U.S. patent application number 15/281007 was filed with the patent office on 2017-03-23 for distributed co-operating nodes using time reversal.
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 | 20170086160 15/281007 |
Document ID | / |
Family ID | 47108036 |
Filed Date | 2017-03-23 |
United States Patent
Application |
20170086160 |
Kind Code |
A1 |
SMITH; David ; et
al. |
March 23, 2017 |
DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL
Abstract
Distributed cooperating nodes of a cluster are used for
communications, object location, and other purposes. The nodes can
move relative to each other and an intended receiver. The nodes are
synchronized and data for transmission from the cluster is
distributed to the nodes. The intended receiver sends a sounding
signal to the nodes. Each node receives the sounding signal,
obtains the channel response between the intended receiver and the
node, and time-reverses the channel response. Each node convolves
its time-reversed channel response with the data to obtain the
node's convolved data. A master node sends a time reference signal
to the other nodes. Each node waits a predetermined time following
the time reference signal, as determined based on a common time
reference. At the expiration of the predetermined time period, the
nodes simultaneously transmit their convolved data. The
transmissions from the nodes combine coherently in time-space at
the intended receiver.
Inventors: |
SMITH; David; (Ellicott
City, MD) ; ACHOUR; Maha; (Encinitas, CA) ;
RODE; Jeremy; (San Diego, CA) ; HUSAIN; Anis;
(San Diego, CA) ; HSU; Mark; (Richmond,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ziva Corporation |
San Diego |
CA |
US |
|
|
Family ID: |
47108036 |
Appl. No.: |
15/281007 |
Filed: |
September 29, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14114901 |
Jan 24, 2014 |
9497722 |
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PCT/US12/36180 |
May 2, 2012 |
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15281007 |
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61481720 |
May 2, 2011 |
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61540307 |
Sep 28, 2011 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04W 56/001 20130101;
H04W 56/0065 20130101; H04J 3/0638 20130101; H04J 3/0682 20130101;
H04W 56/0025 20130101; H04W 56/0015 20130101; H04B 7/024 20130101;
H04W 84/18 20130101 |
International
Class: |
H04W 56/00 20060101
H04W056/00 |
Claims
1. A method of transmitting from a plurality of ad hoc nodes, the
method comprising steps of: synchronizing each node of the
plurality of ad hoc nodes to a common reference of all nodes of the
plurality of ad hoc nodes so that said all nodes of the plurality
of ad hoc nodes are enabled to transmit synchronously at a
predetermined frequency; receiving at said each node one or more
waveforms resulting from a sounding signal; generating a
time-reversed signal at said each node, the step of generating
comprising time-reversing the one or more waveforms received at
said each node, thereby obtaining a plurality of time-reversed
signals, a time-reversed signal of the plurality of time-reversed
signals per said each node of the plurality of ad hoc nodes;
sending a trigger signal from a trigger node of the plurality of ad
hoc nodes; and transmitting synchronously at the predetermined
frequency the time-reversed signals from said all nodes of the
plurality of ad hoc nodes, one of the time-reversed signals per
node of the plurality of ad hoc nodes, so that a combination of the
plurality of time-reversed signals transmitted by said all nodes is
spatially and temporally focused on a subject, the step of
transmitting being performed in response to expiration of a
predetermined synchronization period following sending of the
trigger signal.
2. A method according to claim 1, wherein the step of synchronizing
comprises: determining at said each node of the plurality of ad hoc
nodes, other than a selected node of the plurality of ad hoc nodes,
frequency offset of a physical clock of said each node other than
the selected node from a physical clock of the selected node, and
determining signal flight times between at least some nodes of the
plurality of ad hoc nodes.
3. A method according to claim 2, further comprising measuring the
predetermined synchronization period at said each node based on a
common time reference of said all nodes.
4. A method according to claim 3, further comprising receiving the
trigger signal at said each node of the plurality of ad hoc nodes
other than the trigger node.
5. A method according to claim 4, wherein the common time reference
is based on the physical clock of the trigger node.
6. A method according to claim 5, wherein the subject is an
intended receiver of the time-reversed signals, and every node of
the plurality of ad hoc nodes is within radio frequency (RF)
Line-of-Sight (LoS) of one or more other nodes of the plurality of
ad hoc nodes.
7. A method according to claim 6, wherein the sounding signal is
emitted by the intended receiver, the sounding signal having a good
autocorrelation function approaching an impulse function; said all
nodes of the plurality of ad hoc nodes receive the one or more
waveforms resulting from the sounding signal in an identical
temporal window.
8. A method according to claim 5, wherein the subject is a first
scatterer, the time-reversed signals are intended for an intended
receiver, the intended receiver is not the first scatterer, every
node of the plurality of ad hoc nodes is within radio frequency
(RF) Line-of-Sight (LoS) of one or more other nodes of the
plurality of ad hoc nodes, and the intended receiver is not within
RF LoS of at least one node of the plurality of ad hoc nodes.
9. A method according to claim 8, further comprising: processing
the one or more waveforms using Singular Value Decomposition to
determine first signatures for launching from said all nodes of the
plurality of ad hoc nodes a first transmission temporally and
spatially focused on the first scatterer, each first signature
corresponding to a different node of the plurality of ad hoc nodes;
and generating the time-reversed signal corresponding to said each
node by convolving data intended for the intended receiver with the
signature corresponding to said each node; wherein the sounding
signal comprises one or more channel sounding bursts transmitted
from the intended receiver, the one or more waveforms comprising
reflections of the one or more channel sounding bursts from one or
more scatterers, the one or more scatterers comprising the first
scatterer.
10. A method according to claim 9, further comprising comparing one
or more of the first signatures to a plurality of stored
signatures, wherein the stored signatures are stored in a database
together with identifications of objects in environment of the
plurality of ad hoc nodes corresponding to the stored
signatures.
