FIELD

This invention relates to radio frequency identification (RFID) and to apparatus and methods therefor. BACKGROUND

RFID is a wireless data capturing technology that uses radio frequency (RF) waves for extracting encoded data from remotely placed tags. RFID systems have two main elements, the RFID tag, where data is encoded, and the RFID reader, which is used for extracting the encoded data from the tags. "Tag" refers to a device in which data is encoded and places no limitation on the physical size and shape of the device.

"Active" RFID tags incorporate a battery whereas "passive" RFID tags take their energy from an interrogation signal.

An RFID tag, like a barcode, can be used to identify and characterise an item to which it is attached. At least preferred forms of RFID have numerous advantages over the barcode, including a long reading range, non-line-of-sight reading, and automated identification and tracking.

RFID tags are considered unsuitable for low-cost applications because of their higher price compared to the barcode. The cost of the widely-used passive tags is largely attributable to their Application Specific Integrated Circuit (ASIC). Printable chipless RFID tags are a lower cost option, however, the removal of the chip from the tag makes it inflexible for the encoding of higher numbers of bits within a small tag, and various existing designs require perfect alignment with the reader antennas. The various aspects of the invention aim to provide improvements in and for RFID, or at least to provide alternatives for those concerned with RFID.

It is not admitted that any of the information in this patent specification is common general knowledge, or that the person skilled in the art could be reasonably expected to ascertain or understand it, regard it as relevant or combine it in any way at the priority date.

SUMMARY

A first aspect of the invention provides a reader for reading a chipless RFID tag including a dual polarised antenna arrangement for receiving from the tag a backscattered signal having a first component and a second component, wherein the first component has a polarisation transverse to a polarisation of the second component; and a logic arrangement configured to identify, within each of the first component and the second component, frequencies respectively corresponding to frequencies of a common predetermined set of frequencies to obtain two bits for each frequency of the set. The term "logic arrangement" as used herein refers to any physical arrangement capable of manipulating information, such as a computer Embodiments of the logic arrangement include, but are not limited to, conventional microprocessors and application specific integrated circuits.

Another aspect of the invention provides a reader for reading a chipless RFID tag including an antenna arrangement for receiving from the tag a backscattered signal; and a logic arrangement configured to identify respective frequencies within the backscattered signal; and compensate for an inclination of the tag based on a comparison of at least one of the identified frequencies to at least one reference frequency.

The compensating preferably includes shifting each of one or more of the identified frequencies a respective amount based on the comparison and on the identified frequency. The comparison may be in substance a subtraction.

According to either aspect, the identification may include Fourier analysis, FFT, DFT or other spectral transform.

Another aspect of the invention provides a device for interrogating a chipless RFID tag including the reader of one of the foregoing aspects of the invention and a transmitter for transmitting a radio frequency signal to cause resonators of the tag to resonate to spectrally shape the backscattered signal.

Another aspect of the invention provides a chipless RFID tag including a first set of resonators for resonating in response to a received radio frequency signal to create respective frequencies of a first component of a backscattered signal; and a second set of resonators for resonating in response to the received radio frequency signal to create respective frequencies of a second component of the backscattered signal; wherein the first component has a polarisation transverse to a polarisation of the second component; the backscattered signal is analysable to identify the respective frequencies.

Optionally, the resonators of the first set of resonators may be configured to create selected frequencies of a predetermined set of frequencies; and the resonators of the second set of resonators are configured to create other selected frequencies of the predetermined set of frequencies; such that two bits are conveyed for each frequency of the predetermined set.

Alternatively, the first set of resonators and the second set of resonators may be configured to create the same selected frequencies of a predetermined set of frequencies to allow the tag to be read at various orientations, in which case the first set of resonators and the second set of resonators may be configured to create the same at least one frequency to provide a reference to compensate for an inclination of the tag.

Preferably each of the sets of resonators includes at least two subsets and within each set of resonators the subsets are spaced; and resonators having adjacent frequencies are located in separate subsets for reduced mutual coupling.

Another aspect of the invention provides a chipless RFID tag including at least two subsets of resonators wherein the subsets are spaced and resonators having adjacent frequencies are located in separate subsets for reduced mutual coupling.

According to the foregoing aspects, the resonators are preferably slots, which slots may each substantially consist of a straight line. By way of example, the tag may be a printed tag. Another aspect of the invention provides a method of reading an RFID tag including transmitting a radio frequency signal to cause resonators of the tag to spectrally shape a first component of a backscattered signal; and further resonators of the tag to spectrally shape a second component of the backscattered signal; wherein the first component has a polarisation transverse to a polarisation of the second component; and receiving the backscattered signal; and analysing the backscattered signal to identify respective frequencies.

