Patent Application
20130299579
The present invention relates generally to a method and apparatus for generating dedicated data transmission channels in backscatter 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. Tags use backscatter technology to reflect the reader's RF signal back to the reader, modulating the signal to encode and transmit data.
As a consequence, tag collision in RFID systems occur when the multiple tags are energized by the same RFID reader device, and simultaneously reflect 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 re-radiating the modulated signal backscattered to the reader device is known, using such signal modulation schemes, such as phase shift keying (PSK) and amplitude shift keying (ASK), where the tag changes its reflection coefficient by changing the impedance match between states. However, the adverse effects of tag collisions resulting from overlapping backscattered signals on a given frequency channel still remain.
Provided is an antenna apparatus for backscattering an incoming radio frequency (RF) signal. The antenna apparatus comprises an antenna for backscattering the incoming RF signal in accordance with a reflection coefficient characteristic of the antenna, a variable impedance circuit having an output electrically connected to the antenna, 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 reflection coefficient (Γ) of the antenna is adjusted based on 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 two states to adjust the reflection coefficient.
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 +/−ωo.
The complex modulation signal may consists of any of a GMSK, QPSK, nPSK, nQAM, and an OFDM signal.
In yet another embodiment, the antenna 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 antenna 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, a modulator apparatus for generating interleaved SSB signals based on IQ signal inputs;
b shows a representative output signal of the SSB signals generated using the modulator apparatus of
a shows, in one embodiment, a modulator apparatus for generating a GMSK signal;
b shows a representative output of the GMSK signal generated using the modulator apparatus of
c shows a representation of quadrature errors which may be produced in a reflected signal generated using the modulator apparatus of
a shows a representation of controlling data rate versus power; and
b shows a representation of link budget versus distance between tag and reader.
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 antenna to 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.
Still with regard to
The varying impedance can also be designed to produce a phase shift in the backscattered wave. That is,
Γi=αejφ
where φ has two states, φ1 and φ2 and α is a constant The back scattering impedance is then given by:
where Z1 has two states, Z1 and Z2. Here φi can be designed to have states φ1=0° and φ2=180°. Here, Zs is the impedance of the antenna. Since the antenna impedance adjusts given its environment, the effective Γ is simply rotated and scaled. This can be illustrated by assuming Zs changes to Zsβexp(jφ) where β is the scaling factor and φ is the rotation. Therefore, Γ changes to:
Given this, a change in Zs results in scaling and rotating Z by β−1 and −φ, respectively. In this complex modulation scheme, phase changes in r rather than amplitude changes may be utilized.
In an embodiment, a one filter or more filters may be in the variable impedance circuit to filter out of band noise output from the low pass delta sigma modulator.
Still with regard to
In the examples of
a shows, in one embodiment, modulator apparatus 500 for generating interleaved SSB signals based on IQ signal inputs 508,509.
Single side band (SSB) signals can also be generated, but two low pass ΔΣ modulators 502a, 502b are required. The two ΔΣ modulators 502a, 502b provide signals that either change Γ 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 510 of the SSB signals generated using the modulator apparatus 500 of
The impedance corresponding to the phases may be determined via the equations above. For example if α=1/sqrt(2), φ=0, 90, 180, 270, frf=1 GHz, Zs=50Ω, the impedances become 50+100j, 10+20j, 10−20j, and 50−100j, respectively.
If there are any errors in Z, this will result in an effective IQ offset in the reflected signal. However, this can be corrected within the reader device using known IQ correction schemes. If the antenna impedance changes, one can apply equalization on the RFID reader.
a shows, in one embodiment, modulator apparatus 800 for generating a Gaussian minimum shift keying (GMSK) signal. 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 the low pass delta sigma modulators 802a-b 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 802a-b 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.
b shows a representative output of the GMSK signal generated using modulator apparatus 800 of
c is a representation of the reflected 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 that is used:
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=0o.
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).
a shows a representation of controlling data rate versus power. A power management system may be introduced that depends on the data rate and the modulation type based on the distance between the tag and the reader, r. As the reader gets close enough (i.e. r<rmin) the tag gets enough power to turn on. From rmin to rcritical the tag transmits using a slow clock and ASK or PSK. As r<rcritical the tag can start transmitting using 64QAM. For RF backscattering technology, the power received by the tag (i.e. PRX(r)) is given by:
where λ is the wavelength of the carrier signal, r is the distance between the tag and the reader, PTX is the power of the transmitter, GTX is the antenna gain of the reader, and GRX is the antenna gain of the tag. The modulated power from the tag is then received by the reader.
b shows a representation of link budget versus distance between tag and reader. The link budget between the tag and reader is shown as a function of the tag-reader distance. The power received by the tag is decreased as the tags moves further away form the reader. At some such position, the backscatter power of the tag is attenuated as it travels back to the reader. The signal to noise ratio (SNR) is given by the power received by the reader over the phase noise of the oscillator within the reader.
For example, since the power coming from the reader falls off as (1/r̂2) the complex modulation technology may be applied when the reader is closer. As the reader gets further, lower modulations can be used.
It is understood that application of the complex modulation requires a higher SNR and more power.
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. Thus, other various modifications may be made those skilled in the art without departing from the scope of the invention as defined by the claims.