11. An article of manufacture comprising non-volatile
machine-readable storage medium with program code stored in the
medium, the program code, when executed by processors of a
plurality of ad hoc nodes, configures the plurality of ad hoc nodes
to: synchronize each node of the plurality of ad hoc nodes to a
common reference of all nodes of the plurality of ad hoc nodes so
that said all nodes of the plurality of ad hoc nodes are enabled to
transmit synchronously at a predetermined frequency; receive at
said each node one or more waveforms resulting from a sounding
signal; generate a time-reversed signal at said each node by
time-reversing the one or more waveforms received at said each
node, thereby obtaining a plurality of time-reversed signals, a
time-reversed signal of the plurality of time-reversed signals per
said each node of the plurality of ad hoc nodes; send a trigger
signal from a trigger node of the plurality of ad hoc nodes; and
transmit synchronously at the predetermined frequency the
time-reversed signals from said all nodes of the plurality of ad
hoc nodes, one of the time-reversed signals per node of the
plurality of ad hoc nodes, so that a combination of the plurality
of time-reversed signals transmitted by said all nodes is spatially
and temporally focused on a subject, the step of transmitting being
performed in response to expiration of a predetermined
synchronization period following sending of the trigger signal;
wherein said each node of the plurality of ad hoc nodes comprises
an antenna, a radio frequency transceiver coupled to the antenna, a
local oscillator, and a processor coupled to the transceiver to
control operation of the transceiver.
12. The article of manufacture according to claim 11, wherein the
program code, when executed by the processors of the plurality of
ad hoc nodes, further configures the plurality of ad hoc nodes to
synchronize said each node by determining at said each node, other
than a selected node of the plurality of ad hoc nodes, frequency
offset of a physical clock of said each node other than the
selected node from a physical clock of the selected node, and
determining signal flight times between at least some nodes of the
plurality of ad hoc nodes.
13. The article of manufacture according to claim 12, wherein the
program code, when executed by the processors of the plurality of
ad hoc nodes, further configures the plurality of ad hoc nodes to
measure the predetermined synchronization period at said each node
based on a common time reference of said all nodes.
14. The article of manufacture according to claim 13, wherein the
program code, when executed by the processors of the plurality of
ad hoc nodes, further configures the plurality of ad hoc nodes to
receive the trigger signal at said each node of the plurality of ad
hoc nodes other than the trigger node.
15. The article of manufacture according to claim 14, wherein the
program code, when executed by the processors of the plurality of
ad hoc nodes, further configures the plurality of ad hoc nodes so
that the common time reference is based on a physical clock of the
trigger node.
16. The article of manufacture according to claim 15, wherein the
subject is an intended receiver of the time-reversed signals, and
every node of the plurality of ad hoc nodes is within radio
frequency (RF) Line-of-Sight (LoS) of one or more other nodes of
the plurality of ad hoc nodes.
17. The article of manufacture according to claim 16, wherein the
sounding signal is emitted by the intended receiver; the sounding
signal has a good autocorrelation function approaching an impulse
function; and wherein the program code, when executed by the
processors of the plurality of ad hoc nodes, further configures the
plurality of ad hoc nodes to receive the one or more waveforms
resulting from the sounding signal in an identical temporal
window.
18. The article of manufacture according to claim 15, wherein the
subject is a first scatterer, the time-reversed signals are
intended for an intended receiver, the intended receiver is not the
first scatterer, every node of the plurality of ad hoc nodes is
within radio frequency (RF) Line-of-Sight (LoS) of one or more
other nodes of the plurality of ad hoc nodes, and the intended
receiver is not within RF LoS of at least one node of the plurality
of ad hoc nodes.
19. The article of manufacture according to claim 18, wherein the
program code, when executed by the processors of the plurality of
ad hoc nodes, further configures the plurality of ad hoc nodes to:
process the one or more waveforms using Singular Value
Decomposition to determine first signatures for launching from said
all nodes of the plurality of ad hoc nodes a first transmission
temporally and spatially focused on the first scatterer, each first
signature corresponding to a different node of the plurality of ad
hoc nodes; and generate the time-reversed signal corresponding to
said each node by convolving data intended for the intended
receiver with the signature corresponding to said each node;
wherein the sounding signal comprises one or more channel sounding
bursts transmitted from the intended receiver, the one or more
waveforms comprising reflections of the one or more channel
sounding bursts from one or more scatterers, the one or more
scatterers comprising the first scatterer.
20. The article of manufacture according to claim 19, wherein the
program code, when executed by the processors of the plurality of
ad hoc nodes, further configures the plurality of ad hoc nodes to
compare one or more of the first signatures to a plurality of
stored signatures, wherein the stored signatures are stored in a
database together with identifications of objects in environment of
the plurality of ad hoc nodes corresponding to the stored
signatures.
21. A system comprising a plurality of ad hoc nodes, each node of
the plurality of ad hoc nodes comprising an antenna, a radio
frequency transceiver coupled to the antenna, and a processor
coupled to the transceiver to control operation of the transceiver,
wherein the plurality of ad hoc nodes is configured to: synchronize
each node of the plurality of ad hoc nodes to a common reference of
all nodes of the plurality of ad hoc nodes so that said all nodes
of the plurality of ad hoc nodes are enabled to transmit
synchronously at a same predetermined frequency; receive at said
each node one or more waveforms resulting from a sounding signal;
generate a time-reversed signal at said each node by time-reversing
the one or more waveforms received at said each node, thereby
obtaining a plurality of time-reversed signals, a time-reversed
signal of the plurality of time-reversed signals per said each node
of the plurality of ad hoc nodes; send a trigger signal from a
trigger node of the plurality of ad hoc nodes; and transmit
synchronously at the same predetermined frequency the time-reversed
signals from said all nodes of the plurality of ad hoc nodes, one
of the time-reversed signals per node of the plurality of ad hoc
nodes, in response to expiration of a predetermined synchronization
period following sending of the trigger signal, so that a
combination of the plurality of time-reversed signals transmitted
by said all nodes is spatially and temporally focused on a subject.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority from U.S. patent
application Ser. No. 14/114,901, entitled DISTRIBUTED CO-OPERATING
NODES USING TIME REVERSAL, filed Oct. 30, 2013, now allowed; which
is a National Stage Entry of PCT/US12/36180 (WO/2012/151316); which
claims priority from (1) 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; and from (2) 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. 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
[0002] This document relates generally to radio frequency (RF)
communications using time reversal, RF Geolocation/anti-Geolocation
using time reversal, and scattering object location and
identification.
