Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION
TTFLE OF INVENTION
RFID System Utilizing Parametric Reflective Technology
STATEMENT OF RELATED APPLICATION(S)
[00011 The present application claims priority based on United States
Provisional Patent
Application Serial No. 60/581,384, filed on June 22, 2004, in the name of
inventor Michael
Gregory Pettus, entitled "Millimeter Wave RFID System Using Parametric
Reflective
Encoding," commonly owned herewith, which is hereby incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention is directly generally to the field of radio
frequency
identification (RFID) interrogators and data tags as well as encoding and
decoding methods.
BACKGROUND OF THE INVENTION
[0003} There are many existing technologies in current development and
deployment that
implement the desired function of identifying articles, objects, vehicles and
personnel. Bar codes
and magnetic strips are traditionally familiar as short range devices. More
recently, techniques
for increasing the read reliability are being used in the general area of
radio frequency
identification or RFID.
[00041 RFID technology utilizes a tag transponder, which is placed on an
object, and a
reader, also referred to herein as an interrogator, to read and identify the
tag. RFID technologies
are broadly categorized using "active" tags with the longest range, and
"passive tags" with a
much shorter range (typically less than 20 feet). The industry categorizes
active tags as having a
local power source (such as a battery) so that the active tag sends a signal
to be read by the
interrogator. The industry categorizes passive tags as those whose power is
derived from the
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reader, whereby the passive tag re-transmits or transponds information upon
receiving the signal
from the reader.
[0005] In both of these categories of tags, there is an electronic circuit
that is typically in the
form of an integrated circuit or silicon chip, whereby the circuit stores and
connnunicates
identification data to the reader. In addition to the chip, the tag includes
some form of antenna
that is electrically connected to the chip. Active tags incorporate an antenna
which
communicates with the reader from the tag's own power source. For passive
tags, the antenna
acts as a transducer to convert radio frequency (RF) energy originating from
the reader to
electrical power, whereby the chip becomes energized and performs the
communication function
with the reader.
[0006] Considering that active and passive tags have electronic circuitry in
the fonn of a
chip, the manufacturing costs for each tag is significant. Not only is there a
cost associated with
the chip itself, but there are also numerous processing steps required in
order to place the chip
onto the tag. In addition, existing tags require a method of mechanically and
electrically
connecting the antenna to the chip, which adds to manufacturing costs.
[0007] It should also be noted that active and passive RFID tag technologies
are
fundamentally based on an interrogate-and-then-communicate sequence of
operations. Therefore
there is an amount of time for the interrogator to read the tag which is
dependent on the RF
bandwidth and the data rate of the communications channel between the
interrogator and the tag.
If more than one tag is within range of the interrogator, multiple interfering
transmissions can
result from the interrogator attempting to read a single tag. Also of note, in
the types of RFID
systems thus described, there are no straightforward methods to accurately
locate and track a tag.
The technologies described above provide only a method of identification.
100081 What is needed is a cl-.ipless RF'ID system and method that would
provide greater
range between the interrogator and the tags, lower manufacturing costs of the
tags, and less
aggregate read time for multiple tags in proximity to each other. What is also
needed is a system
which accurately locates and tracks a tag.
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BRIEF DESCRIPTION OF THE INVENTION
[00091 A system and method for encoding and decoding information by use of
radio
frequency antennas includes one or more interrogator devices and RFID data
tags. The RFID
data tags include a plurality of antenna elements which are formed on a
substrate or directly on
an object. The antenna elements are oriented and have dimensions to provide
polarization and
phase information representing the encoded information on the RFID tag. The
interrogator
device scans an area and uses radar imaging technology to create an image of a
scanned area.
The device receives re-radiated RF signals from the antenna elements on the
data tags, with the
data tags represented on the image. The re-radiated RF signals preferably
include polarization
and phase information of each antenna element, whereby the information is
utilized using radar
signal imaging algorithms to decode the information on the RF data tag.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The accompanying drawings, which are incorporated into and constitute a
part of this
specification, illustrate one or more embodiments of the present invention
and, together with the
detailed description, serve to explain the principles and implementations of
the invention.
Figure lA illustrates a radio frequency identification (RFID) system for use
in a baggage
identification setting in accordance with one example embodiment of the
invention.
Figure 1B illustrates a luggage label having the RFID tag of one embodiment of
the
present invention.
Figure 1C illustrates a handheld RFID interrogator in accordance with one
embodiment
of the present invention.
Figure 1D illustrates a personal identification card having the RFID tag
thereon in
accordance with one embodiment of the present invention.
Figure lE illustrates a container having multiple RFID tags thereon in
accordance with
one embodiment of the present invention.
Figure 2 illustrates a schematic of the RFID interrogator in accordance with
one
embodiment of the present invention.
Figure 3A illustrates a schematic of an antenna configuration on a substrate
of an RFID
tag in accordance with one embodiment of the present invention.
Figure 3B illustrates a side view of the RFID tag in accordance with one
embodiment of
the present invention.
Figure 3C schematic of another RFID tag antenna configuration on a substrate
in
accordance with one embodiment of the present invention.
Figure 4A illustrates a schematic of another RFID tag antenna configuration on
a
substrate in accordance with one embodiment of the present invention.
Figure 4B illustrates a schematic of another RFID tag antenna configuration on
a
substrate in accordance with one embodiment of the present invention.
Figure 5 illustrates a block diagram of the decoding process in accordance
with one
embodiment of the present invention.
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Figure 6 illustrates an example table of a data coding scheme for the RFID tag
in
accordance with one embodiment.
Figure 7 illustrates a flow diagram of one example encoding process in
accordance with
one embodiment of the present invention.
