Presentation on theme: "Chapter 14 Cognitive Radio for Broadband Wireless"— Presentation transcript:
1 Chapter 14 Cognitive Radio for Broadband Wireless Access in TV Bands: The IEEEStandard
2 Outline Introduction Overview of the IEEE 802.22 Standard The PHYThe MACSummary
3 IntroductionCognitive Radios (CR) enable flexible, efficient and reliable spectrum use by adapting the radio’s operating characteristics to the real-time conditions of the environmentCRs have the potential to utilize the unused spectrum (e.g. TV White Spaces) in an intelligent way while not interfering with other incumbents
4 Regulatory Scenario for TV White Spaces in the USA IEEE WG starts working on a CR based PHY/MAC specNotice of Proposed Rule MakingNotice of InquiryPublic Notice200320042005Sep. 2006Initial R & OandFurther NPRMReport on Sensing, Interference to DTVs & Other RadiosSource: Monisha Gosh and Dave Cavalcanti, CrownCom 2009 Tutorial “Cognitive Radio Networks in TV White Spaces: Regulation, Theory and Practice “Report on Interference Rej. Cap. of DTV Rx’sOct. 2006Mar. 2007July 2007Sensing Proto TestingMar. 2008IEEE first official LBJune 2008Aug. 2008Dec. 2008Nov. 2008Feb. 2009Mar. 2009June 2009IEEE starts 2nd LBFinal Rule and OrderFinal rules in Federal RegistryField Tests
5 Overview of the IEEE 802.22 Standard Fixed point-to-multipoint wireless regional area networks (WRAN)Reuse of TV broadcast bands on a non-interfering basisSpecify Cognitive Radio based PHY/MAC layersCognitive Radio Entity for spectrum managementMaster-Slave architecture:Base Stations (BS)Cusumer Premise Equipments (CPEs)
6 Service Characteristics Peak-throughput18.72 Mbit/sCommunication RangeTypical: kmup to 100 km depending on EIRPSource: IEEE Draft 2.0, May 2009
7 Protocol Reference Model - BS Source: IEEE Draft 2.0, May 2009
8 Protocol Reference Model - CPE Source: IEEE Draft 2.0, May 2009
9 The 802.22 PHY overview Parameters Specification Frequency range 54~862 MHzBandwidth6 and/or 7, and/or 8 MHzData rate1.51~22.69 Mb/sSpectral Efficiency0.25~3.78 b/s/HzPayload modulationQPSK, 16-QAM, 64-QAMTransmit EIRPDefault 4W for CPEsMultiple AccessOFDMAFFT Mode2048Cyclic Prefix Modes¼, 1/8, 1/16, 1/32DuplexTDD
10 The 802.22 MAC overview TDMA access Superframe structure defined: Frame structure and QoS model similar toSuperframe structure defined:Better self-coexistence, synchronization, and incumbent protectionCoexistence mechanisms:Incumbent avoidance and Spectrum measurementsSpectrum sensing support mechanisms (quiet periods)Channel ManagementCoexistence Beacon Protocol (CBP)Normal mode and coexistence mode with dynamic resource sharing (frame based)
11 Super-frame Structure Source: IEEE Draft 2.0, May 2009
13 Incumbent protectionIncumbents in TV bands:TV broadcasting services (in the US, use 6 MHz channels in VHF and UHF bands)Wireless microphones (in the US, regulated by Part 74 FCC rules, use 50 mW for a 100 m coverage and 200 KHz channel bandwidth)protection mechanisms: combination of incumbent database and spectrum sensing
14 Spectrum sensing requirements ParameterValue for Wireless MicrophonesValue for TV BroadcastingChannel Detection Time 2 secChannel Move TimeChannel Closing Transmission Time100 msecIncumbent Detection Threshold-107 dBm (over 200KHz)-116 dBm (over 6MHz)Probability of Detection90%Probability of False Alarm10%
15 Spectrum Sensing: Key Challenges Incumbents must be reliably detected within the CDTspecifies IDT of -116 dBm and CDT of 2 secReliable sensing requires network wide quiet periods (QP)For typical energy detection approach, integration time is low, however, neighboring WRANs may be detected as incumbentHigher probability of false alarmsFeature detection can accomplish more accurate detection of incumbents, however, the integration time required is the main issueLong integration times mean poor QoS support, especially for VoIPWhy QP? Concurrent transmissions may interfere with sensing (more false alarms)Maximum QP should not be large (possibly < 5 msec)Tradeoff between accuracy/reliability and overhead for sensing algorithms.