BACKGROUND
[0003] The use of multiple transmit/receive antennas in wireless
networks promises mitigation of interference and high spectral
efficiencies through concentrating signals along a designated
direction or transmission path. Compared to
single-antenna-to-single-antenna transmissions, transmit
beamforming may yield increased range (e.g., an N-fold increase for
free space propagation), increased rate (e.g., an N.sup.2-fold
increase in a power-limited regime), increased power efficiency
(e.g., an N-fold decrease in the net transmitted power for a fixed
received power), and/or may allow splitting high data-rate stream
into multiple lower data-rate streams. (Here, N is the number of
cooperative nodes or antenna elements at the transmit side.)
[0004] Distributed coherent transmit beamforming is a form of
cooperative communication in which two or more information sources
simultaneously transmit a common message, controlling the phase of
their transmissions so that the signals constructively combine at
an intended destination. Collective digital beamforming
implementation in a decentralized network may require distributed
algorithms for coordinating the pre-coding matrices used by each
element of the arrays of transmit antennas with low overhead. Such
distributed transmit beamforming methods often rely on complex
weighting algorithms and explicit feedback of the weights from the
receiver to the transmitter based on Line-of-Sight (LoS)
combination, to shape the collected radiation beam. The implicit
transmit beamforming weights may be based on link metrics such as
packet error rate and signal-to-noise ratio (SNR), which are not
effective in MP environment. By fixing the phase and power radiated
by each of the N transmit antennas, up to N.sup.2 fold gain can be
reached at the receiver. Perfect channel state information (CSI) at
the transmitter may be required by conventional transmit
beamforming schemes to generate beamforming coefficients and
achieve phase alignment at the receiver. In full-feedback
closed-loop synchronization, each user uses a single beam and a
linear filter at the receiver, while leveraging perfect channel
state information (CSI) at the transmitters and receivers.
Alternatively, channel training in the forward direction is sent
using the current beam-formers and used to adapt the receive
filters. Training in the reverse direction is sent using the
current receive filters as beams and used to adapt the transmit
beamformers. This approach directly estimates the optimal
beamformer and receive filter parameters, as opposed to estimating
the CSI needed to compute those coefficients. In this approach,
neither the transmitters nor the receiver may have perfect channel
state information, but there is a low-rate feedback link from the
receiver to the transmitters, to adjust nodes' phases for all
radios/sensors simultaneously, in each time slot, to achieve phase
alignment.
[0005] The resultant beam shape at the receiver may resemble a
phased-array radiation pattern, with one main lobe and multiple
undesired side lobes that cause interference at other nodes. With
these techniques, it may be difficult or impossible to support
coherent addition of wave-fronts in MP environments, since most
distributed beamforming approaches assume LoS links between
transmitters and receiver.
[0006] Furthermore, the distributed beamforming algorithms may take
hundreds of iteration cycles before converging, adding delay and
making real-time network adaptation challenging. Also, the
iterative algorithms may fail to converge in dynamic channels and
other challenging environments.
[0007] Thus, selected current distributed communication and
networking approaches may suffer from a number of disadvantages,
including these:
[0008] (1) difficult operation in multipath (MP) and
non-line-of-sight (NLoS) environments;
[0009] (2) need to rely on complex weights and pre-coding matrices
derived from link metrics;
[0010] (3) increased interference because of undesired multiple
side lobes resulting from beamforming;
[0011] (4) additional delay and potential non-converge in dynamic
and challenging environments, due to a large number of iterative
steps;
[0012] (5) reliance on exact channel state information (CSI) at the
transmitters;
[0013] (6) need for channel training in the forward and return
directions to estimate weights and pre-coding matrices; and
[0014] (7) reliance on exact long term and short term
synchronization of carrier, data, and time.
[0015] Needs exist for improved communication techniques for
distributed coherent communications, and for apparatus and articles
of manufacture for such improved communications. Needs also exist
for methods, apparatus, and articles of manufacture for hiding
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. Additional needs exist for improved
methods, apparatus, and articles of manufacture that facilitate
non-invasive imaging, such as ultrasound imaging.
SUMMARY
[0016] 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.
[0017] In embodiments, distributed cooperating nodes of a cluster
can move relative to each other and relative to an intended
receiver of the nodes' data transmissions. The nodes are
synchronized to a common time reference, and data for transmission
from the cluster is distributed to the nodes, using, for example,
relatively low power LoS communications between and among the
nodes. The intended receiver sends a sounding signal to the nodes.
Each node receives the sounding signal, obtains the channel
response between the intended receiver and itself, and
time-reverses the channel response. Each node then convolves its
time-reversed channel response with the data, to obtain the node's
convolved data. A master node sends a time reference signal to the
other nodes. Each node waits a predetermined time following the
time reference signal, as determined based on the common time
reference. At the expiration of the predetermined time period, the
nodes simultaneously transmit their convolved data. The
transmissions from the nodes combine coherently in time-space at
the intended receiver.
[0018] In an embodiment, a method of transmitting from a plurality
of nodes includes synchronizing each node of the plurality of nodes
to a common time reference of all nodes of the plurality of nodes,
so that all nodes of the plurality of nodes are enabled to transmit
synchronously a predetermined synchronization period after a
trigger signal is sent from a trigger node of the plurality of
nodes; obtaining at said each node of the plurality of nodes
information sufficient to transmit from said each node a
time-reversed signal corresponding to said each node, so that when
all nodes of the plurality of nodes synchronously transmit
respective time-reversed signals, the time-reversed signals combine
spatially and temporally to focus on a subject; sending the trigger
signal from the trigger node of the plurality of nodes; and
transmitting the time-reversed signals from all nodes of the
plurality of nodes, a time-reversed signal per node, so that a
combination of the plurality of time-reversed signals is spatially
and temporally focused on the subject, the step of transmitting
being performed in response to expiration of the predetermined
synchronization period following the step of sending the trigger
signal. The nodes of the plurality of nodes are untethered ad hoc
nodes movable relative to each other, a different physical clock
per node of the plurality of nodes.
[0019] In aspects of this embodiment, the step of synchronizing
comprises determining at said each node frequency offset of the
physical clock of said each node other than a selected node of the
plurality of nodes from a physical clock of the selected node. In
further aspects, the method includes measuring the predetermined
synchronization period at said each node based on the common time
reference of all nodes of the plurality of nodes. In further
aspects, the method also includes receiving the trigger signal at
all nodes of the plurality of nodes other than the trigger node. In
further aspects, the common time reference is based on the physical
clock of the trigger node.
