(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2005/0185618 A1
`Friday et al.
`(43) Pub. Date:
`Aug. 25, 2005
`
`US 20050185618A1
`
`(54) WIRELESS NODE LOCATION MECHANISM
`USING ANTENNA PATTERN DIVERSITY TO
`ENHANCE ACCURACY OF LOCATION
`ESTIMATES
`(76) Inventors: Robert J. Friday, Los Gatos, CA (US);
`Paul F. Dietrich, Seattle, WA (US)
`Correspondence Address:
`MARK J. SPOLYAR
`38 FOUNTAIN ST.
`SAN FRANCISCO, CA 94114 (US)
`(21) Appl. No.:
`10/783,186
`
`(22) Filed:
`
`Feb. 20, 2004
`Publication Classification
`
`(51) Int. Cl." ..................................................... H04B 7/185
`(52) U.S. Cl. ...
`370/331; 709/239
`ABSTRACT
`(57)
`A wireleSS node RF Fingerprinting location mechanism that
`uses multiple antenna patterns to enhance the accuracy of
`wireleSS node location in an RF environment. In one imple
`mentation, Substantially non-overlapping antenna pattern
`diversity is used to provide a degree of Sectorization in
`computing the estimated location of a wireleSS node.
`
`
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`Antenna Selector
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`120
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`Transmit
`Receive
`Switch
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`Antenna
`Selection
`Module
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`40
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`Antenna Selector
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`Fig. 2B
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`US 2005/0185618A1
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`()
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`26a
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`122
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`30
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`/
`(>
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`126a
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`124
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`122
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`3O
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`Antenna
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`Selection
`Module
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`Fig. 3B
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`m
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`124
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`Detect Frame
`Preamble?
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`For All
`Antennae Do
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`204
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`Detect Signal
`Attribute Using
`Current Antenna
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`208
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`20
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`22
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`Fig. 4
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`Switch to Selected
`Antenna
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`Provide Selected
`Antenna ID to
`MAC Control Unit
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`Frame
`Complete?
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`Select IRTs
`for Location
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`304
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`Select RF Coverage
`Maps based on
`Antenna Identifiers
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`
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`Estimate Location of
`Node based on
`Coverage Maps and
`Signal Strength Data
`Fig. 5
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`Fig. 6
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`US 2005/0185618A1
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`62
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`Flag Detector
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`Control
`Path
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`Wireless Node
`Data Collector
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`AP Signal
`Strength Matrix
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`RF Physical
`Model Database
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`Fig. 7
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`Fig. 8A
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`Patent Application Publication Aug. 25, 2005
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`Sheet 9 of 9
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`US 2005/0185618 A1
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`IRT1, At
`y1
`y2
`y3
`y4
`
`x1
`ss(x1,y1)
`ss(x1,y2)
`ss(x1,y3)
`ss(x1,y4)
`
`x2
`ss(x2,y1)
`ss(x2,y2)
`ss(x2,y3)
`ss(x2,y4)
`
`x3
`$s(x3,y1)
`ss(x3,y2)
`ss(x3,y3)
`ss(x3,y4)
`
`x4
`ss(x4,y1)
`ss(x4,y2)
`ss(x4,y3)
`ss(x4,y4)
`
`IRT1, A2
`y1
`y2
`y3
`y4
`
`x1
`ss(x1,y1)
`ss(x1,y2)
`ss(x1,y3)
`ss(x1,y4)
`
`x2
`$s(x2,y1)
`$s(x2,y2)
`ss(x2,y3)
`ss(x2,y4)
`
`x3
`ss(x3,y1)
`ss(x3,y2)
`ss(x3,y3)
`ss(x3,y4)
`
`x4
`ss(x4,y1)
`ss(x4,y2)
`ss(x4,y3)
`ss(x4,y4)
`
`IRT2, A1
`y1
`y2
`y3
`y4
`
`x1
`ss(x1,y1)
`ss(x1,y2)
`ss(x1,y3)
`ss(x1,y4)
`
`x2
`$s(x2,y1)
`ss(x2,y2)}
`$s(x2,y3)}
`$s(x2,y4)
`
`x3
`$s(x3,y1)
`$s(x3,y2)
`$s(x3,y3)
`ss(x3,y4)
`
`x4
`ss(x4,y1)
`ss(x4,y2)
`ss(x4,y3)
`ss(x4,y4)
`
`Fig.9
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`...
