U.S. patent application number 11/968592 was filed with the patent office on 2008-07-03 for robust, efficient, localization system.
This patent application is currently assigned to TruePosition, Inc.. Invention is credited to John E. Maloney, James O. Stevenson.
Application Number | 20080161015 11/968592 |
Document ID | / |
Family ID | 26712400 |
Filed Date | 2008-07-03 |
United States Patent
Application |
20080161015 |
Kind Code |
A1 |
Maloney; John E. ; et
al. |
July 3, 2008 |
Robust, Efficient, Localization System
Abstract
Replica correlation processing, and associated representative
signal-data reduction and reconstruction techniques, are used to
detect signals of interest and obtain robust measures of
received-signal parameters, such as time differences of signal
arrival and directional angles of arrival, that can be used to
estimate the location of a cellularized-communications signal
source. The new use in the present invention of signal-correlation
processing for locating communications transmitters. This enables
accurate and efficient extraction of parameters for a particular
signal even in a frequency band that contains multiple received
transmissions, such as occurs with code-division-multiple-access
(CDMA) communications. Correlation processing as disclosed herein
further enables extended processing integration times to facilitate
the effective detection of desired communications-signal effects
and replication measurement of their location-related parameters,
even for the communications signals modulated to convey voice
conversations or those weakened through propagation effects. Using
prior, constructed, signal replicas in the correlation processing
enables elimination of the inter-site communications of the signal
representations that support the correlation analyses. Reduced-data
representations of the modulated signals for voiced conversation,
or for the variable components of data communications, are used to
significantly reduce the inter-site communications that support the
correlation analyses.
Inventors: |
Maloney; John E.;
(Springfield, VA) ; Stevenson; James O.; (Fairfax,
VA) |
Correspondence
Address: |
WOODCOCK WASHBURN LLP
CIRA CENTRE, 12TH FLOOR, 2929 ARCH STREET
PHILADELPHIA
PA
19104-2891
US
|
Assignee: |
TruePosition, Inc.
Berwyn
PA
|
Family ID: |
26712400 |
Appl. No.: |
11/968592 |
Filed: |
January 2, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10347471 |
Jan 17, 2003 |
7340259 |
|
|
11968592 |
|
|
|
|
09240889 |
Feb 1, 1999 |
6546256 |
|
|
10347471 |
|
|
|
|
08855589 |
May 13, 1997 |
6047192 |
|
|
09240889 |
|
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|
60017269 |
May 13, 1996 |
|
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|
60035691 |
Jan 16, 1997 |
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Current U.S.
Class: |
455/456.1 |
Current CPC
Class: |
G01S 5/12 20130101 |
Class at
Publication: |
455/456.1 |
International
Class: |
H04Q 7/20 20060101
H04Q007/20 |
Claims
1. A method for locating a standard, mobile-communications, radio
transmitter in a cellularized communications system, comprising: at
first, second and third sensor stations, receiving a signal
carrying voice/traffic-related data from the mobile radio
transmitter; at the first sensor station, generating replicated
signal data and generating a reduced form of the replicated signal
data, said replicated signal data comprising voice/traffic-related
signal data received at the first sensor station; communicating the
reduced form of the replicated signal data to the second and third
sensor stations; at the second and third sensor stations,
reconstructing the replicated signal data from the reduced form of
the replicated signal data, whereby voice/traffic-related signal
data received at the first sensor station is recovered at the
second and third sensor stations, and performing correlation
processing with the voice/traffic-related signal data received at
the second and third sensor stations and the replicated signal data
communicated by the first sensor station to produce
location-related signal parameters; communicating the
location-related signal parameters from the second and third sensor
stations to a central site; and estimating the position of the
mobile transmitter from the location-related signal parameters.
2. The method of claim 1, further comprising demodulating said
signal carrying voice-related data from the mobile radio
transmitter at the first sensor station to generate said reduced
form of the replicated signal data.
3. The method of claim 2, wherein said signal carrying
voice-related data is communicated in at least one of CDMA, TDMA,
GSM and CDPD systems.
4. The method of claim 1, the sensor stations having an antenna
comprising at least two elements and one of said location-related
signal parameters being an angle of arrival determined in
accordance with: sin ( AOA - bisector ) .apprxeq. 1 kb arg [ 1 S
.intg. R 01 * ( t max ( s ) | s , T ) R 02 ( t max ( s ) | s , T )
s ] ##EQU00002## wherein k is the wavenumber of the signal, b is
the (baseline) inter-element separation, and the "arg" function
extracts a phase of the correlation of the correlation coefficients
themselves.
5. The method of claim 1, one of said location-related signal
parameters being a time difference of arrival between the sensor
stations determined in accordance with: TDOA .apprxeq. t max | R 12
( t max ) = max t 1 T .intg. x 1 * ( s ) x 2 ( s + t ) s
##EQU00003## where t.sub.max is the relative time delay (lag) value
for which the magnitude of the correlation of the signals achieves
a local maximum.
6. A method of locating a standard, mobile-communications, radio
transmitter in a cellular communications system, comprising:
receiving a signal from the mobile radio transmitter at first and
second sensor stations; time-tagging an identified, representative
instant of the received signal to produce time-tagged received
signal data; generating replicated signal data at the first and
second sensor stations; performing matched-replica correlation
processing with the time-tagged received signal data and the
replicated signal data to produce location-related signal
parameters; communicating the location-related signal parameters
from the first and second sensor stations to a central site;
estimating the position of the mobile transmitter from the
location-related signal parameters; and indicating the estimated
position of the mobile transmitter; wherein the step of generating
replicated signal data at the first and second sensor stations
further comprises the step of reconstructing the replica of the
received signal data derived from a prior known form of the
received signal content.
7. A method according to claim 6, wherein the location-related
signal parameters comprise angle-of-arrival information.
8. A method according to claim 6, wherein the location-related
signal parameters comprise time difference of arrival
information.
9. A method according to claim 6, wherein the replicated signal
data comprise voice related data.
10. A method according to claim 6, wherein the replicated signal
data comprise voice/traffic channel overhead data.
11. A system for locating a mobile radio transmitter, comprising:
at least first and second sensor stations, each sensor station
having an antenna for receiving a signal from the mobile radio
transmitter and a timing mechanism to time-tag an identified,
representative instant of the received signal to produce
time-tagged received signal data; at least first and second signal
replica units at the first and second sensor stations,
respectively, each signal replica unit providing replicated signal
data, at least first and second signal correlation and measurement
extraction processing units at the first and second sensor
stations, respectively, each correlation and measurement extraction
processing unit performing matched-replica correlation processing
with the time tagged received signal data and the replicated signal
data to produce location-related signal parameters; a
communications system for communicating the location-related signal
parameters from the sensor stations to a central site; means for
estimating the position of the mobile transmitter from the
location-related signal parameters; and an output for indicating
the estimated position of the mobile transmitter, wherein the
signal replica unit at the first and second sensor stations
reconstructs the replica of the received signal data using a prior,
known form of the received signal content.
