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The Cellular Concept-System Design FundamentalsIntro to Cellular conceptDivide the service area into small cellsTo reduce the transmitting power, no of towers should be at least equal to no.of cellsRe-use the allocated frequency spectrum(channels) as many times as possible.A cluster is group of cells which is replicated throughout the systemAdvantages:Spectral congestionUser capacityDisadvantagesGreater capacity at the expense of large infrastructureOptimal frequency utilization by making system more complicatedUser equipment design made smarter at the cost of circuit complexity and processing powerFrequency Re-useChannel - stands for a pair of radio frequencies used for two way connection between base and mobile subscriberCell- a small geographic area.When considering geometric shapes which cover an entire region without overlap , there are 3 choices:A squareEquilateral Triangle HexagonCells with the same number use the same set of frequencies Introduction (Contd.) Different Type of Possible Cell shapesFor the same Cell Radius (R) (distance from center to Cell boundary) Area of Hexagon is the largestRRRCluster Size1231231231234123412343 Cell Cluster4 Cell ClusterFrequency Reuse for Area Coverage123456712345671234567Base Station (BS)Cellular ConceptS=total number of duplex channels available for use in a given area It depends on K=Group of channels allocated to each cellN=cluster size i.e. number of cells allocated to each clusterTotal number of available channels S=KNCapacityIf cluster is replicated M times within the system, the total number of duplex channels, C, can be used as a measure of system capacityC=MS where S=KNThe capacity of cellular system is directly proportional to the number of times a cluster is replicated in a fixed service areaThe factor N is called cluster sizeTypically N=4,7,12 Larger cluster , means less replication and low capacityFrequency Re-use FactorSmallest possible value of N is desirable in order to maximize capacity over a given coverage areaThe frequency reuse factor is given by 1/N of the total available channels in the system.

Locating Co-Channel CellsIn order to connect without gaps between adjacent cells , the geometry of hexagons is such that the number of cells per cluster, N, can have values which satisfy the following equation:N=i^2+i*j+J^2For typical values of N which are 3,4,7,12 :(i, j)=(1,1)(2,0)(2,1)(2,2)Locating nearest Co-channel NeighborTo find the nearest co-channel neighbor of a particular cell, do the following :Move i cells along any chain of hexagons and thenTurn 60 degrees counter-clockwise andMove j cells Channel Assignment StrategiesChannel Assignment Strategies are used for Efficient Utilization of Radio Spectrum with the main Objectives of:Increasing CapacityMinimizing Interference

Classification of Channel Assignment StrategiesFixed channel assignment strategyDynamic channel assignment strategyContThe choice of channel assignment strategy impacts the performance that includes:Handoff i.e. how calls are managed when a mobile user is handed off from one cell to anotherCall initialization loadMSC processing LoadChannel Assignment Strategies (Contd.)Fixed Channel Assignment StrategyEach cell allocated a predetermined set of voice channelsIf all channels are occupied then calls are blockedSeveral variations of the fixed assignment strategy exist such as Borrowing strategy to tackle call blockageMSC supervises this borrowing ensuring that borrowing of channels does not disrupt or interfere with any other callsOrReserve some channels for handoff Channel Assignment Strategies (Contd.)Dynamic Channel Assignment Strategyvoice channels are not allocated to different cells permanentlyat a call request, the serving base station requests the MSC for a channelMSC allocates the channel following an algorithm that takes into account,Frequency of use of the candidate channelReuse distance, etc.Dynamic Allocation(Cont)MSC only allocates a given frequency if that frequency is not presently in use in the cell or in any other cell which falls within the minimum restricted distance of frequency reuse to avoid co-channel interference.Advantages/Disadvantages:Reduction of blocking probabilityIt increases trunking capacity (traffic intensity/channel) of the system , since available channels in the market are accessible to all of the cellsDynamic Allocation (Cont)In dynamic allocation, real time data is needed for all channels on continuous basis :Channel occupancyTraffic distributionRSSI(Radio signal strength indications) This increases computational load on the system but provides the advantage of increased channel utilization and decreased probability of blocked callHandoff Hand off enables a call to proceed un-interrupted when user moves from one cell to another.Handoff is encouraged in order to maintain call quality as the subscriber moves in and out of range of base stationThe HO operation not only involves identifying a new base station, but also requires that the voice and control signals be allocated to channels associated with the new base station