`...
`...
`...
`
`wee
`...
`...
`...
`...
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`_
`...
`...
`...
`...
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`WIRELESS NODE LOCATION MECHANISM
`USING ANTENNA PATTERN DIVERSITY TO
`ENHANCE ACCURACY OF LOCATION
`ESTIMATES
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`0001. This application makes reference to the following
`commonly owned U.S. patent applications and/or patents,
`which are incorporated herein by reference in their entirety
`for all purposes:
`0002 U.S. patent application Ser. No. 10/155,938 in
`the name of Patrice R. Calhoun, Robert B. O'Hara,
`Jr. and Robert J. Friday, entitled “Method and Sys
`tem for Hierarchical Processing of Protocol Infor
`mation in a Wireless LAN.”
`0003 U.S. application Ser. No. 10/183,704 in the
`name of Robert J. Friday, Patrice R. Calhoun, Robert
`B. O'Hara, Jr., Alexander H. Hills and Paul F.
`Dietrich, and entitled “Method and System for
`Dynamically ASSigning Channels AcroSS Multiple
`Radios in a Wireless LAN;”
`0004 U.S. patent application Ser. No. 10/407.357 in
`the name of Patrice R. Calhoun, Robert B. O'Hara,
`Jr. and Robert J. Friday, entitled “Method and Sys
`tem for Hierarchical Processing of Protocol Infor
`mation in a Wireless LAN.”
`0005 U.S. patent application Ser. No. 10/407,370 in
`the name of Patrice R. Calhoun, Robert B. O'Hara,
`Jr. and David A. Frascone, entitled “Wireless Net
`work System Including Integrated Rogue Access
`Point Detection;”
`0006 U.S. application Ser. No. 10/447,735 in the
`name of Robert B. O'Hara, Jr., Robert J. Friday,
`Patrice R. Calhoun, and Paul F. Dietrich and entitled
`“Wireless Network Infrastructure including Wireless
`Discovery and Communication Mechanism;’ and
`0007 U.S. application Ser. No. 10/611,522 in the
`name of Robert J. Friday and entitled “Non-Over
`lapping Antenna Pattern Diversity In Wireless Net
`work Environments.”
`
`FIELD OF THE INVENTION
`0008. The present invention relates to location of wireless
`nodes in wireleSS network environments and, more particu
`larly, to a wireleSS node location mechanism employing
`antenna pattern diversity to enhance the accuracy of esti
`mating the location of wireleSS nodes.
`
`BACKGROUND OF THE INVENTION
`0009 Market adoption of wireless LAN (WLAN) tech
`nology has exploded, as users from a wide range of back
`grounds and Vertical industries have brought this technology
`into their homes, offices, and increasingly into the public air
`Space. This inflection point has highlighted not only the
`limitations of earlier-generation Systems, but the changing
`role WLAN technology now plays in people's work and
`lifestyles, across the globe. Indeed, WLANs are rapidly
`changing from convenience networks to business-critical
`networks. Increasingly users are depending on WLANs to
`
`improve the timelineSS and productivity of their communi
`cations and applications, and in doing So, require greater
`Visibility, Security, management, and performance from their
`network.
