Source: http://www.patentgenius.com/patent/8565255.html
Timestamp: 2018-11-14 19:28:48
Document Index: 572005550

Matched Legal Cases: ['art 11', 'art 11', 'art 16', 'art 11', 'art 11', 'art 16']

Clear channel assessment in wireless communications - Patent # 8565255 - PatentGenius
Clear channel assessment in wireless communications
8565255 Clear channel assessment in wireless communications
Application: 13/013,142
Inventors: Tang; Kevin Hsiao-Cheng (Milpitas, CA)
Hsu; Yungping (Saratoga, CA)
Hu; Guorong (Sunnyvale, CA)
Feng; Weishi (San Jose, CA)
Primary Examiner: Banks Harold; Marasha D
U.S. Class: 370/445; 370/338; 370/431; 370/441; 370/447; 370/462; 455/234.1; 455/423
International Class: H04W 84/12; H04L 12/44; H04W 28/04
Foreign Patent Documents: 1124337
Other References: IEEE Std 802.11a-1999 (Supplement to IEEE Std 802.11-1999) [Adopted by ISO/IEC and redesignated as ISO/IEC 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 Physical Layer in the 5 GHz Band; LAN/MAN Standards Committee of the IEEE Computer Society; 91 pages. cited by applicant.
IEEE 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 areanetworks--Specific requirements--Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Further Higher Data Rate Extension in the 2.4 GHz Band; LAN/MAN Standards Committee of the IEEE Computer Society; 69 pages. citedby applicant.
IEEE Std 802. 16a-2003 (Amendment to IEEE Std 802.16-2001), IEEE Standard for Local and metropolitan area networks; Part 16: Air Interface for Fixed Broadband Wireless Access Systems--Amendment 2: Medium Access Control Modifications and AdditionalPhysical Layer Specifications for 2-11 GHz; IEEE Computer Society and the IEEE Microwave Theory and Techniquest Society; Apr. 1, 2003; 318 pages. cited by applicant.
IEEE Std. 802.11 b-1999; Sponsor LAN/MAN Standards Committee of IEEE Computer Society, "Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, Higher-Speed Phycisal Layer Extension in 2.4 GHz Band"; Sep. 1999; pp.1-89. cited by applicant.
International Standard, ANSI/IEEE std. 802.11, first edition, Sponsor LAN/MAN Standards Committee of IEEE Computer Society, "Part 11: Wireless LAN Medium Access Control (MAC) and Physical Laver (PHY) specifications"; 1999. cited by applicant.
Geir, Jim, "Wireless LANs, Second Edition", SAMS Publishing; 2000; pp. 137-151. cited by applicant.
IEEE Standards 802.16, "Local and metropolitan area networks; Part 16: Interface for Fixed Broadband Wireless Access Systems"; Oct. 1, 2004; 893 pages. cited by applicant.
Abstract: A transceiver including: an antenna configured to a receive a first signal transmitted on a radio frequency channel; and a peak-to-sidelobe ratio determination unit configured to generate a second signal based on a ratio, in which the ratio is based on a peak value and a sidelobe value, and the peak value and the sidelobe value are determined based on a non-correlated version of the first signal. The transceiver further includes a carrier sense unit configured to, based on the second signal, generate a third signal indicating (i) whether the radio frequency channel is busy or (ii) whether the first signal is valid.
1. A transceiver comprising: a receiver configured to a receive a first signal transmitted on a radio frequency channel; a determination device configured to (i) based on acorrelated version of the first signal, determine a peak value and a sidelobe value, (ii) based on the peak value and the sidelobe value, determine a peak-to-sidelobe ratio, and (ii) generate a second signal based on the peak-to-sidelobe ratio; adistance device configured to determine a consecutive peak-to-peak distance based on the correlated version of the first signal; and a channel assessment device configured to, based on the second signal and the consecutive peak-to-peak distance,generate a third signal indicating (i) whether the radio frequency channel is busy, or (ii) whether the first signal is valid.
