WO2003088499A1

WO2003088499A1 - Method for coding rfid tags

Info

Publication number: WO2003088499A1
Application number: PCT/AU2003/000411
Authority: WO
Inventors: David Malcolm Hall
Original assignee: Tagsys Australia Pty Ltd
Priority date: 2002-04-05
Filing date: 2003-04-04
Publication date: 2003-10-23
Also published as: AUPS151202A0

Abstract

A method is disclosed for coding data communication between an interrogator and electronically coded tags. In the method the data is encoded by an active time period of sub-carrier modulation and an inactive time period of no modulation. In the method a characteristic of the modulation such as phase and/or frequency is varied during the active time period to increase a rate at which the data may be communicated. The method is adapted to minimize collisions between the tags arising from the effect of small signal suppression.

Description

METHOD FOR CODING RFID TAGS

FIELD OF THE INVENTION

The present invention relates to an object management system wherein information bearing electronically coded radio frequency identification (RFID) tags are attached to objects which are to be identified, sorted, controlled and/or audited. In particular the present invention relates to a method for coding RFID tags to minimize effects of small signal suppression in tagging systems incorporating such tags.

BACKGROUND OF THE INVENTION

The object management system of the present invention includes information passing between an interrogator which creates an electromagnetic interrogation field, and the electronically coded tags, which respond by issuing a reply signal that is detected by the interrogator, decoded and consequently supplied to other apparatus in the sorting, controlling or auditing process. The objects to which the tags are attached may be animate or inanimate. In some variants of the system the interrogation medium may be other than electromagnetic, such as optical and/or acoustic.

Under normal operation the tags may be passive, i.e. they may have no internal energy source and may obtain energy for their reply from the interrogation field, or they may be active and may contain an internal energy source, for example a battery. Such tags respond only when they are within or have recently passed through the interrogation field. The interrogation field may include functions such as signalling to an active tag when to commence a reply or series of replies or in the case of passive tags may provide energy, a portion of which may be used in constructing the reply.

One example of an electronic tag reading system is illustrated in Figure 1. In Figure 1 an interrogator 10, containing a transmitter and receiver, both operating under a controller, communicate via electromagnetic means with a code responding electronic tag 11.

A common problem in such systems is that an unknown plurality of tags may be simultaneously present in an interrogation field, and the process of communication between the interrogator and tag must be structured so that all tags present in the interrogation field are detected. A collision is loosely defined as an event in which two or more tags offer their reply simultaneously. Prior art literature has suggested methods or anti-collision protocols which use time- division, frequency-division, or a combination of both in which a diversity in a variable(s) is exploited by the tag in a random or pseudo-random manner to reduce the number of collisions. Inevitably, in a majority of practical protocols a collision will occur.

When a collision occurs it is desirable that tags that have collided are given another opportunity to reply. It is also desirable to use the protocols' diversity variable(s) such that previously colliding tags do not continue to collide. However, there is a problem which derives from the so called small signal suppression effect that most protocols ignore. The problem is common with frequency-modulated or phase-modulated reply signals generally employed in tag replies, and has the result that if two tags reply at the same time with significantly different signal strengths, the tag with the stronger signal will be correctly decoded by the interrogator while the tag with the weaker signal will be ignored, with the result that only one of the tags will be detected, while the other tag may not be offered the chance to reply again and may therefore be missed by the interrogator. This problem will exist whether the tags are differently coded or identically coded.

