unit 5 ep602 wave propagation interference

UNIT 5WAVE PROPAGATION AND INTERFERENCE IN CELLULAR COMMUNICATION SYSTEMS

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COURSE LEARNING OUTCOME

EP602-WIRELESS COMMUNICATION/CHAPTER 5

*CHAPTER OUTLINE


Wave propagation in cellular systems.


*At the end of the lesson, students are able to:Understand the concept of wave propagation in cellular systems. Describe the model of free space propagation. Identify the types of free space propagation phenomena. a. Reflection b. Diffraction c. Scattering Describe the phenomena of free space propagation Describe a microwave line-of-sight. Describe the model of small scale fading. OBJECTIVES


Radio Wave PropagationRadio waves extends from a frequency of 30 kHz to 300 GHz.In free space, radio waves propagate in straight line (LOS) and are reflected off objects. Radio waves on the earth are affected by the terrain of the ground, the atmosphere and the natural and artificial objects on the terrain.


Radio Wave Propagation


CONCEPT OF RADIO WAVE PROPAGATION IN CELLULAR SYSTEMS


RADIO WAVE PROPAGATION - Ground Wavetravels in contact with earths surfacereflection, refraction and scattering by objects on the groundtransmitter and receiver need NOT see each otheraffects all frequenciesat VHF or higher, provides more reliable propagation meanssignal dies off rapidly as distance increases.


Radio Wave Propagation Ground WaveFollows contour of the earthCan Propagate considerable distancesFrequencies up to 2 MHzExample; AM radio


RADIO WAVE PROPAGATION - Ionospheric or Sky Wave Reflected back to earth by ionospheric layer of the earth atmosphereBy repeated reflection, communication can be established over 1000s of milesMainly at frequencies below 30MHzMore effective at times of high sunspot activity


Signal reflected from ionized layer of atmosphere back down to earthSignal can travel a number of hops, back and forth between ionosphere and earths surfaceReflection effect caused by refractionExamples; Amateur radio, CB radioRADIO WAVE PROPAGATION - Ionospheric or Sky Wave


THE MODEL OF FREE SPACE PROPAGATION - Tropospheric WaveBending (refraction) of wave in the lower atmosphereVHF communication possible over a long distancebending increases with frequency so higher frequency more chance of propagationMore of an annoyance for VHF or UHF (cellular)


When electrons move, they create electromagnetic waves that can propagate through the spaceNumber of oscillations per second of an electromagnetic wave is called its frequency, f, measured in Hertz. The distance between two consecutive maxima is called the wavelength, designated by l.*


By attaching an antenna of the appropriate size to an electrical circuit, the electromagnetic waves can be broadcast efficiently and received by a receiver some distance away. In vacuum, all electromagnetic waves travel at the speed of light: c = 3x108 m/sec.In copper or fiber the speed slows down to about 2/3 of this value. Relation between f, l , c: lf = c


The radio, microwave, infrared, and visible light portions of the spectrum can all be used to transmit informationBy modulating the amplitude, frequency, or phase of the waves. *


Radio waves are; Easy to generateCan travel long distancesCan penetrate buildings They are both used for indoor and outdoor communicationThey are omni-directional: can travel in all directionsThey can be narrowly focused at high frequencies (greater than 100MHz) using parabolic antennas (like satellite dish)

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Properties of radio waves are frequency dependentAt low frequencies, they pass through obstacles well, but the power falls off sharply with distance from sourceAt high frequencies, they tend to travel in straight lines and bounce of obstacles (they can also be absorbed by rain)They are subject to interference from other radio wave sources


EP602-WIRELESS COMMUNICATION/CHAPTER 5*At VLF, LF, and MF bands, radio waves follow the ground. AM radio broadcasting uses MF bandAt HF bands, the ground waves tend to be absorbed by the earth. The waves that reach ionosphere (100-500km above earth surface), are refracted and sent back to earth.absorptionreflection Ionosphere


*Directional antennas are usedWaves follow more direct paths- LOS: Line-of-Sight Communication- Reflected wave interfere with the original signalVHF Transmission


Waves behave more like light at higher frequenciesDifficulty in passing obstaclesMore direct pathsThey behave more like radio at lower frequenciesCan pass obstacles*


Transmission path between sender and receiver could beLine-of-Sight (LOS)Non Line-of-Sight (NLOS)- Obstructed by buildings, mountains and foliageEven speed of motion effects the fading characteristics of the channel