[0020] In still further aspects, the subject is an intended
receiver of the time-reversed signals, and every node of the
plurality of nodes is within radio frequency (RF) Line-of-Sight
(LoS) of one or more other nodes of the plurality of nodes. In
still further aspects, the step of obtaining comprises receiving at
said each node a sounding signal from the intended receiver; and
time-reversing at said each node the sounding signal received at
said each node, resulting in said each node obtaining a
time-reversed channel response between said each node and the
intended receiver.
[0021] In aspects of this embodiment, the subject is a first
scatterer, the time-reversed signals are intended for an intended
receiver, the intended receiver not being the first scatterer, and
every node of the plurality of nodes is within radio frequency (RF)
Line-of-Sight (LoS) of one or more other nodes of the plurality of
nodes. In further aspects, the step of obtaining comprises:
receiving at said each node waveforms that resulted from sounding
environment using one or more channel sounding bursts transmitted
from the intended receiver, the waveforms comprising reflections of
the one or more channel sounding bursts from one or more
scatterers, the one or more scatterers comprising the first
scatterer, and processing the waveforms using time-reversal and
Singular Value Decomposition to determine first signatures for
launching from all nodes of the plurality of nodes a first
transmission temporally and spatially focused on the first
scatterer, each first signature corresponding to a different node
of the plurality of nodes; and the method further comprises
generating the time-reversed signal corresponding to said each node
by convolving data intended for the intended receiver with the
signature corresponding to said each node. In further aspects, the
method further comprises comparing one or more of the first
signatures to a plurality of stored signatures, wherein the stored
signatures are stored in a database together with identifications
of objects corresponding to the stored signatures.
[0022] In an embodiment, a method of transmitting from a first node
of a plurality of nodes includes synchronizing the first node to a
common time reference of the plurality of nodes, so that all nodes
of the plurality of nodes are enabled to transmit synchronously a
predetermined synchronization period after a trigger signal is sent
from a trigger node of the plurality of nodes; obtaining at the
first node information sufficient to transmit from the first node a
time-reversed signal corresponding to the first node, so that when
all nodes of the plurality of nodes synchronously transmit
respective time-reversed signals, the time-reversed signals combine
spatially and temporally to focus on a subject; receiving the
trigger signal at the first node; and transmitting from the first
node the time-reversed signal corresponding to the first node in
response to expiration of the predetermined synchronization period
following the trigger node sending the trigger signal. All nodes of
the plurality of nodes are untethered ad hoc nodes movable relative
to each other, a different physical clock per node of the plurality
of nodes.
[0023] In aspects of this embodiment, the step of synchronizing
comprises determining at the first node frequency offset of the
physical clock of the first node from a physical clock of a
selected node of the plurality of nodes. In further aspects of this
embodiment, the method includes measuring the predetermined
synchronization period at the first node based on the common time
reference, the physical clock of the first node being different
from the common clock reference. In further aspects, the subject is
an intended receiver of the time-reversed signals, and the first
node is within radio frequency (RF) Line-of-Sight (LoS) of one or
more other nodes of the plurality of nodes. In further aspects, the
step of obtaining comprises receiving at the first node a sounding
signal from the intended receiver; and time-reversing at the first
node the sounding signal received at the first node, resulting in
the first node obtaining a time-reversed channel response between
the first node and the intended receiver.
[0024] In an embodiment, a node comprises an antenna, a transceiver
coupled to the antenna, and a processor coupled to the transceiver
to control operation of the transceiver. The node is part of a
plurality of cooperative nodes. The node is configured to
synchronize the node to a common time reference of the plurality of
cooperative nodes, so that all nodes of the plurality of
cooperative nodes are enabled to transmit synchronously a
predetermined synchronization period after a trigger signal is sent
from a trigger node of the plurality of cooperative nodes; to
obtain at the node information sufficient to transmit from the node
a time-reversed signal corresponding to the node, so that when all
nodes of the plurality of cooperative nodes synchronously transmit
respective time-reversed signals, the time-reversed signals combine
spatially and temporally to focus on a subject; to receive the
trigger signal at the node; and to transmit from the node the
time-reversed signal corresponding to the node in response to
expiration of the predetermined synchronization period following
the trigger node sending the trigger signal. All nodes of the
plurality of cooperative nodes are untethered ad hoc nodes movable
relative to each other, a different physical clock per node of the
plurality of cooperative nodes.
[0025] In aspects, the node is further configured to synchronize to
the common time reference by determining at the node frequency
offset of the physical clock of the node from a physical clock of a
selected node of the plurality of cooperative nodes. In aspects,
the node is further configured to measure the predetermined
synchronization period at the node based on the common time
reference, the physical clock of the node being different from the
common clock reference. In aspects, the subject is an intended
receiver of the time-reversed signals, and the node is within radio
frequency (RF) Line-of-Sight (LoS) of one or more other nodes of
the plurality of cooperative nodes. In aspects, the node is
configured to obtain the information by receiving a sounding signal
from the intended receiver, and time-reversing the sounding signal
received at the node, resulting in the node obtaining a
time-reversed channel response between the node and the intended
receiver.
[0026] In an embodiment, a system includes a plurality of nodes,
each node of the plurality of nodes comprising an antenna, a
transceiver coupled to the antenna, and a processor coupled to the
transceiver to control operation of the transceiver. The plurality
of nodes is configured to synchronize each node of the plurality of
nodes to a common time reference of all nodes of the plurality of
nodes, so that all nodes of the plurality of nodes are enabled to
transmit synchronously a predetermined synchronization period after
a trigger signal is sent from a trigger node of the plurality of
nodes; to obtain at said each node of the plurality of nodes
information sufficient to transmit from said each node a
time-reversed signal corresponding to said each node, so that when
all nodes of the plurality of nodes synchronously transmit
respective time-reversed signals, the time-reversed signals combine
spatially and temporally to focus on a subject; to send the trigger
signal from the trigger node of the plurality of nodes; and to
transmit the time-reversed signals from all nodes of the plurality
of nodes, a time-reversed signal per node, so that a combination of
the plurality of time-reversed signals is spatially and temporally
focused on the subject, the time-reversed signals being transmitted
in response to expiration of the predetermined synchronization
period following sending of the trigger signal. The nodes of the
plurality of nodes are untethered ad hoc nodes movable relative to
each other, a different physical clock per node of the plurality of
nodes.
