Procedure for reading the data stored in a transponder and a transponder system for the execution of the procedure

For reading the data stored in a transponder by means of an interrogation device, the interrogation device at first receives the background noise for the purpose of detecting interference frequencies present in this background noise. On the basis of the interference frequencies acquired, coefficients for an adaptive filter are computed by means of which this filter may be tuned in such a way as to suppress the interference frequencies. The response signal from the transponder with the superimposed background noise is received by the interrogation device and routed through the adaptive filter which acts to suppress the interference frequencies. The signal available at the output of the filter can then be demodulated for the purpose of reading the data stored. The transponder system for the execution of the procedure comprises a digital signal processor which computes coefficients for an adaptive filter on the basis of the interference frequencies acquired, and tunes the filter in such a way that the interference frequencies within the RF response signal received from the transponder, carrying the superimposed background noise, are suppressed. The output signal from the adaptive filter may then be used for further processing.

BACKGROUND AND BRIEF DESCRIPTION OF THE PRIOR ART
 The invention relates to a procedure for reading the data stored in a
 transponder which derives its supply energy from an RF interrogation pulse
 sent to it by an interrogation device and which transmits the data stored
 in it as an RF response signal modulated by these data. It furthermore
 relates to a transponder system for the execution of this procedure.
 A transponder system is known from the EP 0 681 192 A2 which consists of
 two units, namely a transponder and an interrogation device. Data are
 stored in the transponder which may be read by means of the interrogation
 device. These data may, for example, serve to identify an object within
 which or to which the transponder is attached. The transmission of the
 data is in the form of RF signals, which means that reading the data and
 therefore, to give an example, the identification of the object to which
 the transponder is attached does not involve any physical contact.
 The transponder of the known transponder system is a batteryless
 transponder which derives its supply energy from an RF interrogation pulse
 which is sent by the interrogation device. This RF interrogation pulse is
 rectified in the transponder and used to charge an energy store which in
 turn provides the supply energy required by the transponder in order to
 transmit the data stored in it in the form of RF response signals.
 Due to this particularity it is obvious that the transmitting power of the
 transponder is rather low, which means that the range within which the
 data can still be read correctly by the interrogation device is limited.
 The greater the distance between the transponder and the interrogation
 device, the more feeble is the signal received by the retrieval device, so
 that, of necessity, interference frequencies receivable within the
 operating area of the transponder system tend to swamp the RF
 interrogation signal or at least components of this signal and, therefore,
 make a correct demodulation of the RF interrogation signal impossible.
 SUMMARY OF THE INVENTION
 It is the object of the invention to provide a procedure and a transponder
 system of the type previously indicated which, even in the presence of
 interference frequencies within the operating frequency range and also
 where the distance between the transponder and the retrieval device is
 relatively great, still enables the RF interrogation signals to be
 correctly demodulated.
 According to the invention, this requirement is satisfied in that in the
 interrogation device,
 a) within a time period during which it has not caused the transponder by
 sending an RF interrogation signal to transmit the RF response signal, the
 background noise within the frequency range reserved for the signal
 transmission between the transponder and the interrogation device is
 received, and the background noise received is converted into a digital
 signal,
 b) interference frequencies within the digital signal are acquired with
 which a signal with an amplitude exceeding a pre-determined threshold is
 received,
 c) coefficients for an adaptive filter are computed on the basis of the
 interference frequencies that enable the passband characteristic of the
 filter to be adapted so as to suppress the interference frequencies
 acquired,
 d) after the transmission of the RF interrogation pulse, an aggregate
 signal containing the RF response signal transmitted by the transponder
 and the superimposed background noise is received and converted into a
 digital signal,
 e) the RF signal received is passed through the adaptive filter, tuned by
 means of the computed coefficients and
 f) the filtered output signal from the adaptive filter is demodulated for
 the purpose of reading the data stored.
 By means of the procedure according to the invention, firstly, the
 necessary preconditions are created so that any interference frequencies
 in the RF signal received by the retrieval device within the application
 frequency range are suppressed, so that subsequently an RF signal, freed
 from these interference frequencies, becomes available for demodulation to
 extract the data contained in it.
 The transponder system according to the invention is characterised in that
 the interrogation device comprises the following:
 I. An A/D converter which converts all analogue signals received by the
 interrogation device into digital signals,
 II. an adaptive filter the passband characteristic of which is adjustable
 by means of filter coefficients,
 III. a digital signal processor (DSP) which controls the procedural
 sequence and computes and sets the coefficients of the adaptive filter for
 the suppression of the interference frequencies contained in the
 background noise, and
 IV. a demodulator which demodulates the signals filtered by the adaptive
 filter for the purpose of extracting the data transmitted by them.