The analysing may include respectively identifying frequencies in each of the first component and the second component. Alternatively, the analysing may include compensating for an inclination of the tag based on a comparison of at least one of the identified frequencies to at least one reference frequency, in which case the

compensating preferably includes shifting each of one or more of the identified frequencies a respective amount based on the comparison and on the identified frequency.

In yet another alternative, the method may include comparing at least one of the identified frequencies to at least one reference frequency to determine an orientation of the tag. Accordingly, another aspect of the invention provides a method of determining the orientation of an object including reading an RFID tag wherein the tag is carried by the object.

Another aspect of the invention provides a method of reducing mutual coupling in an RFID system including locating resonators having adjacent frequencies in mutually spaced resonator subsets. The method may include attaching two or more tags to an item, each tag carrying a respective one of the subsets.

BRIEF DESCRIPTION OF DRAWINGS

The figures show various examples related to the invention. Figure 1 is an elevation of an RFID tag; Figure 2 schematically illustrates an RFID system; Figure 3 is an elevation of another RFID tag; Figure 4a is an elevation of an RFID tag; Figure 4b is a graph of the backscattered signal from the tag of Figure 4a; Figure 5a is an elevation of a tag;

Figure 5b is a graph of the backscattered signal from the tag of Figure 5a; Figure 6a is an elevation of a tag;

Figure 6b is a graph of the backscattered signal from the tag of Figure 6a; Figure 7a is an elevation of a tag at a first orientation;

Figure 7b is an elevation of the tag of Figure 7a at a second orientation;

Figure 7c is a graph of the backscattered signals from the tag of Figures 7a and 7b;

Figure 8a is a graph of a shift in resonant frequencies derived from Figure 7c; and

Figure 8b is a normalised variant of the graph of Figure 8a. DESCRIPTION OF EMBODIMENTS

Figure 1 shows an RFID tag consisting of a square of conductive material, in the form of conductive ink, on a vertical planar surface. Preferred forms of the tag are formed by printing. Notably the tag 1 is chipless (i.e. does not include an integrated circuit nor any active elements such as transistors). Tag 1 includes two sets 2, 3 of resonators in the form of simple, straight slots. The resonator slots of set 2 run horizontally and are thus configured to resonate in response to a vertically polarised portion of an interrogation signal. The set 2 is divided into two subsets 2a, 2b. Subsets 2a, 2b are spaced apart, on opposite sides of the tag 1. The length of each slot determines the frequency at which that slot resonates. In tag 1 , subset 2a is a mirror image of subset 2b. Alternatively, slots of differing frequency may be distributed between the subsets 2a, 2b to reduce mutual coupling as in the example of Figure 3 (discussed below). The set 3 likewise includes two subsets 3a, 3b of resonators spaced on opposite sides of the tag 1. In this variant of the invention the slots are arranged so that the shorter slots are positioned towards the centre of the tag and the ends of the slots are spaced from notional diagonals of the square.

Figure 2 illustrates an exemplary RFID system. The system includes the tag 1 and an interrogation system 4. The interrogation system 4 includes a reader 5 and a transmitter 6 which would typically be located within a common housing (not shown).

The reader 5 includes a logic arrangement 5a and an antenna arrangement in the form of a dual polarised antenna 5b. Alternatively, separate polarised antennas may be included. The transmitter 6 includes a dual polarised antenna 6a and logic arrangements integrated with the logic arrangements 5a. In operation, the transmitter 6 outputs an orthogonally polarized interrogation signal including on both polarisations a substantially constant amplitude across an ultra-wide bandwidth (UWB).

In response to the interrogation signal, the resonator slots resonate at their respective resonant frequencies and in doing so absorb energy from the interrogation signal at those resonant frequencies. Thus the tag 1 is a passive tag in which the slots are directly energised by the interrogation signal. Energy at frequencies other than the resonant frequencies of the slot is generally not appreciably absorbed by the tag 1 and is at least partially reflected to the reader as a backscattered signal. The slots serve to 'spectrally shape' the backscattered signal. The resonant frequencies of the slots appear as local minima, or 'notches', in the spectrum of the backscattered signal.

The tag 1 is a frequency domain tag. In the described variant of the invention the

Reader 5 receives the backscattered signal via the antenna 5b and analyses the backscattered signal for the presence of spectral notches at each of a predetermined set of frequencies. Other variants of the invention may look for other features of the backscattered signal to identify respective frequencies therein. By way of example, it is contemplated that respective frequencies may be identifiable by local maxima, or local extrema generally.

