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Rf optimization
 

Rf optimization

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Physical channel - Each timeslot on a carrier is referred to as a physical channel. Per carrier there are 8 physical channels.
Logical channel - Variety of information is transmitted between the MS and BTS. There are different logical channels depending on the information sent. The logical channels are of two types
Traffic channel
Control channel
BCH Channels
BCCH( Broadcast Control Channel )
Downlink only
Broadcasts general information of the serving cell called System Information
BCCH is transmitted on timeslot zero of BCCH carrier
Read only by idle mobile at least once every 30 secs.
SCH( Synchronisation Channel )
Downlink only
Carries information for frame synchronisation. Contains TDMA frame number and BSIC.
FCCH( Frequency Correction Channel )
Downlink only.
Enables MS to synchronise to the frequency.
Also helps mobiles of the ncells to locate TS 0 of BCCH carrier.
RACH( Random Access Channel )
Uplink only
Used by the MS to access the Network.


AGCH( Access Grant Channel )
Downlink only
Used by the network to assign a signalling channel upon successfull decoding of access bursts.


PCH( Paging Channel )
Downlink only.
Used by the Network to contact the MS.

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    Rf optimization Rf optimization Presentation Transcript

    • OPTIMIZATIONOPTIMIZATION
    • BASICSBASICS
    • Physical channelPhysical channel - Each timeslot on a carrier is referred to as a physicalchannel. Per carrier there are 8 physical channels.Logical channelLogical channel - Variety of information is transmitted between the MS andBTS. There are different logical channels depending on the informationsent. The logical channels are of two types• Traffic channel• Control channelDownlinkUplinkCHANNELSCHANNELS
    • GSM Traffic ChannelsGSM Traffic ChannelsTraffic ChannelsTCH/FFull rate 22.8kbits/sTCH/HHalf rate 11.4 kbits/s
    • GSM Control ChannelsGSM Control ChannelsBCH ( Broadcast channels )Downlink onlyControl ChannelsDCCH(Dedicated Channels)Downlink & UplinkCCCH(Common Control Chan)Downlink & UplinkSynch.ChannelsRACHRandomAccess ChannelCBCHCell BroadcastChannelSDCCHStandalonededicatedcontrol channelACCHAssociatedControl ChannelsSACCHSlow associatedControl ChannelFACCHFast AssociatedControl ChannelPCH/AGCHPaging/Access grantFCCHFrequencyCorrection channelSCHSynchronisationchannelBCCHBroadcastcontrol channel
    • BCH ChannelsBCH ChannelsBCCH( Broadcast Control Channel )BCCH( Broadcast Control Channel )• Downlink only• Broadcasts general information of the serving cell called SystemInformation• BCCH is transmitted on timeslot zero of BCCH carrier• Read only by idle mobile at least once every 30 secs.SCH( Synchronisation Channel )SCH( Synchronisation Channel )• Downlink only• Carries information for frame synchronisation. Contains TDMAframe number and BSIC.FCCH( Frequency Correction Channel )FCCH( Frequency Correction Channel )• Downlink only.• Enables MS to synchronise to the frequency.• Also helps mobiles of the ncells to locate TS 0 of BCCH carrier.
    • CCCH ChannelsCCCH ChannelsRACH( Random Access Channel )RACH( Random Access Channel )• Uplink only• Used by the MS to access the Network.AGCH( Access Grant Channel )AGCH( Access Grant Channel )• Downlink only• Used by the network to assign a signalling channel uponsuccessfull decoding of access bursts.PCH( Paging Channel )PCH( Paging Channel )• Downlink only.• Used by the Network to contact the MS.
    • DCCH ChannelsDCCH ChannelsSDCCH( Standalone Dedicated Control Channel )SDCCH( Standalone Dedicated Control Channel )• Uplink and Downlink• Used for call setup, location update and SMS.SACCH( Slow Associated Control Channel )SACCH( Slow Associated Control Channel )• Used on Uplink and Downlink only in dedicated mode.• Uplink SACCH messages - Measurement reports.• Downlink SACCH messages - control info.FACCH( Fast Associated Control Channel )FACCH( Fast Associated Control Channel )• Uplink and Downlink.• Associated with TCH only.• Is used to send fast messages like handover messages.• Works by stealing traffic bursts.
    • T15T5T9T10T11S12T13T14T6T7T8T0T1T2T3T4T16T17T18T19T20T21T22T23T24I2500 11 22 33 44 55 66 77 00 11 22 33 44 55 66 77 00 11 22 33 44 55 66 77120 msec4.615 msec2626 FRAME MULTIFRAME STRUCTUREFRAME MULTIFRAME STRUCTURE• MS on dedicated mode on a TCH uses a 26-frame multiframestructure.• Frame 0-11 and 13-24 used to carry traffic.• Frame 12 used as SACCH to carry control information from and to MSto BTS.• Frame 25 is idle and is used by mobile to decode the BSIC of neighborcells.