`0010. The rapid proliferation of lightweight, portable
`computing devices and high-speed WLANs has enabled
`users to remain connected to various network resources,
`while roaming throughout a building or other physical
`location. The mobility afforded by WLANs has generated a
`lot of interest in applications and Services that are a function
`of a mobile user's physical location. Examples of Such
`applications include: printing a document on the nearest
`printer, locating a mobile user, displaying a map of the
`immediate Surroundings, and guiding a user inside a build
`ing. The required or desired granularity of location infor
`mation varies from one application to another. Indeed, the
`accuracy required by an application that Selects the nearest
`network printer, or locates a rogue access point, often
`requires the ability to determine in what room a mobile
`Station is located. Accordingly, much effort has been dedi
`cated to improving the accuracy of wireleSS node location
`mechanisms.
`0011. The use of radio signals to estimate the location of
`a wireleSS device or node is known. For example, a Global
`Positioning System (GPS) receiver obtains location infor
`mation by triangulating its position relative to four Satellites
`that transmit radio signals. The GPS receiver estimates the
`distance between each Satellite based on the time it takes for
`the radio signals to travel from the Satellite to the receiver.
`Signal propagation time is assessed by determining the time
`shift required to Synchronize the pseudo-random Signal
`transmitted by the Satellite and the Signal received at the
`GPS receiver. Although triangulation only requires distance
`measurements from three points, an additional distance
`measurement from a fourth Satellite is used for error cor
`rection.
`0012. The distance between a wireless transmitter and a
`receiver can also be estimated based on the Strength of the
`received signal, or more accurately the observed attenuation
`of the radio signal. Signal attenuation refers to the weaken
`ing of a Signal over its path of travel due to various factors
`like terrain, obstructions and environmental conditions.
`Generally Speaking, the magnitude or power of a radio
`Signal weakens as it travels from its Source. The attenuation
`undergone by an electromagnetic wave in transit between a
`transmitter and a receiver is referred to as path loSS. Path loSS
`may be due to many effects Such as free-space loSS, refrac
`tion, reflection, aperture-medium coupling loSS, and absorp
`tion.
`0013 In business enterprise environments, most loca
`tion-tracking Systems are based on RF triangulation, RF
`fingerprinting or Time Difference Of Arrival (TDOA) tech
`niques. RF triangulation calculates a mobile user's location
`based upon the detected Signal Strength of nearby acceSS
`points (APS). It assumes that Signal strength represents the
`radius of a circle on which the mobile user is located. Given
`Several circles, whose centers are the access points at known
`locations, it attempts to find the interSection of the circles to
`locate the mobile. However, the multipath phenomenon
`encountered in indoor RF environments does present certain
`difficulties for location Systems using triangulation, Since
`reflection and absorption of RF signals affects the correla
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`tion between Signal Strength and proximity. RF fingerprint
`ing compares a mobile Station's view of the network infra
`Structure (i.e., the strength of Signals transmitted by
`infrastructure access points) with a database that contains an
`RF physical model of the coverage area. This database is
`typically populated by either an extensive site Survey or an
`RF prediction model of the coverage area. For example,
`Bahl et al., “A Software System for Locating Mobile Users:
`Design, Evaluation, and LeSSons,” http://research.microSoft
`.com/-bahl/Papers/Pdf/radar.pdf, describes an RF location
`system (the RADAR system) in a WLAN environment, that
`allows a mobile Station to track its own location relative to
`access points in a WLAN environment.
`0.014. The RADAR system relies on a so-called Radio
`Map, which is a database of locations in a building and the
`Signal Strength of the beacons emanating from the acceSS
`points as observed, or estimated, at those locations. For
`example, an entry in the Radio Map may look like (x, y, z,
`SS (i=1 . . . n)), where (x, y, z) are the physical coordinates
`of the location where the signal is recorded, and SS is the
`Signal Strength of the beacon Signal emanating from the ith
`access point. According to Bahl et al., Radio Maps may be
`empirically created based on heuristic evaluations of the
`Signals transmitted by the infrastructure radioS at various
`locations, or mathematically created using a mathematical
`model of indoor RF signal propagation. To locate the
`position of the mobile user in real-time, the mobile Station
`measures the signal strength of each of access points within
`range. It then Searches a Radio Map database against the
`detected Signal Strengths to find the location with the best
`match. Bahl et al. also describe averaging the detected Signal
`Strength Samples, and using a tracking history-based algo
`rithm, to improve the accuracy of the location estimate. Bahl
`et al. also address fluctuations in RF signal propagation by
`using multiple Radio Maps and choosing the Radio Map
`which best reflects the current RF environment.
