U.S. patent application number 10/225267 was filed with the patent office on 2004-10-14 for method and system for location finding in a wireless local area network.
Invention is credited to Aljadeff, Daniel, Granot, Yair, Tsruya, Shalom.
Application Number | 20040203870 10/225267 |
Document ID | / |
Family ID | 31946297 |
Filed Date | 2004-10-14 |
United States Patent
Application |
20040203870 |
Kind Code |
A1 |
Aljadeff, Daniel ; et
al. |
October 14, 2004 |
Method and system for location finding in a wireless local area
network
Abstract
A method and system for location finding in a wireless local
area network (LAN) enables enhanced security via network intrusion
management and connection access management, as well as providing a
mechanism for physically mapping a wireless network. Multiple
receivers are employed to determine time-difference-of-arrival
(TDOA) of signals transmitted from a wireless device. The location
of the transmitting device is determined by triangulating between
multiple receivers. The receivers may be devices within the
wireless network that have been enhanced to include TDOA
capability, or multiple dedicated location units may be employed
within the wireless network or a wired network that may be a
completely separate infrastructure depending on the requirements of
a particular installation.
Inventors: |
Aljadeff, Daniel; (Kiriat
Ono, IL) ; Granot, Yair; (Modlin, IL) ;
Tsruya, Shalom; (Rishon Lezion, IL) |
Correspondence
Address: |
WEISS & MOY PC
4204 NORTH BROWN AVENUE
SCOTTSDALE
AZ
85251
US
|
Family ID: |
31946297 |
Appl. No.: |
10/225267 |
Filed: |
August 20, 2002 |
Current U.S.
Class: |
455/456.1 ;
455/457 |
Current CPC
Class: |
G01S 5/06 20130101; H04W
64/00 20130101; G01S 5/021 20130101 |
Class at
Publication: |
455/456.1 ;
455/457 |
International
Class: |
H04Q 007/20 |
Claims
What is claimed is:
1. A method for determining the location of a wireless device
within a wireless local area network (LAN), said method comprising:
receiving a wireless LAN signal from said wireless device at
multiple location units; in response to said receiving, determining
multiple times of arrival of said standard wireless LAN signal at
each corresponding one of said multiple location units; sending
said times of arrival from each of said corresponding multiple
location units to a master unit; and determining, within said
master unit, the location of said wireless device by comparing
location information for said multiple location units with said
multiple times of arrival to determine said location of said
wireless device.
2. The method of claim 1, wherein said master unit is one of said
multiple location units, whereby said sending said time of arrival
is not performed for said time of arrival determined at said master
unit.
3. The method of claim 1, further comprising synchronizing a timer
in each of said location units, whereby a precise time relationship
between said multiple times of arrival is maintained.
4. The method of claim 1, wherein each of said location units is
coupled to said master unit via a wired network, and wherein said
sending said times of arrival is performed over said wired
network.
5. The method of claim 1, wherein said sending is performed over
said wireless network.
6. The method of claim 1, further comprising transmitting a
request-to-send signal to said wireless device, and wherein said
receiving receives a clear-to-send from said wireless device issued
in response to said request-to-send signal.
7. The method of claim 1, wherein said receiving further comprises:
filtering said received signal with a matched filter; sampling and
storing quadrature symbol detection outputs of said matched filter;
and subsequently computing an estimate of said time of arrival of
said received signal.
8. The method of claim 7, wherein said computing computes said
estimate of said time of arrival in conformity with a maximal
mean-square profile of said received signal correlated with a
spread-spectrum sequence.
9. The method of claim 6, wherein said computing further computes
said estimate of said time of arrival in conformity with a maximal
mean-square profile of said received signal convolved with a
predetermined message sequence.
10. A wireless local area network (LAN), comprising: a plurality of
location units for receiving a wireless LAN signal transmitted by a
wireless device within said wireless LAN and determining a time of
arrival for said received signal; at least one master unit for
receiving said time of arrival from each of said location units,
whereby said location of said wireless devices is be determined in
conformity with said time of arrival.
11. The wireless LAN of claim 10, wherein said master unit is one
of said multiple location units.
12. The wireless LAN of claim 10, wherein each of said location
units comprises a timebase for maintaining precision in said time
of arrival among said location units.
13. The wireless LAN of claim 12, wherein said master unit further
synchronizes said timebase in each of said location units by
sending a synchronization message to said location units.
14. The wireless LAN of claim 10, wherein each of said location
units is coupled to said master unit via a wired network, and
wherein said time of arrival is sent from said location units to
said master unit over said wired network.
