US007403539B1
`
`a2) United States Patent
`US 7,403,539 B1
`(10) Patent No.:
`
`(45) Date of Patent: Jul. 22, 2008
`Tang etal.
`
`(54) CLEAR CHANNEL ASSESSMENTIN
`WIRELESS COMMUNICATIONS
`
`(75)
`
`Inventors: Hsiao-Cheng Tang, San Jose, CA (US);
`Yungping Hsu, Cupertino, CA (US);
`Guorong Hu, Sunnyvale, CA (US);
`Weishi Feng, San Jose, CA (US)
`
`(73) Assignee: Marvell International Ltd., Hamilton
`(BM)
`
`(*) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 1029 days.
`
`(21) Appl. No.: 10/268,156
`
`(22) Tiled:
`
`Oct. 9, 2002
`
`(51)
`
`Int. Cl.
`(2006.01)
`HOAL 12/413
`(2006.01)
`HOA4L 12/28
`(52) US. CV. eee ce cecceececesseecesstaeseseeaneees 370/445
`(58) Field of Classification Search ................. 370/445,
`370/446, 447, 338, 462
`See application file for complete search history.
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`......... 370/338
`5/2000 Kamerman etal.
`6,067,291 A *
`.
`6,469,997 B1* 10/2002 Dorenboschetal.
`
`.
`6,675,012 B2*
`1/2004 Gray wee eee
`6,834,045 B1* 12/2004 Lappetelainenetal. ..... 370/329
`2002/0061738 Al*
`5/2002 Simmonsetal. ......... 455/234.1
`
`EP
`
`FOREIGN PATENT DOCUMENTS
`1124337 A2
`8/2001
`
`OTHER PUBLICATIONS
`
`Control (MAC) and Physical Layer (PHY) Specifications, Higher-
`Speed Physical Layer Extension in 2.4 GHz Band,” Sep. 1999, pp.
`1-89.
`
`International Standard, ANST/TFFEstd. 802.11, first edition, Sponsor
`LAN MAN Standards Committee ofIEEE Computer Society, “Part
`11: Wireless LAN Medium Access Control (MAC) and Physical
`Layer (PHY) specifications,” 1999,
`Geier, Jim, “Wireless LANs, Second Edition”, S4MS Publishing,
`2000, pp. 137-151.
`and Méctropolitan Arca
`“Local
`802.16,
`TEEE
`Standards
`Networks—Part 16: Interface for Fixed Broadband Wireless Access
`Systems,” Oct, 1, 2004, 893 pages.
`IEEE Std 802.1 la-1999, Sponsor LANMANStandards Committee of
`IEEE Computer Society, “Part 11:Wireless LAN Medium Access
`Control (MAC) and Physical Layer (PHY) specifications: High-
`speed. Physical Layer in the 5 GHZ Band.” Sep. 1999, pp. 1-83.
`
`(Continued)
`
`Primary Examiner—Hassan Kizou
`Assistant Examiner—Betty Lee
`
`(57)
`
`ABSTRACT
`
`Techniques for and apparatus capable of implementing
`packet detection and signal recognition in wireless commu-
`nications systemsare disclosed. In particular, the disclosed
`techniques and apparatus incorporateat least oneof relative
`energy detection operable on assessment of a relative energy
`threshold for an inbound signal borne across an RF channel,
`carrier sense operable upon on assessmentofat least one ofa
`peak-to-sidelobe ratio and peak-to-peak distance defined by
`the inboundsignal, and comparison operable upon demodu-
`lated data correspondingto the inbound signal as compared to
`predetermined preamble data. Clear channel assessmentis
`performed based on determinations undertaken by one or
`moreof the aforementionedrelative energy detection, carrier
`sense and comparison operations.