12. A system according to claim 11, wherein the location-related
signal parameters comprise angle-of-arrival information.
13. A system according to claim 11, wherein the replicated signal
data comprise voice related data.
14. A system according to claim 11, wherein the replicated signal
data comprise voice/traffic channel overhead data.
15. A system according to claim 11, wherein the location-related
signal parameters comprise time-of-arrival information.
16. A system according to claim 11, wherein the location-related
signal parameters comprise time-difference-of-arrival parameters.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 10/347,471, filed Jan. 17, 2003, now pending, which is a
continuation of U.S. application Ser. No. 09/240,889, filed Feb. 1,
1999, now U.S. Pat. No. 6,546,256B1, issued Apr. 8, 2003, which is
a divisional of U.S. application Ser. No. 08/855,589, filed May 13,
1997, now U.S. Pat. No. 6,047,192, issued Apr. 4, 2000, which
claims the benefit of U.S. Provisional Application Nos. 60/017,269,
filed May 13, 1996 and 60/035,691, filed Jan. 16, 1997, the
contents of all of which are hereby incorporated by reference in
their entireties.
BACKGROUND OF THE INVENTION
[0002] Determining the location of standard, wireless radio
frequency (RF), communications transmitter/receivers
("transceivers") based on their communications offers the potential
for emergency response services (fire, rescue and police) to more
rapidly respond to calls for help. Public safety and private
security can all be aided by making available information
concerning position and geographic location. Technologies, such as
those disclosed herein, do not require modifying standard
communications devices in any way to facilitate the real-time
determination of their locations.
SUMMARY OF THE INVENTION
[0003] The communications transceivers most popularly used by the
general public are the mobile units (i.e., "telephones") of
cellularized communications systems. Examples included the
"cellular telephone" and "personal communications service" (PCS)
systems. Cellular communications systems typically use control-data
messages to "manage" the transmitted power level of a mobile unit
to limit its transmitted level to only that needed for successful
communications reception within the controlling local "cell." This
power management can limit the reception of the communications
transmissions at multiple receiver sites and thus make more
difficult the task of determining the transmitter's location.
Furthermore, when in use for communication, the wireless telephones
are dedicated to voice or "traffic" transmission channels rather
than control or "access" channels. Thus, facilities are needed to
locate the transmitters on any type of channel.
[0004] The present invention addresses these issues by providing
robust and efficient means to extract parametric measurements from
either or both of voice and control communications signals. These
measurements can then be used to support the localization
processing that is needed to locate wireless communications
transceivers. Location data can be used to rapidly rout wireless
calls to someone, or some agency, who is in a position to respond
to the call.
[0005] Thus, the location information can support swift response to
wireless emergency "9-1-1" calls. Other requests, such as for
non-emergency assistance or position related, "yellow pages"
information, also can be addressed. Motion data can be generated
from the location data; such data can be used for monitoring
transportation congestion as well as for vehicle fleet
management.
[0006] The present invention can advance the performance and cost
efficiency of a variety of system approaches to the localization of
standard, wireless, mobile communications transmitters. Various
techniques have been disclosed that are intended to provide the
utility and meet the need of such systems. As the pioneer of such
technologies, U.S. Pat. No. 4,728,959 discloses, among other novel
features, a system with a means for measuring a direction angle of
the mobile radio transmitter station from at least two land
stations by phase difference measurement, including means for
performing a phase sensitive weighted integration of a complex
conjugate product of an equivalent signal in each antenna element.
This allows for, among other things, measuring the angle-of-arrival
(AOA) of a signal from a mobile transceiver based on the
covariances of the elemental signals received with phased arrays at
distributed sensor sites, and to thereby obtain and provide the
location of a standard mobile communications transmitter.
[0007] The present invention advances the state of the art in AOA
systems by using matched-replica correlations to enhance their
robustness and to extend the applicability of such fundamental
concepts into the domain of severe co-channel interference.
Co-channel interference is a particular problem and inherent with a
type of digital communication system known as
code-division-multiple-access (CDMA) communications. Systems exist
which purport to provide locations for standard mobile transmitters
by extracting measurements from "beamformed" signals using
time-difference-of-arrival (TDOA) correlations of the direct,
sampled, representations of the signals themselves, given
sufficient signal bandwidth (which is often not available from most
commercial "analog" transceivers). The transmitting may
(adaptively) mitigate some multipath signal propagation effects.
However, to actually implement such correlation processing, the
sampled signal representations must be collected at a common
correlation site. Such signal collection requires supporting
"back-haul" communication of the significant volumes of data that
make up the representations of the sampled signals.
[0008] It is an objective of the present invention to extend the
utility of AOA and TDOA-based localization concepts so as to be
applicable to signals that are not necessarily the product, and do
not entail the expense, of beamforming. It is also an objective of
the present invention to advance the effectiveness of the
correlation processing through the use of matched-replica
processing, which provides a distortion-free representation of the
signal to the correlator for enhanced correlation detectability. It
is also an objective of the present invention to improve the
efficiency of the integrated system processing by eliminating the
need for any inter-site back-haul communication of representative
signal data when the signal replica can be locally derived from the
received signal and/or from a known stored replica. It is also an
objective of the present invention to significantly reduce the
quantity of representative signal data that is transferred between
sites through the extraction and use of the demodulated forms of
the information content that is in the RF transmissions for all
forms of modulation.
[0009] U.S. Pat. No. 5,327,144 discloses a system with purports to
measure signal time-of-arrival (TOA), and associated
time-difference-of-arrival (TDOA) approach using what is described
as correlation processing. However, the technique described
requires extensive inter-site, back-haul communications of sampled
signal representations or the less extensive demodulated
replicas.
[0010] Such communications are apparently used to provide locations
for standard mobile transmitters in cellularized communications
"systems that employ analog control channels," through the
exploitation of the short-duration, "bursty" (control) signals. In
the United States, the "analog" signal formats, for the "air
interface" between the mobile transceiver and the communications
system infrastructure use the Advanced Mobile Phone System (AMPS)
specification. The AMPS control messages occur in bursts that are
approximately one tenth of a second in duration.
[0011] The present invention enhances the utility of the
correlative derivation of any measurements by eliminating
requirements for bursty, analog, control signals and for the
back-haul communications of signal representations. The present
invention further extends the applicability of the matched-replica
processing to enable the processing of signals of "continuous" or
opportunistic (rather than merely induced or transponded)
transmissions as well as of transmissions of digital formats, such
as of voice signals in CDMA systems. Furthermore, the present
invention also extends matched-replica correlative processing to
provide robust and efficient measures of AOAs, as well as TOAs or
TDOAs, for all of the communications signal formats.
