Patent Application
20060098765
The present invention is related to the field of Radio Frequency IDentification (RFID) systems, and more specifically to interference cancellation in RFID readers.
Radio Frequency IDentification (RFID) systems can be used in many ways for locating and identifying objects. RFID systems are particularly useful in product-related and service-related industries for tracking large numbers of objects are being processed, inventoried, or handled. In such applications, an RFID tag is usually attached to individual items, or to their packages or containers.
In principle, RFID techniques entail using a device called an RFID reader to interrogate one or more RFID tags. Interrogation is performed by the reader transmitting a Radio Frequency (RF) wave. A tag that senses the interrogating RF wave responds by transmitting back another RF wave, a process known as backscatter. Backscatter may take place in a number of ways. The response may further encode a number stored internally in the tag. The response, and the number if available, is decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The number can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on. Some RFID tags generate the backscatter so that its spectrum straddles a carrier signal frequency.
An RFID tag typically includes an antenna system, a radio section, a logical section, and a memory. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can generate the backscatter while powered by only the RF signal it receives, enabling some RFID tags to operate without a battery.
An RFID reader typically includes a transceiver. The transmit portion of the transceiver provides a relatively high power carrier signal. The carrier signal may “feed-through” to receive portion of the transceiver, causing undesirable distortion of the received signal and DC offsets in the demodulated signals.
In accordance with the various described embodiments of the present invention, an interference canceller for a RFID reader is provided. In some embodiments, the canceller senses a signal related to the interference signal in the receive path of the reader. The receive path includes a downconverter. The canceller outputs an adjustment signal that depends on the sensed signal, which is coupled into the receive path before the downconverter. By providing cancellation in the receive path before downconversion, the downconverter can be designed to have a relatively small dynamic range without becoming saturated. This can reduce the complexity and cost of the reader. Further, the relatively large transients caused by the carrier feed-through signals as they propagate to the downconverter are eliminated relatively early in the receive process, which can help avoid other noise-related problems in the operation of the reader. Still further, some embodiments do not require a replica path to generate a cancellation signal, which can reduce the cost of the reader.
In some embodiments, the canceller uses a signal derived from the transmit path in generating the adjustment signal. The canceller adjusts the amplitude or phase (or both) of the derived signal to form the adjustment signal so that it cancels a carrier feed-through interference signal when injected into the receive path.
In some other embodiments, the canceller uses in-phase and quadrature component signals in sensing the interference signal. The in-phase and quadrature component signals serve as an error signal in a feedback loop, which can be implemented in hardware using loop filter units. The canceller can also include a vector modulator to adjust the amplitude and/or phase of the derived signal. In an alternative embodiment, the canceller includes a variable attenuator and a variable phase shifter to adjust the amplitude and/or phase of the derived signal.
In still another embodiment, the feedback loop is implemented in software executed by a processor included in the reader. In some alternative embodiments, the loop need not be continuous (e.g., having fixed settings or settings that can be updated as needed or according to a schedule).
Non-limiting and non-exhaustive embodiments are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.
Various embodiments are directed to a system, method and apparatus to cancel an interference signal in which the data signal spectrum of a modulated carrier signal is relatively close to the carrier frequency. For example, this scenario occurs in some RFID systems.
For example, in RFID system 100, the interference signal includes the aforementioned carrier feed-through (i.e., coupling of the carrier signal into receive path propagating the received backscatter signal). In addition, the interference signal may include noise, such as that from a non-coherent noise sources. In this example, canceller 130 is configured to provide the adjustment signal so that when combined with the received backscatter signal, the adjustment signal and the carrier feed-though signal destructively interfere or cancel. In the ideal case, the magnitude of adjustment signal would exactly match that of the carrier feed-through signal, while the phase of the adjustment signal would be exactly π radians out-of-phase with the carrier feed-through signal. In this way, the net energy of the combined adjustment and interference signals would be zero. In a typical application, the cancellation would result in a relatively small net energy. One embodiment of transceiver 110 is described in more detail in conjunction with
In this embodiment, canceller 130 advantageously avoids the use of a replica path to generate an adjustment signal, thereby reducing complexity and cost compared to cancellation systems that require a replica path.