 Further advantageous developments of the invention are specified in the
 sub-claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Before going into detail about the procedure to be described here,
 explanation will be made as to how the data transmission between an
 interrogation device and a batteryless transponder is realised. In the
 simplest case, a batteryless transponder may just store an identification
 code, such as a number consisting of several digits, which permits the
 definite identification of the transponder. This also makes it possible to
 make an unambiguous identification of objects to which the transponder is
 firmly attached. It is the purpose of the interrogation device to read
 this identification number stored in the transponder. Since the
 transponder does not contain its own energy source, the interrogation
 device firstly sends an RF interrogation pulse to initiate the reading
 process, which is used by the transponder to generate the supply energy
 required for the transmission of the identification number. This is
 normally realised in such a way that the RF interrogation pulse is
 rectified in the transponder and then used to charge a capacitor. In the
 case of a full-duplex system, it is also possible to maintain the
 capacitor continuously charged by the continuous transmission of a carrier
 signal. The charge voltage of the capacitor is then used as the supply
 voltage for the transponder.
 FIG. 1A represents an interrogation signal 10 in the form of the field
 strength s generated by it between the points in time t.sub.0 and t.sub.1.
 After a brief delay t, which is used by the transponder to check whether
 sufficient energy is already stored in order to send the data, the
 transponder re-transmits the RF response signal 12 in the time period
 between t.sub.2 and t.sub.3, where this signal is also indicated by its
 field strength in FIG. 1A. This operating mode, whereby the transmission
 of the interrogation pulse and the re-transmission of the RF response
 signal follow each other in time, is generally termed half-duplex
 operation.
 It is, however, equally possible and standard practice to continue sending
 the RF interrogation signal during the retransmission of the RF response
 signal by the transponder, so that both the RF interrogation signal and
 the RF response signal overlap each other in time within an area. It is
 obvious that the transmission of the RF interrogation pulse must start
 before the re-transmission of the response signal can commence, since the
 supply energy for the re-transmission must first be made available in the
 transponder. This is illustrated in FIG. 1B by means of the RF
 interrogation signal 10' and the RF response signal 12'. This operating
 mode is normally known as full-duplex operation.
 Let it be assumed that frequency shift keying modulation (FSK modulation)
 is used for the transmission of the data in the transponder system. This
 means that the H bits and the L bits are expressed each by different
 modulation frequencies. To give an example, the modulation frequency F1
 may be used for the L bit, and the modulation frequency F2 for the H bit.
 This means that, after modulation, apart from the carrier frequency F0,
 the frequencies F0-F1, F0-F2, F0+F1 and F0+F2 are also present within the
 spectrum of the RF response signal. This spectrum is illustrated in FIG.
 2A.
 In practical applications of such a transponder system, fixed interference
 frequencies are usually present. These interference frequencies may
 originate from commercial transmitting devices or also from insufficiently
 screened appliances or similar. These interference frequencies may have
 such a high field strength at the location of the interrogation device
 that it becomes impossible to read the data from the transponder
 correctly, as soon as a certain distance between the transponder and the
 interrogation device is exceeded, or the signal-to-noise ratio becomes
 excessive. As an example, FIG. 2B illustrates a background noise spectrum
 in which the interference frequencies FS1, FS2, FS3, FS4 and FS5 are
 present. FIG. 2C represents the entire frequency spectrum which is covered
 when both the background noise with the interference frequencies of FIG.
 2B as well as the RF response signal of FIG. 2A are present.
 With the aid of the diagram of FIG. 3 an explanation shall now be made as
 to how the data stored in the transponder can still be correctly read when
 there is a greater distance between the transponder and the interrogation
 device and even when interference frequencies are present.
 Before beginning the read process, which is initiated, as previously
 explained, by the transmission of an RF interrogation pulse, the
 interrogation device at first receives the background noise. In this way,
 interference frequencies can be determined which are present within the
 frequency range of the data transmission between the interrogation device
 and the transponder. At this stage, all frequencies are labelled as
 interference frequencies which are received at a field strength exceeding
 a pre-determined threshold.
 On the basis of the interference frequencies so determined, the
 coefficients of an adaptive digital filter can be computed, by means of
 which the filter can be tuned so that exactly these interference
 frequencies are suppressed, whilst differing frequencies are allowed to
 pass.
