U.S. patent application number 14/924699 was filed with the patent office on 2016-02-18 for anti-geolocation.
This patent application is currently assigned to ZIVA CORPORATION. The applicant listed for this patent is Ziva Corporation. Invention is credited to Maha Achour, Mark Hsu, Anis Husain, Jeremy Rode, David Smith.
Application Number | 20160047894 14/924699 |
Document ID | / |
Family ID | 47361849 |
Filed Date | 2016-02-18 |
United States Patent
Application |
20160047894 |
Kind Code |
A1 |
Rode; Jeremy ; et
al. |
February 18, 2016 |
ANTI-GEOLOCATION
Abstract
Methods, apparatus, and articles of manufacture make Geolocation
of a source transmitter more difficult or impossible. Scatterers
common to a source transmitter and an intended receiver are
identified using a variety of techniques, such as iterative time
reversal (ITR) and Singular Value Decomposition (SVD) of a scatter
matrix. The source transmitter then uses time reversal and
knowledge of the signatures of the scatterers to focus its
transmissions on one or more of the scatterers, instead of the
intended receiver. The source transmitter may have multiple
antennas or antenna elements. The source transmitter and/or the
intended receiver may include antenna elements with Near-Field
Scatterers to enable spatial focusing below the diffraction limit
at the frequencies of interest. The source transmitter may be a
plurality of ad hoc nodes cooperating with each other.
Inventors: |
Rode; Jeremy; (San Diego,
CA) ; Achour; Maha; (Encinitas, CA) ; Smith;
David; (Ellicott City, MD) ; Husain; Anis;
(San Diego, CA) ; Hsu; Mark; (La Jolla,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ziva Corporation |
San Diego |
CA |
US |
|
|
Assignee: |
ZIVA CORPORATION
San Diego
CA
|
Family ID: |
47361849 |
Appl. No.: |
14/924699 |
Filed: |
October 27, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13462514 |
May 2, 2012 |
9201132 |
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14924699 |
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13440796 |
Apr 5, 2012 |
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13462514 |
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61481717 |
May 2, 2011 |
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61481720 |
May 2, 2011 |
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61540307 |
Sep 28, 2011 |
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61476205 |
Apr 15, 2011 |
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Current U.S.
Class: |
342/16 |
Current CPC
Class: |
H04B 7/0413 20130101;
G01S 7/282 20130101; G01S 7/4814 20130101; G01S 1/725 20130101 |
International
Class: |
G01S 7/282 20060101
G01S007/282 |
Claims
1. A method of transmitting data from a first radio frequency
communication system to a second radio frequency communication
system, the method comprising steps of: sounding environment using
channel sounding bursts transmitted from a plurality of antennas of
the first radio frequency communication system, wherein reflections
of the channel sounding bursts are received by the plurality of
antennas; creating a scatter matrix from information obtained in
the step of sounding; applying time-reversal to the scatter matrix
to obtain a time-reversed scatter matrix; performing Singular Value
Decomposition (SVD) on the time-reversed scatter matrix to obtain
first automatic focusing parameters for focusing radio frequency
transmissions of the plurality of antennas on a first scatterer;
and sending a first transmission of the data to the second radio
frequency communication system through the plurality of antennas so
that the first transmission is focused on the first scatterer using
the first automatic focusing parameters.
2. A method according to claim 1, wherein the step of creating the
scatter matrix comprises: for each antenna of the plurality of
antennas, transmitting a channel sounding burst from said each
antenna; receiving at said each antenna of reflections of the
channel sounding burst transmitted from all the antennas of the
plurality of antennas; sampling and storing the reflections to
obtain stored reflections; and assembling the stored reflections
into the scatter matrix.
3. A method according to claim 1, wherein the step of creating the
scatter matrix comprises a step for creating the scatter
matrix.
4. A method according to claim 1, wherein: the step of performing
Singular Value Decomposition comprises obtaining second automatic
focusing parameters for focusing radio frequency transmissions of
the plurality of antennas on a second scatterer, the second
scatterer being different from the first scatterer; the method
further comprising step of: sending a second transmission to the
second radio frequency communication system through the plurality
of antennas so that the second transmission is focused on the
second scatterer using the second automatic focusing parameters,
the step of sending the second transmission following the step of
sending the first transmission.
5. A method according to claim 4, wherein: the step of performing
Singular Value Decomposition further comprises obtaining third
automatic focusing parameters for focusing radio frequency
transmissions of the plurality of antennas on a third scatterer,
the third scatterer being different from the first scatterer and
from the second scatterer; the method further comprising step of:
sending a third transmission to the second radio frequency
communication system through the plurality of antennas so that the
third transmission is focused on the third scatterer using the
third automatic focusing parameters, the step of sending the third
transmission following the step of sending the second
transmission.
6. A method according to claim 1, wherein the first communication
system comprises a plurality of ad hoc nodes movable relative to
each other and synchronized with each other, each ad hoc node of
the plurality of ad hoc nodes comprising at least one antenna of
the plurality of antennas.
7. A method according to claim 1, further comprising varying shape
of the channel sounding burst.
8. A method of transmitting data from a first radio frequency
communication system to a second radio frequency communication
system, the method comprising steps of: performing iterative time
reversal (ITR) at the first radio frequency communication system to
obtain information for focusing in space and time transmissions
from a plurality of antennas of the first communication system on a
scatterer; sending a transmission of the data to the second radio
frequency communication system through the plurality of antennas so
that the transmission of data is focused on the scatterer using the
information for focusing.
9. A method according to claim 8, wherein the step of performing
iterative time reversal comprises a step for performing iterative
time reversal.
10. A method according to claim 8, wherein the step of performing
iterative time reversal comprises at least two iterations.
11. A radio frequency communication system, comprising: at least
one transmitter; at least one receiver; a plurality of antennas
coupled to the at least one transmitter and the at least one
receiver; and at least one processor coupled to the at least one
transmitter and the at least one receiver, to control operation of
the at least one receiver and the at least one transmitter, the at
least one processor configured to execute code to cause the radio
frequency communication system to: sound environment using channel
sounding bursts transmitted from the plurality of antennas of the
radio frequency communication system; receive reflections of the
channel sounding bursts by the plurality of antennas; assemble a
scatter matrix from information obtained from sounding the
environment and receiving the reflections; apply time-reversal to
the scatter matrix to obtain a time-reversed scatter matrix;
perform Singular Value Decomposition (SVD) on the time-reversed
scatter matrix to obtain first automatic focusing parameters for
focusing radio frequency transmissions of the plurality of antennas
on a first scatterer in the environment; and send a first
transmission of data to a destination communication system through
the plurality of antennas so that the first transmission is focused
on the first scatterer using the first automatic focusing
parameters.
