Abstract:
In a backscatter interrogator reception method, a continuous carrier signal with a frequency f c  is sent from an interrogator to a tag device over a wireless link, the tag device transmits data to the interrogator using a modulated backscatter signal, and the received modulated backscatter signal is demodulated by the interrogator to retrieve the data transmitted from the tag device. According to the present invention, the interrogator estimates the phase and the amplitude of an inband interferer signal contained in the received modulated backscatter signal, generates a cancellation signal having the opposite phase and the same amplitude as the estimation of the inband interferer signal, and combines said cancellation signal with the received backscatter signal in order to reduce the influence of the inband interferer signal. In addition, a two-stage interferer cancellation scheme is proposed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for backscatter interrogator reception as well as to an interrogator for use in a modulated backscatter system (MBS). 
     2. Description of the Related Art 
     Modulated backscatter systems are known in the art and are the basis for the recently developed so-called radio frequency identification (RFID) systems. These RFID systems are for example used for identifying and/or tracking equipment, inventory, persons or animals. RFID systems are radio communication systems that allow communication between a radio transceiver, called an interrogator, and at least one so-called tag. In an RFID system the interrogator communicates with the tag by transmitting an unmodulated radio signal which is received, modulated and transmitted back by the tag to the interrogator. 
     The general idea underlying a modulated backscatter system is shown in  FIG. 1 . Similar to the field of radar technology, backscatter communication is based on the idea that electromagnetic waves are reflected by an object. The efficiency with which an object reflects electromagnetic waves is described by its radar cross-section. Objects that are in resonance with the wave front that hits them, as it is the case for antennas at the appropriate frequency, for example, have a particularly large radar cross-section. 
     In the general modulated backscatter system shown in  FIG. 1 , a signal P 1  is emitted from a transmitting circuit  101  of interrogator  100 , a small portion P 1 ′ of which reaches the antenna  111  of the tag  110 . 
     A proportion of the incoming power P 1 ′ is reflected by the antenna  111  of the tag  110  and returned as power P 2 . The reflection characteristics of the antenna  111  can be influenced by altering the load connected to the antenna  111 . In order to transmit data from the tag  111  to the interrogator  110 , two different load impedance values Z and Z′ in the embodiment of  FIG. 1  are alternately connected with the antenna  111  in time with the data stream to be transmitted. In this way, the amplitude of the power P 2  reflected from the tag  110  can be modulated resulting in a modulated backscatter signal. 
     The power P 2  reflected from the tag  110  is again radiated into free space wherein a small proportion P 2 ′ of this is picked up by the antenna  103  of the interrogator  100 . The reflected signal therefore travels into the antenna connection of the interrogator  100  in the backwards direction and can be decoupled and transferred to a receiver  102  of the interrogator  100  where the signal is demodulated in order to retrieve the information transmitted by the tag  110 . Alternatively the antenna  103  can be split into a distinct transmit antenna and a separate reception antenna which are spatially separated and thereby eliminating the requirement for a de-coupler in the interrogator  100 . 
     One problem arising with the use of backscatter communication is the appearance of strong so-called inband interferer. In the modulated backscatter system explained above, a part of the emitted radio frequency waveform P 1  from the interrogator  100  is feed back into the interrogator receiver  102 . Here, it will multiply with a local oscillator (LO) signal and generate an unwanted baseband signal, which is proportional to the power of the leakage from the interrogating waveform, differential delay between the interrogator  100  and the tag  110  and etc. As a result, the performance of the interrogator receiver  102  and the useable range for the modulated backscatter system are degraded. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to overcome the problems mentioned above by providing an interrogator for use in a modulated backscatter system wherein the performance of the receiver sensitivity of the interrogator is improved. 
     The present invention proposes a novel idea for reducing the effect of the leakage of the interrogating RF waveform—called inband interferer—allowing to achieve an improved receiver sensitivity. 
     Since inband interferers always exist in modulated backscatter systems and are much stronger than the received backscatter signal itself, it is—according to a first aspect of the present invention—proposed to adopt the principle of a Phase Locked Loop (PLL) in order to track the phase and amplitude of the strong inband interferer and to generate a cancellation signal that is equal in amplitude and 180° out of phase with the inband interferer. After that, this generated cancellation signal is combined with the received signal to implement an inband interferer cancellation before the received signal is forwarded to the next stage in which the signal is finally demodulated in order to retrieve the data transmitted from the tag. By using this technique, the power level of inband interferer is significantly reduced and an improved receiver sensitivity can be obtained. 
     According to the first aspect of the present invention, a backscatter interrogator reception method is proposed wherein
         a continuous unmodulated carrier signal with a frequency f c  is sent from an interrogator to a tag device or multiple tag devices over a wireless link,   a tag device transmits data to the interrogator using a modulated backscatter signal, which is generated by modulating the received carrier signal with or without an intermediate frequency f i  and   the received modulated backscatter signal is demodulated by the interrogator to retrieve the data transmitted from the tag device,
 
