(12) United States Patent
`Zyren
`
`I IIIII IIIIIIII Ill lllll lllll lllll lllll lllll lllll lllll lllll 111111111111111111
`US006377608Bl
`US 6,377,608 Bl
`Apr. 23, 2002
`
`(10) Patent No.:
`(45) Date of Patent:
`
`(54) PULSED BEACON-BASED INTERFERENCE
`REDUCTION MECHANISM FOR WIRELESS
`COMMUNICATION NETWORKS
`
`(75)
`
`Inventor: James G. Zyren, Indialantic, FL (US)
`
`(73) Assignee: Intersil Americas Inc., Irvine, CA (US)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by O days.
`
`(21) Appl. No.: 09/163,802
`
`(22) Filed:
`
`Sep. 30, 1998
`
`Int. Cl.7 .................................................. H04K 1/00
`(51)
`(52) U.S. Cl. ....................................................... 375/132
`(58) Field of Search ................................. 375/130, 132,
`375/140, 141, 220; 370/320, 335, 342,
`441, 479; 455/403, 418, 434, 440, 456,
`462, 556, 515, 501, 524, 63, 67.3
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`5,542,120 A * 7/1996 Smith et al. ................ 455/425
`5,784,368 A * 7/1998 Weigand et al. ............ 370/350
`5,809,421 A * 9/1998 Manssen et al. ............ 455/434
`5,926,470 A * 7/1999 Tiedmann, Jr.
`. ............ 370/334
`
`6,028,853 A * 2/2000 Haartsen ..................... 370/338
`6,128,290 A * 10/2000 Carvey ....................... 370/347
`6,226,317 Bl * 5/2001 Brockert et al. ............ 375/146
`6,246,884 Bl * 6/2001 Karmi et al.
`............... 455/521
`* cited by examiner
`Primary Examiner-Chi Pham
`Assistant Examiner-Emmanuel Bayard
`(74) Attorney, Agent, or Firm-Gary R. Stanford
`
`(57)
`
`ABSTRACT
`
`A repetitively pulsed beacon based mechanism prevents
`interference between wireless communication devices of
`users of a band wireless local area network WLAN, and
`wireless communication devices of ad hoc networks using
`the same ISM band. A beacon generator is installed in the
`vicinity of an access point of the WLAN infrastructure, and
`generates a wireless beacon in a portion of the ISM band that
`does not overlap that portion used by the WLAN. A beacon
`responsive radio control mechanism, installed in each ad hoc
`radio, monitors the beacon channel for the presence of the
`wireless beacon. In response to detecting the beacon, the
`radio control mechanism adjusts the operation of its ad hoc
`radio, to avoid simultaneous use of the same portion of the
`ISM band as the WLAN. In one example, the ad hoc radio
`is adjusted so that it transmits within a selected portion of the
`ISM band other than that used by the WLAN.
`
`44 Claims, 6 Drawing Sheets
`
`20
`
`Infrastructure BSA
`
`21~
`
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`

`

`U.S. Patent
`
`Apr. 23, 2002
`
`Sheet 1 of 6
`
`US 6,377,608 Bl
`
`20
`
`Infrastructure BSA
`
`2 1~ 10 __ _
`
`FIG.1
`
`Bi-directional
`FHSS ad hoc
`piconet
`
`-------------------
`
`FIG. 2
`
`-J i- 1 O MHz guard band on low side of ISM band
`es,.......j~,....,...te..,,..,..,..·~..,..,..,:n I
`t:·1 Restricted;\ :j
`ISM Band
`
`W"""'jj..,.,..,R .......
