WO2008114175A2

WO2008114175A2 - Rfid device, rfid system and equalization process in rfid systems

Info

Publication number: WO2008114175A2
Application number: PCT/IB2008/050917
Authority: WO
Inventors: Harald Witschnig; Johannes Bruckbauer
Original assignee: Nxp B.V.
Priority date: 2007-03-22
Filing date: 2008-03-13
Publication date: 2008-09-25
Also published as: WO2008114175A3

Abstract

In an RFID system an RFID device (2) comprises a device air interface (C2) with a predefined quality factor (Q2) for receiving wireless data signals (DS) being sent by a remote RFID transponder (1) comprising a transponder air interface (C1) with a predefined quality factor (Q1), and data signal processing means (4) for processing the data signal (DS) received by the device air interface (C2).Data signal equalization means (3) are arranged between the device air interface (C2) and the data signal processing means (4), wherein the data signal equalization means (3) are adapted to compensate signal distortions of the data signals (DS) caused by the quality factors (Q2, Q1) of the device air interface (C2) and the transponder air interface (C1) of the RFID device (2) and the RFID transponder(1), respectively.

Description

RFID device, RFID system and equalization process in RFID systems

FIELD OF THE INVENTION

The invention relates to an RFID device comprising a device air interface with a predefined quality factor for receiving wireless data signals being sent by a remote RFID transponder comprising a transponder air interface with a predefined quality factor, and data signal processing means for processing the data signal received by the device air interface. The invention further relates to an RFID system comprising an RFID device and at least one RFID transponder.

The invention further relates to a process for equalizing distortions of wirelessly transmitted data signals in an RFID system comprising an RFID device with a device air interface for receiving wireless data signals and at least one RFID transponder with a transponder air interface.

BACKGROUND OF THE INVENTION

In a conventional high frequency RFID system comprising at least one RFID reader and multiple RFID transponders (e.g. operating at 13,56 MHz, data transmission from the RFID transponders to the RFID reader via load modulation), a preferably high quality factor of the transponders of the RFID system is aimed to receive electric energy with a high energy level, which electric energy is transmitted from the reader to the transponders. However, disadvantageously, a very high quality factor of the transponders has a negative influence on the whole RFID system insofar, as it makes it difficult to achieve a very high data rate between the transponders and the reader. The reason for this behavior of the RFID system is, that increasing the quality factor, or in other words reducing the frequency band width in respect of a given center frequency results in longer swing-out transients of various oscillating circuits employed in the RFID system. This behavior is shown in the charts of Figures IA to ID depicting the impulse response E of data signals over time t at an air interface between the reader and the transponders. Fig. IA shows a low data rate at a high quality factor. It will be appreciated that the envelope Env of the data signal is rather broad, indicating a long swing-out transient. Fig. IB shows a low data rate at a low quality factor resulting in a considerably shortened envelope Env of the data signal providing enough head room between the consecutive data signals for increasing the data rate as is shown in Fig. 1C. However, trying to increase the data rate at the high quality factor of Fig. IA inevitably results in intersymbol interferences Col as depicted in the chart of Fig. ID. As a consequence, in known RFID systems a compromise between an extent of the quality factor and an intended data rate between the transponder and the reader has to be made. Further, national and international standards limit both the theoretically available frequency bandwidths and the energy levels of signals being transmitted in the RFID systems, thereby barring a possible solution of this dilemma between quality factors and data rates in RFID systems.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to provide an RFID device of the type defined in the opening paragraph, an RFID system of the type defined in the second paragraph and a process for equalizing distortions of wirelessly transmitted data signals in an RFID system of the type defined in the third paragraph, in which the disadvantages defined above are avoided.

In order to achieve the object defined above, an RFID device according to the invention can be characterized in the way defined below:

An RFID device comprising a device air interface with a predefined quality factor for receiving wireless data signals being sent by a remote RFID transponder comprising a transponder air interface with a predefined quality factor, and data signal processing means for processing the data signal received by the device air interface, wherein data signal equalization means are arranged between the device air interface and the data signal processing means, wherein the data signal equalization means are adapted to compensate signal distortions of the data signals caused by the quality factors of the device air interface and the transponder air interface of the RFID device and the RFID transponder, respectively. In order to achieve the object defined above, an RFID system according to the invention comprises an RFID device according to the invention and at least one RFID transponder.