DETAILED DESCRIPTION
[0011] Embodiments of the present invention are described herein in the
context of an RFID
system utilizing parametric reflective technology. Those of ordinary skill in
the art will realize
that the following detailed description of the present invention is
illustrative only and is not
intended to be in any way limiting. Other embodiments of the present invention
will readily
suggest themselves to such skilled persons having the benefit of this
disclosure. Reference will
now be made in detail to implementations of the present invention as
illustrated in the
accompanying drawings. The same reference indicators will be used throughout
the drawings
and the following detailed description to refer to the same or like parts.
[0012] The present invention described herein preferably utilizes a very low
cost RFID tag
construction that does not require semiconductor or chip technologies. The
present invention
preferably can read the RFID tags at distances up to 100 meters as well as
read and identify
thousands of RFID tags per second. In addition, the present invention can
preferably provide an
accurate two and/or three dimensional location bearing of an RFID tag. If the
RFID tag is
located on a moving object, velocity and trajectory information can be
computed by the
interrogator of the present invention. The present invention preferably
utilizes frequencies in the
millimeter wave region, which allows detection of RFID tags behind foliage and
non-metallic
building materials. However, it is contemplated that the present invention
utilizes frequencies in
other ranges, and is not limited to the millimeter wave region.
[0013] The present invention can reduce operational expense, improve
efficiencies and
provide features to industry, government, homeland security, military,
healthcare,
education, transportation and consumers. The present invention can be used in
a wide range of
applications including, but not limited to: inventory identification; asset
management tracking
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and shipping container location; vehicular access control (e.g. toll ways);
moving vehicle
identification; healthcare identification and tracking of patients, drugs,
equipment and personnel
identification, tracking and monitoring of personnel and equipment for
security purposes;
identification of luggage and packages at airports; systems for locating lost
objects (e.g. keys,
files, golf balls, clothing articles).
[0014] Figure lA illustrates an RFID system in accordance with one example
embodiment of
the present invention. As shown in Figure 1A, the RFID system 10 includes one
or more
interrogators 100, also referred to as readers, as well as one or more RFID
tags 200. The system
shown in Figure 1A depicts a conveyer belt 97 normally found in a baggage
claim area in an
airport, whereby various suitcases 98 and packages 99 are delivered on the
conveyer belt 97. As
shown in Figure 1A, the packages 99 include one or more RFID tags 200 printed
or affixed
thereon, whereas the suitcases 98 each include an attachment 96 which includes
the RFID tag
200 thereon. Figures 1B, 1D and 1E illustrate examples of the RFID tags 200 on
the suitcases
and packages.
[0015] The interrogators 100 are shown in Figure 1A as stand alone units,
whereby the
interrogators locate, identify and optionally track each tag 200 as the items
98, 99 move on the
conveyer belt 97. In another embodiment, the interrogator is in the form of a
handheld unit 101,
as shown in Figure 1 C. It should be noted that the RFID system of the present
invention is
operable in a multitude of applications and settings, some of which are
discussed below, and is in
no way limiting to the examples shown and described herein.
[0016] The present system 10 preferably utilizes mathematical focus algorithms
in the area
of radar imaging to decode and identify the RFID tags. The type of
mathematical focus
algorithms that are used by the system 10 depends on the application. For
example, as shown in
Figure 1, the present system 10 can use inverse synthetic aperture radar
(ISAR) algorithms to
identify a moving tag 200 (in synthetic aperture radar ternrinology (SAR),
"moving target") on a
conveyor 97 which has a plurality of luggage articles 98, 99, whereby the
luggage articles are
physically moving in a translational direction relative to one or more
interrogators 100. It should
be noted that other mathematical focus algorithms can be used in the
application shown in Figure
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lA and is not limited to SAR. In one embodiment, the interrogators 100 are
fixed and disposed
at various locations along the conveyor 97. The interrogators 100 are
preferably positioned to be
orthogonal and parallel relative to the direction of motion of conveyor 97.
[0017] Figure 2 illustrates a schematic of the RFID interrogator in accordance
with one
embodiment of the present invention. It should be noted that the components in
the interrogator
100 shown in Figure 2 are preferred, and the interrogator 100 can include
other components not
shown. As shown in Figure 2, the interrogator 100 preferably includes an
antenna structure 102,
a polarization and phase transmitting control block 110, a radio frequency
transmitter 112, a
digital signal processor (DSP) 114, a control processor 116, a radio frequency
receiver 118 and a
polarization and phase receiving control block 120. Although not shown, the
interrogator
includes an internal and/or external power source which supplies the necessary
power to the
locate and identify the RFID tags 200 within a specified distance. Preferably,
the antenna
structure 102 of the interrogator 100 is coupled to the transmitting and
receiving control blocks
110 and 120. The transmitting control block 110 is preferably coupled to the
radio frequency
transmitter 112, both of which are preferably coupled to the control processor
116. The
receiving control block 120 is preferably coupled to the radio frequency
receiver 118, both of
which are coupled to the control processor 116, as well. The radio frequency
transmitter 112 and
receiver 118 are preferably coupled to the DSP 114, whereby the DSP 114 is
coupled to the
control processor 116.
[0018] The control processor 116 preferably synchronizes the components of the
interrogator
100 to ensure effective operation of the interrogator 100. In one embodiment,
the control
processor 116 is coupled to a wired or wireless network via hard-wire or
wireless communication
techniques (e.g. Ethernet [such as Power Over Ethernet, POE], Bluetooth, infra-
red, RF wireless
LAN). In one embodiment, the interrogator 100 includes an integrated display
or user control
124, such as in a handheld unit (Figure 1 C), whereby the display 124 is
coupled to the control
processor 116. In one embodiment, the interrogator 100 is coupled to an
external display or user
control 124, such as on a network computer.