[0027] In aspects of this embodiment, the plurality of nodes is
further configured to synchronize by determining at said each node
frequency offset of the physical clock of said each node, other
than a selected node of the plurality of nodes, from a physical
clock of the selected node. In aspects, the plurality of nodes is
further configured to measure the predetermined synchronization
period at said each node based on the common time reference of all
nodes of the plurality of nodes. In aspects, the plurality of nodes
is further configured to receive the trigger signal at all nodes of
the plurality of nodes other than the trigger node. In aspects, the
common time reference is based on the physical clock of the trigger
node.
[0028] In further aspects, the subject is an intended receiver of
the time-reversed signals, and every node of the plurality of nodes
is within radio frequency (RF) Line-of-Sight (LoS) of one or more
other nodes of the plurality of nodes. In further aspects, the
plurality of nodes is configured to obtain the information by
receiving at said each node a sounding signal from the intended
receiver, and time-reversing at said each node the sounding signal
received at said each node, resulting in said each node obtaining a
time-reversed channel response between said each node and the
intended receiver.
[0029] In further aspects, the subject is a first scatterer, the
time-reversed signals are intended for an intended receiver, the
intended receiver not being the first scatterer, and every node of
the plurality of nodes is within radio frequency (RF) Line-of-Sight
(LoS) of one or more other nodes of the plurality of nodes. In
further the plurality of nodes is further configured to obtain the
information by (1) receiving at said each node waveforms that
resulted from sounding environment using one or more channel
sounding bursts transmitted from the intended receiver, the
waveforms comprising reflections of the one or more channel
sounding bursts from one or more scatterers, the one or more
scatterers comprising the first scatterer, and (2) processing the
waveforms using time-reversal and Singular Value Decomposition to
determine first signatures for launching from all nodes of the
plurality of nodes a first transmission temporally and spatially
focused on the first scatterer, each first signature corresponding
to a different node of the plurality of nodes; and generate the
time-reversed signal corresponding to said each node by convolving
data intended for the intended receiver with the signature
corresponding to said each node. In further aspects, the plurality
of nodes is further configured to compare one or more of the first
signatures to a plurality of stored signatures, wherein the stored
signatures are stored in a database together with identifications
of objects corresponding to the stored signatures.
BRIEF DESCRIPTION OF THE FIGURES
[0030] FIG. 1 is a high level block-diagram illustrating selected
components of a communication configuration including an intended
receiver and distributed cooperating nodes;
[0031] FIG. 2 illustrates selected elements of an apparatus, such
as a node or a base station, configured in accordance with one or
more features described in this document;
[0032] FIG. 3 illustrates selected steps of a process for
synchronizing two nodes; and
[0033] FIG. 4 illustrates selected steps of a process for
transmitting data from distributed cooperating nodes to an intended
receiver, using time reversal.
DETAILED DESCRIPTION
[0034] 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 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.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] Other and further explicit and implicit definitions and
clarifications of definitions may be found throughout this
document.
[0039] 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.
[0040] 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, which 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 the 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.
[0041] The sounding burst may be a sharp pulse approaching an
impulse, a Gaussian pulse, chirp, barker code, Gold code, 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).
[0042] FIG. 1 illustrates in a high level, block-diagram manner,
selected components of a communication arrangement 100. This
arrangement includes a cluster of ad hoc nodes 105 that communicate
with each other. As shown, the cluster 105 includes five
distributed cooperating nodes, 105-1 through 105-5. In similar
arrangements, the cluster 105 may include any number of a plurality
of nodes 105, for example, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more. The
arrangement 100 also includes a base station 110. The nodes 105 may
represent transceivers of different soldiers of a squad, and the
base station 110 may be a transceiver of a command center in a
Humvee, tank, or another local headquarters or control center.
[0043] The nodes 105 may be within Line-of-Sight (LoS) of each
other and can communicate directly with each other via cluster
links 120. Although links 120-1, 120-2, 120-4, and 120-5 are shown
as connecting the node 105-3 to each of the remaining node 105,
this is an exemplary arrangement; more generally, any of the nodes
105 may be connected by such cluster link 120 to any of the other
nodes 105. The cluster links 120 may be implemented, for example,
using short-range RF link such as a Bluetooth.RTM. link, WiFi, or
other short-, medium-, and longer-range technologies. The
technologies of the cluster links 120 may be standardized or
proprietary.
[0044] The nodes 105 are ad hoc in the sense that they are free to
move and rotate not only relative to the base station 110, but also
relative to each other. The distances between any two of the nodes
105 are typically much smaller (by a factor of 10, for example)
than the distance between any of the nodes 105 and the base station
110. Additionally, the nodes 105 are not tethered to each other, in
the sense that each of the nodes operates using its own physical
time reference, and the antennas of the different nodes 1-5 are not
electrically connected to each other. Each of the nodes 105 may
have a single antenna, or multiple antennas.
[0045] FIG. 2 illustrates selected elements of an apparatus 200
configured in accordance with one or more features described in
this document. The apparatus may be any of the cooperative
transceivers 105 and/or the base station 110. The apparatus may
include a processor 205; a storage device 210 (which may store
program code for execution by the processor 205); a receiver 215
configured to receive radio frequency transmissions (including
scattered/MP transmissions) from one or more other
transceivers/base stations; a transmitter 220 configured to
transmit radio frequency transmissions to the other
transceivers/base stations; and one or more transmit and receive
antennas 225 coupled to the receiver 215 and the transmitter 220. A
bus 230 couples the processor 205 to the storage device 210, the
receiver 215, and the transmitter 220; and allows the processor 205
to read from and write to these devices, and otherwise to control
operation of these devices.
[0046] The nodes 105 are configured to communicate coherently (in a
synchronized manner) with the base station 110. The communication
is "coherent" in the sense that the nodes 105 can transmit the same
data to the base station 110 in a synchronized manner so that the
radio frequency transmissions (emanations) from all or a plurality
of the nodes 105 add coherently in time and space at the receiving
antenna(s) of the base station 110.
