Abstract:
A Near Field Communication method performed by a tag reader detects whether a tag is present. The method includes stimulating the tag reader&#39;s transmitter to generate an impulse response, evaluating the impulse response generated by the transmitter to obtain an evaluated impulse response, and assessing whether a tag is present based on the evaluated impulse response.

Description:
TECHNICAL FIELD 
     The invention relates to near field communication methods of detection of a tag presence by a tag reader. 
     BACKGROUND 
     According to a first prior art, a Near Field Communication (NFC) reader periodically checks for available nearby tags by sequentially polling for all compatible standards and waits for tag response. In a mobile device, this method will significantly drain battery of the NFC reader as, for each standard, a possible tag must be given time to respond, which means notably at least 5 ms for field powering, during which the reader must send its full power output for enabling the tag to load modulate. This results in a high duty cycle for the polling time with full transmission power causing high average power consumption in the NFC reader, for detecting possible present tag. 
     According to a second prior art, it is known a method called Low Power Tag Detection (LPTD) which is used by the Near Field Communication reader to detect the presence of a nearby tag. The method is based on a chirp stimulus that is used to measure, on-chip, the resonance frequency and the quality factor at the output of the reader transmitter. According to the respective values of the measured resonance frequency and quality factor, the reader detects if a nearby tag is present or not in its vicinity. This second prior art detects tag presence or absence by observing the detuning of the antenna of the reader transmitter, that a possibly present tag represents, to the reader front-end. 
     However, this Low Power Tag Detection method suffers from fault tag detection, as will be illustrated notably by  FIGS. 1 a  and 1 b   . An antenna coupling between reader and tag is simulated. The reader antenna quality factor is reduced with series resistors, and matched to the reader transmitter pins via a three-capacitor matching circuit. The reader transmitter represents a low impedance (voltage) drive to the reader transmitter pins, while also replicating the reader transmitter current waveform into the reader receiver for detecting load modulation and nearby tag presence if any. 
     According to this Low Power Tag Detection method, every couple of hundreds of milliseconds, the reader seeks for a nearby tag. To seek for a nearby tag, the reader transmitter sends out a chirp signal. In the case of the NFC IP of CG2910, the chirp signal frequency is swept from 12 MHz to 15 MHz. In the absence of a nearby tag, due to the reader load across the reader transmitter output terminals, the chirp signal resonates at the resonance frequency of the reader (for example 13.56 MHz for the NFC IP of CG2910). However, in the presence of a nearby tag, the load across the reader transmitter output terminals is impacted by the presence of the tag, which implies that the chirp signal resonates at a frequency different from the reader resonance frequency (for example 13.56 MHz for the NFC IP of CG2910). 
     Since the reader receiver is used to copy the reader transmitter output signal (chirp), this copy is then analyzed to calculate the value of the resonance frequency of the chirp signal. Once a different resonance frequency is detected, meaning a nearby tag is detected, the reader starts a new NFC communication with this newly detected nearby tag. This is how this Low Power Tag Detection method works. 
       FIG. 1 a    shows an example of detuning of an antenna of a reader due to a nearby tag, when reader and nearby tag both resonate at the same frequency. This common resonance frequency is 13.56 MHz. The amplitude A of the reader transmitter current is expressed in decibels dB and plotted as a function of the frequency expressed in Mega Hertz MHz. The detuning of the resonance frequency of the reader, from its original value, is all the more important that the nearby tag becomes closer to the reader and that the coupling factor k increases, what is shown through the multiple curves plotted on  FIG. 1 a   . The corresponding respective values of the coupling factor k are 0-2-4-6-8-10%, the higher peak curves corresponding to the lower coupling factor k values. In this example, the reader quality factor value is 25, the tag quality factor value is 35, and the reader matching circuit comprising two series capacitors of same capacitive value 2*C 1   r , a capacitor in parallel of capacitive value C 2   r , there is the following relation: C 1   r /(C 1   r +C 2   r )=0.9, the current i tx  circulating in the reader matching circuit and in the reader antenna has a value of 100 mA rms. 
       FIG. 1 b    shows an example of detuning of an antenna of a reader due to a nearby tag, when reader and nearby tag each resonate at a different frequency.  FIG. 1 b    is quite similar to  FIG. 1 a   , except that the resonance frequency of the reader is 13.56 MHz, whereas the resonance frequency of the tag is 16 MHz. The amplitude A of the reader transmitter current is expressed in decibels dB and plotted as a function of the frequency expressed in Mega Hertz MHz. The detuning of the resonance frequency of the reader, from its original value, is all the more important that the nearby tag becomes closer to the reader and that the coupling factor k increases, what is shown through the multiple curves plotted on  FIG. 1 b   . The corresponding respective values of the coupling factor k are 0-2-4-6-8-10%, the higher peak curves corresponding to the lower coupling factor k values. In this example, the reader quality factor value is 25, the tag quality factor value is 35. 