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[00191 In one embodiment, a plurality of interrogators 100 are networked to
communicate
and be RF phase synchronous with one another such that information on separate
perspectives of
a common scanned area can be analyzed as an aggregate image. It is
contemplated that one or
more interrogators 100 in the plurality can transmit the RF signals
simultaneously or non-
simultaneously. It is also contemplated, as well, that one or more
interrogators 100 can receive
the reflected signals from the RFID tags 200 simultaneously or non-
simultaneously. These
methods of cooperative signal processing provide the basis for a plurality of
image perspectives
and thereby can create an image of the scanned area showing the locations of
the tags 200 from
different perspectives.
[0020] In the preferred embodiment, the interrogation device 100 transmits and
receives RF
electromagnetic radiation signals utilizing the antenna structure 102, as
shown in Figure 2. In
particular, the antenna structure preferably includes a transmitting antenna
array 102A and a
receiving antenna array 102B which are shown in separate portions of the
antenna structure 102.
In the embodiment shown in Figure 2, the transmitting antenna array 102A is
located in the top
section of the structure 102 whereas the receiving antenna array 102B is
located in the bottom
section. It is contemplated that the individual transmitting and receiving
antennas 102A, 102B
can be alternately arranged from top to bottom of the anten.na structure 102
face. In another
embodiment, one set of antennas serves the transmitting and receiving
functions. In the
embodiment shown in Figure 2, the antenna structure 102 is shown as having a
substantially flat,
planar surface (two dimensional). In another embodiment, the antenna structure
102 has a non-
planar configuration (e.g. conical, bulbed, angled) whereby the individual
antennas in the array
are positioned in three dimensions.
[00211 In general, the interrogator 100 utilizes one or more radio detection
and ranging
(RADAR) technologies to identify tags over a scanned area. The transmitting
array 102A of the
interrogator 100 transmits electromagnetic radiation to a large area at a
desired frequency.
Preferably, the frequency ranges between 30 GHz and 300 GHz, depending on the
application in
which the system 10 is used. However, other frequencies outside this range are
contemplated.
The electromagnetic radiation is received at the RFID tags 200 in the scanned
area, whereby the
antenna structures of the RFID tags 200 resonate at the desired frequency and
re-radiate the
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electromagnetic signals back toward the interrogator 100. The interrogator 100
samples and
stores the received signals from the RFID tag(s), as well as reflected
electromagnetic radiation
from all objects in the scanned area, and builds a signal phase history in a
memory system.
Through mathematical coherent phase analysis, the interrogator 100 preferably
processes the
phase history and polarization samples using general Synthetic Aperture Radar
(SAR) signal
processing algorithms, although other processing algorithms are contemplated.
The interrogator
100 is then able to generate images of the scanned area from the phase history
samples and
associated polarization data to identify the RFID tags 200 in the area. In
other words, the
interrogator 100 is able to "view" the scanned area using RADAR technology and
"see" the
RFID tags and distinguish the tags 200 from other objects and RFID tags 200 by
the orientations
and dimensions of the antenna structures thereon.
[0022] The RFID system 10 of the present invention utilizes polarization and
phase
information, preferably in the millimeter wave range, to detect and identify
the tags 200. As
shown in Figure 2, the antenna structure 102 of the interrogator 100 includes
several vertically
positioned antennas 106 as well several horizontally positioned antennas 108
in the transmitting
and receiving sections 102A, 102B. The individual antennas 106, 108
independently transmit
and receive signals utilizing different RF polarization schemes. For example,
the antenna
structure 102 can transmit RF signals with vertical polarization (V) as well
as horizontal
polarization (H). This is preferably performed by energizing the vertically
oriented antennas 106
for vertically polarized signals, V and energizing the horizontally oriented
antennas 108 for
horizontally polarized signals, H.
[0023] In regards to the antenna structure 102 in Figure 2, the interrogator
100 can transmit
signals utilizing one polarization scheme while the antennas in the receiving
section 102B can be
controlled to receive signals of another polarization scheme. For example
only, the interrogator
100 can transmit vertically polarized signals V and simultaneously receive
signals which are
horizontally polarized, H.
[0024] In addition, the interrogator 100 can control both the vertical and
horizontal antennas
to transmit and receive additional polarization parameters. The interrogator
100 can energize the
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vertically oriented antennas 106 and simultaneously energize the horizontally
oriented antennas
108 with an appropriate phase difference to generate left-hand circular (LC)
or right-hand
circular (RC) polarized signals. Any combination of separate transmit and
receive polarizations
of vertical, horizontal, left-hand circular and right-hand circular (V, H, LC,
RC) are preferably
implemented in the present system 10. This allows the present system 10 to
make use of
polarization differences as well as diversity so that the interrogator 100 can
store received signals
which are created with different transmit and receive polarization parameters,
as will be
discussed in more detail below.
[0025] The interrogator 100 generates and receives radio frequency signals in
a range of
frequencies that are compatible with the frequency range of the resonant
antenna elements on the
RFID tags. The frequency of the transmitting signal is modulated in time by a
modulation signal
generated by the DSP processor 114. The DSP processor provides the modulation
signal to the
radio frequency transmitter 112, whereby the radio frequency transmitter 112
preferably creates
a frequency modulated continuous wave (FMCW) signal. As known in the art, the
distance (z
axis) between the interrogator 100 and the RFID tags 200 can be determined
using the FMCW
signal. In particular, the sweep rate and sweep bandwidth of the transmitter
array antenna 102A
is measured and a beat frequency is created in the baseband by converting the
signals received at
the receiver array antenna 102B and measuring the content of the beat
frequency.