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The physical mechanisms that govern radio propagation are complex and diverse, but generally attributed to the following three factors.Electromagnetic waves propagate through environments where they are reflected, scattered, and diffracted by walls, terrain, buildings, and other objects.*


Radio wave propagation is affected by the following mechanisms:reflection at large obstaclesscattering at small obstaclesdiffraction at edges


ReflectionReflections arise when the plane waves are incident upon a surface with dimensions that are very large compared to the wavelengthExample: reflections from earth and buildingsThese reflections may interfere with the original signal constructively or destructively


Other Types of ReflectionCorner reflectorParabolic reflectorDiffuse Reflection


DiffractionDiffraction occurs according to Huygens's principle when there is an obstruction between the transmitter and receiver antennas, and secondary waves are generated behind the obstructing body.Explains how radio signals can travel urban and rural environments without a line-of-sight path.

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ILLUSTRATION OF DIFFRACTION


ScatteringScattering occurs when the plane waves are incident upon an object whose dimensions are on the order of a wavelength or less, and causes the energy to be redirected in many directions.They are produced by small objects, rough surfaces and other irregularities on the channelFollows same principles with diffractionCauses the transmitter energy to be radiated in many directionsLamp posts and street signs may cause scattering


*Building BlocksDRSR: ReflectionD: DiffractionS: ScatteringtransmitterreceiverDStreet


Rapid fluctuation of the amplitude of a radio signal over a short period of time or travel distance (sub-wavelength)Cause by: multipath waves and Doppler shift


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rapid fluctuations of received signal strength over short time intervals and/or travel distances.

Caused by interference from multiple copies of Tx signal arriving @ Rx at slightly different times


As a mobile moves through a coverage area, these 3 mechanisms have an impact on the instantaneous received signal strength. If a mobile does have a clear line of sight path to the base-station, than diffraction and scattering will not dominate the propagation.If a mobile is at a street level without LOS, then diffraction and scattering will probably dominate the propagation.

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As the mobile moves over small distances, the instantaneous received signal will fluctuate rapidly giving rise to small-scale fading;The reason is that the signal is the sum of many contributors coming from different directions and since the phases of these signals are random, the sum behave like a noise (Rayleigh fading). In small scale fading, the received signal power may change as much as 3 or 4 orders of magnitude (30dB or 40dB), when the receiver is only moved a fraction of the wavelength.

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As the mobile moves away from the transmitter over larger distances, the local average received signal will gradually decrease. This is called large-scale path loss.Typically the local average received power is computed by averaging signal measurements over a measurement track of 5l to 40l. (For PCS, this means 1m-10m track)The models that predict the mean signal strength for an arbitrary-receiver transmitter (T-R) separation distance are called large-scale propagation modelsUseful for estimating the coverage area of transmitters


*14 16 18 20 22 24 26 28T-R Separation (meters)-70-60-50-40-30Received Power (dBm)This figure is just an illustration to show the concept. It is not based on read data.


What is Decibel (dB)What is dB (decibel): A logarithmic unit that is used to describe a ratio.Let say we have two values P1 and P2. The difference (ratio) between them can be expressed in dB and is computed as follows: 10 log (P1/P2) dBExample: transmit power P1 = 100 W, received power P2 = 1 WThe difference is 10log(100/1) = 20 dB.

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dBdB unit can describe very big ratios with numbers of modest size. See some examples: Tx power = 100W, Received power = 1WTx power is 100 times of received powerDifference is 20dBTx power = 100W, Received power = 1mWTx power is 100,000 times of received powerDifference is 50dBTx power = 1000W, Received power = 1mWTx power is million times of received powerDifference is 60dB*


dBmFor power differences, dBm is used to denote a power level with respect to 1mW as the reference power level. Let say Tx power of a system is 100W. Question: What is the Tx power in unit of dBm? Answer: Tx_power(dBm) = 10log(100W/1mW) = 10log(100W/0.001W) = 10log(100,0000) = 50dBm*


dBWFor power differences, dBW is used to denote a power level with respect to 1W as the reference power level. Let say Tx power of a system is 100W. Question: What is the Tx power in unit of dBW? Answer: Tx_power(dBW) = 10log(100W/1W) = 10log(100) = 20dBW. *


*Decibel (dB) versus Power RatioComparison oftwo Sound Systems


Free-Space Propagation Model


Free-Space Propagation ModelFree space power received by a receiver antenna separated from a radiating transmitter antenna by a distance d is given by Friis free space equation: Pr(d) = (PtGtGrl2) / ((4p)2d2L) (5.1) Pt is transmited powerPr(d) is the received powerGt is the trasmitter antenna gain (dimensionless quantity)Gr is the receiver antenna gain (dimensionless quantity)d is T-R separation distance in metersL is system loss factor not related to propagation (L >= 1)L = 1 indicates no loss in system hardware (for our purposes we will take L = 1, so we will igonore it in our calculations). l is wavelength in meters.