     The Low Power Tag Detection method according to second prior art presents weakness and ineffectiveness in the presence of a nearby ground plane or any metallic or magnetic or lossy body. This disadvantage of Low Power Tag Detection method according to second prior art, is that a nearby tag is not the only possible cause for detuning. Through measurements on the same reader as the one used for  FIGS. 1 a  and 1 b   , it was possible to prove that nearby metallic, magnetic, or lossy objects will also detune the reader transmitter circuit response, as can be seen on  FIGS. 2 and 3 . This disadvantage is a major one, since for example in NFC mobile applications, there will be a metallic ground plane on the host platform PCB (“printed circuit board”), and the battery will have an effect similar to a metallic plate too. 
       FIG. 2  shows an example of detuning of an antenna of a reader due to a nearby metal plane. The amplitude A of the reader transmitter current is expressed in decibels dB and plotted as a function of the frequency expressed in Mega Hertz MHz. When an NFC antenna coil is close to a metallic object, the generated magnetic field and the antenna inductance and quality factor will change due to induced circulation currents. The reader transmitter current is respectively measured with a parallel metallic plane at 3, 5, 10, 15, 20 and 25 mm distance.  FIG. 2  shows how the front-end of the reader progressively detunes because of a metallic surface coming closer to the reader. The metallic object has a severe impact on the resonance frequency of the reader, and this will most probably trigger a false tag detection during processing of the Low Power Tag detection method according to the second prior art. Its lack of detection reliability in case of metallic surface in the vicinity of the reader which mistakes it for a tag, is a first major disadvantage of this second prior art. 
       FIG. 3  shows an example of detuning of an antenna of a reader due to a nearby lossy plane. The amplitude A of the reader transmitter current is expressed in decibels dB and plotted as a function of the frequency expressed in Mega Hertz MHz. The nearby lossy plane comes closer to the reader in the same conditions as the metallic surface in  FIG. 2 .  FIG. 3  shows how the front-end of the reader progressively detunes because of a lossy plane coming closer to the reader. The lossy object has a severe amplitude impact on the peak of the reader transmitter current, and this will most probably trigger a false tag detection during processing of the Low Power Tag detection method according to the second prior art. The lack of detection reliability, of the second prior art, in case of lossy object in the vicinity of the reader which mistakes it for a tag, is a second major disadvantage. 
     SUMMARY 
     An object of embodiments of the present invention is to alleviate at least partly the above mentioned drawbacks. 
     More particularly, embodiments of the invention aim to provide an advanced Low Power Tag Detection method which is more reliable than the Low Power Tag Detection method according to the second prior art, especially in case of presence of a metallic surface and or a lossy object in the vicinity of the reader supposed to detect the presence of a tag in its vicinity. This advanced Low Power Tag Detection method evaluates one or more parameters which are different from the parameters evaluated by the Low Power Tag Detection method according to the second prior art. Impulse response (IR) of the reader transmitter is evaluated instead of resonance frequency and quality factor of the reader transmitter. The impulse response, which is the representation of the transfer function in the time domain, contains not only the resonance frequency and quality factor information in the time domain, but it also contains practically all information about the different aspects that impact the signal at the reader transmitter output. That is why, by measuring the impulse response, the reader, according to embodiments of the invention, can detect whether a nearby tag is present or not in the vicinity of the reader. Therefore, much better robustness and very low fault tag detection are possible. 
     Preferably the signal used to stimulate the reader transmitter so as to generate an impulse response of the reader transmitter is different too. In an embodiment, a preferred stimulating signal is a maximal length sequence (MLS), stimulus that is much easier to generate on-chip in the reader than the chirp signal of Low Power Tag Detection method of the second prior art. Using this maximal length sequence as stimulus allows for a simple and cheap way to get at the much better robustness and the very low fault tag detection which are made possible by measuring the impulse response of the reader transmitter. By using the maximal length sequence as stimulus for the reader transmitter, a fully digital design with negligible silicon overhead is made possible, rendering the global system even more cheap and reliable. 
     This object and other objects may be achieved with a Near Field Communication method of detection of a tag ( 6 ) presence by a tag reader ( 5 ), comprising: stimulating (S 2 ) the transmitter ( 1 ,  2 ) of the reader ( 5 ) with a signal representative of a pseudo-random binary sequence so as to generate an output signal based on the impulse response of said transmitter ( 1 ,  2 ), evaluating (S 3 ) the generated output signal in order to extract the impulse response of said transmitter ( 1 ,  2 ), assessing (S 4 ), from the extracted impulse response of said transmitter ( 1 ,  2 ), the presence (S 5 ) or the absence (S 6 ) of a tag ( 6 ). 