One proposed solution that is documented in current and emerging standards for tagging, makes use of a time-division approach (ie. time is the diversity variable) in which tags "randomly" choose a slot from a maximum number of slots in which to offer their reply. The latter solution addresses the effect of small signal suppression by using a tag to interrogator modulation scheme in which each bit is encoded by a total time period that includes a half time period of a single sub-carrier modulation and a half time period of no modulation. This type of scheme is sometimes referred to as sub-carrier Manchester encoding or On-Off encoding. In normal Manchester encoding a 0-bit may be encoded with a high voltage in a first active half period and a low voltage in a second inactive half period as shown in Fig. 2a, and a 1-bit may be encoded with a low voltage in the first inactive half period and a high voltage in the second active half period as shown in Fig. 2(b). In differential Manchester encoding a 0-bit may be encoded by making the first half of the signal equal to the last half of the previous bit's signal and a 1-bit may be encoded by making the first half of the signal opposite to the last half of the previous bit's signal. That is, a 1-bit may be encoded by a transition at the beginning of the bit's signal. Providing that two (or more) simultaneously replying tags vary in their reply by at least one bit, there will generally be a code violation detected by the interrogator due to the weaker reply tag modulating in the non-modulating or inactive time period of the stronger reply tag.

SUMMARY OF THE INVENTION

The present invention may provide a coding method that improves the rate at which data may be communicated between an interrogator and electronically coded tags when compared to conventional Manchester encoding schemes. The coding method of the present invention may increase the data rate by varying one or more characteristics of sub-carrier modulation cycles during each active half period. The characteristics of sub-carrier modulation cycles that may be varied include phase and/or frequency. In one embodiment the coding method may introduce n phases of sub-carrier such that n bits of data may be encoded in the time taken to encode one bit of data in a conventional coding scheme. Thus, where a conventional coding scheme may encode 1 out of 2 symbols in each clock cycle, the coding method of a first embodiment of the present invention may encode n out of 2n symbols in the same clock cycle. The term "clock cycle" denotes a bit rate in conventional Manchester coding schemes. It may also refer to a symbol rate in the method of the present invention. The n phases may be identified from a reference phase in a preamble, start of frame, or start of signalling waveform for normal encoding, or from a phase of the modulating part of a previous symbol for differential encoding. The coding method of the present invention may thus increase the data rate while maintaining robustness in identification in the face of the small signal suppression effect.

In another embodiment of the present invention, different sub-carrier frequencies may be used in lieu of or in conjunction with the n-phases of the sub-carrier. In this embodiment of the present invention the active half period of modulation may include p sub-carrier periods, where p is an integer. In a coding method wherein p may assume m different values or frequencies, m bits may be encoded in the time taken to encode one bit of data in a single frequency (and single phase) scheme. Thus, where a conventional coding scheme may encode 1 out of 2 symbols in each clock cycle, the coding method of the second embodiment of the present invention may encode m out of 2m symbols in the same clock cycle. In a third embodiment of the present invention the m frequencies of the second embodiment may be combined with the n phases of the first embodiment. Thus, if each of the m sub-carrier frequencies has n phases, m x n bits may be encoded in the time taken to encode one bit of data in a single frequency (and single phase) scheme. Thus where a conventional coding scheme may encode 1 out of 2 symbols in each clock cycle, the coding method of the third embodiment of the present invention may encode mn out of 2mn symbols in the same clock cycle.

According to the present invention there is provided a method for coding data communication between an interrogator and electronically coded tags, wherein said data is encoded by an active time period of sub-carrier modulation and an inactive time period of no modulation and wherein a characteristic of said modulation is varied during said active time period to increase a rate at which said data may be communicated.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings wherein: Fig. 1 shows an electronic tag reading system to which the coding method of the present invention may be applied;

Fig. 2(a) shows a prior art scheme for encoding a 0-bit;

Fig. 2(b) shows a prior art scheme for encoding a 1-bit;

Fig. 3 shows an improved scheme for encoding data;

Fig. 4 shows a method for encoding data according to one embodiment of the present invention; and

Fig. 5 shows a method for encoding data according to a further embodiment of the present invention.

Fig. 3 shows a modified Manchester encoding scheme in which the high voltage or active first half period in Fig. 2(a) and the high voltage or active second half period in Fig. 2(b) are each replaced with four sub-carrier cycles (ie. p = 4). This modified scheme has a data rate that is slower than the frequency of the sub-carrier and is preferable because it is less susceptible to noise.