15. The wireless LAN of claim 10, wherein said time of arrival is
sent from said location units to said master unit over said
wireless network.
16. The wireless LAN of claim 10, further comprising an location
measurement initiating unit that transmits a request-to-send signal
to said wireless device, and wherein said wireless LAN signal is a
clear-to-send message sent from said wireless device in response to
said request-to-send signal.
17. The wireless LAN of claim 16, wherein said measurement
initiating unit is one of said location units.
18. The wireless LAN of claim 16, wherein said measurement
initiating unit is said master unit.
19. The wireless LAN of claim 10, wherein each of said location
units comprises a receiver including: a matched filter for
filtering said received signal; a sampler and memory for sampling
and storing quadrature symbol detection outputs of said matched
filter; and a signal processor for computing an estimate of said
time of arrival of said received signal from said stored
samples.
20. The wireless LAN of claim 19, wherein said signal processor
further computes said estimate of said time of arrival in
conformity with a maximal mean-square profile of said received
signal correlated with a spread-spectrum sequence.
21. The wireless LAN of claim 20, wherein said signal processor
further computes said estimate of said time of arrival in
conformity with a maximal mean-square profile of said received
signal convolved with a predetermined message sequence.
22. A location unit, comprising: a receiver for receiving wireless
local area network (LAN) signals from a wireless device in a
wireless network; a time measurement unit coupled to said receiver
for determining the time of arrival of said signals from said
wireless device; and an interface for sending said time of arrival
information to a master unit.
23. The location unit of claim 22, wherein said receiver comprises
a spread-spectrum receiver and wherein said time measurement unit
comprises a processor for determining said time of arrival in
conformity with samples stored from an output of said receiver.
24. The location unit of claim 23, wherein said processor computes
an estimated time of arrival via mean-square estimation.
25. The location unit of claim 22, wherein said wireless LAN signal
is a clear-to-send signal received from said wireless device.
26. A master unit, comprising: a data interface for receiving from
multiple external location units, times of arrival of a standard
wireless local area network (LAN) signal received from a wireless
device by said multiple external location units; a database
including physical location information of said multiple external
location units; and a computation unit for comparing said times of
arrival from said multiple external location units in conformity
with location information retrieved from said database to determine
a location of wireless devices in a wireless LAN.
27. The master unit of claim 26, wherein said computation unit
computes differences between pairs of said time of arrival received
from pairs of said location units, projects a hyperbolic curve for
each of said differences, said curve having foci at physical
locations of said pair of location units associated with a
corresponding difference, and determines said location of said
wireless device in conformity with an intersection of said
hyperbolic curves.
28. The master unit of claim 26, wherein said master unit transmits
a command to issue a request-to-send signal to said wireless device
over said data interface, and wherein said wireless LAN signal is a
clear-to-send signal transmitted by said wireless device in
response to said request-to-send signal.
29. The master unit of claim 26, wherein said master unit includes
a wireless LAN transmitter, and wherein said master unit transmits
a request-to-send signal to said wireless device, and wherein said
wireless LAN signal is a clear-to-send signal transmitted by said
wireless device in response to said request-to-send signal.
30. The master unit of claim 26, wherein said master unit includes
a wireless LAN receiver, and wherein said master unit receives said
wireless LAN signal and measures a time of arrival at said master
unit of said wireless LAN signal, and wherein said computation unit
further compares said time of arrival at said master unit of said
wireless LAN signal.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is related to previously-filed
United States patent applications assigned to the same assignee:
"METHOD AND APPARATUS FOR ENHANCING SECURITY IN A WIRELESS NETWORK
USING DISTANCE MEASUREMENT TECHNIQUES", Ser. No. 10/156,244, filed
May 24, 2002 and "METHOD AND APPARATUS FOR INTRUSION MANAGEMENT IN
A WIRELESS NETWORK USING PHYSICAL LOCATION DETERMINATION", Ser. No.
10/171,427, filed Jun. 13, 2002. The specifications of the
above-referenced U.S. patent applications are herein incorporated
by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to wireless
networks, and more specifically, to a method and system for
determining the physical location of devices within a wireless
network.
[0004] 2. Background of the Invention
[0005] A multitude of wireless communications systems are in common
use today. Mobile telephones, pagers and wireless-connected
computing devices such as personal digital assistants (PDAs) and
laptop computers provide portable communications at virtually any
locality. Wireless local area networks (WLANs) and wireless
personal area networks (WPANs) according to the Institute of
Electrical and Electronic Engineers (IEEE) specifications 802.11
(WLAN) (including 802.11a, 802.11b, etc.), 802.15.1 (WPAN) and
802.15.4 (WPAN-LR) also provide wireless interconnection of
computing devices and personal communications devices, as well as
other devices such as home automation devices.