     Another object may be achieved with a Near Field Communication method of detection of a tag presence by a tag reader, comprising: stimulating the transmitter of the reader so as to generate an impulse response of said transmitter, evaluating the generated impulse response of said transmitter, assessing, from the evaluated impulse response of said transmitter, the presence or the absence of a tag. 
     This object and other objects may also be achieved with a Near Field Communication tag reader, comprising: an emitter adapted to send a stimulating signal representative of a pseudo-random binary sequence to an input of the transmitter of the reader so that said transmitter generates an output signal based on the impulse response of said transmitter, an evaluator adapted to extract the impulse response of said transmitter from said generated output signal, an assessor adapted to assess, from the extracted impulse response of said transmitter, the presence or the absence of a tag. 
     Another object may be achieved with a Near Field Communication tag reader, comprising: an emitter adapted to send a stimulating signal to an input of the transmitter of the reader so as to generate an impulse response of said transmitter, an evaluator adapted to evaluate the generated impulse response of said transmitter, an assessor adapted to assess, from the evaluated impulse response of said transmitter, the presence or the absence of a tag. 
     Preferred embodiments comprise one or more of the following features:
         said assessing comprises: comparing said evaluated impulse response of said transmitter to a predetermined envelope, deducing, either presence of a tag if said evaluated impulse response of said transmitter is completely within said predetermined envelope or absence of a tag if said evaluated impulse response of said transmitter is at least partly outside said predetermined envelope.   said predetermined envelope extends between a predetermined minimal impulse response and a predetermined maximal impulse response, both predetermined impulse responses being such that any impulse response due to any tag of a predetermined set of tags all able to communicate with said reader will be simultaneously above said predetermined minimal impulse response and below said predetermined maximal impulse response.   said evaluated impulse response is digital, absence of a tag is deduced if at least one sample of said digital evaluated impulse response of said transmitter is outside said predetermined envelope.   said evaluating comprises digitally evaluating said generated impulse response.   said evaluating comprises cross-correlating a signal representative of a signal delivered at an output of said transmitter with a signal representative of a signal which has been sent to an input of said transmitter to generate said impulse response.   said stimulating comprises sending, to an input of said transmitter, a signal representative of a pseudo-random binary sequence.   said pseudo-random binary sequence is a maximal length sequence preferably generated by a linear feedback shift register.   said maximal length sequence parameters are chosen such that said reader emits a periodic polling signal for tag detection which emission duration lasts between 1% and 10% of total polling time, preferably between 5% and 10% of total polling time.   the order of said maximal length sequence ranges from 13 to 16.   the clock frequency of said maximal length sequence ranges from 1 Mhz to 1.5 Mhz.   said stimulating comprises sending, to an input of said transmitter, a stimulating signal such as the autocorrelation of said stimulating signal is close enough to a Dirac impulse so that a convolution of a signal representative of a signal delivered at an output of said transmitter with a signal representative of said stimulating signal gives substantially said generated impulse response.   said stimulating, said evaluating and said assessing are all performed on chip of said reader.   a computer program product comprising a computer readable medium, having thereon a computer program comprising program instructions, the computer program being loadable into a data-processing unit and adapted to cause execution of the method according to any embodiment when the computer program is run by the data-processing unit. Preferably, after assessment, from the evaluated impulse response of said transmitter, of the presence of a tag, the tag reader starts communicating with the detected tag, advantageously by starting a communication session with this detected tag.       

     Preferably, after assessment, from the evaluated impulse response of said transmitter, of the absence of a tag, the tag reader does not start any communication session but continues on polling for tag detection. 
     Preferably, the tag reader comprises a transmitter and a receiver. Preferably, the tag reader also comprises, successively connected, current mirrors at the outputs of this transmitter, an envelope detector to derive baseband signal, an analog to digital converter, a cross-correlating device which other input is connected to a Linear Feedback Shift Register. 
     Preferably, the tag reader uses an analog digital converter to evaluate said generated impulse response. Advantageously, this analog digital converter is part of the receiver of said reader and is used during signal reception by this receiver of said reader. 
     Further features and advantages of the invention will appear from the following description of embodiments of the invention, given as non-limiting examples, with reference to the accompanying drawings listed hereunder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    shows an example of detuning of an antenna of a reader due to a nearby tag, when reader and nearby tag both resonate at the same frequency. 
         FIG. 1 b    shows an example of detuning of an antenna of a reader due to a nearby tag, when reader and nearby tag each resonate at a different frequency. 
         FIG. 2  shows an example of detuning of an antenna of a reader due to a nearby metal plane. 
