U.S. patent application number 15/695978 was filed with the patent office on 2018-01-18 for synchronization of distributed nodes.
This patent application is currently assigned to Ziva Corporation. The applicant listed for this patent is Ziva Corporation. Invention is credited to Mark Hsu, Anis Husain, Jeremy Rode, David Smith.
Application Number | 20180020416 15/695978 |
Document ID | / |
Family ID | 51627190 |
Filed Date | 2018-01-18 |
United States Patent
Application |
20180020416 |
Kind Code |
A1 |
Smith; David ; et
al. |
January 18, 2018 |
SYNCHRONIZATION OF DISTRIBUTED NODES
Abstract
Dynamic, untethered array nodes are frequency, phase, and time
aligned/synchronized, and used to focus their transmissions of the
same data coherently on a target or in the target's direction,
using time reversal or directional beamforming. Information for
alignment/synchronization may be sent from a master node of the
array to other nodes, over non-RF links, such as optical and
acoustic links. Some nodes may be connected directly to the master
nodes, while other nodes may be connected to the master node
through one or more transit nodes. A transit nodes may operate to
(2) terminate the link when the alignment/synchronization
information is intended for the node, and (2) pass through the
alignment/synchronization information to another node without
imposing its local clock properties on the passed through
alignment/synchronization information. In this way, an end point
node may be aligned/synchronized to the master node without a
direct link between the two nodes.
Inventors: |
Smith; David; (Ellicott
City, MD) ; Husain; Anis; (San Diego, CA) ;
Rode; Jeremy; (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: |
51627190 |
Appl. No.: |
15/695978 |
Filed: |
September 5, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14476738 |
Sep 4, 2014 |
9794903 |
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15695978 |
|
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61881393 |
Sep 23, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04W 84/18 20130101;
H04W 56/0025 20130101; H04B 7/024 20130101; H04W 56/0015
20130101 |
International
Class: |
H04W 56/00 20090101
H04W056/00 |
Claims
1. A method of synchronizing an array of nodes, the method
comprising steps of: receiving a first non-radio frequency (non-RF)
signal carrying a first radio frequency (RF) signal from a master
node of the array, the step of receiving the first non-RF signal
being performed at a first transit node of the array over a first
non-RF side channel link, the first RF signal including properties
of a local reference of the master node; dividing the first non-RF
signal into a first portion of the first non-RF signal and a second
portion of the first non-RF signal, the first portion of the first
non-RF signal carrying a first portion of the first RF signal, the
second portion of the first non-RF signal carrying a second portion
of the first RF signal, the first portion of the first RF signal
including the properties of the local reference of the master node,
the second portion of the first RF signal including the properties
of the local reference of the master node; passing the first
portion of the first non-RF signal through the first transit node
to at least a first end point node of the array without imposing
clock properties of the first transit node on the first portion of
the first non-RF signal; synchronizing local reference of the first
end point node to the local reference of the master node using the
properties of the local reference of the master node included in
the first portion of the first RF-signal, the step of synchronizing
the local reference of the first end point node to the local
reference of the master node comprising frequency and phase
alignment of the first end point node to the master node; and
synchronizing local reference of the first transit node to the
local reference of the master node using the properties of the
local reference of the master node included in the second portion
of the first RF signal, the step of synchronizing the local
reference of the first transit node to the local reference of the
master node comprising frequency and phase alignment of the first
transit node to the master node; wherein: each node of the master
node, the first transit node, and the end point node comprises a
separate clock; and each of the master node, the first transit
node, and the end point node is free to move in at least one
dimension with respect to other nodes of the master node, the first
transit node, and the end point node.
2. The method as in claim 1, further comprising: terminating the
second portion of the first non-RF signal at the first transit
node.
3. The method as in claim 2, wherein the step of terminating is
performed concurrently with the step of passing the first portion
of the first non-RF signal.
4. The method as in claim 3, wherein the step of passing the first
portion of the first non-RF signal comprises indirectly passing the
first portion of the first non-RF signal to the first end point
node via a second transit node of the array.
5. The method as in claim 4, further comprising synchronizing local
reference of the second transit node to the local reference of the
master node using the properties of the local reference of the
master node included in the first portion of the first RF-signal,
the step of synchronizing the local reference of the second transit
node to the local reference of the master node comprising frequency
and phase alignment of the second transit node to the master
node.
6. The method as in claim 3, wherein the step of passing the first
portion of the first non-RF signal comprises directly passing the
first portion of the first non-RF signal to the first end point
node.
7. The method as in claim 3, further comprising: receiving the
first portion of the first non-RF signal at a second end point node
of the array; and synchronizing local reference of second end point
node to the local reference of the master node using the properties
of the local reference of the master node included in the first
portion of the first RF-signal, the step of synchronizing the local
reference of the second end point node to the local reference of
the master node comprising frequency and phase alignment of the
second end point node.
8. The method as in claim 3, wherein: the step of synchronizing the
local reference of the first end point node to the local reference
of the master node further comprises time alignment of the first
end point node to the master node; and the step of synchronizing
the local reference of the first transit node to the local
reference of the master node further comprises time alignment of
the first transit node to the master node.
9. A method as in claim 3, wherein the step of dividing comprises
separating the first portion of the first non-RF signal from the
second portion of the first non-RF signal using an optical power
splitter.
10. A method as in claim 3, wherein the step of dividing comprises
separating the first portion of the first non-RF signal from the
second portion of the first non-RF signal using an optical
wavelength filter.
11. The method as in claim 3, wherein the each of the master node,
the first transit node, and the end point node is free to move in
at least two dimensions with respect to other nodes of the master
node, the first transit node, and the end point node.
12. The method of claim 3, wherein each of the master node, the
first transit node, and the end point node is free to move in three
dimensions with respect to other nodes of the master node, the
first transit node, and the end point node.
13. The method of claim 12, wherein the first non-RF side channel
link is a free-space optical link, and each node of the array is
free to move in three dimensions and free to rotate around a
plurality of axes.
14. The method as in claim 3, wherein: the step of passing
comprises transmitting the first portion of the first non-RF signal
to the first end point node over a second non-RF side channel link;
and the first non-RF side channel link and the second non-RF side
channel link are RF-over-optics links.
15. The method as in claim 3, wherein: the step of passing
comprises transmitting the first portion of the first non-RF signal
to the end point node over a second non-RF side channel link; and
the first non-RF side channel link and the second non-RF side
channel link are RF-over-acoustic links.
16. A communication method comprising steps of: synchronizing the
array nodes as in claim 3; distributing across the array common
data for transmission to a target; operating all nodes of the array
as a phased array to transmit to the target RF signals carrying the
common data.
17. A communication method comprising steps of: synchronizing the
array nodes as in claim 3; distributing across the array common
data for transmission to a target; coherently transmitting from all
nodes of the array to the target RF signals carrying the common
data, so that the signals carrying the common data add
constructively at the target, the step of coherently transmitting
comprising location-focusing using time-reversal.
18. An array of nodes, comprising: a master node comprising a
master node processor, a master node radio frequency (RF)
transceiver coupled to the master node processor, a master node
local reference, and a master node non-radio frequency (non-RF)
transceiver coupled to the master node processor, wherein the
master node is configured by the master node processor to emit a
non-RF signal over a non-RF side channel link, the non-RF signal
carrying an RF signal including properties of the master node local
reference; a first transit node comprising a first transit node
processor, a first transit node RF transceiver coupled to the first
transit node processor, a first transit node local reference, and a
first transit node non-RF processing module coupled to the first
transit node processor, wherein the first transit node non-RF
processing module comprises a first transit node non-RF transceiver
configured to receive from free-space the non-RF signal, a first
transit node non-RF splitter configured to separate the non-RF
signal into a first non-RF component terminated at the first
transit node and a second non-RF component passed through the first
transit node into free-space without properties of the first
transit node local reference being imposed on the second non-RF
component, and first transit node electronic circuitry configured
to obtain from the first non-RF component data in the non-RF
signal, wherein the first non-RF component comprises a first RF
portion of the RF signal that includes the properties of the master
node local reference and the second non-RF component comprises a
second RF portion of the RF signal that includes the properties of
the master node local reference, and wherein the first transit node
is configured by the first transit node processor to synchronize
the first transit node local reference to the master node local
reference using the properties of the master node local reference
included in the non-RF signal received by the first transit node; a
first end point node comprising a first end point node processor, a
first end point node RF transceiver coupled to the first end point
node processor, a first end point node local reference, and a first
end point node non-RF transceiver coupled to the first end point
node processor, wherein the first end point node is configured by
the first end point node processor to receive from free-space a
first part of the second non-RF component passed through the first
transit node using the first end point node non-RF transceiver and
synchronize the first end point node local reference to the master
node local reference using the properties of the master node local
reference in the second RF portion of the second non-RF component,
the first part of the second non-RF component comprising a first
part of the second RF portion of the RF signal; wherein: each node
of the first transit node, the master node, and the first end point
node is free to move in at least one dimension with respect to
other nodes of the first transit node, the master node, and the
first end point node.
19. The array of nodes as in claim 18, wherein said each node of
the first transit node, the master node, and the first end point
node is free to move in at least two dimensions with respect to the
other nodes of the first transit node, the master node, and the
first end point node.
20. The array of nodes as in claim 18, wherein said each node of
the first transit node, the master node, and the first end point
node is free to move in three dimensions with respect to the other
nodes of the first transit node, the master node, and the first end
point node.
21. The array of nodes as in claim 20, wherein said each node of
the first transit node, the master node, and the first end point
node is free to rotate around a plurality of axes.
22. The array of nodes as in claim 21, wherein: the first transit
node, the master node, and the first end point node are configured
to transmit to a target coherent RF signals carrying common data,
so that the coherent RF signals carrying the common data add
constructively in a general direction from the array to the target,
whereby the array of nodes is configured to operate as a phased
array.
