RF Planning

INTRODUCTION TO RF PLANNING

Designing a cellular system - particularly one that incorporates both Macrocellular and Microcellular networks is a delicate balancing exercise. The goal is to achieve optimum use of resources and maximum revenue potential whilst maintaining a high level of system quality. Full consideration must also be given to cost and spectrum allocation limitations.A properly planned system should allow capacity to be added economically when traffic demand increases. As every urban environment is different, so is every macrocell and microcell network. Hence informed and accurate planning is essential in order to ensure that the system will provide both the increased capacity and the improvement in network quality where required, especially when deploying Microcellular systems.INTRODUCTION TO RF PLANNING

INTRODUCTION TO RF PLANNING

RF planning plays a critical role in the Cellular design process. By doing a proper RF Planning by keeping the future growth plan in mind we can reduce a lot of problems that we may encounter in the future and also reduce substantially the cost of optimization.On the other hand a poorly planned network not only leads to many Network problems , it also increases the optimization costs and still may not ensure the desired quality.

TOOLS USED FOR RF PLANNINGNetwork Planning ToolCW Propagation ToolTraffic Modeling ToolProject Management ToolINTRODUCTION TO RF PLANNING

Network Planning ToolPlanning tool is used to assist engineers in designing and optimizing wireless networks by providing an accurate and reliable prediction of coverage, doing frequency planning automatically, creating neighbor lists etc. With a database that takes into account data such as terrain, clutter, and antenna radiation patterns, as well as an intuitive graphical interface, the Planning tool gives RF engineers a state-of-the-art tool to:Design wireless networksPlan network expansionsOptimize network performance Diagnose system problemsThe major tools available in the market are Planet, Pegasos, Cell Cad. Also many vendors have developed Planning tools of their own like Netplan by Motorola, TEMS by Ericsson and so on.INTRODUCTION TO RF PLANNING

Network Planning Tool (PLANET)INTRODUCTION TO RF PLANNING

Propagaton Test KitThe propagation test kit consists ofTest transmitter.Antenna ( generally Omni ).Receiver to scan the RSS (Received signal levels). The receiver scanning rate should be settable so that it satisfies Lees law.A laptop to collect data.A GPS to get latitude and longitude.Cables and accessories.Wattmeter to check VSWR.A single frequency is transmitted a predetermined power level from the canditate site. These transmitted power levels are then measured and collected by the Drive test kit. This data is then loaded on the Planning tool and used for tuning models.Commonly Graysons or CHASE prop test kits are used.INTRODUCTION TO RF PLANNING

Propagaton Test KitINTRODUCTION TO RF PLANNING

Traffic Modeling ToolTraffic modelling tool is used by the planning engineer for Network modelling and dimensioning.It helps the planning engineer to calculate the number of network elements needed to fulfil coverage, capacity and quality needs.Netdim by Nokia is an example of a Traffic modelling tool.INTRODUCTION TO RF PLANNING

Project Management ToolThough not directly linked to RF Design Planning, it helps in scheduling the RF Design process and also to know the status of the projectSite database : This includes RF data, site acquisition,power, civil ,etc.Inventory Control Fault trackingFinance ManagementINTRODUCTION TO RF PLANNING

RF PLANNINGPROCEDURES

RF PLANNING PROCEDURE

Propagation tool setupSet up the planning tool hardware. This includes the server and or clients which may be UNIX based.Setup the plotter and printer to be used.

Terrain, Clutter, Vector data acquisition and setupProcure the terrain, clutter and vector data in the required resolution.Setup these data on the planning tool.Test to see if they are displayed properly and printed correctly on the plotter.PRELIMINARY WORK

Setup site tracking databaseThis is done using Project management or site management databases.This is the central database which is used by all relevant department, viz. RF, Site acquisition, Power, Civil engineering etc, and avoids data mismatch.

Load master lease site locations in databaseIf predetermined friendly sites that can be used are available, then load this data into the site database.

RF PLANNING PROCEDUREPRELIMINARY WORK

Marketing Analysis and GOS determinationMarketing analysis is mostly done by the customer.Growth plan is provided which lists the projected subscriber growth in phases.GOS is determined in agreement with the customer (generally the GOS is taken as 2%)Based on the marketing analysis, GOS and number of carriers as inputs, the network design is carried out.

Zoning AnalysisThis involves studying the height restrictions for antenna heights in the design area.RF PLANNING PROCEDUREPRELIMINARY WORK

Set Initial Link BudgetLink Budget Analysis is the process of analyzing all major gains and losses in the forward and reverse link radio paths.InputsBase station & mobile receiver sensitivity parametersAntenna gain at the base station & mobile station.Hardware losses(Cable, connector, combiners etc).Target coverage reliabilty.Fade margins.OutputMaximum allowable path loss.RF PLANNING PROCEDUREPRELIMINARY WORK

Initial cell radius calculationUsing link budget calculation, the maximum allowable path loss is calculated.Using Okumura hata emprical formula, the initial cell radius can be calculated.

Initial cell count estimatesOnce the cell radius is known, the area covered by one site can be easily calculated.By dividing the total area to be covered by the area of each cell, a initial estimate of the number of cells can be made.

RF PLANNING PROCEDUREPRELIMINARY WORK

INITIAL SURVEY

Morphology DefinitionMorphology describes the density and height of man made or natural obstructions.Morphology is used to more accurately predict the path loss.Some morphology area definitions are Urban, Suburban, rural, open etc.Density also applies to morphology definitions like dense urban, light suburban, commercial etc.This basically leads to a number of sub-area formation where the link budget will differ and hence the cell radius and cell count will differ.RF PLANNING PROCEDURE

Morphology Drive TestThis drive test is done to prepare generic models for network design.Drive test is done to characterize the propagation and fading effects.The objective is to collect field data to optimize or adjust the prediction model for preliminary simulations.A test transmitter and a receiver is used for this purpose.The received signals are typically sampled ( around 50 samples in 40 ).

RF PLANNING PROCEDURE

PROPAGATION MODEL

Propagation Tool AdjustmentThe data collected by drive testing is used to prepare generic models.For a given network design there may be more than one model like dense-urban, urban, suburban, rural, highway etc.The predicted and measured signal strengths are compared and the model adjusted to produce minimum error.These models are then used for initial design of the network.

INITIAL DESIGN

Complete Initial Cell PlacementPlanning of cell sites sub-area depending on clutter type and traffic required.

Run Propagation AnalysisUsing generic models prepared by drive testing & prop test, run predictions for each cell depending on morphology type to predict the coverage in the given sub-areas.Planning tool calculates the path loss and received signal strength using Co-ordinates of the site location, Ground elevation above mean sea level, Antenna height above ground, Antenna radiation pattern (vertical & horizontal) & antenna orientation, Power radiated from the antenna.RF PLANNING PROCEDURE

Reset Cell Placement( Ideal Sites)According to the predictions change the cell placements to design the network for contigious coverage and appropriate traffic.

System Coverage MapsPrepare presentations as followsBackground on paper showing area MAP which include highways, main roads etc.Phase 1 sites layout on transparency.Phase 1 sites composite coverage prediction.Phase 2 sites layout transparency.Phase 2 composite coverage prediction on transparency.If more phases follow the same procedure. INITIAL DESIGNRF PLANNING PROCEDURE

Design Review With The ClientInitial design review has to be carried out with the client so that he agrees to the basic design of the network.During design review, first put only the background map which is on paper. Then step by step put the site layout and coverage prediction.Display may show some coverage holes in phase 1 which should get solved in phase 2 . INITIAL DESIGNRF PLANNING PROCEDURE

Prepare Initial Search RingNote the latitude and longitude from planning tool.Get the address of the area from mapping software.Release the search ring with details like radius of search ring, height of antenna etc.

Release search rings to project management

Visit friendly site locationsIf there are friendly sites available that can be used (infrastructure sharing), then these sites are to be given preference.If these sites suite the design requirements, then visit these sites first.

SELECTION OF SITESRF PLANNING PROCEDURE

Select Initial Anchor SitesInitial anchor sites are the sites which are very important for the network buildup, Eg - Sites that will also work as a BSC.

Enter Data In Propagation Tool Enter the sites exact location in the planning tool.

Perform Propagation AnalysisNow since the site has been selected and the lat/lon of the actual site ( which will be different from the designed site) is known, put this site in the planning tool and predict coverage.Check to see that the coverage objectives are met as per prediction.


Reset / Review Search RingsIf the prediction shows a coverage hole ( as the actual site may be shifted from the designed site), the surrounding search rings can be resetted and reviewed.Candidate site Visit( Average 3 per ring)For each proposed location, surveys should carried out and at least 3 suitable site candidates identified. Details of each candidate should be recorded on a copy of the Site Proposal Form for that site. Details must include:Site name and option letter Site location (Lat./Long)Building HeightSite address and contact numberHeight of surrounding clutterDetails of potential coverage effecting obstructions or other comments(A, B, C,...)SELECTION OF SITESRF PLANNING PROCEDURE

Drive Test And Review Best CandidateIn order to verify that a candidate site, selected based on its predicted coverage area, is actually covering all objective areas, drive test has to be performed.Drive test also points to potential interference problems or handover problems for the site.The test transmitter has to be placed at the selected location with all parameters that have been determined based on simulations.Drive test all major roads and critical areas like convention centers, major business areas, roads etc.Take a plot of the data and check for sufficient signal strength, sufficient overlaps and splashes( least inteference to other cells).