[0012] The present invention provides a system that effectively
determines location-sensitive parameters for, and locates and/or
tracks, a standard, mobile communications, radio transmitter in a
cellularized communications system. The system uses replica
correlation processing, and associated representative signal-data
reduction and reconstruction techniques, to detect signals of
interest and obtain robust measures of location-related,
received-signal parameters, such as time differences of signal
arrival (TDOAs) and directional angles of arrival (AOAs), for the
estimation of the locations of cellularized-communications signal
sources. The new use in the present invention of signal-correlation
processing to support the localization of the communications
transmitters enables accurate and efficient extraction of
parameters for a particular signal even in a frequency band that
contains multiple received transmissions, such as occurs with
code-division-multiple-access (CDMA) communications.
[0013] The use in the present invention of correlation processing
further enables extended processing integration times to facilitate
the effective detection of desired communications-signal effects
and enhanced measurement of their location-related parameters, even
for the communications signals modulated to convey voice
conversations or those weakened through propagation effects. When
derivable from the received transmissions themselves, such as with
sufficiently strong modulated signals representing digital
information, or when otherwise available, such as with
communications-control or other known-data contents in the received
transmissions, the use in the present invention of reconstructed
signal replicas in the correlation processing enables elimination
of the inter-site communications of the signal representations that
support the correlation analyses. The use in the present invention
of reduced-data representations of the modulated signals for voiced
conversation, or for the variable components of data
communications, significantly reduces the inter-site communications
that support the correlation analyses. Thus, the present invention
significantly enhances the robustness, applicability, and
efficiency, and reduces the cost of implementation, of correlation
techniques for the detection and measurement of signal parameters
to support the localization and tracking of the wireless
communications transmitters used in cellularized or geographically
subdivided communications systems.
[0014] All of the foregoing objectives, features and advantages of
the present invention, and more, are explained below with the aid
of the following illustrative figures and exemplary
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1. An Operational System Configuration is shown with
the reception of the transmissions from a mobile wireless
communications unit at the networked antenna sites of an integrated
localization system.
[0016] FIG. 2. Time-Difference-Of-Arrival Geometric Relations for
two sensor sites are shown as hyperbolic lines representing the
loci of positions ascribable to the distinct, constant range
differences associated with the various time differences.
[0017] FIG. 3. Time-Difference-Of-Arrival Localization is
represented by the intersection of two hyperbolas that are
associated with the time differences associated with two distinct
pairs formed from three sensor sites.
[0018] FIG. 4. Angle-Of-Arrival Localization is represented by the
intersection of two non-colinear lines of constant bearing
associated with angles of signal arrival at two distinct sensor
sites.
[0019] FIG. 5. Autonomous Sensor-Site Operation and data flow is
shown in which the received signal is routed to a correlation with
a locally derived or stored matched replica for the extraction of
angle- and/or time-of-arrival measurements to support centrally
estimated locations.
[0020] FIG. 6. Cooperative Sensor-Site Operation and data flow is
shown in which the received signals at separated sites are routed
to a common correlation with each other for the extraction of
time-difference- and/or angle-of-arrival measurements to support
centrally estimated locations.
[0021] FIG. 7. Signal Replication is represented in the forms of
the signal data that result from the successive stages of
processing that support the preparation of the signal
representations that are applied in the matched-replica
correlations.
[0022] FIGS. 8A and 8B. Localization System Functional Control
Flow, exemplifying one localization system configuration, involves
the assignment by the control site of data collection and reporting
responsibilities for the sensor sites, and calculation of
localization estimates at the control site based on the reported
measurement data.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0023] Locations of transmitters of RF signals can be determined
from the geometric interpretation of measurements of parameters
such as the differences in the times of arrival or the directional
angles of arrival of their signals at multiple receiving sites of
known location. FIG. 1 depicts a RF transmitter 101 transmitting a
signal 102 that is received with antennas 103 serviced by a network
of sensor sites or stations 104 distributed throughout the region
of operation of the transmitter. As indicated for the transmitter
shown, the transmitters to be located may be mobile, wireless,
communications units, such as the telephones used in cellular or
personal communications services systems.
[0024] The sensor stations are connected via "back-haul
communications" links 105 to at least one central site or control
station 106 at which the time-difference or angle data collected
from the sensor sites can be analyzed to provide the estimated
locations, motions, and associated uncertainties for the
transmitters of interest. In a wireless-communications system, the
links 105 are called "back-haul" because they provide the mechanism
for the background, supporting communications of information among
the distributed stations that are necessary to support the primary
RF communications 102 transmitted to and from the communications
units.
1. Time-Related Measurements
[0025] With the time-related measurements, the facility for
localization can be seen in the relation that a
time-difference-of-arrival (TDOA) value between signals received in
common at a pair of sites (e.g., numbered 1 and 2) has to the
ranges or distances between the locations of the signal receivers
and the location of the signal transmitter. With the presumption
that the speed of signal propagation is approximately the speed of
light c, then the TDOA t12 between sites 1 and 2 is
t.sub.12=(r1-r2)/c
[0026] where r1 and r2 are the ranges from the transmitter location
to the locations of the receiving antennas for sites 1 and 2,
respectively. In a simple, flat, two-dimensional representation,
the range difference obtained by multiplying the TDOA by c defines
a hyperbola along which the transmitter is located. That is, a
single TDOA measurement specifies a hyperbolic locus of possible
transmitter locations. FIG. 2 depicts the geometric relations
involved in TDOA measurements obtained with two sensor sites 201
and 202, shown at locations labeled SS1 and SS2 at the top and
bottom of FIG. 2, respectively. The hyperbolas denoted by the dark
lines 203 through 207 are the loci of possible transmitter
locations associated with various TDOA values, which are distinct
for each line. The hyperbolas are symmetric with respect to the two
sides of the inter-site baseline 208 denoted by the dark dashed
line between the two sensor sites. As a simple example, a single
TDOA value of zero would indicate that there is no difference in
the ranges of a transmitter to each of the two sensor sites, and
the associated locus of possible locations for the transmitter
would be the straight-line, perpendicular bisector of the
inter-sensor baseline. The hyperbola 205 nearly approximates this
bisector. Of course, as seen in FIG. 2, when only TDOAs involving
just two sites are available, the transmitter location cannot be
determined more specifically than on a hyperbola extending around
the world. With the reception of the signal at a third,
non-colinear site 303, as shown in FIG. 3, another TDOA
measurement, e.g., between sensor site 1 and 3, can be obtained
that defines another hyperbola 301 which can cross with the first
one 206. The location 302 of intersection of the two distinct
hyperbolas can be calculated from the two associated TDOA
measurements. As with any measurements, the measurements of TDOAs
are obtained with inherent inaccuracies or uncertainties that
accrue from the signal-propagation and measurement-equipment
characteristics. These uncertainties are represented in FIG. 3 by
the light, dashed lines 304, and from these uncertainties the
uncertainty region denoted by the dark ellipse 305 can be
calculated for the intersection of the hyperbolas to represent the
accuracy of the location estimate for the transmitter.