[0026] As shown in Figure 2, the radio frequency transmitter 112 sends the
FMCW signal to
the transmit polarization and phase control block 110, whereby the block 110
conditions the
FMCW signal to a desired polarization and/or phase. The polarization and phase
block 110
outputs the conditioned polarized and phased signal to the transmitting array
antennas 102A.
The signal is then converted into electromagnetic radiation by the
transmitting array antennas
102A. Based on the conditioned signal, the transmitting array antenna 102A can
transmit
vertical (V), horizontal (H), right-hand circular (RC) or left-hand circular
(LC) polarized
electro.n.agrtetic radiation along a beam scanning pattern.
j0027] The transmitting array antennas 102A have the ability to directionally
radiate
electromagnetic radiation indicated by one or more patterns 126 in both the
radial, translational,
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horizontal (x) and/or vertical (y) directions. The interrogator 100 preferably
utilizes a
mathematical focus method used in radar imagery to form a synthetic aperture
in conjunction
with beam scanning methods to interrogate large physical areas for RFID tags
200. Beam
scanning methods known in the art can be utilized to scan physical areas. In
one embodiment,
the interrogator 100 incorporates mechanical movement of the antenna(s) to
radially scan and/or
translate the beam to scan an area. In another embodiment, the interrogator
100 incorporates
phased array beam forming and control to scan an area. In one embodiment, the
transmit array
antenna 102A continuously transmits the electromagnetic radiation signals
simultaneously while
the receive array antenna 102B continuously receives the re-radiated
electromagnetic radiation.
j0028] In another embodiment, the transmit array antenna 102A transmits the
electromagnetic radiation in pulses while the receive array antenna 102B
receives reflected
electromagnetic radiation between pulses from the transmit array 102A. The
interrogator can
utilize an accurate electronic timing clock signal to measure the amount of
time it takes for the
signal to be transmitted and received back from the RFID tag(s) 200. The
distance to the tag(s)
100 is able to be calculated knowing the propagation speed of the transmitted
pulse multiplied
by the measured time difference of the transmitted pulse and the received
pulse. The calculated
distance (z axis) used in conjunction with the spatially scanned data (x, y)
can be used to provide
three dimensional positioning information for the tags 200. It should be noted
that the above is
just an example embodiment, and the present invention can utilize any other
appropriate methods
to accurately transmit and receive the RF signals.
[0029] In one embodiment, the operational distance range between the
interrogator 100 and
various RFID tags 200 can be up to several hundred feet, depending on the
environment and
nature of any obstacles. In applications in which the electromagnetic
radiation is transmitted and
received at millimeter wave frequencies, various materials in the scanned area
may absorb the
RF energy. For example, in applications using the present system, it may be
desired to
interrogate tags 200 through paper and cardboard packaging materials, whereby
the range
between the interrogator 100 and the paper/cardboard packaging materials will
be decreased to
accommodate the attenuation. Considering that the present system 10 can be
used in any
conceivable RFID application, the present system 10 can be configured to
determine the size of
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the desired beam pattern, the power of the RF transmission, and the types of
focusing methods
used, depending on the application.
[0030] The power used by the interrogator 100 depends on the distance range
between the
interrogator 100 and the RFID tag(s) 200 as well as frequency range(s) in
which the system 10
operates as well as other factors. In one embodiment, the interrogator 100 is
powered with 10
mW of transmit power and has an antenna gain of 30 dBi for each antenna array
102A and 102B,
whereby the interrogator 100, after digital signal processing, develops a
signal level margin per
RFID tag antenna element of 10 dB. Larger RFID tags 200 will reflect signals
over longer
distance ranges. This, however, is only one example, and the present
interrogator and/or RFID
tag(s) are not limited to these values. The power utilized by the present
interrogator depends on
the specific application and whether the interrogator is a licensed or
unlicensed device with
reference to spectrum regulations or whether the interrogator is utilized in
military applications.
The transmit power level can be between a few microwatts to 100 mW. Other
power values are
contemplated and are in no way limited to the values provided herein.
[00311 The interrogator 100 preferably receives reflected electromagnetic
radiation at the
receiving array antenna 102B independently of the transmit array antenna 102A.
The
electromagnetic radiation indicated by pattern 128 received by the
interrogator 100 is reflected
from at least one RFID tag 200. As shown in Figure 2, upon the antenna array
102B receiving
the reflected electromagnetic radiation, the signal is converted into a radio
frequency signal by
the antenna array 102B. The received radio frequency signal is passed to the
receive polarization
and phase control block 120. The polarization and phase control block 120
interfaces with the
receive antenna array 102B to create the desired polarization and phase
response of the received
signal and outputs the signal to the radio frequency receiver 118, as shown in
Figure 2. The
radio frequency receiver 118 converts the received signal to baseband signals,
whereby the
baseband signals are provided to the DSP processor 114. The DSP processor 114
then preferably
processes the received baseband signals using radar image signal processing
algorithms to
analyze the baseband signals. Such algorithms are derived from, but not
limited to, algorithmic
calculations utilized in synthetic aperture radar (SAR), inverse synthetic
aperture radar (ISAR),
interferometry SAR (InSAR), poliametric SAR (POLSAR), poliametric
interferometry SAR
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(POLINSAR) and algorithms used in joint time frequency analysis (JTFA)
applications. It
should be noted that other radar signal imaging data processing methods are
contemplated for
use in the present system. Dependent on the applications in which the present
system 10 is used,
applying one or more of these types of image processing methods forms unique
images of the
scanned area. In one embodiment, an individual antenna on an RFID tag may be
represented as
a single pixel, whereby an optical view of the RFID tag(s) 200, along with its
representative
information, is provided on a display screen to identify and track the
particular tag(s) 200. To
better understand how the data is analyzed, a-discussion of the antenna
elements of the RFID tag
200 will first be discussed.