`
`31
`
`2.4boo
`GHz
`
`307
`
`FIG. 3
`
`2.4~35 327
`GHz
`
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`

`U.S. Patent
`
`Apr. 23, 2002
`
`Sheet 2 of 6
`
`US 6,377,608 Bl
`
`DSSS
`
`DSSS
`
`DSSS
`
`1-34
`2.4000
`Ch. 1
`GHz
`
`Ch. 6
`
`Ch. 11
`
`FIG. 4
`
`1-35
`
`2.4835
`GHz
`
`1-34
`2.4000
`Ch. 1
`GHz
`
`Ch. 6
`
`Ch. 11
`
`FIG. 5
`
`2.4835
`GHz
`
`60
`
`2.4000
`GHz
`
`Ch. 1
`
`Ch. 6
`
`Ch. 11
`
`FIG. 6
`
`l-35
`
`2.4835
`GHz
`
`Beacons located at both extremes of ISM band
`~
`~
`72
`
`71
`
`1-34
`2.4000
`Ch. 1
`GHz
`
`Ch. 6
`FIG. 7
`
`Ch. 11
`
`1-35
`2.4835
`GHz
`
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`

`U.S. Patent
`
`Apr. 23, 2002
`
`Sheet 3 of 6
`
`US 6,377,608 Bl
`
`10
`
`20
`
`&25
`
`DSSS
`node
`
`Infrastructure BSA
`21
`80
`
`access point
`
`beacon
`
`2 5~
`DSSS
`node
`
`FIG. 8
`
`24 msec
`beacon
`interval
`
`240 µsec
`beacon - (cid:173)
`duration
`
`20 dBm
`
`90
`
`90
`
`FIG. 9
`
`90
`
`time
`
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`

`U.S. Patent
`
`Apr. 23, 2002
`
`Sheet 4 of 6
`
`US 6,377,608 Bl
`
`112
`111
`113
`Frame Duration/ Infrastructure
`Header
`Beacon Preamble
`Sync
`SFD PLW PSF HEC Control
`ID
`Data
`
`FCS
`
`116
`
`114
`240 bits
`
`FIG. 10
`
`24 msec
`r- beacon --J
`I
`I
`interval
`90
`
`95
`
`r
`
`20 dBm
`
`90
`
`24 msec
`
`interval
`95
`
`I beacon I
`:t (9
`C'-1 co
`'<::t-
`N
`
`:5:!
`(9
`C'-1 co
`'<::t-
`N
`
`FIG. 11
`
`time
`
`133
`
`PROGR.
`SYNTH.
`
`TRANSCVR
`
`135
`
`131
`
`.__~~~--1µCONTROLLER
`
`FIG. 12
`
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`

`U.S. Patent
`
`Apr. 23, 2002
`
`Sheet 5 of 6
`
`US 6,377,608 Bl
`
`Active FHSS Channels
`beacon
`125 ~ I 120 ~ "
`
`60
`
`2.4000
`GHz
`
`Ch. 11
`
`2.4835
`GHz
`
`FIG.13
`
`FIG. 14
`
`145
`
`2.4000
`GHz
`
`60
`
`2.4835
`GHz
`
`Fixed Ad Hoc Channels
`
`beacon " 60
`-- 150 ~
`
`2.4000
`GHz
`
`Ch. 1
`
`Ch. 6
`
`Ch. 11
`
`2.4835
`GHz
`
`FIG.15
`
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`

`U.S. Patent
`
`Apr. 23, 2002
`
`Sheet 6 of 6
`
`US 6,377,608 Bl
`
`TABLE
`
`FHSS Channel Designation
`76
`77
`78
`79
`80
`
`Center Frequency (GHz)
`2.476
`2.477
`2.478
`2.479
`2.480
`
`FIG.16
`
`ethernet
`
`80
`
`beacon
`
`170
`serial link
`
`FIG.17
`
`180
`ethernet
`
`access point
`
`access point
`
`beacon
`
`FIG.18
`
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`US 6,377,608 Bl
`
`1
`PULSED BEACON-BASED INTERFERENCE
`REDUCTION MECHANISM FOR WIRELESS
`COMMUNICATION NETWORKS
`
`FIELD OF THE INVENTION
`
`The present invention relates in general to wireless local
`area networks, and is particularly directed to an access
`point-associated, pulsed beacon-based mechanism for
`reducing the potential for or avoiding interference between
`wireless communication devices of users of a communica(cid:173)
`tion network, such as one employing the unlicensed
`industrial, scientific and medical (ISM) band, and wireless
`communication devices of non-users of the network, who
`are located within a basic service area of the network, and
`are capable of transmitting within the same bandwidth
`portion of the wireless communication spectrum as, and
`thereby potentially interfering with, the users of the network.