16 Example of a sensing architecture and strategy Coarse Sensing(Analog, RSSI, MRSS, FFT…)Begin SensingFine SensingEnd Sensingoccupied?YNMAC(Selectsingle channel)FFTCSFDFieldSyncOptimumRadiometerMulti-cycleDetectorAACSpectralCorrelationSpectrumUsageDatabase(BS)ATSCSegmentOne strategy for sensing: Fast sensing (e.g energy detection) + Fine Sensing (in line with MAC intra and inter-frame QPs)Combine sensing results with incumbent databaseSensing is a continuous process. Need to refresh every 2 (operating ch) or 6 secs (backup).
17 Fine sensing algorithms We discuss two algorithms found in the current IEEE draft specificationFFT-based pilot detectionPilot-energy detection: find maximum of FFT output-squared, and compare to a thresholdPilot-location detection: compare location of maximum of FFT-output between multiple dwellsSpectrum sensing of the DTV in the vicinity of the pilot using higher order statisticsDetect the DTV signals in Gaussian noise using higher order statistics (HOS)Perform non-Gaussianity check in the frequency domain in the vicinity of the pilot of the DTVGive an brief overview of two algorithms recommended byFFT-based: try to find the pilot (feature of ATSC signal)HOS-based: use property of gaussian signals (higher order moments are zero). DTV in frequency domain is not gaussian. Good to identify weak DTV signal from noise.
18 FFT-based pilot detection Demodulate the signal to basebandFilter with a low-pass filter, which should be large enough to accommodate any frequency offsetsDown-sample the filtered signalTake FFT of the down-sampled (FFT size depends on the dwell time)Determine the maximum value, and location, of the FFT output squaredPilot-energy detection: find maximum of FFT output-squared, and compare to a thresholdPilot-location detection: compare location of maximum of FFT-output between multiple dwellsThe FFT size will depend on the dwell-time. With parameters above:1 ms dwell will allow 32-point FFT.5 ms dwell will allow 256-point-FFT.Reference: C. Cordeiro et al, “Spectrum Sensing for Dynamic Spectrum Access of TV Bands,” CrownCom, August 2007
19 Performance of FFT-based pilot detection Low pass filter BW = 40KHz (±20kHz)Sampling rate from MHz to 53.8 kHz (1/400)Dwell time = 1 ms (32-point FFT) and 5 ms (256-point FFT)-20 dB = -116dBmHighlight the average in the figures. (Simulation results)Position location is robust against noise uncertainty (pilot location can be identified even if amplitude is in fading).Real test measurements (later).Note: 12 signals supplied by MSTV to IEEE for testing different sensing algorithms
20 Performance of FFT-based pilot detection w/ 2 dB average noise uncertainty With 2dB average noise uncertainty.Pilot location detection: Pfa is much lower and robust. (Pilot energy detection increases the Pfa)
21 Spectrum sensing of the DTV in the vicinity of the pilot using higher order statistics Main idea: perform non-gaussianity check in the frequency domain in the vicinity of the pilotDTV signals are non-Gaussian in frequency domain (HOS is efficient metric for detecting non-gaussian signals at low SNR.HOS measures (cumulantes=functions of lower order moments) are expected to be zero for gaussian signals.Method compares HO cumulants with the power of 2nd order moment. Uses a threshold, because HO cumulants may not be exactly zero.Reference: A. Mody , IEEE document no. 0370r2, August 2007.
22 Performance of DTV sensing in the vicinity of the pilot using higher order statistics The y parameter can be used to adjust the Pfa. (higher y, lower Pfa)g = 0.8, PD > 0.9 and PFA < 0.05g = 1.05, PD > 0.9 and PFA = 0.01Reference: A. Mody , IEEE document no. 0359r1, August 2007.
23 Spectrum sensing support at the 802.22 MAC layer
24 Self-Coexistence in 802.22 Normal mode Self-coexistence mode no channel sharing when there is spectrum availableSelf-coexistence modeResource sharing on frame basisRandom contention amongst neighboring BS through the CBP mechanism
25 Coexistence Beacon Protocol (CBP) Case 1:CBP packets carry the schedule of the QPs and self-coexistence informationOther cases also possible depending on location of the CPEsCase 2:25
26 Channel management Incumbent detection notification Sensing reportsUCS (Urgent Coexistence Situation) notificationBS controls channel switching procedure through channel management commandsSchedules switching when needed within the required CMT (<2 sec)
27 Spectrum managerThe SM centralizes all the decisions with respect to spectrum management:Maintain up to date spectrum availability information combining incumbent database and spectrum sensing inputs;Classify, prioritize and select channels for operation and backup;Association control;Trigger frequency agility related actions (i.e. channel switch);Manage mechanisms for self-coexistence
28 Chapter 14 Summary IEEE 802.22 Draft standard is under development Fixed point-to-multipoint WRANReuse of TV broadcast bands on a non-interfering basisPHY/MAC air interface specificationSpectrum manager integrating incumbent database and spectrum sensing inputsDraft standard is under developmentCurrently on 2nd working group ballotKey design challengesPrimary protection (e.g., DTV, wireless microphones)Secondary coexistence
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