U.S. patent application number 15/990085 was filed with the patent office on 2019-05-02 for systems and methods for compressive sensing ranging evaluation.
This patent application is currently assigned to Mojix, Inc.. The applicant listed for this patent is Mojix, Inc.. Invention is credited to Andreas Mantik Ali, Christopher Jones, Ramin Sadr, Andres I. Vila Casado.
Application Number | 20190129022 15/990085 |
Document ID | / |
Family ID | 51525009 |
Filed Date | 2019-05-02 |
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United States Patent
Application |
20190129022 |
Kind Code |
A1 |
Sadr; Ramin ; et
al. |
May 2, 2019 |
Systems and Methods for Compressive Sensing Ranging Evaluation
Abstract
RFID systems for locating RFID tags utilizing phased array
antennas and compressed sensing processing techniques in accordance
with embodiments of the invention are disclosed. In one embodiment
of the invention, an RFID system includes at least one exciter that
includes at least one transmit antenna, a phased antenna array that
includes a plurality of receive antennas, and an RFID receiver
system configured to communicate with the at least one exciter and
connected to the phased antenna array, where the RFID receiver
system is configured to locate an RFID tag by performing reads of
the RFD tag at multiple frequencies, generating a measurement
matrix, and determining a line of sight (LOS) distance between the
activated RFID tag and each of the plurality of receive antennas by
eliminating bases from the measurement matrix.
Inventors: |
Sadr; Ramin; (Los Angeles,
CA) ; Ali; Andreas Mantik; (Walnut, CA) ; Vila
Casado; Andres I.; (Los Angeles, CA) ; Jones;
Christopher; (Pacific Palisades, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Mojix, Inc. |
Los Angeles |
CA |
US |
|
|
Assignee: |
Mojix, Inc.
Los Angeles
CA
|
Family ID: |
51525009 |
Appl. No.: |
15/990085 |
Filed: |
May 25, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14796727 |
Jul 10, 2015 |
9983299 |
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15990085 |
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13831938 |
Mar 15, 2013 |
9111156 |
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14796727 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G06K 7/10366 20130101;
G01S 13/75 20130101; G06K 7/10356 20130101; G01S 13/00 20130101;
G01S 11/02 20130101; G01S 2013/0245 20130101; G06K 7/10099
20130101 |
International
Class: |
G01S 11/02 20060101
G01S011/02; G06K 7/10 20060101 G06K007/10; G01S 13/75 20060101
G01S013/75; G01S 13/00 20060101 G01S013/00 |
Claims
1. An RFID system comprising: at least one exciter comprising at
least one transmit antenna configured to transmit an activation
signal to activate an RFID tag; a phased antenna array comprising a
plurality of receive antennas configured to receive a backscattered
response signal from the activated RFID tag; an RFID receiver
system configured to communicate with the at least one exciter and
connected to the phased antenna array, the RFID receiver system is
configured to locate an RFID tag by: performing reads of the RFID
tag at multiple frequencies using the at least one exciter and the
plurality of receive antennas of the phased antenna array;
generating a measurement matrix for each of the plurality of
receive antennas using the phase of the backscattered response
signals from the activated RFID tag at each of the multiple
frequencies; and determining a line of sight (LOS) distance between
the activated RFID tag and each of the plurality of receive
antennas by eliminating bases from the measurement matrix.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 14/796,727, filed Jul. 10, 2015, which is a
continuation of U.S. Patent Application No. U.S. patent application
Ser. No. 13/831,938 filed Mar. 15, 2013, issued on Aug. 18, 2015 as
U.S. Pat. No. 9,111,156, the disclosures of which are incorporated
herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to Radio Frequency
Identification (RFID) systems and more specifically to RFID systems
utilizing phased array antennas.
BACKGROUND
[0003] RFID systems can be used to track, identify, and/or locate
items. Such systems conventionally include RFID tags that are
affixed to the items, an RFID reader that includes a transmit
antenna to send activation signals to the RFID tags and a receive
antenna to receive backscattered response signals from the
activated tags. As a limitation, many RFID systems require that the
RFID reader be within close proximity to the activated RFID tag in
order to correctly receive the response signal. The backscattered
response signal is more vulnerable to interferences as the distance
between the RFID tag and the receive antenna increases. Further,
the backscattered response signal may travel multiple paths to the
receiver antenna creating multipath distortion.