    • BCCHCCCHBCCHCCCHBCCHCCCHBCCHCCCHCCCHCCCHCCCHCCCHCCCHCCCHCCCHCCCH01020304050 50403020100IDLECCCH BLOCKBCCH BLOCKSCH BLOCKFCCH BLOCKBCCH/CCCH NON-COMBINED MULTIFRAMEBCCH/CCCH NON-COMBINED MULTIFRAMERACH BLOCKDownlinkDownlink UplinkUplink
    • 01020304050 50403020100IDLECCCH BLOCKBCCH BLOCKSCH BLOCKFCCH BLOCKBCCH/CCCH COMBINED MULTIFRAMEBCCH/CCCH COMBINED MULTIFRAME51101 10151BCCHCCCHBCCHCCCHCCCHBCCHCCCHCCCHCCCHSACCHSACCHCCCHCCCHCCCHCCCHSDCCHSDCCHSDCCHSDCCHBCCHCCCHBCCHCCCHCCCHBCCHCCCHCCCHCCCHSACCHSACCHCCCHCCCHCCCHCCCHSDCCHSDCCHSDCCHSDCCHBCCHCCCHSDCCHCCCHSACCHCCCHSACCHCCCHCCCHCCCHSDCCHCCCHCCCHCCCHSDCCHCCCHCCCHCCCHSDCCHBCCHCCCHSDCCHCCCHSACCHCCCHSACCHCCCHCCCHCCCHSDCCHCCCHCCCHCCCHSDCCHCCCHCCCHCCCHSDCCHRACH BLOCKSDCCH/4SACCH/4DownlinkDownlink UplinkUplink
    • 01020304050 50403020100IDLEDCCH/8 MULTIFRAMEDCCH/8 MULTIFRAME51101 10151SDCCH/8SACCH/C8BCCHCCCHA2A3BCCHCCCHA0A1CCCHCCCHCCCHD7CCCHCCCHCCCHD6CCCHCCCHCCCHD5CCCHCCCHCCCHD4CCCHCCCHCCCHD3CCCHCCCHCCCHD2CCCHCCCHCCCHD1CCCHCCCHCCCHD0BCCHCCCHA6A7BCCHCCCHA4A5CCCHCCCHCCCHD7CCCHCCCHCCCHD6CCCHCCCHCCCHD5CCCHCCCHCCCHD4CCCHCCCHCCCHD3CCCHCCCHCCCHD2CCCHCCCHCCCHD1CCCHCCCHCCCHD0BCCHA0CCCHCCCHCCCHD7CCCHCCCHCCCHD6CCCHCCCHCCCHD5CCCHCCCHCCCHD4CCCHCCCHCCCHD3CCCHCCCHCCCHD2CCCHCCCHCCCHD1CCCHCCCHCCCHD0BCCHCCCHA6A7CCCHA5BCCHA4CCCHCCCHCCCHD7CCCHCCCHCCCHD6CCCHCCCHCCCHD5CCCHCCCHCCCHD4CCCHCCCHCCCHD3CCCHCCCHCCCHD2CCCHCCCHCCCHD1CCCHCCCHCCCHD0BCCHCCCHA2A3CCCHA1DownlinkDownlink UplinkUplink
    • 00 1100 11 22 20452045 20462046 204720471 Hyperframe = 2048 superframes = 2,715,648 TDMA frames3h 28min 53s 760ms1 Superframe = 1326 TDMAframes = 51(26 fr) 0r 26(51 fr) multiframes11 2 33 494847 5000 11 2424 252500 11 22 2323 2424 2525 00 484811 22 4949 505022 33 44 55 66 776.12s0235.38ms120msControl 51 - Frame MultiframeTraffic 26 - Frame Multiframe4.615msTDMA FrameHYPERFRAME AND SUPERFRAME STRUCTUREHYPERFRAME AND SUPERFRAME STRUCTURE
    • CALL FLOWCALL FLOW
    • Mobile originated callMobile originated callMSChannel Request (RACH)BSS MSCSDCCH SeizureImmediate Assignment [ Reject ] (AGCH)CM Service Request + Connection Request < CMSREQ >Connection [ Confirmed / Refused ]Link EstablishmentAuthentication RequestAuthentication ResponseDT1 <CICMD>Ciphering Mode CommandCiphering Mode CompleteDT1 <CICMP>Identity RequestIdentity ResponseSetupCall ProceedingConnection ManagementAssignment RequestAssignment Request [ Failed ]Assignment CommandAssignment [ Complete / Failure ]Assignment [ Complete / Failure ]TCH SeizureSDCCHTCH
    • MS BSS MSCPagingSDCCH SeizureLink EstablishmentPaging Request (PCH)UDT < PAGIN >PagingChannel Request (RACH)Immediate Assignment [ Reject ] (AGCH)Paging Response + Connection Request < PAGRES >Connection [ Confirmed / Refused ]Authentication RequestAuthentication ResponseSDCCHCiphering Mode CommandCiphering Mode CompleteDT1 <CICMD>DT1 <CICMP>Identity RequestIdentity ResponseSetupCall ConfirmedConnection ManagementAssignment RequestAssignment Request [ Failed ]Assignment CommandAssignment [ Complete / Failure ]TCH TCH SeizureAssignment [ Complete / Failure ]Mobile terminated callMobile terminated call
    • PROPAGATION MECHANISMSPROPAGATION MECHANISMSReflection• Occurs when a wave impinges upon a smooth surface.• Dimensions of the surface are large relative to λ.• Reflections occur from the surface of the earth & from buildings & walls.Diffraction (Shadowing)• Occurs when the path is blocked by an object with large dimensionsrelative to λ and sharp irregularities (edges).• Secondary “wavelets” propagate into the shadowed region.• Diffraction gives rise to bending of waves around the obstacle.Scattering• Occurs when a wave impinges upon an object with dimensions on theorder of λ or less, causing the reflected energy to spread out or“scatter”in many directions.