12. A radio frequency communication system according to claim 11,
wherein the at least one processor is further configured to execute
code to cause the radio frequency communication system to: sound
the environment by transmitting a channel sounding burst from each
antenna of the plurality of antennas; receive at said each antenna
the reflections of the channel sounding burst transmitted from all
the antennas of the plurality of antennas; sample and store the
reflections to obtain stored reflections; and assemble the stored
reflections into the scatter matrix.
13. A radio frequency communication system according to claim 11,
wherein the radio frequency communication system comprises means
for assembling the scatter matrix.
14. A radio frequency communication system according to claim 11,
wherein the at least one processor is further configured to execute
code to cause the radio frequency communication system to: perform
Singular Value Decomposition so that second automatic focusing
parameters for focusing radio frequency transmissions of the
plurality of antennas on a second scatterer are obtained, the
second scatterer being different from the first scatterer; send a
second transmission to the destination communication system through
the plurality of antennas so that the second transmission is
focused on the second scatterer using the second automatic focusing
parameters, the second transmission following the first
transmission.
15. A radio frequency communication system according to claim 14,
wherein the at least one processor is further configured to execute
code to cause the radio frequency communication system to: perform
Singular Value Decomposition so that third automatic focusing
parameters for focusing radio frequency transmissions of the
plurality of antennas on a third scatterer are obtained, the third
scatterer being different from the first scatterer and from the
second scatterer; send a third transmission to the destination
communication system through the plurality of antennas so that the
third transmission is focused on the third scatterer using the
third automatic focusing parameters, the third transmission
following the second transmission.
16. A radio frequency communication system according to claim 11,
wherein the radio frequency communication system comprises a
plurality of ad hoc nodes movable relative to each other and
synchronized with each other, each ad hoc node of the plurality of
ad hoc nodes comprising at least one antenna of the plurality of
antennas.
17. A radio frequency communication system according to claim 11,
wherein the at least one processor is further configured to execute
code to cause the radio frequency communication system to vary
shape of the channel sounding bursts between at least some
soundings.
18. A radio frequency communication system, comprising: at least
one transmitter; at least one receiver; a plurality of antennas
coupled to the at least one transmitter and the at least one
receiver; and at least one processor coupled to the at least one
transmitter and the at least one receiver, to control operation of
the at least one receiver and the at least one transmitter, the at
least one processor configured to execute code to cause the radio
frequency communication system to: perform iterative time reversal
(ITR) at the radio frequency communication system to obtain
information for focusing in space and time transmissions from the
plurality of antennas of the communication system on a scatterer;
send a transmission with data to a destination communication system
through the plurality of antennas so that the transmission is
focused on the scatterer using the information for focusing.
19. A method according to claim 18, wherein the radio frequency
communication system comprises means for performing iterative time
reversal.
20. A method according to claim 18, wherein the ITR comprises at
least two iterations.
Description
CROSS-REFERENCE TO RELATED APLICATIONS
[0001] The present application is a divisional of and claims
priority from U.S. patent application Ser. No. 13/462,514, entitled
ANTI-GEOLOCATION, filed 2 May 2012, now allowed; the U.S. patent
application Ser. No. 13/462,514 claims priority from U.S.
Provisional Patent Application Ser. No. 61/481,717, entitled
ANTI-GEOLOCATION USING TIME REVERSAL (ANGLER), filed on 2 May 2011;
the U.S. patent application Ser. No. 13/462,514 also claims
priority from U.S. Provisional Patent Application Ser. No.
61/481,720, entitled DISTRIBUTED CO-OPERATING NODES USING TIME
REVERSAL FOR COMMUNICATIONS, SENSING & IMAGING, filed on 2 May
2011; the U.S. patent application Ser. No. 13/462,514 claims
priority from U.S. Provisional Patent Application Ser. No.
61/540,307, entitled DISTRIBUTED CO-OPERATING NODES USING TIME
REVERSAL FOR COMMUNICATIONS, SENSING & IMAGING, filed on 28
Sep. 2011; the U.S. patent application Ser. No. 13/462,514 also
claims priority from and is a continuation-in-part of U.S. patent
application Ser. No. 13/440,796, entitled APPARATUS, METHODS, AND
ARTICLES OF MANUFACTURE FOR WIRELESS COMMUNICATIONS, filed on 5
Apr. 2012, which claims priority from U.S. Provisional Patent
Application Ser. No. 61/476,205, entitled TIME REVERSAL
COMMUNICATION SYSTEMS WITH NEAR-FIELD SCATTERERS, filed on 15 Apr.
2011; the present application is also related to patent application
PCT/US12/36180 under the Patent Cooperation Treaty, entitled
DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL, by the same
inventors as the present patent application, attorney docket
reference Ziva003PCT, filed on 2 May 2012. Each of these patent
documents is hereby incorporated by reference in its entirety as if
fully set forth herein, including text, figures, claims, tables,
and computer program listing appendices (if present).
[0002] U.S. Provisional Patent Application Ser. No. 61/481,717,
entitled ANTI-GEOLOCATION USING TIME REVERSAL (ANGLER), filed on 2
May 2011; the U.S. patent application Ser. No. 13/462,514 also
claims priority of U.S. Provisional Patent Application Ser. No.
61/481,720, entitled DISTRIBUTED CO-OPERATING NODES USING TIME
REVERSAL FOR COMMUNICATIONS, SENSING & IMAGING, filed on 2 May
2011; the U.S. patent application Ser. No. 13/462,514 also claims
priority of U.S. Provisional Patent Application Ser. No.
61/540,307, entitled DISTRIBUTED CO-OPERATING NODES USING TIME
REVERSAL FOR COMMUNICATIONS, SENSING & IMAGING, filed on 28
Sep. 2011; (4) U.S. patent application Ser. No. 13/440,796,
entitled APPARATUS, METHODS, AND ARTICLES OF MANUFACTURE FOR
WIRELESS COMMUNICATIONS, filed on 5 Apr. 2012, which claims
priority from U.S. Provisional Patent Application Ser. No.