2. The transceiver of claim 1, further comprising: a signal strength module configured to generate a fourth signal based on the first signal, wherein the fourth signal indicates a gain value; and a detection device configured to generate afifth signal based on (i) a first threshold, and (ii) a difference between the fourth signal and a filtered version of the fourth signal, wherein the channel assessment device is configured to generate the third signal based on the fifth signal.
3. A transceiver comprising: an antenna configured to a receive a first signal transmitted on a radio frequency channel; a determination unit configured to generate a second signal based on a peak-to-sidelobe ratio, wherein thepeak-to-sidelobe ratio is based on a peak value and a sidelobe value, and wherein the peak value and the sidelobe value are determined based on a correlated version of the first signal; a channel assessment device configured to, based on the secondsignal, generate a third signal indicating (i) whether the radio frequency channel is busy, or (ii) whether the first signal is valid; a signal strength module configured to generate a fourth signal based on the first signal; and a detection deviceconfigured to generate a fifth signal to have (i) a first state when a first threshold is greater than a difference between the fourth signal and a filtered version of the fourth signal, or (ii) a second state when the first threshold is less than thedifference between the fourth signal and the filtered version of the fourth signal, wherein the channel assessment device is configured to generate the third signal based on the fifth signal.
4. The transceiver of claim 2, wherein the determination device is configured to generate the second signal based on whether the correlated version of the first signal includes at least one of (i) a valid direct sequence spread spectrum signal,or (ii) a valid header modulated using a Barker spreading sequence.
5. The transceiver of claim 2, further comprising a sixth device configured to: compare demodulated data with predetermined preamble data, wherein the demodulated data is generated based on the first signal; and in response to the demodulateddata matching the predetermined preamble data, generate a sixth signal indicating whether a preamble of the first signal is valid.
6. The transceiver of claim 5, wherein the channel assessment device is configured to generate the third signal based on sixth signal.
7. The transceiver of claim 2, wherein the determination device is configured to generate the second signal to have (i) a first state in response to the peak-to-sidelobe ratio being greater than a second threshold, and (ii) a second state inresponse to the peak-to-sidelobe ratio being less than a third threshold.
8. The transceiver of claim 7, wherein the third threshold is less than the second threshold.
9. The transceiver of claim 7, wherein the determination device is configured to generate the second signal based on (i) whether the consecutive peak-to-peak distance is less than a predetermined distance, and (ii) whether the peak-to-sideloberatio is greater than the second threshold.
10. The transceiver of claim 7, wherein: the difference between the fourth signal and the filtered version of the fourth signal indicates a change in gain for the first signal; and the channel assessment device is configured to generate thefifth signal based on (i) whether the change in gain exceeds a fourth threshold, and (ii) whether the change in gain is less than a fifth threshold.
11. The transceiver of claim 10, wherein the channel assessment device comprises: a timer configured to generate a timeout signal subsequent to a predetermined timer period; and a control device configured to, based on (i) the timeout signal,and (ii) at least one of the second signal or the fifth signal, generate the third signal indicating the radio frequency channel is idle.
12. The transceiver of claim 2, wherein the detection device comprises: a gain device configured to determine a change in gain for the first signal; and a first control device configured to generate the fifth signal based on (i) whether thechange in the gain is greater than a second threshold, and (ii) whether the change in the gain is less than a third threshold.
13. The transceiver of claim 12, wherein the channel assessment device is configured to, based on at least one of the second signal or the fifth signal, generate the third signal indicating the radio frequency channel is idle.
14. The transceiver of claim 12, wherein channel assessment device comprises: a timer configured to generate a timeout signal subsequent to a predetermined time period; and a second control device configured to, based on (i) the timeoutsignal, and (ii) at least one of the second signal or the fifth signal, generate the third signal indicating that the radio frequency channel is idle.