[0047] The nodes 105 may be configured to synchronize their
internal clock references in various way. In embodiments, the
synchronization includes both (1) frequency synchronization,
whereby the nodes 105 acquire knowledge of the relative frequency
differences of their internal clocks, and (2) knowledge of RF
signal time-of-flight between the different nodes 105. FIG. 3
illustrates selected steps of a process 300 for synchronizing a
first node (such as one of the nodes 105) and a second node (such
as another node 105). In the context of describing the
synchronization process, and somewhat arbitrarily, we will refer to
the first and second nodes as a source node and an echo node.
[0048] At flow point 301, both nodes are powered up, initialized,
and ready to communicate with each other.
[0049] In step 305, the source node transmits a frequency
reference, e.g., a continuous wave pulse of a predetermined
duration T at a predetermined frequency F. The frequency F may be
the carrier frequency for the communications between the two nodes,
and possibly also the carrier frequency for the communications
between the cluster 105 and the base station 110. The frequency F
is derived from and directly related to the frequency of the
internal clock reference of the source node, for example, through a
frequency synthesis/PLL circuit. The duration T may be optimized so
that it is long enough to average out short term phase noise, but
not long enough to include substantial frequency drift. For quartz
crystal-controlled F of about 2 GHz, 20-100 ms is a good T
duration; in particular embodiments, T was selected to be about 50
ms for quartz crystal-controlled F of 2 GHz.
[0050] In step 310, the echo node receives the transmission from
the previous step, and measures the duration T as defined by the
known number of received pulses of F. Simultaneously, the echo node
counts the pulses of the frequency F derived from its own
reference. In this way, the echo node acquires knowledge of the
relative frequencies of its own clock and the clock of the source
node. Consider, for example, the case where F is 2 GHz and T is 50
ms. There should be one hundred million pulses
(2,000,000,000.times.0.050), because the source's determination of
T is based on its own clock, and the source's F is also derived
from the same clock. Although the echo node's clock may differ
somewhat, the number of the received discrete pulses should be
counted exactly. But the number of echo node's own clock during the
same period may vary, because of the drift of the two clock
references. Assuming, for example, that the echo node counted
100,020,000 pulses of the frequency F generated by the echo node
the echo node can accurately estimate that its own clock is two
hundred parts per million (20,000/100,000,000=200 per million)
faster than the source node's reference.
[0051] In step 315, the echo node waits a predetermined time
duration W after the receipt of the transmission of the previous
step. In examples, the echo node determines the duration W by
reference to the clock of the source node, rather than its own
clock. Continuing with the previous example and further assuming
that W is 1 ms, the echo node waits for 2,000,400 pulses of F
derived from the echo node's clock. This is so because 1 ms is
equivalent to 2,000,000 pulses at the F based on source node's
clock, and the clock at the echo node is 200 parts per million
(ppm) faster, thus: 2,000,000.times.1.000200=2,000,400.
[0052] In step 320, the echo node transmits its own pulse (echo),
which may be a well-defined and short pulse. This step is performed
in response to the expiration of the predetermined time duration
W.
[0053] In step 325, the source node receives the echo and measures
E, the time elapsed from the time of transmission in the step 305
until the receipt of the echo.
[0054] In step 330, the source node calculates the signal flight
time (SFT) between the two nodes. Because E should be equal to W
plus twice the signal flight time, the signal flight time may be
calculated thus: E=W+(2.times.SFT)=>SFT=(E-W)/2.
[0055] In step 335, the source node may send the SFT information to
the echo node.
[0056] The process 300 terminates in flow point 399, and may be
repeated as needed.
[0057] Although the determination of the frequency difference was
combined with the determination of SFT, the two determinations can
be separated.
[0058] The nodes 105 may also be kept in synchronicity by providing
each node with a very stable clock reference, such as a cesium or
rubidium standard. The clocks may then be re-synchronized at longer
intervals. For example, the clocks of the different communications
equipment of soldiers of a squad may be synchronized before the
beginning of a tactical operation, and the stable references may
continue to be synchronized throughout the operation, without
re-synchronization. The determination of the relative clock
frequencies in the steps 305/310 may then be omitted.
[0059] Once the nodes 105 are synchronized, they can generate
coherent communications. In particular, the nodes 105 can be
configured to send coherently data (or other) transmissions to the
base station 110. FIG. 4 illustrates selected steps of a process
400 for the nodes 105 to transmit data to the base station 110,
using time reversal.
[0060] At flow point 401, the plurality of nodes 105 and the base
station 110 are powered up, initialized, and ready to
communicate.
[0061] In step 405, the nodes 105 are synchronized using processes
such as the process 300 described above. A single node 105 (a
master node) may be used as a source node and the remaining nodes
may act as echo nodes; a master node 105 can also act as an echo
node for all the other nodes 105 (in which case the steps 335 may
become unnecessary). The master node can thus be used to set a
common time reference for all the nodes of the cluster.
Alternatively, the synchronization may be performed otherwise. In
the end, all the nodes 105 are synchronized, and can emit a
simultaneous signal.
[0062] The most centrally located node within the cluster 105, or
one of the more centrally located nodes in the cluster, may be
selected as the master node. Centrality of location may be
determined by reference to all the nodes of the cluster 105.
[0063] In step 410, the base station 110 transmits to the nodes 105
a sounding signal, e.g., a pulse/burst or a pilot signal. The
sounding burst may be a sharp pulse approaching an impulse, a
Gaussian pulse, chirp, barker code, Gold code, 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).
[0064] In step 415, each of the nodes 105 receives, captures,
samples, and stores the received sounding signal. Each of the nodes
105-N will thus store the channel response CR.sub.N between itself
and the base station 110. The same master node as was used for
synchronization (in the step 405) may instruct each of the nodes
105 to use an identical temporal window to collect the sounding
signal from the base station 110. The windows across the cluster
105 may be identical in lengths and may have identical central
times. Each node 105 may be told by the master node when to start
looking for the sounding pulse, and when to stop. This time frame
may be referred to as the "identical window," that is, the time
period during which all the transmitters are attempting to receive
the sounding pulse. There are other ways to receive the sounding
signal at the nodes 105. For example, the receive windows do not
necessarily need to be at the same time or be of the same length,
but should have a common symmetry point on which to mirror their
transmissions.