Drive Test IntegrationThe data obtained from the drive test has to be loaded on the planning tool and overlapped with the prediction. This gives a idea of how close the prediction and actual drive test data match. If they do not match ( say 80 to 90 %) then for that site the model may need tuning.

Visit Site With All Disciplines( SA, Power, Civil etc )A meeting at the selected site takes place in which all concerned departments like RF Engineering, Site acquisition, Power, Civil Engineer, Civil contractor and the site owner is present.Any objections are taken care off at this point itself.SELECTION OF SITESRF PLANNING PROCEDURE

PRE-CONSTRUCTION STUDIES

Select Equipment Type For SiteSelect equipment for the cell depending on channel requirementsSelection of antenna type and accessories.

Locate Equipment On Site For Construction DrawingPlan of the building ( if site located on the building) to be made showing equipment placement, cable runs, battery backup placement and antenna mounting positions.Antenna mounting positions to be shown separately and clearly.Drawings to be checked and signed by the Planner, site acquisition, power planner and project manager.

PRE-CONSTRUCTION STUDIES

Perform Link Balance CalculationsLink balance calculation per cell to be done to balance the uplink and the downlink path.Basically link balance calculation is the same as power budget calculation. The only difference is that on a per cell basis the transmit power of the BTS may be increased or decreased depending on the pathloss on uplink and downlink.

EMI StudiesStudy of RF Radiation exposure to ensure that it is within limits and control of hazardous areas.Data sheet to be prepared per cell signed by RF Planner and project manager to be submitted to the appropriate authority.

SYSTEM DESIGN

Radio Frequency Plan/ PN PlanFrequency planning has to be carried out on the planning tool based on required C/I and C/A and interference probabilities.

System Interference PlotsC/I, C/A, Best server plots etc has to be plotted.These plots have to be reviewed with the customer to get the frequency plan passed.

Final Coverage PlotThis presentation should be the same as design review presentation.This plot is with exact locations of the site in the network.

SYSTEM DESIGN

Identification of coverage holesCoverage holes can be identified from the plots and subsequent action can be taken(like putting a new site) to solve the problem.

RADIO WAVE PROPAGATION

BASIC DEFINITIONS

Isotropic RF SourceA point source that radiates RF energy uniformly in all directions (I.e.: in the shape of a sphere)Theoretical only: does not physically exist.Has a power gain of unity I.e. 0dBi.

Effective Radiated Power (ERP)Has a power gain of unity i.e. 0dBiThe radiated power from a half-wave dipole.A lossless half-wave dipole antenna has a power gain of 0dBd or 2.15dBi.

Effective Isotropic Radiated Power (EIRP)The radiated power from an isotropic source EIRP = ERP + 2.15 dB

BASIC DEFINITIONS

Radio signals travel through space at the Speed of LightC = 3 * 108 meters / secondFrequency (F) is the number of waves per second (unit: Hertz)Wavelength () (length of one wave) = (distance traveled in one second) (waves in one second) = C / FIf frequency is 900MHZ thenwavelength = 3 * 108 900 * 106 = 0.333 meters

BASIC DEFINITIONS

dBdB is a a relative unit of measurement used to describe power gain or loss. The dB value is calculated by taking the log of the ratio of the measured or calculated power (P2) with respect to a reference power (P1). This result is then multiplied by 10 to obtain the value in dB. dB = 10 * log10(P1/P2)The powers P1 ad P2 must be in the same units. If the units are not compatible, then they should be transformed. Equal power corresponds to 0dB.A factor of 2 corresponds to 3dBIf P1 = 30W and P2 = 15 W then10 * log10(P1/P2) = 10 * 10 * log10(30/15) = 2

BASIC DEFINITIONS

dBmThe most common "defined reference" use of the decibel is the dBm, or decibel relative to one milliwatt.It is different from the dB because it uses the same specific, measurable power level as a reference in all cases, whereas the dB is relative to either whatever reference a particular user chooses or to no reference at all.A dB has no particular defined reference while a dBm is referenced to a specific quantity: the milliwatt (1/1000 of a watt).The IEEE definition of dBm is "a unit for expression of power level in decibels with reference to a power of 1 milliwatt." The dBm is merely an expression of power present in a circuit relative to a known fixed amount (i.e., 1 milliwatt) and the circuit impedance is irrelevant.}

BASIC DEFINITIONS

dBmdBm = 10 log (P) (1000 mW/watt)where dBm = Power in dB referenced to 1 milliwattP = Power in watts If power level is 1 milliwatt: Power(dBm) = 10 log (0.001 watt) (1000 mW/watt) = 10 log (1) = 10 (0) = 0 Thus a power level of 1 milliwatt is 0 dBm. If the power level is 1 watt1 watt Power in dBm = 10 log (1 watt) (1000 mW/watt) = 10 (3) = 30

BASIC DEFINITIONS

dBmdBm = 10 log (P) (1000 mW/watt) The dBm can also be negative value.If power level is 1 microwattPower in dBm = 10 log (1 x 10E-6 watt) (1000 mW/watt) = -30 dBm Since the dBm has a defined reference it can be converted back to watts if desired. Since it is in logarithmic form it may also be conveniently combined with other dB terms.

BASIC DEFINITIONS

dBv/mTo convert field strength in dbv/m to received power in dBm with a 50 optimum terminal impedance and effective length of a half wave dipole /0dBu = 10 log[(10-6)2(1000)(/)2/(4*50)] dBm At 850MHZ0dBu = -132 dBm 39dBu = -93 dBm

FREE SPACE PROPAGATION

Friis FormulaPr = Pt GtGr2 (4d)2

Propagation LossLp = 10log [4d / ]2The square term is the propagation exponent. It is greater than 2 when obstructions exist.

Propagation Loss in dB:L p = 32.44 + 20Log(d) +20Log(f)f = MHzd = km PtGtGrPrLpd

PROPAGATION MECHANISMS

ReflectionOccurs when a wave impinges upon a smooth surface.Dimensions of the surface are large relative to .Reflections occur from the surface of the earth and from buildings and walls.

Diffraction (Shadowing)Occurs when the path is blocked by an object with large dimensions relative to and sharp irregularities (edges).Secondary wavelets propagate into the shadowed region.Diffraction gives rise to bending of waves around the obstacle.

ScatteringOccurs when a wave impinges upon an object with dimensions on the order of or less, causing the reflected energy to spread out orscatter in many directions.Small objects such as street lights, signs, & leaves cause scattering

MULTIPATH

Multiple Waves Create MultipathDue to propagation mechanisms, multiple waves arrive at the receiverSometimes this includes a direct Line-of-Sight (LOS) signal

MULTIPATH

Multipath PropagationMultipath propagation causes large and rapid fluctuations in a signalThese fluctuations are not the same as the propagation path loss.

Multipath causes three major thingsRapid changes in signal strength over a short distance or time.Random frequency modulation due to Doppler Shifts on different multipath signals.Time dispersion caused by multipath delaysThese are called fading effectsMultipath propagation results in small-scale fading.

WHAT IS FADING ?

The communication between the base station and mobile station in mobile systems is mostly non-LOS.The LOS path between the transmitter and the receiver is affected by terrain and obstructed by buildings and other objects.The mobile station is also moving in different directions at different speeds.The RF signal from the transmitter is scattered by reflection and diffraction and reaches the receiver through many non-LOS paths.This non-LOS path causes long-term and short term fluctuations in the form of log-normal fading and rayleigh and rician fading, which degrades the performance of the RF channel.

WHAT IS FADING ?

Signal Power (dBm)Large scale fading componentSmall scale fadingcomponent

LONG TERM FADING

Terrain configuration & man made environment causes long-term fading.Due to various shadowing and terrain effects the signal level measured on a circle around base station shows some random fluctuations around the mean value of received signal strength.The long-term fades in signal strength, r, caused by the terrain configuration and man made environments form a log-normal distribution, i.e the mean received signal strength, r, varies log-normally in dB if the signal strength is measured over a distance of at least 40.Experimentally it has been determined that the standard deviation, , of the mean received signal strength, r, lies between 8 to 12 dB with the higher generally found in large urban areas.

RAYLEIGH FADING

This phenomenon is due to multipath propagation of the signal.The Rayleigh fading is applicable to obstructed propagation paths. All the signals are NLOS signals and there is no dominant direct path.Signals from all paths have comparable signal strengths.The instantaneous received power seen by a moving antenna becomes a random variable depending on the location of the antenna.

RICEAN FADING

This phenomenon is due to multipath propagation of the signal.In this case there is a partially scattered field.One dominant signal.Others are weaker.

Dopplers shift is the shift in frequency due to the motion of mobile from the actual carrier frequency.Consider a mobile moving at a constant velocity v along a path segment having a length d between points X and Y while it receives signal from a remote source S.