`0.015 Given the range of potential location-based wire
`leSS applications, further improvements to the accuracy and
`range of wireleSS node location mechanisms are desired. For
`example, while the RADAR system allows a mobile station
`to track its location, it does not disclose a System that allows
`the WLAN infrastructure to track the location of wireless
`nodes without Software installed on the device being
`tracked, allowing devices Such as rogue acceSS points to be
`located. In addition, while the Systems discussed above
`fulfill their respective objectives, they do not disclose or
`Suggest the use of antenna pattern diversity to enhance the
`accuracy of wireleSS node location in an RF environment.
`
`SUMMARY OF THE INVENTION
`0016. The present invention provides methods, appara
`tuses and Systems directed to a wireleSS node RF Finger
`printing location mechanism that uses multiple antenna
`patterns to enhance the accuracy of wireleSS node location in
`an RF environment. In one implementation, Substantially
`non-overlapping antenna pattern diversity is used to provide
`a degree of Sectorization in computing the estimated location
`of a wireleSS node. AS discussed in more detail below, the
`wireleSS node location mechanism can be incorporated into
`wireleSS network environments, Such as 802.11 networks, to
`estimate the location of mobile Stations, rogue access points
`and other wireleSS nodes.
`
`DESCRIPTION OF THE DRAWINGS
`0017 FIG. 1 is a schematic diagram including a wireless
`node location mechanism according to an implementation of
`the present invention.
`0018 FIG. 2A is a functional block diagram illustrating
`an antenna Selector according to an embodiment of the
`present invention.
`0019 FIG. 2B is a functional block diagram showing a
`wireleSS network interface unit according to an embodiment
`of the present invention.
`0020 FIG. 3A is a functional block diagram providing
`an antenna Selector according to a Second embodiment of the
`present invention.
`0021
`FIG. 3B is a functional block diagram setting forth
`an antenna Selector according to a third embodiment of the
`present invention.
`0022 FIG. 4 is a flow chart diagram providing a method,
`according to an embodiment of the present invention,
`directed to the Selection of an antenna during receipt of a
`wireleSS protocol frame.
`0023 FIG. 5 is a flow chart diagram illustrating the
`overall process flow directed to the location of a wireleSS
`node according to an implementation of the present inven
`tion.
`0024 FIG. 6 is a functional block diagram illustrating a
`wireleSS network System according to an implementation of
`the present invention.
`0025 FIG. 7 is a functional block diagram highlighting
`the wireleSS node location functionality of a central control
`element in the wireless network system of FIG. 6.
`0026 FIGS. 8A, 8B and 8C are plots illustrating the
`possible orientation of a plurality of antennas according to
`the offset of peak gain according to different embodiments
`of the present invention.
`0027 FIG. 9 illustrates a Subset of the coverage maps
`asSociated with different direction antennas in the wireleSS
`node location System according to one implementation of
`the present invention.
`
`DESCRIPTION OF PREFERRED
`EMBODIMENT(S)
`0028 A. Wireless Node Location and Antenna Pattern
`Diversity
`0029 FIG. 1 illustrates the basic operating components
`of the wireleSS node location mechanism according to an
`implementation of the present invention. As FIG. 1 shows,
`the wireleSS node location mechanism includes a wireleSS
`node location module 59 and a plurality of infrastructure
`radio transceiverS 58 disposed throughout a physical Space.