23. The array of nodes as in claim 21, wherein the first transit
node, the master node, and the first end point node are configured
as a time-reversal mirror to transmit to a target coherent RF
signals carrying common data so that the coherent RF signals
carrying the common data add constructively at the target.
24. The array of nodes as in claim 18, further comprising: a second
transit node, the second transit node comprising a second transit
node processor, a second transit node RF transceiver coupled to the
second transit node processor, a second transit node local
reference, and a second transit node non-RF processing module
coupled to the second transit node processor, the second transit
node non-RF processing module comprising a second transit node
non-RF transceiver, a second transit node non-RF splitter, and
second transit node electronic circuitry; wherein: the first end
point node receives the first part of the second non-RF component
passed through the first transit node indirectly through the second
transit node; and the second transit node is configured by the
second transit node processor to synchronize the second transit
node local reference to the master node local reference using the
properties of the master node local reference included in the
second non-RF signal.
25. The array of nodes as in claim 18, further comprising: a second
end point node, the second end point node comprising a second end
point node processor, a second end point node RF transceiver
coupled to the second end point node processor, a second end point
node local reference, and a second end point node non-RF processing
module coupled to the second end point node processor; wherein: the
second end point node is configured to receive from free-space a
second part of the second non-RF component passed through the first
transit node, and to perform synchronization of the local reference
of the second end point node to the local reference of the master
node using the properties of the local reference of the master node
carried by the second RF portion, the synchronization comprising
frequency, phase, and time alignment of the second end point
node.
26. The array of nodes as in claim 18, wherein the non-RF side
channel link is an RF-over-optical link.
27. The array of nodes as in claim 18, wherein the non-RF side
channel link is an RF-over-acoustic link.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority from U.S. patent
application Ser. No. 14/476,738, entitled SYNCHRONIZATION OF
DISTRIBUTED NODES, filed on 9 Sep. 2014, now allowed; which claims
priority to U.S. Provisional Patent Application Ser. No.
61/881,393, entitled APPARATUS, METHODS, AND ARTICLES OF
MANUFACTURE FOR COLLABORATIVE ARRAY COMMUNICATIONS INCLUDING
BEAMFOCUSING OF EMISSIONS, filed on 23 Sep. 2013. 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 communications. In
particular, this document relates to alignment (time, frequency,
and/or phase synchronization) of untethered radio frequency (RF)
communication nodes of an array.
BACKGROUND
[0003] The use of multiple transmit/receive antennas in wireless
networks promises mitigation of interference and improved spectral
efficiencies through focusing signals along a designated direction
(directional beamforming or focusing), or on an intended receiver
(location- or spot-focusing). 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 a 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 RF transmit beamforming is a form of
cooperative communication in which two or more nodes (that is,
nodes of a node array) simultaneously transmit a common message,
controlling the phase and timing of their transmissions so that the
transmitted signals constructively combine at an intended
destination.
[0005] In the case of directional beamforming, the individual array
nodes are configured as a phased array to produce a beam that is
approximately collimated in a given direction, but the beam is not
specifically focused to maximize power at a given location of the
target receiver. Phased arrays where the locations of the
individual array elements and the target receiver are known, where
the array elements are interconnected with cables or other
calibrated interconnections (e.g., hardwired), and where a common
centralized clock/time reference can be distributed among the array
elements, can be configured to operate in such directional
beamforming modes.
[0006] Decentralized arrays may be much more difficult to use as
coherent beamforming phased arrays, either in transmit mode or
receive mode. In a decentralized array, the individual nodes are
untethered devices with independent clocks, i.e., without a
distributed/hardwired clock or frequency reference. Additionally,
in a decentralized array the precise positional coordinates of each
node may be unknown and/or varying in time. Decentralized
cooperative arrays and their operation for radio frequency (RF)
communications are described in several commonly-owned and related
patent documents, including the following:
[0007] 1. International Patent Publication WO/2012/151316
(PCT/US2012/36180), entitled DISTRIBUTED CO-OPERATING NODES USING
TIME REVERSAL, filed 2 May 2012;
[0008] 2. U.S. patent application Ser. No. 14/114,901, Publication
Number 2014-0126567, entitled DISTRIBUTED CO-OPERATING NODES USING
TIME REVERSAL, filed on 8 May 2014;
[0009] 3. 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;
[0010] 4. 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;
[0011] 5. U.S. Provisional Patent Application Ser. No. 61/809,370,
entitled APPARATUS, METHODS, AND ARTICLES OF MANUFACTURE FOR
COLLABORATIVE BEAMFOCUSING OF RADIO FREQUENCY EMISSIONS OF RADIO
FREQUENCY EMISSIONS, filed on 7 Apr. 2013;
[0012] 6. U.S. Provisional Patent Application Ser. No. 61/829,208,
entitled APPARATUS, METHODS, AND ARTICLES OF MANUFACTURE FOR
COLLABORATIVE BEAMFOCUSING OF RADIO FREQUENCY EMISSIONS, filed on
30 May 2013;
[0013] 7. International Patent Application PCT/US2014/33234,
entitled DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL, filed
7 Apr. 2014; and
[0014] 8. U.S. patent application Ser. No. 14/247,229, entitled
DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL, filed on 7 Apr.
2014.
[0015] Each of the patent documents described above is hereby
incorporated by reference, including Specification, Claims (if
present), Figures, Tables (if present), and all other matter. We
may refer to these documents collectively as "incorporated
applications" or "related patent documents."
[0016] Several tasks may be necessary or desirable for a
decentralized cooperative array of nodes to operate as a
directional beamforming or spot-focusing array. First, a
decentralized array may need to acquire channel information between
the individual array nodes and the intended target/source, and
provide a mechanism for the nodes to transmit/receive a
correctly-weighted signal at each of the array nodes (or
"elements," or "members," which terms are used interchangeably), so
that beamforming or focusing is achieved to within some
predetermined or variable accuracy required by the system's
specification or applications.
[0017] Second, the information to be transmitted by the
decentralized array to a target may need to be distributed across
the array (i.e., to the individual nodes). Alternatively, when the
array is used for receiving transmissions, the data may need to be
collected from the different nodes of the decentralized array.
[0018] Third, some control operations may need to be performed
across the array.
[0019] Fourth, the individual nodes of the decentralized array
should be phase-aligned, frequency-aligned, and time-aligned, to
enable the array to operate in a coherent manner. Achieving and
maintaining such alignment/synchronization and coordination of the
array nodes is important to the correct operation of the array.
[0020] Some inter-nodal communications are needed in such systems.
The requirements applicable to the procedures used in the
inter-nodal communications may be rather strict, especially those
that are imposed by the need to achieve and maintain
alignment/synchronization of the different nodes. In an array of
nodes, exceeding the clock coherence limit may manifest as a random
scrambling of the phases of the carrier waves utilized in the
beamforming or focusing, and a failure to achieve optimal or even
minimally-acceptable performance. Even with atomic clocks and with
fixed locations of the nodes, the coherence limit is eventually
reached, requiring re-alignment of the clocks. In sum, a method
used for alignment/synchronization should be fast enough to
maintain the alignment required for acceptable communication
operation of the array, given the coherence specifications of the
clocks of the individual nodes. Moreover, there are other factors
that may shorten the time between successive re-alignments, such as
the movement of the nodes and the dynamic changes in the channel
responses.
[0021] Improved techniques for communications between and among
nodes are desirable, in particular improved techniques for
time-phase-, and/or frequency-aligning/synchronizing the nodes and
maintaining their alignment/synchronization in dynamic
environments. Thus, needs exist in the art for improved
node-to-node communication techniques for distributed coherent
communications between an array of nodes and communication
apparatus external to the array; for apparatus, methods, and
articles of manufacture enabling such improved communications; and
for phase/frequency alignment/synchronization techniques that can
be used in ad hoc nodes of a distributed array for coherent
communications.
SUMMARY
[0022] 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.
[0023] In exemplary embodiments, dynamic, untethered nodes of an
array are frequency, phase, and time aligned/synchronized, and used
to focus their transmissions coherently on a target or in the
target's direction, using time reversal for location-focusing or
directional beamforming. Information for alignment/synchronization
may be sent from a master node of the array to other nodes, over
non-RF links, such as optical and acoustic links. These nodes may
operate as Endpoint Nodes (or end nodes) or as Transit Nodes. Some
nodes may be connected by the non-RF links directly (that is,
without the signal traveling through any other array nodes) to the
master node, while other nodes may be connected to the master node
through several non-RF links and one or more intermediate transit
nodes. An end point node may operate to terminate a link when the
alignment/synchronization information on the link is intended for
that node. If the node operates to pass through the
alignment/synchronization information on the link to another node
over another link, without imposing the local clock properties on
the passed-through alignment/synchronization information, the node
operates as a Transit Node. A node may operate as a transit node
and as an end point node. In this way, an end point node may be
aligned/synchronized to the master node without a direct link
between the two nodes.
[0024] In an embodiment, a method of synchronizing an array of at
least three ad hoc nodes includes: (1) receiving from a master node
of the array a first non-radio frequency (non-RF) signal carrying a
first radio frequency (RF) signal, the step of receiving being
performed at a first transit slave node of the array over a first
non-RF side channel link, the first RF signal including properties
of a local time reference of the master node; and (2) passing
through at least a first portion of the first non-RF signal through
the first transit slave to an end-point node without imposing clock
properties of the first transit slave node on the first portion of
the first non-RF signal, the first portion of the non-RF signal
carrying at least a first portion of the first RF-signal, thereby
enabling the end-point node to synchronize to the master node using
the properties of the local time reference of the master node
carried by the first portion of the first RF-signal.
[0025] In aspects, the method also includes terminating a second
portion of the first non-RF signal at the first transit slave node.