2. Direction-Related Measurements
[0027] With the direction-related measurements, the facility for
localization can be seen in the relation that the angle-of-arrival
(AOA) values for signals received in common at a pair of sites
(e.g., numbered 1 and 2) has to the location of the signal
transmitter and the locations of the signal receivers. As
represented in FIG. 4, each angle measurement individually
specifies a line of bearing (LOB), 401 and 402, along which the
probable position of the transmitter may be located. The probable
location 403 can be determined from the intersection of two or more
such LOBs, and the uncertainties 404 in the angles and associated
LOBs can be used to calculate the ellipse 405 representing the
uncertainty region for the location estimate. Without any other
information, (only) two such LOBs associated with the angles from
two antenna sites are required to obtain a location estimate. The
procedures for applying such direction-finding techniques for the
localization of cellularized communications transceivers are
described, for example, by Maloney, et al., in U.S. Pat. No.
4,728,959 ("the '959"), for the Direction Finding Localization
System (DFLS).
3. Correlation Processing
[0028] For a signal source, such as a cellular telephone, the
accuracy of its location determined from measurements of
differences in times of signal arrival or of directions of signal
arrival at known locations is directly related to the accuracy of
the applied TDOA and AOA measurement processes. It is well known
[e.g., Weiss and Weinstein, "Fundamental Limitations in Passive
Time Delay Estimation--Part I: Narrow-Band Systems," IEEE ASSP, $1,
pp. 472-486, 1983, and related references] that the optimum TDOA
measurement accuracy achievable in the processing of received
signals is the Cramer-Rao bound, and that the process of signal
"cross-correlation" (discussed below) inherently achieves the
Cramer-Rao bound with optimal detectability under normal signal and
noise conditions. Thus, the standard signal processing approach
applied in TDOA estimation is the process of signal correlation. It
is further well known [e.g., H. L. Van Trees, "Detection,
Estimation, and Modulation Theory, Part I," New York: Wiley, 1968,
and related references] that the ability to even detect the
presence of a desired signal embedded among the normal additive
noise and interference of other signals is optimized with a use of
correlation processing, to emphasize those signal components that
are "coherent" or "correlated" with (i.e., similar to) the desired
signal and to "integrate out" or "average away" the components that
are not desired or of interest. Signal detection devices employing
the correlation process are referred to as "correlation receivers."
Thus, the correlation process can be used both to achieve
detection, in the presence of co-channel interference or at
multiple receiving stations, and to extract measurements in support
of localization analyses.
[0029] The signal correlation process is simply represented by the
equation for the correlation function of the inter-signal time
delay or "lag," t:
R 12 ( t | t 0 , T ) = 1 T .intg. x 1 * ( s ) x 2 ( s + t ) s /
Norm S ( t 0 , T ) = 1 T t 0 + T / 2 .intg. x 1 * ( s ) x 2 ( s + t
) s / Norm t 0 - T / 2 ##EQU00001##
where x1( ) and x2( ) are the zero-mean analytic signal waveforms
representing the sets of sampled signals between which the time
difference is desired; the integral is a "summation" of the product
of the two signal waveforms; the integrated sum is calculated over
the set S(to,T) representing the time (instants) centered at to
spanning the interval T: i.e., in mathematical set notation,
S(to,T)={s I to-T/2<s<to+T/2); ds is the time differential of
the integration variable; and "Norm" is a normalization factor that
is typically chosen so that the correlation coefficient (i.e., the
function value for any particular lag, t) has a magnitude not
greater than unity: i.e., JR1201 is less than or equal to one.
Without the normalization factor, this correlation is a temporally
averaged estimate of the "covariance" between the two signals. The
efficacy and properties of the correlation function are well known,
as cited in the references above, and are not the subject of the
present patent.
[0030] Although apparently complex in form, the above formulation
for the correlation function provides the desired properties for
the detection of signals and the analysis of the time differences
between signals, as can be seen from the following discussion.
Signal waveforms can have a broad variety of characteristics, but
can be generally characterized as falling between two extremes:
perfectly ordered and perfectly random. In either case the signals
are assumed to have zero mean, since any non-zero constant average
or "DC-bias" value can be subtracted or "blocked" from the signals.
Thus, the signals may be thought of as "bipolar," with
approximately half of their values positive and half negative. The
"ordered" signal may be said to be sinusoidal, such as with an
unmodulated "carrier" of potential communications, while the
"random" signal is entirely unpredictable, such as with a
thermal-noise like signal. With either type of signal, the product
of an arbitrary time alignment of two such signals is generally
also bipolar, and the integration of such a product averages the
positive values with the negative values and results in a small
accumulated sum (i.e., the correlation coefficient magnitude is
close to zero). This could obviously occur in the correlation
described above, for example, when the two signals involved are
entirely random and dissimilar or when the signals are sinusoidal,
but of sufficiently different frequencies. This could also occur
even when the two signals involved in a correlation calculation are
copies of the same random or sinusoidal signal, but are not
properly time aligned. On the other hand, when the two signals
being correlated are effectively the same signal but with a time
offset between them, then the correlation function may be evaluated
for the particular time delay value t21 that causes signal copy 1
to be aligned with signal copy 2 such that whenever the copy 1
value is positive or negative then the corresponding copy 2 value
is likewise positive or negative. For this particular time delay
value, all of the non-zero signal products accumulated in the
integration would be positive (i.e., the product is "unipolar") and
the magnitude of the correlation coefficient would be
correspondingly large (i.e., nearly one). Since each (analog)
receiver produces a signal that is not a perfect copy of the
transmitted signal (due to receiver self noise, as well as to
received interference and signal-propagation distortion), then the
correlation of the signals received at separated sites will not be
perfect (i.e., will not produce a magnitude of exactly one).
Nevertheless, the detection of the presence of a desired signal can
be indicated by the strength or magnitude of the correlation
function, and the fundamental measure of the TDOA between two
signals can be taken to be the inter-signal time delay value that
maximizes the magnitude of the signal cross-correlation
function.