`One skilled in the art will recognize that the System depicted
`in FIG. 1 represents a simple example of the basic compo
`nents of the invention and is mostly for didactic purposes. AS
`discussed more fully below, the functionality generally
`denoted by infrastructure radio transceivers 58 and wireless
`node location module 59 can be integrated into a variety of
`Systems, Such as wireleSS Systems dedicated for location of
`wireless nodes, or WLAN or other wireless network sys
`temS.
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`0030) Infrastructure radio transceivers 58 generally com
`prise at least one antenna, a radio transmit/receive unit, and
`control logic (e.g., a 802.11 control unit) to control the
`transmission and reception of radio Signals according to a
`wireleSS communications protocol. Infrastructure radio
`transceiverS 58 are disposed in known locations throughout
`a physical Space. AS discussed below, at least one of the
`infrastructure radio transceivers 58 includes at least two
`directional antennas with, in one implementation, Substan
`tially non-overlapping patterns, and control functionality
`that Selects one of the antennas for receipt of wireleSS
`protocol frames in the RF signals. Other antenna pattern
`configurations can be used in other embodiments of the
`present invention.
`0.031
`A.1. Infrastructure Radio Transceiver
`0032) Infrastructure radio transceivers 58 are operative to
`detect the strength of received radio-frequency (RF) signals,
`such as the signals 57 transmitted by wireless node 56 and
`by other radio transceivers, and provide the detected Signal
`Strength data for corresponding wireleSS nodes to wireleSS
`node location module 59. In one implementation, infrastruc
`ture radio transceiverS 58 are also operative to transmit and
`receive wireleSS or radio-frequency signals according to a
`wireless communications protocol, such as the IEEE 802.11
`WLAN protocol. Infrastructure radio transceivers 58, in one
`implementation, can operate on a Selected channel from a
`plurality of channels in a given band. In another implemen
`tation, infrastructure radio transceiverS 58 can also operate
`in more than one band. For example, infrastructure radio
`receivers 58 may be configured to operate in either the
`802.11a-5 GHz band, and/or the 802.11b/g-2.4 GHz band. In
`one implementation, infrastructure radio transceiverS 58 can
`be configured to collect the Signal Strength information
`asSociated with wireleSS nodes and transmit the collected
`data in response to SNMP or other requests by wireless node
`location module 59. AS discussed below, other methods for
`collecting Signal Strength data may also be employed.
`0.033
`Identification of wireless nodes depends on the
`wireless communications protocol in use. For 802.11
`WLAN environments, for example, wireless nodes can be
`identified based on MAC address. Furthermore, wireless
`nodes can be authorized mobile Stations, Such as remote
`client elements 16, 18 (see FIG. 6), rogue systems (e.g.,
`rogue access points and/or rogue mobile Stations), as well as
`authorized acceSS points for which no location information
`is known. In other implementations, wireleSS nodes can be
`identified based on a unique property of the RF signal, Such
`as a given frequency channel, or a unique signal pattern, and
`the like. For example, the wireleSS node location function
`ality may be employed to locate a detected Source of
`interference, Such as a non-802.11 compliant device.
`0034.
`In one implementation, infrastructure radio trans
`ceivers 58 are also operable to communicate with one or
`more mobile Stations, Such as wireleSS node 56, according to
`a wireleSS communication protocol. For example, radio
`transceiver 58, in one implementation, is an access point or
`other WLAN component. In one implementation, radio
`transceiver 58 is operably connected to a Local Area Net
`work (LAN), Wide Area Network (WAN) or other wireline
`network to bridge traffic between mobile stations and the
`wireline network. As discussed more fully below, radio
`transceiver 58 may also be an access element or light weight
`
`access point in a wireleSS network featuring hierarchical
`processing of protocol information. In one implementation,
`the radio transceiver 58 implements the 802.11 protocols
`(where 802.11, as used herein, generically refers to the
`IEEE 802.11 standard for wireless LANs and all its amend
`ments). Of course, the present invention can be used in
`connection with any Suitable radio-frequency-based wireleSS
`network or communications protocol.