    • MultipathMultipath• Multiple Waves Create “Multipath”• Due to propagation mechanisms, multiple waves arrive at thereceiver• Sometimes this includes a direct Line-of-Sight(LOS) signal
    • Multipath PropagationMultipath Propagation• Multipath propagation causes large and rapid fluctuations in a signal• These fluctuations are not the same as the propagation path loss.Multipath causes three major thingsMultipath causes three major things• Rapid changes in signal strength over a short distance or time.• Random frequency modulation due to Doppler Shifts on differentmultipath signals.• Time dispersion caused by multipath delays• These are called “fading effects• Multipath propagation results in small-scale fading.
    • FadingFading• The communication between the base station and mobile station inmobile systems is mostly non-LOS.• The LOS path between the transmitter and the receiver is affectedby terrain and obstructed by buildings and other objects.• The mobile station is also moving in different directions at differentspeeds.• The RF signal from the transmitter is scattered by reflection anddiffraction and reaches the receiver through many non-LOS paths.• This non-LOS path causes long-term and short term fluctuations inthe form of log-normal fading and rayleigh and rician fading, whichdegrades the performance of the RF channel.
    • FADINGFADINGSignalPower(dBm)Large scale fading componentSmall scale fadingcomponent
    • Long Term FadingLong Term Fading• Terrain configuration & man made environment causes long-termfading.• Due to various shadowing and terrain effects the signal levelmeasured on a circle around base station shows some randomfluctuations around the mean value of received signal strength.• The long-term fades in signal strength, r, caused by the terrainconfiguration and man made environments form a log-normaldistribution, i.e the mean received signal strength, r, varies log-normally in dB if the signal strength is measured over a distance ofat least 40λ.• Experimentally it has been determined that the standard deviation,σ, of the mean received signal strength, r, lies between 8 to 12 dBwith the higher σ generally found in large urban areas.
    • Rayleigh FadingRayleigh 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 becomesa random variable depending on the location of the antenna.
    • Ricean FadingRicean 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.
    • ANTENNASANTENNAS
    • AntennasAntennas• Antennas form a essential part of any radio communication system.• Antenna is that part of a transmitting or receiving system which isdesigned to radiate or to receive electromagnetic waves.• An antenna can also be viewed as a transitional structure betweenfree-space and a transmission line (such as a coaxial line).• An important property of an antenna is the ability to focus and shapethe radiated power in space e.g.: it enhances the power in somewanted 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.
    • Antenna TypesAntenna Types• In mobile communications two main categories of antennas used are– Omni directional antennaOmni directional antenna• These antennas are mostly used in rural areas.• In all horizontal direction these antennas radiate withequal power.• In the vertical plane these antennas radiate uniformlyacross all azimuth angles and have a main beam withupper and lower side lobes.
    • – Directional antennaDirectional antenna• These antennas are mostly used in mobile cellular systems toget higher gain compared to omnidirectional antenna and tominimise interference effects in the network.• In the vertical plane these antennas radiate uniformly across allazimuth angles and have a main beam with upper and lowerside lobes.• In these type of antennas, the radiation is directed at a specificangle instead of uniformly across all azimuth angles in case ofomni antennas.
    • Radiation PatternRadiation Pattern• The main characteristics of antenna is the radiation pattern.• The antenna pattern is a graphical representation in three dimensions ofthe radiation of the antenna as a function of angular direction.• Antenna radiation performance is usually measured and recorded in twoorthogonal principal planes (E-Plane and H-plane or vertical andhorizontal planes).• The pattern of most base station antennas contains a main lobe andseveral minor lobes, termed side lobes.• A side lobe occurring in space in the direction opposite to the main lobe iscalled back lobe.
