Bandwidth Management
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Transcript of Bandwidth Management
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BANDWIDTH MANAGEMENT
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Any time AnywhereMobility & Roaming High capacity & subs. densityEfficient use of radio spectrumSeamless Network ArchitectureLow costFlexibilityInnovative ServicesStandard InterfacesMOBILE COMMUNICATION OBJECTIVES
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Public Land Mobile NetworkINDIA has adopted GSM standard for PLMN.Digital Cellular System.Operates at 900 MHz.International Roaming facility.Power class 0.8 to 20W.Cell Radius upto 35 Kms.Maximum mobility speed 250 Km/hr.
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BANDWIDTH MANAGEMENT TECHNIQUESFDMA TDMA Cellular Technology & Frequency Re-use SchemeSpeech CodingModulation
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FDMAFrequency Division Multiple Access SchemeUplink Frequency Band = (890 915) MHzDownlink Frequency Band = (935 960) MHzAbsolute Radio Freq Carrier Number (ARFCN)Bandwidth = 915 890 or 960 935 = 25 MHz
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GSM - MULTIPLE ACCESSGSM uses both FDMA & TDMA FDMA Access along Frequency axis Each RF carrier 200khz apart Total 124 RF Channels available. One or more carrier assigned to each base station...
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GSMFDMA
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TDMATime Division Multiple Access SchemeOne Radio Frequency = Eight Time SlotsOne TDMA Frame = Eight Time SlotsOne Time Slot = One Physical ChannelOne Time Slot Duration = 0.577 msec
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GSMTDMA45 MHzFrequencyF2F1(Cell transmit)F2F1(Cell Rx)AmplitudeTypical TDMA/ FDMA frame structure
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FDMA/TDMA Scheme FRAME
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CMCCSMSSS Normal Burst3T 57 Encrypted1S 26 Training1S 57 Encrypted3T8.25 GP TIME SLOT
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87654321FRAME OF 8 TIME SLOTSFRAME REPETITIONPHYSICAL CHANNELS876543211PHYSICAL CHANNELS
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CELLULAR TECHNOLOGY
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Cellular TechnologyCell Site
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CELLULAR MOBILE CONCEPTS
WHAT IS A CELL ? A base station (transmitter) having a number of RF channels is called a cell Each cell covers a a limited number of mobile subscribers within the cell boundaries ( Coverage area) Typical Cell Radius Aprrox = 30 Km (Start up), 1 Km (Mature)
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CELLEach Cell uses a specific set of radio frequenciesEach Base Station contains one or more TRXs
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Types of Cells
OMNI CELLSSECTOR CELLS - Two Sector Cells - Three Sector Cells
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Fundamental problemsRadio range, or coverageno. of channels, or voice circuitsFull, seamless service coverageLarge no. of subscribers in the range of millions
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CELLULAR MOBILE CONCEPTSRADIO IN LOCAL LOOP LIMITED AVAILABILITY OF RF SPECTRUMINTERFERENCE PROBLEMINTERFERENCE AND SYSTEM CAPACITY FREQUENCY REUSE PATTERNCELLULAR PRINCIPLECELLULAR ENVIRONMENTCAPACITY CONSIDERATIONSFUTURE TRENDS
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Frequency Re-use Scheme
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ASSUMPTIONS Traffic /User = 30 mE , GOS = 1%
CELLULAR MOBILE CONCEPTS
CELL RADIUS (KM)
COVERAGE AREA(KM2)
No. Of MOBILE SUBS.
No. of CHANNELS REQD.
1
3.14
100
8
3
28.3
900
38
10
314
10,000
@ 360
25
1960
60,000
@ 2,000
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360 * 25 KHz * 2 = 18 000 KHz = 18 MHz FOR A CELL OF 10 KM RADIUS ONLY THIS IS IMPOSSIBLE TO BE ALLOCATED HENCE FREQUENCY REUSE IS A MUST TO COVER THE TOTAL SERVICE AREA WITH A LIMITED AVAILABLE RF RESOURCES HENCE THE NEED FOR A CELLULAR PRINCIPLE
CELLULAR MOBILE CONCEPTS
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Frequency Re-use Scheme
CLUSTER
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A CLUSTER OF CELLS
4567231
CELLULAR MOBILE CONCEPTS
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FREQUENCY REUSE PATTERN
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1DRCELLULAR PRINCIPLE N=7
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CELLULAR MOBILE CONCEPTS
CO-CHL INTERFERENCE :Interference caused by another cell/mobile using the same frequency DRCo Chl Interference is a Function of Qthe re-use ratio:
Q =D / R Lower Q Increased Co-Chl Interference Higher Q Reduced Co-Chl Interference
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Co Channel Interference Q = D /R = 3N N =Cluster SizeR = Size (Radius of Cell)D = Distance between two Co- Channel Cells
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LOWER Q Higher Interference Increased System Handling Capacity Co Channel Interference
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CLASSIFICATION OF CELLSMICRO CELLMACRO CELLUMBRELLA CELL
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GSM CAPACITY CONSIDERATIONS 8 Access Channels 1--Signaling 7- VoiceWith 2 % GoS 2.94 E 2.94E/25mE=120 Subs120 Subs/Sector 3 = 360 Subscribers
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Techniques adopted in CELLULAR ENVIRONMENTPower Control Channel CodingInterleavingEqualizationSlow Frequency HoppingAntenna Diversity
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Speech CodingAudio CODECRPE : LTP = Regular Pulse Excitation : Long term Prediction
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ModulationGMSK = GAUSSIAN MINIMUM SHIFT KEYING