In operation during a transmit mode, DSP 212 provides in-phase (I) and quadrature (Q) digital data to DACs 214 and 216, respectively. DACs 214 and 216 each convert the received digital data to an analog signal having a voltage (or current in other embodiments) that depends on the digital data. Modulator/upconverter 218 then receives the analog I and Q data signals and performs quadrature modulation to modulate the data onto a carrier signal. In some embodiments, the carrier signal is generated by a local oscillator (LO). In one embodiment, modulator/upconverter 218 uses a vector modulator to perform quadrature modulation to modulate M bits onto the carrier signal using a single “symbol” to be transmitted. In other embodiments, other suitable modulators and/or modulations schemes can be used in implementing modulator/upconverter 218. Although quadrature modulation well known in the art, a brief description of quadrature modulation is provided using modulator 218 and DACs 214 and 216.
The M bits correspond to 2M possible values for the symbol to be modulated on the carrier signal and transmitted. In one example embodiment, each of the possible values is mapped to a preselected set of ranges; i.e., a range of amplitude values of the carrier signal and a range of phase values of the carrier signal. For each set of M bits, DSP 212 provides digital input signals to DACs 214 and 216 so that DACs 214 and 216 output analog signals that cause modulator/upconverter 218 to modulate the phase and amplitude of the carrier signal to fall into the set of ranges of the symbol corresponding to that the set of M bits. For example, responsive to the analog signal received from DAC 214, modulator/upconverter 218 generates an I component signal (e.g., A1 cos(ωCt), where A1 is the amplitude of the I component signal and ωC is the carrier frequency) with the amplitude A1 being dependent on the magnitude (e.g., voltage) of the analog signal from DAC 214.
Similarly, responsive to the analog signal received from DAC 216, modulator/upconverter 218 generates a Q component signal (e.g., as A2 sin((ωCt), where A2 is the amplitude of the Q component signal) with the amplitude A2 being dependent on the magnitude of the analog signal from DAC 216. Because the I and Q signals are based on cosine and sine signals respectively, the I and Q signals are orthogonal and, with proper scaling, can be combined to form a signal with any arbitrary amplitude and phase. Modulator/upconverter 218 then combines the I and Q component signals, so that the resulting signal has an amplitude and a phase that fall within the respective ranges of the desired symbol.
PA 220 then receives the output signal from modulator/upconverter 218 and amplifies it for broadcast via antenna unit 222. In some embodiments, antenna unit 222 includes a duplexer (e.g., directional coupler, circulator, etc.) so that a single antenna can be used for both transmitting and receiving RF signals. In other embodiments, antenna unit 222 may include separate transmit and receive antennas.
As previously described, carrier feed-through (indicated by a dashed arrow 230 in
In this example application, the interference signal to be cancelled is the carrier feed-through previously described. Thus, in one embodiment, canceller 130 obtains the derived signal by coupling a relatively small portion of the carrier signal from the transmit path between PA 220 and antenna unit 222. In other embodiments, canceller 130 obtains the derived signal from the LO used to generate the carrier signal. Canceller 130 then adjusts the amplitude and phase of the derived signal to form the adjustment signal so that when the adjustment signal is coupled into the receive path, the adjustment signal will cancel the carrier feed-through signal. Schemes used to appropriately adjust the adjustment signal are described below in conjunction with
In some RFID applications, an unmodulated or continuous wave carrier signal is transmitted during a receive mode to provide a carrier signal that tags 1041-104N may modulate (i.e. backscatter) and from which tags 1041-104N may scavenge power. In this receive mode, downconverter/demodulator 224 receives a backscatter signal via antenna unit 222. In one embodiment, downconverter/demodulator 224 form a direct conversion receiver to directly obtain baseband I and Q components signals from the received backscatter signal. ADCs 226 and 228 convert the received I and Q component signals to digital signals, which are then processed by DSP 212 to extract data modulated on the carrier signal by one of tags 1041-104N. In some embodiments, tags 1041-104N use the same modulation scheme as transceiver 110, although in other embodiments they may be different.