    • Radiation PatternRadiation Pattern
    • Antenna GainAntenna Gain• Antenna gain is a measure for antennas efficiency.• Gain is the ratio of the maximum radiation in a given direction to that of areference antenna for equal input power.• Generally the reference antenna is a isotropic antenna.• Gain is measured generally in “decibels above isotropic(dBi)” or “decibelsabove a dipole(dBd).• An isotropic radiator is an ideal antenna which radiates power with unitgain uniformly in all directions. dBi = dBd + 2.15• Antenna gain depends on the mechanical size, the effective aperaturearea, the frequency band and the antenna configuration.• Antennas for GSM1800 can achieve some 5 to 6 dB more gain thanantennas for GSM900 while maintaining the same mechanical size.
    • Main Lobe Axis½ Power BeamwidthSide LobeBack LobeFirst Null
    • Front-to-back ratioFront-to-back ratio• It is the ratio of the maximum directivity of an antenna to its directivity in aspecified rearward direction.• Generally antenna with a high front-to-back ratio should be used.First Null BeamwidthFirst Null Beamwidth• The first null beamwidth (FNBW) is the angular span between the firstpattern nulls adjacent to the main lobe.• This term describes the angular coverage of the downtilted cells.
    • Antenna LobesAntenna Lobes• Main lobe is the radiation lobe containing the direction of maximumradiation.• Side lobesHalf-power beamwidthHalf-power beamwidth• The half power beamwidth (HPBW) is the angle between the points onthe main lobe that are 3dB lower in gain compared to the maximum.• Narrow angles mean good focusing of radiated power.PolarisationPolarisation• Polarisation is the propagation of the electric field vector .• Antennas used in cellular communications are usually vertically polarisedor cross polarised.
    • Frequency bandwidthFrequency bandwidth• It is the range of frequencies within which the performance of theantenna, with respect to some characteristics, conforms to a specifiedstandard.• VSWR of an antenna is the main bandwidth limiting factor.Antenna impedanceAntenna impedance• Maximum power coupling into the antennas can be achieved when theantenna impedance matches the cables impedance.• Typical value is 50 ohms.Mechanical sizeMechanical size• Mechanical size is related to achievable antenna gain.• Large antennas provide higher gains but also need care in deploymentand apply high torque to the antenna mast.
    • • Antenna radiation pattern will become superimposed when the distancebetween the antennas becomes too small.• This means the other antenna will mutually influence the individualantenna patterns.• Generally 5 to 10λ horizontal separation provides sufficient decoupling ofantenna patterns.• The vertical distance needed for decoupling is usually much smaller asthe vertical beamwidth is generally less.• A 1λ separation in the vertical direction is sufficient in most cases.
    • • 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 ahorizontal separation between the antennas in order to reduce the TXsignal power at the RX input stages.
    • Antenna downtilt introductionAntenna downtilt introduction• Network planners often have the problem that the base station antennaprovides an overcoverage.• If the overlapping area between two cells is too large, increased switchingbetween the base station (handover) occurs.• There may even be interference of a neighbouring cell with the samefrequency.• If hopping is used in the network, then limiting the overlap is required toreduce the overall hit rate.• In general, the vertical pattern of an antenna radiates the main energytowards the horizon.• Only that part of the energy which is radiated below the horizon can beused for the coverage of the sector.• Downtilting the antenna limits the range by reducing the field strength inthe horizon.
    • Antenna downtiltingAntenna downtilting• Antenna downtilting is the downward tilt of the vertical pattern towards theground by a fixed angle measured w.r.t the horizon.• Downtilting of the antenna changes the position of the half-powerbeamwidth and the first null relative to the horizon.• Normally the maximum gain is at 0•(parallel to the horizon) and neverintersects the horizon.• A small downtilt places the beams maximum at the cell edge• With appropriate downtilt, the received signal strength within the cellimproves due to the placement of the main lobe within the cell radius andfalls off in regions approaching the cell boundary and towards the reusecell.• There are two methods of downtilting– Mechanical downtilting– Electrical downtilting.
    • Mechanical DowntiltMechanical Downtilt• Mechanical downtilting consists of physically rotating an antennadownward about an axis from its vertical position.• In a mechanical downtilt as the front lobe moves downward the back lobemoves upwards.• This is one of the potential drawback as compared to the electricaldowntilt because coverage behind the antenna can be negatively affectedas the back lobe rises above the horizon.• Additionally , mechanical downtilt does not change the gain of theantenna at +/- 90deg from antenna horizon.• As the antenna is given downtilt, the footprint starts changing with a notchbeing formed in the fron’t while it spreads on the sides.• After 10 degrees downtilt the notch effect is quiet visible and the spreadon the sides are high. This may lead to inteference on the sides.
