(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2003/0207699 A1
`Shpak
`(43) Pub. Date:
`Nov. 6, 2003
`
`US 2003O2O7699A1
`
`(54)
`
`(75)
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`(73)
`(21)
`(22)
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`(63)
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`(60)
`
`ENHANCING WIRELESS LAN CAPACITY
`USING TRANSMISSION POWER CONTROL
`
`Inventor: Eran Shpak, Tel Aviv (IL)
`Correspondence Address:
`WELSH & KATZ, LTD
`120 S RIVERSIDE PLAZA
`22ND FLOOR
`CHICAGO, IL 60606 (US)
`Assignee: Extricom Ltd., Herzeliya (IL)
`Appl. No.:
`10/285,869
`
`Filed:
`
`Nov. 1, 2002
`Related U.S. Application Data
`Continuation-in-part of application No. 10/214,271,
`filed on Aug. 7, 2002.
`Provisional application No. 60/377,653, filed on May
`6, 2002.
`
`Publication Classification
`
`(51) Int. Cl." .............................. H04B 1700; H04B 7/00;
`HO4O 7/20
`(52) U.S. Cl. ............................... 455/525; 455/69; 455/70
`
`(57)
`
`ABSTRACT
`
`A method for mobile communication by access points in a
`wireless local area network (WLAN) includes transmitting a
`first downlink Signal on a common frequency channel from
`a first acceSS point to a first mobile Station, at a first downlink
`power level, which is adjusted responsively to a first uplink
`power level transmitted from the first mobile station to the
`first access point. If the first downlink Signal received at a
`Second access point is below a predetermined interference
`threshold, the Second access point may transmit a Second
`downlink Signal on the common frequency channel to a
`Second mobile Station, Simultaneously with transmission of
`the first downlink Signal from the first access point to the
`first mobile station. The capacity of the WLAN may thus be
`Substantially increased.
`
`3)
`
`NEWCRs
`
`28
`
`
`
`ETHERNET
`BUB
`MAC
`COLLABORATION
`
`Hewlett Packard Enterprise Co. Ex. 1009, Page 1 of 10
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`

`Patent Application Publication
`
`Nov. 6, 2003 Sheet 1 of 2
`
`US 2003/0207699 A1
`
`
`
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`Patent Application Publication
`
`Nov. 6, 2003 Sheet 2 of 2
`
`US 2003/0207699 A1
`
`FG, 2
`
`40 ACCESS POINTS TRANSMIT BEACONS ON ASSIGNED CHANNEL
`
`42 MOBILE "A" RECEIVES BEACON AND SENDS ASSOCIATION REQUEST
`
`44
`
`ACCESS POINTS ARBITRATE TO CHOOSE RESPONDER
`
`6 4
`
`ACCESS POINT "A" WINS AND SENDS RESPONSE
`WITH REDUCED POWER
`
`
`
`
`
`
`
`
`
`ACCESS POINT "B" DETERMINES
`THAT SIGNALS BETWEEN ACCESS POINT "A"
`AND MOBILE "A" ARE BELOW
`INTERFERENCE THRESHOLD
`
`50
`
`ACCESS POINTS AGREE ON NETWORK PARTITION
`
`52MOBILE "B" RECEIVES BEACON AND SENDS ASSOCIATION REQUEST
`
`54
`
`
`
`ACCESS POINT "B" WINS AND SENDS RESPONSE
`WITH REDUCED POWER SIMULTANEOUSLY WITH
`ACCESS POINT "A" DOWNLINK
`
`UPLINK POWER
`LEWEL CHANGE
`
`
`
`
`
`58
`CONTINUE
`PARTITION
`
`60
`
`
`
`REWISE
`PARTITION
`
`Hewlett Packard Enterprise Co. Ex. 1009, Page 3 of 10
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`US 2003/0207699 A1
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`Nov. 6, 2003
`
`ENHANCING WIRELESS LAN CAPACITY USING
`TRANSMISSION POWER CONTROL
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`0001) This application claims the benefit of U.S. Provi
`sional Patent Application No. 60/377,653, filed May 6,
`2002, and is a continuation-in-part of a U.S. patent appli
`cation entitled “Collaboration between Wireless LAN
`Access Points, filed Aug. 7, 2002, whose disclosure is
`incorporated herein by reference.