The Change in frequency due to dopplers shift is given by fd = (v/) * cos()It can be seen from the above equation that if the mobile is moving towards the direction of arrival of wave the dopplers shift is positive I.e. the apparent received frequency is increased. .

XYdSDOPPLERS SHIFT

LINK BUDGET PLANNING AND INITIAL CELL ESTIMATES

WHY LINK BUDGET ANALYSIS?Link budget analysis providesCoverage design thresholdsEIRP needed to balance the pathMaximum allowable path lossIt is important that the uplink and downlink paths be balanced, otherwise not enough signal will survive the transmission process to achieve the required signal to noise ratio(SNR) or the bit-error-rate(BER).Path imbalance results from the facts that the gains and losses in the uplink and downlink paths are not the same.The calculations have to be done separately on the uplink and the downlink.

THE RF PATHPBSMSSensitivityPMSBSSensitivityPath LossDownlinkPath LossUplinkNoiseFadingInterference

THE RF PATHINPUTSBase station and Mobile receiver Sensitivity ParametersMinimum acceptable Signal to Noise ratioEnvironmental / Thermal NoiseReceiver Noise figureAntenna gain at the base station and mobile station.Hardware Losses (Cable , Connectors, Combiners, Duplexers etc)Target Coverage reliability.Fade margins.

OUTPUTSBase station ERPMaximum allowable path lossCell size estimatesCell count estimates

GAINS AND LOSSESGainsBase station Antenna gain Mobile antenna gainDiversity gainsLossesHardware lossesCombinerCablesConnectorsDuplexerAir InterfaceFade MarginPenetration LossesIn-carIn-buildingBody Loss

ANTENNA GAINSMobile Station AntennaPortable mobile phones antenna have typically gain of 0 to 1 dBd.Car mounted antenna has a typical gain of 1 to 3 dBd.

Base Station AntennaOmni directional antenna typically have a gain of 0-9dBd.Directional antenna typically have a gain of 9 to 14 dBd.

DIVERSITY GAINDiversity is used on the uplink to overcome deep fades due to multipath by combining multiple uncorrelated signals.Diversity antenna systems are used mostly at the BTS on the uplink.Diversity antenna system can be realised by physically separating two receive antenna in space or by using polarization diversity.Diversity gain should be considered in Link Budget Analysis whenever it is used.Typically a gain of 3dB is considered whenever diversity is used in the Uplink calculation.

CABLESRadio EquipmentMain cableJumper cable

Two types of cables are used, maincable and jumper cable.Cable losses are given in per 100feet.Jumper cable have more loss than main cable.Cable loss is also dependant on frequency

CABLE LOSS

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CABLESIZERECOMMENDEDLOSS/100 Feet

TYPEUSE900MHZ1800MHZ

LDF4-501/2 inch Heliax FoamJumper cable2.160dB

LDF5-507/8 inch Heliax FoamMain cable < 55M1.21dB1.97dB

LDF7-501 5/8 inch Heliax FoamMain cable < 90m0.75dB1.25dB

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CONNECTOR & COMBINER LOSSConnector LossConnectors used to connect RF components have a typical loss of 0.1dB each.

Combiner LossA combiner is a device that enables several transmitters of different frequencies to transmit from the same antenna.Two types of combiners are available.Hybrid combiners combine two inputs to one output.Hybrid combiners have a typical insertion loss of 3dB.Cavity combiners combine more input to one output ( typically 5 inputs)Cavity combiners have around 3dB loss.Cavity combiners cannot be used in cells where synthesizer frequency hopping is used.

DUPLEXERA duplexer enables simultaneous transmission and reception of signals on the same antenna .It provides isolation between the transmitted and received signal.Duplexers typically have a insertion loss of 0.5 to 1 dB

RadioUnitDuplexerTx/ RxTx Rx

BODY LOSSFor all receiving environments a loss associated with the effect of users body on propagation has to be used I.e proximity of the user with the mobile.This effect is in the form of few dB loss in both the uplink and downlink directions.Body loss is typically taken as 2 dB .

PENETRATION LOSSESPenetration losses depend on the location of the subscriber with respect to the site.Generally 3 types of scenarios are taken into consideration viz. In-building, In-car and on street.Body loss is also a type of penetration loss .

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Penetration LossLoss

In-Building Penetration (dB)15

In-car Penetration (dB)3-10

Body Loss (dB)2-5

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Fade Margin Calculation

Cell Area ProbabilityCell area probability (CAP ) is the percentage of the cell area that has signal strength greater than the receiver sensitivity.CAP is dependent on the radio environment, primarily the standard deviation of the log normal faded signal () and the propagation loss constant (n)The CAP is calculated using the following equation

PCA= ( 1+ erf (a) + exp ( )(1 - erf( )))

Where:PCACell area probability

a2ab+1b2ab+1bMFADE


Cell Area ProbabilityB

MFADEFade margin appliedStandard deviation of received signalnPropagation constant10nLog10(e)2


Outdoor Fade MarginThe outdoor fade margin depends on the standard deviation of the lognormal shadowing and the propagation constantThe propagation constant depends on the environment and the frequency.For urban areas propagation constant varies from 2.7 to 5 , with a typical value of 5 for both 850 Mhz and 1900 Mhz.Standard deviation also varies on environment and frequency , and may vary slightly with frequency.The urban areas have higher standard deviation than rural areas. Typical value ranges from 5-12dB with a typical value of 8dBOutdoor fade margin can be calculated using a plot of the CAP equation.The next figure shows the CAP plot for a propagation constant of 3.5 and standard deviation of 5, 8 and 12.From the figure fade margin to be applied to the Link Budget may be selected depending on the standard of the received signal.


Outdoor Fade Margin

RECEIVER SENSITIVITYThis figure is provided by the equipment vendor.Receiver sensitivity is the ability of the receiver to receive signals in the sense that any signal below the sensitivity is considered as noise and is not usable. Receiver sensitivity is given by S = Antenna Noise(dBm) + Receiver Noise Figure(dB) + C/N(dB) S = the receivers sensitivity C/N = Carrier to noise ration required in the presence to achieve a specified BER. Antenna Noise(dBm) = 10log(kTB)Where k = Botlzmann constant 1.38 X 10-20 milli Joules / KelvinT = Room temperature in degrees kelvinB = Bandwidth in Hz

UPLINK MAXIMUM ALLOWABLE PATH LOSS( MAPL) ON UPLINKMS Antenna gain (Gm ) Body Loss (Lbody )Gain of receive antenna (GRA ) In-building / Car penetration Loss (LBldg )Fade Margin (Mfade )Cable,Connector and Combiner losses (LCCC )Diversity gain (GD ) MAPLUP = Pm + Gm - LBody - LBldg - Mfade + GRA + GD - LCCC + RReceiver Sensitivity

IN A NUTSHELL Transmitter powerCombiner lossCable loss(includes jumper and connector loss)Transmit Antenna gainFade marginBody lossMobile antenna gainMobile receiver sensitivity

Mobile Transmit powerMobile antenna gainBody LossFade MarginReceive antenna gainCable loss(includes jumper and connector loss)BTS receiver sensitivity

DOWNLINKUPLINK

LINK BUDGET SHEET

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Linkbudget For General Purpose

Downlink (BTS to MS)Uplink (MS to BTS)

BTS Tx power43dBmMS Tx power33dBm

Combining loss3dBMS antenna gain0dBi

Feeder loss2dBTotal EIRP33dBm

BTS antenna gain17dBi

Total EIRP55dBmBTS antenna gain17dBi

Feeder loss2dB

MS Rx Sensitivity-102dBmDiversity gain3dB

MS antenna gain0dBiBTS Rx Sensitivity-107.00dBm

Fading margin6dBFading margin6dB

Penetration Loss0dBPenetration Loss0dB

Antenna/body loss2dBAntenna/body loss2dB

Max. allowed pathloss149.00dBMax. allowed pathloss150.00dB

Link balance(downlink - uplink)-1.00dB

&F

Page &P

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CELL SIZE/COUNT ESTIMATION Once the Maximum allowable pathloss is known, the achievable cell size can be evaluated.Cell radius is calculated using MAPL and Hatas empirical formula.Cell radius is the distance from base station where the path loss equals MAPL. Beyond this radius, the signal is too weak to be acceptable.Each area has a different correction factor.Also the coverage objectives are usually different for Urban, Suburban and Rural areas.Therefore MAPL has to be calculated for each area and then cell size determined separately.Once the cell radius is calculated, cell count estimates can be made.