`0035) In one implementation, infrastructure radio trans
`ceiverS 58 make use of the Signal Strength detection func
`tionality residing on a wireleSS network interface adapter.
`For example, the IEEE 802.11 standard defines a mechanism
`by which RF energy is measured by the circuitry (e.g., chip
`Set) on a wireless network adapter or interface card. The
`IEEE 802.11 protocol Specifies an optional parameter, the
`receive signal strength indicator (RSSI). This parameter is a
`measure by the PHY layer of the energy observed at the
`antenna used to receive the current packet or frame. RSSI is
`measured between the beginning of the Start frame delimiter
`(SFD) and the end of the PLCP header error check (HEC).
`This numeric value is an integer with an allowable range of
`0-255 (a 1-byte value). Typically, 802.11 chip set vendors
`have chosen not to actually measure 256 different Signal
`levels. Accordingly, each vendor's 802.11 -compliant
`adapter has a specific maximum RSSI value (“RSSI Max”).
`Therefore, the RF energy level reported by a particular
`vendor's wireleSS network adapter will range between 0 and
`RSSI Max. Resolving a given RSSI value reported by a
`given vendor's chip set to an actual power value (dBm) can
`be accomplished by reference to a conversion table. In
`addition, Some wireleSS networking chip Sets actually report
`received signal Strength in dBm units, rather than or in
`addition to RSSI. Other attributes of the signal can also be
`used in combination with received signal Strength or as an
`alternative. For example, the detected Signal-to-Noise Ratio
`(SNR) during packet reception can be used in determining
`overlay Signal transmit power. Again, many chip Sets include
`functionality and corresponding APIs to allow for a deter
`mination of SNRs associated with packets received from
`other transceivers 58 and/or wireless node 56.
`0036 A.1.a. Non-Overlapping Antenna Patterns and
`Antenna Selection
`0037 AS discussed above, at least one infrastructure
`radio transceiver 58 includes a plurality of directional anten
`nas and functionality that Selects one of the antennas for
`receipt of wireleSS frames encoded in RF signals. Infrastruc
`ture radio transceivers 58 that operate in connection with
`only one antenna may include a directional or an omni
`directional antenna.
`0038 U.S. application Ser. No. 10/611,522, incorporated
`by reference herein, discloses antenna Selection in connec
`tion with non-overlapping antenna pattern diversity. FIGS.
`2A and 2B set forth antenna selection and associated
`functionality included in an infrastructure radio transceiver
`58, according to an implementation of the present invention.
`FIG. 2A illustrates an antenna selector 120, according to an
`embodiment of the present invention. As FIG.2B illustrates,
`the antenna Selector 120, in one embodiment, is part of a
`wireleSS network interface unit 160 comprising antennas
`112a & 112b, antenna selector 120, radio module 130, and
`MAC control unit 140. In one embodiment, the functionality
`described herein can be implemented in a wireleSS network
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`interface chip Set, Such as an 802.11 network interface chip
`set. Radio module 130 includes frequency-based modula
`tion/demodulation functionality for, in the receive direction,
`demodulating radio frequency Signals and providing digital
`data Streams, and in the transmit direction, receiving digital
`data Streams and providing frequency modulated Signals
`corresponding to the digital data Stream. In one embodiment,
`radio module 130 is an Orthogonal Frequency Division
`Multiplexed modulation/demodulation unit. In one embodi
`ment, radio module 30 implements the OFDM functionality
`in a manner compliant with the IEEE 802.11a and 802.11g
`protocol. MAC control unit 140 implements data link layer
`functionality, Such as detecting individual frames in the
`digital data Streams, error checking the frames, and the like.