15. The transceiver of claim 1, wherein the channel assessment device is configured to: receive demodulated data corresponding to the first signal; generate a fourth signal indicating whether a preamble of the first signal is valid based on(i) the demodulated data, and (ii) predetermined preamble data; and generate the third signal based on the fourth signal.
16. A method comprising: receiving a first signal transmitted on a radio frequency channel; based on a correlated version of the first signal, determining a peak value and a sidelobe value; based on the peak value and the sidelobe value,determining a peak-to-sidelobe ratio; generating a second signal based on the peak-to-sidelobe ratio; determining a consecutive peak-to-peak distance based on the correlated version of the first signal; and based on the second signal and theconsecutive peak-to-peak distance, generating a third signal indicating (i) whether the radio frequency channel is busy, or (ii) whether the first signal is valid.
17. The method of claim 16, further comprising: generating a fourth signal based on the first signal, wherein the fourth signal indicates a gain value; generating a fifth signal based on (i) a first threshold, and (ii) a difference between thefourth signal and a filtered version of the fourth signal; and generating the third signal based on the fifth signal.
18. A method comprising: receiving a first signal transmitted on a radio frequency channel; generating a second signal based on a ratio, wherein the ratio is based on a peak value and a sidelobe value, and wherein the peak value and thesidelobe value are determined based on a correlated version of the first signal; based on the second signal, generating a third signal indicating (i) whether the radio frequency channel is busy, or (ii) whether the first signal is valid; generating afourth signal based on the first signal; and generating a fifth signal to have (i) a first state when a first threshold is greater than a difference between the fourth signal and a filtered version of the fourth signal, and (ii) a second state when thefirst threshold is less than the difference between the fourth signal and the filtered version of the fourth signal, wherein the third signal is generated based on the fifth signal.
19. The method of claim 17, further comprising generating the second signal based on whether the correlated version of the first signal includes at least one of (i) a valid direct sequence spread spectrum signal, or (ii) a valid headermodulated using a Barker spreading sequence.
20. The method of claim 17, further comprising: generating demodulated data based on the first signal; comparing the demodulated data to predetermined preamble data; and in response to the demodulated data matching the predetermined preambledata, generating a sixth signal indicating whether a preamble of the first signal is valid.
The past few years has witnessed the ever-increasing availability of relatively cheap, low power wireless data communication services, networks and devices, promising near wire speed transmission and reliability. One technology in particular,described in the IEEE Standard 802.11b-1999 Supplement to the ANSI/IEEE Standard 802.11, 1999 edition, collectively incorporated herein fully by reference, and more commonly referred to as "802.11b" or "WiFi", has become the darling of the informationtechnology industry and computer 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 10BaseT Ethernet and other cabled data networking alternatives. With 802.11b, workgroup-sized networks can now be deployed in a building in minutes, a campus in days instead of weeks since the demanding task of pulling cable and wiring existing structuresis eliminated. Moreover, 802.11b compliant wireless networking equipment is backwards compatible with the earlier 802.11 1 M/2 Mbps throughput standard, thereby further reducing deployment costs in legacy wireless systems.
802.11b achieves relatively high payload data transmission rates or effective throughput via the use of orthogonal class modulation in general, and, more particularly, 8-chip complementary code keying ("CCK") and a 11 MHz chipping rate to bearthe 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 of the CCK code symbol is quaternary phase modulated using QPSK("quadrature phase shift keying") modulation 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 analogdomain prepares 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 LayerConvergence 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. Inparticular, the preamble (long format--144 bits, short format--72 bits) is universally modulated using DBPSK ("differential binary phase shift keying") modulation resulting in a 1 Mbps effective throughput, while the header portion may be modulated usingeither DBPSK (long preamble format) or DQPSK (short preamble format) to achieve a 2 Mbps effective throughput.