The step of terminating may be performed concurrently with the step
of passing through.
[0026] In aspects, the step of passing through includes indirectly
passing through the first portion of the first non-RF signal to the
end-point node via at least one additional transit slave node.
[0027] In aspects, the step of passing through includes indirectly
passing through the first portion of the first non-RF signal to the
end-point node via at least one additional transit slave node.
[0028] In aspects, the step of passing through includes separating
the first portion of the first non-RF signal from the second
portion of the first non-RF signal using an optical power splitter
or a a wavelength filter.
[0029] In aspects, the method also includes synchronizing local
time reference of the end-point node to the local time reference of
the master node using the properties of the local time reference of
the master node included in the first portion of the first
RF-signal.
[0030] In aspects, the method also includes synchronizing local
time reference of the first transit slave node to local time
reference of the master node using the properties of the local time
reference of the master node carried by the second portion of the
first RF-signal.
[0031] In aspects, the method also includes transmitting the first
portion of the first non-RF signal to the end-point node over a
second non-RF side channel link; the first non-RF side channel link
and the second non-RF side channel link are RF-over-optical or
acoustic links.
[0032] In embodiments, a communication method includes
synchronizing the array of the at least three ad hoc nodes as is
described above. The method also includes distributing across the
array common data for transmission to a target. The method further
includes coherently transmitting from each node of the at least
three ad hoc nodes of the array to the target RF signals carrying
the common data, so that the signals carrying the common data add
constructively in a general direction from the array to the target
and/or in a general location of the target, the step of coherently
transmitting including directional beamforming or location-focusing
using time-reversal.
[0033] In aspects, synchronizing the local time reference of the
end-point node to the local time reference of the master node
includes frequency, phase, and time alignment of the end-point
node; and synchronizing the local time reference of the first
transit slave node to the local time reference of the master node
includes frequency, phase, and time alignment of the end-point
node.
[0034] In an embodiment, a communication node includes at least one
communication node processor; a communication node radio frequency
(RF) transceiver coupled to the at least one processor; a
communication node local time reference; and a communication node
non-RF processing module coupled to the at least one communication
node processor. The communication node non-RF processing module
includes a non-RF splitter configured to separate a first non-RF
signal received by the communication node into a first component
terminated at the communication node, and a second component passed
through the communication node, without imposing properties of the
communication node local time reference on the second
component.
[0035] In aspects, the non-RF splitter includes an optical power
splitter or an optical wavelength filter.
[0036] In an embodiment, an array of at least three ad hoc nodes
includes the communication node described above; a master node
including at least one master node processor, a master node RF
transceiver coupled to the at least one master node processor, a
master node local time reference, and a master node non-RF
processing module coupled to the at least one master node
processor; and an end-point node including at least one end-point
node processor, an end-point node RF transceiver coupled to the at
least one end-point node processor, an end-point node local time
reference, and an end-point node non-RF processing module coupled
to the at least one end-point node processor. The master node is
configured by the at least one master node processor to emit the
first non-RF signal over a first non-RF side channel link, the
first non-RF signal carrying an RF signal including properties of
the master node local time reference. The communication node is
configured by the at least one communication node processor to
synchronize the communication node local time reference to the
master node local time reference using the properties of the master
node local time reference included in the first non-RF signal
received by the communication node. The end-point node is
configured by the at least one end-point node processor to
synchronize the end-point node local time reference to the master
node local time reference using the properties of the master node
local time reference in the second component of the first non-RF
signal passed through the communication node.
[0037] In aspects, the communication node, the master node, and the
end-point node are configured to transmit coherently to a target RF
signals carrying common data, so that the RF signals carrying the
common data add constructively in a general direction from the
array to the target and/or in a general location of the target.
[0038] In aspects, the communication node non-RF processing module,
the master node non-RF processing module, and the end-point non-RF
processing module are optical processing modules; and the first
non-RF signal is an optical signal carrying an RF signal.
[0039] In aspects, the communication node non-RF processing module,
the master node non-RF processing module, and the end-point non-RF
processing module are acoustic processing modules; and the first
non-RF signal is an acoustic signal carrying an RF signal. These
and other features and aspects of the present invention will be
better understood with reference to the following description,
drawings, and appended claims.
BRIEF DESCRIPTION OF THE FIGURES
[0040] FIG. 1A illustrates selected components of a communication
arrangement including a base station and a collaborative array;
[0041] FIGS. 1B, 1C, and 1D illustrate selected aspects of various
communication layers of an array of nodes;
[0042] FIG. 2 illustrates selected elements of a communication
apparatus configured in accordance with one or more features
described in this document;
[0043] FIG. 3 selected steps of a process of time-reversal
communications between an array of collaborative nodes and a base
station 110;
[0044] FIG. 4 illustrates various array architectures;
[0045] FIG. 5 illustrates selected aspects of a Star
architecture
[0046] FIG. 6 illustrates selected aspects of an End Point example
of an array with Line architecture;
[0047] FIG. 7 illustrates another example of an array with the Line
architecture;
[0048] FIG. 8 illustrates selected components of a node that can be
configured to operate in both a Transit Slave node and an End Point
node configurations;
[0049] FIG. 9 illustrates selected aspects of an optical wavelength
division multiplexing side channel link design; and
[0050] FIG. 10 illustrates selected steps of a Doppler compensation
process.
DETAILED DESCRIPTION
[0051] 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.
[0052] The words "couple," "connect," and similar expressions and
words with their inflectional morphemes do not necessarily import
an immediate or direct connection, but include within their meaning
connections through mediate elements.
[0053] The expression "processing logic" should be understood as
selected steps and decision blocks and/or
hardware/software/firmware 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.
[0054] Array "nodes," "elements," and "members" are used
interchangeably.
[0055] References to "receiver" ("Rx") and "transmitter" ("Tx") are
made in the context of examples of data transmission from a
transmitter to an intended or target receiver. For time-reversal
communication techniques, the intended or target 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 sides. In this document,
the nodes of a cooperative array may be "transmitters" of data,
which they transmit to an "intended receiver" (or "targeted
receiver," "target Rx," or simply "target"), such as a base
station. The roles may be reversed, with the cooperative array (or
any of its nodes) also or instead being the intended or target
receiver for the transmissions from one or more base stations. In
the event that the ascribed meaning is different in a particular
context, we will specify in the context what configuration is being
discussed.
[0056] A "target" thus may be an entity that emits a sounding
pulse, and may generally include both transmit and receive
functionality. Note that although we may occasionally refer to a
target (or equivalent terms, as mentioned above) in the singular,
the general description of the processes and systems involved
applies to multiple targets; as is discussed in this document and
the related patent documents, an array of nodes may transmit to
multiple targets at different times, simultaneously, and/or using
transmissions that partially overlap in time. Note also that a
target may be a source of cooperative and/or opportunistic
transmissions used for "sounding." The "sounding" term is explained
below.
[0057] Selected examples of communication processes and
architectures described in this document and in the related patent
documents allow an array of untethered radio frequency (RF)
transmit-receive (Tx/Rx) nodes with independent and unsynchronized
clocks to achieve phase alignment, frequency alignment, and time
alignment (synchronization), enabling the nodes to operate as a
coherent array. For location-focusing applications, the nodes may
be configured to capture "sounding" signals from one or more
targets, and use time-reversal (TR) to retrodirect energy
automatically back to the target(s). In this way, the array of
nodes may be able to achieve spatio-temporal focusing of the energy
on the one or more targets. In this document, we designate the
nodes of such an array as "ad hoc nodes," to signify that the ad
hoc nodes have their own physical clocks, and the nodes are
"untethered" in the sense that they may be (1) free to move, in
absolute terms (e.g., with respect to a point with fixed GPS
coordinates, and/or with respect to any or all of the targets), and
to move with respect to each other, and (2) free to rotate around
one or more axes. Some constraints on the movements of some or all
of the nodes need not necessarily vitiate their untethered
character; for example, nodes that are free to move in only one or
two dimensions (and not all three dimensions) may still be ad hoc
nodes, whether or not they rotate around any axis. Examples of such
nodes may include radios carried by a squad of soldiers; radios
onboard different aircrafts, water vessels or buoys, land vehicles,
satellites; and similar nodes. The nodes may also be ad hoc even if
they are not used in TR-communications; for example, nodes of a
directional beamforming array may also be ad hoc nodes.
[0058] "Sounding" and its inflectional morphemes refer to
transmitting a signal from the target and capturing the signal by
the nodes of the array for the purpose of obtaining information
about channels from the nodes to the target, for example, for
forming TR signals. There are different modes of sounding. In
accordance with the no-separation sounding approach, targets emit
overlapped sounding signals, but the channel impulse responses
(CIRs, or more generally, channel responses, CRs) might not be
amenable to unique identification at the array nodes, and may be
captured in a buffer at each node of the array. In accordance with
a time-separated sounding approach, each target emits its sounding
signal (e.g., a pulse or a pilot) at times sufficiently separated
so that the nodes can deduce which target emitted the sounding
signal from the timing of the received sounding signal. Each array
node may acquire and store the channel response or the channel
impulse response separately, with an identifier (ID) that
identifies the target that emitted the sounding signal. This
identification may be based on a separate protocol known to the
array nodes. Examples of the protocols include a frequency ID list,
and the protocol that establishes the sequence the targets send
their sounding signals. This can be performed in both the
heterodyne and homodyne modes. Other protocols may also be used. In
accordance with a frequency-separated sounding approach, each
target transmits overlapped or non-overlapped sounding signals, but
the CRs can be uniquely identified by each node from different IF
frequencies. In other words, the frequencies of the sounding
transmissions differ from target to target, and the nodes can
identify the different targets from the frequencies of the sounding
transmissions. Additional sounding approaches include
polarization-separation approach, signal labeling approach in
accordance with which uniquely identifiable pulses from each of the
targets are transmitted, and still other sounding approaches.