4. Correlation and Detectability (Integration Time)
[0031] The ability of the correlation receiver to detect the
desired signal in the presence of noise and interference is limited
by the correlation integration time (CIT) interval, and enhanced
detectability can result from longer "coherent" CITs which further
"average away" interference and noise effects. For the detection of
multi-site reception, e.g., to support localization, the duration
of the CIT can be extended to include whatever duration of signal
is required, to reliably decorrelate non-coherent noise or
interference. This extension in CIT for multisite reception can be
obtained through the use in the correlation of the matched replica
received from a remote site or locally derived or known in advance
at each site, e.g., for a specific communications protocol message
content. For communications reception, the duration of the data
that can be effectively integrated in the correlation interval,
i.e., the maximally useful CIT, is limited to the maximum interval
of the communicated message that is known to the correlation
receiver in advance of the transmission. With the random message
patterns that occur with voice transmissions, this CIT maximum for
communications reception is the duration of the signal used to
transmit one message unit, e.g., bit or bit tuplet.
5. Example
"Matched-Replica" Correlation Inherent in CDMA Reception
[0032] An example of the detection capability of the correlation
function is found in the correlation receivers that are used to
receive code-division-multiple-access (CDMA) RF communications
signals. When the signals are similar, the correlation coefficient
is large, and the converse occurs for dissimilar signals. The
correlation function provides the means to detect and measure the
degree of similarity between signals, as well as the time delay
between the signals. In CDMA and similar "spread-spectrum"
communications, each digital message that is to be transmitted is
"encoded" through the use of a high-bandwidth or to
spread-spectrum, signal that is "known" to the receiver to
represent each bit (or bit pair or bit tuplet) in the message bit
stream. For example, if the message is encoded by individual bits,
then a known signal could be used to represent each "1" and another
known signal (e.g., the inverse or complementary correlation
signal) could represent each "0." The composite signal for
transmission is formed by sequentially stringing together the
representative waveforms for the desired sequence of bits, and this
signal is transmitted. In accord with the Telecommunications
Industry Association and Electronic Industries Association Interim
Standard TIA/EIA/IS-95 specification for CDMA systems in the U.S.,
the encoded bit sequences are transformed for RF transmission using
quaternary quadrature-phase-shift keying (QPSK) for representing
bit pairs ("00," "01," "10," and "11"). A correlation receiver can
correlate its own "matched replica" of the signal "codes," e.g.,
distinctly those for "1" and "0" or for the bit pairs, with its
received signal, and can thus reconstruct (such as by remodulating)
the intended message by creating the bit stream that corresponds to
large correlations of the received signal with the matched replicas
inherent to the system. For the transmission of its particular
message bit stream, each transmitter uses unique "codes" or signals
that do not correlate well with those used by other transmitters in
the system. For the reception of the intended bits, each receiver
can correlate with any replicas in use, and thus can receive a
signal from any of the multiple transmitters in a common frequency
band. Starting from the replica signals used by the transmitters,
the received signal is distorted by the conversion to RF at the
transmitter location and is combined with noise and interference
and distorted in reception at the receiving site. The receiver's
matched replica itself provides a largely uncorrupted form of the
intended transmitted signal for use in the correlation process.
6. Correlation and Measurement Extraction
[0033] When the correlation process described above is used to
establish enhanced signal detection, it can also be adapted to
extract robust measurements of signal parameters other than TDOAs,
such as AOAs, signal strength, and Doppler ratios. For example, in
a preferred embodiment, with the use of a receiving antenna
configured as a "phased array" of two or more elements (from which
the elemental signals are received using phase-locked oscillators,
as described in the '959 patent), the signals, x1(t) and x2(t),
from elements 1 and 2 in the antenna at a single station may be
received by correlation at a small (possibly varying) delay offset
t.sub.max(s) with a replica signal, x0(t), to obtain coefficient
series R01(t.sub.max(s)|s,T) and R02(t.sub.max(s)|s,T),
respectively. The delays tmax(s) are "small" in comparison with the
CIT, T, used to evaluate the correlations and are the delay offsets
associated with the local correlation extrema at which the
successfully detected correlation occurs. Due to noise, distortion,
and signal propagation effects (such as multipath propagation), the
extremal values tmax(s) may vary from one correlation to the next.
Then, through relations such as described in the '959 patent, the
angle of signal arrival can be derived from the phase differences
between the various element correlation coefficients. That is, in a
manner analogous to those presented in the '959 descriptions for
analysis with two elements, the AOA relative to the angle of the
bisector of the inter-element baseline can be derived from the
"argument" of the complex average over a time interval S of the
conjugate product of the correlation coefficients:
sin(AOA-bisector).apprxeq.1arg[1.intg.R.sub.01*(t.sub.max(s)|s,T)R.sub.0-
2(t.sub.max(s)|s,T)ds]kbS
[0034] where k is the wavenumber (two pi divided by the wavelength)
of the signal, b is the (baseline) inter-element separation, and
the "arg" function in this application extracts the phase of the
(e.g., zero-lag) correlation of the correlation coefficients
themselves. Other uses of correlation results, exploiting the
enhanced detectability and accuracy that derive from the
correlation processing, can be applied to equivalently extract AOA
measurements with alternative but related and equivalent
expressions, such as those that adaptively exploit the pairwise
covariances among the multiplicity of correlation coefficients
derivable from the multiple elemental signals from a phased-array
antenna. Thus the correlation results can also be applied in
beamforming with phased-array antennas, e.g., in the same manner as
described above for AOA measurements, to obtain all of the
advantages of spatial separation ("spatial division multiple
access"--SDMA) that accrue through beamforming for both
localization and communication.
[0035] Similarly, to further support localization determinations,
the correlation results can be used to extract measurements of
other signal parameters, such as measures of signal strength, which
are directly related to the correlation coefficients, or Doppler
ratios, which are directly related to rates of change of the time
differences.
7. Autonomous vs. Cooperative Processing ("Back Haul")
[0036] In order to apply the correlation process to detect the
joint or common reception of a signal at separate sites and/or to
measure the TDOA or AOAs for two separate signals received at
separate sites, both signals must be available in common to the
con-elator or the "known" signal waveform must be available in
common to separate correlators. For the measurement of a direction
of signal arrival at each site, the results of the correlated
reception of signals from the multiple "phased" elements of its
directional receiving antenna are used to derive the "phase-based"
AOA measurement, in the manner discussed above and analogous to
that described in the '959 patent. For the measurement of the TDOA
between the signal representations received at two sites, both
signals are used in a common correlator or the "known" signal
waveform is used in separate correlators, each of which determines
a Time Of Arrival (TOA) from which the difference can be obtained
by subtraction. FIG. 5 represents the functional component
configuration and data flows applied in the autonomous sensor-site
operation in which a "known," locally derived or stored replica is
used in the correlation processing to obtain AOA or TOA
measurements, as described above and elaborated further below. In
the represented embodiment, the antenna element responds to the RF
signal and produces the varying voltage of the analog signal that
is "conditioned," received, and routed to the analog-to-digital
converter (ADC) for "digitization" into sequential, "time-series"
samples. The correlation measurements are derived from the digital
correlation of the received signal samples with the sampled,
modulated, matched replica of the transmitted signal. As discussed
further below, this operation can be applied where a known message
or bit-string replica is used in correlation with a priori known
parts of the control/access-channel sampled signal, or where
overhead bit sequences such as are involved with communications
synchronization, command acknowledgment, and/or contact management
are used in correlation with the voice/traffic-channel sampled
signal. This operation can even be applied where bit-pair replicas
are used in correlating with the sampled voice-channel signal
(i.e., with all digital air-interface formats to obtain TOAs, and
with CDMA signals to obtain AOAs). Where reliable demodulation can
be achieved and extended CITs for enhanced time-tag accuracy are
warranted, the modulated replica can be derived locally from the
demodulation of the received signal, in analogy with the
demodulation flow discussed below for cooperative site processing.