An IEEE 802.11b compliant receiver receives and downconverts 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, headerand payload portions in sequence. The preamble and header portions are Barker correlated and then either DBPSK or DQPSK demodulated based on the preamble format used to recover synchronization data and definitional information concerning the receivedPLCP frame, including the data rate (Signal field in the PLCP header) and octet length (Length field in the PLCP header) of the variable-length payload or PSDU portion. The CCK encoded symbols forming the PLCP payload portion are each correlated against64 candidate waveforms in received per symbol sequence in combination with DQPSK demodulation to verify and reverse map each into the underlying bitstream data of interest, 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 GHz ISM band and must therefore coexist with quite an array of dissimilar signals operating in the same frequency, including microwave ovens and digital phones. Bydefinition, there are no licensure restrictions within the available RF channels of the ISM band, so 802.11 and 802.11b compliant transceivers must employ clear channel assessment techniques to determine if it is safe to transmit. In particular, thereis an expected amount of ambient noise that the 802.11/802.11b 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 channel useand 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 isdesirable that these transceivers should be free to transmit on the operating channel 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 802.11 and 802.11b 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 donot meet these CCA guidelines are considered to bear either corrupted frames, or represent interference or noise in the channel. The 802.11/802.11b CCA guidelines are organized in modes as follows: CCA Mode 1: Energy above threshold. CCA shall report abusy medium upon detecting any received energy above the ED threshold. CCA Mode 2: Carrier Sense only. CCA shall report a busy medium only upon detection of a DSSS signal. This signal may be above or below the ED threshold. CCA Mode 3: Carrier Sensewith 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 mediumupon 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): Acombination 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 according to at least oneof 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 thresholding inbound signal energy, and so this guideline is believed important. However, conventional transceivers simply compare inbound signal energy against the specified threshold, andreport an energy threshold validation signal whenever the threshold is exceeded. Thus, the presence of strong interference in the operating channel, will cause (in the case of a CCA mode 1 implementation) or potentially may (in the case of a CCA mode 3or 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 2-5, conventional CCA carrier sense techniques are used to determine if a DSSS or High Rate PHY inbound signal is present, typically by thresholding a measure of the perceived Barker code lock. However, knowntechniques are relatively complex and are thus power inefficient and expensive to implement. Both cost and power consumption reduction are critical design goals in 802.11/802.11b transceiver implementation, it would be advantageous if simpler carriersense techniques could be incorporated without materially affecting carrier sense sensitivity 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 preamble. Checking for valid preamble receipt would be advantageous,especially where the inbound signal fades potentially below the inbound signal energy threshold, but the receiver is still able to successfully recover recognizable preamble 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 capableof handling further CCA modes other than those defined by the 802.11/802.11b standards, preferably while retaining backwards compatibility with such standards.
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 of relative energy detection operable on assessment of arelative energy threshold for an inbound signal borne across an RF channel, carrier sense operable upon on assessment of at least one of a peak-to-sidelobe ratio and peak-to-peak distance defined by the inbound signal, and comparison operable upondemodulated 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 andcomparison operations.
Further aspects of the present invention include a transceiver, network interface apparatus, and information processor incorporating this packet detection unit, as well as a computer program product including computer readable program codecapable of causing an information processor to perform one or more of these signal recognition aspects.
Turning now to the figures, FIG. 1 illustrates a wireless communications transceiver 100 according to an embodiment of the invention. In this embodiment, inbound RF signals potentially conveying an 802.11 or 802.11b compliant PLCP frame arepicked up by the duplex antenna 110 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 down-conversion and automatic gain control of the inboundRF 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, includingpacket detection and channel busy consistent with the present invention, 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 receiversynchronization (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 of each received 802.11b PLCP frame is delivered to a network interface such as the MAC layer interface 125 and then on to higher layer applications anddevices being serviced by the transceiver 100.