`
`FIELD OF THE INVENTION
`0002 The present invention relates generally to wireless
`communications, and Specifically to methods and devices
`for improving the performance of wireleSS local area net
`WorkS.
`
`BACKGROUND OF THE INVENTION
`0003 Wireless local area networks (WLANS) are gain
`ing in popularity, and new wireleSS applications are being
`developed. The original WLAN standards, such as “Blue
`tooth” and IEEE 802.11, were designed to enable commu
`nications at 1-2 Mbps in a band around 2.4 GHz. More
`recently, IEEE working groups have defined the 802.11a,
`802.11b and 802.11g extensions to the original standard, in
`order to enable higher data rates. The 802.11a standard, for
`example, envisions data rates up to 54 Mbps over short
`distances in a band near 5 GHZ, while 802.11b defines data
`rates up to 22 Mbps in the 2.4 GHz band. In the context of
`the present patent application and in the claims, the term
`“802.11” is used to refer collectively to the original IEEE
`802.11 Standard and all its variants and extensions, unless
`Specifically noted otherwise.
`0004) The theoretical capability of new WLAN technolo
`gies to offer enormous communication bandwidth to mobile
`users is Severely hampered by the practical limitations of
`wireleSS communications. Indoor propagation of radio fre
`quencies is not isotropic, because radio waves are influenced
`by building layout and furnishings. Therefore, even when
`wireless access points are carefully positioned throughout a
`building, Some “black holes' generally remain-areas with
`little or no radio reception. Furthermore, 802.11 wireless
`linkScan operate at full Speed only under conditions of high
`Signal/noise ratio. Signal Strength Scales inversely with the
`distance of the mobile Station from its acceSS point, and
`therefore So does communication Speed. A single mobile
`Station with poor reception due to distance or radio propa
`gation problems can slow down WLAN access for all other
`users in its basic service set (BSS-the group of mobile
`Stations communicating with the same access point).
`0005 The natural response to these practical difficulties
`would be to distribute a greater number of acceSS points
`within the area to be served. If a receiver receives signals
`Simultaneously from two Sources of Similar Strength on the
`Same frequency channel, however, it is generally unable to
`decipher either signal. The 802.11 standard provides a
`mechanism for collision avoidance known as clear channel
`assessment (CCA), which requires a station to refrain from
`transmitting when it Senses other transmissions on its fre
`quency channel. In practice, this mechanism is of limited
`
`utility and can place a heavy burden on different BSSs
`operating on the same frequency channel.
`0006. Therefore, in 802.11 WLANs known in the art,
`access points in mutual proximity must use different fre
`quency channels. Theoretically, the 802.11b and 802.11g
`standards define 14 frequency channels in the 2.4 GHz band,
`but because of bandwidth and regulatory limitations,
`WLANs operating according to these Standards in the
`United States and most European countries actually have
`only three non-overlapping frequency channels from which
`to choose. (In other countries, Such as Spain, France and
`Japan, only one channel is available.) As a result, in com
`plex, indoor environments, it becomes practically impos
`Sible to distribute wireleSS access points closely enough to
`give Strong Signals throughout the environment without
`Substantial Overlap in the coverage areas of different access
`points operating on the same frequency channel.
`0007 Some WLAN standards provide for transmission
`power control (also known as “transmit power control,” or
`TPC). TPC is applied by access points in order to determine
`the power level of the signals they transmit to mobile
`Stations. It may also be applied by the mobile Stations, as
`well, in transmission to the access points. Typically, in a
`WLAN, TPC limits the power transmitted by the access
`point to the minimum needed to reach the farthest mobile
`Station, and it similarly may limit the power transmitted by
`the mobile station to the minimum needed to reach the
`access point that is Serving it. TPC is mandated for use by
`access points in the 5 GHz band by IEEE Draft Supplement
`802.11 h, entitled “Spectrum and Transmit Power Manage
`ment Extensions in the 5 GHz Band in Europe” (publication
`P802.11h/D2.1 of the IEEE Standards Department, Piscat
`away, N.J., July 2002), which is incorporated herein by
`reference. TPC in the 5 GHz band is required in some
`European countries in order to reduce interference with
`radar. It can also be used for interference reduction, range
`control and reduction of power consumption by access
`points and mobile Stations.