         FIG. 3  shows an example of detuning of an antenna of a reader due to a nearby lossy plane. 
         FIG. 4  shows an example of steps of a method of detection of a tag presence by a tag reader according to an embodiment of the invention. 
         FIG. 5  shows an example of circuits of a tag reader and of a tag according to an embodiment of the invention. 
         FIG. 6  shows an example of a detailed part of circuits of a tag reader as shown in  FIG. 5 . 
         FIG. 7  shows an example of a more detailed part of circuits of a tag reader as shown in  FIG. 6 . 
         FIG. 8  shows an example of a predetermined envelope which can be used in an assessing step of a method of detection of a tag presence by a tag reader according to an embodiment of the invention. 
         FIG. 9  shows an example of a linear feedback shift register which can be used in a tag reader according to an embodiment of the invention. 
         FIG. 10  shows an example of a maximum length sequence which can be produced by a linear feedback shift register as shown in  FIG. 9 . 
         FIG. 11  shows an example of a first type of linear feedback shift register which can be used in a tag reader according to an embodiment of the invention. 
         FIG. 12  shows an example of a second type of linear feedback shift register which can be used in a tag reader according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 4  shows an example of steps of a method of detection of a tag presence by a tag reader according to an embodiment of the invention. In a step S 1 , a maximal length sequence MLS is generated on chip in the reader. In a group S 2  of steps S 21  and S 22 , this maximal length sequence MLS will be used to stimulate the reader transmitter so that the reader transmitter generates an impulse response. Generating on chip a digitized signal such as this maximal length sequence MLS is much more practical than would be an analog generation of sine waves. The generated impulse response depends on the presence or the absence of a tag in the vicinity of the reader. In a group S 3  of steps S 31 , S 32  and S 33 , the generated impulse response will be evaluated by the reader. In a group S 4  of steps S 41  and S 42 , the evaluated impulse response will be assessed by the reader in order either to give as result S 5  that a tag is present in the vicinity of the reader, or to give as result S 6  that there is no tag present in the vicinity of the reader. The groups S 2 , S 3  and S 4  of steps are performed successively. 
     Stimulation group S 2  successively comprises step S 21  and step S 22 . In step S 21 , the maximal length sequence is sent to stimulate the reader transmitter which emits a signal which impacts on a tag if there is one in the vicinity of the reader, which tag in turn load modulates the reader transmitter. Because of the coupling factor between the reader and the tag, the impulse response generated by the reader transmitter in step S 22  is changed compared to the impulse response the reader transmitter would have generated in the absence of such a tag. 
     Evaluation group S 3  successively comprises step S 31 , step S 32  and step S 33 . In step S 31 , the generated impulse response is processed so that the envelope of the generated impulse response can be detected by separating the base band signal from the carrier frequency. In step S 32 , an analog digital conversion of the detected envelope is performed so that a digitized envelope can be obtained. In step S 33 , a cross-correlation between on the one hand this digitized envelope and on the other hand the maximal length sequence MLS which has been generated on chip in the reader in former step S 1 . This cross-correlation is performed between two base band signals. 
     Assessment group S 4  successively comprises step  41  and step  42 . In step S 41 , the previous result of the cross-correlation step S 33  is compared to a predetermined envelope, in order to see whether this result is either fully within the predetermined envelope or at least partly outside the predetermined envelope. In step  42 , from the result of the comparison, a deduction is performed to know whether there is a tag presence or is a tag absence. If the result of the comparison of step  41  is that the result of the cross-correlation step S 33  is fully within the predetermined envelope, then the result of the deduction step S 42  is an indication of a tag presence in step S 5 . On the contrary, if the result of the comparison of step  41  is that the result of the cross-correlation step S 33  is at least partly outside the predetermined envelope, then the result of the deduction step S 42  is an indication of a tag absence in step S 6 . 
       FIG. 5  shows an example of circuits of a tag reader and of a tag according to an embodiment of the invention. There is a coupling, more particularly an antenna coupling between a reader  5  and a tag  6 . The tag  6  successively comprises, from upside to downside with respect to signal transmission, a tag matching circuit  7 , a tag rectifier  8  and a tag load  9 . The tag matching circuit  7  successively comprises, from upside to downside with respect to signal transmission, a tag antenna  71  which is essentially an inductance, two series resistors  72  and  73  of same resistive value, one on each side of the tag antenna  71 , a capacitor  74  in parallel. 
     The reader  5  successively comprises, from upside to downside with respect to signal transmission, a reader pretreatment circuit  1 , a reader transmitter circuit  2 , a reader matching circuit  3 , one circuit being electrically connected to the next one. The reader  5  also comprises an additional reader evaluation circuit  4  which has an input fed by the output of the reader transmitter circuit  2 , and which has one of its outputs feeding the input of the reader pretreatment circuit  1 . 