[00321 Figures 3-4 illustrate different embodiments of the RFID tag 200 of the
present
invention. Figures 3A and 3B illustrate one embodiment of the RFID tag 200 in
accordance with
the present invention. As shown in Figure 3A, the tag 200 includes a substrate
layer 202 having
one or more conductive antenna elements 204, 206, 208, 210, 212, 214, 216, 218
thereon. For
sake of brevity, the antenna elements shown in relation to Figure 3A are
generally referred to as
having reference numeral 204, whereas the antenna elements will be described
with their
individual reference numerals of 206, 208, 210, 212, 214, 216, 218 where
needed. It should be
noted that the relative sizes, number and positions of the antenna elements
204 are illustrated in
Figure 3A as a non-limiting example, and are not limited thereto. It should
also be noted that the
antenna elements 204 are greatly exaggerated and not illustrated to scale,
either individually,
relative to each other, or relative to the substrate layer 202. Although the
present description
discusses several different tag configurations, any of the tag configurations
are applicable even if
only one tag reference numeral is discussed.
(0033] In one embodiment, the substrate layer 202 is disposed on or is
integral with an
optional conductive ground plane 203. The ground plane 203 can increase the
radiation
efficiency as well as allow greater control of the antenna element resonance,
amplitude response,
phase response and polarization response reflection parameters. However,
operation without the
use of a ground plane 203 will provide adequate response in many applications,
including but not
limited to, directly printing the antenna elements on the packaging containers
99 (Figure lE) or
directly embedding the antenna structures into a manufactured product.
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[0034] The antenna elements 204 are shown in Figure 3A as having various
length and width
dimensions, whereby the width dimensions are preferably measured at the ends
of the antenna
dipoles. The length dimensions are preferably on the order of %z wavelength,
V2. The width
dimensions of the antenna elements are preferably on the order of V10. As an
example, at a
frequency of 60 GHz, the dimension of a,%/2 antenna element is approximately
2.50 mm in free
space. It is preferred that the antenna elements 204, 206, 208, 210, 212, 214,
216, 218 are
positioned at least X /2 apart from one another. It is contemplated, however,
that the antenna
elements can be separated less than k /2 apart from one another.
[0035] The substrate layer material 202 provides an effect of decreasing the
physical size of
the wavelength which is associated with surface conductive elements according
to equation (1).
[0036] a,g = )+, / 4r (1)
[0037] As shown in equation 1, kg is physical wavelength (guide length), k is
the free space
wavelength and s, is the relative permittivity or dielectric constant of the
substrate layer 202
material. For example, if the material of the substrate layer 202 has a
dielectric constant of 2.0,
the physical wavelength (7,,g) of a conductive element on the surface at a
frequency of 60 GHz
would be 3.54 mm, and the ,%g /2 element 206 would be 1.77 mm along the length
dimension of
the antenna element, according to equation (1). The thickness dimension of the
substrate layer
202 is preferably on the order of k /10 to X /50, although other dimensions
are contemplated.
[0038] The antenna elements 204, 206, 208, 210, 212, 214, 216, 218 of the tag
200 respond
to incident electromagnetic radiation transmitted from the interrogator 100.
In particular, the
antenna elements 204 will resonate and convert the incident electromagnetic
radiation into
electrical signals if the frequency of the incident electromagnetic radiation
corresponds with the
wavelength characteristic of that antenna element 204. Upon being energized,
the electrical
signal produced by the antenna element 204 will flow through the conductive
structure of the
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antenna element 204 as well as any terminating transmission line or electrical
impedance which
is coupled thereto.
[0039] Assuming the antenna element 204 is terminated directly into a low
electrical
impedance, as compared to the termination impedance of the antenna element
itself, the antenna
element 204 will immediately convert the electrical signal into
electromagnetic radiation which
is then re-radiated, also referred to as reflected, from the antenna element
204 to be received by
the interrogator 100. However, the reflected radiation has parameters and
characteristics such as
amplitude, phase and polarization which are used by the interrogator 100 to
identify each
particular antenna element 204, the aggregate of which allows the interrogator
100 to identify the
RFID tag 200 as well as any information related to the tag 200. These
parameters are controlled
by the physical characteristics of the antenna element 204 that produces the
reflection.
[0040] As shown in the embodiment in Figure 3A, the antenna elements 204, 206,
208, 210,
212, 214, 216, 218 have various dimensional lengths and rotational
orientations. The
dimensional lengths and orientations of the antenna elements 204, 206, 208,
210, 212, 214, 216,
218 control their particular phase and polarization responses, respectively,
as well as allow the
antenna elements 204, 206, 208, 210, 212, 214, 216, 218 to provide the
information needed by
the interrogator 100 to identify the antenna elements. Antenna element 204 in
the upper left
corner of the tag 200 in Figure 3A preferably serves as a reference antenna
element, whereby the
orientations and thus polarization characteristics of the antenna elements
206, 208, 210, 212,
214, 216, 218 are determined relative to the reference antenna element 204.
This allows the
overall rotational orientation of the tag 200 (and thus the object to which
the tag 200 is affixed)
to be in elevant in reading the antenna elements. In other words, the
interrogator 100 will be able
to effectively identify a tag 200 by virtue of its antenna elements 204, 206,
208, 210, 212, 214,
216, 218 irrespective of whether the tag 200 is right-side-up or upside down.
It should be noted
that the reference antenna element 204 can be located anywhere on the tag 200,
or object to
which the tag 200 is affixed to, and is not limitecl to the upper left corner.
It should be noted that
there can be more than one reference antenna element per tag 200 without
departing from the
scope of the present invention.