`SUMMARY OF THE INVENTION
`0008. It is an object of some aspects of the present
`invention to provide methods and devices for enhancing the
`coverage of WLAN systems.
`0009. The above-mentioned U.S. patent application,
`entitled “Collaboration between Wireless LAN Access
`Points,” describes a WLAN system comprising multiple
`wireleSS access points distributed within a Service region. In
`order to provide complete coverage of the Service region,
`with Strong communication Signals throughout the region,
`the access points are preferably closely spaced. The areas of
`coverage of the access points, at least when operating at full
`power, may Substantially overlap one another. In order to
`deal with this overlap, the acceSS points communicate
`among themselves using a novel protocol over a high-speed,
`low-latency communication medium. When a mobile Station
`Sends an uplink message attempting to initiate communica
`tions in a given frequency channel, the acceSS points receiv
`ing the message arbitrate among themselves over the
`medium, in order to decide which of the access points will
`communicate with this mobile station. Problems of overlap
`ping coverage areas and collisions are thus resolved.
`0010. In preferred embodiments of the present invention,
`transmit power control (TPC) is used to increase the com
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`munication capacity of the WLAN system still further. This
`additional capacity can be achieved using existing network
`resources, without necessarily increasing the number of
`available transceivers or adding frequency spectrum. After a
`first access point is chosen by arbitration to begin commu
`nicating with a first mobile Station, the access point reduces
`the power level of the downlink Signals that it transmits to
`the mobile station, using a suitable TPC algorithm. Since the
`“winner of the arbitration is typically the closest access
`point to the given mobile Station, and the power measure
`ments are available in real time, it is often possible to reduce
`the transmitted power Substantially, with no power-speed
`tradeoff. The first access point preferably notifies the
`remaining access points of the periods during which it is
`transmitting downlink Signals to the first mobile Station.
`0.011
`Under these conditions, a second access point may
`determine that the downlink Signals from the first acceSS
`point are Sufficiently weak So that the first and Second acceSS
`points can transmit Simultaneously, on the same frequency
`channel, without mutual interference. This determination
`may be made by the Second access point, for example, by
`detecting the weak signals, identifying the Signature of the
`transmitting access point (in accordance with the applicable
`Standard), and ascertaining that a Sufficient signal/interfer
`ence margin exists for its own transmissions even in the
`presence of the weak signal. Then, when a Second mobile
`Station sends an uplink message, and the Second access point
`wins the arbitration with respect to this second mobile
`Station, the Second access point can transmit downlink
`Signals to the Second mobile Station simultaneously with the
`downlink transmissions of the first access point to the first
`mobile Station. The Second access point applies TPC, as
`well, in order not to interfere with the transmissions of the
`first access point.
`0012. This cooperative TPC procedure thus enables the
`access points to divide the WLAN into dynamic, non
`interfering Sub-networks. This sub-network structure allows
`frequency channels to be spatially reused among the acceSS
`points, thus increasing the capacity of the WLAN.
`0013 There is therefore provided, in accordance with a
`preferred embodiment of the present invention, a method for
`mobile communication by access points in a wireless local
`area network (WLAN), wherein the access points include at
`least first and Second acceSS points and are configured to
`communicate on a common frequency channel with mobile
`Stations, including at least first and Second mobile Stations,
`the method including:
`0014 receiving a first uplink signal from the first
`mobile Station at the first acceSS point with a first
`uplink power level;
`0015 transmitting a first downlink signal on the
`common frequency channel from the first access
`point to the first mobile Station in response to the first
`uplink Signal, at a first downlink power level, which
`is adjusted responsively to the first uplink power
`level to be less than a maximum downlink power
`level;
`0016 making a determination that while the first
`access point is transmitting the first downlink Signal
`at the first downlink power level, the first downlink
`Signal received at the Second access point is below a
`predetermined interference threshold;
`
`0017 receiving a second uplink signal from the
`Second mobile Station at the Second acceSS point; and
`0018 responsive to the determination, transmitting
`a Second downlink Signal on the common frequency
`channel from the Second acceSS point to the Second
`mobile Station in response to the Second uplink
`Signal, Simultaneously with transmission of the first
`downlink Signal from the first access point to the first
`mobile Station.
