draw the power necessary for their operation from a supply circuit 9 (ALIM) connected, for example, the electric distribution network.

transponder side 10, an inductor L2 in parallel with a capacitor C2 form a parallel resonant circuit (called a reception resonant circuit), for sensing the magnetic field generated by the oscillating circuit series L1, C1 of the terminal 1. The circuit resonant (L2, C2) of the transponder 10 is tuned to the resonant frequency of the oscillating circuit of the terminal 1.

Terminals 11 and 12 of the resonant circuit L2, C2 corresponding to the terminals of capacitor C2 are connected to two AC input terminals of a bridge rectifier 13 whose rectified output terminals 14 and 15 are connected to terminals of a capacitor Ca energy storage and smoothing the rectified voltage provided by the bridge 13. the bridge 13 is single or double alternation.

When the transponder 10 is in the field of the terminal 1, a high frequency voltage is generated across the resonant circuit L2, C2. The voltage rectified by bridge 13 is smoothed by the capacitor Ca, which provides a supply voltage to electronic circuits of the transponder via a voltage regulator 16 (REG). These circuits generally include, essentially a microprocessor 17 (.mu.P) associated with a memory not shown, a demodulator 18 (DEM) possibly received from terminal 1 signals, and a modulator 19 (MOD) for transmitting information to terminal 1. the transponder is generally synchronized by means of a clock (CLK) derived by a block 20 of the high-frequency signal recovered across capacitor C2 before rectification. Most often, all the electronic circuits of the transponder 10 are integrated into a single chip.

To transmit data from transponder 10 to terminal 1, modulator 19 controls a stage of modulation (back modulation) of resonant circuit L2, C2. This modulation stage is generally formed of an electronic switch (e.g., a transistor T) and a resistor R in series between terminals 14 and 15.

The transistor T is controlled at a frequency (e.g., 847.5 kHz) said subcarrier, much lower (usually, with a ratio of at least 10) than the frequency of the excitation signal of the oscillating circuit of the terminal 1 (eg, 13.56 MHz). When the switch T is closed, the oscillating circuit of the transponder is submitted to an additional damping with respect to the load constituted by the circuits 16 to 20, so that the transponder samples a greater amount of power from the high frequency magnetic field . terminal side 1, the amplifier 3 maintains the amplitude of the excitation high frequency signal. Therefore, the power variation of the transponder resulting in a change of amplitude and current phase in the antenna L1. This variation is detected by demodulator 7 of the terminal which is either a phase demodulator or an amplitude demodulator.

In some cases, the floor retromodulation (transistor T, resistor R) is situated upstream of the bridge 13, that is to say side of its AC input.

The terminal typically transmits no data as it receives from the transponder, the transmission being alternately in one direction and then the other.

Figure 2 illustrates a classic example of data transmission from terminal 1 to transponder 10. This figure shows an example of the shape of the excitation of antenna L1 signal to a transmission of a code 0101. The modulation commonly used is an amplitude modulation with a rate of 106 kilobits per second (one bit is transmitted in approximately 9.5 microseconds) much smaller than the frequency (e.g., 13.56 MHz) of the carrier from the oscillator 5 (period of approximately 74 nanoseconds). The amplitude modulation is performed either in all or nothing or with a modulation rate (defined as the difference of the peak amplitudes (a, b) between the two states (0 and 1) divided by the sum of these amplitudes) of less than unity because of the need for supply of transponder 10. in the example of Figure 2, the carrier at 13.56 MHz is modulated with a flow rate of 106 kilobits per second with an amplitude modulation rate tm, for example, 10%.

Figure 3 illustrates a classic example of data transmission from transponder 10 to terminal 1. This figure shows an example of the shape of the signal V_T control of the transistor T, supplied by the modulator 19, and the corresponding signal Rx received by the terminal 1. On the transponder side, the modulation is generally of resistive type with a so-called carrier subcarrier, for example, 847.5 kHz ( period of about 1.18 ms). The modulation is, for example, based on a BPSK type of encoding (binary encoding by phase shift keying) with a flow rate of around 106 kilobits per second well below the frequency of the subcarrier. In Figure 3, the signal Rx has been shown "smooth", that is to say without showing the undulations of the high frequency carrier (13.56 MHz). In the example of FIG 3, it was considered that each of the three bits represented was different from the previous bit. Thus, there is a transmission of a 010 code.

Whatever type of modulation or back modulation used (eg, amplitude, phase, frequency) and whatever the data encoding (NRZ, NRZI, Manchester, ASK, BPSK, etc.), modulation is performed digitally, by jumping between two binary levels.

As illustrated in Figure 3, the signal V_T consists of a pulse train to the frequency of the subcarrier, a phase jump occurring at each change of state of one bit to the next bit.

If several transponders are in the field of the same terminal, different communications can be initiated between each transponder and the terminal read-write. Most often, the transponders transmit identifiers that allow the terminal to individualize messages to their respective destinations.