61/476,205, entitled TIME REVERSAL COMMUNICATION SYSTEMS WITH
NEAR-FIELD SCATTERERS, filed on 15 April 2011; and (5) patent
application under the Patent Cooperation Treaty, entitled
DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL, by the same
inventors as the present patent application, attorney docket
reference Ziva003PCT, to be filed on 2 May 2012. Each of these
patent documents is hereby incorporated by reference in its
entirety as if fully set forth herein, including text, figures,
claims, tables, and computer program listing appendices (if
present).
FIELD OF THE INVENTION
[0003] This document relates generally to the field of Geolocation
and Geolocation prevention.
BACKGROUND
[0004] Geolocation refers to the process of determining geographic
location of a subject of the process. The concept of Geolocation
includes both source Geolocation, i.e., Geolocation of a
transmitter (such as a cellular telephone, communication radio,
radar); and self-Geolocation, i.e., Geolocation of the subject by
the subject itself.
[0005] Source Geolocation is a classical problem for identifying
the location of a transmitter, often done using triangulation.
Passive Geolocation is commonly used to locate accurately a
transmitter using multiple receivers (or, more precisely, receiver
antennas) in non-multipath environments. Several triangulation
methods may be used to locate signal sources in line-of-sight (LoS)
environments. Geolocation of transmitters may use multilateration
(e.g., triangulation) approaches, often with Time of Arrival (ToA),
angle-of-arrival (AoA), power-of-arrival (PoA), frequency of
arrival (FoA), and/or Time Difference of Arrival (TDoA) estimation
techniques. Angle-of-Arrival, PoA, and FoA metrics may require
favorable operating conditions. If the signal start time is known,
ToA can be converted to range estimation. Emission time, however,
is generally not available in the case of geolocation of a
non-cooperative (hostile) transmitter.
[0006] In TDoA measurements, the signal of the transmitter is
received at multiple receivers with distance-dependent time delays.
Correlation analysis provides a time delay of the transmit signal
corresponding to the path length difference to receiver pairs. When
the signal is received at two receivers at known locations and
TDoA, the intersection of possible transmitter locations lies on
one half of a two-sheeted hyperboloid. Adding a third receiver at a
third known location provides a second TDoA measurement (i.e., a
second hyperboloid) with the location of the transmitter at the
intersection of these two hyperboloids in two dimensions. A fourth
receiver may enable measurement of a third hyperboloid, resulting
in a determination of the transmitter location in three dimensions.
Time Difference of Arrival techniques can be quite accurate for
passive location estimation of a transmitter, including
non-cooperative transmitter, when three or more receivers are
available.
[0007] Geolocation may use Channel Impulse Response (CIR)
estimations. In non-multipath environments, the CIR exhibits a
single peak corresponding to the direct LoS. In this case, TDoA may
be employed, because time of arrival can be determined directly
from the peak.
[0008] In a multipath (MP) environment, a transmitted signal may be
subjected to multiple scattering, resulting in a linear combination
of delayed, attenuated, and Doppler-shifted versions of the
original transmitted signal detected by the receivers along
different paths. It follows that the CIR will exhibit multiple
peaks and consequential uncertainty regarding which peak is the LoS
peak, if LoS is present at all. Therefore, with significant MP
contributions and/or without LoS, the CIR may have multiple peaks,
and measuring time of arrival becomes more difficult. Accurate and
robust location estimation is thus challenging in harsh MP
environments, with their time-varying MP fading and co-channel
interference.
[0009] Low probability of intercept (LPI) communication techniques,
such as direct sequence spread spectrum and frequency hopping,
operate at instantaneous or average power levels that may be lower
than ambient noise power levels. Such communication techniques
present difficult scenarios for geolocation of non-cooperative
transmitters. Low Probability of Intercept techniques may not
communicate reliably in Non-Line-of-Sight (NLoS) multipath
environments without incorporating Multiple-Input-Multiple-Output
(MIMO) technologies. Geolocating signal transmitters in NLoS
environments is more difficult, but possible with MP scattering.
Multiple-Input-Multiple-Output systems generally have to ensure
adequate decorrelated paths through the multipath environment,
which means that they may transmit signals in all directions,
allowing hostile receivers not only to detect but also to locate
them, e.g., by combining triangulation and TDoA techniques.
[0010] Geolocation of hidden transmitters in an MP environment may
be possible with recently developed processing techniques. The
reason for this is that when the source is an NLoS source, a
triangulation, by the hostile observer employing an array may first
be used to locate the primary scatterer locations (from the point
of view of the observer), and then iteratively find other
scatterers and the original transmitter. This process is described
in U.S. Provisional Patent Application Ser. No. 61/586,675,
entitled Geolocation, filed on Jan. 13, 2012; and in U.S.
Provisional Patent Application Ser. No. 61/597,492, entitled
Geolocation, filed on Feb. 10, 2012. Each of these provisional
patent applications (which are commonly owned with the present
application and which were filed in the name of one of the
inventors herein) is hereby incorporated by reference in its
entirety as if fully set forth herein, including text, figures,
claims, tables, computer program listing appendices (if present).
In sum, TDoA techniques make geolocation possible when there are
three or more discrete scatterers in the field. Each of these
scatterers can be treated as a virtual antenna to enable both
direction and distance of the source to be determined uniquely.
[0011] There is a need for techniques to hide transmitter locations
in multipath environments in real-time, to prevent hostile
receivers from locating signal transmitters, without unduly
disrupting communications between the transmitters and their
intended receivers.
SUMMARY
[0012] Embodiments, variants, and examples described in this
document are directed to methods, apparatus, and articles of
manufacture that may satisfy one or more of the above described
and/or other needs.