`In one embodiment, MAC control unit 140 implements the
`802.11 wireless network protocol. Other suitable wireless
`protocols can be used in the present invention.
`0.039 Antenna selector 120 is operative to receive signals
`transduced by antennas 112a, 112b, Select an antenna based
`on detected Signal attributes associated with the antennas,
`and provide the Signal corresponding to the Selected antenna
`to radio module 130. In one implementation, the directional
`antennas have a wide beamwidth directed away from each
`other. In one implementation, each directional antenna has a
`horizontal beamwidth of about 180 degrees. Of course, other
`deployments may require narrower or wider horizontal
`beamwidths. In one implementation, the directional anten
`nas also feature Suitable front-to-back isolation to minimize
`the RF energy that is detected from Signals emanating from
`behind the intended coverage area of the antenna. In one
`implementation, antennas 112a, 112b are directional anten
`nas having Substantially, non-overlapping patterns to
`enhance the Sectorization capabilities discussed herein.
`Although the various Figures Show two antennas, the
`present invention can operate in conjunction with more than
`two directional antennas having Substantially non-overlap
`ping patterns. Antennas 112a, 112b can be any Suitable
`directional antennas, Such as patch antennas, yagi antennas,
`parabolic and dish antennas.
`0040. In one embodiment, the peak gains of the antennas
`are offset from one another in a manner that maximizes
`coverage in all directions. In one embodiment, the peak
`gains of the antennas are oriented relative to each other at an
`angle A about the vertical or Z-axis, where A is equal to
`360/n degreest 10 degrees (where n is the number of anten
`nas). Accordingly, for a two-antenna System (see FIG. 8A),
`the peak gains PG of the antennas are oriented at about 180
`degrees from each other about the Vertical axis. For a
`three-antenna system (see FIG. 8B), the peak gains PG of
`the antennas are oriented at about 120 degrees from each
`other, and So on. In other embodiments, the peak gains of the
`antennas can be offset from one another at other angles
`determined according to other factors or criteria. For
`example, the peak gains of two antennas located at the end
`of a room may be offset at 90 degrees relative to each other
`(see FIG. 8C). As one skilled in the art will appreciate,
`embodiments of the present invention essentially effect a
`Sectorization capability to the infrastructure radio trans
`ceiver 58 including the antenna Selection functionality
`described herein. Embodiments of the present invention
`enhance performance under load conditions in that, by
`Selecting a given antenna, the effect of noise and other Signal
`interference Sources emanating from behind the Selected
`antenna are greatly attenuated or cutoff. Furthermore, this
`
`Sectorization also results in increased performance. For
`example and in one embodiment, the use of a directional
`antenna can result in coverage gains of 6 to 8 dBi, while the
`typical gain associated with an omni directional antenna is
`0 to 2 dBi. Furthermore, the use of directional antennas
`enhances the accuracy of estimating the location of a wire
`leSS node, as the Sectorization provided by the antenna
`computationally eliminates, or reduces the effect of, regions
`outside the beamwidth and/or behind the intended coverage
`area of the Selected antenna. Accordingly, wireleSS node
`location can be enhanced by Strategically placing the infra
`Structure radio transceiverS relative to each other and the
`physical deployment location, and orienting the respective
`directional antennas of the infrastructure radio transceivers
`to correspond to various Sectors of the physical deployment
`location.
`0041 AS FIG. 2A illustrates, antenna selector 120, in one
`embodiment, comprises Switch 122, antenna Selection mod
`ule 124 and detector 126. Switch 122 is operative to Switch
`between a plurality of antennas, Such as antennas 112a,
`112b, under control Signals provided by antenna Selection
`module 124. Detector 126 detects at least one attribute of the
`Signal received at the antennas, as discussed more fully
`below. Antenna Selection module 124 receives signal
`attributes from the detector 126 and provides control signals
`to Switch 122 to Switch among the available antennas.
