WO2007012891A2

WO2007012891A2 - A radio frequency identification interrogator

Info

Publication number: WO2007012891A2
Application number: PCT/GB2006/050198
Authority: WO
Inventors: John Domokos
Original assignee: Siemens Aktiengesellschaft
Priority date: 2005-07-25
Filing date: 2006-07-11
Publication date: 2007-02-01
Also published as: WO2007012891A3

Abstract

A radio frequency identification interrogator comprises a transmitter (27, 28, 29) and a receiver (32). The transmitter transmits an interrogation signal and the receiver receives a backscattered signal from one or more tags. The transmitter includes a modulator (28) which further modulates signalling information from a signalling channel onto a continuous wave signal (27). The modulated signalling channel signals are orthogonal to reader to tag and tag to reader data transmissions.

Description

A RADIO FREQUENCY IDENTIFICATION INTERROGATOR

This invention relates to a radio frequency identification (RFID) interrogator and associated method for operating an interrogation cycle.

In passive RFID systems, tags are energized by a continuous wave (CW) radio frequency (RF) field which is transmitted via an antenna of a reader.

The interrogation cycle starts with a short CW signal to energize the tags located in the field of the reader. This is followed by a modulated CW signal transmitted on frequency fl that contains a message transmitted from the reader to the tag(s). After this message, the reader continues to transmit CW to supply energy to the tag(s). This CW signal is backscatter modulated by the tag(s) and a tag response is sent on a frequency, fio, the modulation containing the tag to reader message, which is then demodulated within a certain bandwidth. During an interrogation cycle several such messages are exchanged between the reader and the tag(s). Once the interrogation is completed, the transmitter is usually turned off until the next interrogation is required.

In channelised systems, several readers operate asynchronously at different frequencies. Prior to transmission in any given channel, the reader first listens to the channel to check if the channel is free. If the channel is free, the reader starts transmission. Should the channel be occupied, the reader then scans the other channels until an empty channel is found. This mode is called listen before talk or scanning mode. The purpose of the listen mode is to ensure that once in a given channel an interrogation cycle started, that session is not corrupted by other readers.

The RF collisions in a multi-channel system can be very costly. For example, it is quite possible that towards the end of a long interrogation cycle , involving several tags, the whole process is destroyed and the information is corrupted because of an unwanted transmission by another reader. For this reason the standards set very a strict criterion, that is, a very low listen mode threshold, before transmission can be allowed to commence in that channel. In table 1 the ETSI EN302 208- 1 listen mode thresholds are shown.

Table 1

The values in table 1 are intended to ensure the integrity of systems operational in three different power classes, i.e. in 10OmW, in 50OmW and in 2W categories. These levels are checked at the channel centre and at +/-75 KHz offsets from the channel centre to ensure that the whole channel is empty.

A problem with setting such stringent thresholds based upon measuring the power of the CW signal and/or other emissions in the channel is that frequently noise and interference exceed the thresholds when there is no CW signal present, so that channel is excluded from consideration when it could have quite validly been used. The ETSI criteria are based on power measurements only. This means that genuine reader to transmitter communication cannot be distinguished from noise and other interference (as illustrated in Figs. 1 and 2) and the channel could have been used. In accordance with a first aspect of the present invention, a radio frequency identification interrogator comprises a transmitter and a receiver; wherein the transmitter transmits an interrogation signal; wherein the receiver leceives a backscattered signal from one or more tags; wherein the transmitter includes a modulator and wherein the modulator further modulates signalling information from a signalling channel onto a continuous wave signal; wherein the modulated signalling channel signals are orthogonal to reader to tag and tag to reader data transmissions.

In addition to normal modulation which is backscattered by the tag, the transmitter modulates signalling information onto a CW signal to provide a unique identifier.

Preferably, the modulated signalling channel signals include an identifier of the interrogator to be transmitted with the carrier signal sent to interrogate one or more radio frequency identification tags. Preferably, the signalling information is modulated as a frequency modulation or phase modulation signal.

Preferably, the signalling information is modulated as an upper sideband of a single sideband modulator. Preferably, the signalling information is modulated and coded in a robust manner in order to be distinguished from noise and other interference.