wherein the interrogator estimates the phase and amplitude of an inband interferer signal contained in the received modulated backscatter signal and having a frequency f c  corresponding to the carrier frequency,
 
the interrogator generates a cancellation signal having the opposite phase and the same amplitude as the estimation of the inband interferer signal, and combines said cancellation signal with the received modulated backscatter signal in order to reduce the influence of the inband interferer signal.
       

     Also in accordance with this first aspect of the present invention, an interrogator for use in a modulated backscatter system is provided comprising
         transmitting means for transmitting a continuous unmodulated carrier signal with a frequency f c  to a tag device,   receiving means for receiving a backscatter signal obtained by a modulation of the carrier signal and   demodulating means for retrieving data transmitted from a tag device,
 
wherein the interrogator further comprises inband interferer cancellation means for
       a) estimating the phase and the amplitude of an inband interferer signal contained in the received modulated backscatter signal and having a frequency f c  corresponding to the carrier frequency,   b) generating a cancellation signal having the opposite phase and the same amplitude as the estimation of the inband interferer signal and   c) combining the cancellation signal with the received backscatter signal in order to reduce the influence of the inband interferer signal.   

     For long distance backscatter transmissions from the tag to the interrogator (e.g. more than 5 m), the inband interferer can be more than 60 db stronger than the received backscatter signal. Here, an one-stage inband interferer cancellation as commonly used in known prior art RFID systems cannot cancel the inband interferer efficiently enough. Accordingly, a good receiver sensitivity and a large dynamic range cannot be guaranteed. 
     The present invention therefore proposes according to a second aspect an improved receiver structure, which concatenates a two-stage inband interferer cancellation scheme efficiently to eliminate the inband interferer. In particular, the present invention proposes an improved super-heterodyne receiver structure with which a good receiver sensitivity can be achieved. 
     According to this second aspect of the present invention, a backscatter interrogator reception method is proposed wherein
         a continuous unmodulated carrier signal with a frequency f c  is sent from an interrogator to a tag device or multiple tag devices over a wireless link,   a tag device transmits data to the interrogator using a modulated backscatter signal, which is generated by modulating the received carrier signal with or without an intermediate frequency f i  and   the received modulated backscatter signal is demodulated by the interrogator to retrieve the data transmitted from the tag device,
 
wherein the influence of an inband interferer signal contained in the received modulated backscatter signal is reduced in two consecutive stages. Preferably, the received modulated backscatter signal is down-converted to an intermediate frequency f 0 , bandpass filtered at said intermediate frequency f 0  and filtered, again, with a pair of notch filters in the baseband in order to reduce the influence of the inband interferer signal.
       