Fig. 4 shows a method for encoding data according to one embodiment of the present invention. The coding method is^' similar to Fig. 3 but includes two phases (o, π) whereby four sub-carrier cycles in the first active half period may encode two different symbols. A first phase angle (o) may encode symbol 1 (eg. 00) as shown in Fig. 4(a) and a second phase angle (π) may encode symbol 3 (eg.10) as shown in Fig. 4(c). Similarly, four sub-carrier cycles in the second active half period may encode two different symbols. A first phase angle (o) may encode symbol 2 (eg. 10) as shown in Fig. 4(b) and a second phase angle (π) may encode symbol 4 (eg. 11 ) as shown in Fig. 4(d).

The number of phase angles (o, π) in Fig. 4 is 2 (ie. n = 2). Hence, the number of symbols that may be encoded is 2n = 4. Other phases intermediate angles (o) and (π) may be used to encode further symbols. The number of phases that may be adopted is limited only by the resolution of the carrier frequency, although in practical embodiment the number of phase angles that may be reliably resolved may be more limited. For example, in one practical embodiment the number of phase angles may be limited to about 16 (ie. n = 16).

Fig. 5 shows a method for encoding data according to a further embodiment of the present invention. The coding method is also similar to Fig. 3 but includes a frequency component whereby the sub-carrier cycles in the first active period may encode two different symbols. Four sub-carrier cycles may encode symbol 1 (eg. 000) as shown in Fig. 5(a) and three sub-carrier cycles may encode symbol 5 (eg. 100) as shown in Fig. 5(e).

The sub-carrier cycles in the active second period may also encode two different symbols. Four sub-carrier cycles may encode symbol 2 (eg. 001) as shown in Fig. 5(b) and three sub-carrier cycles may encode symbol 6 (eg.101 ) as shown in Fig. 5(f).

Fig. 5 also makes use of two phase components (o, π) whereby each active half period for each frequency (value of p) may encode two different symbols. Considering the active half periods wherein p = 4, a first phase angle (o) may encode symbol 1 (eg. 000) as shown in Fig. 5(a) and a second phase angle (π) may encode symbol 3 (eg. 010) as shown in Fig. 5(c). Phase angle (o) may also encode symbol 2 (eg. 001 ) as shown in Fig. 5(b) and phase angle (π) may also encode symbol 4 (eg. 011) as shown in Fig. 5(d). Considering the active half periods wherein p = 3, a first phase angle (o) may encode symbol 5 (eg. 100) as shown in Fig. 5(e) and a second phase angle (π) may encode symbol 7 (eg. 110) as shown in Fig. 5(g). Phase angle o may also encode symbol 6 (eg. 101) as shown in Fig. 5(f) and phase angle π may also encode symbol 8 (eg. 111 ) as shown in Fig. 5 (h). The number of phase angles (o, π) in Fig. 5 is 2 (ie. n = 2) and the number of frequencies (p = 3, 4) is also 2 (ie. m = 2). Hence, the number of symbols that may be encoded is 2mn = 8.

Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.

Claims

1. A method for coding data communication between an interrogator and electronically coded tags, wherein said data is encoded by an active time period of sub-carrier modulation and an inactive time period of no modulation and wherein a characteristic of said modulation is varied during said active time period to increase a rate at which said data may be communicated.

2. A method according to claim 1 wherein said characteristic includes n phases associated with said sub-carrier modulation for increasing the data rate by a factor of n, wherein n is an integer.

3. A method according to claim 2 wherein n is at least 2.

4. A method according to claim 2 or 3 wherein n is not greater than 16.

5. A method according to any one of claims 2 to 4 wherein each phase is identified from a reference phase in a preamble, start of frame or start of signalling waveform.

6. A method according to any one of the preceding claims wherein said characteristic includes m frequencies associated with said sub-carrier modulation for increasing the data rate by a factor of m, wherein m is an integer.

7. A method according to claim 6 wherein m is at least 2.

8. A method for coding data communication between an interrogator and electronically coded tags substantially as herein described with reference to Figure 4 or 5 of the accompanying drawings.

9. A method according to any one of the preceding claims, wherein said method is adapted to minimize collisions between said tags arising from an effect of small signal suppression.