[0013] Selected examples described in this document rely on
obtaining steering vectors for scatterers common to the intended
receiver and the transmit array. One of the methods employed for
this purpose is the application of Singular Value Decomposition
(SVD) processing to the time reversal operator. It may be a
frequency domain operation using, for example, Fast Fourier
Transforms (FFTs) and inverse FFTs (IFFTs). There are other methods
to attribute groups of pulses to scatterers across a distributed
array using the positional layout of the antenna array and
identifying patterns. Using knowledge of the spatial layout of the
receiver array, one can identify which pulses come from which
scatterer, and effectively null energy on that scatterer in the
time domain. This may be done by first detecting patterns in the
arrival times consistent with the known array layout and
determining commonality across the array. After identification of
time of arrival of the scatterer to be nulled across the array, the
peak at that time slot may be removed in the time-reversed
transmission. Upon transmission of the new time-reversed signal,
energy to that target scatterer will not align coherently in time
and space, resulting in suppression of energy at that scatterer.
This is equivalent to nulling the steering vector to that scatterer
without frequency domain processing. Other methods may include
using common symmetric time axes across the nodes, which may
eliminate the need for positional information. The ability to null
energy on a particular scatterer (and consequently the ability to
focus on a particular scatterer by nulling energy on other
scatterers) may be used to make a transmission appear (to a hostile
receiver) to come from the focused-on scatterer, instead of the
actual transmitter.
[0014] In an embodiment, a method of transmitting data from a first
radio frequency communication system to a second radio frequency
communication system is disclosed. The method includes receiving by
a plurality of antennas of the first radio frequency communication
system waveforms that resulted from sounding environment using one
or more channel sounding bursts transmitted from the second radio
frequency communication system, the waveforms including reflections
of the one or more channel sounding bursts from one or more
scatterers. The method also includes processing the waveforms using
time-reversal and Singular Value Decomposition to (1) select a
first selected scatterer from the one or more scatterers, and (2)
determine first signatures for launching from the plurality of
antennas a first transmission temporally and spatially focused on
the first selected scatterer, each first signature corresponding to
a different antenna of the plurality of antennas. The method
additionally includes convolving first data with each first
signature to obtain first transmission waveforms, a first
transmission waveform per antenna of the plurality of antennas. The
method further includes transmitting the first transmission
waveforms from the antennas of the plurality of antennas, each
first transmission waveform transmitted through the antenna
corresponding to each first transmission waveform, so that the
first data is temporally and spatially focused on the first
selected scatterer.
[0015] In an embodiment, a method of transmitting data from a first
radio frequency communication system to a second radio frequency
communication system is disclosed. The method includes sounding
environment using channel sounding bursts transmitted from a
plurality of antennas of the first radio frequency communication
system, wherein reflections of the channel sounding bursts are
received by the plurality of antennas. The method also includes
assembling a scatter matrix from information obtained in the step
of sounding. The method additionally includes applying
time-reversal to the scatter matrix to obtain a time-reversed
scatter matrix. The method further includes performing Singular
Value Decomposition (SVD) to the time-reversed scatter matrix to
obtain first automatic focusing parameters for focusing radio
frequency transmission of the plurality of antennas on a first
scatterer. The method further includes making a first transmission
of the data to the second radio frequency communication system
through the plurality of antennas so that the first transmission is
focused on the first scatter using the first automatic focusing
parameters.
[0016] In an embodiment, a method of transmitting data from a first
radio frequency communication system to a second radio frequency
communication system is described. The method includes performing
iterative time reversal (ITR) at the first radio frequency
communication system to obtain information for focusing in space
and time a plurality of antennas of the first communication system
on a scatterer. The method also includes making a transmission of
the data to the second radio frequency communication system through
the plurality of antennas so that the transmission is focused on
the scatterer using the information for focusing.
[0017] In an embodiment, a radio frequency communication system has
at least one transmitter, at least one receiver, a plurality of
antennas coupled to the transmitter and the receiver; and at least
one processor coupled to the at least one transmitter and the at
least one receiver, to control operation of the at least one
receiver and the at least one transmitter. The at least one
processor is configured to execute code to cause the radio
frequency communication system to receive by the plurality of
antennas waveforms that resulted from sounding environment using
one or more channel sounding bursts transmitted from an intended
receiver, the waveforms including reflections of the one or more
channel sounding bursts from one or more scatterers; to process the
waveforms using time-reversal and Singular Value Decomposition to
(1) select a first selected scatterer from the one or more
scatterers, and (2) determine first signatures for launching from
the plurality of antennas a first transmission temporally and
spatially focused on the first selected scatterer, each first
signature corresponding to a different antenna of the plurality of
antennas; to convolve first data with each first signature to
obtain first transmission waveforms, a first transmission waveform
per antenna of the plurality of antennas; and to transmit to the
intended receiver the first transmission waveforms from the
antennas of the plurality of antennas, each first transmission
waveform transmitted through the antenna corresponding to each
first transmission waveform, so that the first data is temporally
and spatially focused on the first selected scatterer.
[0018] These and other features and aspects of selected embodiments
not inconsistent with the present invention will be better
understood with reference to the following description, drawings,
and appended claims.
BRIEF DESCRIPTION OF THE FIGURES
[0019] FIGS. 1 and 2 illustrate selective steps of an Iterative
Time Reversal process;
[0020] FIG. 3 illustrates selected steps of a process for masking a
node's location using Singular Value Decomposition and time
reversal with sounding from the transmitter node;
[0021] FIGS. 4A and 4B show an exemplary high level representation
of an environment including cooperative nodes, hostile receivers,
and scatterers, for facilitating discussion of the process of FIG.
3;
[0022] FIG. 5 shows an exemplary high level representation of an
environment including cooperative nodes, hostile receivers, and
scatterers, for facilitating discussion of a process of the
following Figure;
[0023] FIG. 6 illustrates selected steps of a process for masking a
node's location using Singular Value Decomposition and time
reversal with sounding from the intended receiver node; and
[0024] FIG. 7 illustrates selected elements of an apparatus
configured in accordance with one or more features described in
this document.
DETAILED DESCRIPTION
[0025] In this document, the words "embodiment," "variant,"
"example," and similar words and expressions refer to a particular
apparatus, process, or article of manufacture, and not necessarily
to the same apparatus, process, or article of manufacture. Thus,
"one embodiment" (or a similar expression) used in one place or
context may refer to a particular apparatus, process, or article of
manufacture; the same or a similar expression in a different place
or context may refer to a different apparatus, process, or article
of manufacture. The expression "alternative embodiment" and similar
words and expressions are used to indicate one of a number of
different possible embodiments, variants, or examples. The number
of possible embodiments, variants, or examples is not necessarily
limited to two or any other quantity. Characterization of an item
as "exemplary" means that the item is used as an example. Such
characterization of an embodiment, variant, or example does not
necessarily mean that the embodiment, variant, or example is
preferred; the embodiment, variant, or example may but need not be
a currently preferred embodiment, variant, or example. All
embodiments, variants, and examples are described for illustration
purposes and are not necessarily strictly limiting.