     The reader pretreatment circuit  1 , successively comprises, from upside to downside with respect to signal transmission, a digital analog converter  11 , a filter  12 , a pre-amplifier  13 . A clock signal is generated and sent on one input of the digital analog converter  11 , whereas a maximal length sequence MLS, generated in the reader evaluation circuit  4 , is sent on the other input of the digital analog converter  11 . Both outputs of digital analog converter  11  are connected to both inputs of filter  12 . Both outputs of filter  12  are connected to both inputs of pre-amplifier  13 . Filter  12  is a low pass filter with for example a cutoff frequency at 27 MHz. 
     The reader transmitter circuit  2 , successively comprises, from upside to downside with respect to signal transmission, two transmitter buffers  21  and  22 , two transmitter outputs  27  and  28 , respectively linked to two connections  29  and  30 . Thanks to two current mirrors  23  and  24 , the two transmitter outputs  27  and  28  are replicated, on the one hand towards the reader receiver inputs rx 1  and rx 2 , the reader receiver being not shown on  FIG. 5  for sake of simplicity, and on the other hand towards the reader evaluation circuit  4  inputs  25  and  26 . The two current mirrors  23  and  24  fulfill a double function which is to simultaneously replicate the signal towards the reader receiver and towards the additional reader evaluation circuit  4 . The reader transmitter comprises the reader pretreatment circuit  1  and the reader transmitter circuit  2 . 
     The reader matching circuit  3 , successively comprises, from upside to downside with respect to signal transmission, two connections  29  and  30 , two series capacitors of same capacitive value  31  and  32 , a capacitor  33  in parallel, two series resistors  34  and  35  of same resistive value, one on each side of the capacitor  33 , a reader antenna  36 . The reader antenna  36  is inductively coupled to the tag antenna  71  by a coupling factor k. The reader antenna  36  is de-Q′ed, which means its quality factor Q is reduced, with series resistors  34  and  35  each having a resistive value of RQr/2, and matched to the transmitter connections  29  and  30  via a three-capacitor  31  to  33  matching circuit, respectively having a capacitive value of 2Clr and C 2   r . The reader transmitter represents a low-impedance drive, which is a voltage drive, to the transmitter connections  29  and  30 , while also replicating the reader transmitter current waveform into the reader receiver for detecting load modulation, as well as in the additional reader evaluation circuit  4  for detecting nearby tag presence via impulse response evaluation. 
     The additional reader evaluation circuit  4 , successively comprises, from upside to downside with respect to signal transmission, the reader evaluation circuit  4  inputs  25  and  26 , the envelope detector  41 , the analogue digital converter  42 , the cross-correlation device  44 . This analogue digital converter  42  can be shared with the reader receiver which also requires such an analogue digital converter. The linear feedback shift register  43  also feeds the cross-correlation device  44 . At a first input of the cross-correlation device  44 , there is a digitized signal y(k), which is representative of the impulse response generated by the reader transmitter, and at a second input of the cross-correlation device  44 , there is a digitized signal x(k), which is the maximum length signal MLS, so that, at the output of the cross-correlation device  44 , there is a digitized signal h(k), which is the impulse response of the reader transmitter, which has been extracted from the digitized signal y(k). The linear feedback shift register  43  also feeds the reader pretreatment circuit  1  with the maximum length signal MLS it generates. 
     Indeed, to measure the impulse response, a certain stimulus x(k), with k being the discrete time domain, is applied at the unloaded output of the reader transmitter. However, the reader transmitter is always loaded. Due to the reader transmitter output load, x(k) gets convoluted by the impulse response h(k) of the output load. Due to this convolution, a different signal y(t) is output by the reader transmitter such that y(k)=x(k)*h(k), where h(k) is the impulse response of the reader transmitter and where * is the convolution operation. Then, the impulse response can be obtained according to the cross-correlation method, because the input/output cross-correlation φ xy (k) of x(k) and y(k) is the convolution of the output y(k) with the time reverse of the input x(k). This is derived as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             
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     The condition φ xx (k)≅δ(k) is satisfied when x(k) is a white noise or a long maximal length sequence. Equation 1 shows how the input/output cross-correlation φ xy  (k) is derived to be equal to the impulse response h(k) when the stimulus is a white noise or a maximal length sequence. 