     Also in accordance with this second aspect of the present invention, an interrogator for use in a modulated backscatter system is provided comprising
         transmitting means for transmitting a continuous unmodulated carrier signal with a frequency f c  to a tag device,   receiving means for receiving a backscatter signal obtained by a modulation of the carrier signal and   demodulating means for retrieving data transmitted from a tag device,
 
wherein the interrogator further comprises two cancellation circuits for reducing the influence of an inband interferer signal contained in the received modulated backscatter signal in two following stages.
       

     Preferred embodiments of the invention are described in the following with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows schematically the principles of a modulated backscatter system. 
         FIG. 2  is a chart showing typical intermediate frequency modulated backscatter modulation schemes. 
         FIG. 3  shows a typical spectrum of an intermediate frequency MBS signal. 
         FIG. 4  shows a known interrogator receiver structure for use in a modulated backscatter system. 
         FIG. 5  shows an improved interrogator receiver structure in accordance with a first aspect of the present invention. 
         FIG. 6  shows schematically three different possibilities for a two-stage inband interferer cancellation, where single arrow means a real scalar signal and double arrow means a complex signal with in phase and quadrature phase components. 
         FIG. 7  shows an embodiment of an interrogator receiver structure in accordance with the second aspect of the present invention in which an additional intermediate frequency f 0  is introduced and a two-stage inband interferer cancellation is realised. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principles of modulated backscatter communication have already been explained with reference to  FIG. 1 . Again, in order to achieve a modulation of the signal P 1  emitted from the interrogator  100 , the load impedance of the antenna  111  of the tag  110  is changed for example between values representing the data signals “1” and “0”. From the point of view of the interrogator  100 , the interrogating radio frequency waveform reflected by the tag  110  is switched in its amplitude (and/or phase) between those two values. When the switch is closed the antenna  111  becomes the electrical equivalent of a single half-wave length antenna that reflects a portion of the power; when the switch is open, the antenna  111  becomes the electrical equivalent of a two quarter wave length antenna that reflects very little of the power. This effect corresponds to an amplitude-shift-keying (ASK) modulation with a small modulation depth. 
     The modulated reflected signal P 2  is called the MBS signal, which will be received by the interrogator  100  together with a strong inband interferer. In order to reduce the effect of the inband interferer and phase noise, flicker noise and jitter introduced by a local oscillator (LO) of the interrogator  100 , the spectrum of the MBS signal shall be far away from the leakage of the interrogating RF carrier frequency (called the inband interferer), which can be implemented by the introduction of an intermediate frequency. Examples of various intermediate frequency MBS modulation schemes are shown in  FIG. 2 . 
       FIG. 3  shows the typical spectrum of the intermediate frequency MBS signal received by the antenna  101  of the interrogator  100  where f c  is the carrier frequency of the interrogating carrier signal P 1  and f i  is the MBS intermediate frequency. In a typical RFID scenario, the power of the inband interferer is approximately 25 db higher than that of the MBS reflected signal. If the range between the interrogator  100  and the tag  110  is extended, for example to more than 5 m, the power of the reflected MBS signal is further reduced by approximately 35 dB or even more. Thus, the power of the inband interferer can be about 60 dB higher than that of the MBS signal. 
     At the interrogator  100 , the inband interferer will be multiplied with the local oscillator and an unwanted baseband noise is generated. Part of this noise overlaps with the spectrum of the MBS signal, which is proportional to the power level of the inband interferer, the round trip delay between the interrogator  100  and the tag  110  and etc. If the noise level approximates that of the MBS signal, the reflected MBS signal cannot be demodulated and a reliable data transmission cannot be guaranteed. 
     The present invention now provides an improved interrogator receiver structure that allows to expand the operating range of classical modulated backscatter systems by reducing the effect of the inband interferer. 