Embodiments of canceller 130 of transceiver 110 can provide several advantages over other techniques used to solve interference problems. For example, by providing cancellation in the receive path before downconverter/demodulator 224, the performance of transceiver 110 can be improved and/or the complexity and cost of transceiver 110 can be reduced. Without cancellation before downconverter/demodulator 224, carrier feed-through signals will reach downconverter/demodulator 224. As a result, downconverter/demodulator 224 will undesirably output the I and Q component signals with a relatively large DC component. Further, because the backscatter signals are typically small relative to the carrier feed-through signal, downconverter/demodulator 224 and/or the LNA should have a relatively large dynamic range or else become saturated, resulting in distortion of the received signal. Increasing the dynamic range of these components can significantly increase complexity and cost of transceiver 110 (
In this embodiment, downconverter/demodulator 224 is a part of a direct conversion receiver and thus, is used to downconvert backscatter signals received via antenna unit 222 (
In an operation 402, a signal is derived from a transmit path of a transceiver. In one embodiment, canceller 130 (
In an operation 404, a signal that is propagating on a receive path of a transceiver is downconverted. In one embodiment, downconverter/demodulator 224 downconverts the backscatter signal received via antenna unit 222 (
In an operation 406, the downconverted signal from operation 404 is sensed. In one embodiment, canceller 130 senses the downconverted I and Q component signals outputted by downconverter/demodulator 224. In another embodiment, the I and Q component signals are sensed by DSP 212 (
In an operation 408, the derived signal from operation 402 is adjusted in response to the sensed signal from operation 406. In one embodiment, canceller 130 then adjusts the amplitude and/or phase of the derived signal to form an adjustment signal in response to the sensed signal. Embodiments of canceller 130 and how canceller 130 can adjust the derived signal are described in more detail below. For example, Cartesian cancellation embodiments of canceller 130 are described in conjunction with
In an operation 410, the adjusted signal from operation 408 is then injected into the receive path before downconverter/demodulator 224 so that the carrier feed-through signal can be canceled prior to downconverting a received backscattered signal. In one embodiment, coupler 302 (
In operation, coupler 502 splits off a relatively small portion of the carrier signal propagating in transmit path 300. This split off signal serves as the derived signal in this embodiment. In one embodiment, coupler 502 is a directional coupler although other types of couplers can be used to implement coupler 502 in other embodiments.
Ninety-degree splitter 504 then divides the derived signal into two signals having substantially the same power but with a ninety-degree phase difference. One of the output signals corresponds to the I component signal of the derived signal and the other output signal corresponds to the Q component signal of the derived signal. In one embodiment, ninety-degree splitter 504 is implemented with a quadrature coupler (or 3 dB hybrid coupler). Other types of ninety-degree splitters can be used in other embodiments. Further, in some embodiments, ninety-degree splitter 504 can be omitted, with the derived signal being obtained directly from the I and Q component signals generated by modulator/upconverter 218.
Mixers 506 and 508 then multiply the two signals being input to each mixer. In this embodiment, mixer 506 receives the Q component signal from ninety-degree splitter 504 and the output signal from loop filter unit 512. Mixer 508 receives the I component signal from ninety-degree splitter 504 and the output signal from loop filter unit 514. Mixers 506 and 508 then each output a signal that represents the product of its input signals. Mixers 506 and 508 may be implemented using any suitable design.