    • Mechanical DowntiltMechanical Downtilt
    • Mechanical DowntiltMechanical Downtilt
    • Vertical antenna pattern at 0°Vertical antenna pattern at 15° downtiltBacklobe shoots over the horizonMechanical DowntiltMechanical Downtilt
    • Electrical downtiltElectrical downtilt• Electrical downtilt uses a phase taper in the antenna array to angle thepattern downwards.• This allows the the antenna to be mounted vertically.• Electrical downtilt is the only practical way to achieve patterndowntilting with omnidirectional antennas.• Electrical downtilt affects both front and back lobes.• If the front lobe is downtilted the back lobe is also downtilted by equalamount.• Electrical downtilting also reduces the gain equally at all angles on thehorizon. The that adjusted downtilt angle is constant over the wholeazimuth range.• Variable electrical downtilt antennas are very costly.
    • Electrical downtiltElectrical downtilt
    • Electrical downtiltElectrical downtilt
    • Obstacle requirementObstacle requirement• Nearby obstacles are those reflecting or shadowing materials that canobstruct the radio beam both in horizontal and vertical planes.• When mounting the antenna on a roof top, the dominating obstacle inthe vertical plane is the roof edge itself and in the horizontal plane,obstacles further away like surrounding buildings, can act as reflectingor shadowing material.• The antenna beam will be distorted if the antenna is too close to theroof. Hence the antenna must be mounted at a minimum height abovethe rooftop or other obstacles.• If antennas are wall mounted, a safety margin of 15 degrees betweenthe reflecting surface and the 3-dB lobe should be kept.
    • Main RadiationDirectionHalf PowerBeamwidthSafety Margin15 DegreesBuildingObstacle requirementObstacle requirement
    • Optimal DowntiltOptimal Downtilt• Although the use of downtilt can be a effective tool for controllinginterference, there is a optimum amount by which the antenna can bedowntilted whereby both the coverage losses and the interference atthe reuse cell can be kept at a minimum.downtilt angle (D)3 dB BeamwidthMain lobeHeight (H)CellmaxΦΦ
    • • The figure shows a cells coverage area.• The primary illumination area is the area on the ground that receives thesignal contained within the 3dB vertical beamwidth of the antenna.• The distance from the base station to the outer limit of the illuminationarea is denoted by Cellmax.• It should be noted that the cellmax can be different from the cellboundary area which is customer defined.• Ideally in a well planned network Cellmax should always be less thanthe co-channel reuse distance to minimise interference.• We now derive the relation between height (H), downtilt angle (D), 3dBvertical beamwidth and Cellmax.• As shown in the schematic φ is the angle between the upper limit of the3dB beamwidth and the horizon.Optimal DowntiltOptimal Downtilt
    • • tan (Φ ) = Cellmax / HΦ = 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 morethan half of the 3dB vertical beamwidth.• When the downtilt angle is less than half of the 3dB beamwidth, part ofthe signal from the main beam shoots over the horizon .• The signal directed towards or above the horizon can potentially causeinterference at the reuse sites.Optimal DowntiltOptimal Downtilt
    • INTERFERENCEINTERFERENCE
    • WHAT IS INTERFERNCE ?WHAT IS INTERFERNCE ?• Interference is the sum of all signal contributions that are neither noisenot the wanted signal.
    • EFFECTS OF INTERFERNCEEFFECTS OF INTERFERNCE• Interference is a major limiting factor in the performance of cellularsystems.• 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 errorcorrection mechanisms.• The interference situation is not reciprocal in the uplink and downlinkdirection.• Mobile stations and base stations are exposed to different interferencesituation.
    • SOURCES OF INTERFERNCESOURCES 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 frequencyband.
    • TYPES OF INTERFERNCETYPES OF INTERFERNCE• There are two types of system generated interference– Co-channel interference– Adjacent channel interferenceCo-Channel InterferenceCo-Channel Interference• This type of interference is the due to frequency reuse , i.e. severalcells use the same set of frequency.• These cells are called co-channel cells.• Co-channel interference cannot be combated by increasing the powerof the transmitter. This is because an increase in carrier transmit powerincreases the interference to neighboring co-channel cells.• To reduce co-channel interference, co-channel cells must be physicallyseparated by a minimum distance to provide sufficient isolation due topropagation or reduce the footprint of the cell.
    • Co-Channel InterferenceCo-Channel Interference• Some factors other then reuse distance that influence co-channelinterference are antenna type, directionality, height, site position etc,• GSM specifies C/I > 9dB.Carrier f1 Interferer f1dBDistanceCI
    • Co-Channel InterferenceCo-Channel Interference• In a cellular system, when the size of each cell is approximately thesame, co-channel interference is independent of the transmitted powerand becomes a function of cell radius(R) and the distance to the centreof the nearest co-channel cell (D).C1C2C3C1C2C3D
    • Co-Channel InterferenceCo-Channel Interference• Q = D / R = √3N• By increasing the ratio of D/R, the spatial seperation between the co-channel cells relative to the coverage distance of a cell is increased. Inthis way interference is reduced from improved isolation of RF energyfrom the co-channel cell.• The parameter Q , called the co-channel reuse ratio, is related to thecluster size.• A small value of Q provides larger capacity since the cluster size N issmall whereas a large value of Q improves the transmission quality.