Outbound data intended for wireless transmission originating from the device(s) or application(s) being serviced by the transceiver 100 are delivered to the transmit baseband processor 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 transmit baseband processor aswell 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 802.11b PLCP frame. As the frame orpacket 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.
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 processingapparatus such as a desktop or laptop computer, and may be integrated within and constitute a part of such information processing apparatus. This network interface apparatus may alternatively form an operational component of a wireless communicationsaccess 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 of the receive baseband processor 120 shown in FIG. 1. So as not to obfuscate the teachings of the present invention, several 802.11 and 802.11b compliant directives and signals arenot 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 theassistance of a feedback loop tied to the input of the FIR filter 315, the AGC 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 theinbound 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 the gain adjusted inbound analog signal into digital form. With the aid of the 44 MHz clock, the ADC produces a correspondingdigital signal sampled at 44 MHz. Next, 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 MHz signalrepresenting the preamble and header portions of the inbound frame for symbol demodulation by the combination DBPSK/DQPSK demodulator 375. This first demodulation pathway-demodulator combination 375 is also used for symbol decoding a base 802.11 PLCPframe payload in 1 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 transmission 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 modulation pathway selection mux 370 based on which portion of the inbound frame is being demodulated, as well whether 5.5 Mbps+payloadmodulation modes are specified. The combination DBPSK/DQPSK demodulator 375 is used to recover the symbol encoded inbound data presented in the preamble, header and payload portions. The DBPSK/DQPSK demodulator is clocked at the symbol rate; i.e., 1MHz for 1 Mb and 2 Mb modes, and 1.375 MHz for 5.5 Mb and 11 Mb modes. After 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 recovered from an RF channel tuned by the RF receiver 115 (FIG. 1) is a valid signal at least capable of bearing an 802.11b compliantPLCP frame, and if so, keep the transceiver 100 in receive mode for the duration of the frame.
To implement relative energy threshold detection consistent 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 theinbound baseband signal generated by the AGC 210 to a 2 MHz gain signal E, with E.sub.k representing the signal F 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 anoise floor unit 230. The noise floor unit 230 here constitutes an IIR low pass filter to generate a 0.5 .mu.sec delayed signal n which tracks the original signal E using the following relationship: n.sub.k=.alpha.E.sub.k+(1-.alpha.)n.sub.k-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 differential unit 810 shown in FIG. 8) as a gain change over time. Through the controlunit 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 relativeenergy threshold detection unit 235 of the present embodiment operates on a 1 MHz synchronized clock, and so E and n are effectively downsampled at a 1 MHz rate before their difference is compared against one of these thresholds. As shown in FIG. 6, thecontrol 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 receivedon the operating RF channel). As such, the energy threshold validation signal (ED) 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. Thecontrol unit 820 remains in state 610 while the gain remains relatively stable (transition or trans. 3). If the gain change over time exceeds a first energy detection threshold, meaning that the gain is changing rapidly with the AGC 210 in a gainunlock 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 transitions to state 820 (trans. 2) and the energy threshold validationsignal transitions to true (logic level 1). In turn, assertion of true energy threshold validation signal may cause assertion of the channel busy signal (CCA) true by the CCA unit 250, depending on the CCA mode being implemented.
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 validation signal true. However, once the gain change over time settles andthe 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, inalternative 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 handlingrelatively 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 inbound signal,thus at least potentially causing a conventional CCA unit implementing legacy CCA modes to consider the channel to be busy for the duration of the interference and erroneously holding the transceiver in receive mode. Take, for example, the gain curvesshown in FIG. 7. The left curve 710 illustrates the RSSI output E, 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 hasunlocked 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 E crosses the absolute threshold 730 and would remain so until the inboundsignal 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 validation signal is only held high between the point from where the difference between the n 720 and E 710 curves exceed the first energydetection threshold T.sub.RED1 to where it no longer exceeds the second energy detection threshold T.sub.RED2. This helps the CCA unit discriminate between interference and a valid signal and respond more quickly with an idle channel determination inthe presence of such interference, especially where additional validating criteria based on the content of the inbound signal such as preamble, carrier sense and header validation is assessed.