[0006] Within the above-listed networks and wireless networks in
general, it is desirable to know the location of devices for
operation of location-based services, mapping of network
facilities, and security. The above-incorporated patent
applications describe wireless networks in which intrusion
management and connection access control use a physical location
determination as an indicator of the desirability of a particular
connection to a wireless device. It is further desirable to provide
services based on the location of a device, such as coupling of a
portable device to a workstation when the devices are in proximity,
or provision of a financial transaction menu when the portable
device is brought in proximity with a transaction terminal.
[0007] Techniques that may be used to determine location are
disclosed in the above-incorporated patent applications. The
techniques include loop delay measurement for distance
determination or received signal strength measurement (RSSI),
time-difference-of-arrival techniques (TDOA), and angle-of-arrival
techniques (AOA) for location finding. However, the infrastructure
of present-day wireless networks has not been adapted to provide
physical location determination.
[0008] Existing systems that determine the physical location of
assets (that may include wireless network devices) typically use a
separate radio-frequency identification (RFID) tag attached to the
asset. The RFID tag broadcasts a signal, separate from the wireless
network signals and protocols, that can be received at a short
distance by a specially adapted receiver.
[0009] The use of tags for locating wireless network devices adds
cost and complexity to the wireless network. Also, the tags are
typically battery-operated devices that are attached to the asset,
and as such have a limited life or will require replacement of the
power source. Further, the separate attachment of tags is an
inconvenience and is subject to incorrect tagging or tampering such
as removal from the asset or relocation to another asset.
[0010] Therefore, it would be desirable to provide a method and
system for location finding in a wireless network, so that the
physical location of wireless network devices may be determined,
and without adding tags or using special signals for determining
the location of wireless network devices.
SUMMARY OF THE INVENTION
[0011] The above objectives of physically locating devices in a
wireless network is achieved in a method and system. The method is
embodied in a system that determines a physical location of a
wireless device by comparing the time difference between signals
received from the wireless device at multiple receiving stations.
The arrival times of the signals are sent from each of the multiple
receiving stations to a master unit, where they are compared to
known physical location information for the receiving stations
stored within the master unit. The master unit then determines the
location of the wireless device in conformity with the differences
between arrival times at the multiple receiving stations and their
known locations. The location of standard wireless network devices
having no special location-finding circuitry can be determined by
the location units and standard network signaling can be detected
and time difference measurements performed thereon to determine the
locations of standard network devices.
[0012] The receiving stations may be transceivers, receive only
devices or devices performing other network functions that have
been enhanced to include embodiments of the present invention. The
receiving stations further may be hard-wired to a network channel
to provide secure sharing of time difference data, or the time
difference data may be communicated via the wireless network
channels or via other wireless means. The master device may also be
one of the receiving stations.
[0013] The foregoing and other objectives, features, and advantages
of the invention will be apparent from the following, more
particular, description of the preferred embodiment of the
invention, as illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a pictorial diagram depicting a wireless network
in accordance with an embodiment of the present invention.
[0015] FIG. 2 is a block diagram of a location finding unit in
accordance with an embodiment of the present invention.
[0016] FIG. 3 is a graph depicting operation of multiple location
finding units in accordance with an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT
[0017] The present invention provides location finding within a
wireless network, such as a WLAN (e.g., IEEE 802.11) or WPAN
network, by determining a time-difference-of-arrival (TDOA) profile
for signals received from wireless devices connected to or
attempting to connect to the wireless network. Once the location of
a wireless device is determined via the TDOA profile,
location-based services can be provided, the device can be mapped
in a network facility map, and security can be managed in
conformity with the device's location. In contrast to the use of
RFID tags, the present invention provides a wireless network with
location-finding capability where no special signaling (e.g., the
separate channels used by the RFID tags) and no separate device or
tag is required.
[0018] Existing wireless network devices (generally the access
points or "APs") may be enhanced to provide a TDOA measurement of
physical device location without adding a separate infrastructure,
thereby providing position determination and consequent enhanced
network security with low incremental cost. Alternatively, a
separate infrastructure employing a series of dedicated location
finding units may be added to a wireless network facility for
providing device location information, avoiding the need to replace
installed devices or otherwise reconfigure the wireless network. By
adding a set of dedicated location finding units, standard network
signals (e.g., request-to-send (RTS), clear-to-send (CTS) and
beacon signals (broadcast) can be observed and used to determine
the location of the transmitting devices) and standard wireless
network devices can be located without requiring any modification
to the located devices.