[0004] The theory of compressive sampling, also known as compressed
sensing or CS, is a novel sensing/sampling paradigm that allows one
to recover signals from far fewer samples or measurements than once
thought to be possible. The following overview of CS is largely
drawn from Emmanuel J. Candes and Michael B. Wakin, An Introduction
to Compressive Sampling, IEEE Signal Processing Magazine 21 (March
2008).
[0005] CS in practice allows for designing sampling protocols that
allow for capturing less data while still maintaining the ability
to reconstruct the signal of interest. The two fundamental
requirements for CS protocols are that (1) the signals of interest
must be "sparse" and (2) the sensing modality must have a
sufficient degree of "incoherence".
[0006] By way of background, sparsity expresses the idea that the
"information rate" of a continuous time signal may be much smaller
than suggested by its bandwidth, or that a discrete-time signal
depends on a number of degrees of freedom, which is comparably much
smaller than its (finite) length. More precisely, CS exploits the
fact that many natural signals are sparse or compressible in the
sense that they have concise representations when expressed in an
appropriate basis.
[0007] Incoherence extends the duality between time and frequency
and expresses the idea that objects have a sparse representation in
one domain can be spread out in the domain in which they are
acquired, just as a Dirac or spike in the time domain is spread out
in the frequency domain. Put differently, incoherence says that
unlike the signal of interest, the sampling/sensing waveforms are
capable of having an extremely dense representation in an
appropriate domain.
Sparsity
[0008] Systems that perform CS typically are faced with the problem
in which information about a signal f(t) is obtained by linear
functionals recording the values:
y.sub.k=f, .phi..sub.k
[0009] In a standard configuration, the objects that the system
acquires are correlated with the waveform .phi..sub.k (t). If the
sensing waveforms are Dirac delta functions (spikes), for example,
then y is a vector of sampled values of f in the time or space
domain. If the sensing waveforms are sinusoids, then y is a vector
of Fourier coefficients; this is the sensing modality used in
magnetic resonance imaging MRI.
[0010] Systems can apply CS to recover information in undersampled
situations. Undersampling refers to a circumstance in which the
number M of available measurements is much smaller than the
dimension N of the signal f. In such situations, a CS protocol is
tasked with solving an underdetermined linear system of equations.
Letting A denote the M.times.N sensing or measurement matrix with
the vectors .phi.*.sub.1, . . . , .phi.*.sub.M as rows (a* is the
complex transpose of a), the process of recovering f.di-elect
cons..sup.N from y=Af.di-elect cons..sup.M is ill-posed in general
when M<N: there are infinitely many candidate signals for f.
Shannon's theory indicates that, if f(t) has low bandwidth, then a
small number of (uniform) samples will suffice for recovery. Using
CS, signal recovery can actually be made possible using a broader
class of signals.
[0011] Many natural signals have concise representations when
expressed in a convenient basis. Mathematically speaking, a vector
f.di-elect cons..sup.N can be expanded in an orthonormal basis
.PSI.=[.psi..sub.1.psi..sub.2 . . . .psi..sub.N] as follows:
f ( t ) = i = 0 N x i .psi. i ( t ) ##EQU00001##
where x is the coefficient sequence of f, x.sub.i=f,
.psi..sub.k.
[0012] It can be convenient to express f as .PSI. (where .PSI. is
the N.times.N matrix with .psi..sub.1, . . . , .psi..sub.n as
columns). The implication of sparsity is now clear: when a signal
is a sparse expansion, the small coefficients can be discarded
without much perceptual loss. Formally, consider f.sub.s (t)
obtained by keeping only the terms corresponding to the S largest
values of (x.sub.i). By definition f.sub.s:=.PSI.x.sub.s, where
x.sub.s is the vector of coefficients (x.sub.i) with all but the
largest S set to zero. This vector is sparse in a strict sense
since all but a few of its entries are zero. Since .PSI. is an
orthonormal basis,
.parallel.f-f.sub.S.parallel.=.parallel.x-x.sub.S.parallel..sub.t2,
and if x is sparse or compressible in the sense that the sorted
magnitudes of the (x.sub.i) decay quickly, then x is well
approximated by x.sub.s and, therefore, the error
.parallel.f-f.sub.S.parallel.=.parallel.x-x.sub.S.parallel..sub.t2
is small. In plain terms, one can "throw away" a large fraction of
the coefficients without much loss. As can be appreciated, sparsity
is a fundamental modeling tool which permits efficient fundamental
signal processing; e.g., accurate statistical estimation and
classification, efficient data compression, etc. Sparsity has more
surprising and far-reaching implications, however, which is that
sparsity has significant bearing on the acquisition process itself.