`
`BACKGROUND OF THE INVENTION
`
`2
`two beacons alternatively may be generated at lower and
`upper unused band regions of the ISM band. For example,
`by placing a first warning beacon at 2.401 GHz and a second
`warning beacon at 2.482 GHz, the probability of loss of
`5 beacon signal due to simultaneous fading on two non(cid:173)
`correlated channels is significantly reduced.
`The warning beacon is sourced from a beacon generator
`that is spatially located in close proximity to and linked with
`infrastructure access point. By close proximity is meant that
`10 the beacon generator and access point are spaced sufficiently
`far apart to provide isolation that minimizes adjacent chan(cid:173)
`nel interference; yet they are still close enough to ensure the
`AP and beacon coverage areas are approximately the same.
`The beacon generator is operative to periodically generate a
`15 pulsed beacon signal which, because of its physical prox(cid:173)
`imity to access point, serves to indicate the presence of
`infrastructure WLAN and its associated BSA The beacon
`generator may be serially interfaced with the access point in
`order to convey information about AP spectrum utilization to
`20 the mobile nodes. Alternatively, the beacon generator may
`communicate with the access point via a wired LAN
`(Ethernet). Communication from the access point to the
`beacon generator need not occur frequently, since it is only
`necessary to communicate when the access point is turned
`25 on or off, or when the beacon frequency channel is changed.
`By periodically tuning the receive frequency synthesizer
`of its transceiver to this frequency, a node in an ad hoc
`network radio, such as an FHSS radio, is able to monitor
`whether it is in close proximity to an infrastructure network.
`30 Because FCC regulations permit a low power transmitter to
`periodically increase its transmit power by as much as
`20+dBm, as long as average power remains below O dbm,
`the beacon signal emitted by beacon generator preferably
`has a pulsed beacon profile. The beacon may also be
`35 modulated with information relating the operation of the
`infrastructure network, such as the center frequency of the
`occupied channel, where the BSA is that of a DSSS infra(cid:173)
`structure WLAN. In the case of an infrastructure FHSS
`network, the hop sequence and system clock data are embed-
`40 ded in the beacon signal.
`In order to monitor and respond to the pulsed warning
`beacon that is sourced from the vicinity of the access point
`of the infrastructure of a WLAN, it is only necessary to
`modify the control software employed by the microcontrol-
`45 ler of an ad hoc participant's radio, to incorporate a synthe(cid:173)
`sizer tuning mechanism that controls a programmable
`synthesizer, through which operation of the radio's trans(cid:173)
`ceiver is controlled, so that operation of each of the ad hoc
`radio and the WLAN may proceed on a non-interfering
`50 basis. No modification to infrastructure mode operation is
`required. Non-limiting examples of such non-interfering
`operation include deferral mode, altered hop pattern mode,
`fixed frequency mode, deactivation mode, and a reduced
`transmit power mode.
`In deferral mode, the tuning control mechanism employed
`by the ad hoc radio network is operative to defer
`transmission, in response to detecting that ad hoc radio and
`the infrastructure networks occupy the same frequency. This
`technique may be employed regardless of which type of
`60 infrastructure network is encountered. The ad hoc radio uses
`information contained in the monitored beacon which
`informs FHSS radios operating in the ad hoc mode of the
`operating parameters of the infrastructure network. For an
`IEEE 802.11 standards DSSS network, which occupies fixed
`65 channels which are about 25 MHz apart, to avoid
`interference, the FHSS ad hoc radio defers on 25 out of 79
`1 MHz channels, or about 30% of the FHSS channels. For
`
`Because the (2.400-2.4835 GHz) industrial, scientific and
`medical (ISM) band is unlicensed and reasonably wide, it is
`anticipated to soon undergo substantial crowding, as wire(cid:173)
`less devices capable of transmitting at data rates in excess of
`10 Mbps are expected to become increasingly common in
`the enterprise environment in the next few years. For
`example, as illustrated diagrammatically in FIGS. 1 and 2,
`it can be expected that one or more ad hoc networks ( or
`'piconets') 10, such as those comprised of a wireless phone
`11 and laptop computer 13 wirelessly linked to each other
`( e.g., via associated Blue tooth radios), will be carried into a
`building having an infrastructure high speed wireless local
`area network (WLAN). Note that ad hoc network 10 is one
`which is temporary in nature, and typically contains two or
`more mobile nodes which communicate with each other and
`do not make use of the infrastructure WLAN.