In accordance with a second aspect of the present invention, a method of operating an interrogation cycle of an interrogator comprises listening for emissions on a channel at a first frequency; determining a first constraint, whether emissions on the channel exceed a first threshold; determining a second constraint, whether the emissions include an identifier; and refraining from transmitting from the interrogator, only if both constraints are met.

Preferably , a second threshold is set; and wherein, if emissions exceed the second threshold, the interrogator moves to another channel, without transmitting on the first channel.

Preferably, the emissions comprise one or more of background noise, interference and a continuous wave carrier radio frequency signal.

Preferably, the identifier is modulated onto an interrogator carrier frequency. Preferably, the carrier modulation is a spread spectrum modulation. Alternatively, the identifier is modulated onto transmitted data signals.

Preferably, the interrogator is a radio frequency identification interrogator. In accordance with a third aspect of the present invention, a radio frequency identification interrogator carrier signal modulated with an interrogator identifier.

Preferably, the identifier identifies a class of interrogators. In accordance with a fourth aspect of the present invention, a method of discriminating between tag readings by location comprises determining an identity of a tag response and the identity of a reader; selecting the highest power response amongst a plurality of readers; allocating the tag to the reader with the higher power response and deleting the tag response from any other reader inventory. An example of the method of the present invertion will now be described with reference to the accompanying drawings in which:

Figure 1 is a single shot measurement of an 865 MHz spectrum measured at a dipole; Figure 2 is a measurement over a 10 second interval of an 865 MHz spectrum measured at a dipole; and,

Figure 3 is an example of an analogue implementation of an interrogator for carrying out the method of the present invention; Figure 4 is an example of a typical RFID system installation in three bays;

Figure 5 is an example of an RFID reader according to the present invention with AM/FM modulation;

Figure 6 is and example of an RFID reader according to the present invention with SSB implementation; Figure 7 is and example of a reader waveform in DIE mode, SSB modulation; and,

Figure 8 is an example of RF Spectra in DIE mode from a reader according to the present invention.

Passive RFID systems operate in the 860 MHz to960 MHz unlicensed industrial, scientific and medical (ISM) radio bands. In these bands there is a lot of interference present, which is illustrated in Figs. 1 and figure 2. The plots show the spectrum in a 100 KHz bandwidth at the output of a dipole antenna which is located in a typical industrial environment. In this environment there are no tag readers operating at the time of these measurements.

Fig. 1 shows a single shot measurement. The bottom trace 1 is the noise floor, which is the level that would be measured in an interference free environment, such as a screened anechoic chamber and the top trace 2 is the level of interference in the environment. Fig. 1 illustrates that the -96dBm threshold for example is exceeded at three frequencies at any instant by the general interference in the environment, even though there are no interrogator transmissions present at that time.

Fig. 2 shows a peak hold measurement over a period of 10 seconds. This illustrates that the snapshot in Fig. 1 is not a unique occurrence and in fact over a 10 second period almost all channels exhibit interference levels 2 exceeding the required listen mode threshold of -96 dBm.

Listen before talk (LBT) operation is based on power measurements of the CW signal at the channel centre and at +/-75 KHz offsets. Figs. 1 and 2 illustrate that in a typical environment, due to the interference present there, an interrogation system operating in this way will frequently skip through channels in which, actually, there are no interrogations taking place at the time. On the other hand, a sensitive receiver is necessary to ensure that an ongoing interrogation cycle is never blocked. The present invention provide s a method in which adequate sensitivity and robust decision making is achieved even in the presence of interference and noise.

The present invention uses a robust signalling channel and coding scheme which enhances the LBT mode, so that transmissions from the RFID system can be distinguished from other sources of interference and noise. A key aspect of the present invention is that the CW signal of the interrogator is uniquely identified in the system by a signature, or interrogator identifier. To apply the signature to the CW signal, in the example described below with respect to Fig.3, the carrier in the interrogator is phase or frequency modulated with the signalling information. There is no conflict with tag modulation applied to the carrier signal because backscatter has different properties, specifically, it is orthogonal to the modes of modulation used to apply the identifier. It is also possible to apply the modulation to transmit data within a digital signal processor (DSP), not shown, rather than to the CW carrier signal Furthermore, although generally, it is sufficient to identify a signal as being an interrogator transmission, or not, spread spectrum modulation can be provided to uniquely identify the carrier, or transmit data, with such a signature. If a carrier, or transmit data, has a unique digital signature, the test receiver can look for the special format of the signature, recognise channels where the signature is present and reuse the channel when appropriate e.g. for dense mode.

