FULL-DUPLEX RADIO FREQUENCY ECHO CANCELLATION
A system comprising a transmitter element creating an interrogation signal and transmitting the interrogation signal and a receiver element receiving a reflection signal of the interrogation signal and combining the reflection signal and a feedback signal to cancel at least a portion of radio frequency echo signals in the reflection signal.
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Radio frequency identification (“RFID”) systems are used in a plethora of commercial contexts requiring a unique identification system for large numbers of items. Such contexts include everything from department store inventory and check-out systems to the tracking of military supplies to and from the front lines. Similar in utility to bar code technology, RFID systems are often preferred due to their increased range, lack of a line of sight requirement between a tag and its reader and the high multi-tag throughput of RFID readers (i.e., RFID readers may read many tags in their large field of view at very high transport speeds).
A problem that arises is that optimal performance of RFID systems is often hampered by the reflection and coupling which inevitably occur in RF transceivers, in which a significant portion of the transmitted interrogation signal is reflected by the antenna and objects in the environment into the receiving portion of the transceiver. These problems are quantified in a measure called the voltage standing wave ratio (“VSWR”), measured as the non-transmitted (i.e. coupled or reflected from the antenna or non-RFID objects in the environment) power over the total transmitted power of the transceiver. A high VSWR interferes with efficient transceiver performance and may even result in a “blinding” or complete saturation of the receiver. Transceivers designed to minimize VSWR are often unacceptable because of their high cost in terms of size and power, especially in the context of mobile devices.
SUMMARY OF THE INVENTIONA system comprising a transmitter element creating an interrogation signal and transmitting the interrogation signal and a receiver element receiving a reflection signal of the interrogation signal and combining the reflection signal and a feedback signal to cancel at least a portion of radio frequency echo signals in the reflection signal.
A method, comprising the steps of receiving a reflection signal, deriving a feedback signal from the reflection signal by isolating an error component of the reflection signal and combining the reflection signal and the feedback signal to cancel at least a portion of radio frequency echo signals in the reflection signal.
Furthermore, a method comprising the steps of demodulating a reflection signal into an in-phase signal and a quadrature signal, filtering the in-phase signal to isolate an in-phase error signal, filtering the quadrature signal to isolate a quadrature error signal, modulating the in-phase error signal and the quadrature error signal to create a feedback signal and combining the reflection signal and the feedback signal to cancel at least a portion of radio frequency echo signals in the reflection signal.
In addition, a system, comprising a demodulator to demodulate a reflection signal into an in-phase signal and a quadrature signal, a first filter to isolate an in-phase error signal from the in-phase signal, a second filter to isolate a quadrature error signal from the quadrature signal, a modulator to modulate the in-phase error signal and the quadrature error signal to create a feedback signal and a combiner element to combine the reflection signal and the feedback signal to cancel at least a portion of radio frequency echo signals in the reflection signal.
The transmitter portion of an RFID transceiver may create an interrogation signal for transmission by using a modulator 105 and a variable gain amplifier (“VGA”) 110 to modulate a carrier signal. Use of the VGA 110 may result in an amplitude modulated (AM) carrier wave. This modulated carrier wave may then be sent to a power amplifier 115 and band-pass filter 120. This amplified and filtered modulated carrier wave may then be sent to a circulator or coupler element 125 for transmission to the antenna 130.
This transmitted interrogation signal may then reflect off of an RF tag which has been attached to or associated with a piece of equipment or other commodity. These reflections, which carry information to identify the tag, may be received by the antenna 130. In an ideal RFID tranceiver, these received reflections constitute the whole of the signal received by the antenna 130. However, in deployed RFID systems, the received signal also contains an error component comprised of interrogation signal energy which has been coupled from the transmitter, reflected from the antenna 130, and reflected from objects in the environment other than the RF tag.
The incoming signal may arrive at the antenna 130 containing both valuable information from an RF tag and an error signal. In the exemplary embodiment of the present invention, this composite signal may be sent through a circulator 125 which may route the incoming signal into one input of an RF combiner 140. The combiner 140 may add this incoming signal to the output of the feedback circuit discussed below, and may feed the sum of these two signals into a band-pass filter 145. The band-pass filter 145 removes signal components outside of the frequency range of the modulated data signal of interest.