In either approach for this autonomous operation, no "back-haul"
communications are needed to provide the replica information for
the support of the correlation analyses.
[0037] Of course, the joint correlation is needed for the
processing of any signals for which the waveform is not known in
advance of reception and cannot be derived from the received
signal. Correlation with the replica information obtainable by
demodulation from a strong reception can also be used to
cooperatively establish detection with and extract localization
measurements from a weak reception that would otherwise not have
been useful. In typical distributed, "cellularized" communications
systems, the signals are received at cell base stations at discrete
site locations distributed throughout the geographic region over
which the communications services are provided, and the cell
stations are linked through a communications backbone to central
facilities to support the distributed communications services. In
like manner, FIG. 6 represents the functional component
configuration and data flows applied in cooperative sensor-site
operation to support joint correlation analyses of potentially
common signals received at two separated sites. In this cooperative
operation, for correlation, a (digital) representation of one of
the signals, i.e., the stronger of the two if significantly
different in signal-to-noise power ratio (SNR), is communicated to
the site at which the other signal is received. This supporting
type of "back-haul" communication of signal representations to the
common site for joint correlation constitutes an expensive
component of the typical localization system that derives TDOA
measurements for its localization determinations. The present
invention applies replica data storage and reduction to eliminate
or minimize the back-haul communications load.
[0038] The most direct (digital) representation of a signal is a
direct copy of the (sampled) signal itself. In accord with the
fundamental Nyquist theorem of signal processing, a signal must be
sampled at a rate equivalent to at least twice the bandwidth of its
information content in order to accurately represent that
content.
[0039] In the case of the frequency-modulated (FM) signals
transmitted in the format of the Advanced Mobile Phone System
(AMPS) specified in Electronics Industries
Association/Telecommunications Industries Association specification
EIA/TIA553 and used as the "analog" cellular system standard in the
U.S., the signal channels are separated by 30 kHz and hence can be
represented by approximately 60 thousand samples per second. If
approximately 50 dB of dynamic range is desired for each sample of
the signal representation, then each sample would be 8 "bits (`b`)"
of information and the signal representation could consist of 480
thousand bits (480 kb) for each second of signal duration. The
communication of such a quantity of data to support signal
correlations is a burden, and an objective of the present invention
is to alleviate or eliminate this back-haul communication load
whenever possible.
8. "Matched Replicas"
[0040] The present invention provides a simple method and means to
enhance the detectability of communications signals at one site or
multiple sites, and to minimize or eliminate the need for excessive
back-haul communications to support the correlation processing used
for the detection of signal arrivals and the derivation of
measurements for the localization of communications transceivers.
In particular, the present invention applies effective and
efficient matched-replica correlations to support the optimal
detection and measurement of common signal localization parameters.
In the application of the matched-replica approach, the potential
received signal is "known" or derived at or provided to each
receiving site, when the transmitted waveform can be inferred, or
is communicated to the common correlation site(s), when the locally
"unknown" signal waveform(s) are received and interpreted remotely.
For a remotely received signal, the present invention uses a
"reduced-data" form of the communicated waveform (e.g., the
demodulated signal) to efficiently support correlation processing
with a representation that does not require the interstation
transfer of a "high-fidelity image" of the waveform as transmitted.
The use of correlations enables the extension of the correlation
integration interval over a duration that significantly exceeds the
interval used to detect the individual units or bits of
"communication." The use of reduced-data representations of the
signal replicas to support the correlations obviates the need to
communicate complete signal copies to a common site for the
correlation processing.
[0041] More specifically, in the AMPS networks employed in the
U.S., the mobile unit communications occur on separate frequency
channels spaced at 30 kHz intervals and centered at approximately
835 MHz. Two types of communications occur: those on "Control
Channels (CCs)" and those on "Voice Channels (VCs)." Whenever the
mobile-unit user keys into the mobile unit a telephone number to be
called and initiates the call, the embedded mobile-unit data
processor causes the transceiver to transmit a CC message that has
a duration of approximately 100 milliseconds (msec) and consists of
data bits that are transmitted by frequency-shift-key (FSK)
modulation at a rate of 10 thousand bits per second (bps), i.e., 10
kbps. Similarly, when the mobile unit is "called" by another
caller, the communications system "pages" the mobile unit with a CC
message, and the mobile unit responds with a CC FSK message also of
approximately 100 msec duration and with an information rate of 10
kbps. In either case, upon receiving the CC message broadcast by
the mobile unit, the communications system then selects a VC for
the conduct of the conversation and transmits back to the mobile
unit a message assigning the selected VC. The ensuing conversations
then follow on the initial and subsequently assigned VCs. The voice
signals are, of course, unknown in advance of reception, and are
communicated by frequency modulation (FM) as they occur. Before
transmission, the voice signals are companded and filtered, which
reduces even further the bandwidth of signals that are already
inherently limited by the range of the frequency content of the
human voice. Thus, the initial CC message from the mobile unit is
typically characterized by a signal of significantly greater
bandwidth than that of the VC. Since it is well known (e.g., Weiss
and Weinstein, op. cit.] that the accuracy with which a TDOA
measurement can be estimated is inversely proportional to the
signal bandwidth and to the square root of the time-bandwidth
product, it is the CC message of a mobile unit operating under AMPS
communications standards that provides the primary opportunity for
its adequate localization through TDOA measurement techniques. In a
fashion similar to AMPS, the voice communications provided by
typical Specialized Mobile Radio (SMR) systems are also FM
modulated, in 25 kHz channels, and are thus similarly limited in
their effectiveness for TDOA determination. Nevertheless, to the
extent that a VC signal will support the determination of TDOAs,
its replica for correlation processing may be represented by either
a segment of the sampled signal or a sampled segment of the
FM-demodulated voice signal, which itself may be characterized by a
bandwidth that is reduced relative to that of the FM transmission.
Indeed even the data content of the sampled voice signal
representation may be further reduced through linear predictive
coding (LPC) and dynamic range companding, albeit with sufficient
fidelity for accurate FM waveform reconstruction.