`Although not directly communicating with the access
`point (AP) 21 of the WLAN infrastructure's basic service
`area (BSA) 20, by transmitting in the ISM band, wireless
`communications between the cell phone 11 and laptop
`computer 13, such as downloading of e-mail to the laptop,
`for example, may interfere with, or suffer interference from,
`the infrastructure WLAN. Namely, as there will be multiple
`devices operating in the same unlicensed frequency band
`(i.e., 2.400-2.4835 GHz) as Bluetooth, HomerRF, and IEEE
`802.11 specification-based frequency hopped spread spec(cid:173)
`trum (FHSS) radios, it is essential that the various radio
`systems utilizing the ISM band be capable of at least some
`degree of coexistence. This interference issue is of concern
`to all users of the band and, because most wireless commu(cid:173)
`nication devices are mobile, it is currently substantially
`impossible to predict the severity of the interference prob(cid:173)
`lem. Indeed, at present, none of the above-referenced sys(cid:173)
`tems are capable of operating in the presence of any of the
`others without causing potentially serious levels of interfer- 55
`ence.
`
`SUMMARY OF THE INVENTION
`
`In accordance with the present invention, this problem is
`effectively obviated by taking advantage of the availability
`of a prescribed unused region of portion of the ISM band ( at
`either or both ends of the ISM band) to generate a pulsed
`warning beacon, that serves to alert ad hoc network radios
`(such as frequency hopped spread spectrum (FHSS) radios),
`that they are spatially close to a WLAN infrastructure access
`point, and thereby in the range of potential interference with
`the WLAN. To combat potential fading of the beacon signal,
`
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`

`US 6,377,608 Bl
`
`4
`FIG. 12 diagrammatically illustrates an ad hoc radio;
`FIGS. 13 and 14 are ISM spectrum diagrams associated
`with deferral mode of operation of an ad hoc radio;
`FIGS. 15 and 16 are ISM spectrum diagrams associated
`5 with fixed frequency mode of operation of an ad hoc radio;
`FIG. 17 diagrammatically shows a beacon generator seri(cid:173)
`ally linked to a WLAN infrastructure access point; and
`FIG. 18 diagrammatically shows a beacon generator
`10 linked to a WLAN infrastructure access point by means of
`a wired LAN.
`
`DETAILED DESCRIPTION
`
`3
`an FHSS WLAN infrastructure, in which each channel has
`an instantaneous bandwidth of only 1 MHz, a lower reduc(cid:173)
`tion in throughput is obtained, as the ad hoc network radio
`defers on only 3 out of 79 channels. As a result, throughput
`reduction is less than 4%.
`Altered hop pattern mode may be used for FHSS systems,
`such as Bluetooth, that have nominal RF output of O dBm,
`and are able to operate in accordance with FCC low power
`rules, where there is no requirement to employ spread
`spectrum modulation, so that changing hop sequences to
`avoid interference is permissible. The advantage to this
`approach is that the band occupied by the infrastructure
`network may be avoided completely. Also, there is no
`reduction in throughput in either the ad hoc radio or the
`infrastructure network.
`Fixed frequency mode is employed for avoiding interfer(cid:173)
`ing with DSSS WLANS, which use center frequencies
`shifted toward the lower end of the ISM band to avoid
`excessive out-of-band emissions in the lower restricted
`band. This frequency shift provides for the use of a set of 1 20
`MHz center frequencies in a range of from 2.476 to 2.480
`GHz, so as to provide interference free operation for both
`Bluetooth and DSSS WLAN radios. The fixed frequency
`scheme only requires that the Bluetooth radio detect a
`transmitter on the beacon frequency. It is unnecessary for the 25
`beacon signal to be encoded with any other information. The
`fixed frequencies do not interfere with any DSSS system,
`regardless of which channels the infrastructure DSSS net(cid:173)
`work is using.