Figure 3 shows an example for an analogue implementation A transmitter 4 transmits from an antenna 5 a signal from a voltage controlled oscillator 6, or synthesiser, which has been modulated with a low frequency signal, S(t) during CW tag to reader transmission and during reader to tag transmission, for which a switch 7 is closed In a receiver 8, a signal received at antenna 9 is combined with a local oscillator (LO) signal 10 which also carries the modulation which carries the signature and the backscatter and therefore phase modulated (PM) or frequency modulated (FM) sidebands are stripped off during backscatter reception, hence the modulation does not affect the normal operation of the reader. A baseband signal 11 is input to a complex correlator 12, together with a signature signal ?m(t) 13 from a signature generator 14. The correlated signal is input to a band limiting filter 15 to limit the bandwidth before passed through to a power detector 16. The filter separates the signature from the data, that is separates the product of the correlation from the other signals present. The output from this is applied to a decision algorithm 17, together with a channel power measurement 18 which has also been passed through a power detector 19, in order to make a decision on whether, or not to skip 20 the channel in question.

By contrast, during listen before talk mode, the switch 3 is open, so that modulation is not applied to the LO. The incoming PM or FM sidebands are correlated locally, filtered and applied to the decision algorithm together with the channel power measurement. In a typical system, the rate of the PM or FM modulation is slow enough to ensure that spectral components fall below the spectral components of the data. However, the rate is fast enough to provide enough information for the correlation within a window of a few milliseconds. The modulating frequency for this is typically in the order of 10Hz to 1 KHz. The criteria, for whether, or not to transmit on a channel in listen mode is based on two conditions. Firstly, does the detected CW power exceed a threshold and secondly does the detected CW, or transmit data, signal carry a signature, i.e. the appropriate modulation format.

This method allow s distinctions between readers, which are legitimately using a channel and interference which does not carry the signature and therefore is not harmonized with the system or is not an authorised user of that channel. In this sense, the improved system provides interference detection whilst also maintaining reliable channel sharing between members of a system. An example of such interference detection is described below. A channel is always assigned initially for interrogation, even if the channel power exceeds the threshold limits of table 1, provided that the detected signal does not carry the signature modulation. This means that in the presence of interference, attempts are made to use that channel.

In the case where the interrogation is unsuccessful, due to the magnitude and persistence of the interference, then after one or more attempts , a new channel is assigned, according to the parameters of a decision algorithm. The issue of failure due to excessive interference can be addressed by applying a further feature of a second, higher threshold. Above this threshold level, the channel is skipped even if the signal does not carry the expected signature because it is assumed that the interference level is too high in that channelfor successful communication.

In a different application, readers in a system are specifically designed for frequency re-use within that system. This means that they can transmit on the same channel simultaneously providing that they are members of the same system. In this case, it is preferable to allocate the same channel within that system and leave other channels free for other systems. By contrast, the purpose of the signature in this application is to re-allocate the same channel to all members of the same system and share the frequency band on a system to system basis. An example for this is the Gen 2 RFED system operating in dense- interrogatory-environment (DIE) in Europe

In addition to modulating just a static signature, it is possible to use modulation for creating a high speed signalling channel that is orthogonal to the other spectral components within the RFID system. Two ways for implementing orthogonal signalling in the current RFID systems operating in dense -interrogatory-environment (DEE) and in normal mode are described below.

The operation of the LBT mode is mandatory in Europe. As mentioned above, the purpose of the LBT mode is to ensure that readers do not interfere with each other and with other devices using the same frequency and this mode is specified in the ETSl EN 302-208. In LBT mode, the reader first listens to a channel for a period of 5mS to 1OmS.

If the channel is empty the reader can start transmitting on that channel for up to 4 seconds. At the end of the 4 second period the reader must release the channel for at least a lOOmS period before it can resume the transmission on that channel. If the channel is occupied, the reader must search for the nearest empty channel and again may use the next channel for no longer than 4 seconds. This ensures that the readers do not block each other and also they share the frequency resource equally. Readers that operate in DIE mode may transmit at the same frequency at the same time without interfering with each other.