HATAS EMPIRICAL FORMULA PL = 69.55 +26.6log10fc - 13.82log10hb + (44.9 - 6.55log10hb) log10R - a(hm) -CFwhere fc - Frequency in MHZ hb - Transmitter antenna height hm - Receiver antenna height R - Radius in Km a(hm) is the correction factor for effective mobile antenna heightSolving backwards the cell radius is given bylog10R = MAPL +CF - 69.55 +26.6log10fc + 13.82log10hb + a(hm) (44.9 - 6.55log10hb)

INITIAL CELL COUNT Once the cell radius for each area is calculated, then the minimum number of cells required to provide coverage can be determined.For each area A = 2.6R2Where R - radius of cell A - Area of the corresponding hexagon.Cell count = Urban Area(Km2) + Suburban area(Km2) + Rural Area(Km2) Aurban(Km2) Asuburban(Km2) Arural(Km2)

DRIVE TEST FOR MODEL TUNING

Predesign drive test for measurement integrationThis is at beginning of design when no site has been built or even selected. All test sites are temporary.Drive test is performed mostly for characterization of propagation and fading effects in the channel. The object is to collect field data to optimize and adjust the prediction model for preliminary simulations.

Post design drive test for site verification / optimizationDrive test is performed to verify if they meet the coverage objectives.Overlaps are checked for hand-offs.Predesign drive testPostdesign drive testDrive test typesINTRODUCTION

INTRODUCTION In field measurement we have to collect variations due to propagation and slow fading.The received signals are typically sampled and averaged over spatial windows called bins.There are several sampling issues to be considered likeSampling rateAveraging windowNumber of bins to be measured

SAMPLING CRITEREA When measuring the RF signal strength certain sampling criterea must be met to eliminate the short-term fading components from the long-term component ( I.e. log normal fading )The RF signal strength measurements must be taken over a radio path or mobile path distance interval of 40, where is the wavelength of the RF signal.If the distance interval is too short, the short term variation cannot be smoothed out and will affect the local mean.If the distance interval is too long, the averaged output cannot represent the local mean since it washes out the detailed signal changes due to the terrain variations.The number of RF measurements taken within the 40 distance should be greater than 50.Depending on the speed of the vehicle during the drive test, the sampling interval in time is selected.Measurements have to be stopped whenever the vehicle is not moving.

SAMPLING CRITEREA If f = 1900MHZ, then = 3 * 108 / 1900 * 106 = 0.158 m 40 = 40 * 0.158 = 6.32 m50 measurements must be recorded every 6.32m or 1 measurement every 0.1264mThe conversion from sampling distance to mobile velocity can be done as followsminimum sampling rate ( per second ) = v / (0.1264 m/sample)If velocity of vehicle is 50 kph thenSampling rate( per second ) = (50000/ 3600) / 0.1264 = 110 samples / secTEMS kit cannot be used for this purpose as it can report RF signal strength measurements at a maximum rate of 1 sample per second

WINDOW SIZE In field measurements the interest is on local averages of received signals.The size of averaging window have to be small enough to capture slow variations due to shadowing and large enough to average out the fast variations due to multipath.A typical range is 20 to 1500 m.The bin size is typically selected in 40 to 1500m, i.e. all measurements in this size square are averaged to one value.Normally the post processing tool takes care of averaging the collected data over different bins.

NUMBER OF BINS The predicted and measured signal strengths for all bins within the drive route is compared and the best set of correction factors to minimize the prediction errors is determined.All the bins within the coverage area cannot be drive tested. So a large enough sample set should be considered.The more the number of bins, the larger the confidence level of results.Generally for acceptable confidence at least 300 to 400 bins have to be considered.Bin

PROPAGATION KIT The propagation test kit consists ofTest transmitter.Antenna ( generally Omni ).Receiver to scan the RSS (Received signal levels). The receiver scanning rate should be settable so that it satisfies Lees law.A laptop to collect data.A GPS to get latitude and longitude.Cables and accessories.Wattmeter to check VSWR.

TransmitterRECEIVERLAPTOPGPS AntennaReceiver AntennaTransmit Antenna

CABLE INTEGRITY TEST TESTTRANSMITTERWATTMETER50 Ohms terminationCable integrity is checked by measuring VSWR.A good known cable is connected from the test transmitter to the wattmeter.The cable under test is placed between the wattmeter & 50 Ohms load.Forward and reverse power levels are measured and VSWR can be calculated as VSWR = ( 1 + PR/PF)/(1 - PR/PF).Each cable should be checked and any that exhibit greater than 1.2 : 1 VSWR must be replaced.Cable integrity check must be performed on all jumpers and antenna feedlines.

CABLE LOSS All cables used for testing must be measured and clearly labelled with insertion loss before they are used.These are normal wear items and so it is important to replace any cables that become kinked or frayed or that are worn or damaged connectors.Step 1 Connected a cable A directly to the wattmeter. Terminate the other end of the wattmeter with a 50 ohms resistance. Insert 1 Watt element in forward position. Adjust to full deflection . Record reading A

TESTTRANSMITTERWATTMETER50 Ohms terminationCable A

CABLE LOSS Step 2Connect cable B as shown. Turn transmitter on. Record reading B.Cable Loss for cable B in dB = 10 log10(B/A)

TESTTRANSMITTERWATTMETER50 Ohms terminationCable ACable Bto be tested

TRANSMITTER SETUP If the propagation test is being done for model tuning to produce a generic model for macro cells, then a high point in the particular area has to be selected.The transmitter and the transmit antenna will be placed at this point (say the roof of the building ).The transmit antenna is connected to the transmitter via a RF cable.Check to see that the cable is connected properly and tight.Loosely connected or faulty cable can increase the VSWR.A test frequency has to selected from the frequency band allocated to the operator. Set the transmitter to this test frequency.TESTTRANSMITTER

TEST SITE SELECTIONSite selection is based on a number of criteria. It may not be possible to satisfy all these criteria at the same time, but it is important to select the best sites available.Drive test sites should be selected to give a good representative sample of the system coverage area. The exact number of sites required will depend on the size of the system coverage area and the variability of the characteristics of the coverage area.All terrain and clutter types in the area should be represented in the drive test data for proper prediction tuning.Typical terrain types are: Flat, Rolling Hills, Large Hills, MountainsTypical clutter types are: Water, Open Land, Forest, Commercial / Industrial, Low Density Urban, Medium Density Urban, High Density Urban, City Center, Airport.City maps, topographical maps and aerial photographs can be useful in determining the terrain and clutter types for an area. It may also be necessary to drive the area and observe building types and density.

TEST SITE SELECTIONSite AvailabilityTest sites must be available for use during the drive test.The site owner/supervisor should approve access to the site for as long as needed to complete the testing. This may involve multiple visits to the site, possibly on short notice.Test sites must also be physically accessible to allow setup of the transmitter equipment and mounting of the antenna. For this reason building top sites are preferred to tower sites.

Site VisitEach site selected should be visited before testing to verify that is suitable for use. The inspection should be done by the same people who will be doing the site setup for the actual drive test. Familiarity with the site should speed up the site setup during the drive test.

BUILDING SITE SELECTIONWhen inspecting a building site the rooftop should be checked for any obstructions that would interfere with signal propagation. This could include objects on the rooftop itself or other nearby buildings or structures.The antenna location should be selected and a sketch of the rooftop made to identify this location relative to other objects nearby.Photographs should be taken of the location where the antenna will be mounted and in all directions looking away from the site.

BUILDING SITE SELECTION

TOWER SITE SELECTIONWhen inspecting a tower site the best location to mount the antenna to the tower must be deter-mined.This should be selected such that the tower doesnt interfere with the propagation pattern of the transmit antenna. This will usually require that the antenna be above the tower or on an arm extending from the side of the tower.The area around the tower should be checked for any obstructions that would interfere with signal propagation.

TEMPORARY STRUCTUREGenerally cranes are used for temporary structure.When cranes are used power generators have to be arranged in advance.The location should be selected such that the antenna will be above any nearby obstacles.

TEMPORARY STRUCTURE

DRIVE TEST PLAN verification to check that the positioning information in the drive test file is correct. A separate map should be prepared for each route.Both line of site (LOS) and non-LOS points have to be included in the drive test.The data collected should represent typical coverage scenarios.In urban area the effect of street orientations have to be considered.The selection of drive test route should be based on the terrain variations, Major highways and throughfares, potential shadowing areas and handoff region.Each drive route should be marked on a detailed road map showing the exact route to be driven.These maps should be used during the actual drive for navigation of the test vehicle. They can also be used during the drive test

DRIVE TEST PROCEDUREThe actual dive test must be performed carefully to insure that the data collected is accurate. It is important that all equipment used be tested and all setup information be recorded.If any of the procedures are not followed or any of the data is not properly recorded then the drive test data will not be usable and the drive will have to be repeated.Engineer should study the drive test plan ahead of time and highlight the intended drive test routes.For each drive test a team of two people should get involved.The measurement process should be stopped the car stops ( eg near traffic lights) or whenever the sampling and measurements look suspect.

DRIVE TEST OUTPUTThe result of drive test is a collection of data files which has lat, long, Received Signal Strength Indicator(RSSI) at that point and the frequency.The location information ( lat, long ) is used by the post processing tools as a reference of correlation between the measured vs. predicted signal levels for measurement integration.This file has to be transferred onto the planning tool either by a floppy or by data transfer using LAN.