`
`1999, Sponsor LANMANStandards Committee
`JEEEstd. 802.11b
`ofIEEE Computer Society, “Part 11: Wireless LAN Medium Access
`
`145 Claims, 7 Drawing Sheets
`
`
`FRAME DETECTION |
`
`
`120
`
`x
`
`to MAC WF 425
`“x37
`
`inbound data
`
`
`
`
`375
`
`TT
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`HI_LRATE_PSDU
`
`
`
`Page 1 of 26
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`SAMSUNG EXHIBIT 1032
`
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`SAMSUNG EXHIBIT 1032
`
`

`

`US7,403,539 B1
`Page 2
`
`OTHER PUBLICATIONS
`
`TEEE P802.112/D8.2-Apr. 2003, Sponsor LAN/MAN Standards
`Committee of the IEEE Computer Society, Part 11:Wireless LAN
`Medium Access Control (MAC) and Physical Layer (PHY)specifi-
`cations, Further Higher Data Rate Extension in the 2.4 GHZ Band,
`2003, pp. 1-69.
`TEEEStd 802.16a-2003, Sponsor LAN/MANStandards Committee of
`IEEE Computer Society and the IEEE Microwave Theoryand Tech-
`niques Society, “Part 16: Air Interface for Fixed Broadband Wireless
`Access Systems-Amendment 2: Medium Access Control Modifica-
`tions and Additional Physical Layer Specifications for 2-11 GHz.”
`2003, pp. 1-292.
`TEEE Std 802.11a-1999(Supplement to TEEE Std 802.11-1999)
`[Adopted by ISO/IEC and redesignated as ISO/TEC 8802-11: 1999/
`Amd. 1:2000(E)|; Supplement to IEEE Standard for Information
`technology - Telecommunications
`and information exchange
`between systems - Local and metropolitan area networks - Specific
`requirements - Part 11: Wireless LAN Medium Access Control
`
`(MAC) and Physical Layer (PHY)specifications High-speed Physi-
`cal Layerin the 5 GHz Band; LAN/MAN Standards Committee of
`the IEEE ComputerSociety; 91 pages.
`TEEE P802.11g/D8.2, Apr. 2003 (Supplement to ANSI/IEEE Std.
`802.11-1999(Reaff 2003)); DRAFT Supplement to STANDARD
`[for] Information Technology - Telecommunications and information
`exchange between systems - Local and metropolitan area networks-
`Specific requirements - Part 11: Wireless LAN Medidum Access
`Control (MAC) and. Physical Layer (PHY) specifications: Further
`Higher Data Rate Extension in the 2.4 GHz Band; T.AN/MANStan-
`dards Committee of the IEEE Computer Society; 69 pages.
`IEEE Std 802.16a-2003 (Amendment to IEEE Std 802.16-2001),
`IEEEStandard for Local andmetropolitan area networks; Part 16: Air
`Interface for Fixed Broadband Wireless Access Systems -- Amend-
`ment 2: Medium Access Control Modifications and Additional Physi-
`cal Layer Specifications for 2-11 GHz; IEEE Computer Society and
`the IEEE Microwave Theoryand Techniquest Society; Apr. 1, 2003;
`318 pages.
`
`* cited by examiner
`
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`U.S. Patent
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`Jul. 22, 2008
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`U.S. Patent
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`Jul. 22, 2008
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`U.S. Patent
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`U.S. Patent
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`Jul. 22, 2008
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`U.S. Patent
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`US7,403,539 B1
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`1
`CLEAR CHANNEL ASSESSMENTIN
`WIRELESS COMMUNICATIONS
`
`TECHNICALFIELD
`
`The presentinvention generally relates to wireless commu-
`nications, and is specifically concerned with clear channel
`assessment techniques to determine the presence or absence
`of a valid inboundsignal.
