Cellular Wireless NetworksOverviewCellular Frequency ReuseCharacterizing Frequency ReuseIncreasing CapacityCell SplittingSlide 7Overview of Cellular SystemCall StagesOther FunctionsReflection, Diffraction, ScatteringReflection, Diffraction and ScatteringMultipath PropagationTypes of FadingError Compensation MechanismsWireless GenerationsFirst Generation AnalogSpectral Allocation In North AmericaOperationCall SequenceAMPS Control ChannelsSecond GenerationCdmaOne3GWCDMACDMA2000TD-SCDMAEvolution of Cellular TechnologiesSummaryReading Assignment12-1©2005 Raj JainCSE473sWashington University in St. LouisCellular Wireless Cellular Wireless NetworksNetworksRaj Jain Washington UniversitySaint Louis, MO [email protected] slides are available on-line at:http://www.cse.wustl.edu/~jain/cse473-05/12-2©2005 Raj JainCSE473sWashington University in St. LouisOverview of Cellular SystemPHY Issues in Cellular SystemsWireless GenerationsFirst Generation - AMPS Second Generation: CdmaOne3G Evolution of Cellular TechnologiesOverviewOverview12-3©2005 Raj JainCSE473sWashington University in St. LouisCellular Frequency Reuse Cellular Frequency Reuse12-4©2005 Raj JainCSE473sWashington University in St. LouisCharacterizing Frequency ReuseCharacterizing Frequency ReuseD = minimum distance between centers of cells that use the same band of frequencies (called co-channels)R = radius of a celld = distance between centers of adjacent cells (d = R)N = number of cells in repetitious patternReuse factorEach cell in pattern uses unique band of frequenciesHexagonal cell pattern, following values of N possible N = I2 + J2 + (I x J), I, J = 0, 1, 2, 3, … Possible values of N are 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, …D/R=D/d = N3N12-5©2005 Raj JainCSE473sWashington University in St. LouisIncreasing CapacityIncreasing CapacityAdd new channelsNot all channels used to start withFrequency borrowingTaken from adjacent cells by congested cellsOr assign frequencies dynamicallyCell splittingNon-uniform distribution of topography and trafficSmaller cells in high use areasMore frequent handoff, More base stations12-6©2005 Raj JainCSE473sWashington University in St. LouisCell SplittingCell Splitting12-7©2005 Raj JainCSE473sWashington University in St. LouisIncreasing CapacityIncreasing CapacityCell SectoringCell divided into wedge shaped sectors3 – 6 sectors per cell, Each with own channel setSubsets of cell’s channels, Directional antennasMicro cellsMove antennas to tops of small buildings Even lamp postsForm micro cells, Reduced powerGood for city streets, along roads and inside large buildings12-9©2005 Raj JainCSE473sWashington University in St. LouisOverview of Cellular SystemOverview of Cellular SystemBase station (BS) at center of each cellAntenna, controller, transceiversMTSO handles channel assignment, call connection, billing and handoff12-10©2005 Raj JainCSE473sWashington University in St. LouisCall StagesCall StagesMonitor for strongest SignalRequest for connectionCall acceptedHandoffOngoingCallPaging12-11©2005 Raj JainCSE473sWashington University in St. LouisOther FunctionsOther FunctionsCall blockingOn mobile-initiated calls, if all channels busy, mobile retries After number of retries, busy tone returnedCall termination: User hangs upMTSO informed. Traffic channels at two BSs releasedCall drop: BS cannot maintain required signal strengthTraffic channel dropped and MTSO informedCalls to/from fixed and remote mobile subscriberMTSO can connect mobile user and fixed subscriber via PSTNMTSO can connect to remote MTSO via PSTN or via dedicated lines Can connect mobile user in its area and remote mobile user12-12©2005 Raj JainCSE473sWashington University in St. LouisReflection, Diffraction, ScatteringReflection, Diffraction, ScatteringEflection Phase shiftifferactioncattering12-13©2005 Raj JainCSE473sWashington University in St. LouisReflection, DiffraReflection, Diffraction and Scatteringction and ScatteringReflection: Surface large relative to wavelength of signalMay have phase shift from originalMay cancel out original or increase itDiffraction: Edge of impenetrable body that is large relative to May receive signal even if no line of sight (LOS) to transmitterScatteringObstacle size on order of wavelength. Lamp posts etc.If LOS, diffracted and scattered signals not significantReflected signals may beIf no LOS, diffraction and scattering are primary means of reception12-14©2005 Raj JainCSE473sWashington University in St. LouisMultipath PropMultipath PropagationagationInter-symbol Interference12-15©2005 Raj JainCSE473sWashington University in St. LouisTypes of FadingTypes of FadingFast fadingRapid changes in strength over distances about half 900MHz wavelength is 0.33m. 20-30dBSlow fadingSlower changes due to user passing different height buildings, gaps in buildings etc.Over longer distances than fast fadingFlat fadingNon-selectiveAffects all frequencies in same proportionSelective fadingDifferent frequency components affected differently12-16©2005 Raj JainCSE473sWashington University in St. LouisError Compensation MechanismsError Compensation MechanismsForward error correctionTypically, ratio of total bits sent to data bits between 2 and 3Big overhead: Capacity one-half or one-thirdAdaptive equalizationUsed to combat inter-symbol interferenceGathering the dispersed symbol energy back together into its original time intervalTechniques include so-called lumped analog circuits and sophisticated digital signal processing algorithmsAntenna Diversity: Multiple antenna in, Multiple Antenna outSpace Diversity: Directional antennas with multiple beamsFrequency Diversity: OFDM12-17©2005 Raj JainCSE473sWashington University in St. LouisWireless GenerationsWireless Generations1G: Analog Cellular Phones. Needs a modem. 9.6 kbps max.2G: Digital Cellular Phones. No modem required. 19.3 kbps max. GSM, CDMA2.5G: GPRS. 144kbps. Data only.3G: Future high-speed data with Voice. 64 kbps to 2 Mbps.1G:AnalogVoice2G:DigitalVoice + Low Speed DataTDMAFDMA CDMAAMPS(U.S.)TACS(Europe)IS-54 D-AMPSIS-136 US TDMAGSMDCS1800DCS1900NA GSMIS-95IS-54 has analog control channel for compatibility with AMPS. Did not succeed.12-18©2005 Raj JainCSE473sWashington