    • Adjacent-Channel InterferenceAdjacent-Channel Interference• Interference resulting from signals which are adjacent in frequency tothe desired signal is called adjacent channel interference.• Adjacent channel interference results from imperfect receiver filterswhich allow nearby frequencies to leak into the passband.• Adjacent channel interference can be minimized through carefulfiltering and channel assignments.• By keeping the frequency separation between each channel in a givencell as large as possible , the adjacent interference may be reducedconsiderably.
    • Adjacent-Channel InterferenceAdjacent-Channel InterferenceCarrier f1 Interferer f2dBACDistance
    • POWER CONTROLPOWER CONTROL• RF power control is employed to minimise the transmit power requiredby MS or BS while maintaining the quality of the radio links.• By minimising the transmit power levels, interference to co-channelusers 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 measurementreport.• Similarly uplink power received from the MS by the BTS is measuredby the BTS.• Complex algorithm evaluate this measurements and take a decisionsubsequently reducing or increasing the power in the Uplink or thedownlink.
    • SECTORIZATIONSECTORIZATION• For 120 degrees sectored site as compared to an omni site almost1/3rd interference is received in the uplink.• The more selective and directional is the antenna, the smaller is theinterference.• Reduction in interference results in higher capacity in both links.
    • DIVERSITY ANTENNADIVERSITY ANTENNASYSTEMSSYSTEMS
    • NEED OF DIVERSITYBuildingBuildingBuilding
    • NEED OF DIVERSITY• In a typical cellular radio environment, the communication between thecell site and mobile is not by a direct radio path but via many paths.• The direct path between the transmitter and the receiver is obstructedby buildings and other objects.• Hence the signal that arrives at the receiver is either by reflection fromthe flat sides of buildings or by diffraction around man made or naturalobstructions.• When various incoming radiowaves arrive at the receiver antenna,they combine constructively or destructively, which leads to a rapidvariation in signal strength.• The signal fluctuations are known as ‘multipath fading’.
    • Multipath PropagationMultipath Propagation• Multipath propagation causes large and rapid fluctuations in a signal• These fluctuations are not the same as the propagation path loss.Multipath causes three major thingsMultipath causes three major things• Rapid changes in signal strength over a short distance or time.• Random frequency modulation due to Doppler Shifts on differentmultipath signals.• Time dispersion caused by multipath delays• These are called “fading effects• Multipath propagation results in small-scale fading.
    • DIVERSITY TECHNIQUE• Diversity techniques have been recognised as an effective meanswhich enhances the immunity of the communication system to themultipath fading. GSM therefore extensively adopts diversitytechniques that includeDiversity techniquesInterleavingIn time domainFrequency HoppingIn Frequency domainSpatial diversityIn spatial domainPolarisation diversityIn polarisation domain
    • CONCEPT OF DIVERSITY ANTENNA SYSTEMS• Spatial and polarisation diversity techniques are realised throughantenna systems.• A diversity antenna system provides a number of receiving branchesor ports from which the diversified signals are derived and fed to areceiver. The receiver then combines the incoming signals from thebranches to produce a combined signal with improved quality interms of signal strength or signal-to-noise ratio (S/N).• The performance of a diversity antenna system primarily relies onthe branch correlation and signal level difference between branches.
    • Transmissionmedia 1TransmissionTmedia 2PeakFadeReceiverInformationCONCEPT OF DIVERSITY ANTENNA SYSTEMS
    • SPATIAL DIVERSITY ANTENNA SYSTEMS• The spatial diversity antenna system is constructed by physicallyseparating two receiving base station antennas.• Once they are separated far enough, both antennas receiveindependent fading signals. As a result, the signals captured by theantennas are most likely uncorrelated.• The further apart are the antennas, the more likely that the signalsare uncorrelated.• The types of the configuration used in GSM networks are:• horizontal separation• vertical separation
    • TYPICAL SPATIAL ANTENNA DIVERSITY CONFIGURATIONSHorizontal Separation Vertical Separation
    • THREE ANTENNA SPATIAL CONFIGURATION10λ SeparationReceive 1 Transmit Receive 2
    • TWO ANTENNA SPATIAL CONFIGURATION10λ SeparationReceive 2Tx RxTransmit Receive 1Duplexer
    • POLARISATION DIVERSITY ANTENNA SYSTEMS• A single (say vertical) polarised electromagnetic wave is convertedto a wave with two orthogonal polarised fields while it is propagatingthrough scattering environment.• It has also been found that the two fields exhibit some extent ofdecorrelation.