 The interrogation device now transmits the RF interrogation pulse, which
 triggers the transmission of the data stored in the transponder. The
 interrogation device receives the RF response signal from the transponder,
 onto which is superimposed the background noise with the interference
 frequencies contained therein. This signal is now routed through the
 adaptive filter, which, with the aid of the previously computed
 coefficients, has been tuned so as to suppress the interference
 frequencies. The output of the filter therefore provides a signal which
 now only contains the frequency components of the RF response signal
 transmitted by the transponder, so that this signal may then be subjected
 to further processing, such as demodulation, to give an example.
 Since, as has been explained, the interference frequencies in the output
 signal of the adaptive filter have been suppressed as far as possible, the
 possible range between the transponder and the interrogation device is
 thereby increased. Because of the absence of the interference components,
 the interrogation device is still capable of correctly processing the
 signals although they are weakened by distance.
 Should it appear that an interference frequency is very close to a sideband
 frequency within the RF response signal transmitted by the transponder,
 this then signifies that even this sideband frequency is suppressed by the
 adaptive filter. The output of the filter therefore outputs a signal
 which, although no longer containing any interference frequencies, also no
 longer contains a sideband frequency needed for the FSK demodulation. This
 fact can be taken advantage of in that the demodulation procedure for the
 data transmitted is modified. It is known that an FSK modulated signal,
 when using amplitude demodulation, can still be correctly demodulated when
 one of the two sideband frequencies is missing in the spectrum of the
 signal received.
 It has been indicated in the above-described procedural sequence that the
 background noise is received and analysed before the RF interrogation
 pulse is transmitted. However, it is equally feasible to invert this
 sequence, in other words to receive the background noise only after
 reception of the RF response signal transmitted by the transponder
 together with the superimposed background noise.
 If a particularly high reading reliability is required, the reception of
 the background noise and its analysis with respect to the presence of
 interference frequencies can take place before or after each time an RF
 response signal is received from the transponder. This ensures that the
 required suppression of interference frequencies is always achieved, even
 when interference conditions change with different application locations.
 If, however, it may be assumed that the environmental conditions with
 respect to the presence of interference frequencies is not likely to
 change, it will be sufficient to receive and analyse the background noise
 only once, so that the adaptive filter can be permanently tuned to the
 interference frequencies identified in each case. This makes it possible
 to speed up the reading process, since there will then be no need for the
 reception of the background noise.
 FIG. 4 illustrates the fundamental structure of an interrogation device for
 the execution of the procedure described above, by means of a simple block
 diagram. The interrogation device comprises an RF input stage 14 which can
 both transmit and receive RF signals by means of an aerial 16. The signals
 output by the RF input stage 14 are converted into digital signals in the
 analogue-to-digital converter 18, which are then analysed by a digital
 signal processor 20 for the presence of interference frequencies. The
 digital signal processor 20 generates coefficients for an adaptive filter
 22 which, by means of these coefficients, can be tuned in such a way as to
 suppress the interference frequencies detected. The digital signal
 processor 20 also controls the entire procedural sequence of the retrieval
 device. After analysing the background noise, it initiates the
 transmission of the RF interrogation pulse by the RF output stage 24 via
 the aerial 16, which prompts the transponder, not shown in the figure, to
 transmit the RF response signal. The RF response signal received from the
 transponder by the RF input stage 14 is also digitised by the
 analogue-to-digital converter 18, and routed through the adaptive filter
 22, so that a signal which is freed from interference frequencies becomes
 available at its output. This signal is then demodulated in a demodulator
 26, so that the required data become available at its output 28.
 Although the adaptive filter 22 and the demodulator 26 are shown as
 individual blocks in FIG. 4, these units may in praxis be equally well
 realised as software modules, which are stored in the digital signal
 processor 20 and are then processed by it. The digital signal processor 20
 processes the signals output by the analogue-to-digital converter 18 in
 such a way as to obtain the required filtering and demodulation. This type
 of signal processing is conventional and known to the expert, so that
 there is no need here to explain it in more detail. It is even possible to
 integrate the analogue-to-digital conversion into the digital signal
 processor 20, so that the entire processing of the signal received by the
 RF input stage 14 can be effected by the digital signal processor 20.
 In practical terms, the RF signal transmitted by the transponder is a
 carrier at a frequency of 13.56 MHz which is modulated by the two FSK
 frequencies 423 kHz and 484 kHz. The frequency spectrum of this signal
 therefore contains, apart from the carrier frequency, the frequencies
 13.56 MHz+423 kHz and 13.56 MHz+484 kHz as the upper sideband, as well as
 the frequencies 13.56 MHz-423 kHz and 13.56 MHz-484 kHz as the lower
 sideband.