In the sense terminal to the transponder, the transponders determine whether their messages are intended respectively from their identifier contained in the message, they detect after demodulation.

However, it can be a problem when several transponders simultaneously transmit to the same terminal, so that they are within the scope of this terminal. Such conflicts may be poorly detected by the terminal read-write, which is detrimental to system reliability.

Furthermore, in some applications it may be desired that the transponders exchange information. In such a case the terminal that mediates communication by receiving information from a transponder to relay them to another by having the previously demodulated and then remodulated.

EP-A-1136940 discloses a smart card wherein there are provided means for detecting the presence of a signal from another card. This doccument does not provide another card This document does not provide interpretation decode signals.

The document "Radio frequency power and signal interface." ISO / IEC 14443-2 / IDENTIFICATION CARDS-CONTACTLESS INTEGRATED CIRCUIT (S) CARDS - PROXIMITY CARDS: March 26, 1999, describes a standard that applies for example to the present invention.

The present invention aims to propose a new solution exchange of information between two transponders in the field of a terminal read-write.

The invention also aims at providing a solution that requires no modification of existing read-write terminals.

The invention also aims at providing a solution that would resolve conflict problems when two transponders are present in the field of a terminal read-write with which they communicate

To achieve these objects and other, the present invention provides an electromagnetic transponder intended to draw the energy necessary for its operation from a field radiated by a transmitting terminal to a carrier at a radiated by a transmitting terminal a carrier at a first frequency remote supply and rétromoduler the received signal at the rate of a subcarrier at a second frequency lower than the first, and comprising means capable of demodulating and decoding signals modulated by said sub-carrier .

According to one embodiment of the present invention, the transponder comprises an oscillating circuit upstream of a rectifying means own to provide a DC voltage of a power supply, electronic circuitry the electronic circuit means for transmitting digitally-coded data , and the transponder comprising own undémodulateur differentiating information received at the rate of the subcarrier retromodulation of another transponder with respect to information received, at the rate of a third still lower frequency, from the read-write terminal .

According to one embodiment of the present invention, said demodulator comprises two parallel branches each having a filter respectively centered on the second and third frequencies, each filter being associated with a digital decoder.

According to one embodiment of the present invention, a first decoder associated with the filter centered on the back-modulation frequency is a decoder of phase shift type, a second decoder associated with the third frequency being a decoder type by amplitude jump .

The invention also provides contactless and wireless communication system between at least two electromagnetic transponders no independent power supply, each transponder including means adapted to collect the energy needed to power its circuits, field electronics at a first frequency remote supply radiated by at least one read-write terminal, and means for demodulating and decoding signals transmitted by another transponder in modulation of a subcarrier at a second frequency.

According to one embodiment of the present invention, each transponder comprises separate demodulators and decoders dedicated signal receiving respectively transmitted by another transponder and to the reception of signals transmitted by the terminal to read-write.

According to one embodiment of the present invention, the first frequency is at 13.56 MHz, the second frequency being 847.5 kHz, and the third frequency being 106.5 kHz.

These objects, features and advantages, and others of the present invention will be discussed in detail in the following description of specific embodiments in non-limiting in connection with the accompanying drawings: <ul><li>Figures 1 to 3 described above are intended to show the state of the art and the problem encountered;</li><li>4 shows, very schematically, a communication between transponder system according to one embodiment of the present invention;</li><li>5 shows a first embodiment of an electromagnetic transponder according to the present invention; and</li><li>6 shows a second embodiment of a transponder according to the invention.</li></ul>

The same elements have been designated by the same references in the different figures. For reasons of clarity, only the elements which are necessary for understanding the invention have been shown in the drawings and will be described subsequently. In particular, the creation and coding of messages with different protocols have not been detailed and are not the subject of the present invention.

A feature of the present invention is to provide a direct communication between two electromagnetic transponders present in the field of a terminal read-write from which they derive their power.

Figure 4 illustrates very schematically and in block form, an embodiment of a communication system according to the present invention.

As before, a drive 1 (STA) generates by means of an antenna inductance L1 an electromagnetic field at a frequency corresponding to one transponder remote supply carrier 10 (T1, T2). Each transponder itself comprises an antenna (inductances L21, L22) for detecting electromagnetic radiation from the terminal to derive a power of its circuit.

The transponders 10 are, according to the invention, able not only to communicate with the terminal 1 but also directly with each other as illustrated by two-way arrows in Figure 4.

When two transponders are within the scope of the same terminal, and a transponder transmits information back modulation, this affects the electromagnetic field available for the other transponder. It may therefore be able to detect information transmitted subject to be capable of demodulating or decoding them.

According to a first embodiment of the invention, the same demodulator for receiving information from the terminal (for example, at a frequency of 106 kHz) and information from another transponder back modulation (e.g. at a frequency of 847.5 kHz). In this case, it is expected downstream of the transponder receiver, several decoders depending on the type of information to detect. It is these decoders that differentiate transmissions 106 kilobits transmissions at the frequency of 847.5 kHz.