In order to achieve the object defined above, with a process for equalizing distortions of wirelessly transmitted data signals in an RFID system according to the invention characteristic features are provided so that an equalization process according to the invention can be characterized in the way defined below, that is:

A process for equalizing distortions of wirelessly transmitted data signals in an RFID system comprising an RFID device with a device air interface for receiving wireless data signals and at least one RFID transponder with a transponder air interface, wherein the equalization process comprises determining a quality factor of the device air interface of the RFID device, determining a quality factor of the transponder air interface of the RFID transponder, determining an equalization function being adapted to compensate signal distortions of the data signals received at the device air interface caused by the quality factors of the transponder air interface and the device air interface and applying the equalization function to the received data signals.

The characteristic features according to the invention provide the advantage that in an RFID system with the inventive data signal equalization for a given quality factor of the RFID transponder the bandwidth limitation for data transmission can be better compensated for than with known systems. This results in higher achievable data rates compared to known systems.

Very good results can be expected for the RFID device according to the invention by configuring the data signal equalization means with a filter with inverse channel characteristics, e.g. having a transfer function following a Zero Forcing (ZF) Criterion, in respect of the transfer function of the air interfaces of the RFID transponder and the RFID device and optionally of an air transmission path between said air interfaces.

In an alternative approach of the RFID device according to the invention, the data signal equalization means are configured with a transfer function following a Minimum Mean Square Error (MMSE) Criterion in respect of the transfer function of the air interfaces of the RFID transponder and the RFID device and optionally of the air transmission path. Compared with the Zero Forcing Criterion, the present embodiment of the invention yields reduced noise for low frequency signals.

In an easy to implement embodiment of the invention, the data signal equalization means comprise an analog/digital converter and a digital filter. This embodiment provides comparably low complexity in respect of circuit design. Good filtering results can be achieved by designing the digital filter as a Finite Impulse Response filter.

Alternatively, the data signal equalization means may comprise an analog filter with inverse channel characteristics. The analog filter may be configured as an active or passive filter. For instance, with an active or passive analog filter having a specific bandpass characteristic the criterion of inverse channel characteristic can be met. In one embodiment of the invention, the inductance necessary for a bandpass filter may be implemented by a coil or a winding at the antenna itself. At the moment it is preferred to configure the RFID device according to the invention either as an RFID reader or an NFC device. However, with very low power consumption A/D converters, the present solution may also be applied to RFID transponders.

The aspects defined above and further aspects of the invention are apparent from the exemplary embodiment to be described hereinafter and are explained with reference to this exemplary embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail hereinafter with reference to an exemplary embodiment. However, the invention is not limited to this exemplary embodiment.

Figures IA to ID show graphs of the impulse response E of data signals over time t at an air interface between the reader and the transponders.

Fig. 2 shows a schematic block diagram of an RFID system according to the invention.

Fig. 3 shows Bode diagrams of the data signal transmission path and various equalizing criterions.

Fig. 4 shows signal timing diagrams of an undistorted data signal, a data signal distorted due to the quality factors of air interfaces and a data signal restored according to the invention, respectively.