[0059] If the CRs (which term subsumes CIR) resulting from the
different targets are separated at the nodes of the cooperative
array by separate soundings (e.g., by different IF frequencies, or
otherwise), the array can apply different data streams to the
different CIR/CR sets corresponding to the different targets. For
example, if a first target emits a signal and each node identifies
and stores its copy of the CR resulting from this signal, then each
node can convolve a first data stream with the node's copy of the
CR of the first target. When the convolved signals of the array
nodes are transmitted, after upconverting to the correct carrier
frequency, the signals carrying the first data stream should be
coherently focused (spatially and temporally) on the first target.
Note that all the signals may be at different IF frequencies at the
nodes, but arrive back at the target at the same frequency; that
same frequency may be the frequency of the sounding transmission
from the first target, or it may be another frequency. (Preferably,
the array transmits on the same frequency as the sounding
frequency, or a frequency sufficiently close to the sounding
frequency so that the channel response does not differ appreciably
between the two frequencies.) At other locations, including the
locations of the other targets, the transmissions from the nodes of
the cooperative array will generally not combine constructively,
and an observer at the other locations should see only incoherent
signals without coherent data that can be detected and decoded. In
general, the signals will combine constructively only at the
location of the target that emitted the sounding pulse and whose
time-reversed CRs were utilized by the array for the retrodirected
transmission of the data stream.
[0060] If the CRs/CIRs from the different targets are not separable
and consequently are combined as a composite sounding signal in a
buffer at each node of the cooperative array, then a single data
stream may be broadcast to all the targets. To do so, each of the
nodes is configured to convolve the data stream with the node's
composite (and time-reversed) CR which is the combination of the
sounding signals from all the different targets that emitted the
sounding signals. The TR transmissions from the multiple nodes of
the array will then focus the same data stream at the multiple
targets whose time-reversed CRs/CIRs were included in the composite
sounding signals in the nodes' buffers. If a target did not emit a
sounding signal, no coherent data stream will generally appear at
that target.
[0061] The sounding signals from the different targets may not be
easily "separable" at the nodes of the cooperative array. An
example of such circumstances is when the sounding signals are
overlapping in time, use the same frequency, and do not carry
information from which the targets can be distinguished. Although
the resultant multipath signal may appear chaotic and complex at
the nodes of the array, the spatial distribution of the antennas
may create a deterministic signature which can be used to identify
the component of the signal arriving from each target. At each
node, the composite signal can be deconstructed using an eigenvalue
decomposition method that is capable of separating out the
different signal components. For example, Singular Value
Decomposition or SVD may be used for separation of the different
components. Singular Value Decomposition and identification of
specific sources of emission (e.g., the targets) from a combined
signal are described in the related patent documents and in a
commonly-owned U.S. patent application Ser. No. 13/462,514,
entitled ANTI-GEOLOCATION, 2 May 2012, which is also incorporated
herein by reference in its entirety.
[0062] Briefly, the application of eigenvalue decomposition to a
composite signal can decompose the signal into its individual
components, which, when time-reversed, focus on the multiple
sources (e.g., A, B, C and D) independently. It is then possible
selectively to choose to omit or modify the properties (e.g., gain)
of each eigenstate independently. If the TR version of eigenstate
corresponding to the source B (by way of example) is not launched
from each node (or a subset of nodes) of the array, then no signal
is focused at the source B. In this example, a gain of zero is
effectively applied to the eigenstate B, and unity gain to each of
the other eigentstates. The node may be configured so that it has
the ability to apply independently to each eigenstate any arbitrary
gain from zero to essentially infinity, or gain modulated by a data
sequence. Other nodes of the array may be similarly configured and
perform analogous processes. The individual gain functions can be
different, but the data sequences applied at each node may be the
same.
[0063] As should be apparent to a person skilled in the art after
perusal of this document, the TR communication techniques operate
to suppress the signal at unauthorized receivers (snoopers),
whether the location of the intended target is known or not known.
A snooper may be able to remove the scrambling if the snooper uses
a directional antenna to observe a single node of the cooperative
array. The snooper's gain, however, may then be much lower than the
gain on the authorized channel (that is, the channel to the
intended target or one of the intended targets). It can be
estimated that, in many situations, the difference in gain may
often be 20-30 dB. In examples, the cooperative array is configured
to operate a channel to an intended target at the SNR threshold for
error-free performance, or slightly above (e.g., 1-3 dB) above the
threshold. In the above example this may result in the snooper
receiving a signal 20-30 dB below the SNR threshold for error-free
performance, thereby reducing the snooper's opportunity to
eavesdrop, because the snooper does not benefit from the coherent
gain that the array provides to the intended target(s).
[0064] A more complex coding system may be implemented to add
protection to the data transmitted by the cooperative array so the
data stream cannot be acquired by a snooper observing a single
array node. For example, a data stream to be transmitted from the
array may first be split into I (two or more) separate and uniquely
different streams using a code that has the property that when the
original data stream is broken into I different substreams, and
then the I substreams sum together with equal amplitude, the sum
re-creates the original data stream. A TR-enabled array may emit
different substreams from each array member (or from a different
subset of array members). However, since TR can enable I substreams
to overlap and add spatially at the target, TR with the additional
requirement of amplitude balancing of the emitted signals may
recreate the original data stream at the target.
[0065] The amplitudes of the different substreams arriving at the
target may be balanced, but at the same time they may appear
unbalanced when they are emitted. Assuming (.alpha..sub.m0,
.alpha..sub.m1, . . . , .alpha..sub.mn) are the amplitudes of
impulses received at the array and which originated as a sounding
pulse from the target m, then emitting the time reversed version of
the sequence
K ( .gamma. m 0 .alpha. m 0 , .gamma. m 1 .alpha. m 1 , , .gamma.
mn .alpha. mn ) ##EQU00001##
ensures that all the signals arriving at the intended target are of
(substantially) equal amplitude and should spatially sum to provide
a coherent signal. Here, K is an amplitude adjustment constant
applied equally across all array nodes. This allows the system to
ensure that signal levels lie within the range appropriate for the
RF electronics at the nodes. The terms .gamma..sub.mn allow
individual nodes to apply local adjustments for signal optimization
purposes. The benefit of this technique is that the spatial code
ensures that no single substream contains all the information in
the original data stream, and generally the only location where
amplitude balance and constructive time alignment are possible is
the intended target that emitted the sounding signal.
[0066] Note that TR may provide multipath gain in addition to the
array gain.
[0067] Time-reversal techniques may be applied not only to data
communications, but also to power focusing (electro-magnetic pulse
or EMP), and any other applications requiring selectivity.
[0068] Control and configuration of the array for cooperative tasks
(such as transmission of data to target(s)) may rely on
communications between and among the array nodes. For example, the
array nodes may need to agree on which target to send data to, what
power levels should be used in transmitting to the target, and
exchange other information needed for various communication layers.
The inter-nodal communications are also needed to synchronize/align
the nodes to make coherent transmissions possible, including clock
synchronization/alignment, phase synchronization/alignment, and/or
frequency synchronization/alignment. Other tasks for which
inter-nodal communication may be needed include distribution across
the array of the data for transmission to the target(s), and
collection of data received by the array cooperatively from the
target(s).
[0069] In some embodiments, one of the nodes in the array is
defined as the master node which can be set as a de-facto reference
for alignment/synchronization of the other nodes of the array,
referred to as slave nodes. Embodiments of the cooperative array
implement a procedure where the slave nodes are phase, frequency,
and time aligned/synchronized to the master node. When this is
done, the array may be set up to ensure that, if the cooperative
array protocol is used, the array will automatically location-focus
the signals on an external target, with fading eliminated or
reduced and without requiring knowledge of the target's position.
The node designated as the mater node may change during operation,
for example, in response to the varying dynamic conditions of the
environment and/or of the array. The master node may also be
responsible for other functions, such as control and coordination
of the array, distribution of data for transmission to the target,
collection of data received from the target, and still other
functions.
[0070] In TR-based location-focusing embodiments, the cooperative
array may emit signals at the same start and finish times across
the array, based on a common time reference. The accuracy of the
time synchronization across the cooperative array need not be
perfect, but may be accurate to a reasonable fraction (e.g., 1/10)
of the sounding pulse envelope--not of the carrier frequency
period. This feature may permit, for example, nano-second alignment
accuracy, instead of femto-second alignment accuracy. The array
nodes do not necessarily require to measure the arrival times of
the sounding pulses at each of their receivers, and each node does
not necessarily require knowledge of the arrival times of the
sounding pulses at any other node of the array. But the nodes do
need to capture the sounding pulse and may need to agree on the
"capture window," that is, the time period within which the nodes
of the array attempt to capture the sounding pulse. The master node
(or another assigned node) may be given the responsibility for
determining the start and the duration of the capture window (or,
alternatively, the start and finish times), to ensure that the
array nodes are "listening" when the sounding pulse is emitted, and
that the capture window (which is essentially defined as the time
period over which the nodes are capturing any arriving signals
generated by the sounding pulse) is long enough so that every node
of the cooperative array will capture the sounding signal,
including its significant multipath components, if present. In
embodiments, the master node can ensure that the capture window is
long enough by assuming the window is longer than the worst case of
the sum of the two longest propagation delays between the master
node and the slave nodes. This may be particularly useful for cases
where the target is Line-of-Sight (LOS) or weakly Non-LOS (NLOS) to
the cooperative array.
[0071] For situations in which there is severe multipath that
extends over time periods that greatly exceed (e.g., by a factor of
10 or more) the inter-nodal propagation time, the master node may
be configured to extend the capture window by the excess time, or
longer. There are many techniques by which the master node can
acquire the knowledge regarding the length of multipath and
node-to-node delays. For example, the master node can be configured
to set an initial capture window of a very long duration, and share
this information with the other nodes of the cooperative array.
