USOO7403539B1
`
`(12) United States Patent
`US 7,403,539 B1
`(10) Patent N0.:
`
`(45) Date of Patent: Jul. 22, 2008
`Tang et a1.
`
`(54) CLEAR CHANNEL ASSESSMENT IN
`WIRELESS COMMUNICATION S
`
`(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(1)) by 1029 days.
`
`(21) Appl.No.: 10/268,156
`
`(22) Filed:
`
`Oct. 9, 2002
`
`(51)
`
`Int. Cl.
`(2006.01)
`H04L 12/413
`(2006.01)
`H04L 12/28
`(52) US. Cl.
`...................................................... 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
`
`5/2000 Kamerrnan et a1.
`6,067,291 A *
`......... 370/338
`6.469,997 B1* 10/2002 Dorenbosch et a1.
`.. 370/337
`
`6,675,012 B2 *
`l/2004 Gray ....................
`.. 455/423
`6.834,045 B1* 12/2004 Lappetelainen et a1.
`..... 370/329
`......... 455/2341
`2002/0061738 A1*
`5/2002 Simmons et al.
`
`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, ANSI/lEEE std. 802.1 1, first edition, Sponsor
`LAN MAN Standards Committee oflEbE Computer Society, “Part
`11: Wireless LAN Medium Access Control (MAC) and Physical
`Laycr (PHY) specifications,” 1999.
`Geier, Jim, “Wireless LANs, Second Edition", SAMS Publishing,
`2000,1311 137—151.
`and Mctropolitan Arca
`“Local
`802.16,
`IEEE
`Standards
`NetworksiPart 16: Interface for Fixed Broadband Wireless Access
`Systems,” Oct, 1, 2004, 893 pages.
`IEEE Std 802.11a-1999,SponsorLANMIINStandards Committee of
`lEEE 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 ExamineriHassan Kizou
`Assistant ExamineriBetty Lee
`
`(57)
`
`ABSTRACT
`
`Techniques for and apparatus capable of implementing
`packet detection and signal recognition in Wireless commu-
`nications systems are disclosed. In particular, the disclosed
`techniques and apparatus incorporate at least one of relative
`energy detection operable on assessment of a relative energy
`threshold for an inbound signal borne across an RF chalmel;
`carrier sense operable upon on assessment of at lea st one of a
`peak-to-sidelobe ratio and peak-to-peak distance defined by
`the inbound signal, and comparison operable upon demodu-
`lated data corresponding to the inbound signal as compared to
`predetermined preamble data. Clear channel assessment is
`performed based on determinations undertaken by one or
`more of the aforementioned relative energy detection, carrier
`sense and comparison operations.
`
`1999, SponsorLANA/[ANStandards Committee
`IEEE std. 802.11b
`ofIEEE Computer Society, “Part 1 1: Wireless LAN Medium Access
`
`145 Claims, 7 Drawing Sheets
`
`
`
`
`225
`
`ME fificfiofi‘ '1
`CCA UNIT
`
`
`
`
`
`to MAC IIF 125
`-——>
`Inbound data
`
`120
`
`/
`
`375
`
`
`
`HI_RATE_PSDU
`
`
`
`Page 1 of 26
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`SAMSUNG EXHIBIT 1032
`
`Page 1 of 26
`
`SAMSUNG EXHIBIT 1032
`
`

`

`US 7,403,539 B1
`
`Page 2
`
`OTHER PUBLICATIONS
`
`IEEE P802.11g/D8.2-Apr. 2003, Sponsor LAN/AJAN 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.
`IEEE Std 802 . 1 6a—2003 , Sponsor LAN/MdNStandards Committee of
`IEEE Computer Society and the IEEE Microwave Theory and Tech7
`niques Society, “Part 16: Air Interface for Fixed BroadbandVVirelcss
`Access Systems-Amendment 2: Medium Access Control Modifica-
`tions and Additional Physical Layer Specifications for 2-11 GHZ.”
`2003, pp. 1-292.