Free-Space Propagation ModelThe gain of an antenna G is related to its affective aperture Ae by: G = 4pAe / l2 [5.2]The effective aperture of Ae is related to the physical size of the antenna, l is related to the carrier frequency by: l = c/f = 2pc / wc [5.3]f is carrier frequency in Hertzwc is carrier frequency in radians per second. c is speed of light in meters/sec


Free-Space Propagation ModelAn isotropic radiator is an ideal antenna that radiates power with unit gain uniformly in all directions. It is as the reference antenna in wireless systems. The effective isotropic radiated power (EIRP) is defined as: EIRP = PtGt [5.4]Antenna gains are given in units of dBi (dB gain with respect to an isotropic antenna) or units of dBd (dB gain with respect to a half-wave dipole antenna). Unity gain means: G is 1 or 0dBi


Free-Space Propagation ModelPath loss, which represents signal attenuation as positive quantity measured in dB, is defined as the difference (in dB) between the effective transmitted power and the received power. PL(dB) = 10 log (Pt/Pr) = -10log[(GtGrl2)/(4p)2d2] [5.5](You can drive this from equation (5.1)) If antennas have unity gains (exclude them)

PL(dB) = 10 log (Pt/Pr) = -10log[l2/(4p)2d2] [5.6]


Free-Space Propagation ModelFor Friis equation to hold, distance d should be in the far-field of the transmitting antenna.The far-field, or Fraunhofer region, of a transmitting antenna is defined as the region beyond the far-field distance df given by: df = 2D2/l [5.7]D is the largest physical dimension of the antenna. Additionally, df >> D and df >> l


Free-Space Propagation Model Reference Distance d0It is clear the Equation 1 does not hold for d = 0. For this reason, models use a close-in distance d0 as the receiver power reference point.d0 should be >= dfd0 should be smaller than any practical distance a mobile system usesReceived power Pr(d), at a distance d > d0 from a transmitter, is related to Pr at d0, which is expressed as Pr(d0).The power received in free space at a distance greater than d0 is given by: Pr(d) = Pr(d0)(d0/d)2 d >= d0 >= df [5.8]


Free-Space Propagation ModelExpressing the received power in dBm and dBWPr(d) (dBm) = 10 log [Pr(d0)/0.001W] + 20log(d0/d) where d >= d0 >= df and Pr(d0) is in units of watts. [5.9]Pr(d) (dBW) = 10 log [Pr(d0)/1W] + 20log(d0/d) where d >= d0 >= df and Pr(d0) is in units of watts. [5.10]

Reference distance d0 for practical systems: For frequncies in the range 1-2 GHz1 m in indoor environments100m-1km in outdoor environments


Example QuestionA transmitter produces 50W of power. A) Express the transmit power in dBmB) Express the transmit power in dBWC) If d0 is 100 m and the received power at that distance is 0.0035 mW, then find the received power level at a distance of 10 km. Assume that the transmit and receive antennas have unity gains.


SolutionA) Pt(W) is 50W. Pt(dBm) = 10log[Pt(mW)/1mW)] Pt(dBm) = 10log(50x1000) Pt(dBm) = 47 dBmB)Pt(dBW) = 10log[Pt(W)/1W)] Pt(dBW) = 10log(50) Pt(dBW) = 17 dBW


SolutionPr(d) = Pr(d0)(d0/d)2Substitute the values into the equation: Pr(10 km) = Pr(100 m)(100 m/10 km)2 Pr(10 km) = 0.0035 mW(10-4) Pr(10 km) = 3.5x10-10 WPr(10 km) [dBm] = 10log(3.5x10-10 W/1 mW) = 10log(3.5x10-7) = -64.5 dBm


INTERFERENCE IN CELLULAR COMMUNICATION SYSTEMS


Understand interference in cellular communication system. Describe co-channel and adjacent channel. Describe the types of interference in cellular communication system. a. Co-channel Interference (CCI)b. Adjacent channel interference (ACI) Explain the method to reduce CCI Calculate co-channel reuse ratio by using the formula. List the causes of interference.OBJECTIVES