[0019] In TDOA techniques, the location of a transmitting source
can be determined by triangulation based on the timing between the
signal arrivals at the multiple receivers. Referring now to the
figures and in particular to FIG. 1, a wireless network 10 within
which the present invention is embodied is depicted in a pictorial
diagram. Access points (APs) 14A-14C include time-of-arrival (TOA)
electronics and software for measuring the arrival time of signals
from other wireless network devices. One such device, wireless
device 16 is depicted at the intersection of two hyperbolic curves
18A and 18B. Curves 18A and 18B represent points for which the
difference between TOAs for a pair of locating device (APs 14A and
14C for curve 18B and APs 14B and 14C for curve 18A) is a constant.
Therefore, once a pair of TOAs are determined by two of APs 14A-C,
a curve may be drawn that intersects the location of the device
that transmitted the signal received by the pair of APs. The TOA
difference from another pair of APs is then used to determine a
second curve and the intersection of the two curves yields the
physical location of the transmitting device. The curves are
hyperbolic with foci a the location of each of the APs in the pair,
since the hyperbolic curve represents the set of points for which
the difference between the distance to the two foci is a constant.
A particular TOA difference determines the particular curve (i.e.,
a new curve is generated for each measurement) and represents all
of the possible positions of the transmitting device for the
determined TOA difference (TDOA) between a pair of receivers. There
are two hyperbolic curves that satisfy the absolute value time
difference location equation, but the sign of the time difference
determines the proper curve, as for negative signs, the hyperbolic
curve on which the transmitting device lies is the one closest to
the base station for which the time or arrival was subtracted.
[0020] Extending the above-described technique, if at least three
receivers are employed, it is possible to locate a transmitting
device in two dimensions via the intersection of the two curves
generated by two pairs of receivers is detected. The technique can
be further extended to three-dimensional space using hyperbolic
sections and/or additional pairs of receivers can be used to reduce
error in measurement by interpolating between location results or
rejecting location results that are bad statistical fits for the
measurement.
[0021] The curve calculations described above are performed by a
master unit MST (which may be one of the location units or software
executing within of another network device) that receives the TOA
information from each of the location units (APs 14A-C in the
exemplary embodiment), calculates the differences and determines
the location of the transmitting device via the intersection of the
above-described hyperbolic curves. Master unit MST may also provide
synchronization between the location units (or an independent
synchronization mechanism may be employed) and control of the
location finding process by requesting that the location units
capture TOA information and send the TOA information to master unit
MST.
[0022] Generally, the present invention uses wireless network
signals that are already in place for network communications and
while the system of the present invention may monitor
communications without intervening in wireless network operation,
active location finding may be performed in accordance with an
embodiment of the present invention. A useful protocol is to
transmit a request-to-send to a particular wireless network device
to be located. The TDOA computations can be performed on the
clear-to-send response generated by the particular device. In the
above-described manner, the wireless network (or a particular
device only) may be polled in order to obtain low-latency location
information for a device, the entire network or a portion
thereof.
[0023] Referring now to FIG. 2, an enhanced wireless network
device, in which an embodiment of the present invention is
included, is depicted in a block diagram. Location unit 20 may be a
dedicated location unit, or may be a wireless network device having
enhanced features for location determination according to TDOA
measurements. A WLAN transmitter/receiver 22A is coupled to an
antenna 21 for receiving wireless network signals, which will
generally be digital spread-spectrum signals (DSSS). A DSSS
baseband processor 24A detects and decodes the DSSS signals and
passes the decoded information to a media access control (MAC)
processor 26A that generates MAC (layer 3) network packets and
passes then to a transmission control protocol/Internet protocol
(TCP/IP) interface 28 for conversion to the TCP/IP (layer 4)
packets for communication with the network-coupled device. In the
return direction, TCP/IP packets received from the network-coupled
device at TCP/IP interface 28 are converted to MAC packets by MAC
processor 26A and are passed for encoding (DSSS modulation) to DSSS
baseband processor 24A, which provides a signal input to the
transmit portion of WLAN transmitter/receiver 22A. WLAN
transmitter/receiver 22A transmits a wireless network signal to
other network devices via antenna 21.