`0019 Typically, the access points have respective service
`areas within a region served by the WLAN, and the access
`points are arranged So that at least Some of the Service areas
`Substantially overlap. Preferably, transmitting the Second
`downlink Signal includes partitioning the region Served by
`the WLAN So as to define non-overlapping first and second
`Sub-regions Served respectively by the first and Second
`access points. Typically, the access points are arranged So
`that when the first access point transmits the first downlink
`Signal at the maximum power level, the first downlink Signal
`received at the Second access point is above the predeter
`mined interference threshold.
`0020. In a preferred embodiment, the first and second
`downlink Signals are transmitted Substantially in accordance
`with IEEE Standard 802.11.
`0021
`Preferably, transmitting the first downlink signal
`includes arbitrating among the access points So as to Select
`the first access point to transmit the first downlink Signal.
`Most preferably, arbitrating among the access points
`includes Sending broadcast messages from the access points
`receiving the first uplink Signal, and Selecting the first access
`point responsive to the broadcast messages.
`0022. Additionally or alternatively, transmitting the first
`downlink Signal includes making a first association between
`the first access point and the first mobile Station, and
`informing the Second access point of the first association,
`and making the determination includes measuring the first
`downlink Signal received at the Second access point respon
`sively to the first association. Preferably, informing the
`Second acceSS point of the first association includes notify
`ing the Second access point of one or more time intervals
`during which the first access point is to transmit the first
`downlink Signal, and measuring the first downlink Signal
`includes monitoring Signals received by the Second access
`point during at least one of the time intervals. Most prefer
`ably, transmitting the Second downlink Signal includes trans
`mitting the Second downlink signal during the one or more
`time intervals after determining that the first downlink Signal
`received at the Second access point is below the predeter
`mined interference threshold.
`0023. Additionally or alternatively, making the determi
`nation includes receiving the first downlink Signal at the
`Second acceSS point, and identifying a signature of the first
`access point in the received first downlink Signal.
`0024 Preferably, transmitting the second downlink sig
`nal includes Sending a message from the Second access point
`to the first access point So as to inform the first access point
`that the Second access point is transmitting Simultaneously
`with the first acceSS point.
`0025) Further preferably, receiving the second uplink
`Signal includes receiving the Second uplink Signal at the
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`Second access point with a Second uplink power level, and
`transmitting the Second downlink Signal includes adjusting a
`Second downlink power level of the Second downlink Signal
`in response to the Second uplink power level, So that the
`Second downlink Signal received at the first access point is
`below the predetermined interference threshold. Typically,
`receiving the Second uplink Signal includes Sensing a change
`in the Second uplink power level, and transmitting the
`Second downlink Signal includes, responsive to the change,
`ceasing to transmit the Second downlink Signal Simulta
`neously with the transmission of the first downlink Signal.
`0026. There is also provided, in accordance with a pre
`ferred embodiment of the present invention, apparatus for
`mobile communication, including a plurality of acceSS
`points arranged in a wireless local area network (WLAN)
`and configured to communicate on a common frequency
`channel with mobile Stations, including at least first and
`Second mobile Stations, the acceSS points including at least
`first and Second access points,
`0027 wherein in response to receiving a first uplink
`Signal from the first mobile Station at the first access
`point with a first uplink power level, the first acceSS
`point transmits a first downlink Signal on the com
`mon frequency channel to the first mobile Station at
`a first downlink power level, which is adjusted
`responsively to the first uplink power level to be less
`than a maximum downlink power level, and
`0028 wherein the second access point is adapted to
`make a determination that while the first access point
`is transmitting the first downlink Signal at the first
`downlink power level, the first downlink Signal
`received at the Second access point is below a
`predetermined interference threshold, and
`0029 wherein in response to receiving a second
`uplink Signal from the Second mobile Station at the
`Second access point, and responsive to the determi
`nation, the Second access point transmits a Second
`downlink Signal on the common frequency channel
`to the Second mobile Station Simultaneously with
`transmission of the first downlink Signal from the
`first access point to the first mobile Station.