`
`10
`
`
`
`BACKGROUND OF THE INVENTION
`
`
`
`Thepast few years has witnessed the ever-increasing avail-
`ability of relatively cheap, low power wireless data commu-
`nication services, networks and devices, promising near wire
`speed transmission and reliability. One technologyin particu-
`lar, described in the IEEE Standard 802.11b-1999 Supple-
`
`mentto the ANSI/IEEE Standard 802.11, 1999 edition, col-
`lectively incorporated herein fully by reference, and more
`commonlyreferred to as “802.11b” or “WiFi”, has become
`the darling of the information technologyindustry and com-
`puter enthusiasts alike as a wired LAN/WANalternative
`because of its potential 11 Mbpseffective throughput, ease of
`installation and use, and transceiver component costs makeit
`areal and convenientalternative to wired 10 BaseT Ethernet
`and other cabled data networking alternatives. With 802.11b,
`workgroup-sized networks can now be deployedina building,
`in minutes, a campus in days instead of weeks since the
`demanding task of pulling cable and wiring existing struc-
`tures is eliminated. Moreover, 802.11b compliant wireless
`networking equipmentis backwards compatible with the ear-
`lier 802.11 1 M/2 Mbpsthroughput standard, thereby further
`reducing deploymentcosts in legacy wireless systems.
`802.11b achievesrelatively high payload data transmission
`rates or effective throughput via the use of orthogonal class
`modulation in general, and, moreparticularly, 8-chip comple-
`mentary code keying (“CCK”) anda 11 MHz chippingrate to
`bear the payload. As such, previously whitened or scrambled
`bitstream data of interest is mapped into nearly orthogonal
`sequences (or CCK code symbols) to be transmitted, where
`each chip ofthe CCK code symbolis quaternary phase modu-
`lated using QPSK (“quadrature phase shift keying”) modula-
`tion techniques. Meanwhile the common phase of each CCK.
`symbol is jointly determined by the current and previous
`symbols using differential QPSK or DQPSK modulation
`scheme. Subsequent conversion inta the analog domain pre-
`pares these CCK symbols for delivery over a wireless
`medium RF modulated on a carrier frequency within the
`internationally recognized 2.4 GHz ISM band to form the
`payload or PLCP Service Data Unit of an 802.11b compliant
`Physical Layer Convergence Procedure (“PLCP”) frame, a
`type of packet. The high-rate physical layer PLCP preamble
`and header portions forming the frame overheadarestill
`modulated using the 802.11 compliant Barker spreading
`sequence at an 11 MHz chippingrate. In particular, the pre-
`amble (long format—1 44bits, short format—72bits) is uni-
`versally modulated using DBPSK (“differential binary phase
`shift keying”) modulation resulting in a 1 Mbps effective
`throughput, while the headerportion may be modulated using
`either DBPSK (long preamble format) or DQPSK(short pre-
`amble format) to achieve a 2 Mbps effective throughput.
`An IEEE 802.11b compliant receiver receives and down-
`converts an incident inbound RI’ signal to recover an analog
`baseband signal bearing the PLCP frame, and then digitizes
`and despreads this signal to recover the constituent PLCP
`preamble, header and payload portions in sequence. Thepre-
`amble and header portions are Barker correlated and then
`
`40
`
`45
`
`5
`
`5
`
`Page 10 of 26
`
`2
`either DBPSK or DQPSK demodulated based on the pre-
`amble format used to recover synchronization data and defi-
`nitional information concerning the received PLCP frame,
`includingthe data rate (Signal field in the PLCP header) and
`octet length (Length field in the PLCP header) ofthe variable-
`length payload or PSDU portion. The CCK encoded symbols
`forming the PLCPpayloadportion are each correlated against
`64 candidate waveformsin received per symbol sequence in
`combination with DQPSK demodulation to verify and
`reverse map each into the underlying bitstream data ofinter-
`est, at either 4 bits per symbol (5.5 Mbps)or 8 bits per symbol
`(11 Mbps) increments.
`It should be appreciated that 802.11 and 802.11b signals
`operate in the 2.4 GI1z ISM band and must therefore coexist
`with quite an array ofdissimilar signals operating in the same
`frequency, including microwave ovensanddigital phones. By
`definition, there are no licensurerestrictions within the avail-
`able RF channels of the ISM band, so 802.11 and 802.11b
`compliant transceivers must employ clear channel assess-
`ment techniques to determine if it is safe to transmit. In
`particular, there is an expected amount of ambientnoise that
`the 802.11/802.11b transceivers must tolerate butstill be able
`to transmit, but should not attempt to transmit while another
`in-range 802.1 1/802.11b transceiver is transmitting so as to
`maximize channel use and system throughput. In other
`words, it is desirable for 802.11 and 802.11b transceivers to
`know when the operating channel is occupied with valid
`traffic, and thus enter receive mode without attempting to
`transmit over suchtraffic. Likewise, it is desirable that these
`transceivers should befree to transmit onthe operating chan-
`nel while that channelis free of 802.11/802.11b traffic, even
`in the presence of a tolerable amountof noise or interference.