    • DUAL POLARISED ANTENNAS• A dual-polarisation antenna consists of two sets of radiatingelements which radiate or, in reciprocal, receive two orthogonalpolarised fields.• The antenna has two input connectors which separately connects toeach set of the elements.• The antenna has therefore the ability to simultaneously transmit andreceive two orthogonally polarised fields.H / V Slant 45°
    • ADVANTAGES OF DUAL POLARISED ANTENNAS• The best advantage of using the dual polarisation antenna is thereduction in the number of antennas per sector.• Reduced size of the headframe of the supporting structure• Reduced windload and weight.• Reduced difficulty in site acquisition and installation.• Cost saving– Requiring slim tower– Requiring less installation time.– Cost of one dual polarisation antenna is generally lower than thatof two– Single polarised antennas
    • DUAL POLARISED ANTENNA CONFIGURATIONSDUALPOLEANTENNAT RTX RX RXDUALPOLEANTENNASINGLEPOLEARX RXTXDUALPOLEANTENNAT TR RTX RX TX RX
    • SYSTEM INFORMATIONSYSTEM INFORMATIONMESSAGESMESSAGES
    • BROADCAST MESSAGESBROADCAST MESSAGES• System information is data about the network which the MSneeds to be able to communicate with the network in aappropriate manner.• System information messages are sent on the BCCH andSACCH.• There are six different types of system information messages.• System information messages 1 to 4 are broadcast on the BCCHand are read by the MS in idle mode.• System information message 5 and 6 are sent on the SACCH tothe MS in dedicated mode.• System information messages 1 to 4 are broadcast on the BCCHin a cyclic mode over 8 BCCH multiframes, i.e. 8 * 51 frames.• Every message is sent at least after every 1.8 sec.
    • What is sent is optional on BCCH Multiframe 4 and 5• System information 5 and 6 are sent on the SACCH immediatelyafter HO or whenever nothing else is being sent.• Downlink SACCH is used for system information messages whileUplink SACCH is used for measurement reports.BROADCAST MESSAGESBROADCAST MESSAGESSystemInformationBCCHMultiframe1 02 13 2 and 64 3 and 7
    • SYSTEM INFORMATION 1SYSTEM INFORMATION 1When frequency hopping is used in cell MS needs to know whichfrequency band to use and what frequency within the band it shoulduse in hopping algorithm.Cell Channel DescriptionCell allocation number :- Informs the band number of thefrequency channels used.00 - Band 0 ( Current GSM band )Cell allocation ARFCN :- ARFCN’s used for hopping. It is codedin a bitmap of 124 bits.124 123 122 121016 015 014 013 012 011 010 009008 007 006 005 004 003 002 001
    • SYSTEM INFORMATION 1SYSTEM INFORMATION 1RACH Control ParametersAccess Control Class :- Bitmap with 16 bits. All MS spread out onclass 0 - 9. Priority groups use class 11-15. A bit set to 1 barresaccess for that class. Bit 10 is used to tell the MS if emergency callis allowed or not.0 - All MS can make emergency call.1 - MS with class 11-15 only can make emergency calls.Cell barred for access :-0 - Yes1 - No
    • RACH Control ParametersRe-establishment allowed :-0 – Yes1 - Nomax_retransmissions :- Number of times the MS attempts toaccess the Network [ 1,2,4 or 7 ].tx_integer :- Number of slots to spread access retransmissionswhen a MS attempts to access the system.Emergency Call Allowed :- Yes / NoSYSTEM INFORMATION 1SYSTEM INFORMATION 1
    • • Contains list of BCCH frequencies used in neighbor cells.• MS uses this list to measures the signal strength of the neighbors.Neighbor Cell DescriptionBA Indicator :- Allows to differentiate measurement results relatedto different list of BCCH frequencies sent to the MS.BCCH Allocation number :- Band 0 is used.BCCH ARFCN number :- Bitmap 1 -1241 = Set0 = Not setPLMN permittedRACH Control ParametersSYSTEM INFORMATIONSYSTEM INFORMATION 22
    • SYSTEM INFORMATION 3SYSTEM INFORMATION 3Location Area IdentityCell Identity8 7 6 5 4 3 2 1Octet A1 1 1 1 Octet B BCDOctet COctet DOctet EMCC DIG 1MCC DIG 2MCC DIG 3MNC DIG 1MNC DIG 2LACLACBinary8 7 6 5 4 3 2 1Octet FOctet GCICIBinary
    • SYSTEM INFORMATION 3SYSTEM INFORMATION 3Control Channel DescriptionAttach / Detach0 = Allowed1 = Not allowedcch_conf :- Defines multiframe struturebs_agblk :- Number of block reserved for AGCH [ 0-7 ].Ba_pmfrms :- Number of 51 frame multiframes betweentransmisiion of paging messages to MS of the same group.T3212 :- Periodic location update timer [ 1-255 deci hours].cch_conf Physical Channels Combined No of CCH0 1 timeslot (0) NO 91 1 timeslot (0) YES 32 2 timeslots (0, 2) NO 184 3 timeslots (0, 2, 4) NO 276 4 timeslots (0, 2, 4, 6) NO 36