Sparsity determines how efficiently one can acquire signals
nonadaptively.
Incoherent Sampling
[0013] Consider a pair (.phi., .PSI.) of orthonormal bases or
orthobases of .sup.N. The first basis .phi. is used for sensing the
object f and the second .PSI. is used to represent f. The coherence
between the sensing basis .phi. and the representation basis .PSI.
is
.mu. ( .PHI. , .PSI. ) = N max 1 .ltoreq. k , j .ltoreq. N .PHI. k
, .psi. j ##EQU00002##
[0014] In plain English, coherence measures the largest correlation
between any two elements of .PHI. and .PSI.. If .PHI. and .PSI.
contain correlated elements, the coherence is large. Otherwise, it
is small. As for how large and how small, it follows from linear
algebra that .mu.(.PHI., .PSI.).di-elect cons.[1, N].
[0015] Compressive sampling is mainly concerned with low coherence
pairs of bases. Such bases include the time frequency pair where
.phi. is the canonical or spike basis and .PSI. is the Fourier
basis, and wavelet bases for .PSI. and noiselet basis for .phi..
Random matrices are largely incoherent with any fixed basis .PSI..
Select an orthobasis .phi. uniformly at random, then with high
probability, the coherence between .phi. and .PSI. is about (2 log
N). In terms of hardware cost and complexity, it is desirable if
the signal basis, .PSI., does not need to be known a priori in
order to determine a viable sensing matrix .phi.. Fortunately,
random sensing matrices with sufficient sample size exhibit low
coherence with any fixed basis. This means that a random sensing
matrix can acquire sufficient measurements to enable signal
reconstruction of a sparse signal without knowing a priori the
proper basis .PSI. for the signal.
Undersampling and Sparse Signal Recovery
[0016] Ideally, the N coefficients of f are observed, but in
reality a CS system can only observe a subset of these and collect
the data
y.sub.k=f, .PHI..sub.k, k.di-elect cons.M
where M.di-elect cons.[1, . . . , n] is a subset of cardinality
M<N.
[0017] With this information, a conventional approach is to recover
the signal by I.sub.1-norm minimization. Essentially, for all
objects consistent with the data, find the object with the
coefficient sequence that minimizes the I.sub.1-norm. The use of
the I.sub.1-norm as a sparsity-promoting function traces back
several decades. A leading early application was reflection
seismology, in which a sparse reflection function (indicating
meaningful changes between subsurface layers) was sought from
bandlimited data. However I.sub.1-norm minimization is not the only
way to recover sparse solutions; other methods, such as greedy
algorithms, or Orthogonal Matching Pursuit can also be
utilized.
[0018] In view of the above, CS suggests a very concrete
acquisition protocol: sample nonadaptively in an incoherent domain
and invoke linear programming after the acquisition step. Following
this protocol enables the acquisition of a signal in a compressed
form. A decoder can then "decompress" this data.
SUMMARY OF THE INVENTION
[0019] RFID systems for locating RFID tags utilizing phased array
antennas and compressed sensing processing techniques in accordance
with embodiments of the invention are disclosed. In one embodiment
of the invention, an RFID system includes at least one exciter that
includes at least one transmit antenna configured to transmit an
activation signal to activate an RFID tag; a phased antenna array
that includes a plurality of receive antennas configured to receive
a backscattered response signal from the activated RFID tag; and an
RFID receiver system configured to communicate with the at least
one exciter and connected to the phased antenna array, where the
RFID receiver system is configured to locate an RFID tag by
performing reads of the RFD tag at multiple frequencies using the
at least one exciter and the plurality of receive antennas of the
phased antenna array, generating a measurement matrix for each of
the plurality of receive antennas using the phase of the
backscattered response signals from the activated RFID tag at each
of the multiple frequencies, and determining a line of sight (LOS)
distance between the activated RFID tag and each of the plurality
of receive antennas by eliminating bases from the measurement
matrix.