DESCRIPTION OF EMBODIMENTS

An implementation of the RFID system according to the invention is now explained with reference to the schematic block diagram of Fig. 2. The RFID system comprises at least one RFID transponder 1 and an RFID device 2. In this embodiment the RFID transponder 1 is configured as a passive RFID transponder, also called tag, being wirelessly powered by a high frequency electromagnetic field that is generated by the RFID device 2. The electromagnetic field has carrier signal waves CS having a given frequency, e.g. 13,56 MHz. It should be emphasized that the term "electromagnetic field" as used herein comprises electric, magnetic and mixed electromagnetic fields depending on the frequency of the field. In the 13,56 MHz range the magnetic field in the near field is prevailing, whereas in UHF systems between 800 and 900 MHz a mixed electromagnetic field is propagating. The RFID transponder 1 comprises a transponder air interface Cl being implemented as a coil which is adapted to receive the electromagnetic field. In order to receive as much energy from the electromagnetic field as possible, the transponder air interface Cl has a relatively high quality factor Ql adjusted to the frequency of the carrier signal CS. The RFID transponder 1 is further adapted to transmit data signals DS via the transponder air interface Cl by means of load modulating the carrier signals CS of the received electromagnetic field. A typical signal sequence of the data signals DS appearing at the transponder air interface Cl is shown in line A of the signal timing diagram of Fig. 4. It will be appreciated that it reveals a typical clean load modulation signal sequence.

The RFID device 2 is configured as an RFID reader. It comprises a device air interface C2 with a predefined high quality factor Q2 for both transmitting the high frequency electromagnetic field and receiving wireless data signals DS from the remote RFID transponder 1. It should be noted, that in the present embodiment the data rate within the data signals DS is set to a high level that goes beyond present standards for 13,56 MHz RFID systems. The results of the high data rate in combination with the high quality factors Ql, Q2 of the transponder air interface Cl and the device air interface C2 can be seen in line B of the signal timing diagram of Fig. 4 which reveals the signals received by the device air interface C2. It will be appreciated, that the signals are completely distorted so that the data comprised therein are not any longer recognizable as load modulated data bits. However, the present invention copes with the problem of such distorted data signals DS by providing data signal equalization means 3 arranged between the device air interface C2 of the RFID device 2 and data signal processing means 4, wherein the data signal equalization means 3 are adapted to compensate signal distortions of the data signals DS caused by the quality factors Q2, Ql of the device air interface C2 and the transponder air interface Cl of the RFID device 2 and the RFID transponder 1 , respectively. It should be observed that the data signal processing means 4 are well known to those skilled in the art and therefore need no further explanation. According to the present idea, energy transmission and the data transmission rate are at least to some extent decoupled from each other.

The positive impact of equalizing the distorted data signal DS according to the invention can be seen from the timing diagram in line C of Fig. 4. It will be appreciated that the load modulated data bits within the signal sequence have been restored to such an extent that they can easily be processed by the data signal processing means 4.

The quality factors Ql, Q2 of the transponder air interface Cl and the device air interface C2 contribute to an overall quality factor Q of a signal transmission path that also includes an air transmission path 7 between the two air interfaces Cl, C2. The resulting transfer function H(f) is shown in the Bode diagram of Fig. 3. It will be recognized that this transfer function H(f) is responsible for the heavy signal distortions to the data signal DS. According to the invention, the data signal equalization means 3 follow an equalization criterion that provides equalization of the data signals DS that have been distorted due to the high quality factors Ql, Q2 of the air interfaces Cl, C2. In a first approach, the data signal equalization means 3 have a transfer function E(f) following a Zero Forcing (ZF) Criterion in respect of the transfer function H(f) of the air interfaces Cl, C2 of the RFID transponder 1 and the RFID device 2 and optionally of the air transmission path 7. Applying a Zero Forcing (ZF) Criterion means that the equalizing transfer function E(f) is calculated as:

E(f) = l / H(f)

The equalizing transfer function E(f) is depicted in the Bode diagram of Fig. 3. In theory, it fully linearizes the transfer function H(f). However, in practice applying the Zero Forcing (ZF) Criterion may cause signal noise at lower signal frequencies due to the high signal gain at said lower frequencies. Hence, it may be advantageous to apply a different equalization criterion, e.g. a Minimum Mean Square Error (MMSE) Criterion, resulting in a transfer function E'(f) of the data signal equalization means 3 that is also shown in the Bode diagram of Fig. 3. Calculating the Minimum Mean Square Error (MMSE) Criterion is well known to those skilled in the art. The result is a signal that has a slight distortion over the entire frequency bandwidth but with a very low noise portion of the signal.