Sheet1

LatLongRSSIFreq

X1Y1M1F

X2Y2M2F

X3Y3M3F

Sheet2

Sheet3

DRIVE TEST OUTPUT

COMPUTER MODELING

Models are used to predict path loss.Different models are used for different purpose. Eg:- Rural Macro-cell -Okumura hata model, Microcells - Ray tracingModels have to be tuned using data collected by drive testing.Good propagation tool + Sound engineering ingenuity = Sound RF design.Some of the popular prediction models are Okumura hata, Walfisch Ikegemi, COST231, Ray tracing etc. INTRODUCTION

COMPUTER MODELLINGOkumura Hata ModelThis is used for Macro cell modeling.It has become the most popular propagation model for mobile environments.It is best applicable for cell ranges of 5 to 20 kms.Below a range of 1 km it becomes very unreliable since obstacles in the close vicinity of receiver and transmitter become the dominant scattering influences which are not taken into account in the formula.Path loss = K1 + K2log(d) + K3log(Heff) + K4 * Diff + K5log(Heff)log(d) + K6log(Hmeff) +K7log(f) + Kmorphology K1 - 1Km intercept value. Upto this point model assumes free space loss K2 - Slope value K3 - Effective height coefficient K4 - Coefficient for diffraction calculation K5 - Hata model multiplier K6 - Multiplier for mobile height K7 - factor for frequency

COMPUTER MODELLING

Walfish-Ikegami ModelWalfish and Ikegami is a propagation model used in urban environment. It takes into account near-by building structures. It assumes a regular grid pattern throughout the city.The model has four main parameters: Building separation (in meters)(b) : It is the distance between the centre of two buildings. Average building height (h) : This is the average height of all buildings in the cells coverage area.Road width (w)Road orientation angle ( )The model has separate equations for LOS and NLOS conditions. In NLOS roof-top-to-street diffraction and scatter loss and multi-screen diffraction loss for the immediate surroundings at mobiles location are taken into account.

COMPUTER MODELLING

hwbdWalfish Ikegami Model

COMPUTER MODELLING

Ray Tracing In microcellular environments cells are generally location below rooftops and average cell coverage area is around 1 Km or less. This makes it difficult to do coverage predictions. Most of the popular models like Okumara Hata model have been found to be unreliable in such conditions. In any statistical analysis a large sample is required inorder to get reliable results. In microcellular environment situations change rapidly in the small coverage area making it difficult to provide reliable results using any statistical models.Deterministic models have been created inorder to overcome this problem.

COMPUTER MODELLING

Ray Tracing Ray tracing is one such method and is used commonly in microcellular environments. Ray tracing follows a certain number of rays from the point of transmission to the point under calculation.There are different threshold for the number of rays to be taken in consideration as the ray becomes negligible after it has experienced a certain number of reflections. There are primarily two methods for ray tracing : Ray launching and mirror image methods.For the method of ray launching a receiving circle is defined . The rays that cross this circle are taken into account when evaluating field strength level at the centre of the circle.The limitations of this method are that the map has to be precise, the buildings have to be modelled reliably and the computational load might turn out to be enormous. This is especially the case when a large number of reflections have to be taken into account.

COMPUTER MODELLING

Ray Launching MethodReceivingCirclesingle pointsignal source

COMPUTER MODELLING

MODEL TUNINGPropagation models use clutter and terrain data to predict cell coverage at a site. However usually the terrain and clutter data available from the maps are not perfect. This means that the actual cell coverage could be different from the predicted cell coverage. This could in turn result in wrong cell designing. To avoid this model tuning is done.In model tuning data collected from the propagation test is loaded on the planning tool. This data represents the real life condition cell coverage. The prediction for that cell is then done using the same conditions as were for the propagation tests (i.e. using the same antenna type, same height of the antenna at the site, same downtilts, same transmit power etc. ).

COMPUTER MODELLING

MODEL TUNINGIdeally both the propagation test cell coverage and the predicted cell coverage should match. If they match then the model does not require to be tuned. If the models do not match then the certain parameters in the propagation model equation are altered so that they both match. Once both the cell coverage match the model is then said to be tuned. Now the actual antenna type, height of antenna, transmit power are used and prediction done. This prediction can then be assumed to be correct. Cell designing is then done using this prediction.

COMPUTER MODELLING

MODEL TUNINGIdeally model tuning needs to be done for all the sites. However in many cases , the Network is divided into different clutter types (around 7 to 8) (e.g. urban,dense urban, semi urban, rural etc.) and models are tuned for each clutter types. The sites are then categorized in these clutter types and then fitted in the model tuned for that clutter type. This method though not perfect is widely accepted and saves lot of time and money for the operator.

COMPUTER MODELING

Tuning Of Model In PlaNETWe will have a look at a general process of model tuning in PLANET using Okumura Hata Model.Load the Prop Test Survey data on PLANETDo a prediction using same antenna type, height, transmit power, etc. as used during survey.Check if surveyed data matches that of prediction, if yes model tuning not required, if not we proceed with model tuning.Planet has analysis capability with which it can compare predicted and surveyed data to give RMS and mean error for the predicted with respect to the surveyed data.It displays it with a Log d versus slope graph.To tune the model we should try to bring mean error to zero and the slope to zero. We do this by varying the values of K1 and K2

COMPUTER MODELING

Tuning Of Model In PlaNET Steps In TuningSet Okumura Hata constants to initial valuesK1 = -20K2 = -44.9K3 = -5.83K4 = 0.5K5 = 6.55K6 = 0Set all clutter values to zeroAnalyse model with surveyed data, PLANET will give you new suggested values for K1 and K2 and RMS and mean errors.This is also displayed on Log(d) versus error graph. Our objective is to get a line with no slope and centred on zero.Use suggested values for K1 and K2 and analyse again.Repeat this procedure till we get mean error to be zero.After getting mean error to be zero our next aim is to obtain the line on the graph with no slope. This is done by varying the value of K2 positively or negatively depending on the graph.Once we get the line with no slope, note the suggested value of K1 for that K2.

COMPUTER MODELING

Tuning Of Model In PlaNET Steps In TuningAnalyse again with these values for K1 and K2. This should centre the line and will have no slope.Now add clutter factors and analyse again.If the line remains centred on zero and has no slope ,the model is tuned.If not we need to repeat the process of varying the values of K1 and K2 till we get the line centred on zero and with no slope to tune the model.

ANTENNAS

ANTENNAS

Antennas form a essential part of any radio communication system.Antenna is that part of a transmitting or receiving system which is designed to radiate or to receive electromagnetic waves. An antenna can also be viewed as a transitional structure between free-space and a transmission line (such as a coaxial line). An important property of an antenna is the ability to focus and shape the radiated power in space e.g.: it enhances the power in some wanted directions and suppresses the power in other directions.Many different types and mechanical forms of antennas exist.Each type is specifically designed for special purposes.

ANTENNAS TYPES

In mobile communications two main categories of antennas used areOmni directional antennaThese antennas are mostly used in rural areas.In all horizontal direction these antennas radiate with equal power. In the vertical plane these antennas radiate uniformly across all azimuth angles and have a main beam with upper and lower side lobes.

ANTENNAS TYPES

Directional antennaThese antennas are mostly used in mobile cellular systems to get higher gain compared to omnidirectional antenna and to minimise interference effects in the network.In the vertical plane these antennas radiate uniformly across all azimuth angles and have a main beam with upper and lower side lobes.In these type of antennas, the radiation is directed at a specific angle instead of uniformly across all azimuth angles in case of omni antennas.

ANTENNA CHARACTERISTICS

Radiation PatternThe main characteristics of antenna is the radiation pattern.The antenna pattern is a graphical representation in three dimensions of the radiation of the antenna as a function of angular direction. Antenna radiation performance is usually measured and recorded in two orthogonal principal planes (E-Plane and H-plane or vertical and horizontal planes). The pattern of most base station antennas contains a main lobe and several minor lobes, termed side lobes. A side lobe occurring in space in the direction opposite to the main lobe is called back lobe.


Radiation Pattern


Antenna GainAntenna gain is a measure for antennas efficiency.Gain is the ratio of the maximum radiation in a given direction to that of a reference antenna for equal input power.Generally the reference antenna is a isotropic antenna.Gain is measured generally in decibels above isotropic(dBi) or decibels above a dipole(dBd).An isotropic radiator is an ideal antenna which radiates power with unit gain uniformly in all directions. dBi = dBd + 2.15Antenna gain depends on the mechanical size, the effective aperature area, the frequency band and the antenna configuration.Antennas for GSM1800 can achieve some 5 to 6 dB more gain than antennas for GSM900 while maintaining the same mechanical size.

ANTENNA CHARACTERISTICSMain Lobe Axis Power BeamwidthSide LobeBack LobeFirst Null


Front-to-back ratio It is the ratio of the maximum directivity of an antenna to its directivity in a specified rearward direction.Generally antenna with a high front-to-back ratio should be used.

First Null BeamwidthThe first null beamwidth (FNBW) is the angular span between the first pattern nulls adjacent to the main lobe. This term describes the angular coverage of the downtilted cells.


Antenna LobesMain lobe is the radiation lobe containing the direction of maximum radiation.Side lobes Half-power beamwidthThe half power beamwidth (HPBW) is the angle between the points on the main lobe that are 3dB lower in gain compared to the maximum.Narrow angles mean good focusing of radiated power.PolarisationPolarisation is the propagation of the electric field vector .Antennas used in cellular communications are usually vertically polarised or cross polarised.