[0020] In a further embodiment, the RFID system of claim 1, where
performing reads of the RFID tag at multiple frequencies also
includes selecting a new transmit carrier frequency for the
activation signal and instructing the at least one exciter to send
the activation signal at the new transmit carrier frequency.
[0021] In another embodiment, the RFID system of claim 2, where
performing reads of the RFID tag at multiple frequencies also
includes receiving the backscattered response signal from the
activated RFID tag using each of the plurality of receive antennas
of the phased antenna array and measuring at least a phase
associated with the received backscattered response signal.
[0022] In a still further embodiment, the RFID system of claim 3,
where generating a measurement matrix also includes selecting a
basis function representing the distance travelled from the exciter
to the RFID tag to each of the plurality of receive antennas of the
phased antenna array.
[0023] In still another embodiment, the RFID system of claim 1,
where eliminating bases from the measurement matrix also includes
deconvolving the measurement matrix; sequentially eliminating a
basis from the basis function corresponding to distance outward
from the RFID receiver system; calculating and minimizing error
after elimination of each successive basis; and determining if the
calculated error is greater than a threshold value.
[0024] In a yet further embodiment, the RFID system of claim 5,
where the threshold value can be determined using a stopping
rule.
[0025] In yet another embodiment, the RFID system of claim 6, where
sequentially eliminating a basis also includes eliminating the
shortest remaining basis by a predetermined distance each time.
[0026] In a further embodiment again, the RFID system of claim 7,
where calculating and minimizing error after elimination of each
successive basis also includes forcing a convex optimization
process to fit with the remaining basis.
[0027] In another embodiment again, the RFID system of claim 1,
where eliminating bases from the measurement matrix also includes
placing an upper limit on the estimate of the line of sight
distance.
[0028] In a further additional embodiment, the RFID system of claim
1, where the RFID receiver system is also configured to locate an
RFID tag by defining a plurality of elliptical representations
using the at least one exciter, the RFID tag, and each of the
plurality of receive antennas of the phased antenna array.
[0029] In another additional embodiment, the RFID system of claim
10, where the RFID receiver system is also configured to locate an
RFID tag using the determined line of sight distance and the
plurality of elliptical representations to locate the RFID tag as
the intersection of a first ellipse and a second ellipse.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a schematic diagram of a distributed exciter
architecture showing transmit and receive coverage areas and
exciter interrogation spaces for an RFID system in accordance with
an embodiment of the invention.
[0031] FIG. 2 illustrates an RFID system utilizing elliptical
representations for determining the location of an RFID tag in
accordance with the prior art.
[0032] FIG. 3 illustrates an exciter, RFID tag, and RFID receiver
and a multipath propagation of a backscattered response signal from
an activated RFID tag to the RFID receiver in accordance with an
embodiment of the invention.
[0033] FIG. 4 is a flow chart illustrating a process for locating
RFID tags utilizing CS techniques in accordance with an embodiment
of the invention.
[0034] FIG. 5 is a flow chart illustrating a process for activating
RFID tags using multiple transmit frequencies in accordance with an
embodiment of the invention.
[0035] FIG. 6 is a flow chart illustrating a process for
eliminating multipath distortion and locating the line of sight
(LOS) distance between RFID receivers and RFID tags in accordance
with an embodiment of the invention.
DETAILED DESCRIPTION
[0036] Referring now to the drawings, systems and methods for
locating radio frequency identification (RFID) tags utilizing
phased array antennas and compressed sensing (CS) processing
techniques are described. The systems and methods utilize group
delay measurements and CS techniques to reduce the effects of
multipath distortion on backscattered response signals received at
the RFID receiver. The systems include estimating the line of sight
(LOS) distance between RFID tags and RFID receivers by creating a
measurement matrix and selecting appropriate basis functions to
eliminate multipath distortion. By successively eliminating a basis
and observing the effects on error calculations, the system is able
to accurately determine the LOS distance from the RFID receiver to
the RFID tag. The LOS distance is determined for each receive
antenna of a phased antenna array and the RFID tag is located using
the system and method described in U.S. Pat. No. 8,082,311 entitled
"Radio Frequency Identification Tag Location Estimation and
Tracking System and Method", issued Dec. 6, 2011, the disclosure of
which is incorporated by reference herein in its entirety.