In the present embodiment, the data signal equalization means 3 comprise an analog/digital converter 5 and a digital filter 6, wherein the digital filter 6 is designed as a Finite Impulse Response FIR filter. However, such a filter design is not mandatory and other filter designs may also be applicable, for instance an analog filter with an inverse channel characteristics.

It should be observed that although in the present embodiment of the invention the RFID device 2 has been configured as an RFID reader it is nevertheless also be configurable as an NFC device and even as an RFID transponder, provided that very low power consuming A/D converters become available. Finally, it should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. In a device claim enumerating several means, several of these means may be embodied by one and the same item of software or hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. An RFID device (2) comprising a device air interface (C2) with a predefined quality factor (Q2) for receiving wireless data signals (DS) being sent by a remote RFID transponder (1) comprising a transponder air interface (Cl) with a predefined quality factor (Ql), and data signal processing means (4) for processing the data signal (DS) received by the device air interface (C2), wherein data signal equalization means (3) are arranged between the device air interface (C2) and the data signal processing means (4), wherein the data signal equalization means (3) are adapted to compensate signal distortions of the data signals (DS) caused by the quality factors (Q2, Ql) of the device air interface (C2) and the transponder air interface (Cl) of the RFID device (2) and the RFID transponder (1), respectively.

2. The RFID device as claimed in claim 1, wherein the data signal equalization means (3) comprise a filter with inverse channel characteristics, e.g. having a transfer function (E(f)) following a Zero Forcing (ZF) Criterion, in respect of the transfer function (H(f)) of the air interfaces of the RFID transponder and the RFID device and optionally of an air transmission path (7) between said air interfaces.

3. The RFID device as claimed in claim 1, wherein the data signal equalization means (3) have a transfer function (E'(f)) following a Minimum Mean Square Error (MMSE) Criterion in respect of the transfer function (H(f)) of the air interfaces of the RFID transponder and the RFID device and optionally of the air transmission path (7).

4. The RFID device as claimed in claim 1, wherein the data signal equalization means (3) comprise an analog/digital converter (5) and a digital filter (6).

5. The RFID device as claimed in claim 4, wherein the digital filter (6) is designed as a Finite Impulse Response (FIR) filter.

6. The RFID device as claimed in claim 1, wherein the data signal equalization means (3) comprise an analog filter with an inverse channel characteristics.

7. The RFID device as claimed in claim 1, being configured either as an RFID reader, an NFC device, or an RFID transponder.

8. An RFID system comprising an RFID device (2) as claimed in any of claims 1 to 7 and at least one RFID transponder (1).

9. A process for equalizing distortions of wirelessly transmitted data signals in an

RFID system comprising an RFID device (2) with a device air interface (C2) for receiving wireless data signals (DS) and at least one RFID transponder (1) with a transponder air interface (Cl), wherein the equalization process comprises determining a quality factor (Q2) of the device air interface (C2) of the RFID device (2), determining a quality factor (Ql) of the transponder air interface (Cl) of the RFID transponder (1), determining an equalization function being adapted to compensate signal distortions of the data signals (DS) received at the device air interface (C2) caused by the quality factors (Ql, Q2) of the transponder air interface (Cl) and the device air interface (C2) and applying the equalization function to the received data signals (DS).

10. The equalization process as claimed in claim 9, wherein the equalization function has an inverse channel characteristics, e.g. is determined to have a transfer function E(f) following a Zero Forcing (ZF) Criterion, in respect of the transfer function (H(f)) of the air interfaces of the RFID transponder and the RFID device and optionally of an air transmission path (7).

11. The equalization process as claimed in claim 9, wherein the equalization function is determined to have a transfer function (E'(f)) following a Minimum Mean Square Error (MMSE) Criterion in respect of the transfer function (H(f)) of the air interfaces of the RFID transponder and the RFID device and optionally of the air transmission path (7).

12. The equalization process as claimed in claim 9, wherein the equalization process is carried out by use of analog/digital conversion and digital filtering of the data signals (DS).

13. The equalization process as claimed in claim 9, wherein the equalization process is carried out by use of an analog filter having an inverse channel characteristics.