Frequency bandwidthIt is the range of frequencies within which the performance of the antenna, with respect to some characteristics, conforms to a specified standard.VSWR of an antenna is the main bandwidth limiting factor.Antenna impedanceMaximum power coupling into the antennas can be achieved when the antenna impedance matches the cables impedance.Typical value is 50 ohms.Mechanical sizeMechanical size is related to achievable antenna gain.Large antennas provide higher gains but also need care in deployment and apply high torque to the antenna mast.

COUPLING BETWEEN ANTENNAS

Antenna radiation pattern will become superimposed when the distance between the antennas becomes too small.This means the other antenna will mutually influence the individual antenna patterns.Generally 5 to 10 horizontal separation provides sufficient decoupling of antenna patterns. The vertical distance needed for decoupling is usually much smaller as the vertical beamwidth is generally less.A 1 separation in the vertical direction is sufficient in most cases.

ANTENNA INSTALLATION

Antenna installation configurations depend on the operators preferences.It is important to keep sufficient decoupling distances between antennas.If TX and RX direction use separated antennas, it is advisable to keep a horizontal separation between the antennas in order to reduce the TX signal power at the RX input stages.

ANTENNA DOWNTILTING

Network planners often have the problem that the base station antenna provides an overcoverage.If the overlapping area between two cells is too large, increased switching between the base station (handover) occurs.There may even be interference of a neighbouring cell with the same frequency. If hopping is used in the network, then limiting the overlap is required to reduce the overall hit rate.In general, the vertical pattern of an antenna radiates the main energy towards the horizon.Only that part of the energy which is radiated below the horizon can be used for the coverage of the sector. Downtilting the antenna limits the range by reducing the field strength in the horizon.

ANTENNA DOWNTILTING

Antenna downtilting is the downward tilt of the vertical pattern towards the ground by a fixed angle measured w.r.t the horizon.Downtilting of the antenna changes the position of the half-power beamwidth and the first null relative to the horizon.Normally the maximum gain is at 0 (parallel to the horizon) and never intersects the horizon.A small downtilt places the beams maximum at the cell edgeWith appropriate downtilt, the received signal strength within the cell improves due to the placement of the main lobe within the cell radius and falls off in regions approaching the cell boundary and towards the reuse cell.There are two methods of downtiltingMechanical downtiltingElectrical downtilting.

MECHANICAL DOWNTILTING

Mechanical downtilting consists of physically rotating an antenna downward about an axis from its vertical position.In a mechanical downtilt as the front lobe moves downward the back lobe moves upwards.This is one of the potential drawback as compared to the electrical downtilt because coverage behind the antenna can be negatively affected as the back lobe rises above the horizon.Additionally , mechanical downtilt does not change the gain of the antenna at +/- 90deg from antenna horizon. As the antenna is given downtilt, the footprint starts changing with a notch being formed in the front while it spreads on the sides.After 10 degrees downtilt the notch effect is quiet visible and the spread on the sides are high. This may lead to inteference on the sides.

MECHANICAL DOWNTILTING

MECHANICAL DOWNTILTINGVertical antenna pattern at 0Vertical antenna pattern at 15 downtiltBacklobe shoots over the horizon

ELECTRICAL DOWNTILT

Electrical downtilt uses a phase taper in the antenna array to angle the pattern downwards.This allows the the antenna to be mounted vertically.Electrical downtilt is the only practical way to achieve pattern downtilting with omnidirectional antennas.Electrical downtilt affects both front and back lobes. If the front lobe is downtilted the back lobe is also downtilted by equal amount.Electrical downtilting also reduces the gain equally at all angles on the horizon. The that adjusted downtilt angle is constant over the whole azimuth range. Variable electrical downtilt antennas are very costly.

ELECTRICAL DOWNTILT

ELECTRICAL DOWNTILT

Horizontal and vertical pattern for allgon 7144 antennaHorizontal Beamwidth = 90 Vertical Beamwidth = 16 Electrical Downtilt = 16

OBSTACLE REQUIREMENT

Nearby obstacles are those reflecting or shadowing materials that can obstruct the radio beam both in horizontal and vertical planes.When mounting the antenna on a roof top, the dominating obstacle in the vertical plane is the roof edge itself and in the horizontal plane, obstacles further away like surrounding buildings, can act as reflecting or shadowing material.The antenna beam will be distorted if the antenna is too close to the roof. Hence the antenna must be mounted at a minimum height above the rooftop or other obstacles.If antennas are wall mounted, a safety margin of 15 degrees between the reflecting surface and the 3-dB lobe should be kept.

OBSTACLE REQUIREMENT

Main RadiationDirectionHalf Power BeamwidthSafety Margin15 DegreesBuilding

OPTIMAL DOWNTILT

Although the use of downtilt can be a effective tool for controlling interference, there is a optimum amount by which the antenna can be downtilted whereby both the coverage losses and the interference at the reuse cell can be kept at a minimum.

downtilt angle (D)3 dB BeamwidthMain lobeHeight (H)Cellmax

OPTIMAL DOWNTILTThe figure shows a cells coverage area. The primary illumination area is the area on the ground that receives the signal contained within the 3dB vertical beamwidth of the antenna.The distance from the base station to the outer limit of the illumination area is denoted by Cellmax. It should be noted that the cellmax can be different from the cell boundary area which is customer defined. Ideally in a well planned network Cellmax should always be less than the co-channel reuse distance to minimise interference.We now derive the relation between height (H), downtilt angle (D), 3dB vertical beamwidth and Cellmax.As shown in the schematic is the angle between the upper limit of the 3dB beamwidth and the horizon.

OPTIMAL DOWNTILTtan ( ) = Cellmax / H = D - 0.5 * 3dB vertical beamwidthCellmax = H * tan (D - 0.5 * 3dB vertical beamwidth)For the Cellmax to be a positive quantity , downtilt angle must be more than half of the 3dB vertical beamwidth.When the downtilt angle is less than half of the 3dB beamwidth, part of the signal from the main beam shoots over the horizon .The signal directed towards or above the horizon can potentially cause interference at the reuse sites.

DIVERSITY ANTENNA SYSTEMS

Diversity Antenna Systems

NEED OF DIVERSITYBuildingBuildingBuilding


NEED OF DIVERSITYIn a typical cellular radio environment, the communication between the cell site and mobile is not by a direct radio path but via many paths.The direct path between the transmitter and the receiver is obstructed by buildings and other objects. Hence the signal that arrives at the receiver is either by reflection from the flat sides of buildings or by diffraction around man made or natural obstructions.When various incoming radiowaves arrive at the receiver antenna, they combine constructively or destructively, which leads to a rapid variation in signal strength. The signal fluctuations are known as multipath fading.


Multipath PropagationMultipath propagation causes large and rapid fluctuations in a signalThese fluctuations are not the same as the propagation path loss.

Multipath causes three major thingsRapid changes in signal strength over a short distance or time.Random frequency modulation due to Doppler Shifts on different multipath signals.Time dispersion caused by multipath delaysThese are called fading effectsMultipath propagation results in small-scale fading.


DIVERSITY TECHNIQUEDiversity techniques have been recognised as an effective means which enhances the immunity of the communication system to the multipath fading. GSM therefore extensively adopts diversity techniques that include

Diversity techniquesInterleavingIn time domainFrequency HoppingIn Frequency domainSpatial diversityIn spatial domainPolarisation diversityIn polarisation domain


CONCEPT OF DIVERSITY ANTENNA SYSTEMSSpatial and polarisation diversity techniques are realised through antenna systems.A diversity antenna system provides a number of receiving branches or ports from which the diversified signals are derived and fed to a receiver. The receiver then combines the incoming signals from the branches to produce a combined signal with improved quality in terms of signal strength or signal-to-noise ratio (S/N).The performance of a diversity antenna system primarily relies on the branch correlation and signal level difference between branches.


Transmission media 1Transmission Tmedia 2PeakFadeReceiverInformationCONCEPT OF DIVERSITY ANTENNA SYSTEMS


CORRELATION BETWEEN BRANCHESThe branch correlation coefficient (r) represents the degree of similarity between the signals from two different receiving branches. The correlation coefficient ranges from 0 to 1. r=1 means the signals from two different branches behave exactly the same. In this case, the signals are coherent.r=0 means the signals from two different branches behave completely different. In this case, the signals are uncorrelated.To achieve the best performance, a diversity antenna system is required to provide uncorrelated signals.For r=1, the diversity antenna becomes ineffective in combating the multipath fading.In reality, however, it is not always practical to have a diversity antenna system which guarantees r=0. Extensive research in this field has revealed that a diversity antenna system can perform satisfactorily provided that r 0.7.