[0037] A variety of RFID reader configurations can be used in
accordance with embodiments of the invention including, but not
limited to, configurations in which the transmit and receive
functions of the reader are decoupled and can be performed by
separate exciters and RFID receivers as described in U.S. patent
application Ser. No. 12/054,331, filed Mar. 23, 2007 and allowed
Oct. 5, 2012, entitled "RFID Systems Using Distributed Exciter
Network", the disclosure of which is incorporated by reference as
if set forth in full herein.
Distributed Architecture
[0038] An RFID system including a distributed exciter architecture
in accordance with an embodiment of the invention is shown in FIG.
1. The RFID system (1-1) includes an RFID receiver system (1-2)
connected to a phased antenna array (1-4) and a plurality of
exciters (1-6, 1-14, 1-18, 1-23, 1-28) that are daisy chained to
the RFID receiver system via cables (1-10, 1-9, 1-16, 1-22, 1-26).
The RFID receiver system (1-2) is also connected to a LAN (1-32)
via connection (1-34). An RFID application server (1-30) is
connected to the LAN via connection (1-36). Although the plurality
of exciters are shown as wired, in many embodiments exciters
communicate wirelessly with the RFID receiver system.
[0039] In operation, the RFID receiver system (1-2) controls the
activation of exciters. The cable segments (1-10, 1-9, 1-16, 1-22,
1-26) carry both direct current (DC) power and control commands
from the RFID receiver system (1-2) to each exciter. The
transmitted "backhaul signal" from the RFID receiver system (1-2)
to the exciters embeds signal characteristics and parameters that
can be used to generate a desired waveform output from the exciter
module to an RFID tag. In several embodiments, each exciter can be
commanded and addressed by an N-bit address, N-ranging from
16-to-32 bit. The exciters (1-6, 1-14, 1-18, 1-23, 1-28) can be
operated sequentially or concurrently, depending on the number of
possible beams the RFID receiver system can support. In the
illustrated embodiment, the RFID receiver system (1-2) includes a
single phased antenna array (1-4) and is capable of generating a
single beam. In other embodiments, the RFID receiver system
includes multiple antenna arrays and is capable of generating
multiple beams.
[0040] The interrogation space and transmitted power of each
exciter can be managed and controlled by the RFID receiver system
(1-2). In the illustrated embodiment, the RFID receiver system
(1-2) controls the exciters to create interrogation space (1-8,
1-15, 1-20, 1-24, 1-29) of different sizes. In addition, the
received coverage area is configurable. The RFID receiver system
can receive signals from the complete coverage area (1-11).
Alternatively, the RFID receiver system can adaptively beam-form to
the specified exciter interrogation spaces (1-12,1-21).
[0041] The RFID application server (1-30) schedules each exciter to
operate harmoniously in multiple dimensions, which are time,
frequency and space. In a number of embodiments, the RFID
application server (1-30) includes a scheduler for S/T/FDM (Space,
Time and Frequency Division Multiplexing), which utilizes an
optimization algorithm to maximize the probability of successful
manipulation of all the RFID tags within a target interrogation
space. In addition, the controller may utilize frequency hopping in
scheduling the frequency channel for each exciter in order to
satisfy various regulatory constraints. Although specific RFID
systems including a distributed architecture are discussed above
with respect to FIG. 2, any of a variety of RFID system
architectures as appropriate to the requirements of a specific
application can be utilized in accordance with embodiments of the
invention. Processes for determining RFID tag locations using
elliptical representation are discussed below.
RFID Tag Location Using Elliptical Representation
[0042] In several embodiments of the invention, the RFID system
observes a backscattered response signal from activated RFID tags
including the signal's phase information. Phase differences
observed at various transmit frequencies can provide range
information. The ratio of phase difference to frequency difference,
referred to as group delay, can provide estimates of the path
length between exciters, RFID tags and receive antennas. Using the
path lengths and known relative distances between exciters and RFID
receivers, an elliptical representation can be utilized to locate
RFID tags.
[0043] An RFID system utilizing elliptical representations for
determining the location of an RFID tag using a receiver antenna
array in accordance with the prior art is shown in FIG. 2. The RFID
system (200) includes an RFID receiver antenna array (1-4) with a
first receive antenna RX.sub.1 (202) and N-1 additional antennas
such that the last antenna is RX.sub.N (204). The ellipse (210) is
formed using exciter (206) and RX.sub.1 (202) as the focus points.