In the embodiment shown in FIG. 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 AGC is not in the aforementioned gain unlock state (as indicated by theGAIN.sup.--UNLOCK signal). When not operational the output of the noise floor unit 230, n is frozen to the last tracked value. For example, if the inbound energy is a valid 802.11/802.11b signal, the noise floor computation will stop upon gain unlockor CCA assertion. Thus, in this case and though not shown in FIG. 7, curve 720 representing n would remain constantly high while the inbound energy is detected. Thus, the energy threshold validation signal would hold true for the duration of the802.11/802.11b packet.
If, however, strong noise is instead detected, the noise floor unit 230 is initially held high due to the AGC 210 unlocking (GAIN_UNLOCK is asserted true). However, if the channel busy signal (CCA) remains idle for a few microseconds while theAGC is still in the unlock state, the AGC 210 of the present embodiment resets the GAIN_UNLOCK signal to false and the noise floor unit 230 begins to track (n curve 720) the falling gain signal E 710 as shown in FIG. 7. Before the noise floor unit 230completely tracks the noise level, the energy threshold validation signal is temporarily asserted true--from where the difference between the n 720 and E 710 curves exceed the first energy detection threshold T.sub.RED1 to where it no longer exceeds thesecond energy detection threshold T.sub.RED2, as previously discussed. Thus, CCA can still temporarily report a false busy depending on the CCA mode being utilized.
In particular, if 802.11/802.11b CCA mode 1 is supported (ED only) the CCA will report a false busy once the difference between n and E exceed the first energy detection threshold T.sub.RED1. In the configuration shown in FIG. 2, this willcause the noise floor unit 230 to suspend tracking of the gain signal E. To counteract this, the CCA unit 250, and the comparison unit 1020 in particular, can be used verify that the inbound signal is 802.11/802.11b compliant through preamble or headerverification, such as through looking for the presence of the Start Frame Delimiter ("SFD") situated at the end of a proper PLCP frame preamble. If the selected field is found in the correct sequence, e.g. the SFD field is confirmed immediately afterthe end of the sync symbols, approximately 128 .mu.sec (long preamble format) or 28 .mu.sec (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 theremaining 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 transition CCA false, indicating that thechannel is free and the transceiver 100 may transmit.
Alternatively, if the selected CCA mode also includes carrier 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 sensethreshold 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.
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 of Barker correlationto 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 inboundbaseband 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. Inparticular, the determination unit 910 calculates a peak-to-sidelobe average ratio as follows:
.times..times..times..times..times. ##EQU00001## where i=half-chip index=0 . . . 21. Or, alternatively:
.times..times..times..times..times..times. ##EQU00002## where i, k are half-chip indexes each ranging from 0 . . . 21, 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 SQ1 computation to counteract potential multipath interference and more effectively validate the inbound signal as being 802.11/80211b compliant.
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 Barkercorrelation results provided by the Barker correlator 330. In particular, the consecutive peak-to-peak distance is calculated as follows: pp.sub.n=|max_ind.sub.n-max.sub.--ind.sub.n-1|
Referring to FIG. 5, the control unit 930 of the carrier sense unit 240 is initialized to state 510 at the beginning of each frame detection processing sequence. At state 510, the control unit 930 issues the carrier sense validation signal (CS)as false. While in state 510, the control unit 930 monitors the peak-to-sidelobe average ratio (SQ1) generated by the peak-to-sidelobe determination unit 910 along with the consecutive peak-to-peak distance (pp.sub.n) calculated by the peak-to-peakdistance determination unit 920 (trans. 3). In this embodiment, if the SQ1 signal meets or exceeds a first signal quality threshold at least 3 out of 4 preceding SQ1 calculation iterations undertaken by the determination unit 910 and the pp.sub.n isless than a maximum acceptable distance in at least 3 out of the 4 preceding pp.sub.n calculation iterations undertaken by the determination unit 920, the control unit 930 transitions to state 520 (trans. 1) and the carrier sense validation signaltransitions to logic level 1 or true, thereby indicating that an 802.11/802.11b DSSS signal (e.g. PLCP preamble/header) has been perceived. Note, that in alternative embodiments consistent with the teachings of the present invention, satisfaction ofeither these SQ1 or pp.sub.n conditions alone and/or setting different threshold criteria for the SQ1 or pp.sub.n may be used to trigger the transition from state 510 to state 520.