`To this end, the 802.11 and 802.11b standards specifyclear
`channel assessment (CCA) guidelines which are used to
`determine if a tuned RF channel contains valid PLCP frame
`traffic. Inbound signals in the tuned or operating RI’ channel
`which do not meet these CCA guidelines are considered to
`bear either corrupted frames, or represent interference or
`noise inthe channel. The 802.11/802.11b CCA guidelines are
`organized in modesas follows:
`CCA Mode1: Energyabove threshold. CCA shall report a
`busy medium upon detecting any received energy above
`the ED threshold.
`
`CCA Mode2: Carrier Sense only. CCA shall report a busy
`medium only upon detection of a DSSS signal. ‘lhis
`signal may be above or below the ED threshold.
`CCA Mode 3: Carrier Sense with energy above threshold.
`CCAshall report a busy medium upon detection of a
`DSSSsignal with energy above the ED threshold.
`CCA Mode4 (802.11b): Carrier sense with mer. CCA
`shall start a timer whose duration is 3.65 ms and report a
`busy medium upon the detection of a High Rate PHY
`signal. CCA shall report an IDLE mediumafterthe timer
`expires and no High Rate PHY signal is detected. The
`3.65 ms timeout is the duration of the longest possible
`5.5 Mbps PSDU.
`CCA Mode 5 (802.11b): A combination of carrier sense
`and energy above threshold. CCA shall report busy at
`least while a High Rate PPDU with energy above the ED
`threshold is being received at the antenna.
`
`
`
`The 802.11 DSSS PHYreceiver must perform CCA accord-
`ing to at least one of modes 1 3, and the 802.11b High Rate
`PHY must perform CCA according to modes1, 4 or 5.
`Three of the five conventional CCA modesrequire thresh-
`olding inboundsignalenergy, and so this guideline is believed
`important. However, conventional transceivers simply com-
`
`Page 10 of 26
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`US7,403,539 B1
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`4
`3
`FIG.2 is a more detailed block diagram ofa receive base-
`pare inbound signal energy against the specified threshold,
`band processing unit shownin FIG.1.
`and report an energy threshold validation signal wheneverthe
`FIG.3 is adetailed block diagram of a packet detection unit
`threshold is exceeded. Thus, the presence of strong interfer-
`according to an alternative embodimentof the invention.
`encein the operating channel, will cause (in the case ofaCCA
`FIG.4 is a state transition diagram for the CCA unit shown
`mode | implementation) or potentially may (in the case of a
`in FIG,2.
`CCA mode3 or 5) cause a false busy to be reported, and thus
`
`preventthe transceiver from transmitting, which may in turn T'IG. 5isastate transition diagram for the CS unit shown in
`FIG.2.
`cause transmission delay and lowereffective data throughput.
`
`Moreover, to implement CCA modes 2—5, conventional
`FIG. 6 isa state transition diagram for the ED unit shown in
`FIG,2.
`CCAcarrier sense techniquesare used to determine ifa DSSS
`or High Rate PHYinboundsignal is present, typically by
`FIG. 7 is a sample plot of gain perceived by the ED unit
`over time.
`thresholding a measure of the perceived Barker code lock.
`However, known techniques are relatively complex and are
`thus powerinefficient and expensive to implement. Both cost
`and power consumptionreductionarecritical design goals in
`802.11/802.11b transceiver implementation,
`it would be
`advantageous if simpler carrier sense techniques could be
`incorporated without materially affecting carrier sense sensi-
`livily or recognition performance.
`Further, while conventional CCA techniques look for valid
`PLCPheaderinformation (via CRC validation), there is no
`post-demodulation confirmation during receipt of the pre-
`amble. Checking for valid preamble receipt would be advan-
`tageous, especially where the inbound signal fades poten-
`tially below the inbound signal cnergy threshold, but the
`receiveris still able to successfully recover recognizable pre-
`amble information from the signal.