`to IEEE Std 802.11-1999)
`IEEE Std 802.11a-1999(Supp1ement
`[Adopted by ISO/IEC and redesignated as ISO/IEC 8802-11: 1999/
`Amd 1:2000(E)J; 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 Layer in the 5 GHZ Band; LAN/MAN Standards Committee of
`the IEEE Computer Society, 91 pages.
`IEEE P802.11g/D8.2. Apr. 2003 (Supplement to ANSI/IEEE Std.
`802.11-1999(Rea1T 2003)); DRAFT Supplement to STANDARD
`[for] Information Technology - Telecommunications and information
`exchange between systems - Local and metropolitan area networks -
`Specific requirements - Part 11: Wireless LANT Medidum Access
`Control (MAC) and Physical Layer (PHY) specifications: Further
`Higher Data Rate Extension in the 214 GHZ Band; LAN/M AN Stan-
`dards Committee ofthe IEEE Computer Society; 69 pages.
`IEEE Std 802.1621-2003 (Amendment to IEEE Std 80216-2001),
`IEEE Standard 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 Theory and Techniquest Society; Apr. 1, 2003;
`3 18 pages.
`
`* cited by examiner
`
`Page 2 of 26
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`US 7,403,539 B1
`
`1
`CLEAR CHANNEL ASSESSMENT IN
`WIRELESS COlVlMUNICATIONS
`
`TECHNICAL FIELD
`
`The present invention generally relates to wireless commu—
`nications, and is specifically concerned with clear channel
`assessment techniques to determine the presence or absence
`of a valid inbound signal.
`
`
`
`BACKGROUND OF THE INVENTION
`
`
`
`The past few years has witnes sed 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 technology inparticu-
`lar, described in the IEEE Standard 802.11b-1999 Supple-
`
`ment to the ANSI/IEEE Standard 802.11, 1999 edition, col—
`lectively incorporated herein fully by reference, and more
`commonly referred to as “802.11b” or “WiFi”, has become
`the darling of the information technology industry and com-
`puter enthusiasts alike as a wired LAN/WAN alternative
`because of its potential 11 Mbps effective throughput, ease of
`installation and use, and transceiver component costs make it
`a real and convenient alternative to wired 10 BaseT Ethernet
`and othcr cablcd data networking altcrnativcs. With 802.1 lb,
`workgroup-sized networks can now be deployed in a Juilding
`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 equipment is backwards compatible with the ear-
`lier 802.11 1 W2 Mbps throughput standard, thereby further
`reducing deployment costs in legacy wireless systems.
`802.1 lb achieves relatively highpayload data transmission
`rates or effective throughput via the use of orthogonal class
`modulation in general, and, more particularly, 8-chip comple-
`mentary code keying (“CCK”) and a 1 1 MHZ chipping rate 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 symbol is 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 into the analog domain pre-
`pares these CCK symbols for delivery over a wireless
`mcdium 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 overhead are still
`modulated using the 802.11 compliant Barker spreading
`sequence at an 11 MHZ chipping rate. In particular, the pre—
`amble (long formatil 44 bits, short formati72 bits) is uni-
`versally modulated using DBPSK (“differential binary phase
`shift keying”) modulation resulting in a 1 Mbps effective
`throughput, while the header portion may be modulated using
`either DBPSK (long preamble fonnat) or DQPSK (short pre-
`amble format) to achieve a 2 Mbps effective throughput.
`An IEEE 802.1 lb compliant receiver receives and down—
`converts an incident inbound RF 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. The pre—
`amble and header portions are Barker correlated and then
`
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`Page 10 of 26
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`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,
`including the 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 PLCP payloadportion are each correlated against
`64 candidate wavefomrs in received per symbol sequence in
`combination with DQPSK demodulation to verify and
`reverse map each into the underlying bitstream data of inter—
`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 GI Iz ISM band and must therefore coexist
`with quite an array ofdissimilar signals operating in the same
`frequency, including microwave ovens and digital phones. By
`definition, there are no licensure restrictions 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, thcrc is an expected amormt of ambicnt noise that
`the 802.1 1/802.1 lb transceivers must tolerate but still be able
`to transmit, but should not attempt to transmit while another
`in-range 802.11/802.11b transceiver is transmitting so as to
`maximize chalmel 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 such traffic. Likewise, it is desirable that these
`transceivers should be free to transmit on the operating chan-
`nel while that channel is free of 802.11/802.11b traffic, even
`in the presence of a tolerable amount of noise or interference.