DEE mode is implemented in Gen 2 tags as described in the EPCglobal protocol. In this mode, the reader configures the tags to transmit the backscattered signal in the adjacent channels. This is achieved by a sub-carrier, the Miller carrier, which chops the return signal within the tag(s). Given that the downlink and the uplink frequencies are different, half -duplex operation is achieved and therefore the same frequency can be reused indefinitely.

LBT mode requires special attention in dense environment because in this mode several readers (which are configured for DIE mode) may transmit at the same frequency simultaneously without interfering with each other. However, DIE readers must also use LBT to ensure that they do not block other neighbouring RFID systems or other devices which are not configured for frequency re-use or operate on a different channel. Consequently, it is necessary to identify the readers that are configured for DIE, so that frequency re -use is allowed in that case. Furthermore, the ETSI protocol requires that after 4 seconds of transmission the channel is released for other systems. This means that a system comprising DIE readers should behave like a single reader operating in basic LBT mode.

A solution to this problem has been proposed by CISC in which a recognition pattern is transmitted by the "master reader" every 200ms. DIE readers recognising the recognition pattern (RecPattern) become "slave readers" and the slave readers transmit at the same frequency for a period of time up to the overall 4 second time limit applicable for the whole system. The modulation format for the RecPattern is the same as that of the reader to tag data modulation which means that the period during which data is sent is shared with the time that is used for RecPattern broadcasting. The implication of the proposed system is that some overhead is needed for the RecPattern and the algorithm response is semi real-time. This means that some of the 4 second timeslot is wasted. Furthermore, it has been recognised that the RecPattern may confuse the tags and other short range devices (SRD).

Another technical problem is the reading of unwanted tags that are located in adjacent reading bays or portals. This is mainly due to the large variation of tag sensitivity and variation of signal levels caused by tag orientation and/or multipath propagation. In Fig.4, a typical installation is shown for 3 portals operating next to each other. Three pallets, A, B , C are moved through bays 21, 22, 23 each having readers 24, 25, 26 If the reader 25 in bay 22 reads some stronger tags, in pallet A, as well as tags in its own bay, then some tags are read by both, readeis 24, 25 leading to confusion about their actual location. This still remains an unsolved problem in RFID systems. To minimise the chance of unwanted readings, proposals have been made to screen the portals and line the surrounding walls w ith absorbing materials reduce the number of incidents for duplicate readings , to reduce antenna beam widths, but this has other disadvantages. Setting readers in each portal A, B, C to be synchronised, so that they transmit at the same time, the number of unwanted readings is reduced even further.

Readers operating in DIE mode transmit on the same frequency. However, due to the tolerance of the local oscillators, the tags see a beat note at the rate of the frequency difference between the readers. This causes a reduction in reliability in the reader to tag communication, resulting in lower reading speed. Screening and lining the portals may improve reliability, but this is expensive and does not entirely eliminate the problem.

Thus, the present invention provides a further improvement in addition to providing an identifier, which is that a high speed signalling channel is created which is orthogonal to both the reader to tag transmissions and the tag to reader backscatter. Orthogonality in this sense means that the signalling channel continuously exists alongside the data channel without interfering with the data transmissions and without occupying additional spectrum outside the allocated channel. The high speed signalling channel in this example provides a way to synchronise, or phase lock, the adjacent readers, thus eliminating the beat note.

In prior art systems, non- orthogonal transmissions are used, for example, the RecPattern is transmitted using the same modulation format as the data and is therefore transmitted in a semi-real time fashion. By contrast, due to the orthogonality in the present invention, the signalling channel is a real time medium that is continuously available for transmissions.

The orthogonal arrangement also means that the capacity of the signalling channel can be high, similar to that of the data capacity. This opens the way for new applications in which system reliability is improved and the unwanted readings are reduced. Furthermore, systems which are using a broadband signalling scheme do not interfere with other Gen2 systems that can not operate in this mode.