RF summer 510 then combines the two signals received from mixers 506 and 508 without introducing any phase difference between the input signals. In this embodiment, RF summer 510 is implemented using any suitable zero-degree combiner. In effect, ninety-degree splitter 504, mixers 506 and 508 and RF summer 510 implement a vector modulator receiving “data signals” from loop filter units 512 and 514. As previously described, with appropriate selection of the data signals provided to a vector modulator, the carrier signal (i.e., the derived signal in this case) can be adjusted to have any desired amplitude and phase. In this embodiment, loop filter units 512 and 514 provide the data signals to cancel the carrier feed-through signal using a negative feedback technique as described below.
Loop filter units 512 and 514 are used to control the magnitude of the “data signals” inputted to mixers 508 and 506, respectively. That is, loop filter unit 512 filters out the backscatter spectrum from the Q component signal extracted by downconverter/demodulator 224 from the receive signal, passing, in effect the DC component which results from the carrier feed-through signal. Loop filter unit 512 then outputs a data signal to mixer 508 so that the Q component signal of the adjustment signal will cancel out the Q component signal of the carrier feed-through signal. Similarly, loop filter unit 514 filters out the backscatter spectrum from the I component signal extracted by downconverter/demodulator 224 from the receive signal. Loop filter unit 514 then outputs a data signal to mixer 506 so that the I component signal of the adjustment signal will cancel out the I component signal of the carrier feed-through signal. In one embodiment, loop filter units 512 and 514 are implemented using integrators circuits. In this embodiment, loop filter units 512 and 514 have a relatively low bandwidth to keep the feedback loop stabilized on the carrier frequency rather than the spectrum of the modulation of backscatter signals.
In applications in which the carrier feed through interference and canceller performance is expected to stay relatively stable over time, the cancellation system need not use continuous feedback. For example, DSP 212 can have a look-up table (LUT) that is determined empirically to cancel out the carrier feed-through interference. In some embodiments, the settings for the canceller (for both Cartesian-based and polar-based embodiments) can be determined when the system is initiated using training techniques. The settings can be updated to account for changes in performance due to environmental factors such as aging, wear, temperature cycling, etc. Still further, other embodiments use an analog implementation of the non-continuous feedback loop. For example, an adjustable voltage regulator can be used to provide analog control of canceller settings.
In an alternative embodiment, canceller 130 is substantially similar to canceller 130 of
Reference has been made throughout this specification to “one embodiment,” “an embodiment,” or “an example embodiment” meaning that a particular described feature, structure, or characteristic is included in at least one embodiment of the present invention. Thus, usage of such phrases may refer to more than just one embodiment. Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
In addition, embodiments of the present invention may be implemented not only with physical components (e.g., within a semiconductor chip), but also within machine-readable media. For example, the designs described above may be stored upon and/or embedded with machine readable media associated with a design tool used for designing semiconductor devices. Examples include designs defined/formatted in VHSIC Hardware Description Language (VHDL), Verilog language and SPICE language. Some netlist examples include: a behavior level netlist, a register transfer level (RTL) netlist, a gate level netlist, and a transistor level netlist. Machine readable media also include media having layout information such as a GDS-II file. Further, netlist files or other machine-readable media for semiconductor chip design may be used in a simulation to perform the methods of the embodiments disclosed herein.
Thus, embodiments of the present invention may be used as or to support software program executed upon some form of processing core (e.g., a CPU of a computer) or otherwise implemented or realized upon or within a machine-readable medium. A machine-readable medium includes any mechanism for storing or transmitting information in a form readable by a machine (e.g. a computer). For example, a machine-readable medium can include read only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, etc. In addition, machine-readable media can include propagated signals such as electrical, optical, acoustical or other form of propagated signal (e.g., carrier wave signals, infrared signals, digital signals, etc.)
One skilled in the relevant art may recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, resources, materials, etc. In other instances, well known structures, resources, or operations have not been shown or described in detail merely to avoid obscuring aspects of the invention.
While example embodiments and applications have been illustrated and described, it is to be understood that the invention is not limited to the precise configuration and resources described above. Various modifications, changes, and variations apparent to those skilled in the art may be made in the arrangement, operation, and details of the methods and systems of the present invention disclosed herein without departing from the scope of the claimed invention.