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[00411 As shown in Figure 3A, the antenna element 204 is shown having an
orientation of 0
and being vertically polarized (V). The adjacent antenna element 206 in Figure
3A is shown
having an orientation of 90 and being horizontally polarized (H). The antenna
elements 208 and
210 are shown at 45 and 135 (8) and are polarized at their respective
angles. Similarly,
antenna elements 212 and 214 are shown to be vertically polarized (V), whereas
antenna element
216 is shown to be horizontally polarized (H). Antenna element 218 is shown to
be polarized at
45 . It should be noted that the particular rotational orientations shown and
described are for
example purposes and should not be limited to only those angles described. It
is apparent that
any angle, and thus polarization characteristic, is contemplated within the
scope of the present
invention.
[0042] For example purposes only, the antenna structures 204, 206, 208 and 210
have the
same length dimension and thus re-radiate the RF signal to be in at the same
phase. The antenna
elements 212, 214, 216 and 218 in Figure 3A are shown to have different length
dimensions.
Although antenna elements 212 and 214 are vertically polarized (0 ) like the
antenna element
204, elements 212 and 214 have different length dimensions from each other as
well as element
206 such that elements 212 and 214 have different phase response
characteristics. Antenna
elements 212 and 214 thus respond and reflect different phases of
electromagnetic radiation from
one another as well as antenna element 204. Antenna element 216 is
horizontally polarized at
90 , like antenna element 206, however, antenna element 216 will respond and
reflect a different
phase than element 206 due to the difference in the length dimension. The same
theory applies
to antenna element 218.
[0043] Figure 3C illustrates another embodiment of the RFID tag 300 according
to the
present invention. As shown in Figure 3C, the antenna elements 304, 306, 308,
310, 312, 314,
316, 318 are shown to be a strip of rectangular or square conductive material.
The antenna
elements are configured to be on the order of X~/2 along both, the length and
width dimensions,
whereby the width dimension is such that the rectangular antenna elements
respond to the
resonant frequency. The antenna elements are also preferably positioned at
least one ?y /2 apart
from one another. As with the RFID tag 200 in Figure 3A, the physical
dimensions and
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orientations of the antenna elements vary in order to control individual
antenna element phase
and polarization reflective parameters.
[0044] Figure 4A illustrates another embodiment of an RFID tag 400 according
to the
present invention. As shown in Figure 4A, the antenna elements 404, 406, 408,
410, 412, 414,
416, 418 have the same length and width dimensions. In another embodiment, one
or more of
the antenna elements 404, 406, 408, 410, 412, 414, 416, 418 have length and
width dimensions
different from one another. Although eight antenna elements 404, 406, 408,
410, 412, 414, 416,
418 are shown on the substrate 402 in Figure 4A, it should be noted that any
number of antenna
elements, including only one, is contemplated. For sake of brevity, the
antenna elements shown
in relation to Figure 4A are generally referred to as having reference numeral
404, whereas the
antenna elements will be described with their individual reference numerals of
404, 406, 408,
410, 412, 414, 416, 418 when needed.
[0045] Each antenna element shown in Figure 4A includes a transmission line
element
extending therefrom. In particular, antenna element 404 includes transmission
line element 420
whereas antenna element 406 includes transmission line element 422, and so on.
As shown in
Figure 4A, the transmission line elements 420, 422, 424, 426, 428, 430, 432
and 434 vary in
length and position with respect to one another, whereby the length and
position of the
transmission line elements control the phase and polarization parameters at
which the antenna
elements respond to the incident electromagnetic radiation. The transmission
line element for
each antenna element 404 in Figure 4A includes a first transmission line 420A
and a second
transmission line 420B, whereby the first transmission line 420A extends out
perpendicularly
from the antenna element 404. The second transmission line 420B extends from
the first
transmission line 420A at a right angle and is parallel to the side of the
antenna element 404.
[0046] The position and overall length of the transmission line element 420
controls the
antenna element's polarization and phase response when receiving the incident
electrornagnetic
radiation. In Figure 4A, the orientation of the transmission line element 420
is 0 and is
referenced as being vertically polarized (V). The orientation of the
transmission line element
422 is 90 and is referenced as being horizontally polarized (H). For antenna
elements 408 and
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410, the orientation of the transmission line elements 424 and 426 are 135
and 45 ,
respectively.
[0047] As stated, the overall length of the transmission line element 420 also
controls the
response of the phase parameter when receiving the incident electromagnetic
radiation. As
shown in Figure 4A, the transmission line element 420 has a greater length
dimension than the
transmission line element 428, but has a smaller length dimension than the
transmission line
element 430. The difference in the overall length dimension causes the antenna
elements 404,
412 and 414 to reflect the incident electromagnetic radiation at a different
phase. The other
antenna elements 406, 408, 410, 416 and 418 have various length dimensions and
orientations of
the transmission line elements that affect the phase and polarization
reflective parameters
respectively.
[0048] Figure 4B illustrates another embodiment of the RFID tag 500 which
preferably
includes antenna elements 506, 508, 510, 512, 514, 516, 518 and 520. As shown
in Figure 4B,
the antenna elements each include one or more transmission line elements
whereby the
transmission line elements vary in length, position and number with respect to
one another.
Antenna elements 508, 510, 512, 516, 518 and 520 are similar to the antenna
elements discussed
in Figure 4A and will not be discussed again. However, antenna elements 504
and 512 each
include two sets of transmission line elements 520 and 522, respectively.