[0065] In step 420, each of the nodes 105 performs time reversal on
its stored sounding signal, obtaining and storing its time-reversed
channel response TR-CR.sub.N. In practice, this step is an
approximation of time-reversal, because causality limits the length
of each recorded CR.sub.N. The TR-CR.sub.N of a particular node 105
is a time-reversed captured window of the particular node 105. The
centers of the windows may be aligned across the nodes of the
cluster 105.
[0066] In step 425, each of the nodes 105 obtains payload data that
the cluster 105 intends to transmit to the base station 110. The
payload data may be distributed, for example, from one or several
of the nodes 105 to the remaining nodes, using the cluster links
120. The payload data may be distributed from the master node. The
data may originate at the master node, another node 105, two or
more of the nodes 105, or all of the nodes 105.
[0067] In step 430, each of the nodes 105 convolves its respective
time-reversed channel response TR-CR.sub.N with the payload data,
to obtain its respective data for transmission DT.sub.N.
[0068] In step 435, a selected node 105 sends a reference
transmission, such as a short pulse, to the remaining nodes 105.
The selected node 105 may be the master node or another node.
Several or each of the nodes 105 may be capable of being the master
node, and the selection or rotation of the master node role among
the nodes 105 capable of being the master node may be predetermined
or performed in the field using various algorithms.
[0069] In step 440, each of the nodes 105 transmits its respective
convolved data DT.sub.N. This step is performed by the nodes 105
simultaneously, for example, following a predetermined time
duration S after the emission of the reference transmission, as
measured at a selected node 105 (e.g., the master node). The length
of the predetermined time duration S is preferably longer than the
SFT between the selected node 105 (e.g., the master node) and each
of the remaining nodes 105. In this way, all of the nodes 105
transmit their respective data DT.sub.N coherently, at the same
time. All of the nodes 105 can measure the length of the time
period S based on the same time reference; for example, each of the
nodes 105 measures S based on the clock of the selected node 105.
Recall that the nodes have the information needed to correct their
internal time references, because of the synchronization in the
step 405. (The frequency offsets are determined as discussed above
in relation to the process 300.) The time-reversal process now
equalizes both the delays of the multipath signatures and the
propagation delay differences, so that all the pulses arrive at the
base station 110 at substantially the same times, creating array
gain and multipath gain. No intentional alignment or gain is
obtained at an unintended (hostile) transceiver, because no matched
filtering or alignment generally occurs at any other location but
the intended receiver (the base station 110). A multi-node transmit
cluster may thus permit significant power reduction and/or range
increase in an NLoS multipath channel.
[0070] In step 445, the base station 110 (the intended receiver)
receives the transmissions from the nodes 105. Because of the
properties of time-reversal communications, the received
transmissions add coherently in time-space at the base station 110.
The properties of time reversal communications cause such coherent
focusing, without the need to synchronize the nodes 105 to the base
station 110. The time reversal process removes many of the sources
of timing errors. The data pulse shape is derived from recording
the sounding, so fixed timing delays are removed during the time
reversal process.
[0071] In effect, the ad hoc distributed nodes 105 act similarly to
a single transmitter with multiple spatially diverse antenna
elements. Among potential benefits of this technique is the ability
to communicate with the base station 110 where a single node 105
may not be powerful enough to close the connection on its own, for
example, due to insufficient signal strength, high noise or
interference levels, and/or other reasons for poor link
reliability.
[0072] The process 400 may then terminate at flow point 499, and be
repeated as needed.
[0073] A node 105 may have two or more antennas, and associated
transmitters. Such a node 105 can be treated as multiple virtual
nodes 105, according to the number of antennas used in cooperative
transmissions to the base station 110. The separate virtual nodes,
however, may be synchronized together, and share a single clock
reference. For added spatial diversity, the antennas can be
implemented using Near-Field Scatterers, as is described in more
detail in U.S. patent application Ser. No. 13/440,796, entitled
APPARATUS, METHODS, AND ARTICLES OF MANUFACTURE FOR WIRELESS
COMMUNICATIONS, filed on 5 Apr. 2012; and in U.S. Provisional
Patent Application Ser. No. 61/476,205, entitled TIME REVERSAL
COMMUNICATION SYSTEMS WITH NEAR-FIELD SCATTERERS, filed on 15 Apr.
2011. Each of these patent applications is commonly owned with the
present patent application, and 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). Briefly, a node 105 may have an antenna array (for
the separate virtual nodes) where adjacent antenna array elements
are separated by less than the diffraction limit (.lamda./2) of the
radio frequency communication band in which the apparatus and
methods operate. A plurality or multiplicity of near-field
scatterers are asymmetrically placed in the immediate vicinity of
each of the antenna array elements, to perturb the pattern of each
of the antenna elements, making the patterns different even below
the diffraction limit. In embodiments, the spacing between the two
antenna elements may be less than .lamda./5 (wavelength over 5),
.lamda./10, .lamda./15, .lamda./30 intervals, or even less, for all
wavelengths (or the longest wavelength, or the center wavelength)
of the design band of the communication system. The polarization of
the near-field scatterers may be the same as or substantially the
same as the polarization of the elements of the antenna array, so
that the scatterers interact efficiently (in the electromagnetic
sense) with the antenna elements of the array. The spacing of at
least some (one or more) of the near-field scatterers from one or
more of the antenna elements of the array may be less than the
spacing between the adjacent antenna elements of the array. The
spacing of at least some of the scatterers from one or more antenna
elements of the array may be less than 0.2.lamda., less than
0.15.lamda., less than 0.1.lamda., less than 0.05.lamda., less than
0.03.lamda., or less than 0.01.lamda..
[0074] The method 400 may further include one or more steps for
selecting the nodes for inclusion in the cluster 105, from among
the nodes available for clustering. In performing such selection,
the nodes with signal-to-noise ratio (SNR) or signal-to-noise and
interference ratio (SNIR) falling below a predetermined limit may
be omitted and not included in the cluster 105. The selection may
be performed dynamically, because the nodes may move and the
environment may change in real time. Furthermore, the time-reversed
channel responses TR-CR.sub.N of the nodes 105 may be analyzed for
"disjointedness," i.e., excessive delays in the channels between
the base station 110 and some of the nodes 105. The channels with
delays over a predetermined limit can be eliminated from the
cluster 105, equalizing the (dominant) delays across the cluster.