[0042] For a digital data message such as a communications control
message, the simplest example of a reduced-data representation of
the transmitted waveform is the communicated, extracted,
demodulated data message itself. As mentioned previously, such a
message may be represented by the relatively small average bit rate
of approximately 10 kbps, whereas the transmitted waveform
representation would require a much larger bit rate. The replica
waveform is constructed from the message content through the use of
the message-to-waveform transformation appropriate to the
specifications of the particular communications system of interest.
These transformations include various forms, such as the forms of
FSK, QPSK, and DQPSK described in EIA/TIA and other specifications,
as mentioned above and discussed further below.
[0043] The present invention can efficiently provide a location for
a wireless communications transmitter, e.g., for an emergency 911
call to a Public Safety Answering Point (PSAP), by enabling the
correlation detection of times of signal arrival (TOAs) and angles
of signal arrival (AOAs) of the call-initiating CC transmission
without the need for any communication of signal representations to
a common correlator site. The necessary detections can each be
derived with correlations at their respective receiving sites by
using a matched replica of the CC signal that is reconstructed from
the received CC message content, detected in real time or known in
advance. In accord with the AMPS format standards [EIA/TIA-553] for
all control messages, the transmitted CC signals begin with a
"syncing" bit pattern, then follow with defined bits of information
in a specific sequence that is repeated five times for
communications reliability, and finally terminate with error
detection and correction bit patterns. Thus, although each message
may be uniquely composed of individual called and calling telephone
numbers and an identifying serial number, the successful reception
and demodulation of the message content provides the data stream at
each site from which the FSK replica can be reconstructed for
effective detection and parameter determination through correlation
analysis. Indeed, it is the extracted representation of the
demodulated control message that defines the transmitted replica,
and enables its detection and parametric measurement to be
determined with optimum robustness and accuracy through the
correlator. Since the duration of the CC message is short and since
the entire message can be received and decoded, all or any part of
the replica can be reconstructed for use in the correlation to
accurately identify the time of a selected specific instant in the
content of the message (e.g., the end of the sync pattern or the
beginning of the error detection pattern or the beginning of the
first bit in the third repetition of the data content). As
mentioned above, the parametric measurement accuracy is improved
with the use of longer signal duration in the correlation process.
With successful determination of location-related parameters, e.g.,
TOAs and/or AOAs, only the very small information content
describing the measured parameter values and uncertainties, along
with their site and time of measurement, needs to be communicated
to a common site, where the differences in the TOAs, i.e., the
TDOAs, can be calculated and/or where all location-related
parametric data can be used to estimate the associated location of
the transmitter.
[0044] A growing number of communications systems are emerging in
which the voice content of the communications is "digitized" and
then communicated via techniques such as
code-division-multiple-access (CDMA) and
time-division-multiple-access (TDMA), in either the North American
(NA) TDMA or the Global System for Mobile Communications (GSM) TDMA
forms, rather than via the analog FM techniques of the AMPS
systems. Similarly, wireless data communications devices such as
those used in Cellular Digital Packet Data (CDPD) systems transmit
digitized information in accord with air-interface specifications
that define their individual replica signal formats. With such
"digital" systems, the digitized voice or data information content
can be used to adequately represent the signal waveform which is
needed for correlation to determine the desired estimates of TDOAs,
AOAs, or other localization parameters. As described above, the
digitized information content may consist of an information rate of
approximately 10 kbps (ten thousand bits per second) or less, while
the direct representation of the RIF signal waveform would
constitute several hundred thousand bits per second (or even
several million bits per second, in the case of CDMA signals with
coded bandwidths in excess of 1 MHz--one million Hertz). Thus, with
the transmission to a common site of only the reduced "digital"
information content in a segment of the voice communications
received at separated sites, the information content can then be
used to is reconstruct the equivalent transmitted signal waveforms
for application in the necessary correlation processing.
9. Specific Matched-Replica Construction/Reconstruction
[0045] To support the correlation analyses, the signal
reconstruction process is conducted in accord with the appropriate
signal specification, which defines the particular system for
representation of the zeros and ones in the information bit stream
of each communications system. This signal replication process is
summarized in FIG. 7 and is discussed in more detail in the
following.
[0046] As mentioned above, the communications for the AMPS CC use
FSK transitions specified in EIAlTIA-553. In accord with Manchester
encoding techniques, these signals use a frequency transition from
8 kHz below the signal carrier frequency to 8 kHz above the carrier
frequency to represent a "one," and a transition from 8 kHz above
to 8 kHz below the carrier to represent a "zero." Such bits of
information are communicated at a rate of 10 kbps for the AMPS CC
standard.
[0047] For the CDMA communications described in TIA/EIAIIS-95, the
message content bits are first encoded with "uncorrelated" bit
streams unique to each transmitter, and then are transmitted as
QPSK signals in which each pair of encoding bits is represented by
one of four selected quadrature phases of the transmitted signal.
While the message bits occur at rates up to 9600 bps, the encoded
bit "chips" are transmitted at a rate of 1.2288 million chips per
second (Mcps).
[0048] For TDMA transmissions in accord with the NA TDMA
specifications in EIAfTIA/IS-54, the bits for a message occur at an
average rate of 7800 bps and are transmitted in time bursts (with
time-division access controlled by the managing system) at a burst
rate of 24.3 thousand symbols per second (ksps), in which the
message bits are represented in symbol pairs by the technique of
differential-quadrature-phase-shift keying (DQPSK). With this
method, each pair of bits is represented by a transition or
difference in phase that is equal to one of a set of four selected
phase changes. Similarly, a smoothed form of binary, offset DQPSK
called Gaussian minimum shift keying (GMSK) is used for
transforming the bit sequences to the TDMA transmissions used in
the Global System for Mobile Communications (GSM) [e.g., as
described by Michael Mouly and Marie-Bernadette Pautet in "The GSM
System for Mobile Communications," Cell & Sys, 1992]. For CDPD
transmissions, GMSK is used for the transformation and transmission
of the message data bits at a rate of 19.2 kbps and the
transmissions are overlaid into the voice channels of the AMPS
configuration, with their 30 kHz channel spacing. Each of these
system-specific signal waveforms can be appropriately constructed
from the message bit stream that is intended for transmission.
Thus, with the fully reconstructed and filtered representations of
the transmitted signal waveforms, the signals applied in the
correlation processing possess the full signal bandwidth that
supports signal detection and parameter determination with optimal
accuracy.
[0049] Through the use described above of the reduced-data signal
representation and matched-replica reconstruction techniques, and
the associated matched-replica correlation processing, the
back-haul communications that support the correlation analyses can
be significantly reduced, or even eliminated. The matched replica
correlation processing also enables extended processing integration
times to facilitate the detection of desired signal effects, even
at distant sites in environments of strong, local, interfering
signals. Thus, the present invention significantly enhances the
robustness and efficiency and reduces the cost of implementation of
correlation techniques for the detection and measurement of signal
parameters at multiple sites to support the localization of
communications signal transmitters, such as the wireless
communications transmitters in cellularized communications
systems.