The signal may then be amplified by an automatic gain control (“AGC”) 150. This amplified signal may then be carrier-demodulated in quadrature using a demodulator 155. Both of the resulting demodulated signals (the in-phase signal Irx and the quadrature signal Qrx) may then be split. Two separate branches may take the in-phase and quadrature signals through band-pass filters 180I and 180Q before continuing towards the transceiver output for further processing by the base-band decoder 20.
Each of these branches includes a second path as input for a feedback loop. The feedback loop achieves echo cancellation in the transceiver by isolating the noise (error) component of the incoming signal using low-pass filters 160I and 160Q, subjecting this signal to a phase inversion, and then combining it with the incoming signal using another input of the RF combiner 140. The required phase inversion may be accomplished by modulating the physical path length of the return loop. For example, the path length may be controlled by either controlling the microwave traces on the circuit board at the design phase, or by adding a variable delay element for adaptive control. The feedback loop may be designed to converge with the incoming signal within the impulse response time of the low-pass filter, which is usually within a few cycles of the carrier signal.
After beginning the feedback loop, both the in-phase signal Irx and the quadrature signal Qrx may first be passed through low-pass filters 160I and 160Q. These low-pass filters may isolate the undesirable echo signal since the majority of the base band error signal is of a lower frequency than the signal of interest. In this example, the error signal is of a lower frequency and therefore low pass filters are used. However, there may be other implementations where the error signal is in a defined range of frequencies and a band-pass filter may be used or where the error signal is a higher frequency signal and a high pass filter is used. The outputs of these low-pass filters 160I and 160Q may then be modulated using modulator 165. The two signals may then be combined using a summing element 170. The resulting signal may then be passed through feedback amplifier 175 and the amplified signal may be fed into another input of RF combiner 140. This closes the feedback loop. The feedback signal may combine with the incoming signal in a manner which cancels out the noise component of the incoming signal, leaving only the modulated data reflected from the RF tag.
The exemplary embodiment of the present invention shown in
In the feedback portion of the signal path, these digital signals may then be filtered using digital low-pass filters 420I and 420Q contained in the base-band digital radio 410. The output of these filters may then be converted back into analog signals using digital-to-analog converters 425I and 425Q. These converters may inherently perform the echo cancellation performed by the sample-and-hold circuit 305 in
In the present exemplary embodiment the incoming signal may again be passed through a band-pass filter 145 and an AGC element 150. The signal may then be demodulated in-phase and in-quadrature using demodulators 155I and 155Q. The resulting base band signals may be passed through low-pass filters 160I and 160Q. The low pass filters 160I and 160Q are anti-aliasing filters for the D/A converters. This exemplary embodiment may utilize the digital sub-system (shown in detail in
The RF echo cancellation low pass filters may be digitally implemented in the baseband portion of the system. The multi-path signal propagation conditions change the nature of the echo signals from non-RFID elements that may be moving around in the environment. Thus, a digitally implemented adaptive filter may be advantageous. The inputs for adaptation may be a calibration period that sends out a known signal while obtaining reflections from known tags. For example, a known tag may be affixed to a known location on the wall near a docking bay portal. The digital system may also continuously re-calibrate the feedback loop by monitoring the video signals 520I and 520Q for imbalances. When such imbalances are detected the digital system may compute gain, phase, and offset correction factors, and then apply these factors to the feedback loop using Icancel and Qcancel signals 525I and 525Q.
The embodiment of
It will be apparent to those skilled in the art that various modifications and variations can be made in the structure and the methodology of the present invention, without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
Claims
1-19. (canceled)
20. A receiver, comprising:
- a demodulator configured to demodulate a signal into an in-phase signal and a quadrature signal; and
- a base-band digital radio configured to receive the in-phase signal and the quadrature signal and implement a feedback loop thereon, the feedback loop configured to provide adaptation to cancel a portion of radio frequency echo signals in the signal.
21. The receiver of claim 20, wherein the adaptation comprises real-time adaptation.
22. The receiver of claim 20, wherein the base-band digital radio comprises digital components configured to digitally implement echo cancellation, and wherein the digital components are configured to adapt to cancel the portion of radio frequency echo signals in the signal.