[00491 The antenna elements 504 and 512 shown in Figure 4B are circularly
polarized,
whereas the remaining shown antenna elements are linearly polarized. For
example, in antenna
element 504, the transmission line element 520 is oriented and shown to be
greater in the length
dimension than the transmission line element 522. The orientation and greater
length dimension
preferably causes the transmission line element 520 to have a phase delay that
is r/2 radians
greater than the phase delay of the other transniission line element 522. This
difference in phase
delay generates a quadrature phase condition on the adjacent sides of the
antenna element 504
which creates a circular polarization parameter (e.g. LC, RC). In particular,
the direction of the
circular polarization (either right-hand or left-hand) is determined by which
side of the square
patch antenna is either leading or lagging in quadrature (7r/2 radians). For
example, the
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transmission line element 524 of the antenna element 512 is shown in Figure 4B
to have a
greater length dimension that the transmission line element 526. This
difference in the length
dimension between the transmission line elements 524 and 526 creates a
quadrature phase
condition on the adjacent sides of the antenna element 512 that is opposite in
the circular
polarization direction as compared to that of the antenna element 504. These
double tuned
antenna elements 504, 512 allow the elements 504, 512 to provide additional
information in the
form of circular polarization of the electromagnetic radiation, as discussed
below.
[0050] For one or more of the above discussed antenna structures, the
structures are
preferably made of conductive ink which can be printed on the substrate or
other surface. The
antenna structures, and thus the RFID tags, can be produced very inexpensively
from a laser, ink
jet or commercial printer. The antenna structures can also be printed using
other conventional
methods of making RFID tags. In another embodiment, the antenna structures can
be etched,
deposited or applied using any other appropriate method.
[0051] Figure 5 illustrates a flow diagram of one example method in which the
DSP
processor 114 of the interrogator 100 processes and analyzes the received
signal to decode and
identify and retrieve information from the RFID tag 200. As discussed above in
relation to
Figure 2, the transmit polarization and phase control block 110, in
conjunction with transmitting
array antenna 102A and DSP processor 114, create and transmit incident
electromagnetic
radiation that has polarization parameters sequenced in time. The polarization
of the
electromagnetic radiation can be vertical V, horizontal H, right-handed
circular RC or left-
handed circular LC. Once the incident electromagnetic radiation resonates the
antenna structures
of the RFID tag(s) (Figures 3-4), the tag 200 reflects the radiation which is
then received at the
receiving array antenna 102B of the interrogator 100. The receive polarization
and phase control
block 120 (Figure 2), in conjunction and the DSP processor 114, creates
predetermined antenna
configurations which are more sensitive to certain reflected electromagnetic
radiations. The
predeternnined polarization configurati_ans are sequenced and stored such that
interrogator 100 is
able to compare the polarization diversity of the received reflected signals
to that of the stored
configurations to aid in identifying and decoding the tag 200.
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[0052] Ordinary backscatter reflection from objects other than RFID tags will
be received by
the interrogator 100 and will have random polarization and phase compared to
the antenna
elements of on the tag(s). Thus, antennas with known phase and polarization
parameters are
printed into the coded pattern at known relative locations to establish an a
priori reference return
signal. This technique provides an effective method to lock onto the tag 200
and create a phase
and polarization decoding reference for the interrogator.
[0053] By controlling how the polarized radiation is transmitted and received,
the
polarization configuration associated with the transmitted and received
radiation controls how
the sampled received signal information is stored in a time-indexed manner. In
one embodiment,
such received signal information is sequenced in time and indexed in memory.
As shown in
Figure 5, the baseband signal is received from the radio frequency receiver
block 118 (Figure 2).
In one embodiment, the baseband signal is received in analog form, whereby the
analog signal is
converted into digital form (step 602). The converted digital signal is
preferably made up of
time sequences of transmit-receive polarization combinations of Vt-V,, Ht-Hi,
Ht-Vr, LCt-LCr,
RC,-RCi, LCt-RCr, Vt-LC,, Vt-RC,, Ht-LCi and Ht-RCr at given times. For
example, the
polarization combination Vt-LC, represents data received when vertically
polarized radiation was
transmitted and left-circular polarized radiation was received for a
particular time. Each
polarization combination set of time-indexed polarization samples is
preferably stored in one of
separate memory locations known as polarization memory image panes, shown as
604A-604J. It
should be noted that the present invention is not limited to the polarization
combinations shown
and described herein and any number of individual or combination of
polarization radiation that
is transmitted and received is contemplated.
[0054] Radar image signal processing is applied to each memory pane in step
606, preferably
by performing Fourier Transform algorithms on the received data. Fourier
transformed data is
symbolized by two data locations in memory for each sample, whereby magnitude
and phase
information of each sample is able to be computed from the data. By computing
the magnitude
and phase information for each sample, the polarization and phase information
of each antenna
element in any number of RFID tags 200 in the scanned area can be determined.
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[0055] As shown in Figure 5, once the data is processed using Fourier
Transform algorithms,
synthetic aperture radar calculations are performed on the data to calculate
compressed elevation
data (y) samples 608. If needed, phase correction calculations are then
performed on the
elevation data 608, as sho,%sm in 610. The corrected phase data 612 is then
preferably processed
again using Fourier Transform algorithms to calculate compressed azimuth data
(x) samples 616.
From the elevation 608 and azimuth 616 data samples, a preliminary digital
image 618 of the
scanned area can be formed, whereby the multiple memory panes of data 620A-
620J are then
used to decode the tags 200 and/or output a visual image 624 of both the tags
200 and the entire
scanned area containing the tags 200.
(0056] By creating multiple memory panes of data 620A-620J, the present system
is able to
correlate and recognize tag pattern data. For example, if the transmitted
array antenna 102A is
vertically polarized V and the received array antenna 102B is also vertically
polarized V at a
given time, the data samples taken at that period are stored in a unique
memory location, indexed
by time, designated as Vt-Vr. If the transmitter is H and the receiver is V, a
separate memory
pane location is used and is distinguished as Ht-Vr, etc. Through valid
mathematical
permutations, the set of memory panes necessary to analyze the different
combinations of
transmit and receive polarizations comes out to be the set of Vi-Vr, Ht-Hr, Ht-
V, LCt-LCr, RCt-
RCr, LCt-RC,, Vt-LCi, V,-RC,, Ht-LCr and Ht-RCr. By mathematically correlating
between these
image panes, recognition of the tag's polarization and phase can be deduced.