Interference is the limiting factor in performance of all cellular radio systemsWhat are the sources of interference for a mobile receiver?Interference is in bothvoice channelscontrol channels

Interference In Cellular Communication System


Interference in Cellular System


Co-channel Interference(CCI). First we look at CCIFrequency ReuseMany cells in a given coverage area use the same set of channel frequencies to increase system capacity (C)VC & CC traffic in co-channel cells is an interfering source to mobiles in Several different cells


Co-Channel InterferenceCells using the same frequency cause interference to each otherCalled co-channel interference (CCI)CCI increases as the cluster size N decreasesImportant factor for signal quality is the Carrier to Interference Ratio C/IMost interference comes from the first tier of co-channel cellsEP602-WIRELESS COMMUNICATION/CHAPTER 5*


Co-Channel Interference


C/I is calculated as:The maximum number of K in the first tier is 6 and knowing thatInterfering signal,The above equation becomes:Co-Channel InterferenceKI = number of interfering cells


Rearranging:andThe qk is the co-channel interference reduction factor with kth co-channel interfering cell.Co-Channel Interference


Co-Channel InterferenceAs N decreases the number of frequency channels per cell increases but C/I decreasesC/I is improved by different methodsSectored antennas: reduces KIBeam tilting: Reduces power to co-channel cellsChannel assignment: minimizes activation of co-channel frequencies, which reduces KI*


Adjacent Channel InterferenceAdjacent Channel Interference (ACI)ACI happens because Imperfect Rx filters allow energy from adjacent channels to leak into the passband of other channelsEP602-WIRELESS COMMUNICATION/CHAPTER 5*


actual filter response

EP602-WIRELESS COMMUNICATION/CHAPTER 5* desired filter response Adjacent Channel Interference


This affects both forward & reverse linksForward Link base-to-mobileinterference @ mobile Rx from a ______ Tx (another mobile or another base station that is not the one the mobile is listening to) when mobile Rx is ___ away from base station.signal from base station is weak and others are somewhat strong.Reverse Link mobile-to-baseinterference @ base station Rx from nearby mobile Tx when desired mobile Tx is far away from base station Adjacent Channel Interference


Near/Far Effectinterfering source is near some Rx when desired source is far awayACI is primarily from mobiles in the same cellsome cell-to-cell ACI does occur as well but a secondary sourceControl of ACIdont allocate channels within a given cell from a contiguous band of frequenciesfor example, use channels 1, 4, 7, and 10 for a cell.no channels next to each other

Adjacent Channel Interference


maximize channel separationseparation of as many as N channel bandwidthssome schemes also seek to minimize ACI from neighboring cells by not assigning adjacent channels in neighboring cells



*EP602-WIRELESS COMMUNICATION/CHAPTER 5


Originally 666 channels, then 10 MHz of spectrum was added666+166 = 832 channels 395 VC plus 21 CC per service provider (providers A & B)395*2 = 790, plus 42 control channelsProvider A is a company that has not traditionally provided telephone serviceProvider B is a traditional wireline operator21 VC groups with 19 channels/groupat least 21 channel separation for each group Adjacent Channel Interference


for N = 7 3 VC groups/cellFor example, choose groups 1A, 1B, and 1C for a cell so channels 1, 8, 15, 22, 29, 36, etc. are used. 57 channels/cellat least 7 channel separation for each cell groupto have high quality on control channels, 21 cell reuse is used for CCsinstead of reusing a CC every 7 cells, as for VCs, reuse every 21 cells (after every three clusters)greater distance between control channels, so less CCI

* Adjacent Channel Interference


use high quality filters in base stationsbetter filters are possible in base stations since they are not constrained by physical size and power as much as in the mobile Rxmakes reverse link ACI less of a concern than forward link ACIalso true because of power control (discussed below)choice of modulation schemesdifferent modulation schemes provide less or more energy outside their passband.