     The receiver structure proposed in accordance with the present invention is based on a known receiver structure, which is shown in  FIG. 4 . This known structure combines the benefits of a direct down-conversion architecture with that of a low intermediate frequency (IF) architecture. Before the improvements in accordance with the present invention are explained, the function of the known receiver structure of  FIG. 4  is shortly summarised in the following. 
     In this known receiver structure, the MBS signal received by the antenna  2  passes at first a pre-selection filter  3  in order to remove out-of-band signal energy as well as partially reject image signals. After this filter, the signal is amplified by a low noise amplifier (LNA)  4 . 
     This signal is now mixed by two quadrature mixers  5   a  and  5   b  and down-converted to an intermediate frequency resulting in a real and an imaginary part of the down-converted signal, the so-called in phase component (upper branch of the receiver structure) and the quadrature phase component (lower part). 
     Both signal components are passed through two DC notch filters  6   a ,  6   b  to two automatic gain controllers (AGC)  7   a ,  7   b  which forward the amplified signal components to lowpass filters  8   a ,  8   b  and further amplifiers  9   a  and  9   b . The signal components amplified by amplifiers  9   a  and  9   b  are now converted by two analog/digital converters (A/D)  10   a  and  10   b  and again complex-mixed in a complex mixing configuration  11  comprising four mixers  11   a  to  11   d  and two adders  11   e  and  11   f . Finally, the signals are passed again through two baseband selection filters  12   a  and  12   b  and demodulated by a digital signal processor  13 . 
     This receiver architecture shown in  FIG. 4  has been described in J. Crols and M. S. J. Steyaert “Low-IF Topologies for High-Performance Analogue Front Ends of Fully Interrogated Receivers” and Shahriar Mirabbasi and Ken Martin “Classical and Modern Receiver Architectures”. This receiver topology is a combination of known heterodyne and homodyne receiver structures and allows a reliable demodulation of the received signals. For further information regarding the detailed function of the several components of the receiver structure, it is referred to the two above-mentioned publications. 
     The first aspect of the present invention now provides a further development of the known receiver structure, which is shown in  FIG. 5 . In particular, this first aspect of the present invention relates to the shadowed part which is located between the low noise amplifier (LNA)  4  and the first down-conversion stage consisting of the two mixers  5   a  and  5   b  and serves to reduce the power of inband interference before the signal enters the next stage, which is similar to that already known from  FIG. 4 . 
     The shadowed part provides an inband interferer pre-cancellation  20  structure for significantly reducing the power of the inband interferer. Elements of the pre-cancellation structure  20  are at first tracking means  21  and power estimation means  22  for estimating the phase and the amplitude of an inband interferer signal contained in the signal received from the low noise amplifier  4 . On the basis of the information obtained from the tracking means  21  and the power estimation means  22 , a cancellation signal is generated by a cancellation signal generator  23  which cancellation signal is combined with the delayed MBS signal forwarded by the low noise amplifier  4 . Thus, the pre-cancellation structure  20  further comprise a delay circuit  24  for delaying the MBS signal and a combining circuit  25  for combining the two signals. 
     As a result of the combination of both signals (the delayed MBS signal and the cancellation signal), the power level of the inband interferer is significantly reduced since the cancellation signal is equivalent in amplitude to the original inband interferer but 180° out of phase. Thus, before the received MBS signal is forwarded to the next stage, the effect of inband interferer is reduced and the overall receiver performance is improved. 
     It has to be noted that the pre-cancellation of the inband interfere has been shown as an improvement of the known receiver structure of  FIG. 4 . However, the present invention is not in any way limited to this specific receiver structure but can also be used with other receiver structures in order to reduce—already at the beginning of the signal demodulation—the influence of inband interferer. 
     Coming back to the receiver structure shown in  FIG. 4 , the one-stage inband interferer cancellation scheme used in this known structure is simple and good enough for conventional RFID systems since the required receiver sensitivity is about −70 dBm and the inband interferer is about 25 dB stronger than the MBS signal. However, the performance of the known structure is not good enough to support a long range MBS transmission, which would require a receiver sensitivity better than −100 dBm under the existence of inband interferer which can be about 60 dB stronger than the MBS signal. 