`Deactivation mode is intended to be employed where 30
`operation of an ad hoc network would result in unacceptable
`levels of interference to an infrastructure WLAN or other
`equipment, and local use of ad hoc radio networks could be
`prohibited and might be desirable. To accommodate this
`preference, the beacon includes information that causes the 35
`ad hoc radio detecting the beacon to simply disable its
`transmitter, for as long as the radio is within range of the
`beacon.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 diagrammatically illustrates a wireless local area
`network basic service area containing ad hoc wireless
`radios;
`FIG. 2 shows an example of an ad hoc wireless radio
`network;
`FIG. 3 is a spectrum diagram of the ISM band and
`adjacent restricted bands;
`FIG. 4 is an ISM spectrum diagram showing center
`frequencies for direct-sequence spread spectrum (DSSS)
`WLANs skewed toward the low end of the ISM band;
`FIG. 5 shows the ISM spectrum diagram of FIG. 4 with
`a warning beacon placed at 2.401 GHz;
`FIG. 6 shows the ISM spectrum diagram of FIG. 4 with
`a warning beacon placed at 2.482 GHz;
`FIG. 7 shows the ISM spectrum diagram of FIG. 4 with
`a pair of warning beacons placed at 2.401 GHz and 2.482
`GHz;
`FIG. 8 diagrammatically illustrates a wireless local area
`network basic service area containing a beacon generator in
`the vicinity of an infrastructure access point;
`FIG. 9 is a timing diagram of a non-limiting example of
`a pulsed beacon profile that may be employed in accordance
`with the present invention;
`FIG. 10 is an example of a beacon signal framing format; 65
`FIG. 11 is a timing diagram showing a pair of time(cid:173)
`interleaved or alternating beacon signals;
`
`Before describing in detail the repetitively pulsed beacon-
`15 based, wireless interference prevention mechanism of the
`present invention, it should be observed that the invention
`resides primarily in what is effectively a prescribed arrange(cid:173)
`ment of conventional communication circuits and associated
`digital signal processing components and attendant super(cid:173)
`visory control programs therefor, for controllably generating
`a periodically pulsed warning beacon in the vicinity of a
`network access point, and a prescribed software tuning
`mechanism that is installable in the microcontroller of a
`respective wireless radio for monitoring and responding to
`the warning beacon.
`Consequently, the configuration of such communication
`and signal processing circuits and components and the
`manner in which they are interfaced with other communi(cid:173)
`cation system equipment have, for the most part, been
`illustrated in the drawings by readily understandable block
`diagrams, which show only those specific details that are
`pertinent to the present invention, so as not to obscure the
`disclosure with details which will be readily apparent to
`those skilled in the art having the benefit of the description
`herein. Thus, the block diagram illustrations and functional
`and spectral diagrams associated therewith are primarily
`intended to show the major components of the invention in
`a convenient functional grouping and communication signal
`processing scenario, whereby the present invention may be
`40 more readily understood.
`In order to facilitate an understanding of the interference
`avoidance mechanism of the present invention, particularly
`in the context of the spectral characteristics of the ISM band,
`45 it is initially useful to review current usage of the ISM band,
`and FCC restrictions on bands adjacent to this band. As
`diagrammatically illustrated in the spectrum diagram of
`FIG. 3, according to FCC regulation 15.205, only spurious
`emissions are allowed in a band 31, having a spectral range
`50 from 2.310 to 2.390 GHz, that is immediately adjacent to the
`lower end of the ISM band 30, and in a band 32, having a
`spectral range from 2.4835 to 2.500 GHz, that is immedi(cid:173)
`ately adjacent to the upper end of the ISM band 30.
`Power limitations are also quite restrictive, in that total
`55 spurious power emissions from intentional radiators are
`limited to less than -41 dBm in these bands. In may noted
`that the lower restricted band 31 (2.300-2.390 GHz) is
`spaced 10 MHz below the lower end (2.400 GHz) of the ISM
`band 30. However, the upper restricted band 32
`60 (2.4835-2.500 GHz) actually borders the top end of the ISM
`band. The (2.4835-2.500 GHz) restricted band 32 therefore
`poses a much more severe problem to users of the ISM band
`30.