Specific examples of orthogonal modulation in RFID systems are described below. Fig.5 is a block diagram of a typical reader operating in double sideband amplitude modulation (AM) mode. This modulation format is used in most RFID systems for transmitting data from the reader to the tag. The additional signalling of the present is arranged by frequency modulation (FM), or phase modulation (PM) of the carrie r. The reader comprises a CW source, a modulator 28, transmitter 29 and transmit antenna 30. On the receive side, a receive antenna 31 feeds into a receiver 32, which also receives a signal from the CW source and the signal is demodulated through a listen before talk demodulator 33, a low pass filter 34 and signature (FM or PM) demodulator 35, or a high pass filter 36 and data (AM) demodulator according to the type of signal received. All the demodulated signals are input for processing by a control algorithm 38. The receiver in Fig. 5 operates as an ordinary RFID receiver, in that the backscattered signal arriving from the tag is high pass filtered 36 and AM demodulated 37. The functionality of the LBT demodulator 33 is unchanged, in that it measures the overall power received in the channel, but in addition to the LBT receiver, there is now a low pass filter 34 and an FM (or PM) demodulator 35, which is specifically configured to demodulate the signalling information. The low pass filter in this sense fulfils the same purpose as the filter 15 in Fig. 3, i.e. to separate the signalling channel (or signature) from the data.

In the downlink, the phase of the FM (or PM) sidebands is orthogonal to the AM sidebands and therefore it will not have any effect on the tag's operation. This means that the signalling can be continuously transmitted alongside the AM reader to tag data using FM (or PM) modulation. In the uplink, the phase of the back scattered signal and the phase of the modulated CW that carries the signalling information from another reader is arbitrary. This means that the phase of AM sidebands from the tag and the FM (or PM) sidebands from the reader(s) are not orthogonal. However, the response from the tag is spectrally separated from the signalling information and the two signals can therefore be separated with the high pass low pass arrangement as shown in Fig.5.

In Fig.6, an implementation is shown using single side band (SSB) modulation as specified in the EPCglobal Gen 2 standard, dense mode. As before, a CW source 27, transmitter 29 and antenna 30 are provided, but the modulator is an SSB modulator 39 in this case. The data to tag messages are modulated on to the carrier in the lower side band (LSB) and the transmitter reference time interval for a data 0 in interrogator-to-tag signalling (TARI) is 25 μS. The Miller carrier for the back-scatter is 160 KHz, m=4 case enabled in the tag. The upper side band (USB) is used to carry the signalling information for the DIE mode. The signalling information is first modulated onto a 60 KHz sub -carrier. In this example BPSK modulation is used at a rate of 40 Kb/S.

In the receiver, in addition to the receiver 32, LSB data demodulator 40and LBT demodulator 33, there is a demodulator 41 for the USB of the SSB modulation. This is configured to decode the reader to reader signalling information. The USB demodulator is locked on to the 60 KHz sub-carrier. The spectrum of the sub-carrier is clearly separated from other components and therefore can easily be filtered out in the same way as filter 15 operates in Fig. 3. The sub-carrier modulates the RF carrier at a low modulation index as shown in Fig. 7 which is a graph of amplitude of the transmitted signal in millivolts against time in milliseconds. This produces only a small amount of ripple on the transmit waveform that will not effect the tag. Fig.8 shows the spectra as amplitude in dB against frequency in KHz, for the downlink (receiver to tag data 43) , for the uplink (tag to receiver back scatter 44) and for the signalling 42, illustrating the change in spectral density with frequency.

The spectrum 42 of the signalling channel is located within the guard band. This is an unused space between the modulated transmitter spectrum 43 and the backscattered spectrum 44. The signalling spectrum fits within the spectral mask 45 of the EPCglobal Gen2 standard and therefore the system is compatible with the standard DIE operation.

The broadband wireless signalling channel has many applications. It is particularly advantageous for hand held and mobile (fork lift mounted) readers where a wired arrangement is not practical. A flexible network, very similar to that of a wireless local area network (WLAN), could be set up on an ad-hoc basis between readers operating close by.

The signalling channel has a number of benefits. Tag location identification is a very desirable feature for future systems and a method based on time difference of arrival measurements and triangulation techniques between different receivers can have significantly improved cross correlation and therefore accuracy, if all readers sharing the same frequency are synchronised, that is their CW sources are coherent. The signalling channel facilitates this by providing means for the phase locking of the local oscillators of the readers. Because of the high bandwidth of the signalling channel, the phase coherence of the readers is synchronised to a high accuracy, which is essential for accurate tag location.