The ellipse (212) is formed using exciter (206) and RX.sub.N (204)
as the focus points. Additional ellipses are formed using the
exciter (206) and the additional N-1 receive antennas of the
antenna array (1-4).
[0044] The location of an RFID tag (208) is also shown. The exciter
is configured to transmit interrogation signals and the receive
antennas are configured to receive signals backscattered by the
RFID tag. Each receive antenna is a known distance from the
exciter, for example RX.sub.1 (202) and RX.sub.N (204) are spaced a
distance d1 and d'1, respectively relative to the exciter (206).
Path length from the exciter to tag to receiver, also known as the
ETR distance, can be represented as the distance d2+d3 to receive
antenna RX.sub.1 (202) and d'2+d'3 to receive antennas RX.sub.N
(204). The ETR distance can be determined using group delay
observations and the systems and methods described in U.S. Pat. No.
8,082,311 entitled "Radio Frequency Identification Tag Location
Estimation and Tracking System and Method", issued Dec. 6, 2011,
incorporated by reference above. Accordingly, the ETR distances can
be used with a priori known receive antenna and exciter locations
to create elliptical representations such that the RFID system can
locate RFID tags. In many embodiments of the invention, the RFID
tag (208) is located as the intersection of a first ellipse (210)
and a second ellipse (212). The method of locating RFID tags
utilizing group delay observations and elliptical representation
becomes more accurate with additional receive antennas utilized.
However, interferences can negatively affect locating RFID tags as
further discussed below.
[0045] The backscattered response signal of an activated RFID tag
can take multiple paths to reach the RFID receiver. The receive
antenna cannot decipher how many paths, if any, a backscattered
response signal has traveled and thus leads to so called multipath
distortion. An illustration of multipath distortion in accordance
with an embodiment of the invention is shown in FIG. 3. The
backscattered response signal bounces off obstacle (302) in route
to the receive antenna RX.sub.1 (202) and thus travels via two
paths, d3 and d4+d5. As discussed above, the additional paths
negatively impact correctly determining the LOS distance between
RFID tag and RFID receiver. Although not illustrated in FIG. 3,
there can be more than one obstacle and thus increased multipath
distortion.
[0046] Although specific process for determining the location of an
RFID tag using elliptical representation utilizing a phased antenna
array are discussed above with respect to FIG. 2, any of a variety
RFID receiver antenna array configurations as appropriate for
specific applications can be utilized in accordance with
embodiments of the invention. Processes for locating RFID tags
utilizing compressed sensing techniques in accordance with
embodiments of the invention are discussed further below.
Locating RFID Tags Utilizing CS Techniques
[0047] In a compressed sensing approach, the signal received at
each receive antenna is assumed to be a sum of the multipath with
different distances and phases. In several embodiments of the
invention, the received signal wave is deconvolved to express the
received signal as the sum of n different distances that the
backscattered signal travelled through such that:
y = i = 0 n .alpha. i e jf + k ##EQU00003##
for each of the transmit frequencies.
[0048] In several embodiments of the invention, the RFID system
measures phase of a received signal at 50 frequency channels (the
number of channels allowed in the United States that are open for
RFID air communications), where more channels increase sparsity.
The CS technique calls for selecting as few basis vectors as
possible that still satisfy a given constraint. Knowing a priori
that the signal of interest lies in the LOS path, many embodiments
of the invention use a successive initial basis elimination (SIBE)
approach to give an upper bound on the positive error. In various
embodiments, error statistics can be computed via Monte-Carlo
simulations and tabulated for the first and second moments for
several noise figures.
[0049] A process for locating RFID tags utilizing CS techniques in
accordance with an embodiment of the invention is shown in FIG. 4.
The process (400) includes performing (404) reads of RFID tags at
multiple frequencies as discussed further below. Using the
backscattered response signals, a measurement matrix is generated
(406). The process (400) includes reducing (408) multipath effects
and locating (410) the LOS distance. In a number of embodiments,
multipath effects are reduced by using a successive initial basis
elimination (SIBE) approach, which is discussed further below.
Although, in other embodiments any of a variety of processes can be
used to eliminate bases from the bases used to construct the
measurement matrix to determine the LOS distance as appropriate to
the requirements of specific applications. Using the LOS distance,
the RFID tag can be located (412). Although specific processes for
locating RFID tags utilizing CS techniques are discussed above with
respect to FIG. 4, any of a variety of processes for locating RFID
tags utilizing CS techniques as appropriate to the requirements of
a specific application can be utilized in accordance with
embodiments of the invention. Processes for performing RFID tags
reads using multiple transmit frequencies are discussed further
below.