`As further shown in the spectrum diagram of FIG. 4, in
`order to conform to out-of-band emissions, center frequen(cid:173)
`cies for direct-sequence spread spectrum (DSSS) WLANs
`are commonly skewed toward the low end of the ISM band
`
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`US 6,377,608 Bl
`
`5
`30. Although there are fourteen overlapping channels
`defined for DSSS operation in IEEE specification 802.11,
`only the lowest eleven channels are suitable for use in the
`U.S., due to the restricted band 32 at 2.4835-2.500 GHz
`described above. Channel 11, the highest channel allowed in
`the U.S., has a center frequency of 2.,462 GHz. In order to
`squeeze three non-overlapping channels into the ISM band,
`Channels 1, 6, and 11 (having center frequencies of 2.412
`GHz, 2.437 GHz, and 2.462 GHz, respectively) are typically
`employed, as shown in FIG. 4, thereby providing a channel
`spacing of 25 MHz.
`As can be seen in FIG. 4, the channel usage of the ISM
`band provides a pair of unused regions 34 and 35 at
`respective upper and lower ends of the ISM band for
`placement of a pulsed warning beacon, to alert ad hoc 15
`network radios (such as frequency hopped spread spectrum
`(FHSS) radios), that they are spatially close to a WLAN
`infrastructure access point. In particular, there is a relatively
`narrow spectral spacing or gap 34 between DSSS channel 1
`and lower end of the ISM band 30.
`As diagrammatically illustrated in FIG. 5, gap 34 allows
`for the placement of a beacon 50 at 2.401 GHz, for example,
`which is 12 MHz from the nearest DSSS channel center
`frequency (4.412. GHz of channel 10). On the other hand, as
`shown in FIG. 6, the substantial spectral spacing 35 between 25
`DSSS channel 11 and the upper end of the ISM band 30,
`allows for placement of a beacon 60 at 2.482 GHz, which is
`20 MHz away from the center of channel 11 and 1.5 MHz
`below the upper end of the ISM band. This spacing effec(cid:173)
`tively prevents the possibility that the beacon 60 would 30
`disrupt DSSS systems in either the U.S. or Europe.
`Frequency hopped spread spectrum (FHSS) systems are
`also affected by the presence of the restricted band 31
`(2.4835-2.500 GHz), but to a lesser degree. Their 1.0 MHz
`occupied bandwidth allows FHSS radios to employ center
`frequencies from 2.402 GHz to 2.480 GHz. As a
`consequence, placing a 1.0 MHz fixed beacon within the
`lower spectral gap 34 ( e.g., at 2.401 GHz) or in the upper gap
`35 above 2.481 GHz would not interfere with FHSS chan-
`nels operating at lower frequencies.
`To combat potential fading of the beacon signal, two
`beacons shown at 71 and 72 in FIG. 7, may be located within
`the lower and upper spacings 34 and 35, respectively, of the
`ISM band. For example, by placing beacon 71 at 2.401 GHz 45
`and beacon 72 at a prescribed frequency near the top of the
`band (at 2.482 GHz for example), the probability of loss of
`beacon signal due to simultaneous fading on two non(cid:173)
`correlated channels is significantly reduced.
`In accordance with a preferred embodiment of the
`invention, not only is the beacon placed at a fixed frequency
`within the ISM band, but, as diagrammatically illustrated in
`FIG. 8, it is sourced from a beacon generator 80 that is
`physically or spatially located in close proximity to and
`linked with infrastructure access point 21. The beacon 55
`generator 80 is operative to periodically generate a pulsed
`beacon signal which, because of its physical proximity to
`access point, serves to indicate the presence of infrastructure
`WLAN and its associated BSA 20 serving DSSS nodes 25.
`As will be described, by periodically tuning the receive 60
`frequency synthesizer of its transceiver to this frequency, an
`ad hoc network radio, such as an FHSS radio, is able to
`monitor whether it is in close proximity to an infrastructure
`network.
`Because FCC regulations (FCC 15.249) permit a low 65
`power transmitter to periodically increase ( or pulse) its
`transmit power by as much as 20 dB, as long as average
`
`6
`power remains below O dbm, the beacon signal emitted by
`beacon generator 80 preferably has a pulsed beacon profile.