     The reader represented in  FIG. 5  shows an implementation of a new advanced Low Power Tag Detection method using impulse response and maximal length sequence as stimulus to generate this impulse response. During performance of this method, the linear feedback shift register  43  injects a maximal length sequence MLS at the transmitting data input of the digital analog converter  11 . At the outputs of the reader transmitter  27  and  28 , the differential signal is the analog version of the injected maximal length sequence MLS after being amplified, up-converted to the resonance frequency of the reader which is 13.56 MHz, and then convoluted with the impulse response h(t) available at the connections  29  and  30  and being created by the output load of the reader transmitter. According to Equation 1 and according to the auto-correlation property φ xx (k)≅δ(k) of the maximal length sequence MLS, h(k), a sampled version of h(t), can be calculated by cross-correlating x(k) and y(k). Indeed, as explained before, the result of this cross-correlation gives the impulse response h(k) which was searched for. 
       FIG. 6  shows an example of a detailed part of circuits of a tag reader as shown in  FIG. 5 . The reader transmitter  50  comprises both the reader pretreatment circuit  1  and the reader transmitter circuit  2 . Envelope detector  41  and analogue digital converter  42  are regrouped. Both the linear feedback shift register  43  and the cross-correlation device  44  are detailed. The processing of the different signals, which are the maximal length sequence MLS, the digitized signal x(k), the generated digitized signal y(k) and the searched for digitized impulse response h(k), is shown in more detail than on  FIG. 5 . 
     The linear feedback shift register  43  comprises several delay lines  431  connected to one another in series, and an adder  432 . The output of the adder  432  is connected to the input of the first delay line  431 . One input of the adder  432  is connected to the output of the last delay line  431 . The other input of the adder  432  is connected to an intermediate position in the series of delay lines  431 . 
     The cross-correlation device  44  comprises several simplified correlation cells  441  connected to one another in series. Each simplified correlation cell comprises two inputs and one output. At the first input of each simplified correlation cell, there is a different sample of the digitized signal x(k) which is the maximal length sequence MLS. At the second input of each simplified correlation cell  441 , there is a different sample of the digitized signal y(k) which is generated at the output of the analog digital converter  42 . At the output of each simplified correlation cell, there is a different sample of the digitized signal h(k) which is the digitized impulse response. 
       FIG. 7  shows an example of a more detailed part of circuits of a tag reader as shown in  FIG. 6 . Each simplified correlation cell  441  comprises a multiplexer  442 , an inverter  443 , an accumulator  447  comprising an adder  444  and a delay line  445 , a divider  446 . At the output of this simplified correlation cell  441 , there is one sample h(k) of the digitized impulse response. At the first input of this simplified correlation cell  441 , there is one sample x(j-k) of the maximal length sequence. At the second input of this simplified correlation cell  441 , there is one sample y(j) of the signal generated at the output of the analogue digital converter  42 . 
     The second input of this simplified correlation cell  441  is simultaneously directly connected to one input of the multiplexer  442  and indirectly connected to another input of the multiplexer  442  via the inverter  443 . The first input of this simplified correlation cell  441  is directly connected to the control input of the multiplexer  442 . The output of the multiplexer  442  is directly connected to one input of the adder  444 . The output of the adder  444  is on the one side connected to the input of the divider  446  and on the other side feedbacks another input of the adder  444  via a delay line  445 . The output of the divider  446  is the output of the simplified correlation cell  441 . 
     Performing the cross-correlation of x(k) and of y(k) is indeed relatively simple because the products are replaced by sums since x(k) is a maximal length sequence. Each simplified correlation cell  441  is used to obtain one of the components of the impulse response h(k). Each sample of the output sequence y(j) is multiplied by 1 or −1, which are the maximal length sequence analog levels at the output of the digital analogue converter  11  shown in  FIG. 5 , in the multiplexer  442  controlled by the input sequence x(j−k), and the result is added to the sum stored in the adder  444  of the accumulator  447 . The value obtained at the end of the calculation loop realized by the delay line  445  is divided by L+1 in the divider  446 . In the end, the equation of cross-correlation, between x(k) and y(k), results in: 
     
       
         
           
             
               
                 
                   
                     
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     The first m components of the impulse response which are h(k), with k varying from 0 to m−1, can be obtained with the help of the circuit shown in  FIG. 6 , each of these m components corresponding to the output of a simplified correlation cell  441  as the one shown in  FIG. 7 . The on-chip implementation shown on circuits of  FIGS. 6 and 7  does not give the overall impulse response but only the first m samples of h(k), at the sampling frequency of the injected digitized signal x(k). These first m samples of h(k) are called the h(k) pattern. This measured h(k) pattern is then compared to the nominal patterns, what will be explained in more detail with respect to  FIG. 8 . If samples beyond the above set would be required, longer linear feedback shift registers could be used, but this would result in increasing the time needed for performance of this advanced Low Power Tag Detection method. 