After the nodes of the array capture the sounding signal, each
slave node calculates the extent of the sounding pulse plus any
multipath decay spreads and communicates this information to the
master node. The master node then determines the actual capture
window required, possibly including an additional time margin, and
communicates the new capture window parameters (start/stop times,
or one of the start/stop times and duration, for example) to the
slave nodes. The nodes of the array then proceed to acquire a new
sounding pulse using the new capture window. In an alternative
approach, the nodes actually capture the sounding signal over a
long time period and the master node simply decides how to reduce
this period into a shorter capture window by using only the segment
between different time stamps applied to the whole signal, without
using a new sounding for this purpose.
[0072] One of the more challenging cases is presented when a large
array with a wide spatial distribution of nodes is used to focus a
signal on a target whose presence is known (from the signals
emitted by the target, for example), but whose location is not
known to the array. A directional beamforming array may attempt to
send a beam in the general direction of the target with the beam's
power maximally concentrated in that general direction (as the
direction is estimated). A TR node array, however, may be
configured to focus its collaborative transmission on the desired
target, without actual knowledge of the location of the target.
Thus, when the full angle Field of View (FOV) of the cooperative
array (as observed from the target) becomes large, and in
particular when the array is randomly distributed through space, it
may be difficult for a conventional system to produce even
directional beamforming in a meaningful manner. Time-reversal
communications, however, may work well for the high FOV distributed
arrays. Furthermore, a cooperative TR array with its nodes
distributed in three dimensions may be able automatically to focus
its transmissions in three dimensions, including the elevation
dimension.
[0073] The captured signals used in the sounding process may be
"cooperative signals" or "collaborative signals," that is, signals
sent by the target for the purpose (or one of the purposes) of
allowing the nodes of the array to obtain estimates of the channels
between the target and the nodes; the captured signals may also be
"opportunistic," sent from the target for some other purpose.
[0074] FIG. 1A illustrates in a high level, block-diagram manner,
selected components of a communication arrangement 100. This
arrangement includes an array of ad hoc nodes 105 that communicate
with each other. As shown, the array 105 includes five distributed
cooperating nodes, 105-1 through 105-5. In similar arrangements,
the array 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, a target.
[0075] The nodes 105 may be within Line-of-Sight or
Non-Line-of-Sight of each other, and may communicate directly with
each other via side channel links 120. As shown, the links 120-1,
120-2, and 120-4 connect the node 105-3 to each of the nodes 105-1,
105-2, and 105-4, respectively; and the link 120-5 connects the
node 105-4 to the node 105-5. The node 105-3 may thus communicate
directly with each of the nodes 105-1, 105-2, and 105-4. The node
105-3 may communicate with the node 105-5 indirectly, through the
node 105-4 and the links 120-4 and 120-5. This is just one example.
More generally, any of the nodes 105 may be connected by such side
channel link 120 to any of the other nodes 105, and any of the
nodes may lack a direct link to any other node (or nodes), and
communicate with such other nodes through intermediate nodes and
multiple (two or more) links. The side channel 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 RF technologies. The side channel links 120 may also
be implemented using non-RF technologies and transmission media,
including optical technologies, such as free-space or guided
optics, and sound/acoustic (ultrasound) technologies. A more
detailed discussion of the architecture of the side channel links
120 and their underlying technologies, with examples, will be
provided below.
[0076] 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 nodes
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); an RF receiver 220 configured
to receive radio frequency signals, such as sounding pulses; an RF
transmitter 215 configured to transmit radio frequency signals,
such as collaborative communications to a base station; one or more
RF transmit and receive antennas 225 coupled to the receiver 220
and the transmitter 215; and a non-RF processing module 227, such
as an optical or acoustic transceiver and associated signal
processing devices. A bus 230 couples the processor 205 to the
storage device 210, the receiver 220, the transmitter 215, and the
non-RF processing module 227; and allows the processor 205 to read
from and write to these devices, and otherwise to control operation
of these devices. In embodiments, additional receivers and/or
transmitters are present and coupled to the processor 205.
[0077] The nodes 105 may be configured to communicate coherently
(in a synchronized and coherent 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 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; such coherent
communications include directional beamforming and
location-focusing.
[0078] FIG. 3 illustrates selected steps of a process 300 for an
array of collaborative nodes, such as the nodes 105, to transmit
data to a target, such as the base station 110, using time
reversal.
[0079] At flow point 301, the nodes of the array 105 and the base
station 110 are powered up, initialized, and ready to
communicate.
[0080] In step 305, the nodes 105 are aligned/synchronized. A
single node 105 (for example, a selected master node) may be used
to set a common time/frequency/phase reference for all the nodes of
the array. Alignment/synchronization of nodes may be performed as
is described in the related patent documents, particularly in (1)
International Patent Publication WO/2012/151316 (PCT/US2012/36180),
entitled DISTRIBUTED CO-OPERATING NODES USING TIME REVERSAL, filed
2 May 2012; (2) U.S. patent application Ser. No. 14/114,901,
Publication Number 2014-0126567, entitled DISTRIBUTED CO-OPERATING
NODES USING TIME REVERSAL, filed on 8 May 2014; and (3) U.S. patent
application Ser. No. 14/247,229, entitled DISTRIBUTED CO-OPERATING
NODES USING TIME REVERSAL, filed on 7 Apr. 2014. Alternatively, the
alignment/synchronization may be performed otherwise. In the end,
all the nodes 105 are aligned/synchronized, and can emit
simultaneous signals for coherent communications such as
directional beamforming and location-focusing.
[0081] In step 310, the base station 110 transmits to the nodes 105
a sounding signal. The sounding signal may be a cooperative signal
or an opportunistic signal. The sounding signal 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 of interest. The
sounding signal may be selected to have 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).
[0082] In step 315, each of the nodes 105 receives, captures,
samples, and stores the received sounding signal. Each of the nodes
105-N may thus store the channel response CR.sub.N between itself
and the base station 110 (or analogous information). The same
master node as was used for synchronization in the step 305 may
instruct each of the nodes 105 to use the same or substantially
same temporal window to receive/capture/sample the sounding signal
from the base station 110. The windows across the array 105 may be
identical in lengths and may have the same center times. Each node
105 may be instructed by the master node when to start looking for
the sounding pulse, and when to stop. This timeframe may be
referred to as the "capture window," that is, the time period
during which all the transmitters are attempting to receive the
sounding signal. There are other ways to receive the sounding
signal at the nodes 105. For example, the capture windows do not
necessarily need to be at the same times or be of the same lengths,
but should have a common symmetry point on which to mirror their
transmissions.
[0083] In step 320, 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
time-reversal may be performed at carrier frequency, an
intermediate frequency, or baseband.
[0084] In step 325, each of the nodes 105 obtains payload data that
the array 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 side channel
links 120. The payload data may be distributed from the master
node. The payload data may originate at the master node, another
node 105, two or more of the nodes 105, or all of the nodes
105.
[0085] In step 330, each of the nodes 105 may generate its data for
collaborative transmission to the target. For example, each node
may convolve its respective time-reversed channel response
TR-CR.sub.N with the common payload data, to obtain its respective
data for transmission, DT.sub.N.
[0086] In step 335, a selected node 105 may send a reference
transmission, such as a short pulse, to the remaining nodes 105;
the remaining nodes 105 may receive the reference transmission sent
by the selected node 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 predetermined conventions.
[0087] In step 340, each of the nodes 105 transmits its carrier
with the 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. The length of the predetermined time duration S is
preferably longer than the Signal Flight Time (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. The nodes 105 have the
information needed to correct their internal time/clock references,
because of the synchronization/alignment in the step 305.
[0088] The time-reversal process equalizes both the delays of the
multipath signatures and the propagation delay differences, so that
all the signals from the nodes arrive at the base station 110 at
substantially the same times, creating array gain and multipath
gain. No intentional 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, the target). A multi-node transmit array may thus
permit significant power reduction and/or range increase in an NLoS
multipath channel.
[0089] In step 345, the base station 110 (the intended
receiver/target) receives the transmissions from the nodes 105.
Because of the properties of time-reversal communications, the
received transmissions add constructively 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.
[0090] 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 collectively with the base station 110 in situations
where a single node 105 may not be powerful enough to close the
connection to the base station 110 on its own, for example, due to
insufficient signal strength, high noise or interference levels,
and/or other reasons for poor link reliability.
[0091] The process 300 may then terminate at flow point 399, and be
repeated as needed.
[0092] The side channel links 120 may be RF links. In embodiments,
the links 120 operate on one or more RF frequencies that are
different from the frequency (frequencies) of the "main" RF
channel, that is, the channel between the array and the target.
While the main RF channel is by definition in the RF domain, it is
possible to operate the side channel links by which the array is
aligned/synchronized using other communication media, for example,
optics and acoustic. One reason why non-RF media may be desirable
for the operation of the side channel links 120 is to guard against
RF emissions, which can be detected and can even be used for the
purposes of geolocation or hostile targeting of the array. Another
reason is that highly collimated optical beams may exhibit
substantial immunity to interference. Yet another reason is that
there are various weather conditions that may prevent RF
communications, while allowing other media communications that are
sufficiently robust for the purposes of aligning the array via the
side channel links 120. For example, heavy rain may seriously
impact some RF propagation modes, while allowing short-range
optical communications.