`To this end, the 802.1 1 and 802.1 lb standards specify clear
`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 RF channel
`which do not meet these CCA guidelines are considered to
`bear either corrupted frames, or represent interference or
`noise int 1e channel. The 802.11/802.1 1b CCA guidelines are
`organized in modes as follows:
`CCA Mode 1: Energy above threshold. CCA shall report a
`busy medium upon detecting any received energy above
`the 3D threshold.
`
`CCA Mode 2: Carrier Sense only. CCA shall report a busy
`mecium only upon detection of a DSSS signal. This
`signal may be above or below the ED threshold.
`CCA Mode 3: Carrier Sense with energy above threshold.
`CCA shall report a busy medium upon detection of a
`DSSS signal with energy above the ED threshold.
`CCA Mode 4 (802.11b): Carrier sense with timer. 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 medium after the 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 PHY receiver 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 modes 1, 4 or 5.
`Three of the five conventional CCA modes require thresh-
`olding inbound signal energy, and so this guideline is believed
`important. However, conventional transceivers simply c0111-
`
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`US 7,403,539 B1
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`3
`pare inbound signal energy against the specified threshold,
`and report an energy threshold validation signal whenever the
`threshold is exceeded. Thus, the presence of strong interfer-
`ence in the operating channel, will cause (in the case ofa CCA
`mode 1 implementation) or potentially may (in the case of a
`CCA mode 3 or 5) cause a false busy to be reported, and thus
`prevent the transceiver from transmitting. which may in turn
`cause transmission delay and lower effective data throughput.
`Moreover, to implement CCA modes 275, conventional
`CCA carrier sense techniques are used to determine ifa DSSS
`or High Rate PHY inbound signal is present, typically by
`thresholding a measure of the perceived Barker code lock.
`However, known techniques are relatively complex and are
`thus power inefficient and expensive to implement. Both cost
`and power consumption reduction are critical design goals in
`802.11/8021 lb transceiver implementation,
`it would be
`advantageous if simpler carrier sense techniques could be
`incorporated without materially affecting carrier sense sensi-
`tivity or recognition performance.
`Further, while conventional CCA techniques look for valid
`PLCP header information (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 energy threshold, but the
`receiver is 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.
`
`SUMMARY OF THE INVENTION
`
`To address these and other perceived shortcomings and
`desires, the present invention is directed in part to a packet
`detection unit and signal recognition method that includes at
`least one ofrelative energy detection operable on assessment
`of a relative energy threshold for an inbound signal borne
`across an RF channel, carrier sense operable upon on assess-
`ment of at least one of a peak-to-sidelobe ratio and peak-to-
`peak distance defined by the inbound signal, and comparison
`operable upon demodulated data corresponding to the
`inbound signal as compared to predetermined preamble data.
`Clear channel assessment is performed based on detennina-
`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 more of these signal recognition aspects.
`Additional aspects and advantages of this 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 embodiment of the invention.
`
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`FIG. 2 is a more detailed block diagram of a receive base-
`band processing unit shown in FIG. 1.
`FIG. 3 is a detailed block diagram of a packet detection unit
`according to an altemative embodiment of the invention.
`FIG. 4 is a state transition diagram for the CCA unit shown
`in FIG. 2.
`FIG. 5 is a state transition diagram for the CS unit shown in
`FIG. 2.
`
`FIG. 6 is a state transition diagram for the ED unit shown in
`FIG. 2.
`FIG. 7 is a sample plot of gain perceived by the ED unit
`over time.