[0026] The words "couple," "connect," and similar expressions with
their inflectional morphemes do not necessarily import an immediate
or direct connection, but include within their meaning connections
through mediate elements.
[0027] References to "receiver" ("Rx") and "transmitter" ("Tx") are
made in the context of examples of data transmission from the
transmitter to the intended receiver. For time reversal
communication techniques, the intended receiver may need to
transmit to the transmitter a sounding signal, e.g., a pulse/burst
or a pilot signal, and the transmitter may need to receive the
sounding signal. Moreover, data communications can be
bi-directional, with transceivers on both end nodes.
[0028] The expression "processing logic" should be understood as
selected steps and decision blocks and/or hardware for implementing
the selected steps and decision blocks. "Decision block" means a
step in which a decision is made based on some condition, and
process flow may be altered based on whether the condition is met
or not.
[0029] Other and further explicit and implicit definitions and
clarifications of definitions may be found throughout this
document.
[0030] Reference will be made in detail to several embodiments that
are illustrated in the accompanying drawings. Same reference
numerals may be used in the drawings and this description to refer
to the same apparatus elements and method steps. The drawings are
in a simplified form, not to scale, and omit apparatus elements and
method steps that can be added to the described systems and
methods, while possibly including certain optional elements and/or
steps.
[0031] In order to locate a hidden source by observing its
emissions, a geolocation procedure may first use triangulation to
determine the position of the multipath scatterers reflecting the
energy, and then use the identified scatterers as a secondary
virtual antenna array to geolocate the hidden source. This may
require N+1 discrete scatterers, where N is the number of spatial
dimensions to which we are constrained, to enable both the
direction and the distance of the source to be determined uniquely.
For signals propagating in a 2D plane approximately parallel to the
ground, three scatterers may be needed. For signals from all
directions in a 3D space, four scatterers are generally needed.
Dynamic and controlled manipulation of a time-reversed multipath
scattering field can therefore be used for focusing transmit
signals onto a single scatterer (or sequentially on single
scatterers) and null or significantly reduce the signal at other
scatterers, inhibiting geolocation. If a transmitter reduces the
number of scatterers it illuminates with its signal to fewer than
three or four, it becomes more difficult to use triangulation from
the scattered signal to locate the transmitter. This can be used to
fool an enemy into thinking the signal is being emitted from the
location of the scatterer (e.g., a building) rather than the true
source. It may also have an advantage of preventing application of
triangulation and TDoA techniques to geolocate the transmitter.
Further, the target scatterer can be used as a virtual "repeater"
node to relay information to the base station. The technique uses
an array of antenna elements at the transmitter and may sacrifice
some MP gain. Furthermore, some commonality between multipath
scatterers observed by the intended receiver and the hostile
observer may need to be present.
[0032] Time Reversal (TR) is a set of communication techniques that
uses the reciprocity property of wave equations. Time reversal is
described, for example, in U.S. patent application Ser. No.
13/142,236, entitled TECHNIQUES AND SYSTEMS FOR COMMUNICATIONS
BASED ON TIME REVERSAL PRE-CODING, filed on 3 Sep. 2010, by David
F. Smith and Anis Husain. This application is hereby incorporated
by reference in its entirety, as if fully set forth herein,
including text, figures, claims, tables, and computer program
listing appendix (if present). Briefly, in a system that uses time
reversal, a pilot (e.g., a sounding burst) is sent from the target
antenna of the Rx to the Tx; the Tx receives the pilot and captures
in its analog-to-digital converter (ADC) the Channel Response (CR)
of the channel between the Rx antenna and the Tx. The Tx may then
be configured to send data back to the Rx by convolving the data
with the time-reversed version of the captured CR. Standard
modulation techniques can be used to apply the data to the signal
by convolving a binary data stream with the time-reversed CR
(TR-CR). For example, the Tx may be configured to use the TR-CR as
its data pulse/burst. When the TR-CR is launched back down the same
channel by the Tx, the actual physical channel that created the
multipath now acts as its ideal (or near ideal, as the case may be
in the real world) spatial-temporal matched filter and becomes a
perfect (or near perfect) equalizer for the signal, creating a
pulse at the intended receiver that captures much of the energy
present in the original CR. In effect, this can create significant
multipath gain. Communication systems employing TR also have the
flexibility to operate in 1.times.N or M.times.N antenna
configurations, with the ability to derive additional gain over and
above the MP gain. The systems can focus a signal both spatially
and temporally at a designated point in space within the
diffraction limits. They can operate with no LoS visibility of the
receiver, no knowledge of the location of receiver, and no array or
dish antenna at the transmit end of the link. Additionally, there
is no requirement to sweep or scan the Tx array, and the process
does not require complex space-time algorithmic processing or
calculation, or implementation of a Rake filter to remove the
signal distortion created by long MP decay times.
[0033] The sounding burst may be a sharp pulse approaching an
impulse, a Gaussian burst, or another appropriate burst with
substantially flat frequency response in the communication band,
and having a good autocorrelation function (i.e., approaching that
of an impulse function), as is known in communication theory and
related fields (e.g., CDMA, autocorrelation radar).
[0034] Iterative Time Reversal (ITR) is a method for using TR to
focus on a scatterer (as opposed to the eventual intended receiver)
in the field. The scatterer may be, for example, the strongest
scatterer in the field as viewed from the transmitter's location.
FIG. 1 shows selective steps of an ITR process 100, and FIG. 2
illustrates some of the steps in a graphic manner.