Power Controltechnique to minimize ACIbase station & MSC constantly monitor mobile received signal strengthmobile Tx power varied (controlled) so that smallest Tx power necessary for a quality reverse link signal is used (lower power for the closer the mobile is to the base station)also helps battery life on mobile Adjacent Channel Interference


dramatically improves adjacent channel S / I ratio, since mobiles in other cells only transmit at high enough power as transmitter controls (not at full power)most beneficial for ACI on reverse linkwill see later that this is especially important for CDMA systems



Method To Reduce Co-channel InterferencePossible Solutions?1) Increase base station Tx power to improve radio signal reception? __this will also increase interference from co-channel cells by the same amountno net improvement2) Separate co-channel cells by some minimum distance to provide sufficient isolation from propagation of radio signals? if all cell sizes, transmit powers, and coverage patterns same co-channel interference is independent of Tx power


co-channel interference depends on:R : cell radiusD : distance to base station of nearest co-channel cellif D / R then spatial separation relative to cell coverage area improved isolation from co-channel RF energyQ = D / R : co-channel reuse ratiohexagonal cells Q = D/R =

Method To Reduce Co-channel Interference


CCI Reduction: Cell SectoringShown 120 sectored antennasChannel per cell are divided among 3 sectorsCCI decreased. Sector 0 gets interference from sectors 4, 5 and 6 only60 degrees sectored also possible

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CCI Reduction: Beam TiltingBy tilting down the antenna beam, the power outside the cell, causing CCI reduces*


CCI Reduction: Channel AssignmentFixed Channel AssignmentCell allocated predetermined set of channelsAny call within the cell must use one of the unused channels assigned to cellIf all channels used, call is blockedChannel BorrowingIf all channels are used in a cell has, it can, temporarily, borrow from neighboring cellsMSC supervises borrowing Should not cause high CCI to other cell*


CCI Reduction: Channel Assignment Dynamic Channel AssignmentChannels not permanently assigned to cellsBSC requests channel from MSC when call madeMSC allocates channel to call based on algorithm that takes into accountProbability of future blocking within cellFrequency of use of candidate channelReuse distance of channelMSC assigns channel that will not interfere with existing callsReduces probability of blocking &Increases channel utilization*


CCI Reduction: Cell splittingIf higher capacity is needed in a spot, we need to go, locally, to smaller cluster size NEach cell can be split into multiple microcells with own BSRescaling system to smaller cell sizeTransmit power of BS reduced to obtain smaller coverage area than original BSEnables more spatial reuse greater system capacityCell splitting preserves original frequency reuse planCell splitting causes increased handoffCan use umbrella cells where fast-moving mobiles covered by original cell and slower mobiles covered by microcells


CCI Reduction: Cell Splitting Example


CCI Reduction: Cell Splitting Example...*


CCI Reduction: Frequency ReuseDesign cells to be non-overlapping and cover entire regionCells depicted as hexagonsConceptual design allowing easy analysis of systemClose to circular shape achieved by omnidirectional antennasFootprint: actual radio coverage of a cellDetermined from field measurements or propagation prediction modelsAmorphous in natureUse hexagon to approximate shape


CCI Reduction: Frequency ReuseDue to Co-channel Interference (CCI), cannot use same frequency in adjacent cellCells that use same frequencies must be separated by distances large enough to keep interference levels lowFrequencies assigned to different cells using frequency reuse planAdjacent cells assigned different frequencies to avoid interference or crosstalk*


CCI Reduction: Frequency ReuseObjective is to reuse frequency in nearby cells10 to 50 frequencies assigned to each cellTransmission power controlled to limit power at that frequency escaping to adjacent cellsThe issue is to determine how many cells must intervene between two cells using the same frequency

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Frequency ReuseCells with same letter use the same set of frequency channelsUsing hexagonal cells, BS located at center of cellMS at edge of cell receives weak signal from BS, i.e., low Carrier to Interference ratio (C/I)

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CCI Reduction: Frequency ReuseSuppose system has S total channels & k channels per cell (k < S)Channels divided among N cells into disjoint groups, S = kN, N cells which use all S channels called cluster (N = cluster size, typically 4, 7, 12)Clusters replicated in systemTypically cluster size N = i2 + ij + j2N=7 i=2, j=1N=3 i=1, j=1Move i cells in any directionTurn 60o CCWMove j cells in this direction

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CCI Reduction: Frequency Reuse


231N=312434136527N=4N=71111N=1


Cell GeometryDRRR


Causes of interferenceanother mobile in the same cella call in progress in the neighboring cellother base stations operating in the same frequency bandNon cellular system leaks energy into the cellular frequency band*


*There are 3 mains of wave propagation on the earth i.e. ground wave, ionespheric or sky wave and trophospheric wave.*This is the illustration of how radio wave propagates on earth.*Ground wave: the wave travels in contact with earths surface.*The wave propagation follows contour of the earth***Reference for Fading: Mobile Wireless Communications by Mischa Schwartz****Nearby, far ********