Patent Application
20130324032
The present invention relates generally to a method and apparatus for generating dedicated data transmission channels in inductive or magnetic coupled radio frequency communication networks.
Radio Frequency Identification (RFID) systems are commonly used to locate and track items in a near-field communication network including a reader device and at least one wireless terminal, or tag. Energized time-varying electromagnetic radio frequency (RF) waves, which comprise the carrier signal, are transmitted from the reader to the tags in a given RFID network or system. Inductive coupling may be used to transfer energy from one circuit (such as a conductive antenna coil and associated circuitry) to another by means of mutual inductance between the two circuits. A voltage is induced in the tag that can be rectified and used to power the tag circuitry. RFID networks may include tags and readers which exchange information using such inductive coupling between their inductive coupling coils (or antenna coils). To enable data to be passed from the tag to the reader, the tag circuitry changes the load, which is referred to herein as the coupled impedance, associated with its inductive coupling coil. This change can be detected by the reader as a result of the mutual inductive coupling, whereby a reader-originated RF signal can be modified back to the reader, the modified signal being modulated by the tag to transmit encoded data.
As a consequence, tag collision in RFID systems occur when the multiple tags are energized by the same RFID reader device, and simultaneously couple their respective, overlapping signals back to the reader using the given frequency channel. Thus the tag collision problem is exacerbated whenever a large number of tags must be read together in the same RF field. The reader is unable to differentiate these signals when the simultaneously generated signals collide. The tag collisions confuse the reader, generate data transmission errors, and generally reduce data throughput within the RFID system or network.
Various systems have been proposed to isolate individual tags. For example, in one technique aimed at reducing collision errors, when the reader recognizes that tag collision has taken place, it sends a special “gap pulse” signal. Upon receiving this signal, each tag consults a random number counter to determine the interval to wait before sending its data. Since each tag gets a unique number interval, the tags send their data at different times. The adverse impact on overall RFID system performance, in terms of data throughput rate, however, still exists.
Modulating the signal received by the tag and inductively coupling the modulated signal to the reader device is known, using such signal modulation schemes as phase shift keying (PSK) and amplitude shift keying (ASK), where the tag changes its coupled impedance by changing the impedance match between states. However, the adverse effects of tag collisions resulting from overlapping modified signals on a given frequency channel still remain.
Provided is an inductive coupling apparatus for modifying an incoming radio frequency (RF) signal. The inductive coupling apparatus comprises an inductive element, a variable impedance circuit having an output electrically coupled to the inductive element, and at least one low pass delta sigma (ΔΣ) modulator coupled to the variable impedance circuit and digitally controlling the output of the variable impedance circuit, wherein the for modifying the incoming RF signal is modified as the coupled impedance of the inductive element is adjusted in accordance with the output of the variable impedance circuit.
In one embodiment, an output of the at least one low pass delta sigma modulator switches the output of the variable impedance circuit between at least two states to adjust the coupled impedance Z.
In another embodiment, an input signal applied to the low pass delta sigma modulator consists of one of a complex modulation signal offset from the incoming radio frequency signal by +/−ω0.
The complex modulation signal may consists of any of a GMSK, QPSK, nPSK, nQAM, and an OFDM signal.
In yet another embodiment, the inductive coupling apparatus further comprises at least a second low pass delta sigma modulator coupled to the variable impedance circuit, wherein the output of the variable impedance circuit is further digitally controlled by the second low pass delta sigma modulator.
In a further embodiment, input signals to the first and second low pass delta sigma modulators of the inductive coupling apparatus comprise in-phase (I) and quadrature (Q) signals respectively.
The invention will now be described by way of example only with reference to the following drawings in which:
a shows, in one embodiment, an interleaved modulator apparatus for generating SSB signals based on IQ signal inputs;
b shows a representative output signal of the SSB signals generated using the interleaved modulator apparatus of
a shows, in one embodiment, an inductive coupling modulator apparatus for generating a QAM signal; and
b shows a representation of quadrature errors which may be produced in a modified signal generated using the modulator apparatus of
The term modulation as used herein refers to the process by which the radio frequency identification (RFID) wireless terminal, or tag, changes the carrier radio frequency (RF) signal of the reader inductive coupling apparatus to encode and convey information. For instance, in phase modulation, data being transmitted from the reader device to the tag is encoded in changes in the phase of the carrier wave sent out by the RFID reader device.