`Finally, while the defined 802.11/802.11b CCA modes
`account accommodate a range of operational environments,
`they are not appropriate for every environment and channel
`condition. Therefore,
`it would advantageous to provide a
`transceiver capable of handling further CCA modes other
`than those defined by the 802.11/802.11b standards, prefer-
`ably while retaining backwards compatibility with such stan-
`dards.
`
`10
`
`15
`
`20
`
`30
`
`35
`
`SUMMARYOF THE INVENTION
`
`To address these and other perceived shortcomings and
`desires, the present inventionis directed in part to a packet
`detection unit and signal recognition methodthat includesat
`least one ofrelative energydetection operable on assessment
`of a relative energy threshold for an inbound signal borne
`across an RF channel, carrier sense operable upon on assess-
`mentof at least one of a peak-to-sidelobe ratio and peak-to-
`peak distance defined bythe inboundsignal, and comparison
`operable upon demodulated data corresponding to the
`inbound signal as compared to predetermined preamble data.
`Clear channel assessment is performed based on determina-
`tions undertaken by one or more of the aforementioned rela-
`tive energy detection, carrier sense and comparison opera-
`tions.
`
`Further aspects of the present invention include a trans-
`ceiver, network interface apparatus, and information proces-
`sor incorporating this packet detection unit, as well as a
`computer program product including computer readable pro-
`gram code capable of causing an information processor to
`perform one or moreof these signal recognition aspects.
`Additional aspects and advantagesofthis invention will be
`apparent from the following detailed description of certain
`embodiments thereof, which proceeds with reference to the
`accompanying drawings.
`
`
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG.1 is a high level block diagram of a wireless trans-
`ceiver in accordance with an embodimentof the invention.
`
`40
`
`45
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`Page 11 of 26
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`FIG. 8 is a block diagram for the ED unit shownin FIG.2.
`FIG. 9 is a block diagram of the CS unit shownin FIG.2.
`FIG. 10 is a block diagramof the CCA unit shownin FIG.
`
`2.
`
`DETAILED DESCRIPTION OF THE
`
`
`LEMBODIMINTS
`
`Turning nowto the figures, FIG. 1 illustrates a wireless
`communications transceiver 100 according to an embodi-
`ment of the invention. In this embodiment, inbound RF sig-
`nals potentially conveying an 802.11 or 802.11b compliant
`PLCP frame are picked up by the duplex antenna 10 and
`routed to the RF receiver unit 115 of a reeciver 150 arranged
`in a Manner consistent with the present invention. The RF
`receiver unit 115 performs routine downconversion and auto-
`matic gain control ofthe inbound RFsignals, and presents an
`analog baseband signal containing the aforementioned
`802.11b PLCP frameto the receive baseband processor 120.
`The functions of the receive baseband processor 120 will be
`detailed below with reference to FIG. 2, including packet
`detection and channel busyconsistent with the present inven-
`tion, along with conventional symbol correlation and/or
`demodulation of the preamble, header and payload portions
`of each inbound 802.11b PLCP frame to recover bitstream
`data for receiver synchronization (preamble), frame or packet
`definition (header), or the actual inbound data of interest
`(payload).
`Oncerecovered by the receive baseband processor 120, the
`inbound data contained in the PSDUofeach received 802.1 1b
`PLCPframeis delivered to a network interface such as the
`MAClayerinterface 125 and then on to higher layer applica-
`tions and devices being serviced by the transceiver 100.
`Outbounddata intended for wireless transmission originat-
`ing from the device(s) or application(s) being serviced by the
`transceiver 100 are delivered to the transmit bascband pro-
`cessor 135 of the transmitter 160 from the MAC interface
`125. Directives from the PMD sublayer (not shown) forming
`part of the MAC interface 125 and expressing the desired
`transmission mode, including the 802.11b 1, 2, 5.5 and 11
`Mbps effective throughput modesare transferredto the trans-
`mit baseband processoras well for each PLCP frame/packet.