`
`FIG. 8 is a block diagram for the ED unit shown in FIG. 2.
`FIG. 9 is a block diagram ofthc CS unit shown in FIG. 2.
`FIG. 10 is a block diagram of the CCA unit shown in FIG.
`
`2.
`
`DETAILED DESCRIPTION OF THE
`
`
`EMBODIMENTS
`
`Turning now to 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 802.1 lb compliant
`PLCP frame are picked up by the duplex antenna 10 and
`routed to the RF receiver unit 115 of a receiver 150 arranged
`in a manner consistent with the present invention. The RF
`receiver unit 115 performs routine downconversion and auto-
`matic gain control of the inbound RF signals, and presents an
`analog baseband signal containing the aforementioned
`802.11b PLCP frame to 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 chalmel busy consistent 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).
`Once recovered by the receive baseband processor 120, the
`inbound data contained in the PSDU ofeach received 802.1 lb
`PLCP frame is delivered to a network interface such as the
`MAC layer interface 125 and then on to higher layer applica-
`tions and devices being serviced by the transceiver 100.
`Outbound data intended for wireless transmission originat-
`ing from the device(s) or application(s) being serviced by the
`transceiver 100 are delivered to the transmit baseband 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 modes are transferred to the trans-
`mit baseband processor as well for each PLCP frame/packet.
`The transmit baseband processor 135 formulates appropriate
`802.11b PLCP frame, and symbol encodes the outbound data
`as specified by the PMD sublayer to generate a complete
`outbound 802.1 lb 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 802.1 lb physical layer requirements.
`Though not shown in FIG. 1, the transceiver 100 may form
`an operational part 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 infonnation
`
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`5
`processing apparatus. This network interface apparatus may
`alternatively form an operational component of a wireless
`communications access point such as a base station as will be
`appreciated by these ordinarily skilled in the art.
`Turning now to FIG. 2, FIG. 2 is a more detailed block
`diagram ofthe receive baseband processor 120 shown in 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
`PLCP frame recovered by the RF receiver unit 110 of the
`receiver 150 is fed to the input of the automatic gain control
`unit (AGC) 210. With the assistance ofa feedback loop tied to
`the input ofthe FIR filter 315, thcAGC 210 adjusts the gain of
`the inbound baseband signal to maximize the dynamic range
`and performance of the analog to digital converter 310, as is
`known in the art, assuming the inbound signal is valid. The
`AGC 210 also reports the gain adjusted signal to the RSSI unit
`220 of the frame detection unit 200 for channel busy, as will
`be described in greater detail below.
`The gain adjusted inbound analog signal generated by the
`AGC 210 is then sent to the digital converter 310 to convert it
`into digital form. With the aid ofthe 44 MHZ clock, the ADC
`produces a corresponding digital signal sampled at 44 MHZ.
`cht, this digital signal passes through the digital FIR LPF
`315 to reject out-of—band interference, and the down sampler
`320 to provide a 22 Mhz digital signal potentially bearing a
`PLCP frame of interest.
`This 22 MHZ signal next encounters two parallel baseband
`demodulation pathways. The first demodulation pathway,
`including the Barker correlator 330, down sampler 335, Rake
`340 and the down sampler 345 is used to recover a despread
`1 MI Iz signal representing the preamble and header portions
`of the inbound frame for symbol demodulation by the com-
`bination DBPSK/DQPSK demodulator 375. This
`first
`demodulation pathway—demodulator combination 375 is also
`used for symbol decoding a base 802.1 1 PLCP frame payload
`in l Mbps/2 Mbps modes. The second demodulation pathway
`is used to symbol demodulate a high rate 802.11b payload
`portion of the inbound frame, and includes a 22 MHZ to 11
`MHZ down sampler 350 following by a decision feedback
`equalizer 355. To begin the CCK symbol decode process for
`802.11b compliant payloads at 11 Mbps or 5.5 Mbps trans-
`mission modes, a CCK correlator 360 is provided.