Co-channel interference is a problem in DIE systems. The interference is caused by the frequency tolerance between the close- by readers working simultaneously. A beat note at the rate of the frequency difference appears at the tags location which greatly reduces the ability of the tag(s) to decode the data correctly. This problem is completely eliminated when the readers are phase locked as the tags see a constant CW power. The elimination of the beat note also reduces the unwanted readings to a large extent. In a similar way to the RecPattern concept, arbitration between DIE mode and normal mode can be achieved but by using the signalling channel of the present invention The advantage is that the algorithm runs in real time and therefore the system responds quicker to changing circumstances, such as handover between master and slave readers. This means that the time slot of 4 seconds is fully utilised. Furthermore, given that the modulation is orthogonal to the normal data modulation, there is no chance for confusing the tags of other systems (e.g. other SRD devices) having unrelated / unrecognisable commands.

Unwanted readings occur in neighbouring bays or in situations where a mobile reader approaches a fixed installation, like a gate or a portal. In such circumstances, to exclude all duplicate readings, the signalling channel facilitates a hand-over algorithm between readers similar to that used in cellular systems. The elimination of unwanted readings using the signalling channel requires the steps of identifying nearby readers by measuring the their transmitted power , requesting inventories from nearby readers which have been identified, the nearby readers report the tag IDs in their field and their power leveland inventory lists from nearby readers are compared with the reader's own inventory, power levels of duplicate IDs are compared and duplicate IDs which have lower received power are removed.

The identification of the nearby readers is an important step and it requires the orthogonal signalling channel. Without this step the inventories would have to be taken over the whole site. This means that unnecessary transmissions would be required and a large amount of data processed to remove the unwanted readings.

There are applications in which two or more gate readers operate in close proximity. For example a lorry, equipped with a gate reader, is approaching a loading bay which is also equipped with a reader system. As the goods pass over from one system to another, both systems are taking inventory. In this or similar mobile applications the wireless signalling channel offers significant advantages for synchronisation, data transfer and arbitration between two or more adjacent systems.

Claims

1. A radio frequency identification interrogator comprising a transmitter and a receiver; wherein the transmitter transmits an interrogation signal; wherein the receiver receives a backscattered signal from one or more tags; wherein the transmitter includes a modulator and wherein the modulator further modulates signalling information from a signalling channel onto a continuous wave signal; wherein the modulated signalling channel signals are orthogonal to reader to tag and tag to reader data transmissions.

2. An interrogator according to claim 1, wherein the modulated signalling channel signals include an identifier of the interrogator to be transmitted with the carrier signal sent to interrogate one or more radio frequency identification tags.

3. An interrogator according to claim 1 or claim 2, wherein the signalling information is modulated as a frequency modulation or phase modulation signal.

4. An interrogator according to claim 1 or claim 2, wherein the signalling information is modulated as an upper sideband of a single sideband modulator.

5. An interrogator according to any preceding claim, wherein the signalling information is modulated and coded in a robust manner in order to be distinguished from noise and other interference.

6. A method of operating an interrogation cycle of an interrogator , the method comprising listening for emissions on a channel at a first frequency, determining a first constraint, whether emissions on the channel exceed a first threshold; determining a second constraint, whether the emissions include an identifier; and refraining from transmitting from the interrogator, only if both constraints are met.

7. A method according to claim 6, wherein a second threshold is set; and wherein, if emissions exceed the second threshold, the interrogator moves to another channel, without transmitting on the first channel.

8. A method according to claim 6 or claim 7, wherein the emissions comprise one or more of background noise, interference and a continuous wave carrier radio frequency signal.

9. A method according to any of claims 6 to 8, wherein the identifier is modulated onto an interrogator carrier frequency.

10. A method according to any of claims 6 to 8, wherein the carrier modulation is a spread spectrum modulation.

11. A method according to any of claims 6 to 8, wherein the identifier is modulated onto transmitted data signals.

12. A method according to any of claims 6 to 11, wherein the interrogator is a radio frequency identification interrogator.

13. A radio frequency identification interrogator carrier signal modulated with an interrogator identifier.

14. An interrogator according to claim B, wherein the identifier identifies a class of interrogators.

15. A method of discriminating between tag readings by location, the method comprising determining an identity of a tag response and the identity of a reader; selecting the highest power response amongst a plurality of readers; allocating the tag to the reader with the higher power response and deleting the tag response from any other reader inventory.