`The transmit baseband processor 135 formulates appropriate
`802.11b PLCPframe, and symbol encodesthe outbound data
`as specified by the PMD sublayer to generate a complete
`outbound 802.11b PLCP frame. As the frame or packet is
`being developed, it is converted into analog form suitable for
`upconversion and RF transmission by the RF transmitter unit
`140 consistent with 802.11b physical layer requirements.
`Thoughnot shownin FIG.1, the transceiver 100 may form
`an operationalpart of a network interface apparatus such as a
`PC card or network interface card capable of interfacing with
`the CPU or information processor of an information process-
`ing apparatus such as a desktop or laptop computer, and may
`be integrated within and constitute a part of such information
`
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`an IIR lowpassfilter to generate a 0.5 psec delayed signal n
`whichtracks the original signal E using the following rela-
`tionship:
`n=aE,+(1-a)nz_;
`
`40
`
`45
`
`6
`the inbound basebandsignal generated by the AGC 210 to a2
`MHzgainsignal E, with E, representing the signal E at the kth
`2 MHz sample in time.E is sent directly to one input ofthe
`relative energy detection unit 235, and is concurrently sentto
`anoise floor unit 230. The noise floor unit 230 here constitutes
`
`5
`processing apparatus. This network interface apparatus may
`alternatively form an operational component of a wireless
`communications access point suchas a basestation as will be
`appreciated by these ordinarily skilled in theart.
`Turning nowto FIG. 2, FIG. 2 is a more detailed block
`diagram ofthe receive baseband processor 120 shownin FIG.
`1. So as not to obfuscate the teachings of the present inven-
`tion, several 802.11 and 802.11b compliant directives and
`signals are not shown. As such, an inbound analog baseband
`signal potentially conveying an inbound 802.11/802.11b
`‘Lhe relative energy threshold detection unit 235 receives
`PLCP frame recovered by the RF receiver unit 110 of the
`
`both E and n,and calculates the difference between the gain
`receiver 150 is fed to the input of the automatic gain control
`values each represents (via the gain differential unit 810
`unit (AGC) 210. Withthe assistance ofa feedbacklooptied to
`shown in FIG. 8) as a gain change over time. Through the
`the input ofthe FIR filter 315, the AGC 210 adjusts the gain of
`control unit 820, the relative energy threshold detection unit
`the inbound basebandsignal to maximize the dynamic range
`235 compares the gain change over time against one of two
`and performanceofthe analog to digital converter 310, as is
`preferably programmable energydetection thresholds, based
`knownin the art, assuming the bound signalis valid. The
`on the current state of the contro] unit 820. Though not
`AGC210 also reports the gain adjusted signal to the RSST unit
`required, the relative energy threshold detection unit 235 of
`220 ofthe frame detection unit 200 for channel busy, as will
`20
`the present embodiment operates on a 1 MHz synchronized
`be described in greater detail below.
`clock, and so F andnare effectively downsampled at a 1 MHz.
`The gain adjusted inbound analog signal generated by the
`rate before their difference is compared against one of these
`AGC 210 is thensent to the digital converter 310 to convert it
`thresholds. As showninFIG.6, the control unit 820 functions
`into digital form. With the aid ofthe 44 MHz clock, the ADC
`as a finite state machine capable of switching, between two
`producesa correspondingdigital signal sampled at 44 MHz.
`states 610, 620.
`Noxt, this digital signal passes through the digital FIR LPF
`Referring to FIG. 6, the control unit 820 (FIG. 8) of the
`315 to reject out-of-band interference, and the down sampler
`energy threshold detection unit 235 (FIG. 2) is initialized to
`320 to provide a 22 Mhz digital signal potentially bearing a
`PLCPframeofinterest.
