System and method for communicating with multiple transponders

[0002] The present invention relates to transponder systems and more particularly to systems that contain several transponders closed to each other.

[0003] Transponder arrangements are used to identify and interrogate data in a contactless manner. Transponder systems typically include an interrogator that transmits radio frequency pulses and a plurality of transponders that receive the pulses and respond with data in the form of a modulated radio frequency carrier. However, a problem arises when multiple transponders units are present within the field of an interrogation unit and responding simultaneously as shown in FIG. 1. In this case either none or only the transponder with the strongest field strength is detected. Accordingly, what is needed is a system and method for detecting the signal of multiple transponders. The present invention addresses such a need.

[0004] The system and method in accordance with the present invention solves the problem of several transponders in the same reading field with a minimum of effort on the transponder side. This is important because in most of applications the transponder is the cost critical item. The arbitration among the transponders is solved by the fact that the reader unit sends a burst interrogation signal instead of a continuous field. The end of burst signal is detected by each transponder, and is used to launch a random delay generator. At the end of the delay, the transponder with the shortest delay sends a short signal to the reader that is interpreted as a Request To Send signal from the transponder. The reader responds with another burst that causes this transponder to send the main data and the other transponders to stop the transmission. After the first transponder has finished the data transmission, it goes to the sleep mode. The whole process is repeated until there are no more transponders left in the field. If the random delay generated in two or more transponders is the same, they start the transmission of the Request To Send signal at the same time. This collision is detected at the reader, which ignores this signal and repeats the process.

[0005] FIG. 1 illustrates multiple transponders in the same interrogation field.

[0006] FIG. 2 illustrates the principle of the multi-transponder protocol with the interrogation bursts of the reader and the example of two transponders in the reader field.

[0007] FIG. 3 illustrates the principle of the multi-transponder protocol, collision case.

[0008] FIG. 4 illustrates state-diagram for the multi-transponder protocol, reader side.

[0009] FIG. 5 illustrates state-diagram for the multi-transponder protocol, transponder side.

[0010] FIG. 6 illustrates the block diagram of the hardware of the reader and the transponder required for the multi-transponder protocol.

[0011] The present invention relates to transponder systems and more particularly to systems that contain several transponders close to each other. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest possible scope consistent with the principles and features described herein.

[0012] The principal method of interrogation of several transponders in the same reader field is shown in FIG. 2 for the simplified case of only 2 transponders. The reader transmits field burst in a periodical manner. Assuming the transponders enter the reader field at time t1, the End Of Burst signal (EOB) is detected in the transponders at time t2 and is used to trigger a Random Delay (RD) generator, which causes a delay RD1 for the transponder 1 and delay RD2 for the transponder 2. After the delay, the transponder sends a Request To Send (RTS) signal back to the reader, which consists of a defined pattern plus a random value. Assuming RD2>RD1, the transponder 1 will send the RTS signal first. As the RTS signal is detected in the reader at time t3, it forces immediately another field gap in the current field burst. The beginning of the next burst is interpreted in the transponder 1 as a Clear To Send (CTS) signal, which causes the transponder 1 to respond with the main data. However, the CTS signal is interpreted in the transponder 2 as a stop signal, which causes the transponder 2 to not to transmit the RTS signal, which the transponder would otherwise send at time RD2. The transponder 1 accomplishes now the transmission of the main data, until receiving another End Of Burst signal from the reader at time t4. After t4, transponder 1 goes into a sleep mode, and only the transponder 2 will be active in the field. Hence after another arbitration phase (t4-t5) the data from the transponder 2 is transmitted to the reader.

[0013] FIG. 3 shows the collision case, when the random delay generators of the transponders generate the same delay (RD1=RD2) after the EOB signal at t2. In this case the reader because of the collision of the signals can detect no valid RTS signal. The collision detection at the reader is ensured because there is a random pattern encoded in the RTS signal. Since no valid RTS signal is detected, the reader continues the periodic field burst transmission until the random delays generated in the transponders are different, i.e. at t3. At this time the protocol follows the same way as in FIG. 2 until both transponders are detected.

[0014] The described method is primarily suitable for passive transponders, which take their required energy from the interrogation field. In this case, the gap for generating the End Of Burst signal must be so short that the transponder can be supplied by a storage capacitor on the transponder. However, the described protocol can also be used for active transponders, which contain an active power source.

[0015] FIGS. 4 and 5 describe possible state diagrams of the reader and transponders to realize the described protocol. FIG. 6 shows the block diagram of the hardware of the reader and the transponder required for the multi-tag protocol.

[0016] Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.