CombiningCombined signalfed to receiverSignal 2Signal 1

_1056878693.doc

Time

Signal Strength

Combined signal

Signal 1

Signal 2

_1056878757.doc

Time

Signal Strength

Combined signal

Signal 1

Signal 2

_1056878926.doc

Time

Signal Strength

Combined signal

Signal 1

Signal 2

_1056878183.doc

Time

Signal Strength

Combined signal

Signal 1

Signal 2


SIGNAL LEVEL DIFFERENCEThe second key parameter for a good diversity antenna system is the mean signal level difference. The difference is a statistical parameter which indicates the balance of the signal strengths from the two receiving branches.In a real system, the statistical balance can be verified by comparing the mean values of the two signals measured over a lengthy period. If the ratio betn the median values is 0dB, the two receiving branches are statistically balanced. The performance of the diversity system will deteriorate while the ratio increases or decreases from 0dB.


SIGNAL LEVEL DIFFERENCE

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Signal level difference

Signal strength

Time

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Signal strength

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_1056879987.doc


Signal strength

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SPATIAL DIVERSITY ANTENNA SYSTEMSThe spatial diversity antenna system is constructed by physically separating two receiving base station antennas.Once they are separated far enough, both antennas receive independent fading signals. As a result, the signals captured by the antennas are most likely uncorrelated.The further apart are the antennas, the more likely that the signals are uncorrelated.The types of the configuration used in GSM networks are: horizontal separation vertical separationcomposite separation.


TYPICAL SPATIAL ANTENNA DIVERSITY CONFIGURATIONS

Horizontal SeparationVertical Separation


Branch correlationThe physical limitation of the supporting structure should also be considered while selecting the spatial diversity antenna configuration. For example, if a wide framework is not permitted on top of a mounting tower, vertical separation is a alternative to be considered.To achieve the required correlation coefficient (r 0.7) different configurations require different separations.The separation indicated in Table below shows that low values of correlation are more easily obtained with horizontal rather than vertical separation. That is why most of the diversity antenna systems in GSM networks use horizontal separation.

CRITERIA FOR SELECTING TYPE OF SPATIAL SEPARATION

Sheet1

Horizontal SeparationVertical Separation

d/900MHZ1800MHZd/900MHZ1800MHZ

Separation103.3m1.7m175.7m2.8m

Sheet2

Sheet3


CRITERIA FOR SELECTION OF SPATIAL SEPARATIONSignal level differenceA system using horizontally separated diversity antennas has a symmetrical configuration and is therefore able to provide balanced signal strengths.A system using vertically separated antennas needs large separation to meet the required correlation. The consequence is that the two antennas have different antenna height gains, which may result in imbalance between the two signal strengths.


CRITERIA FOR SELECTION OF SPATIAL SEPARATIONAngular dependenceAngular dependence reflects the dependence of the performance of a diversity antenna system on the angular position of a mobile relative to the boresight of the antenna.Horizontally separated antenna system has high dependence on the mobiles angular position. The effective separation reduces as the mobile moves away from the antenna boresight. As the mobile is 90 off the antenna boresight, the effective separation becomes zero. In such a case, the signals from two antennas are very likely coherent which will then lead to a deterioration of the diversity performance.


ANGULAR DEPENDANCE

Most of the GSM cell sites are 3 sectored cell sites.The maximum angular offset is therefore approximately 60.Simulation shows that the performance of a horizontally separated antenna system experiences noticeable deterioration only when the angular offset exceeds 70 .

SeparationReduced SeparationZeroSeparationView from boresightView from 45 deg off boresightView from 90 deg off boresight


PROS AND CONS OF HORIZONTAL CONFIGURATIONAdvantagesEasier to achieve low values of correlation and balance between the signals. Hence widely used.DisadvantagesHigh angular dependence. The impact is however marginal for sectorised applications. Require sizeable headframe on the supporting structure.


PROS AND CONS OF VERTICAL CONFIGURATIONAdvantagesSlim supporting structure.Angular independenceDisadvantagesRequire large separation for low values of correlation.May cause imbalance between the two diversity branches.Generally not used.


THREE ANTENNA SPATIAL CONFIGURATION

10 SeparationReceive 1TransmitReceive 2


TWO ANTENNA SPATIAL CONFIGURATION

10 SeparationReceive 2TxRxTransmit Receive 1Duplexer


POLARISATION DIVERSITY ANTENNA SYSTEMSA single (say vertical) polarised electromagnetic wave is converted to a wave with two orthogonal polarised fields while it is propagating through scattering environment.It has also been found that the two fields exhibit some extent of decorrelation.


DUAL POLARISED ANTENNASA dual-polarisation antenna consists of two sets of radiating elements which radiate or, in reciprocal, receive two orthogonal polarised fields. The antenna has two input connectors which separately connects to each set of the elements. The antenna has therefore the ability to simultaneously transmit and receive two orthogonally polarised fields.

H / VSlant 45


ADVANTAGES OF DUAL POLARISED ANTENNASThe best advantage of using the dual polarisation antenna is the reduction in the number of antennas per sector.Reduced size of the headframe of the supporting structureReduced windload and weight.Reduced difficulty in site acquisition and installation.Cost savingRequiring slim towerRequiring less installation time.Cost of one dual polarisation antenna is generally lower than that of two Single polarised antennas


DUAL POLARISED ANTENNA CONFIGURATIONS

DUAL POLE ANTENNATRTX RXRXDUAL POLE ANTENNASINGLE POLE ANTENNARXRXTXDUAL POLE ANTENNATTRRTX RXTX RX

INTERFERENCE

WHAT IS INTERFERNCE ?Interference is the sum of all signal contributions that are neither noise not the wanted signal.

EFFECTS OF INTERFERNCE Interference is a major limiting factor in the performance of cellular systems.It causes degradation of signal quality.It introduces bit errors in the received signal.Bit errors are partly recoverable by means of channel coding and error correction mechanisms.The interference situation is not reciprocal in the uplink and downlink direction.Mobile stations and base stations are exposed to different interference situation.

SOURCES OF INTERFERNCE Another mobile in the same cell.A call in progress in the neighboring cell.Other base stations operating on the same frequency.Any non-cellular system which leaks energy into the cellular frequency band.

TYPES OF INTERFERNCE There are two types of system generated interferenceCo-channel interferenceAdjacent channel interference

Co-Channel InterferenceThis type of interference is the due to frequency reuse , i.e. several cells use the same set of frequency.These cells are called co-channel cells.Co-channel interference cannot be combated by increasing the power of the transmitter. This is because an increase in carrier transmit power increases the interference to neighboring co-channel cells.To reduce co-channel interference, co-channel cells must be physically separated by a minimum distance to provide sufficient isolation due to propagation or reduce the footprint of the cell.

TYPES OF INTERFERNCE Co-Channel InterferenceSome factors other then reuse distance that influence co-channel interference are antenna type, directionality, height, site position etc,GSM specifies C/I > 9dB.Carrier f1Interferer f1dBDistanceCI

TYPES OF INTERFERNCE Co-Channel Interference

In a cellular system, when the size of each cell is approximately the same, co-channel interference is independent of the transmitted power and becomes a function of cell radius(R) and the distance to the centre of the nearest co-channel cell (D).C1C2C3C1C2C3D

TYPES OF INTERFERNCE Co-Channel InterferenceQ = D / R = 3NBy increasing the ratio of D/R, the spatial seperation between the co-channel cells relative to the coverage distance of a cell is increased. In this way interference is reduced from improved isolation of RF energy from the co-channel cell.The parameter Q , called the co-channel reuse ratio, is related to the cluster size.A small value of Q provides larger capacity since the cluster size N is small whereas a large value of Q improves the transmission quality.

TYPES OF INTERFERNCE Adjacent-Channel InterferenceInterference resulting from signals which are adjacent in frequency to the desired signal is called adjacent channel interference.Adjacent channel interference results from imperfect receiver filters which allow nearby frequencies to leak into the passband.Adjacent channel interference can be minimized through careful filtering and channel assignments.By keeping the frequency separation between each channel in a given cell as large as possible , the adjacent interference may be reduced considerably.

TYPES OF INTERFERNCE Adjacent-Channel InterferenceCarrier f1Interferer f2dBACDistance

COUNTERING INTERFERENCE POWER CONTROLRF power control is employed to minimise the transmit power required by MS or BS while maintaining the quality of the radio links.By minimising the transmit power levels, interference to co-channel users is reduced.Power control is implemented in the MS as well as the BSS.Power control on the Uplink also helps to increase the battery life.Power received by the MS is continously sent in the measurement report.Similarly uplink power received from the MS by the BTS is measured by the BTS.Complex algorithm evaluate this measurements and take a decision subsequently reducing or increasing the power in the Uplink or the downlink.

COUNTERING INTERFERENCE SECTORIZATIONFor 120 degrees sectored site as compared to an omni site almost 1/3rd interference is received in the uplink.The more selective and directional is the antenna, the smaller is the interference.Reduction in interference results in higher capacity in both links.

FREQUENCY PLANNING

INTRODUCTIONThe objective of a cellular system is to provide quality communication to the maximum number of users in a defined area.The number of users supported by the system can be increased by using more frequencies.Frequency resources are however always limited.Hence RF Planning engineers are required to maximise spectrum efficiency. In order to accommodate a maximum number of subscribers per network, the available frequencies need to be reused as often as possible.This creates interference towards other cells, which have detrimental impact to the link quality.Finding the optimum compromise between dense re-use and least interference is the objective of frequency planning.