`state 610 at the beginning of each framedetection processing
`sequence (such as when a new inboundsignalis first recerved
`This 22 MHz signal next encounters twoparallel baseband
`on the operating RF channel). As such, the energy threshold
`demodulation pathways. The first demodulation pathway,
`validation signal (ED)isset to false (e.g. logic level 0). While
`including the Barkercorrelator 330, down sampler 335, Rake
`in state 610, the control unit 820 monitors the gain change
`340 and the down sampler 345 is used to recover a despread
`over time generated by the gain differential unit 810. ‘he
`1 MIIz signal representing the preamble and headerportions
`control unit 820 remains in state 610 while the gain remains
`of the inbound frame for symbol demodulation by the com-
`relativelystable (transition or trans. 3). Ifthe gain change over
`bination DBPSK/DQPSK demodulator 375. This
`first
`time exceedsa first energy detection threshold, meaning that
`demodulation pathway-demodulator combination 375is also
`the gain is changing rapidly with the AGC 210 in a gain
`used for symbol decoding a base 802.11 PLCP frame payload
`unlock condition and attemptingto transition to receive mode
`in 1 Mbps/2 Mbps modes. The second demodulation pathway
`from transmit protect mode in response to an inbound signal
`is used to symbo! demodulate a high rate 802.11b payload
`havingrelatively significant energy, the control unit 820 tran-
`portion of the inbound frame, and includes a 22 MHzto 11
`sitions to state $20 (trans. 2) and the energy threshold valida-
`MHz downsampler 350 following, by a decision feedback
`tion signaltransitionsto true (logic level 1). In turn,assertion
`equalizer 355. To begin the CCK symbol decode process for
`oftrue energy threshold validation signal may cause assertion
`802.11b compliant payloads at 11 Mbps or 5.5 Mbps trans-
`of the channel busy signal (CCA)true by the CCA unit 250,
`mission modes, a CCK correlator 360 is provided.
`depending on the CCA modebeing implemented.
`An 802.11b compliant receive state machine (not shown)
`The control unit 820 remainsat state 620 (trans. 4) while
`issues the HI_LRATE_PSDU semaphoreto control the modu-
`the gain change over time continues to exceed a second
`lation pathway selection mux 370 based on whichportion of
`energy detection threshold to hold energy threshold valida-
`the inbound frameis being demodulated, as well whether 5.5
`tion signal true. However, once the gain change over time
`Mbps+payload modulation modesare specified. The combi-
`settles and the gain stabilizes, the control unit 820 transitions
`nation DBPSK/DQPSK demodulator 375 is used to recover
`back to state 610 (2), and the energy threshold validation
`the symbol encoded inbound data presented in the preamble,
`signal transitions backto logic level 0 or false.
`header and payload portions. The DBPSK/DQPSK demodu-
`Note that in this embodiment the second energy detection
`lator is clocked at the symbolrate; i.e., 1 MHz for 1 Mb and
`2 Mb modes, and 1.375 MHzfor 5.5 Mb and 11 Mb modes.
`threshold is less than the first energy detection threshold to
`After symbol demodulation, the recovered inbound data is
`lengthenthe windowin which the energy threshold validation
`signal is asserted high by the control unit 820. Ilowever, in
`descrambled by the descrambler 380 in a knownfashion, and
`alternative embodiments, the first and second thresholds may
`delivered to the MAC interface 125 (FIG. 1).
`be equal or even reversed depending on particular CCA
`Still referring to FIG. 2, the frame detection unit 200 is
`provided to determine if the inbound basebandsignal recov-
`implementation goals.
`ered from an RF channel tuned by the RF receiver 115 (FIG.
`Due to this “relative energy thresholding”, certain gain
`1) is a valid signal at least capable of bearing an 802.11b
`signal transitioning rather than the receive/transmit state of
`compliant PLCPframe, and if'so, keep the transceiver 100 in
`the AGC 210is used to toggle the energy threshold validation
`receive modefor the duration ofthe frame.
`signal. This difference is subtle, yet important in handling
`To implementrelative energy threshold detection consis-
`relatively strong, persistent
`interference in the operating
`tent with the invention,in the frame detection unit 200 of the
`channel. In conventional absolute RSSI thresholding, the
`embodiment shownin FIG.2, an RSSI unit 220is provided to
`energy threshold validation signal would be held as long as
`receive and down samplethe digital adjusted gain signal for
`the strong interference is perceived by the AGC 210 as the
`
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`the inbound signal to verify the presence of a valid DSSS
`signal. In particular, the Barker correlator 330, in addition to
`feeding the div2 downconverter 335 and RAKE filter 340,
`presents the correlation result of the digital form of the
`inbound basebandsignal against the 802.11 Barker PN code
`to the input of the carrier sense unit 240.