[0024] A location signal section is provided by a second WLAN
transmitter/receiver 22B (or a single transmitter/receiver can be
used for the location section and network section of the location
unit as long as the TOA measurement requirements are fulfilled by
the receiver design). WLAN transmitter/receiver 22B receives a
signal from antenna 21 and sends it to a special DSSS processor 24B
that determines the TOA of the received signal. The TOA information
is passed to a location controller that includes a MAC interface
26B coupled to MAC processor 26A in the network section, so that
the TOA information can be communicated to a master unit within the
wireless network. Alternatively, the communications path from MAC
interface 26B can be passed to a non-wireless Ethernet interface or
other wired LAN interface for communicating the TOA information to
the master unit.
[0025] The signaling components of location unit 20 are depicted as
two separate subsystems coupled to the same antenna 21, but the
structure of a wireless network device in accordance with
embodiments of the present invention may be varied. For a
location-only unit (i.e., a device that provides only the
location-finding capability of the present invention without
serving as a wireless network access point or other devices) may be
implemented by including only the location signal section
comprising WLAN transmitter/receiver 22B coupled to antenna 21 and
DSSS processor 24B. Alternatively a fully network capable wireless
device may include all of the depicted elements, but the
transmitter/receiver blocks and DSSS processing blocks may be
merged as mentioned above with respect to the transmitter/receiver,
so that the location finding capabilities of the present invention
are integrated within the standard wireless network device
electronics. However, while standard signal processing blocks for a
wireless network device generally process only the messages encoded
for the address of the device (message level detection), location
units decode the symbol level of these otherwise undeciphered
messages to determine time of arrival information. If the location
unit itself is addressed, or the location unit knows the address of
another wireless device, message level detection can further
enhance the signal to noise ratio of the location finding. The
message level detection improves the signal to noise ratio of the
location measurement by using known address information to further
decode the message, permitting rejection of spurious signals, and
raising the confidence of the measurement.
[0026] Referring now to FIG. 3, details of DSSS processor 24B are
depicted in a block diagram. The decode input accepts signals from
a receiver (WLAN transmitter/receiver 22B) and a PN sequence
matched filter 32 correlates the location signal to provides a
series of samples in (I,Q) pairs that are stored in a sample buffer
33. Matched filter 32 provides increased immunity from multipath
effects due to reflections within the network facility. Therefore,
the locations of access points (or another network device used to
perform location measurements) do not need to be optimized to
achieve accurate location finding results. If the immunity to
multipath effects was lower than that provided by the system of the
present invention, the position of the transmitting devices
(generally access points being observer) and location units would
require careful control of placement in order to avoid location
error due to multipath effects. In some multipath environments, it
would not be possible to locate all of the devices such that
multipath error could be sufficiently reduced.
[0027] A Time Slot Start signal is provided by WLAN
transmitter/receiver 22B and is used to start the sampling process
via a timer latch 35. A timebase 36 provides synchronization of the
location unit containing DSSS processor 24B to the other location
units, so that the TOA information is precisely related among the
locating units and the TDOA differences computed are accurate. A
digital signal processor (DSP) 34 computes the TOA of a received
signal and transmits the TOA information to the master unit over
the wireless network. Location controller and MAC interface 26B
sends the TOA information to MAC processor 26A which formats the
TOA message and TCP/IP interface 28 sends the message through the
wireless network to the master station.
[0028] DSP 34 calculates a best-estimate of the TOA for the
received signal by performing coherent or non-coherent detection.
Coherent detection at the message level is preferred if information
about the transmitted message and signal is available such as
frequency deviation of the signal and content of the message. In
either case, coherent detection is performed at the symbol level by
matched filter 32, providing a high signal to noise ratio (SNR) for
the TOA measurement.
[0029] The signal/message detection techniques differ in complexity
and performance. While coherent detection provides the best
theoretical performance, the non-coherent detector represents a
simpler implementation with potentially reduced performance. One of
the important advantages of the location method described above is
its ability to perform well in low signal to noise ratios (SNR).
Even if the receiver cannot decode the WLAN message due to noise,
the TOA may still be determined with adequate accuracy. Signal
energy detection techniques using mean-square estimation as are
well known in the art of signal detection may be used to estimate
the greatest likelihood arrival time. Other suitable detection
algorithms may also be used depending on the type of signals used
and the desired complexity of the detection hardware and/or
processors.
[0030] While the invention has been particularly shown and
described with reference to the preferred embodiments thereof, it
will be understood by those skilled in the art that the foregoing
and other changes in form, and details may be made therein without
departing from the spirit and scope of the invention.
* * * * *