Returning to the embodiment shown in FIGS. 5 and 9, the control unit 930 maintains this state (trans. 4) and continues monitor SQ1 and pp.sub.n while asserting the carrier sense validation signal true. If, at state 520, the SQ1 signal meets orexceeds a second signal quality threshold no more than once out of the preceding four calculation iterations undertaken by the determination unit 910, and/or (depending on the desired carrier sense tolerance) the pp.sub.n meets or exceeds the maximumacceptable distance more than once during the preceding 4 calculation iterations undertaken by the determination unit 920, the control unit 930 will transition back to state 510 and the carrier sense validation signal will transition back to false. Thiscan occur in the case of a corrupted PLCP frame preamble or header, or in the case of a valid PLCP frame that has transitioned to the High Rate PPDU. The latter case is expected, and consistent with 802.11 and 802.11b CCA guidelines, the CCA unit 250will nevertheless report the channel busy until the end of the packet (as indicated by the packet length in the received PLCP frame header) has been reached, and CS is ignored.
Note here that the number of calculation iterations used to assess the SQ1 and pp.sub.n signals in this embodiment is a matter of design choice, and, consistent with the present invention any number of calculation iterations and thresholding maybe used as long as a valid symbol-modulated signal can be recognized within relevant CCA tolerances such as those specified in the 802.11/802.11b standards.
Returning to FIG. 2, the ED and CS validation signals are sent to the CCA unit 250 of the embodiment shown in FIG. 2 in order to at least assist in determining whether the transceiver 100 should consider the inbound signal as valid. This CCAunit 250 is configured to operate and support at least a subset of the conventional 802.11/802.11b CCA modes 1-5 described above as well as support additional signal validation modes A-E as detailed below. As shown in FIG. 10, the CCA unit includes acontrol unit 1010 configured as a finite state machine (as depicted in FIG. 4) operating in one of two states 410, 420 based on either the ED and/or CS validation signal semaphores in isolation, or in combination with a PLCP preamble validation to bedescribed below, depending on the desired signal validation mode A-E. In this embodiment, though not required, the CCA control unit 1010 assesses the ED, CS and/or preamble validation signals every microsecond.
The operation of the CCA unit 250, including the CCA control unit 1010 will now be described with reference to the following signal validation modes. State 410 is in the initial state at the beginning of each frame detection processingsequence.
The ED validation signal is initially assumed to be false. While in state 410 (trans. 3), the CCA control unit 1010 monitors or polls ED, preferably every microsecond. The channel busy signal is held false, indicating that the operating RFchannel is free of traffic and is idle. If the ED validation signal transitions to true (logic level 1), The CCA control unit 1010 transitions from state 410 to state 420 (trans. 1), and the channel busy signal transitions to true, indicating that theoperating RF channel is considered to be busy with valid traffic. In this embodiment, State 420 is maintained (trans. 4) and the channel busy signal is held true until ED transitions false after the earlier of either antenna selection in a diversityimplementation is complete or the CCA timer (default of 12 .mu.sec) expires, the SFD field in the PLCP preamble is not found and the comparison unit 1020 of the CCA unit 250 determines that valid PLCP frame preamble symbols are not being received. TheCCA comparison unit 1020 determines this in this embodiment by comparing the output of the descrambler 380 against predetermined preamble data including strings of 8 consecutive 1's or 0's identifiable with the 802.11/802.11b long preamble format orstrings of 16 consecutive 1's or 0's identifiable with 802.11b's short preamble format. Consistent with the 802.11/802.11b standards, if the PLCP header is found to be corrupted or the SFD field is omitted, the CCA control unit 1010 will consider theinbound signal as invalid noise and consider the channel to be idle, thus transitioning the control unit 1010 back to step 410. Further, if frame transmission is deemed complete, transition to state 410 will occur.