`A non-limiting example of a pulsed beacon profile that may
`be employed in accordance with the present invention is
`5 diagrammatically illustrated in the beacon pulse signal tim(cid:173)
`ing diagram of FIG. 9. As shown therein, to conform with
`the above-referenced FCC regulations, and to ensure
`adequate range, each periodically generated beacon pulse 90
`may have a peak transmit power of +20 dBm and pulse
`10 repetition interval of 24 msec. To achieve an average power
`of O dBm, the beacon has a one percent duty cycle, or a pulse
`width of 240 µsec., as shown.
`In order to convey information relating the operation of
`the infrastructure network with which the beacon generator
`is associated, the beacon signal may employ 2-FSK modu(cid:173)
`lation at 1 Mbps. As a non-limiting example, where the BSA
`is that of a DSSS infrastructure WLAN, the information
`contained in the beacon modulation includes the center
`frequency of the occupied channel. In the case of an infra-
`20 structure FHSS network, the hop sequence and system clock
`data are embedded in the beacon modulation.
`While the beacon message structure is not limited to any
`particular format, for the purposes of providing a non(cid:173)
`limiting example, a (240 bit) framing format diagrammati(cid:173)
`cally illustrated in FIG. 10, that conforms with IEEE 802.11
`FHSS PLCP frame format, may be used. As shown therein,
`following a beacon preamble 111 and header 112 is a
`fourteen byte message 113, that includes a Frame Control
`segment 114, a Duration/AP identification segment 115, an
`infrastructure data segment 116, which may include modu(cid:173)
`lation scheme, channel, and timing information, and a check
`sum segment 117.
`The present example of a beacon pulse repetition interval
`of 24 msec. implies that an ad hoc FHSS radio must tune to
`the beacon frequency and dwell on that channel for at least
`24 msec. Where the tuning algorithm of the FHSS radio
`causes its transceiver's receive synthesizer to tune itself to
`the beacon channel every five seconds, as a non-limiting
`example, the resulting reduction in system throughput for ad
`hoc network 10 due to beacon monitoring is extremely
`small. For the present example, the percentage reduction in
`throughput=24 msec/5 sec=0.48%.
`For enhanced reliability, where two beacons are employed
`to combat potential fading of the beacon signal, as described
`above with reference to FIG. 7, beacon generator 80 may be
`configured to generate a pair of time-interleaved or alter(cid:173)
`nating pulsed beacon signals, as shown at 90 and 95 in the
`dual beacon timing diagram of FIG. 11. For the present
`50 example of using a beacon period of 24 msec. with a 1 %
`duty cycle, interleaving two beacons provides ample time to
`switch to the alternate beacon channel. In this dual mode
`operation, there is no additional throughput penalty, and
`there is a reasonably high probability that both beacon
`channels will not fade simultaneously. In dual beacon mode,
`beacon detection would take ten seconds, at most.
`As shown in FIGS. 4-6, channel assignments of the ISM
`band provide for spectral placement of the beacon frequency
`at either or both the upper and lower ends of the band. For
`purposes of the present description, it has been assumed that
`all FHSS systems employ the same channel definition
`scheme. Bluetooth, IEEE 802.11 FHSS, and HomeRF
`employ 1 MHz occupied bandwidths. All three systems use
`79 channels spaced at 1 MHz starting at 2.402 GHz up to
`2.480 GHz. This means that the pulsed beacon may located
`at 2.401 GHz in the lower spectral gap 34 (e.g., at 2.401
`GHz) or in the upper gap 35 above 2.481 GHz.
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`35
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`Marvell Semiconductor, Inc. - Ex. 1010, Page 0010
`IPR2019-01350 (Marvell Semiconductor, Inc. v. Uniloc 2017 LLC)
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`US 6,377,608 Bl
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`5
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`10
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`7
`The substantial spectral spacing 35 at the upper end of the
`ISM band allows for placement of a beacon 60 at 2.482
`GHz, which is 20 MHz away from the center of channel 11
`and 1.5 MHz below the upper end of the ISM band. While
`locating the beacon near the top of the ISM band requires the
`use of more precision filtering to avoid excessive out-of(cid:173)
`band emissions, such spectral placement virtually guaran(cid:173)
`tees that the beacon will not interfere with FHSS or DSSS
`radios. While the use of a high precision filter entails an
`increase in cost, the cost is only associated with the signal
`processing functionality of the beacon generator, not with
`the ad hoc networks. No additional filter requirements are
`imposed on the ad hoc radios, since these mobil nodes
`operate in a "listen only" mode on the beacon frequency. The
`overall impact on cost is therefore modest.