    • SYSTEM INFORMATION 3SYSTEM INFORMATION 3Cell Optionsdtxpwrc :- Power control on the downlink.0 = Not used1 = UsedRadio link timeout :- Sets the timer T100 in the MS.Cell Selection ParametersRxlev_access_min :- Minimum received signal level at the MS forwhich it is permitted to access the system.0-63 = -110 dBm to -47dBmmx_txpwr_cch :- Maximum power the MS will use when accessingthe system.Cell_reselect_hysteresis :- Used for cell reselection.RACH Control Parameters
    • SYSTEM INFORMATION 4SYSTEM INFORMATION 4Location Area IdentificationCell Selection ParametersRxlev_access_minmx_txpwr_cchCell_reselect_hysteresisRACH Control Parametersmax_retransmissionstx_integerCell barred for accessRe-establishment allowedEmergency Call AllowedAccess Control Class
    • SYSTEM INFORMATION 4SYSTEM INFORMATION 4Channel DescriptionChannel type :- Indi. channel type SDCCH or CBCH( SDCCH/8).Subchannel number :- Indicates the subchannel.Timeslot number :- Indicates the timeslot for CBCH [0 - 7].Training Sequence Code :- The BCC part of BSIC[0 - 7 ].Hopping Channel(H) :- Informs if CBCH channel is hopping orsingle. 0 - Single RF Channel 1 - RF hopping channelARFCN :- If H = 0MAIO :- If H = 1 , informs the MS where to start hopping. Values [0- 63].HSN :- If H = 1 , informs the MS in what order in what order thehopping should take place. Values [ 0 - 63]. HSN = 0 CyclicHopping.MA :- Indicates which RF Channels are used for hopping. ARFCNnumbers coded in bitmap.
    • SYSTEM INFORMATION 5SYSTEM INFORMATION 5Sent on the SACCH on the downlink to the MS in dedicated mode.Neighbour Cell DescriptionBA-IND :- Used by the Network to discriminate measurementsresults related to different lists of BCCH carriers sent by theMS( Type 2 or 5).Values 0 or 1 ( different from type 2).BCCH Allocation number :- 00 - Band 0 (Current GSM band).BCCH ARFCN :- Neighboring cells ARFCN’s. Sent as a bitmap.0 = ARFCN not used1 = ARFCN used124 123 122 121016 015 014 013 012 011 010 009008 007 006 005 004 003 002 001
    • SYSTEM INFORMATION 6SYSTEM INFORMATION 6• MS in dedicated mode needs to know if the LA has changed.• MS may change between cells with different Radio link timeoutand DTX.Cell IdentityLocation Area IdentificationCell OptionsdtxpwrcRadio link timeoutPLMN permitted
    • PAGINGPAGING• Whenever the Network wants to contact the MS, it sendsmessages on the paging channel.• Paging is sent on the PCH and it occupies 4 bursts.• MS has to monitor the paging channel to receive pagingmessages.• MS does not monitor all paging channel but only specific pagingchannels.• There are three types of paging messagesPagingTypeNo of MSusing IMSINo of MSusing TMSITotal no ofMS1 2 - 22 1 2 33 - 4 4
    • CALCULATION OF PAGING GROUPCALCULATION OF PAGING GROUPFollowing factors are used for calculation of paging group• CCCH_group– cch_conf in System Information 3 defines the number ofCCCH used in the cell.– CCCH can be allocated only TN 0, 2, 4, 6.– Each CCCH carries its own paging group of MS.– MS will listen to paging messages of its specific group.• bs_pa_mfrms• bs_ag_blk_res
    • CALCULATION OF PAGING GROUPCALCULATION OF PAGING GROUPTotal number of paging groups on 1 CCCH_GROUP(N)No of paging groups N = Paging blocks * Repitition of paging blocks= [ CCCH - bs_ag_blk_res ] * bs_pa_mfrmsRange of Paging Groups on 1 CCCH_GroupMinimum available Paging Groups = Min pag blocks * min bs_pa_mfrms= 2 * 2= 4Maximum available Paging Groups = Max pag blocks * max bs_pa_mfrms= 9 * 9= 81
    • AVAILABLE PAGING BLOCKS ON 1 CCCH_GROUPAVAILABLE PAGING BLOCKS ON 1 CCCH_GROUPMaximum AGCH reservation for non-combined multiframe = 7Available paging blocks = 2Maximum AGCH reservation for combined multiframe = 1Available paging blocks = 2Minimum AGCH reservation for non-combined multiframe = 0Available paging blocks = 9Minimum AGCH reservation for combined multiframe = 0Available paging blocks = 3No of paging blocks will have a range of 2 - 9
    • CALCULATION OF CCCH AND PAGING GROUP NOCALCULATION OF CCCH AND PAGING GROUP NOCCCH_GROUP = [ ( IMSI mod 1000) mod (BS_CC_CHANS * N ) ]div NPaging group no = [ ( IMSI mod 1000) mod (BS_CC_CHANS *N ) ] mod N
    • ENDEND