`0030 The present invention will be more fully under
`stood from the following detailed description of the pre
`ferred embodiments thereof, taken together with the draw
`ings in which:
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`0.031
`FIG. 1 is a block diagram that schematically illus
`trates a WLAN system, in accordance with a preferred
`embodiment of the present invention; and
`0.032
`FIG. 2 is a flow chart that schematically illustrates
`a method for communication between multiple mobile Sta
`tions and multiple wireleSS access points, in accordance with
`a preferred embodiment of the present invention.
`
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS
`FIG. 1 is a block diagram that schematically illus
`0.033
`trates a wireless LAN (WLAN) system 20, in accordance
`with a preferred embodiment of the present invention.
`
`System 20 comprises multiple access points 22, 23, 25, 27,
`which are configured for data communication with multiple
`mobile stations 24, 29, 31. The mobile stations typically
`comprise computing devices, Such as desktop, portable or
`handheld devices, as shown in the figure. In the exemplary
`embodiments described hereinbelow, it is assumed that the
`access points and mobile Stations communicate with one
`another in accordance with one of the standards in the IEEE
`802.11 family and observe the 802.11 medium access con
`trol (MAC) layer conventions. Details of the 802.11 MAC
`layer are described in ANSI/IEEE Standard 801.11 (1999
`Edition), and specifically in Part 11: Wireless LAN Medium
`Access Control (MAC) and Physical Layer (PHY) Specifi
`cations, which is incorporated herein by reference. The
`principles of the present invention, however, are not limited
`to the 802.11 standards, and may likewise be applied to
`substantially any type of WLAN, including HiperLAN,
`Bluetooth and hiswan-based systems.
`0034. Access points 22, 23, ..., are typically connected
`to an Ethernet hub 26 by a wired LAN 28. The LAN serves
`as a distribution system (DS) for exchanging data between
`the access points and the hub. This arrangement enables
`mobile Stations 24 to Send and receive data through the
`access points to and from an external network 30, Such as the
`Internet, via an access line 32 connected to hub 26. LAN 28
`is typically capable of carrying data at high Speeds, but
`message latency on the LAN is high, due mainly to collision
`avoidance mechanisms used in Ethernet and other conven
`tional LANs.
`0035. The present invention, however, requires the access
`points to exchange high-speed messages with low latency,
`using a novel protocol to provide MAC-level collaboration
`between the access points. For the purpose of this protocol,
`the access points are preferably interconnected by a shared
`communication medium 34 to a MAC collaboration hub 36,
`in addition to their connection to LAN 28. Medium 34
`allows all access points 22 to broadcast and receive low
`latency messages to and from all other access points. Exem
`plary implementations of medium 34 and hub 36 are
`described in the above-mentioned U.S. patent application,
`entitled “Collaboration between Wireless LAN Access
`Points.
`0036) Alternatively, instead of using dedicated medium
`34 for low-latency messaging, the access points may use a
`low-latency protocol over LAN 28 itself. This alternative
`messaging method is described in detail in another U.S.
`patent application, filed Oct. 17, 2002, entitled “Collabora
`tion Between Wireless LAN Access Points. Using Wired
`LAN Infrastructure,” which is assigned to the assignee of the
`present patent application and is incorporated herein by
`reference. Further alternatively, Substantially any means
`known in the art may be used for communication between
`the access points, as long as it meets the Speed and latency
`constraints imposed by the applicable WLAN standard or
`other requirements.