`Antenna Selection module 124, in one embodiment, further
`includes control logic for Selecting an antenna for receipt of
`a Signal corresponding to a packet or frame, as discussed
`more fully below. AS FIG. 2A illustrates, antenna selector
`120 may further include transmit/receive Switch 128 to
`allow Signals in the transmit direction to by-pass detector
`126. AS discussed below, other architectures are possible.
`0042. Detector 126 can detect one to a plurality of signal
`attributes, Such as Signal Strength, Signal-to-noise ratio, etc.
`In one embodiment, the functionality of detector 126 is
`embodied within an integrated circuit. One skilled in the art
`will recognize that Such signal attribute detection function
`ality is part of Standard 802.11 wireleSS chip Sets, as dis
`cussed above. AS to Signal Strength, the detector 126 can
`provide absolute Signal Strength values, Such as decibels
`(dBs) or relative indicators, such as RSSI values.
`0043 Antenna selection module 124, during the prelimi
`nary or preamble portion of the RF signal, evaluates the
`Signals received at each antenna, Such as antenna 112a and
`112b, and Selects an antenna for receipt of the remaining
`Signal data corresponding to the wireleSS packet or frame.
`For example, according to the 802.11 protocol, MAC sub
`layer data units are mapped into a framing format Suitable
`for wireless transmission. The MAC sublayer data units,
`according to the 802.11 protocol, are essentially encapsu
`lated by a PLCP preamble and a PLCP header, thereby
`forming a PLCP protocol data unit (PPDU). The PLCP
`header generally includes a SYNC field and Start Frame
`Delimiter (SFD). The SYNC field allows the receiver to
`perform necessary operations for Synchronization, while the
`SFD indicates the start of PHY layer-dependent parameters
`in the PLCP header. According to the 802.11 protocol, once
`the Signal associated with the Synchronization field is
`detected, the PHY layer functionality of the receiver
`searches for the SFD to begin processing the PHY-depen
`dent parameters in the PLCP header. In one embodiment,
`during receipt of the preamble, antenna Selection module
`
`Page 14 of 21
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`US 2005/0185618 A1
`
`Aug. 25, 2005
`
`124 evaluates the signals transduced by antennas 112a, 112b
`(as provided by detector 126) and Selects an antenna based
`on the detected Signal attributes. The Selected antenna is then
`used to receive the signal for the remainder of the PPDU.
`0044 FIG. 4 illustrates a method, according to an
`embodiment of the present invention, directed to Selecting
`an antenna during receipt of the frame preamble. In the
`listening mode, the infrastructure radio transceiver 58 can
`operate in either a slow or fast receive diversity Scheme
`when listening for wireleSS frames. For example, in a slow
`receive diversity Scheme, the radio Switches to another
`antenna if no signal is detected on the current antenna within
`a threshold period of time. In a fast receive diversity Scheme,
`the infrastructure radio transceiver 58 in the listen state
`Switches frequently (e.g., every 1 to 3 microSeconds)
`between the available antennas. AS FIG. 4 shows, when a
`frame preamble is detected (202), antenna Selection module
`124 Selects a first antenna and transmits control Signals to
`Switch 122 which Switches the circuit to allow signals
`received at the Selected antenna to pass to detector 126.
`Detector 126, as discussed above, detects at least one
`attribute of the received signal (206). Antenna selection
`module 124 then Selects another antenna, transmitting con
`trol Signals to Switch 122. This proceSS is repeated, in one
`embodiment, for all antennas connected to Switch 122 (204).
`The time spent detecting the signal attribute(s) for each
`antenna depends on both the number of antennas and the
`length of the frame preamble (as defined by the wireless
`networking protocol employed). For example, in a wireless
`network employing the IEEE 802.11g protocol, the long
`PLCP preamble is 128 microseconds. Accordingly, assum
`ing that two antennas are used, antenna Selection module
`124 can allocate a maximum of about 128 microSeconds to
`detect the Signal attributes for each antenna and