Each resonant slot in tag 1 corresponds to a respective frequency and polarisation, whereby the presence or absence of a respective slot results in the presence or absence of a spectral notch at that frequency in the backscattered signal. By selectively forming the tag with or without a slot corresponding to a given frequency, the tag can convey a single bit of information which will be carried by the backscattered signal. In tag 1 , each slot of the set 2 has the same length as a corresponding slot (or "potential slot") in the set 3 whereby each slot and its corresponding slot resonate at the same frequency but at different polarisations, and in turn for each frequency two bits of information may be conveyed. The reader 5 applies a Fourier analysis to identify whether spectral notches are present at the predefined frequencies in the spectrum of the backscattered signal. Figures 4 to 6 illustrate simulated results of Fourier analysis. The downward vertices of the graphs, or "notches", indicate the presence of a frequency in the backscattered signal. Notionally the presence of a notch is assigned the binary value of 1. The absence of an expected vertex is assigned the binary value of 0. Figure 4 illustrates a tag in which all of the slots are present (i.e. wherein the tag is encoded to deliver all 1 's) and the backscattered signal therefrom. In this graph the polarisations are plotted on respective axes. Figures 5a and 5b relate to a tag in which three vertically polarised (i.e. horizontally extending) slots have been omitted to encode three zeros. Three spectral notches are absent from Figure 5b, enabling the reader to correctly read the 01 1 1 1 1 1 1 1 1011 1101 code of this subset of the tag. Similarly, Figure 6 relates to a patch encoded with three zeros in the horizontal polarisation.

As an alternative to being encoded to convey two bits of information for each frequency, the tag 1 may be encoded so that the vertically and horizontally polarised slots resonate at the same frequencies (i.e. each slot of the set 2 has a slot of corresponding length in the set 3). Each subset of slots thus presents the same code, but at differing polarity, improving the prospects that such a tag can be read at a variety of orientations.

Figure 7 illustrates the backscattered signal from such a tag encoded with all 1 's when the tag is upright (Figure 7a) and when the tag is inclined to 40° from vertical (Figure 7b). When the tag is rotated, the spectral notches in the lower trace in Figure 7 are spectrally shifted so that each appears at a lower frequency (i.e. the notches shift to the left from the top trace to the bottom trace in Figure 7). As charted in Figure 8a, this shift varies approximately in proportion to frequency. By dividing the shift value by the frequency in the backscattered signal from the inclined tag, a "normalised shift" value can be calculated. The normalised shift value is a dimensionless quantity. As shown in Figure 8, for a given inclination the normalised shift value is approximately constant across the frequencies (i.e. across the different bits).

According to a preferred form of the invention, all tags used in the system are encoded with a common reference bit at a known frequency. The reference bit may be separated in frequency from the other bits for ready identification of any spectral "shift" in the reference frequency in the backscattered signal from the tag. By subtracting the identified shifted reference frequency from the reference frequency, a shift value can be calculated. This calculated shift value may then be divided by the shifted reference value to calculate a normalised shift value. In turn, the other frequencies identified in the backscattered signal may be multiplied by the calculated normalised shift value to determine respective shift values for each of the identified frequencies which may be added to the identified frequencies to compensate for the inclination of the tag. Also, if knowledge of the tag orientation is of interest, the shift value, or normalised shift value, may be compared to a chart relating shift values to inclinations such as Figure 8b (or equivalent data in an alternative format, e.g. in a lookup table) to determine the angle of inclination, and thus the orientation of an object to which a tag is attached.

Thus a preferred form of the tag is linearly polarised but rotation independent, is single sided, has higher data density than various existing printable chipless tags, and can be used in banknotes and ID cards, etc. It is contemplated that tags may be printed directly onto packaging for item level tagging. For the purposes of orientation independence or determining orientation, the reader antenna does not need to be linearly polarised.

Figure 3 illustrates an alternate exemplary RFID tag V in which the resonator subsets 2a', 2b', 3a', 3b' of sets 2', 3' are respective rectangular patches of conductive ink printed in a square array on a square common substrate. The resonators take the form of nested U-shaped slots. Each set 2', 3' of resonators includes a series of 1 to N resonators. Through the series the resonators reduce in length and correspondingly increase in resonant frequency. The resonators of the series are alternately placed between the subsets so that adjacent frequencies are placed in separate patches for reduced mutual coupling. By way of example, odd-numbered resonators 1 , 3, 5, etc are placed in patches 2a', 3a' and even-numbered resonators 2, 4, 6, etc are placed in patches 2b', 3b'. "Mutual coupling" refers to a slot resonating to produce a backscattered signal at its frequency in response to the resonance of another slot.

It is also contemplated that the patches 2a', 2b', 3a', 3b' might be separately applied to an object to be tagged, although this is not a preferred form of the invention.