`0037 For the sake of the description that follows, it is
`assumed that access points 22, 23, 25 and 27 all transmit and
`receive signals on the same frequency channel, to which
`mobile stations 24, 29 and 31 are likewise tuned. Typically,
`as noted in the Background of the Invention, WLAN system
`20 includes additional access points operating on other
`frequency channels, but these additional acceSS points do not
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`interfere with communications on the frequency channel of
`access points 22, 23, 25 and 27, and therefore are not of
`concern here. Rather, the methods of access point collabo
`ration of the present invention, as described hereinbelow
`with reference to access points 22, 23, 25 and 27, may be
`carried out independently by the Set of access points on each
`of the operative frequency channels in System 20.
`0.038. Downlink signals transmitted at full power by any
`of acceSS points 22, 23, 25 and 27 can, in principle, be
`received by any of mobile stations 24, 29 and 31. In WLAN
`Systems known in the art, if adjacent access points 22 and 23
`were to transmit Simultaneously on the same frequency
`channel, for example, mobile Station 24 would receive
`downlink Signals from both access points. This overlap
`would probably result in inability of the mobile station to
`communicate with any of the access points. In preferred
`embodiments of the present invention, however, acceSS
`points 22, 23, 25 and 27 communicate with one another over
`medium 34 (or LAN 28) in order to resolve this conflict
`using a MAC-level collaboration protocol, as described in
`the above-mentioned patent applications.
`0039. Furthermore, the MAC-level collaboration proto
`col of the present invention allows the access points to
`dynamically define portions of the service area of system 20
`as Spatial Sub-networks, Such as a Sub-network 38 shown in
`FIG. 1. The method by which these sub-networks are
`defined is described in detail below with reference to FIG.
`2. In brief, by way of example, after mobile station 24 has
`exchanged association messages with access point 22 (as
`required in order to begin communications under the 802.11
`Standards), access point 22 uses transmit power control
`(TPC) to reduce its transmission power in downlink mes
`Sages to mobile Station 24. The power is preferably reduced
`to a minimum level that will allow the mobile station to
`receive the downlink messages reliably at the highest poS
`Sible Speed.
`0040. At this transmission power level of access point 22,
`nearby access point 23 and mobile station 29 may still
`receive the downlink messages from access point 22, but
`access points 25 and 27 and mobile station 31 will not.
`Therefore, if mobile station 31 becomes associated with
`access point 27, for example, it is then possible for acceSS
`point 27 to transmit downlink messages to mobile Station 31,
`with power level reduced by TPC, simultaneously with the
`downlink transmission by access point 22 to mobile Station
`24. System 20 is thus partitioned dynamically into two
`Sub-networks, each with its own Service Sub-region, oper
`ating Simultaneously. Larger numbers of Simultaneous Sub
`networks may be defined in like fashion. These sub-net
`Works are used when the participating acceSS points transmit
`downlink Signals at low power to nearby mobile Stations.
`Such simultaneous downlink communications may be inhib
`ited when one of the acceSS points transmits downlink
`Signals to a more distant mobile station (Such as when access
`point 23 transmits to mobile Station 29), requiring higher
`power. The Sub-network partition may be restored thereafter,
`as acceSS points 22 and 27 resume their respective low
`power transmissions to mobile stations 24 and 31.
`0041
`FIG. 2 is a flow chart that schematically illustrates
`a method for collaboration between the access points in
`WLAN system 20, in accordance with a preferred embodi
`ment of the present invention. Access points 22, 23, 25 and
`
`27 transmit beacon Signals on their common frequency
`channel, at a beacon transmission Step 40. In accordance
`with the 802.11 standard, the beacon signals transmitted by
`any given access point provide the time base with which the
`mobile Station should synchronize its communications and
`indicate the BSS identification (BSSID) of the access point.
`The BSSID can be regarded as the MAC address of the
`access point. In 802.11 WLAN systems known in the art,
`each access point has its own unique BSSID. In system 20,
`however, access points 22, 23, 25 and 27 share the same
`ESSID, so that they appear logically to the mobile stations
`to be a single, extended, distributed acceSS point, which has
`multiple antennas at different locations. The time bases of
`the access points are mutually Synchronized using medium
`34 (or using low-latency messages on LAN 28), and the
`beacon Signals transmitted by the access points are inter
`laced to avoid collision between them.