[0035] The process 100 begins at flow point 101, at which point the
apparatus performing the process 100 (a transceiver, for example)
is powered up and initialized. In steps 155 and 160, respectively,
the channel is sounded using an initial sounding burst (e.g., a
burst substantially flat in frequency response and with a good
autocorrelation function) with one or more antenna elements, and
the scattered reflections of the burst are received by all antenna
elements of the transceiver. The signals received by the antenna
elements are then time-reversed in step 165. In decision block 170,
a test is performed and a decision is made whether the process
should continue, that is, whether the scatterer's signature has not
yet been determined with sufficient precision. If the test
indicates that the process should continue, the latest
time-reversed reflections are transmitted in step 175, and then the
process flow returns to the step 160. Thus, the steps 160, 165, and
175 may be repeated sequentially, i.e., each time each of the
return signals is time-reversed and again re-transmitted from the
antenna element corresponding to the signal.
[0036] After several iterations, as determined in decision block
170, the signal energy should automatically focus on the strongest
scatterer in the field, and the process terminates at flow point
199. In some embodiments, the number of iterations is
predetermined, e.g., 1, 2, 3, 4, 5, or greater. In other
embodiments, the iterations continue for a predetermined time
period. In still other embodiments, the transceiver performing the
ITR process 100 looks at the changes in the reflected or time
reversed signals, to detect when the signals stabilize from one
iteration to the next. There may be other approaches and
variants/combinations of the approaches discussed in this paragraph
for determining the number or duration of the iterations.
[0037] Once focused, the latest signatures for the antenna elements
can be used to focus a subsequent data transmission on the
scatterer. In other words, the data stream can be convolved with
the signatures, and the resulting waveforms can then be transmitted
from the respective antenna elements. For example, if the
time-reversed signature for an antenna element n is Sn, and the
data stream is D, then the product of the convolution of Sn and D
can be transmitted from the antenna element n. This is repeated for
other antenna elements, and the transmissions are made
simultaneously.
[0038] The process 100 then terminates at flow point 199, and may
be repeated as needed.
[0039] Another way to focus on a scatterer (as opposed to the
eventual intended receiver) in the field is by a process of
Singular Value Decomposition (SVD) applied to a scatter matrix (a k
a environment transfer response matrix). In accordance with this
approach, time reversal signals of an array at the transmit node
can be mathematically decomposed into a sum of distinct
eigenfunctions/eigenstates/eigenvalues or singular states; each
eigenstate represents a path from the transmit node to the receiver
through only one scatterer. If the transmit antenna (array)
transmits only one of these singular states, then the signal is
focused at the scatterer represented by that state and the signals
incident on the other scatterers are nulled or partially nulled. We
refer to such focusing as "Selective Focusing by Singular Value
Decomposition," or "Selective Focusing by SVD." Although the system
may lose multipath TR gain in this mode, it can select which
scatterer(s) to illuminate.
[0040] FIG. 3 illustrates selected steps of a process 300 for using
SVD to determine the information needed by a node to focus one or
more scatterers, to mask the node's location while communicating
with friendly base station transceiver(s). FIGS. 4A and 4B, which
are used to illustrate further the process 300, show an exemplary
high level representation of the node 405, a base station 410 with
which the node 405 intentionally communicates, scatterers 420 (all
round objects in this and other Figures), and a hostile radio
430.
[0041] The process 300 begins with flow point 301, where the node
405 is powered up and ready to communicate with the base station
410. In step 305, the node 405 transmits a channel sounding burst
from one of its transmit antenna elements (here shown to be the
topmost antenna element). In step 310, the plurality of antenna
elements of the node 405 receive reflections/returns generated by
the scatterers 420 as a result of the sounding burst emitted from
the antenna element in the preceding step 305. If the steps have
been repeated (during the current process) for all the antenna
elements, the process flow continues to step 320.
[0042] Decision block 315 causes the steps 305 and 310 to be
repeated for each individual antenna element, transmitting a
sounding burst from a selected antenna element, and receiving the
resulting reflections at all the antenna elements.
[0043] In the step 320, the results from the steps 305/310 are
assembled into a scatter matrix with each pair-wise sounding. In
other words, a matrix is assembled so that where a reflection
received at nth antenna element due to a sounding burst emitted
from mth antenna element is located at the intersection of nth
column and mth row (or vice versa).
[0044] In step 325, the scatter matrix is time-reversed, resulting
in a TR scatter matrix.
[0045] In step 330, Singular Value Decomposition is performed on
the TR scatter matrix to calculate
eigenvalues/eigenfunctions/eigenvectors corresponding to the
scatterers in the field. Singular Value Decomposition is known in
linear algebra. Briefly, SVD is a factorization process of an
m.times.n matrix M. The matrix is decomposed or factorized thus:
M=U.SIGMA.V*. In this formula, U stands for a unitary matrix of
m.times.m dimensions; .SIGMA. stands for a rectangular diagonal
matrix of m.times.n dimensions with non-negative real numbers on
its diagonal; and V* is a unitary matrix of n by n dimensions.
[0046] Each scatterer is identified by corresponding eigenvalue and
eigenvector. Each eigenvalue provides scattering strength of the
scatterer to which it relates; the eigenvectors provide automatic
focusing parameters to focus energy onto the scatterer without the
need for sweeping the array to focus.
[0047] The process ends at flow point 399. The process may be
repeated as needed, for example, periodically, at predetermined
times, or otherwise.
[0048] In other embodiments, a node (e.g., a cooperative
transceiver 505 shown in FIG. 5, or a plurality/cluster of such
cooperative transceivers 505) identifies scatterers (e.g.,
scatterers 520) in the channel between the node and another node
(the base station 510) using a burst sent from the other node (the
base station 510). The cooperative transceiver(s) 505 can then
focus on an individual scatterer 520 when transmitting to the base
station 510, creating a virtual source heard by a hostile
transceiver 530. FIG. 6 illustrates selective steps of a process
600 using this technique.
[0049] At flow point 601, the cooperative transceiver 505 (which
term also applies to a plurality of transceivers 505) and the base
station 510 are powered up, initialized, and ready to perform the
steps of the process 600. In step 605, the base station 510
transmits a sounding burst to the cooperative transceiver 505. The
base station 510 may transmit the burst using a single antenna or a
plurality of antennas or antenna elements. The burst can be a sharp
pulse approaching an impulse, a Gaussian burst, or another
appropriate burst with substantially flat frequency response in the
communication band, and having a good autocorrelation function
approaching that of an impulse function, as is known in the
communication theory and related fields (e.g., CDMA,
autocorrelation radar).
[0050] In step 610, the cooperative transceiver 505 receives,
captures, samples, and stores the received sounding burst at a
plurality of antennas (antenna elements). The burst is captured,
sampled, and stored for each of the individual antennas or antenna
elements.