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GSM Specifications GSM 900 Mobile to BS(UP-LINK) -890 to 915 MHzBS to Mobile (DOWN -LINK) - 935 to 960 MHz Bandwidth - 25 MHz GSM 1800 ( DCS ) Mobile to Cell(UP-LINK) -1710 to 1785 MHz Cell to Mobile (DOWN -LINK) - 1805 to 1880 MHz Bandwidth - 75 MHz RF Spectrum :
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GSM Specifications Carrier Separation -200 kHzNo. of RF Carriers -124Access Method -TDMA/FDMAModulation Method -GMSKTransmission Rate -270.833 KbpsSpeech Coding -Full rate 13 Kbps Half rate 6.5 KbpsDuplex Distance -45 MHz
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GSMDigital Voice Transmission - In GSM speech coding a block of 20 ms is encoded in one set of 260 bits. - This calculates as 50X 260 = 13 kbps. Thus GSM speech coder produces a bit rate of 13 kbps per subscriber. - This provides speech quality which is acceptable for mobile telephony and comparable with wire-line PSTN phones.Speech Coding
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GSMDigital Voice Transmission - It uses 260 bits from speech coding as input and outputs 456 encoded bits. - In one burst one block of 57 bits from one sample and another block from another sample are sent together. - These 456 bits for every 20 ms of speech are interleaved forming eight blocks of 57 bits each.Channel CodingInterleaving
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GSMDigital Voice Transmission - Additional bits as training sequence added to basic speech/data. - Total of 136 bits added, bringing overall total to 592 bits. - To counteract the problems encountered in radio path.Burst Formatting - Each TS of TDMA frame is 0.577 ms long and during this time 156.25 bits are transmitted. - One burst contains only 148 bits. Rest of the space, 8.25 bits time, is empty and is called Guard Period ( GP ). - GP enables MS/BTS to ramp up and ramp down.
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GSMGaussian Minimum Shift Keying (GMSK)The modulation method in GSM is GMSK which facilitates the use of narrow bandwidth and coherent detection capability.Rectangular pulses are passed through a Gaussian filter prior to their passing through a modulator.The modulation scheme almost satisfies the adjacent channel power spectrum density requirements of -60dB specified by CCIR.
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GSM Speech to Radio waves200kHz BW
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GSM Radio Interface - CYCLES
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Organisation of Speech & DataFrames 0-11 : TCHFrames 12 : SACCHFrames 13-24 : TCHFrames 25 : Unused26 frame multiframeDuration: 120 msTDMA frameDuration: 60/13 ms=4.615 msTailbitsData bitsStealingbitTraining sequenceStealingbitData bitsTailbitsGuardbitsNormal burstDuration 15/26 ms
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GSM-- TDMA STRUCTURE TDMA 8 Time Slots / RF Channel Time slot duration 0.577m sec or 15 / 26 m sec Frame 8 Burst Periods ( Time Slots) = 8 15/26 = 4.615 m sec Multi Frame Traffic 26 4.615 = 120 msec Control 51 4.615 = 235.365 m sec Super Frame 51 Traffic Multi frames 26 Control Multi frames Hyper Frame 2048 Super Frames = 3 28 52.76 hr min sec
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GSMLOGICAL CHANNELS USER INFORMATION( TRAFFIC) SIGNALLING INFORMATION (CONTROL)
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GSM CONTOL CHHANELS OVER LOGICAL CHANNELS THREE TYPES OF CONTROL CHANNELS Intended to carry signalling and synchronisation Broadcast control channel BCCH Common control channel CCCH Dedicated control channel DCCH
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OPERATIONAL CONCEPTS Hence IDLE MODE & DEDICATED MODE DEDICATED MODE -- When a full Bi -directional P to P CHL has been allocated during an established call IDLE MODE MODE -- When MS is powered on (active) without being in dedicated mode
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GSM THREE TYPES OF CONTROL CHANNELS
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OPERATIONAL CONCEPTS IDLE MODE -- When MS is powered on (active) without being in dedicated mode MS stays continuously in touch with BS Listens to transmissions from BS to intercept Paging Messages ( for incoming calls) Monitors Radio Environment in order to evaluate Chl Quality & choose the most suitable BS Listens to BS to avail short message broadcast service
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OPERATIONAL CONCEPTS ACCESS PROCEDURE -- Access to system ( switch over from IDLE to DEDICATED Mode) MS indicates to BS that it needs a connection BS accepts the request & indicates which traffic CHL it may use For above purpose specific transmission is done over Common Channels
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OPERATIONAL CONCEPTS MOBILE O/G Call MS sends access over RACH System allocates SDCCH through AGCH Lastly TCH is assigned through SDCCH when a conversation can start Set up information exchanged over SDCCH ( Authentication , Measurement Reports, Power Control)
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OPERATIONAL CONCEPTS MOBILE I/C Call Paging to MS through PCH since MS is monitoring PAGCH MS responds by sending a page response over RACH Set up information exchanged over SDCCH ( Authentication, Call set-up messages , Power Control) As a result system allocates SDCCH to MS over AGCH Lastly TCH is allocated to mobile over SDCCH . Mobile starts conversation.
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OPERATIONAL CONCEPTS IDLE MODE IDLE MODE ACCESS PROCDURE DEDICATED MODE MS O/G Call MS I/C Call----FCCH---- SCH----BCCH----RACH----AGCH----SDCCH----TCH----PCH----RACH----AGCH----SDCCH----TCH