     As a preferred numerical example to implement an important capability to detect a nearby tag while using this advanced Low Power Tag Detection method, the parameters have been chosen as follows. We have m=16 which makes L=2 16 −1=65535. The digitization frequency of the maximal length sequence is F c =1 MHz, which makes the duration of the maximum length sequence MLS to be T MLS =65535/1 M=65.5 ms. This duration of 65.5 ms presents the advantage of being less than 100 ms which would correspond to emission duration of about 10% of the overall time which is still a low proportion of overall time corresponding to power emission time, what is one key advantage of this advanced Low Power Tag Detection method. Choosing m&gt;16 would result in a duration emission longer than 10% of the overall time, what would make this advanced Low Power Tag Detection method more power consumptive and therefore somewhat less interesting. On the contrary, choosing m&lt;13 would result in a shorter sequence of impulse response samples, in an impulse response with less content of information, what would make this advanced Low Power Tag Detection method less precise and therefore somewhat less interesting. The digitization frequency F c  of the maximal length sequence is will preferably be kept equal to or lower than 1.5 MHz, since above 2 MHz, it becomes less easy to extract the impulse response. 
       FIG. 8  shows an example of a predetermined envelope which can be used in an assessing step of a method of detection of a tag presence by a tag reader according to an embodiment of the invention. A value V, which is unit-less, of the impulse response, is plotted as a function of n, the number of measured samples of the impulse response. There can be seen a minimum curve, the lower limit of the envelope LE, and a maximum curve, the upper limit of the envelope UE. This minimum curve, the lower limit of the envelope LE, and this maximum curve, the upper limit of the envelope UE, delimit an envelope, also called interval, which is situated between the curves UE and LE, and in which the measured impulse response is supposed to be in case of a nearby tag. Indeed, once the h(k) pattern has been measured, the h(k) pattern is compared to these lower limit of the envelope LE and upper limit of the envelope UE. If the measured h(k) pattern is totally found to be within the interval located between this lower limit of the envelope LE and this upper limit of the envelope UE, this implies that there is at least a nearby tag present in the vicinity of the reader which then can detect it, this nearby tag impacting the loading across the reader antenna, and thus changing the form of the impulse response h(k). If at least one sample of the measured h(k) pattern is found to be out of this interval, this implies that there is no nearby tag in the vicinity of the reader. Typically, presence of a lossy or metallic object, of simply absence of tag and of any other object, would lead to an impulse response being, at least partly, outside this interval. 
     One way to determinate these lower limit of the envelope LE and upper limit of the envelope UE can be the following one, using the same maximum length sequence parameters as the ones depicted in the preferred numerical example related to  FIGS. 6 and 7 . First, one-time parametric transient simulations were performed by sweeping four variables. First variable is the coupling factor k, within the operating volume of Near Field Communication. Second variable is the tag resonance frequency, going from 13.56 MHz to 23 MHz. Third variable is the tag quality factor Q, going from Q=15 to Q=24. Fourth variable is the tag load, going from 50 to 5000 ohms. Those simulations were performed at many, here preferably at all, Near Field Communication standards, data rates, type A, Type B and Type F protocols, and in many, here preferably at all available power modes. 
     As a result of the parametric simulation, it was possible to have the h i (k) patterns that correspond to each simulation, with i being the simulation number. The lower limit of the envelope LE and the upper limit of the envelope UE were then chosen such that for all h i (k), none of h i (k) samples is outside the envelope interval limited by LE and UE and encompassing all measured impulse responses. Preferably, the lower limit of the envelope LE and the upper limit of the envelope UE were also chosen such that the interval is made as small as possible. Once this interval is obtained, it is saved in a register on-chip of the reader. When in the field, at each time the reader starts performance of the advanced Low Power Tag Detection method, the first 16 samples of the sampled impulse response h(k), also called h(k) pattern, are measured using the technique previously described. In the digital part of the reader circuit, the h(k) measured pattern is compared to the [LE, UE] interval. As already mentioned, if at least one sample of the h(k) measured pattern is out of this interval, there is no nearby tag, whereas there is a nearby tag if all samples of the h(k) measured pattern are within this interval. 
       FIG. 9  shows an example of a linear feedback shift register which can be used in a tag reader according to an embodiment of the invention. The maximum length sequences, which are also called pseudo-random sequences, or pseudo-noise sequences or m-sequences, are certain binary sequences of length L=2 m −1 with m denoting the order of the sequence. These sequences are already known in areas such as range-finding, scrambling, fault detection, modulation, synchronization and acoustic measurements. To construct a maximum length sequence MLS of a given length L, a primitive polynomial p(x) of a degree m is used. An example of such a polynomial is given by the following expression:
 
 p ( x )= x   m   +x   n +1, 0 &lt;n&lt;m.  