[0051] In step 615, the cooperative transceiver 505 assembles a
scatter matrix. For example, the sounding burst samples for each of
the antenna elements of the cooperative transceiver 505 becomes a
row (or a column) of the scatter matrix. Note that the rows (or
columns, as the case may be) are in time domain, because they
contain individual time-ordered samples.
[0052] In step 620, the cooperative transceiver 505 transforms each
of the rows (or columns, as the case may be) into the frequency
domain, resulting in a frequency domain scatter matrix.
[0053] In step 622, the cooperative transceiver 505 conjugates
(applies complex conjugation to) the frequency domain scatter
matrix, to obtain a conjugate frequency domain scatter matrix. This
step in the frequency domain is analogous to time reversal in the
time domain.
[0054] In step 625, the cooperative transceiver 505 processes the
conjugate of the frequency domain scatter matrix using SVD, to
identify the signature of each scatterer by its eigenvalue and
eigenvector. Having thus identified the signatures of the
individual scatterers, the cooperative transceiver 505 has the
ability to launch signals temporally and spatially focused on
individual scatterers.
[0055] In step 630, the cooperative transceiver 505 selects a
scatterer 510 for focusing the transmission. The cooperative
transceiver 505 may make this selection, for example, by selecting
the strongest scatterer, or by randomly or pseudo-randomly cycling
through the N strongest scatterers (N being a predetermined
number). In cycling through the scatterers, the cooperative
transceiver 505 may create a "walking" appearance, by sequentially
changing focus from one scatterer to another. To the hostile
transceiver, this may create an appearance of a moving transmission
source. Other selection methods may also be used.
[0056] In step 635, the cooperative transceiver 505 transforms the
vector corresponding to the selected scatterer back into time
domain. We will refer to the resulting time domain vector as
selected time domain vectors.
[0057] In step 645, the cooperative transceiver 505 convolves the
data (intended for transmission to the base station 510) with the
selected time domain vectors, obtaining the waveforms for
transmission from each of the antennas or antenna elements.
[0058] In step 650, the cooperative transceiver 505 transmits, from
its respective antennas or antenna elements, the waveforms obtained
in the step 645.
[0059] The process flow then terminates at flow point 699. The
steps 645-650 may be repeated for additional data, as needed;
furthermore, all the steps may be repeated as needed, to enable the
cooperative transceiver 505 to focus on the individual scatterers
520 in a dynamically changing environment, or to create a "walking"
appearance.
[0060] In variants, the process 600 is modified so that the
processing takes place in the time domain. For example, the steps
620 and 635 may be omitted, the complex conjugation of the step 622
may be replaced by time-reversal in the time domain, and the
processing of the step 625 may be performed in the time domain. The
cooperative transceiver 505 is thus configured to processes the
scatter matrix using SVD in the time domain, to identify the
signature of each scatterer by its eigenvalue, eigenvector, and/or
eigenfunction.
[0061] In variants, the step 625 (whether performed in the
frequency or the time domain) is modified to identify the signature
of each scatterer using one or more techniques other than SVD.
[0062] The transmissions made by the cooperative transceiver 505
using the process 600 (including its variants) can thus be used to
create a virtual source that may be heard by both the base station
510 and the hostile transceiver 530. The base station 510 detects
the transmissions scattered off of the selected (targeted)
scatterer. The hostile transceiver 530 may also detect the
scattered signal transmissions, but may lack information sufficient
to geolocate the real source of the transmissions.
[0063] Further, the cooperative transceiver 505 may in fact be made
up of two or more ad hoc transceivers that are collaborating with
each other, as is described in U.S. Provisional Patent Applications
with Ser. Nos. 61/540,307 and 61/481,720, as well as in a
counterpart patent application under the Patent Cooperation Treaty
(attorney docket reference Ziva003PCT, claiming priority to these
provisional applications) to be filed on 2 May 2012, the same date
as the present application. The nodes are ad hoc in the sense that
they are free to move and rotate not only relative to each other.
The distances between any two of the ad hoc nodes are typically
much smaller (by a factor of 10, for example) than the distance
between any of the nodes and the base station 510. Additionally,
the ad hoc nodes are not tethered to each other, so that each of
the nodes operates using its own physical time reference, and the
antennas of the different ad hoc nodes are not electrically
connected to each other. In this case of ad hoc nodes, the
scattering of the transmissions may mislead the hostile transceiver
530 into believing that fewer cooperative transceivers are present
than their actual number. For example, the plurality of cooperative
transceivers 505 may appear to the hostile transceiver 530 as a
single transmitter.
[0064] For enhanced use of scatterers to communicate from the
cooperative transceiver 505 to the base station 510 while
maintaining stealth, the base station 510 may also be configured to
perform an SVD (or analogous) operation on the environment, albeit
from a different geometry, and identify the eigenvalues and
eigenvectors of the scatterers from its position. The base station
510 may use any of the processes described in this document with
respect to the cooperative transceiver 505, for example, ITR and
SVD of a scatter matrix obtained by self-sounding the environment.
A mutual scatterer (seen by both the cooperative transceiver 505
and the base station 510) can be identified as an efficient or
strong virtual "repeater" node between the cooperative transceiver
505 and base station 510. Strong scatterers of the cooperative
transceiver 505 may differ from those of the base station 510.
Since the geometry of the scattering field (and scatterers) will be
different for the cooperative transceiver 505 and the base station
510, the scatterers may not be able to be easily mapped one-to-one,
since scatterers detected from different geometries may not be
identical. Therefore, a search technique may be implemented to
determine a desirable or optimal scatterer to be used as the
virtual repeater node. For example, the Tx (which can be the
cooperative transceiver 505 or the base station 510) can cycle
through focusing on each scatterer in the intermediate field, while
the Rx (conversely, the base station 510 or the cooperative
transceiver 505) identifies the strongest return from its
perspective, and relays to the Tx the selection of the scatterer to
focus on. The cooperative transceiver or the base station
transceiver can be used interchangeably to probe scatterers.
[0065] An optimal scatterer can also be identified roughly by the
product of its scatter strength to the cooperative and base station
transceivers. Many intermediate scatterers may be mutually
identified by the cooperative and base station transceivers. It
follows that overlapping scatterers may be identified as
potentially optimized "repeater" nodes for communication.