     According to a second aspect of the present invention, an efficient concatenation of a two-stage inband interferer cancellation working at different frequencies is proposed and explained in the following. 
     The cancellation circuit can be understood as a filter, whereby it can be implemented as either one IF SAW (surface acoustic wave) bandpass filter in the conventional super heterodyne structure or a pair of DC notch filters in the conventional homodyne structure (for in-phase and quadrature signal component). Both of the two alternatives have some benefits and drawbacks. For example, a pair of DC notch filters can reduce the inband interferer and at the same time eliminate the common mode noise but suffers in dynamic range and channel selectivity. The IF bandpass filter on the other hand can reduce the inband interferer and at the same time obtain good dynamic range and channel selectivity; however, it is expensive, difficult to be integrated into a chip design and cannot eliminate the common mode noise. 
     If a two-stage inband interferer cancellation scheme is introduced, the different components can be arranged in different ways as shown in  FIG. 6 : 
     According to the first possibility shown in  FIG. 6   a , two consecutive cancellation circuits are used after a second mixer in order to reduce the generated noise level. The cancellation circuits are very cheep since they work in or near the baseband. However, since the whole RF link before the two cancellation circuits should be linear under the strong inband interferer, it seems to be unrealistic to adopt the structure of  FIG. 6   a.    
     In the second alternative shown in  FIG. 6   b , two consecutive cancellation circuits are placed just after the first mixer wherein the background noise level is generated by only one mixer and is partly overlapping with the spectrum of the MBS signal. The design of said mixer is difficult to realise assuming that the noise figure budget shall be kept unchanged. In addition, two IF SAW bandpass filters are more expensive and common mode noise cannot be eliminated. 
     The novel and more efficient concatenation of a two-stage cancellation circuit is shown in  FIG. 6   c , wherein the noise level can be reduced without additional complexity increase. A non-quadrature cancellation circuit is used after the first down-conversion mixer, which is cheaper than quadrature cancellation circuits as shown in  FIG. 6   a  and no image interference overlapping happens. The second cancellation circuit uses quadrature cancellation circuits (a pair of DC-notch filters) since they work in or near the baseband, which are less costly. Therefore, common mode noise can be reduced. 
     In summary, with the structure of  FIG. 6   c  the receiver performance is improved in the following points:
     a) The cancellation of the inband interferer can be efficiently concatenated and therefore the effect of a more than 60 dB higher inband interferer can be eliminated thoroughly. Accordingly, the receiver sensitivity can be obtained.   b) A combination of the benefits of an IF SAW bandpass filter and a pair of DC notch filters, i.e. good dynamic range, channel selectivity and the cancellation of common mode noise are achieved.   c) In comparison with the super heterodyne receiver architecture using a single IF SAW filter, the part count and the cost are only slightly increased.   

     FIG.  7 —in which similar reference signs refer to similar components as in the structures of FIGS.  4  and  5 —finally shows a block diagram of the improved two-stage interferer cancellation receiver architecture explained above. An additional intermediate frequency f 0  is inserted to implement the first stage interferer cancellation circuit and the channel selection filter. At this stage, a non quadrature solution is preferably adopted since the filter is not cheap. The signal obtained from the low noise amplifier  4  is thus at first mixed in a mixer  30  with a first mixing frequency f c -f 0  and then forwarded to the first stage inband interferer cancellation filter  31  and an amplifier  32 . The second interferer cancellation circuits however, are implemented by a pair of DC notch filters  33   a ,  33   b  located at the outputs of the two mixers  5   a ,  5   b , whereby quadrature processing is adopted for both inband interferer reduction and common mode noise cancellation. 
     In summary, the present invention provides a simple and relatively cheap solution for significantly increasing the receiver sensitivity of an interrogator for a MBS system. Accordingly, a reliable data transmission can be obtained and the useable range of MBS systems is extended.