`0042. When mobile station 24, for example, receives a
`beacon signal of sufficient strength, it extracts the BSSID
`and time base from the Signal, at a first beacon processing
`step 42. (Mobile station 24 is identified as “MOBILE A” in
`FIG. 2.) This step, as well as subsequent steps taken by the
`mobile Stations in System 20, is completely in accordance
`with the 802.11 standard. In other words, the present inven
`tion can be implemented in a manner that is transparent to
`and requires no modification of legacy mobile Stations.
`Using the time base and BSSID it has acquired, mobile
`Station 24 Sends an uplink Signal, in the form of an asso
`ciation request message that is addressed to the BSSID and
`indicates the MAC address of the mobile Station.
`0043. To determine which of the access points will
`respond to the association request message, the access points
`carry out an arbitration procedure using low-latency mes
`saging over medium 34 or LAN 28, at an arbitration step 44.
`For this purpose, all acceSS points that received the asso
`ciation request message from mobile Station 24 broadcast
`messages, giving notice to the other access points that they
`have received an uplink message. The messaging and arbi
`tration procedures are described in detail in the above
`mentioned patent application entitled “Collaboration
`between Wireless LAN Access Points.”
`0044. Each access point is able to determine whether it
`was first to Send its message, or whether another access point
`preceded it, by comparing the time of receipt of these
`broadcast messages to the time at which the access point Sent
`its own broadcast message. Typically, the access point that
`was able to be first in Sending out its broadcast message in
`response to an uplink message from a given mobile Station
`is in the best position to continue communications with the
`mobile Station, Since this access point is generally the closest
`one to the mobile Station. Therefore, all the acceSS points
`independently choose this first access point to respond to
`mobile Station 24. Alternatively, other criteria, Such as
`received signal power, may be applied in choosing the
`“winning acceSS point, as long as the criteria are applied
`uniformly by all the access points. Preferably, if a deadlock
`occurs (such as when two access points send their broadcast
`messages at the same instant), a predetermined formula is
`applied by all the acceSS points to resolve the deadlock
`uniformly.
`004.5 The winning access point sends an acknowledg
`ment (ACK) message to mobile Station 24, at a first response
`
`Hewlett Packard Enterprise Co. Ex. 1009, Page 7 of 10
`Hewlett Packard Enterprise Co. v. Intellectual Ventures II LLC
`IPR2021-01376
`
`

`

`US 2003/0207699 A1
`
`Nov. 6, 2003
`
`step 46, as required by the applicable WLAN standard. In
`the present example, we assume that access point 22 (iden
`tified as “ACCESS POINTA” in FIG.2) is the winner. After
`Sending the ACK, access point 22 typically sends an asso
`ciation response message to mobile Station 24, and then
`continues its downlink transmission to the mobile Station as
`appropriate. To determine the power level at which to Send
`its downlink messages to mobile Station 24, acceSS point 22
`measures the Signal power of the uplink messages it has
`received from this mobile Station. If the uplink Signal power
`is significantly Stronger than a predetermined minimum
`threshold, the acceSS point applies a TPC procedure to
`determine the power level at which it should transmit
`downlink Signals to this mobile Station.
`0046) Any suitable TPC algorithm may be used for this
`purpose, including the TPC algorithm defined in the above
`mentioned 802.11h Draft Standard. As a rule, the downlink
`power transmitted by the acceSS point to a given mobile
`Station is inversely proportional to the uplink power that it
`received from that mobile Station. Thus, for example, acceSS
`point 22 may transmit downlink signals to mobile station 29
`with higher power (possibly even full power) than it trans
`mits to mobile station 24. The optimal downlink power level
`in each case can be adjusted by trial-and-error transmission
`to each different mobile station. If there is no limitation on
`the downlink power imposed by any other acceSS point, it is
`possible (for the sake of communication robustness) for
`access point 22 to continue transmitting at times at its
`maximum level. Otherwise, access point 22 preferably esti
`mates the interference its transmissions will cause at other
`access points, and takes this interference into account in
`Setting its own downlink transmission level.
`0047 While access point 22 transmits downlink signals
`to mobile Station 24, other acceSS points, Such as acceSS
`point 27 (identified in FIG. 2 as “ACC