In an embodiment, one or more filters may be in the variable impedance circuit to filter out of band noise output from low pass delta sigma modulator 302.
In the examples of
a shows, in one embodiment, interleaved modulator apparatus 500a for generating SSB signals based on IQ signal inputs 508,509. Single side band (SSB) signals can be generated using two interleaved low pass ΔΣ modulators 502a, 502b.
The two interleaved ΔΣ modulators 502a, 502b provide signals that either change Z by 0, 90, 180,or 270° (or in general offset+0, offset+90, offset+180, or offset+270); see
Still in regard to
b shows a representative output signal 500b of the SSB signals generated using the modulator apparatus 500a of
If there are any errors in Z, this will result in an effective IQ offset in the modified signal. However, this can be corrected within the reader device using known IQ correction schemes. If the inductive element impedance changes, one can apply equalization on the RFID reader.
a shows, in one embodiment, modulator apparatus 700 for generating quadrature amplitude (QAM) signals. Data bits are applied to LUT (Look Up Table) 701 and then applied to ΔΣ modulators 702a, 702b.
By applying the SSB scheme, complex modulation signals like GMSK, nPSK, quadrature phase shift keying (QPSK), OFDM, nQAM, etc. may be generated, where n represents an integer.
In one embodiment, the output of low pass delta sigma modulators 702a, 702b may be a return to zero (RTZ), so if the data is 1101101, the output would be 10100010100010; note there is a zero between each bit. In an alternate embodiment, the output of low pass delta sigma modulator 702a, 702b may be a non-return to zero (NRZ) type signal; for example, if the data is 1101101, the output is 1101101, and nothing is added to the data stream.
The phases of Zin may have errors; i.e. Z0exp(j0°), (Z0+ε1)exp(j(180°+φ1), (Z0+ε2)exp(j(90°+φ2), and (Z0+ε3)exp(j(270°+φ3), where ε1, φ1, ε2, φ2, ε3, and Φ3 represent the errors, and Z0 is some reference impedance. These errors produce a quadrature error in the signal modified back by the inductive element.
b is a representation of the modified signal if a SSB is generated at an offset of δf. The error tone at −δf is produced due to this error; ideally the error signal would not exist. This quadrature error can be corrected by adjustments either (i) to the I and Q signals applied to the low pass delta sigma modulators, or (ii) within the reader of the RFID communication network itself.
For instance, in the reader what is measured is E(Q̂2)-E(Î2) and E(IQ), where E(x) is the average expected value. The term E(Q̂2)-E(Î2) is a measure of the gain mismatch, and E(IQ) is a measure of the phase mismatch. The gain on the I (or Q) channel may be corrected until E(Q̂2)-E(Î2)=0, and the phase so E(IQ)=0. This may be done in a closed loop scheme, for example using a Least Mean Square filter.
The matrix used to product the corrected I and Q values is :
Icorrected=I*D
Qcorrected=sin(phase_error)*I+cos(phase_error)*Q
where D is a measure of the gain mismatch between I and Q, and phase_error is the error in phase between I and Q. Without any error, D=1 and phase_error=0°.
With regard to the clocking function utilized by the wireless tag terminal, such as for driving the low pass delta sigma modulator, generation of the clocking function may be provided by a clock circuit within the tag reader, or via a clock circuit generation based on the frequency of the incoming RF signal provided by the reader device of the RFID network.
For example, in the instance of using the signal from the read as the clock, if the reader is at frf, the clock used by the tag will frf, or some frequency, frf/N, where N is some integer (that is frf is divided by N to generate a clock).
Although preferred embodiments of the invention have been described herein with regard to passive and semi-passive RFID communication networks, it is contemplated, and indeed it will be understood by those skilled in the art, that the solutions presented herein may be applied to other aspects of wireless communication. Accordingly, a person of ordinary skill in the art would understand that the specific embodiments described herein, while illustrative are not necessarily comprehensive. For instance, it is contemplated that the bitstream of the low pass delta sigma modulator described herein may be substituted by a similar bitstream from a pulse width modulated system. These, and other various modifications, may be made those skilled in the art without departing from the scope of the invention as defined by the claims.