Furthermore, one or more additional clusters may be formed from
such nodes. If, for example, there are nodes on either side of a
building, it might be better to split them into one cluster on one
side of the building, and a second cluster on the other side of the
building.
[0075] U.S. patent application Ser. No. 13/462,514, entitled
Anti-Geolocation, filed on 2 May 2012, attorney docket number
Ziva002UTL, is commonly owned with the present application. The
Anti-Geolocation application describes methods for preventing or
making more difficult Geolocation of a source transmitter having an
array of antennas. The Anti-Geolocation application 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). The Geolocation methods
described there can be practiced with the cluster 105, synchronized
as described above. See the description of the process 300 above.
In particular, each of the nodes 105 can act as an antenna element,
and the nodes 105 can be configured to use time reversal to
illuminate an external scatterer instead of the intended receiver
(such as the base station 105), in effect creating a virtual
transmitter from the illuminated scatterer. The nodes 105 can be
configured to illuminate sequentially a plurality of scatterers,
creating an appearance of a walking transmitter. The cluster may
create a scatter matrix, and apply Singular Value Decomposition
(SVD) to the matrix to obtain eigenvalues, and
eigenvectors/eigenfunctions of the matrix, which correspond to
signatures of different scatterers. Illuminating a particular
scatterer may cause a hostile transceiver to become confused as to
the location of the actual nodes 105 and/or their number.
Additionally, collective focusing of multiple nodes 105 may be used
to focus simultaneously on multiple scatterers, simultaneously
creating multiple virtual sources, for an extra layer of
misdirection.
[0076] Creating virtual transmitters by focusing on scatterers in
the operating environment may also be useful for locating concealed
objects and determining their properties. Thus, a strong scatterer
identified on an apparent civilian person by a squad of soldiers
may indicate a weapon concealed under the clothing of the person.
Furthermore, illumination by a spatially diverse cluster, from
different angles, allows a better look at the concealed object. The
signature of the concealed object thus obtained may be compared to
signatures in a library storing many signatures of various more or
less common objects. When a match of predetermined degree (or
greater) between the analyzed signature and a stored signature is
present, an indication can be provided, for example, a visual
and/or audio warning as to the possible nature of the object.
[0077] Above, we have focused on radio frequency operations. The
same principles may apply to ultrasound, for example, in a medical
context. Applying the techniques described above, including those
in the Anti-Geolocation application, allows better scanning. For
instance, today's brain scanning relies on implanting a transmitter
inside the skull. Using multiple ultrasound sources along with time
reversal and SVD, multiple virtual sources may be created inside
the skull to image the brain. Near-Field scatterers can also be
used to increase the density of ultrasound sources and increase TR
gain to enable signal penetration through the skull.
[0078] Synchronization of the nodes 105 may also be advantageous
for implementing a low probability of intercept (LPI) communication
mode. Since time reversal sends a "messy" signal with significant
inter-symbol interference (ISI), an observer at a location other
than the intended receiver may find it difficult to deconvolve the
data. It is possible, however, to add additional security to a
cooperative cluster through distributed coding. For example, a
common source node (e.g., the base station 110) generates a data
stream, applies Forward Error Correction (FEC) coding to the
stream, and then demultiplexes the stream by attaching each
successive bit to a different TR-CRi on each transmit antenna. The
different streams are then summed and transmitted using time
reversal. Each successive bit will arrive at the correct receiver,
but no one data stream will have the correct bits to decode the FEC
correctly since the FEC code is spread across all the channels at
the same frequency. If one attempts to decode a single stream,
(N-1)/N.times.100% of the FEC data is missing, making it impossible
to know how to decode the sequence. However, in the designated TR
channel, if time synchronization of the receivers (e.g., the nodes
105) is pre-established, each receiver will automatically recover
the correct data sequence at the correct receiver, and these can
now be cooperatively combined to re-establish the original FEC
coded sequence, which can now be decoded. If this total channel is
operated at a very poor bit error rate (BER), requiring strong FEC,
then the sequence can only be recovered error free if it is first
completely regenerated at the physical layer by the TR process.
This sequence can now be successfully decoded. However, it is
impossible for a hostile observer to perform the time reversal,
because the hostile observer has no way to recover the transmitted
data sequence and decode the FEC, particularly if the cooperative
receivers (105) are placed at such a distance from each other that
makes it impossible for the hostile observer to be physically close
to more than one of them simultaneously. Not only will the hostile
observer not be able to physically reconstruct the signal, but even
if the hostile observer could somehow isolate all the different
sequences heading to each receive node, the hostile observer is at
a different location than the correct receiver node (105) and so
has no way to know what order or delays to use to align the
streams.
[0079] Another layer of security is added if the sub-lambda
antennas (with near-field scatterers) are used, because then the
hostile observer cannot resolve the separate transmissions from
each antenna and attempt to deconstruct the signals.
[0080] This can be applied to at least two cases. The first one is
accomplished by using one or multiple Tx radios supporting high
data rates and desiring to communicate with multiple receivers
operating with much lower data rates. The second one is
accomplished by using one or multiple Rx radios supporting high
data rates desiring to communicate with multiple transmitters
operating with much lower data rates.
[0081] In both cases, the clustering of Tx and Rx radios based on
their capabilities, locations, and MP signatures is achieved at the
network level followed by splitting the high-data signals to
multiple lower data rate signals in a time division multiplexed
(TDM) fashion. Each of the lower data streams is transmitted to or
received by the designated radio. Then, the high-data rate signal
is reconstructed at the network layer.
[0082] 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.
[0083] 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.
[0084] 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.
[0085] This document describes in detail the inventive apparatus,
methods, and articles of manufacture for communications and other
techniques using distributed cooperating nodes. This was done for
illustration purposes only and, therefore, the foregoing
description is not necessarily intended to limit the spirit and
scope of the invention(s) described. 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 embodiments described
above are illustrative and not necessarily limiting, although they
or their selected features may be limiting for some claims. The
illustrative examples therefore do not necessarily define the metes
and bounds of the invention(s) and the legal protection afforded
the invention(s).
* * * * *