`An 802.11b compliant receive state machine (not shown)
`issues the HI_RATE_PSDU semaphore to control the modu-
`lation pathway selection mux 370 based on which portion of
`the inbound frame is being demodulated, as well whether 5.5
`Mbps+ payload modulation modes are specified. The combi-
`nation DBPSK’DQPSK demodulator 375 is used to recover
`the symbol encoded inbound data presented in the preamble,
`header and payload portions. The DBPSK/DQPSK demodu-
`lator is clocked at the symbol rate; i.e., l MHZ for 1 Mb and
`2 Mb modes, and 1.375 MHZ for 5.5 Mb and 11 Mb modes.
`Aftcr symbol demodulation, the recovered inbound data is
`descrambled by the descrambler 380 in a known fashion, and
`delivered to the MAC interface 125 (FIG. 1).
`Still referring to FIG. 2, the frame detection unit 200 is
`provided to determine if the inbound baseband signal recov-
`ered from an RF channel tuned by the RF receiver 115 (FIG.
`1) is a valid signal at least capable of bearing an 802.1 lb
`compliant PLCP frame, and if so, keep the transceiver 100 in
`receive mode for the duration of the frame.
`To implement relative energy threshold detection consis-
`tent with the invention, in the frame detection unit 200 of the
`embodiment shown in FIG. 2, an RSSI unit 220 is provided to
`receive and down sample the digital adjusted gain signal for
`
`5
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`10
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`15
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`20
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`30
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`35
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`40
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`45
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`60
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`6
`the inbound baseband signal generated by the AGC 210 to a 2
`MHZ gain signal E, with Ek representing the signal E at the kth
`2 MHZ sample in time. E is sent directly to one input of the
`relative energy detection unit 235, and is concurrently sent to
`a noise floor unit 230. The noise floor unit 23 0 here constitutes
`
`an IIR low pass filter to generate a 0.5 usec delayed signal 11
`which tracks the original signal E using the following rela-
`tionship:
`Hk:(‘LEk+(l 4011),,1
`
`The relative energy threshold detection unit 235 receives
`
`both E and n, and calculates the difference between the gain
`values each represents (via the gain dilferential unit 810
`shown in FIG. 8) as a gain change over time. Through the
`control unit 820, the relative energy threshold detection unit
`235 compares the gain change over time against one of two
`preferably programmable energy detection thresholds, based
`on the current state of the control unit 820. Though not
`required, the relative energy threshold detection unit 235 of
`the present embodiment operates on a 1 MHZ synchronized
`clock, and so F, and n are effectively downsampled at a l MHZ
`rate before their difference is compared against one of these
`thresholds. As shown in FIG. 6, the control unit 820 functions
`as a finite state machine capable of switching between two
`states 610, 620.
`Referring to FIG. 6, the control unit 820 (FIG. 8) of the
`energy threshold detection unit 235 (FIG. 2) is initialized to
`state 610 at the beginning of each frame detection processing
`sequence (such as when a new inbound signal is first received
`on the operating RF channel). As such, the energy threshold
`validation signal GED) is set to false (e. g. logic level 0). While
`in state 610, the control unit 820 monitors the gain change
`over time generated by the gain differential unit 810. The
`control unit 820 remains in state 610 while the gain remains
`relatively stable (transition or trans. 3). Ifthe gain change over
`time exceeds a first energy detection threshold, meaning that
`the gain is changing rapidly with the AGC 210 in a gain
`unlock condition and attempting to transition to receive mode
`from transmit protect mode in response to an inbound signal
`having relatively significant energy, the control unit 820 tran-
`sitions to state 820 (trans. 2) and the energy threshold valida—
`tion signal transitions to true (logic level 1). In turn, assertion
`oftrue energy threshold validation signal may cause assertion
`of the chalmel busy signal (CCA) true by the CCA unit 250,
`depending on the CCA modc bcing implcmcntcd.