[0093] The non-RF media may place various restrictions on the side
channel links 120 and on the ways the signals propagating through
these links may be used. The typically omnidirectional behavior of
RF antennas usually means that a single RF signal may be emitted
from one of the nodes 105 (e.g., the master node) and, if there are
no path blockages, the signal will likely reach another node 105
independently of what is happening at any other node 105. Even if
multiple nodes lie in a line, it is rare for any one node 105 to
block the RF signal and prevent other nodes 105 from receiving the
signal. Thus, one node 105 emits the signal, and each remaining
node 105 can receive some component of the signal without
significantly attenuating the signal reaching the remaining nodes
105. This situation may change with a move away from RF
frequencies, especially in the case of optical signals. Optical
signals may be focused into collimated free-space beams or be sent
along fiber optic or other waveguides. Techniques for fractional
tapping of guided signals have been developed, whereby some of the
signal is tapped and terminated at a node and the remaining
untapped fraction is propagated onwards, to other nodes. This same
technique can be used for free-space beams. Attempting this type of
splitting in field-deployed free-space optical links, particularly
links where the end point nodes are in motion, is a challenging
problem, however. Additionally, placing an optical detector
directly in the path of another node tends to cause severe
shadowing at the short wavelengths of optical signals. Optical
detectors typically absorb nearly all the energy falling on them,
and, unlike a dipole antenna, do not re-emit a meaningful fraction
of the signal. Optical beams also experience little diffraction,
which means that they usually do not re-converge behind the object
and do not fill-in the shadow.
[0094] FIGS. 1B, 1C, and 1D illustrate at a high-level an example
of the communication layer architecture of an array of nodes 155-N.
In these figures, four nodes a shown: a master node 155-1 that is
responsible for alignment/synchronization of the array in this
example; and slave nodes 155-2, 155-3, and 155-4. The
communications between the nodes 155 may be implemented using
free-space optics, for example. FIG. 1B shows an exemplary physical
layout of the array; note, however, that the physical arrangement
may be a dynamically-varying one, with the nodes 155 moving with
respect to each other in any or all three dimensions. FIG. 1C shows
the data link layer of the array of the nodes 155. FIG. 1D shows
the physical layer of the array. In the latter figure, the nodes
155-2, 105-3, and 105-4 include optical directional couplers (used
as splitters) 158-2, 158-3, and 158-4, respectively. The master
node 155-1 may also include such an optical directional coupler
(not shown).
[0095] In operation, the master node 155-1 may attempt to create an
independent connection (over side channel link) to each of the
slave nodes 155-2/3/4 at the transport and network layer. At the
data link layer, however, the signals from the master node 155-1
may pass through intermediate nodes, and each of the slave nodes
155 may decide whether to terminate a received signal at that node,
or pass it through because the signal is intended for another slave
node 155. As shown, a signal from the master node 155-1 may be
terminated at the slave node 155-2, or it may be passed to the
slave node 155-3. If the signal is intended for the slave node
155-3, the slave node 155-3 may terminate the signal; if the signal
is not intended for the slave node 155-3, the slave node 155-3 may
pass it to the slave node 155-4. The slave node 155-4 may in its
turn determine whether the signal is intended for it, and terminate
the signal in this case; or pass it still further to another node
(not shown).
[0096] Similarly, the signals from some of the slave nodes 155 may
pass through the intermediate nodes on their way to the master node
155-1 or another slave node.
[0097] At the physical layer, FIG. 1D, the system is shown as a
single light pipe; the pipe may have the ability to run many (two
or more) channels at different wavelengths and/or different
bandwidths. Each slave node 155-2/3/4 has an embedded optical
directional coupler (used as a splitter) 158-2/3/4 that enables the
node to tap into the signal and select out a component of the
signal that is of interest to that node, and pass the other
components (possibly after amplification) to another node. In some
embodiments, a fraction of the total power of signal is removed and
used by the intermediate/transit slave node. In some embodiments,
the node may be frequency selective and tap (or partially tap) one
or more wavelengths of the signal, allowing other signals
substantially to pass through. Other possible examples of tapping
modes include polarization-based and optical mode-based
selectivity.
[0098] The directional couplers 158 may be bidirectional, and the
nodes 155 may be configured to determine whether a tapped signal
component is travelling from a slave node 155-N to the master node
155-1, or from the master node 155-1 to one or more of the slave
nodes 155-N. More generally, the side channel link hardware of the
slave nodes may be direction-sensitive. Thus, a transit slave node
may pass through (directly or indirectly) a signal from the master
node to an end point slave node without imposing the clock
properties of the transit node on the signal that is passed through
to the end point slave; similarly, the transit slave node may pass
through (directly or indirectly) a signal from the slave node to
the master node without imposing the clock properties of the
transit node on the signal that is passed through to the master
node.
[0099] In embodiments, the nodes 155 are configured so that the
optical signal which passes through an intermediate node 155 and is
destined for another node 155 does not undergo termination and
regeneration at the intermediate node 155, but passes transparently
through the intermediate node 155, although optical amplification
or other process at the analog level may be permitted as long as
they do not affect (that is, do not substantially distort) the
clock information embedded in the signal from the node which
generated the signal. Thus, the intermediate nodes do not terminate
the optical signal by converting it to an electrical signal and
then retransmit the digital data of the signal on a new optical
carrier. Such "termination and regeneration" of the signal imposes
properties of the local clock of the intermediate node 155 which is
performing the regeneration of the signal; as has been indicated,
this is not done here. Thus, the clock information that is
intrinsic to the node that generated the signal is preserved as the
signal travels through the intermediate nodes 155, allowing one end
node 155 (e.g., the node 155-4) to compare its clock to the clock
of another node (e.g., the master node 155-1).
[0100] Given the high bandwidth of optical transmission systems, it
is feasible to place onto an optical channel link an entire RF
signal, both carrier and information-containing modulation
envelope. In embodiments, the side channel links are implemented
using such "RF-over-optics" communications. In other words, the
side channel links are optical, with the RF signals of the nodes
being placed onto (e.g., modulating) the optical carriers of the
side channel links. In embodiments, the side channel links are
implemented using "IF-over-optics," with an intermediate frequency
(IF) generated in the RF domain being placed onto (e.g.,
modulating) the optical carriers of the side channel links. In
embodiments, the side channel links are implemented with the
baseband signal placed onto (e.g., modulating) the optical
carrier.
[0101] Many different physical network architectures may be used to
interconnect the nodes of the collaborative array. FIG. 4 shows
selected aspects of examples of the following physical array
configurations/architectures: (1) Star/Round Robin
configuration/architecture, (2) Ring configuration/architecture,
(3) Partially-Connected Mesh configuration/architecture, (4)
Fully-Connected Mesh configuration/architecture, (5) Line
configuration/architecture, (6) Tree configuration/architecture,
and (7) Bus configuration/architecture. Note that FIG. 4
illustrates simple interconnectivity, and not Open System
Interconnection (OSI) model hierarchy.
[0102] Recall that the side channel links may be used by the array
nodes to align/synchronize their local phases, frequencies, and
reference clock times; for example, each slave node 105-2/3/4 may
align/synchronize to the reference of the master node 105-1; and
the slave nodes 155-2/3/4 may align synchronize to the master node
155-1. In some cases, the actual node which plays the role of
master node may be selected arbitrarily or randomly and each array
node (e.g., 105, 155) can interact with any other node of the
array. The Bus and the Fully Connected Mesh architectures are
examples of such cases. The Star architecture is an example of a
network architecture where the master node may be the central node
and the slaves nodes do not connect to each other directly, but
rather indirectly through the central master node. The Star
architecture does not necessarily exclude configurations where the
master node is not the central node and communicates with all or
some of the slave nodes through the central node.
[0103] FIG. 5 illustrates selected aspects of the Star
architecture, where a master node 505-1 is connected directly (no
intermediate/transit nodes) with each of the slave nodes 505-2,
505-3, and 505-4. Here, the side channel links 520-2, 520-3, and
520-4 may implement RF-over-optics communications between the
master node 505-1 and, respectively, the slave nodes 505-2, 505-3,
and 505-4. The alignment/synchronization of the array may be
performed in a sequential round-robin fashion, using time-division
duplexing (TDD). The array may also be aligned/synchronized
simultaneously or substantially simultaneously, using
communications on different RF frequencies (i.e., frequency
Division Duplexing or FDD). In some embodiments, the simultaneous
communications may be carried out on different optical wavelengths,
using wavelength division multiplexing (WDM), whether or not the
optical signal was modulated with baseband, IF, or RF carrier.
Simultaneous alignment/synchronization is particularly useful for
arrays with large numbers of nodes, high frequency clocks, and/or
low-cost clocks using quartz crystals, because sequential process
may exceed the available clock coherence time or other time limits
that constrain the alignment process of the array.
[0104] An "End Point" variant of the Line architecture is shown in
FIG. 6. Here, a master node 605-1 is the first (or last, depending
on the vantage point) node in the line, and the slave nodes
605-2/3/4 may align/synchronize to the master node 605-1. It is the
alignment/synchronization process that defines a node as an End
Point node, meaning that the master node is attempting to
synchronize the clock at that node with the master node's own
clock.
[0105] The slave node 605-2 is the nearest neighbor to the master
node 605-1 and aligns/synchronizes directly to the master node
605-1, over a side channel link 620-2. Other slave nodes may also
align/synchronize to the same master node 605-1, either directly or
indirectly; we will return to this below in the description of FIG.
7.
[0106] FIG. 7 shows another Line architecture example. Here, a
slave node 705-5 has no direct connection to a master node 705-1,
and the side channel link connection between the nodes 705-1 and
705-5 passes through other slave nodes (705-2, 705-3, 705-4, with
the latter two also not having a direct connection to the master
node). This may be typical in an optical interconnect environment.