10. Equipment and Processing
[0050] Equipment configurations for the reception of standard
wireless communications for the purpose of localization will be
largely composed of the same devices that are used in the
implementation of the communications system itself. For example,
the antenna configurations and signal reception components shown in
FIGS. 5 and 6 may actually be the same as those applied in
providing the communications services. The phased arrays used to
support AOA measurements employ the same technology and may be the
same as the "smart arrays" currently being implemented to provide
"spatial-division multiple-access" communications services in some
locales with enhanced capacity and frequency reuse. To support TOA
and TDOA determinations of useful locations, the digitization or
sampling of the signals at the distributed sensor sites must be
synchronized and time-tagged to within (at most) one-half
microsecond. This may be achieved through the use of stable,
calibrated oscillators, such as those in rubidium clocks or Global
Positioning System (GPS) time bases, and maintained with periodic
recalibration of the timing standards in each sensor site. The
stability or drift rate of the oscillator standard determines how
often the recalibration with signals from known locations must be
performed. Similarly, for the phased-array determination of useful
AOAs, periodic recalibration of equipment-specific, inter-element,
phase-difference offsets must be performed, but only as often as
needed to account for temperature and other environmental drift
effects on the analog RF equipment, and inter-site time
synchronization needs to be maintained, but to only approximately
one-half second.
[0051] The digital correlation signal processing for the present
invention is similar, or identical in the case of CDMA, to that
applied in the "software radio" reception equipment employed for
the provision of communications services.
[0052] This processing, for the correlation measurement extractors
shown in FIGS. 5 and 6, may be accomplished by digital signal
processing devices that are specially designed for efficient
communications processing, or may alternatively be performed with
general purpose signal processing devices, such as the scaleable
multiprocessor board manufactured by Pentek, Inc. of Upper Saddle
River, N.J., and designed to use four TMS320C6201 digital signal
processing chips manufactured by Texas Instruments, Inc. of Dallas,
Tex. As the capabilities of digital signal processing facilities
advance and the price-to-performance ratio declines, increasingly
more of the functionality currently allocated to the analog signal
conditioning equipment described above will be allocated to digital
signal processing devices. With the digital signal processing
approaches, signal integrity is maintained or significantly
enhanced while increased functionality and flexibility is
added.
[0053] The control flow for the functional direction of the
multiple sensor site (SS) assets by the central site (CS) may be
represented for one embodiment as shown in FIGS. 8A and 8B. In this
flow, the control site distributes the responsibility for obtaining
and reporting location-related measurements on the communications
calls of interest (COIs). The sensor sites also report the
detection of initiated communications on the reverse control
channels (RCCs) and the voice channel assignments (VCAs) provided
on the forward control channel (FCC) and subsequently on the
forward voice channel (FVC) by the communications system to the
mobile caller unit. In response to VCAs, the SSs coordinate their
assignments for reporting with the CS and also tune to and follow
the COls to produce ongoing location-related measurements to the
CS, until the CS terminates such an assignment or the signal of
interest is lost. In the embodiment shown in FIGS. 8A and 8B, the
location-related measurements are derived from the voice signals
alone. In an alternative embodiment, the sensor sites may
continuously monitor communications control signals for the
derivation of location-related measurements from them when they
occur. In such an embodiment, the sensor sites also report such
localization data to the CS when detected at the time of initiation
of the communications.
[0054] Through the application of standard statistical analysis
procedures [e.g., as described in Jazwinski, "Stochastic Processes
and Filtering Theory," Academic Press, 1970], the TDOA-based
range-difference measurements and AOA-based measurements of LOBs,
and their associated uncertainty information, can be analyzed to
provide estimates of the associated mobile unit locations and
velocities. The knowledge representation of the measurement
information and its uncertainty can take numerous forms, such as
discrete attribute vectors in which each element of the vector
represents the value of a particular discrete attribute where the
values may be boolean, integer, floating point, or symbolic, and
particular choices of the values will have attendant confidences;
continuous numeric parameters with associated statistical errors;
and/or fuzzy logic parameters. The localization evaluation
processing can employ any or a combination of numerous analysis and
uncertainty management systems, each suited to the appropriate
knowledge representation. Examples of such analysis approaches
include maximum likelihood or least squares estimators, joint
probabilistic data association algorithms, probability density
function multi-target tracking systems for continuous parameters,
multi hypothesis uncertainty management systems, rule-based expert
systems with multi-confidence production rules that combine
discrete logical assertions with continuous numeric information,
fuzzy logic engines, and causal belief networks. The specific
method, form, or implementation of the analyses that are applied to
obtain a location estimate from the location-related data is not
the subject of the present invention.
[0055] The localization estimates resulting from such analyses may
be represented in graphical, tabular, or internal processor-data
format, and may be presented or displayed either on displays that
are integral to the data collection and analysis equipment or that
are embodied in equipment that is remote from such equipment. The
particular method, form, and location for representation of the
localization results are also not the subject of the present
invention.
[0056] The processing and display facilities required for the
execution of the sensor site control and management, the
localization and tracking calculations, the storage and retrieval
of the localization data, and the display and interaction with
users of the localization and system management data are readily
implemented with an integrated set of current versions of general
purpose personal computer configurations. These configurations may
include a network of processors and workstations that are based,
for example, on the Intel Pentium or Motorola Power PC processor
chips.
[0057] In the interests of public benefit, the locations obtained
through the efficient application of the above techniques can be
most beneficially applied to rapidly direct a wireless call for
assistance to the Public Safety Answering Point (PSAP) that is
closest to the location of need or has jurisdictional
responsibility for calls for help from that location. In
particular, the location-related measurement data derived from
correlations that are performed at the receiving sites with locally
derived replicas, or with stored replicas of those portions of the
initiating control messages that are known in advance, can be
accomplished more rapidly than can processing that requires data
from another site after signal reception. Thus, the present
invention can further apply location-related measurements, derived
without the need for cooperative inter-site transfer of
signal-replica data, to more quickly evaluate the localization
calculations and obtain the location for the call-routing algorithm
to rapidly and accurately direct the call to the appropriate
location-determined response point.
[0058] The principles, preferred embodiments and modes of operation
of the present invention have been set forth. in the foregoing
specification. The embodiment disclosed herein should be
interpreted as illustrating the present invention and not as
restricting it. The foregoing disclosure is not intended to limit
the range of equivalent structure available to a person of ordinary
skill in the art in any way, but rather to expand the range of
equivalent structures in ways not previously thought of. Numerous
variations and changes can be made to the foregoing illustrative
embodiments without departing from the scope and spirit of the
present invention as set forth in the appended claims.
* * * * *