Cross-correlation of intermediate scatterer information may then
provide a faster estimation of optimal scatterer nodes, reducing
search transmissions detectable by hostile geolocation
transceivers.
[0066] Singular Value Decomposition can be extended to multiple
steps for added anti-geolocation functionality. In the previous
description of SVD for anti-geolocation, energy was focused onto an
individual scatterer to create a single virtual Tx node. Because
SVD enables selectively focusing energy to create virtual Tx nodes,
SVD can be performed using virtual Tx nodes with the original Rx
array, to selectively focus energy on scatterers with secondary
scattering. The transceiver 505 may thus be configured to focus on
scatterers to generate secondary scatter response of the
environment to the original Rx array. Similar SVD processing
results in singular values using secondary echoes. Additional steps
of SVD can in principle be applied.
[0067] In variants, the shape of the sounding burst is dynamically
varied, to make Geolocation still more difficult. The transceiver
signal is a convolution of the time reversed channel response and
the data content, which may have significant intersymbol
interference (ISI). This complex signal is unraveled automatically
by TR at the intended receiver. For hostile observers, however, the
signal is further scrambled by the multipath of the hostile
observer channel--possibly preventing the use of higher network
layer information to enhance the physical layer geolocation
process. With TR, the pulse can be shape-altered dynamically during
communication, without penalty to the intended Rx, but making
synchronization for the hostile observers difficult, preventing (or
making more difficult) accurate geolocation from triangulation and
decoding of content. The sounding bursts can be varied, for
example, in accordance with a predetermined sequence of shapes.
[0068] Another technique for increasing the difficulty of
Geolocation is the use of Near-Field scatterers by the transceiver
505 or a similar node. As described above, the geolocation process
may use TDoA information in the Source-Scatterer-Observer channel
to perform multilateration, to identify the source location. The
hostile observer, however, can also look at reflections or
remissions from the receive node in the Source-Base Station primary
channel. If TR is used in this channel, each TR pulse actually
consists of a temporal mapping of all the TDoA values from the main
channel. Hence, if the base station has four or more receive
antennas, the hostile observer may be able to recover the TDoA
information from each receive antenna and use that array to
triangulate the source. The use of Near-Field Scatterers allows
resolution of separate channels on antennas with sub-lambda/2
spacing, and thus helps to defeat this countermeasure. Since the
hostile observer is not part of the TR channel, the hostile
observer may be unable to resolve different signals from antennas
spaced less (and substantially less) than 1/2 wavelength apart,
when the hostile observer is observing from the far-field of those
antennas. Hence, the hostile observer may be unable to use the
scattering from the sub-lambda/2 receiver antennas to triangulate
the source. The antennas with Near-Field scatterers may be employed
at the transceiver 505 and/or at the base station 510.
[0069] FIG. 7 illustrates selected elements of an apparatus
configured in accordance with one or more features described in
this disclosure. The apparatus, which may be a cooperative
transceiver or a base station, may include a processor 705; a
storage device 710 (which may store program code for execution by
the processor 705); a receiver 715 configured to receive radio
frequency transmissions (including scattered/MP transmissions) from
one or more other transceivers/base stations; a transmitter 720
configured to transmit radio frequency transmissions to the other
transceivers/base stations and to produce scatterer reflections;
and one or more transmit and receive antennas 725 coupled to the
receiver 715 and the transmitter 720. A bus 730 couples the
processor 705 to the storage device 710, the receiver 715, and the
transmitter 720, and allows the processor 705 to control operation
of these elements.
[0070] The embodiments described above are illustrative and not
necessarily limiting, although they or their selected features may
be limiting for some claims.
[0071] The features described throughout this document may be
present individually, or in any combination or permutation, except
where presence or absence of specific elements/limitations is
inherently required, explicitly indicated, or otherwise made clear
from the context.
[0072] Although the process steps and decisions (if decision blocks
are present) may be described serially in this document, certain
steps and/or decisions may be performed by separate elements in
conjunction or in parallel, asynchronously or synchronously, in a
pipelined manner, or otherwise. There is no particular requirement
that the steps and decisions be performed in the same order in
which this description lists them or the Figures show them, except
where a specific order is inherently required, explicitly
indicated, or is otherwise made clear from the context.
Furthermore, not every illustrated step and decision block may be
required in every embodiment in accordance with the concepts
described in this document, while some steps and decision blocks
that have not been specifically illustrated may be desirable or
necessary in some embodiments in accordance with the concepts. It
should be noted, however, that specific
embodiments/variants/examples use the particular order(s) in which
the steps and decisions (if applicable) are shown and/or
described.
[0073] The instructions (machine executable code) corresponding to
the method steps of the embodiments, variants, and examples
disclosed in this document may be embodied directly in hardware, in
software, in firmware, or in combinations thereof. A software
module may be stored in volatile memory, flash memory, Read Only
Memory (ROM), Electrically Programmable ROM (EPROM), Electrically
Erasable Programmable ROM (EEPROM), hard disk, a CD-ROM, a DVD-ROM,
or other form of non-transitory storage medium known in the art.
Exemplary storage medium or media may be coupled to one or more
processors so that the one or more processors can read information
from, and write information to, the storage medium or media. In an
alternative, the storage medium or media may be integral to one or
more processors.
[0074] Having thus described in detail selected embodiments, it is
to be understood that the foregoing description is not necessarily
intended to limit the spirit and scope of the invention(s).
[0075] This document describes in detail the inventive apparatus,
methods, and articles of manufacture for making Geolocation
impossible or more difficult. This was done for illustration
purposes only. Neither the specific embodiments of the invention(s)
as a whole, nor those of their features necessarily limit the
general principles underlying the invention(s). The specific
features described herein may be used in some embodiments, but not
in others, without departure from the spirit and scope of the
invention(s) as set forth herein. Various physical arrangements of
components and various step sequences also fall within the intended
scope of the invention(s). Many additional modifications are
intended in the foregoing disclosure, and it will be appreciated by
those of ordinary skill in the pertinent art that in some instances
some features will be employed in the absence of a corresponding
use of other features. The illustrative examples therefore do not
necessarily define the metes and bounds of the invention(s) and the
legal protection afforded the invention(s).
* * * * *