The slave nodes 705-2/3/4 here play a new role, which we refer to
as the "Transit Slave" node role. In this case, the optical (or
other) signal from the master node 705-1 to the slave node 705-5 is
routed to bypass the clocks located at the Transit Slave nodes
705-2/3/4. The Transit Slave nodes 705-2/3/4 are now considered
part of the optical pipe that enables the master node 705-1 to
connect to the slave node 705-5 further down the chain, with the
slave node 705-5 operating as an End Point node. The goal is that
the slave node 705-5 and the master node 705-5 can
align/synchronize clocks without substantial distortion caused by
the Transit Slave nodes 705-2/3/4 to the clock information passed
from the master node 705-1 to the End Point slave 705-5. In other
words, the slave node 705-5 obtains the clock information of the
master node 705-1 from the optical (or other) signal which has
traversed the Transit Slave nodes 705-2/3/4 on its way from the
master node 705-1. To achieve this, each of the Transit Slave nodes
705-2/3/4 includes module(s) to enable it to be configured in a
bypass mode, which permits the Transit Slave node to bypass its
internal clock process in allowing the optical (or other) signal to
transit as an analog signal whose transit delay is not corrupted by
the clock of the Transit Slave nodes. The Transit Slave nodes
705-2/3/4 thus do not impose their internal clock properties on the
signal that aligns/synchronizes the slave node 705-5 to the master
node 705-1.
[0107] The slave node 705-5 may also receive other information from
the master node 705-1, for example, payload data for transmission
to the base station, and control information. It is not necessary
that such other information be unaffected by the clocks of the
Transit Nodes. In other words, the payload information may be
re-clocked at the Transit Slave nodes.
[0108] Some nodes, such as the nodes 705-2/3/4, may be configured
to function simultaneously as End Point nodes and Transit Slave
nodes. Indeed, when a slave node itself is aligned to the master
node, it is an End Point node; the same node may also serve as a
Transit Node for passing signals to another slave node. A node may
be configured, for example, to function as an End Point node for
one signal on a first optical wavelength (where the side channel
links are WDM links), while at the same time to function as a
Transit Slave node on a second and different optical wavelength. As
another example, a node may be configured to split the same optical
signal of a side channel link into two components and act as an End
Point node for one of the components, while at the same time
functioning as a Transit Slave node for another component. In still
other examples, a node may be configured to split an optical beam
into two signals based on polarization (or optical propagation
mode), and simultaneously act as an End Point node for a first of
the two polarizations (or modes) and as a Transit Slave node for
the second and different polarization (or mode).
[0109] In some embodiments, the side channel links are acoustic.
Some properties of acoustic signals may be similar to those of RF
signals in a star configuration, where the acoustic waves propagate
omnidirectionally. The acoustic signals, however, may exhibit
dispersion more significant than dispersion of RF signals. Acoustic
signals may also operate in full or partial waveguide modes and, in
such cases, they may behave more like optical signals.
[0110] In the examples given throughout this document, it is the
master node that is aligning/synchronizing the clocks at the slave
nodes with its own clock. In variants, however, the master node and
one or more slave nodes align/synchronize to a common external
reference clock. Moreover, the role of the master node, that is,
the node that directs the synchronization operations and/or
distributes across the array the data to be transmitted to the
target, may be assigned to different nodes of the array at
different times. In embodiments, the role of the master node may be
assigned to different nodes depending on their physical position
relative to other nodes of the array. For example, the master node
may be selected to be the node that is closest to the center of
gravity of the array, giving equal weight to each of the array
nodes. The master node may be selected from among the nodes that
are capable of communicating with all the other nodes of the array
over the side channel links, and the selection of a new master node
may be triggered as the current node loses access to one (or
another predetermined number) of the other nodes of the array over
the side channel links. Here, "loses access" means that at least
one metric representative of the quality of the side channel link,
such as signal-to-noise ratio (SNR) or error rate (e.g., bit error
rate or BER) fails to meet a predetermine standard. The at least
one metric may be derived from multiple other metrics.
[0111] In embodiments, two or more nodes of the array concurrently
perform some functions of the master node. For example, one master
node can align/synchronize a second master node (and possibly one
or more of the remaining nodes) to itself, and the second master
node can then align/synchronize some or all of the remaining nodes
of the array to itself. Data to be collaboratively transmitted from
the array may be distributed across the array by one master node or
jointly by multiple master nodes, and/or by one or more slave
nodes. Generally, a master node is a node to which one or more
other nodes are aligned/synchronized.
[0112] FIG. 8 shows selected components of an exemplary array node
805, which can be configured to operate in both a Transit Slave
node and an End Point node. As a person of skill in the art would
understand after perusal of this document, other components would
typically also be present in the node 805, such as the components
205 (processor(s)), 210 (storage), 215 (RF transmitter), and 220
(RF receiver) of the apparatus 200 illustrated in FIG. 2.
[0113] As shown in FIG. 8, the node 805 includes a receive coupling
lens 815 that is configured to receive optical signals from free
space and couple them into a receive optical amplifier 820. The
receive optical amplifier 820 amplifies the received signal and
sends it into an optical splitter 825. The splitter 825 separates
the signal into a first received component 816 and a second
received component 817. The first received component is processed
by the optical and electronic circuitry of the node 805 that
terminate this signal (if needed), including terminating TX/RX
optics 830, RX electronics 835, and internal node clock 840. For
example, the first component may be used to align/synchronize the
node 805, and to receive the data to be transmitted to the target.
The second component 817 is guided to a transmit optical amplifier
845, which amplifies it and sends it toward a transmit coupling
lens 850. The transmit coupling lens 850 couples the signal to free
space, towards another node, which can operate as an End Point
node, a Transit Slave node, or both.
[0114] As noted above, the optical splitter 825 may be a splitter
in the conventional sense, that is, a splitter that separates the
power of the received signal into two components 816/817 based on
partially-transparent, partially-reflecting prism, with the optical
properties of the two components (such as polarization, wavelength,
mode) being the same or substantially the same. Another device may
be substituted for the optical splitter 825, to separate the
components 816/817 based on the optical properties (again,
polarization, wavelength, optical propagation mode, etc.).
[0115] Importantly, the internal node clock 840 does not affect the
second component 817 of the optical signal, and consequently the
signal coupled by the lens 850 to free space preserves the clock
properties of the received signal coupled by the receive coupling
lens 815.
[0116] The architecture illustrated in FIG. 8 may be used as part
of a free-space optical transmission link. The optical signal is
converted back to a fiber system within the node 805, for separate
Transit Slave and End Point processing. This also allows the
inclusion of optical amplifiers 820 and 845 into a free-space
architecture. In embodiments, these devices may be implemented as
single-mode Erbium doped fiber amplifiers (EDFAs), Raman optical
amplifiers, semiconductor optical amplifiers (SOAs), and or other
optical amplifiers and/or combinations of different types of
optical amplifiers.
[0117] Since the ad hoc nodes of the array may be in motion
relative to each other, the array may implement a method for
continually or continuously beam-steering or tracking the
free-space optical side channel links with the precision necessary
for reliable inter-nodal connectivity through the side channel
links. When the various nodes of the array are in relative motion,
the coupling of the free-space links into a fiber or other optical
waveguide of the individual nodes is likely to experience
significant variations due to a number of factors, including
vibration and rapid random motion. This will be particularly true
if the free-space coupling elements are mechanical gimbals with
mechanically-limited response speeds. The amplifiers 820 and 845
may be configured to play an important role, that of equalizing the
coupling variations. For example, the receive optical amplifier 820
may have a large margin of excess gain, and include or be followed
by a high-speed programmable optical attenuator. The electronics
portion of the node 805 can then analyze the power of the received
signal, for example, the power of the first component 816 or the
second component 817, and adjust the programmable optical
attenuator to keep the signal strength between predetermined
limits. Other automatic gain control and similar techniques may be
used.
[0118] A feature of the embodiments described above is that the
Transit Slave mode is a bypass mode, in which the signal (or the
second component 817 of the signal) of the side channel link
through the optical splitter remains in the analog domain, with
reasonable power control. If the transit signal were converted to a
digital signal, the clock properties of the intermediate nodes
would be superimposed on the signal transmitted by the node, making
it difficult or impossible to align accurately the clocks at the
end point. This problem is avoided if the transit portion of the
node through which the side channel link signal travels remains
independent of the internal clock of the node through which the
signal passes.
[0119] As has already been mentioned, the alignment/synchronization
information may be transmitted over the side channel links as
analog RF-over-optical signals. While distribution of data across
the nodes of the array may also be needed (e.g., data to be
transmitted cooperatively by the nodes to a target, array
maintenance data, etc.), it is the RF carrier that is used to
align/synchronize the nodes at either end of an optical channel
link.
[0120] Digital signals, including high data rate communication
signals, may also be sent directly over the optical side channel
links. For example, the back-and-forth array
alignment/synchronization signals may be sent as RF-over-optics,
while higher-layer control signals and the data for the target may
be sent as binary codes over a separate channel and have no need to
be part of the RF-over-optics channel, though still travelling over
the same optical side channel link. Examples of the higher-layer
control signals include the signaling for keeping the optical power
at predetermined levels, and signals used to turn the array on and
off. The high speed communication data from the array to the target
may also be distributed on separate high performance inter-nodal
channels that are distinct from the side channel links used for
alignment/synchronization and/or higher-layer control signals.
[0121] In summary, the array operational process may need to
perform three basic functions with widely-varying bandwidth and
timing requirements: (1) node alignment/synchronization, (2)
control signaling, and (3) data distribution or collection. It may
be quite challenging to place all the pipes onto a single RF link.
An optical link, however, may be more suited for multiple pipes.
For example, an optical link can put the different pipes on
separate high bandwidth WDM wavelengths.
[0122] FIG. 9 illustrates selected aspects of such design. Here, n
separate side channel links are shown, one side channel link per
slave node. (The side channel links may be the links 120/520/620 or
any other optical side channel links such as shown in the figures
and/or discussed in this document.) Each side channel link has
three separate optical pipes. First, there are end-to-end analog
optical pipes 911 on a first wavelength .lamda..sub.1, with no
components clocked by Transit Slave nodes. The optical pipes 911
may be implemented with RF-over-optics, and may be used for the End
Point slave node clock alignment as explained above.