`The control unit 820 remains at state 620 (trans. 4) while
`the gain change over time continues to exceed a second
`energy detection threshold to hold energy threshold valida—
`tion signal true. However, once the gain change over time
`settles and the gain stabilizes, the control unit 820 transitions
`back to state 610 (2), and the energy threshold validation
`signal transitions back to logic level 0 or false.
`Note that in this embodiment the second energy detection
`threshold is less than the first energy detection threshold to
`lengthen the window in which the energy threshold validation
`signal is asserted high by the control unit 820. However, in
`altemative embodiments, the first and second thresholds may
`be equal or even reversed depending on particular CCA
`implementation goals.
`Due to this “relative energy thresholding”, certain gain
`signal transitioning rather than the receive/transmit state of
`the AGC 210 is used to toggle the energy threshold validation
`signal. This difference is subtle, yet important in handling
`relatively strong, persistent
`interference in the operating
`channel. In conventional absolute RSSI thresholding, the
`energy threshold validation signal would be held as long as
`the strong interference is perceived by the AGC 210 as the
`
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`US 7,403,539 B1
`
`8
`found in the correct sequence, e.g. the SFD field is confirmed
`immediately after the end ofthe sync symbols, approximately
`128 usec (long preamble format) or 28 usec (short preamble
`format) after the signal begins, it may be assumed that the
`inbound signal is valid, and that CCA unit 250 will report a
`busy channel for the remaining duration of the packet. If,
`however, the SFD or other selected field not found at its
`predesignated place, the inbound signal is presumed invalid
`and the CCA unit will override the ED mode logic and tran-
`sition CCA false, indicating that the channel is free and the
`transceiver 100 may transmit.
`Altematively, if the selected CCA mode also includes car-
`rier sense thresholding (e.g. 802.11/802.11b modes 3 or 5), it
`is less likely, though still possible, for the CCA unit to still
`report a false busy, particularly where the carrier sense thresh-
`old levels are kept low. Again, SFD or other predefined field
`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 shown in FIG. 2, an absolute energy thresh-
`olding unit may be provided in addition to or as an altemative
`to the relative energy detection unit 235 to perform routine
`absolute energy thresholding of the inbound signal.
`Referring back to FIG. 2, the frame detection unit 200 also
`includes the capability of providing carrier sense feedback to
`the CCA unit 250 through the carrier sense unit 240. This
`carrier sense unit 240 takes the results ofBarker correlation to
`
`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 baseband signal 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 (I) and complex (Q) components of this
`correlation result to help determine that, in fact, 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:
`
`m§X(|1i|+|Qi|)
`SQ]:—1'
`as lit-I + IQzI 72xngm<llil + law]
`
`,
`
`where i:half-chip index:0 .
`
`.
`
`. 21. Or, alternatively:
`
`,
`
`‘ 1
`
`maxtllzl +|Qz|)
`
`SQ]:
`
`E[Z_(|1z|+lQ;|)—2><maxtllz| +|Qz|)- Zt|1k|+|Qk|)]
`
`. 21,
`.
`where i, k are half-chip indexes each ranging from 0 .
`where k is the index of four sidelobes relatively distant from
`a local peak in the received signal. In the latter case, near-peak
`sidelobes are excluded from the SQl computation to coun—
`teract potential multipath interference and more effectively
`validate the inbound signal as being 802.1 1/802.1 lb compli-
`ant.
`
`In order to better validate the inbound signal as bearing
`valid Barker modulated information, the carrier sense unit
`240 also includes a peak—to—peak detection unit 920 (FIG. 9)
`to calculate the distance between consecutive peaks in the
`
`
`
`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 curation of the interfer-
`ence and erroneously holding the transceiver in receive mode.
`Take, for example, the gain curves shown in FIG. 7. The left
`curve 710 illustrates the R881 output 3, and the conventional
`absolute threshold is shown as horizontal line 730. As E
`transitions from a relatively high value to a relatively low
`value over time (indicating that the AGC has unlocked and is
`transitioning from the high gain transmit protect state to a low
`gain receive state) a conventional energy threshold validation
`signal would transition true once F crosses the absolute
`threshold 730 and would remain so until the i