5 shows, in block form and very schematically, a transponder according to the first embodiment of the invention.

It is found constituted by the inductance L2 in parallel resonant circuit with a capacitor C2 between terminals 11 and 12 input of a bridge rectifier 13 (full wave here). The output terminals 14 and 15 of the bridge 13 provide adjusted supply voltage across a storage capacitor and smoothing Ca. There is also the controller 16 for feeding the different circuits of the transponder and in particular the demodulator 28, of the modulator 19 and the logic circuits 17 to control and which are constituted either a microprocessor or a circuit in wired logic. 5 illustrates a variant of the back-modulation circuit in which the transistor T of modulation is only placed in parallel on the capacitor Ca, its series resistance in the on state resistance component retromodulation.

For simplicity, all components have not been shown in Figure 5. In particular, the clock is, of course, if necessary, present. Similarly, the various circuits supply connections have not all been indicated.

According to the invention, the output of demodulator 28 is sent to two decoders 25 and 26 (DEC1 and DEC2) for decoding each one of the types of signals that could be received, namely those received at the rate of 106 kilobits per a second terminal 1 and those received at the rate of 847.5 kHz from another transponder.

The demodulator 28 specific to the invention comprises an analog head 29 (ADH) providing the envelope of the received signal and driven by the frequency of 13.56 MHz. This envelope is either modulated in time with the carrier retromodulation 847.5 kilohertz, or the rate of transmission from the (106 kHz). As a result, each decoder DEC1 and DEC2 is capable of detecting variations in levels according to whether they are at the rate of 106 kHz or at a rate of 847.5 kHz. The respective outputs of decoders provide the demodulated signals respectively from the terminal or from another transponder in the field and in close coupling with the transponder concerned.

6 shows a second embodiment of a decoder 30 according to the invention.

According to this embodiment, the output 14 of the rectifier bridge is connected to the respective inputs of two filters 31 and 32 respectively centered on the frequencies of 847.5 kHz and 106 kHz, for example, corresponding to the frequencies back modulation and modulation of the thick headed. The respective outputs of filters 31 and 32 therefore restore the demodulated digital signals only if the received signal includes information at the corresponding frequency. The filter 31 is followed by a decoder 33 of BPSK type whose output provides a bit stream to the circuit 17. The filter 32 is associated with a decoder 34 of ASK type whose output provides a decoded bit stream to the circuit 17.

The embodiment of Figure 6 takes advantage of the fact that in the transponder by electromagnetic transmission systems, the transmission from the reader to a transponder is generally performed by an ASK modulation (amplitude jump) at a frequency of 106 kHz while transmission from a transponder to a reader (or for the invention to another transponder) is effected by a phase shift keying (BPSK) with a 847 kHz modulation frequency. transponder side of the modulation types can therefore distinguish for decoding both types of signals.

It will be appreciated of course that, in both cases, that is 13.56 MHz carrier which is modulated by the modulation reader to transponder and transponder to reader.

An advantage of the present invention is that it requires no modification of the transponders as regards the modulation part. The invention requires only a modification of the demodulation part of the transponders to be able to interpret information from another transponder in the field of a terminal, close enough to a mutual coupling between two transponders.

Another advantage of the invention is that by allowing direct communication between two transponders, we can resolve any conflict problems when several transponders are present in the field of the same terminal. Indeed, from the moment a transponder directly captures information from another transponder thanks to the invention, one can provide for this prohibition to issue the same time so that the time management communications transponders to a single terminal is managed directly by the transponders.

Of course, the present invention is capable of various variants and modifications that occur to those skilled in the art. In particular, the practical embodiment of a demodulator for the implementation of the invention is within the scope of those skilled in the art based on the functional indications given above and depending on the desired scope of coupling between two transponders neighbors.

Note that in a simplified embodiment, it is possible by means of a single demodulator, to differentiate messages by analyzing the transmitted code. However, such an embodiment is reserved in case of transponders with microprocessor significant computing and storage of transmitted bits. Indeed, to be able to differentiate by analysis of the received code must be able to store sufficient code length and have an efficient calculation tool. The advantage of the preferred embodiment of the invention of filtering by means of two distinct analog heads of the demodulator avoids these computing elements and additional storage and makes the invention applicable in transponders using wired logic circuits interpreting transmissions.

Among the applications of this invention, especially smart cards will signal without contact (for example, identification cards for access control, electronic purse cards, store cards information on the owner of the card, the consumer loyalty cards, toll television cards, etc.), and playback systems or reading and / or writing of these cards (for example, terminals or access control portals the vending product, computer terminals, telephone terminals, televisions or satellite decoders, etc.). In this application, the invention may allow, for example, charging a transport card from a card electronic purse by coupling the last two in the same field (the reader then serving no more that 'to generate a magnetic field of feeding these cards). The corresponding safety devices can then remain in the cards, which improves the reliability of systems against hackers. Direct communication between two cards may also allow communication priority detection when they are in the field of the same terminal.