`A peak-to-sidelobe ratio determination unit 910 (FIG. 9)
`examines the real (1) and complex (Q) components ofthis
`correlation result to help determinethat, infact, the inbound
`signal presents a valid Barker modulated preamble or header.
`In particular, the determination unit 910 calculates a peak-to-
`sidelobe average ratio as follows:
`
`max(|/;| + |Q;))
`SQ] = ———___________,
`seEll +101 -2xmoxtt1 +1210}
`
`8
`foundin the correct sequence,e.g. the SFD field is confirmed
`immediately after the end ofthe syne symbols, approximately
`128 usec (long preamble format) or 28 usec (short preamble
`format) after the signal begins, it may be assumed that the
`inboundsignalis valid, and that CCA unit 250 will report a
`busy channel for the remaining duration of the packet. If,
`however, the SID or other selected field not found at its
`predesignated place, the inbound signal is presumedinvalid
`and the CCA unit will override the ED modelogic and tran-
`sition CCA false, indicating that the channelis free and the
`transceiver 100 may transmit.
`Alternatively, if the selected CCA modealso includes car-
`rier sense thresholding (e.g. 802.11/802.11b modes3 or5), it
`is less likely, though still possible, for the CCA unitto still
`report a false busy, particularly wherethecarrier sense thresh-
`old levels are kept low. Again, SFD or other predefinedfield
`verification can be used to clear a false busy a number of
`microseconds after perception of the inbound signal by the
`AGC 210 begins.
`Though not shownin FIG. 2, an absolute energy thresh-
`olding unit maybe provided in addition to or as an alternative
`to the relative energy detection unit 235 to perform routine
`absolute energy thresholding of the inboundsignal.
`Referring back to FIG.2, the frame detection unit 200 also
`includesthe capability of providing carrier sense feedback to
`the CCA unit 250 through the carrier sense unit 240. This
`carrier sense unit 240 takesthe results ofBarker correlation to
`
`7
`inbound signal, thus at least potentially causing a conven-
`tional CCA unit implementing legacy CCA modes to con-
`sider the channel to be busy for the duration of the interfer-
`ence and erroneously holding the transceiver in receive mode.
`Take, for example, the gain curves shown in FIG.7. Theleft
`curve 710 illustrates the RSSI output E, and the conventional
`absolute threshold is shown as horizontal line 730. As L
`transitions from a relatively high value to a relatively low
`value overtime (indicating that the AGC has unlocked andis
`transitioning from the high gain transmitprotect state to a low
`gain receivestate) a conventional energy threshold validation
`signal would transition true once F crosses the absolute
`threshold 730 and would remain so until the inbound signal
`diminishes significantly or disappears, thereby permitting E
`to drift upward towards high gain or receive mode.
`However, in “relative energy thresholding” according to
`the present embodiment, the energy threshold validationsig-
`nal is only held high between the point from where the dif-
`ference between the n 720 and E 710 curves exceedthe first
`energy detection threshold Tz,,, to where it no longer
`exceeds the second energy detection threshold Tz... This
`helps the CCA unit discriminate between interference and a
`valid signal and respond more quickly with an idle channel
`determination in the presenceof such interference, especially
`where additional validating criteria based on the content of
`the inboundsignal suchas preamble, carrier sense and header
`validation is assessed.
`In the embodiment shownin FG.2, the noise floor unit 230
`is only operational(and so tracks the RSSI output E) while the
`channel busy signal (CCA)is false and the AGCis notin the
`aforementioned gain unlock state (as indicated by the
`GAIN_UNLOCKsignal). Whennotoperational the output of
`the noise floor unit 230, n is frozen to the last tracked value.
`For example, if the inbound energyis a valid 802.11/