 
     This polynomial specifies a linear feedback shift register as shown in  FIG. 9 , comprising unit-sample delays  433 , also called delay lines  433 , produced by memory elements, as well as a XOR device  434 . The linear feedback shift register presents an output  438  at the level of the output of the last delay line  435 . The linear feedback shift register is clocked at a certain fixed frequency F c . The values of n and m are chosen so that the linear feedback shift register generates a maximal length sequence and then this obtained linear feedback shift register can be used in the circuit of  FIG. 6 . 
       FIG. 10  shows an example of a maximum length sequence which can be produced by a linear feedback shift register as shown in  FIG. 9 . This maximum length sequence MLS has a duration of L*Δt, with Δt the period equal to 1/F c . 
       FIG. 11  shows an example of a first type of linear feedback shift register which can be used in a tag reader according to an embodiment of the invention. This first type of linear feedback shift register comprises delay lines  435 , XOR devices  436  and weightings  437 . The linear feedback shift register presents an output  438  at the level of the output of the last delay line  435 . Delays lines  435  are connected in series to one another. XOR devices  436  are connected in series to one another. Output of last delay line  435  is connected to one of the input of first XOR device  436 , whereas input of first delay line  435  is connected to output of last XOR device  436 . Weightings  437  are connected in parallel, each time between the output of a delay line  435  and the other input, the free input, of a corresponding XOR device  436 . The first type of linear feedback shift register corresponds to a Fibonacci implementation consisting of a simple shift register in which a binary-weighted modulo-2 sum of the taps is fed back to the input. For the generation of binary sequences, the coefficients {g 0 , g 1 , . . . , g m } belong to {0, 1}. 
       FIG. 12  shows an example of a second type of linear feedback shift register which can be used in a tag reader according to an embodiment of the invention. This second type of linear feedback shift register comprises delay lines  435 , XOR devices  436  and weightings  437 . The linear feedback shift register presents an output  438  at the level of the output of the last delay line  435 . Couples consisting of a delay line  435  and of a XOR device  436  are connected in series to one another. Output of last delay line  435  is connected to input of first delay line  435 . Weightings  437  are connected in parallel, each time between the output  438  of the linear feedback shift register and the other input, the free input, of a corresponding XOR device  436 . The second type of linear feedback shift register corresponds to a Galois implementation consisting of a shift register, the contents of which are modified at every step by a binary-weighted value of the output stage. For the generation of binary sequences, the coefficients {g 0 , g 1 , . . . , g m } belong to {0, 1}. It can be seen that the order of the Galois weightings  437  is opposite to that of the Fibonacci weightings  437 . 
     Whatever the linear feedback shift register used to generate the desired maximum length sequence MLS, this maximum length sequence MLS will present the following property, which is used to obtain equation 1, and which is: if an m-sequence is mapped to an analog time-varying waveform, by mapping each binary zero to −1 and each binary one to +1, then the autocorrelation function will have a periodic triangular shape, with period equal to T c  and with unity for zero delay and 1/L=−1/(2 m −1) for any delay greater that one bit. It can be noticed that, for a long maximum length sequence MLS at small period T c , the autocorrelation is almost an impulse function of period equal to LT c . This property is used to obtain equation 1 (see before), since it proves that the impulse response equals the input/output cross-correlation when the stimulating signal is a maximum length sequence. 
     A maximum length sequence also presents other properties, among which there are:
         the modulo-2 sum of a maximum length sequence MLS and another phase of the same sequence yield a third phase of the sequence.   a maximum length sequence MLS is deterministic and periodic of period L=2 m −1, where m is the length of the linear feedback shift register.   an m-sequence contains exactly 2 m-1  ones and 2 m-1 −1 zeros.   each node of an m-sequence generator runs through a certain phase of the sequence. This is always true with a Fibonacci linear feedback shift register, this is not always true with a Galois linear feedback shift register.   a sliding window of length m, passed along an m-sequence for 2 m −1 positions, will span every possible m-bit number, except all zeros, once and only once.   if it is defined a series of length r to be a sequence of r consecutive identical numbers, then, in any maximum length sequence MLS, there are: one series of 1s of length m, one series of 0s of length m−1, one series of 1s and one series of zeros each of length m−2, two series of 1s and two series of 0s each of length m−3, four series of ones and four series of zeros each of length m−4, and so on to 2 m-3  series of 1s and 2 m-3  series of 0s each of length 1.   the power spectrum of a maximum length sequence MLS is a discrete spectrum whose upper 3 dB roll-off frequency is about 0.45 F c . By adjusting the clock frequency, a broadband signal over a wide frequency range can be generated.   if the order of the feedback taps is reversed, the resulting sequence will be the time reversal of the original sequence, and will also be an m-sequence.   the tap numbers of any given m-sequence linear feedback shift register will all be relatively prime.       

     The invention has been described with reference to preferred embodiments. However, many variations are possible within the scope of the invention.