`In order to monitor and respond to the pulsed warning 15
`beacon that is sourced from the vicinity of the access point
`of the infrastructure of a WLAN, it is only necessary to
`modify the control software employed by the microcontrol-
`ler of an ad hoc participant's radio. Namely, as diagram(cid:173)
`matically illustrated in FIG. 12, the ad hoc radio's to 20
`microcontroller 131 is upgraded to incorporate a synthesizer
`tuning mechanism that controls a programmable synthesizer
`133, through which operation of the radio's transceiver 135
`is controlled, so that operation of each of the ad hoc radio
`and the WLAN may proceed on a non-interfering basis. No 25
`modification to infrastructure mode operation is required.
`Non-limiting examples of such non-interfering operation
`include the following interference avoidance modes:
`1----deferral mode; 2-altered hop pattern mode; 3-fixed
`frequency mode; 4----deactivation mode; and 5-low trans- 30
`mit power mode. Each of these respective modes will be
`described individually below.
`1-Deferral Mode (FIGS. 13 and 14)
`As described previously, embedded within the beacon
`signal is information relating the operation of the infrastruc- 35
`ture network with which the beacon generator is associated.
`For example of a DSSS infrastructure WLAN, that infor(cid:173)
`mation includes the center frequency of the occupied chan(cid:173)
`nel. For an infrastructure FHSS network, the hop sequence
`and system clock data are embedded in the beacon modu- 40
`lation. Because the monitored beacon informs FHSS radios
`operating in the ad hoc mode of the operating parameters of
`the infrastructure network, in this mode of operation, the
`tuning control mechanism employed by the ad hoc radio
`network is operative to defer transmission in response to
`detecting that ad hoc radio and the infrastructure networks
`occupy the same frequency. This technique may be
`employed regardless of which type of infrastructure network
`is encountered.
`Where the infrastructure network is an IEEE 802.11 50
`DSSS network, deferral results in approximately a 30%
`reduction in throughput for the ad hoc radio. As mentioned
`previously with reference to FIG. 3, and as shown in the
`spectrum diagram of FIG. 13, a DSSS network occupies
`fixed channels 120 which are about 25 MHz apart, as shown
`for DSSS channel 11. In order to avoid interference, the
`FHSS ad hoc radio defers on 25 out of 79 1 MHz channels
`125, or about 30% of the FHSS channels. For an FHSS
`WLAN infrastructure, in which each channel has an instan(cid:173)
`taneous bandwidth of only 1 MHz, as shown in the spectrum
`diagram of FIG. 14, a lower reduction in throughput is
`obtained. In such an FHSS system, since the ad hoc network
`radio needs to defer on only 3 out of 79 channels 145,
`throughput reduction is less than 4%.
`2-Altered Hop Pattern Mode
`Some FHSS systems, such as Bluetooth, have nominal RF
`output of O dBm. As a consequence they are able to operate
`
`8
`in accordance with FCC low power rules (FCC reg. no.
`15.249). Under these rules, there is no requirement to
`employ spread spectrum modulation, so that changing hop
`sequences to avoid interference is permissible. The advan-
`tage to this approach is that the band occupied by the
`infrastructure network may be avoided completely. Also,
`there is no reduction in throughput to either the ad hoc or the
`infrastructure network.
`3-Fixed Frequency Mode (FIGS. 15 and 16)
`In order to avoid interfering with DSSS WLANS, a low
`power FHSS radio (e.g. Bluetooth radio) detecting the
`warning beacon enters into a fixed frequency operating
`mode. DSSS WLANs use center frequencies shifted toward
`the lower end of the ISM band to avoid excessive out-of-
`band emissions in the lower restricted band. This frequency
`shift provides for the use of a set of 1 MHz center frequen(cid:17