23. The receiver of claim 20, wherein the digital components are configured to adapt depending on multi-path signal propagation conditions due to movement of objects and the receiver relative to one another.
24. The receiver of claim 20, wherein the wherein the base-band digital radio is configured to continuously re-calibrate the feedback loop by monitoring the in-phase signal and the quadrature signal for imbalances.
25. The receiver of claim 24, wherein the wherein the base-band digital radio is configured to, upon detecting the imbalances, compute gain, phase, and offset correction factors, and to apply these factors to the feedback loop.
26. The receiver of claim 22, wherein the digital components are configured to adapt for a calibration period by receiving a reflection signal from a known signal interrogating a known tag.
27. The receiver of claim 22, further comprising:
- filters disposed between the in-phase signal and the base-band digital radio and between the quadrature signal and the base-band digital radio.
28. The receiver of claim 22, wherein the feedback loop outputs an in-phase cancellation signal and a quadrature cancellation signal.
29. The receiver of claim 28, further comprising:
- a modulator receiving the in-phase cancellation signal and the quadrature cancellation signal and a combiner forming a feedback signal as a combination of the in-phase cancellation signal and the quadrature cancellation signal.
30. The receiver of claim 29, further comprising:
- a coupler combining the signal and the feedback signal to cancel the portion of radio frequency echo signals in the signal.
31. The receiver of claim 30, further comprising:
- an amplifier disposed between the combiner and the coupler, the amplifier configured to provide impedance matching.
32. The receiver of claim 31, further comprising:
- a low pass filter disposed between the amplifier and the coupler, the amplifier providing impedance matching to the low pass filter.
33. A base-band digital radio, comprising:
- inputs for an in-phase signal and a quadrature signal;
- outputs for an in-phase cancellation signal and a quadrature cancellation signal;
- digital components configured to monitor the in-phase signal and the quadrature signal in order to drive the in-phase cancellation signal and the quadrature cancellation signal;
- wherein the in-phase cancellation signal and the quadrature cancellation signal are configured to combine with the in-phase signal and the quadrature signal to adaptively cancel a portion of radio frequency echo signals therein.
34. The base-band digital radio of claim 33, wherein the digital components are configured to adapt in real-time to cancel the portion of radio frequency echo signals in a signal formed by the in-phase signal and the quadrature signal.
35. The base-band digital radio of claim 33, wherein the digital components are configured to adapt depending on multi-path signal propagation conditions due to movement of objects and the base-band digital radio relative to one another.
36. The base-band digital radio of claim 33, wherein the digital components are configured to continuously re-calibrate the in-phase cancellation signal and the quadrature cancellation signal by monitoring the in-phase signal and the quadrature signal for imbalances.
37. The base-band digital radio of claim 36, wherein the wherein the digital components are configured to, upon detecting the imbalances, compute gain, phase, and offset correction factors, and to apply these factors to the in-phase cancellation signal and the quadrature cancellation signal.
38. A method, comprising:
- demodulating an incoming signal into an in-phase signal and a quadrature signal, the incoming signal comprising echo signals;
- digitally processing the in-phase signal and the quadrature signal to provide real-time adaptation based on the echo signals;
- continuously re-calibrating an in-phase cancellation signal and a quadrature cancellation signal based on the processing; and
- coupling the incoming signal with a combination of the in-phase cancellation signal and the quadrature cancellation signal.
39. The method of claim 38, wherein the echo signals comprising an error component based on any of signal energy coupled from a transmitter, signal energy reflected from an antenna, and signal energy reflected from non-radio frequency identification (RFID) elements in an environment.
Type: Application
Filed: Nov 3, 2011
Publication Date: Mar 1, 2012
Applicant: SYMBOL TECHNOLOGIES, INC. (Holtsville, NY)
Inventors: MARK DURON (EAST PATCHOGUE, NY), RAJ BRIDGELALL (MOUNT SINAI, NY)
Application Number: 13/288,622
International Classification: H04L 27/06 (20060101); H04B 17/00 (20060101); H03D 1/04 (20060101);