Though not required, the CCA control unit 1010 can be configured to delay a transition back to state 410 if, for example a recent antenna switch was performed in a receiver selection diversity implementation so as to allow e.g. the AGC 210 toretrain and settle.
signal validation mode B: ED & CS.fwdarw.busy, ED & CS.fwdarw.idle
In this mode, the CCA control unit 1010 will transition from idle state 410 to busy state 420 if both the ED and CS validation signals are concurrently asserted true. In this embodiment, state 420 will be held until both the ED and CSvalidation signals transition false, the SFD field in the PLCP header is not found and the comparison unit 1020 of the CCA unit 250 determines that valid PLCP frame preamble data is not being received. Again, a recent antenna selection in a diversityimplementation can forestall, at least temporarily, the transition back to state 410.
signal validation mode C: ED& CS.fwdarw.busy, ED.parallel.CS.fwdarw.idle
The transition from idle state 410 to busy state 420 occurs similarly to that described previously for signal validation mode A. However, the CCA control unit 1010 remains in busy state 420 with the input validation signal held true unit eitherthe first of an 802.11/802.11b standards compliant 3.65 millisecond timer (e.g. timer 1030 shown in FIG. 10) initiated at the beginning of the frame detection processing sequence times out or the end of the recognized frame is reached.
The transition from idle state 410 to busy state 420 occurs similarly to that described previously for signal validation mode B, and the transition back from busy state 420 to idle state 410 occurs when the first of either the 3.65 millisecondtimer expires or the end of the recognized frame is reached.
The above embodiments were described with reference to an 802.11/802.11b PLCP frame format. However, several aspects and features of the present invention are not limited to the particular wireless frame format chosen. For example, sincerelative energy thresholding according to the present invention is not dependent on the content of the inbound signal, it could be applied to a range of wireless communications which could benefit from clear channel assessment, including, but not limitedto, those compliant with one or more of IEEE 802.11a, IEEE 802.16a, and the forthcoming IEEE 802.11g High Rate PHY supplement to the 802.11 standards.
Moreover, carrier sense determination according to the present invention, though dependent on the type of modulation used, can accommodate a wide range of orthogonal class modulated communications, including, but not limited to, theaforementioned Barker modulated communications, CCK modulated communications consistent with 802.11b, and OFDM modulated communications consistent with IEEE 802.11a, IEEE 802.16a, and the forthcoming IEEE 802.11g High Rate PHY supplement to the 802.11standard.
In the case where the inbound signal may be modulated in one of several ways at the outset, such as proposed in draft IEEE standard 802.11g, a frame detection unit such as that shown in FIG. 3 as frame detection unit 390 may be used whichincludes plural carrier sense units 240 and 380, to determine the inbound signal bears either Barker or OFDM modulated information. In such case, the CCA unit 385 here may include appropriate processing logic to handle plural carrier sense validationsignals (which in this case would have an XOR relationship).
Moreover, while the above described embodiments describe certain componentry in terms of function and functional relationships, it should be realized that such functions and relationships can be conveniently implemented using a wide variety ofdiscrete circuitry and logic, in combination with or alternatively through one or more general purpose or specific purpose information processors such as a microprocessor or digital signal processor programmed in accordance with these functions andrelationships, as will be appreciated by those of ordinary skill in the art.
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