Performing RFID Tag Reads at Multiple Transmit Frequencies
[0050] In several embodiments of the invention, the exciter
transmits activation signals to the RFID tag using multiple
frequencies. A process for performing reads of RFID tags using
multiple transmit frequencies in accordance with an embodiment of
the invention is shown in FIG. 5. The process (500) includes
selecting (504) a new transmit frequency and instructing (506) the
exciter to send an activation signal to the RFID tag. The RFID
receiver receives (508) the backscattered information signal from
the activated RFID tag. The process further includes measuring
(510) the phase of the received information signal. If an
additional frequency is available then process (500) is repeated
from step (504). If no additional frequency is available then
process (500) is complete. Although specific processes for
performing reads of RFID tags with multiple transmit frequencies
are discussed above with respect to FIG. 5, any of a variety of
processes for performing reads of RFID tags using multiple transmit
frequencies as appropriate to the requirements of a specific
application can be utilized in accordance with embodiments of the
invention. Processes for eliminating multipath effects using a
successive initial basis elimination approach are discussed further
below.
Eliminating Multipath Distortion
[0051] CS techniques can be utilized to eliminate multipath
distortion and determine the LOS distance. A successive initial
basis elimination (SIBE) approach is posed by minimizing the
following expression:
(1-.gamma.).parallel.Ax-b.parallel..sub.1+.gamma..parallel.x.parallel..s-
ub.1
where A is a M_basis.times.N_frequency matrix consisting of the
basis for each frequency, x is a M_basis by 1 complex weight
vector, and b is a N_frequency.times.1 complex vector of
beamforming coefficients that are measured from each of the RFID
tag reads. The vector b includes the observations from the response
signal and L1 norm of Ax-b describes how well the Ax matches the
observations. In several embodiments, a L2 norm of Ax-b can be
utilized to describe how well the Ax matches the observations.
Using a convex optimization process, the system determines the
lowest coefficient that contributes the most to the observation and
once that coefficient is removed from the observation vector, the
error significantly increases. When no noise is present, the
shortest (nearest to 0) component of the estimate often corresponds
to the true LOS path. The choice of .gamma. within 0.1 to 0.9 does
not give significant difference in the simulated multipath. The
SIBE approach exploits the LOS by successively eliminating the
shortest basis by a predetermined distance away from the RFID
receiver each time and hence forcing the convex optimization
process to fit with the remaining basis.
[0052] A process for eliminating the effects of multipath in
determining the LOS distance in accordance with an embodiment of
the invention is shown in FIG. 6. The process (600) includes
deconvolving (604) the measurement matrix. A basis is sequentially
eliminated (608) outward from the RFID receiver. After elimination
of each successive basis, the process includes calculating and
minimizing (610) error as described below. The change in error is
compared (612) to a threshold value. In several embodiments, the
threshold value can be determined by a stopping rule that can be
predetermine or determined in real-time. If the change in error is
not greater than a determined threshold, the process (600) is
repeated from step (604). If the change in error is greater than
threshold, the LOS is distance is determined (614) as described
below. In several embodiments of the invention, the LOS distance is
then used to locate the RFID tag utilizing elliptical
representation.
[0053] Process (600) sequentially eliminates the shortest basis by
a predetermined distance each time and generates a misalignment
which increases the mean square error (MSE) fit if the true LOS
path is eliminated. By detecting the pivot point where significant
error occurs by comparing the error to a threshold value, the RFID
system can estimate the true LOS distance. Process (600) also puts
an upper limit on the estimate since the error significantly
increases once the LOS path is removed from the basis. Although
specific processes for locating RFID tags by eliminating multipath
distortion using CS techniques are discussed above with respect to
FIG. 6, any of a variety of processes for locating RFID tags by
eliminating multipath distortion using CS techniques as appropriate
to the requirements of a specific application can be utilized in
accordance with embodiments of the invention.
[0054] While the above description contains many specific
embodiments of the invention, these should not be construed as
limitations on the scope of the invention, but rather as an example
of one embodiment thereof. Accordingly, the scope of the invention
should be determined not by the embodiments illustrated, but by the
appended claims and their equivalents.
* * * * *