Inter memory pane
mathematical correlation is then applied to derive image detail that
emphasizes polarization
content within the image. Unique images are then able to be formed for each
set of time-indexed
polarization combinations.
[0057) Accurate SAR imaging of the objects, walls or buildings around the
tag(s) 200 can
also be useful in processing the location of the tag(s) 200 within the context
of the objects
represented in scanned area. In one embodiment, the interrogators can be used
in conjunction
with a GPS system and graphical mapping software to locate and track the
position of the RFID
tag relative to geographic coordinates. It is contemplated that the RFID tag
200 as well as one or
more interrogator's location can be indicated on a mapped graphical display
for user recognition
of the target's location. In one embodiment, this is performed by the system
10 mapping the
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interrogator's position by scanning the boundary area, such as the walls of a
room. Once the
boundary perimeter is established, the location of the interrogator within the
perimeter can be
determined and the bearings of the tags 200 can be determined as well. It
should be noted that
the locations of the interrogator 100 as well as the tags 200 can be
determined in a stationary
setting, such as a room, or while the interrogator and/or the tags 200 are
moving.
[0058] As shown in Figure 5, mathematical focus calculations preferably
utilizing radar
image signal processing are performed on the recognized individual tag
structures 200 to deduce
the frequency and phase response of one or more given tag antenna structures
200. In deducing
the polarization, frequency and phase response, the information on the tag 200
can be decoded
622. For example purposes only, Figure 6 illustrates a table showing an
example RFID tag
antenna structure code decoding scheme. It should be noted that the table
shown in Figure 6
only takes into account the polarization and phase characteristics of the
antenna structures to
decode the structures. For example purposes, the table in Figure 6 will be
explained in relation
to the RFID tag discussed in Figure 3A. However, it should be noted that the
table can be used
with any other antenna configuration and is not limited to that shown in
Figure 3A.
[0059] As shown in Figure 6, the table provides a four-level polarization code
and a four-
level phase code. The table is thus set up such that, together, there can be
16 different
combinations or states represented from the polarization and phase information
received from the
RFID tag 200. In binary data terms, the bit size of the code is defined by log
2 16 = 4.
Therefore, the table in Figure 6 provides all possible combinations of a 4-bit
code in terms of
polarization and phase.
[0060] As stated above, in one embodiment, the interrogator 100 transmits RF
signals as a
modulated frequency carrier (e.g. FMCW) that is swept over a large frequency
range many times
per second. Considering that the tag antenna structures can be tuned to re-
radiate different
wavelengths of signals by changing the geometry of the structures, the
represented polarization
and phase combinations in the sample table in Figure 6 can be repeated at
different, separated
frequencies. This, in turn, allows for more bits to be encoded, and thus
decoded, for a given
antenna structure. The degree of frequency separation will depend on the
antenna structure,
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electrical conductivity, and the substrate dielectric losses which both affect
the resonance
sharpness (i.e. Q factor). For example, if four frequency "bins" were used to
analyze separate 4-
bit polarization and phase data in each bin, a total of 4x 16 or 64 total
combinations would be
feasible. Since log Z 64 = 6, a 6-bit code would thus result for each antenna
structure utilizing a
4-bin frequency separation.
[00611 In one embodiment, the present system can accommodate for higher
density codes in
applications which desire higher quantities of coding of bits per antenna
structure, thereby
resulting in more encoded data per tag area. As a non limiting example of the
number of data
bits that can be coded into a given tag area, one square inch of tag can
accommodate
approximately 25 individual X/2 microstrip patch antennas spaced at one
wavelength apart (X), or
mm at a frequency of 60 GHz. Using a 6-bit coding schema, the total number of
bits encoded
into a square inch of area would be 6x25 = 150 bits. Using the same coding
schema at 92 GHz
(A = 3.2 mm), approximately 60 individual patch antennas fit within one square
inch. Thus, 360
bits (6 x 60) can be encoded.
[0062] Figure 7 illustrates one example method in how the antenna elements can
be encoded.
As shown in Figure 7, whatever information that is to be coded is initially
converted from
alphanumeric characters into a data coded format (e.g. ASCII) (step 700). The
data characters
are then preferably converted into appropriate phase and polarization values
which can be
standard or proprietary (step 702). Following, the phase and polarization
values for each
character is then preferably converted into the appropriate antenna structure
(step 704). This
entire process can be performed by a software program which then sends the
information to a
printer (706) to produce the antenna structure configuration. It is
contemplated that reference
antenna elements will be placed in the printed antenna configuration, as
discussed above. In one
embodiment, error correction coding methods (e.g. parity coding, turbo coding)
are applied to
the data which is to be eventually coded into the antenna elements. In one
embodiment, the
antenna elements are encoded and decoded utilizing encryption/decryption
methods for security.
Such methods include, but are not limited to, use of hash algorithms, digital
signatures such as
MD2, MD4,1NfD5 and/or SHA algorithms. Hash algorithms can also be used in the
context of
the present invention to iinprove signal randomness for detection enhancement.
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[00631 While embodiments and applications of this invention have been shown
and
described, it would be apparent to those skilled in the art having the benefit
of this disclosure that
many more modifications than mentioned above are possible without departing
from the
inventive concepts herein. The invention, therefore, is not to be restricted
except in the spirit of
the appended claims.
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