INTRODUCTIONThe system design and planning of the system has to be done so as to reuse the frequencies as often as possible while keeping the co-channel and adjacent channel interference within acceptable limits.Also a minimum received signal level has to be provided throughout the coverage area of the network.Frequency planning can be doneManually by skilled expert RF Engineers.With powerful planning tool having the option of automated frequency planning.

FREQUENCY PLANNING STEPSThe steps to be followed in manual frequency planning areCalculating the frequency reuse distance theoratically.Determining the cell repeat patternPlanning the frequency groups.Inputting the planned frequency into the planning tool.Generating the C/I and C/A plots and checking out the results.Rectifying the fault areas.

FREQUENCY PLANNING STEPSDetermining the cell repeat patternFrequencies have to be reused at different cells throughout the network to maximise capacity.The distance cells using the same set of frequencies is called the frequency reuse distance.This reuse distance depends on the number of frequency reuse groups N.Once N has been determined every Nth cell will be assigned the same frequencies.Also a minimum received signal level has to be provided throughout the coverage area of the network.The cell repeat pattern is dependent on the frequency spectrum available, the traffic required and most important on the way the network is planned.Generally 7/21 or 7 site repeat pattern and 4 site repeat patterns are used.

FREQUENCY PLANNING STEPSCell reuse patternThe distribution of the C/I ratio desired in a system determines the number of frequency groups, F, which may be used.If we have N carrier frequencies then No of carriers / group = N/FSince the number of frequency groups are fixed, a smaller number of frequency groups(F) results in more carriers per set and per cell.Hence a reduction in the number of frequency groups would allow each site to carry more traffic.However decreasing the number of frequency groups and reducing the frequency reuse distance results in lower average C/I distribution in the system.Generally 7/21 and 4/12 reuse patterns are used.

FREQUENCY PLANNING STEPS7/21 Cell reuse patternSay we have 42 frequencies and we require 2 carriers per site then we can use 7 site repeat pattern. Hence a cluster will be formed of 7 sites. The frequencies for manual frequency planning for a cluster size of 7 are arranged a s shown below

Sheet1

A1B1C1D1E1F1G1A2B2C2D2E2F2G2A3B3C3D3E3F3G3

Carrier1123456789101112131415161718192021

Carrier2222324252627282930313233343536373839404142

Sheet2

Sheet3

FREQUENCY PLANNING STEPS7/21 Cell reuse pattern

B1B2B3C1C2C3A1A2A3G3G2G1F1F2F3D3D2D1E1E2E3B1B2B3C1C2C3A1A2A3G3G2G1F1F2F3D3D2D1E1E2E3

FREQUENCY PLANNING STEPS4/12 Cell reuse patternSay we have 48 frequencies and we require 4 carriers per site then we can use 4 site repeat pattern. Hence a cluster will be formed of 4 sites. The frequencies for manual frequency planning for a cluster size of 4 are arranged a s shown below

Sheet1

A1B1C1D1E1F1G1A2B2C2D2E2F2G2A3B3C3D3E3F3G3

Carrier1123456789101112131415161718192021

Carrier2222324252627282930313233343536373839404142

A1B1C1D1A2B2C2D2A3B3C3D3

Carrier1123456789101112

Carrier2131415161718192021222324

Carrier3252627282930313233343536

Carrier4373839404142434445464748

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FREQUENCY PLANNING STEPS4/12 Cell reuse pattern

D1D2D3B1B2B3C1C2C3A1A2A3D1D2D3B1B2B3C1C2C3B1B2B3D1D2D3B1B2B3C1C2C3A1A2A3A1A2A3B1B2B3C1C2C3

INTERFERENCE PREDICTIONOnce the repeat pattern is determined the frequencies should be entered in the planning tool.Enter the C/I threshold and C/A threshold. For GSM put 12dB(GSM specifies > 9dB) as C/I and 0dB( GSM specifies C/A > -9dB) as C/A.Generate a C/I and C/A plot.Analyse the plot and check for problems.Debug and solve the interference problems.Note that to get a correct C/I and C/A plot, all the sites prediction on the tool should be completed.Morever the models should be correctly tuned and the coverage predicted by the propagation model should match the coverage on the field.

AUTOMATIC FREQUENCY PLANPlanning tools nowadays have automatic frequency planning options.This tool uses predictions. Hence the models have to be accurately tuned.Morever Co-cell and co-site separations, allowed frequency bands, target levels for allowed co-channel and adjacent channel interference need to be defined.

FREQUENCY CO-ORDINATIONOn international borders frequencies are commonly co-ordinated with neighboring countries to avoid mutual interference.Generally sets of reserved or preferential frequencies are negotiated between the national authorities of the respective countries.

SITE SELECTION

INTRODUCTIONSite LocationProper site location determines usefulness of its cells.Site are expensive and have to be chosen carefully.The planner needs to visit each and every site.

Good Site SelectionWe need to understand various factors we must take into account to ensure that the selected site is good. Simple way is to ask yourself three questions1) Why am I putting this site ?2) Will this selected site serve that purpose ?3) Are there any possible problems that might arise if i select this site. If yes can I solve or avoid those problems ?

BAD SITE SELECTIONDesired cell boundaryUncontrolled interferenceInterleaved coverage areaweak own signal, strong foreign signalHilltop locations for a BTS site should be avoided as they causeUncontrolled interferenceInterleaved coverageAwkward HO behavioursBut are good for microwave links

GOOD SITE SELECTIONSites off hilltop locations are preferable for a BTS site as hills can be used to separate cellsinterference can be easily controlledminimum overlapping will resultcan face problems for microwave linksDesired cell boundary

SITE SELECTION CRITERIARadio criteria for site selectionGood view in the main beam directionNo surrounding nearby high obstacleGood visibilty of terrainRoom for antenna mountingLOS to the two microwave site and if possible to the BSCShort cabling distance

Non-radio criteriaSpace for equipmentAvailaibilty of lease lines or microwave linksPower supplyAccess restrictionsRental costsEase of acquisition

REPEATERS

REPEATERS

Repeater units are designed to receive signals from a donor site, amplify and rebroadcast the donor sites signals into poor coverage areas or to extend the coverage range of the donor site.These repeater are bi-directional and do not translate frequency and subsequently are limited in output power and gain.Repeaters provide between 50 to 80 dB of gain.Donor CellINTRODUCTIONDonor side antennaMobile side antennaPoor Coverage areaRepeater receives Donor signal at~ -90dBmRepeater amplifiesthe signal andrebroadcasts thesignal

REPEATERSINTRODUCTIONThere are two types of repeater band selective and channel selective.Band selective repeater amplifies a band of frequency. Hence it amplifies any frequency that falls within its band.Channel selective repeater allows selection of a number of individual channels to amplify and rebroadcast. Typically a channel selective repeater allows selection of 2 to 4 channels. If the GSM900 or DCS1800 network incorporates frequency hopping, then only band selective repeaters should be used.

REPEATERSANTENNA ISOLATIONSince repeaters are non-frequency translating, the isolation between the donor-side and mobile-side antenna needs to be much greater than the gain of the repeater.If antenna isolation is less than or equal to the repeater gain, the repeater will begin to amplify its own feedback and oscillate.This condition must be avoided.The isolation between the antenna needs to be at least 10 dB greater than the gain of the repeater.Iso = Grep + 10dBFollowing equation can be used to calculate antenna isolationVertical separation Iso = 28 + 40log( d / )Horizontal separation Iso = 22 + 20log( d / ) - (GA + GB)where d = Separation distance(feet) = WavelengthGA = Gain of donor-side antenna in direction of Mobile side antenna.GB = Gain of mobile-side antenna in direction of donor- side antenna.

REPEATERSANTENNA ISOLATIONSeparation type is based on site type. If the site is a roof, then horizontal separation should be used while if the site is on a tower vertical separation should be used.If directional antennas are used then the required horizontal spacing is given byIso = 22 + 20log( d / ) - (Gamax - Gaback) - (Gbmax - Gbback ) andIso = Grep + 10dBIf gain of the directional antenna is 17dBi and front-to-back >25dB and the repeater( gain of 80dB) is used on GSM900 then80 + 10 = 22 + 20log(d/1) - (17-25) - (17-25)90 = 22 +20log(d) + 8 + 822+20log(d) = 7420log(d) = 52log(d) = 2.6d = 398 feet

REPEATERSANTENNA ISOLATIONHence a horizontal separation of 398 feet is required to provide a isolation of 90dB between the donor-side and the mobile-side antenna which is quite unreasonable.Large antenna separation produce large cable losses and reduce repeater output power.Some obstruction should be in between both the antennas to reduce this separation.If vertical spacing is given byIso = 28 + 40log( d / ) andIso = Grep + 10dBif the repeater is used on GSM900 then80 + 10 = 28 + 40log(d/1)90 = 28 + 40log(d)40log(d) = 62d = 35.48 feet

RF Planning

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