[0123] Second, there are relatively low-bandwidth (e.g., 10 MHz)
pipes 912 on a second wavelength .lamda..sub.2, for sending back
and forth data for the higher level control processes that are used
during the node alignment/synchronization. If the corresponding
analog pipe channel (the channel on the analog pipe 911 that goes
to the same End Point node to perform frequency and time rate
alignment/synchronization) is also used to align the clocks, that
is, to set the clocks to the same actual time, time-stamp data may
be sent over the pipes 912.
[0124] Third pipes 913 may be relatively high-bandwidth optical
pipes for distributing communication data that is to be transmitted
by the array to the target, or collecting the data received by the
array, or other data-intensive inter-nodal communications. These
pipes may allow a single node (which may be the master node, or
another node) to receive the data to be transmitted and then to
distribute this data to the different nodes that are members of the
array. In examples, the pipes 913 may operate at 1 Gb/s and higher
rates.
[0125] The embodiments of FIGS. 8 and 9 are optical interconnect
implementations; analogous functions can be defined for guided
acoustic and even guided RF modes as well as hard wired systems, as
long as the medium can be used simultaneously in both the Transit
and the End Point modes. Acoustic interconnects may be particularly
important in underwater applications.
[0126] In embodiments, some or all of the nodes of the array may be
configured to function as Transit Slaves. In embodiments, some or
all of the nodes may be configured to function as both End Point
slave nodes and Transit Slave nodes. In embodiments, each node may
be configured to function as an End Point slave node, as a Transit
Slave node, and as a master node.
[0127] One of the challenges that need to be addressed for analog
RF-over-optics channel implementation is that an RF signal may be
bipolar (i.e., positive and negative), whereas an optical signal is
a power signal (rather than amplitude signal) and consequently has
no negative component. This mismatch may be resolved by using an
input RF signal to modulate an optical power signal, as follows.
First, an input RF signal is signal-conditioned. This includes
normalizing the input RF signal to a predetermined amplitude "A" at
about 0 VDC average, and level-shifting the normalized RF signal by
a predetermined DC value. Normalization may be performed, for
example, by an automatic gain control (AGC) circuit, and/or a
saturated amplifier. Level-shifting may be performed by adding a
predetermined DC voltage, for example, adding a voltage of the
amplitude A or greater, to raise the signal to a level where it can
be directly mapped/modulated onto optical power, without a negative
component. The modulated optical component may then be transmitted
by an appropriate WDM channel link, for example. At the end of the
link (e.g., fiber, free-space optical) the received signal may be
converted back into the RF domain. Conversion may be performed, for
example, by a photodiode. The signal's DC component may then be
removed by an appropriate bandpass filter (BPF), at which point the
RF signal is available to the End Point node. This technique
eliminates any direct effects from the optical phase. Coherent
detection need not be used in this process.
[0128] Doppler shift of optical and RF signals may become a
complication in applications where the collaborative array nodes
are in motion relative to each other. In particular, Doppler shift
may create problems for clock alignment/synchronization, because
there is no simple way to distinguish between a Doppler-shifted
frequency offset and a frequency offset due to clock misalignment.
Fortunately, there are ways to compensate for the Doppler effects
during alignment/synchronization.
[0129] For example, it may be possible to measure the Doppler shift
due to the relative motion of the array nodes independently, by a
Doppler detection system that may include velocity sensors, such as
GPS sensors. The measured frequency offsets may then be corrected
before alignment is performed.
[0130] As another example, a known pilot frequency pair may be
added to each signal before transmission between array members. The
frequency difference between the pair should remain constant if the
clocks are simply offset. On the other hand, the difference should
increase if the clock frequency shift is due to Doppler. The
Doppler shift may then be computed from the measured change in the
frequency difference between the pilots.
[0131] In still another example, the array implements a Doppler
monitor using optical signals carried on the optical side channel
links. Doppler measurements using optical and RF signals are known
(for example, in traffic law enforcement), but they usually require
not only high-precision clocks, but also availability of a
reflected signal that can be mixed with the emitted signal to
produce a beat frequency. If the optical signal is terminated at a
photodiode, Doppler may not be readily measured, because of the
absence of a reflected signal. Nevertheless, because of the
wavelength stability of optical sources and because Doppler shift
is proportional to the frequency, a node may measure/estimate
Doppler shift of the received optical signal with a relatively high
degree of precision.
[0132] Doppler shift is proportional to both (1) the relative
velocity, and (2) the frequency of the signal, as shown by the
equation
.DELTA. f = f v c , ##EQU00002##
where .DELTA.f represents the Doppler shift, f represents the
frequency, c is the speed of light, and a is the relative velocity
along the line between the two nodes. The optical signal should
therefore exhibit a much greater Doppler shift than the RF signal,
which in turn should exhibit a greater Doppler shift than the
baseband data carried by the RF signal. There may be five orders of
magnitude difference between the optical and RF frequencies. An
optical Doppler monitor may operate using an optical wavelength of
one of the side channel links. Alternatively, an additional optical
wavelength may be used on the side channel link as a Doppler
channel, to measure node displacement and speed during
alignment/synchronization.
[0133] Once the Doppler shift is known at an optical frequency, it
can be scaled down to the RF frequency, and then inverted to
correct the Doppler shift of the RF signal. FIG. 10 illustrates
selected steps of an exemplary process 1000 for Doppler
compensation. In step 1005, the End Point node determines the
uncorrected frequency offset between its clock reference (e.g., LO)
and the clock reference (e.g., LO) of the Master node. For example,
the End Point node may count the number of RF oscillations of a
known, predetermined frequency emitted by the Master node, within a
predetermined time period. As is discussed throughout this
document, the RF oscillations may be received directly from the
Master node (for example, on an RF-over-optical side channel link);
or through one or more intermediate nodes, but without the
intermediate nodes re-clocking the signal. If the RF is 1 GHz and
the period is 1 second, for instance, one billion oscillations
would be expected. If the End Point node counted 1,000,002,000
oscillations, it could be concluded that the uncorrected offset was
two parts per million, with the End Point having the slower of the
two clocks. But this number has not yet been corrected for the
Doppler shift; thus, the uncorrected offset at RF frequency
(.DELTA.f.sub.RFuncor) is equal to the sum of the actual clock
offset at RF (.DELTA.f.sub.RFclock) and the Doppler shift at RF
(.DELTA.f.sub.RFD):
.DELTA.f.sub.RFuncor=.DELTA.f.sub.RFclock+.DELTA.f.sub.RFD.
Equivalently,
.DELTA.f.sub.RFclock=.DELTA.f.sub.RFuncor-.DELTA.f.sub.RFD.
[0134] In step 1210, the End Point node would determine the Doppler
shift between itself and the Master node at an optical frequency.
For example, the End Point node may be configured to measure and
store the long-term frequency offset between its optical source at
a specific wavelength, and the counterpart optical source of the
Master node, and then measure the difference between the current
offset and the long-term offset. The optical frequency difference
(.DELTA.f.sub.Opt) between the current optical offset and the
long-term optical offset may be attributable to the Doppler
shift.
[0135] In step 1215, the End Point node scales the optical
frequency difference to the equivalent Doppler shift at the RF
frequency (.DELTA.f.sub.RFD@1 GHz in this example). Because the
Doppler shift is proportional to frequency, this is a matter of
division:
.DELTA.f.sub.RFD=.DELTA.f.sub.Opt.times.(F.sub.RF/F.sub.Opt), where
F.sub.RF is the RF frequency (again, 1 GHz here), and F.sub.Opt is
the optical frequency at which the Doppler shift was measured (for
example, 10.sup.14 Hz).
[0136] In step 1220, the actual clock offset at RF
(.DELTA.f.sub.RFclock) is computed from the measured uncorrected
offset at RF frequency (.DELTA.f.sub.RFuncor) and the measured and
scaled Doppler offset at RF frequency (.DELTA.f.sub.RFD):
.DELTA.f.sub.RFclock=.DELTA.f.sub.RFuncor-.DELTA.f.sub.RFD.
[0137] In step 1225, the LO of the End Point node is corrected.
This may be done in various ways, actually (by adjusting the LO or
a local synthesizer) and/or computationally. For example, the LO
may be tuned to reduce or eliminate the LO offset from the LO of
the master; a programmable synthesizer may be programmed to effect
the required frequency offset; a mixing process may be used to
obtain the compensated LO at the End Point node.
[0138] The process 1000 then ends in flow point 1099, to be
repeated as needed.
[0139] Once the nodes of the array are aligned/synchronized, they
may use time-reversal (TR) to retrodirect energy automatically back
to the target(s). In this way, the array of nodes may be able to
capture signals from the target or targets, and achieve
spatio-temporal location-focusing of the energy on the targets. The
nodes of the array may also or instead be configured for
directional beamforming.
[0140] Some definitions have been explicitly provided in this
document. Other and further implicit definitions and clarifications
of definitions may be found throughout this document.
[0141] While the examples in this document focus on transmission
from the array of untethered ad hoc nodes to one or more targets,
analogous alignment of untethered ad hoc nodes of an array may also
serve to receive a transmission from the target to the array.
Furthermore, examples of inter-nodal communications may be useful
for other nodes, such as nodes that are tethered, that is, nodes
that are generally stationary with respect to each other, but which
have individual clocks. The RF channels between such nodes may be
varying, for example, due to changes in the environment surrounding
the nodes, such as movement of objects that cause signal
reflections, signal phase shift, and signal attenuation.
[0142] 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.
[0143] 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.
[0144] 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.
[0145] This document describes in detail the inventive apparatus,
methods, and articles of manufacture for communications and other
techniques using distributed cooperating/collaborative nodes. This
was done for illustration purposes 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).
* * * * *