Cont..When mobile is on the edge of cellRSL of the mobile in that cell falls below a set levelBase station of the cell originates a handoff requestMSC gets RSL information from all candidate cellsMSC asks the originating cell and strongest candidate cell to co-ordinateIn case the handoff is successful, the mobile is asked to switch to another Voice channel

Handoff Strategies (Contd..)Many HO strategies prioritize HO requests over call initiation requests when allocating unused channelsHO must be performed Successfully, infrequently and should be imperceptible to the usersThreshold signal level DD = Pr handoff Pr minimum usable Pr handoff specifies the optimum signal level at which to initiate handoffPr minimum usable minimum usable signal for acceptable voice quality at the base station receiverIf D to large: unnecessary HOsIf D to small: calls may be lost due to insufficient time for HO

Handoff (Cont)The call will be dropped when there is an excessive delay by MSC in assigning handoff or if D is too small for the handoff time in the system.Excessive delay can occur during high traffic conditions due to computational loading at the MSC or the fact that no channels are available on any of the nearby stations Handoff Strategies (Contd..)BS monitors the signal level for a certain period of time before a HO is initiatedAverage measurement of signal strength should be optimized so that unnecessary handoffs are avoided, while ensuring that necessary handoffs are completed before a call is terminated.Dwell timeThe time over which a call may be maintained within a cell, without HO is call Dwell timeStatistics of dwell time are important in the practical design of HO algorithmsDwell timeThe dwell time of a particular user is governed by a number of factors:PropagationInterferenceDistance between the subscriber and base stationTime varying effects Dwell time vary greatly depending upon the speed of user and type of radio coverageVehicular Highway users concentrated around the mean dwell timeIn cluttered microcell environment , there is a large variation around the mean

Handoff Strategies (Contd..)In 1G analog cellular systems, signal strengths measurements are made by BS and supervised by MSC. BS measures signal strength of all Reverse voice channels.In each BS a spare receiver or locator receiver measures signal strengths of channels in neighboring cellsBased on the information from all the locator receivers the MSC decides if HO is necessary or notHandoff Strategies (Contd..)In 2G cellular systems that uses digital TDMA HO decisions are mobile assisted; mobile assisted handoff (MAHO) In MAHO every mobile measures the received power from the surrounding BSs and continually reports the results to the serving BSA handoff is initiated when the power received from the base station of a neighboring cell begins to exceed the power received from the current base stationMAHO allows much faster HOs, as the HO measurements are made by each mobile and the MSC no longer constantly monitors the signal strength MAHO is particularly suited for microcellular environments where HOs are more frequent Handoff Strategies (Contd..) Intersystem HandoffDuring the course of a call if a mobile moves from one cellular system to a different cellular system controlled by a different MSC and intersystem handoff becomes necessaryConditions for Intersystem HOWhen a mobile signal becomes weak in a given cell and the MSC cannot find another cell within its system to which it can transfer the call in progressIssues to be addressed when Implementing Intersystem HOA local call may become long distance call (billing issue) Compatibility between the two MSCsHandoff (Cont)Some systems handle handoff requests in the same way as they handle originating calls.In such systems, the probability that a handoff request will not be served by a new base station is equal to the blocking probability of incoming cellsHowever from the users point of view, handoff requests are prioritized over call initiation requests.Prioritizing HandoffsDifferent systems have different policies and methods for managing handoff requests. Some systems give priority to HO over call initiation others deal them at same priority Prioritizing HandoffsGuard channel conceptDisadvantage reduces total carried trafficAdvantage efficient spectrum utilization with dynamic channel assignment Queuing of Handoff requestsPossible because a finite time interval between the time the signal drops below the HO threshold and the time the call is terminated due to insufficient signal levelQueuing does not guarantee a zero probability of forced termination in case of large delays which will cause the received signal level to drop below the minimum required level to maintain communication. Practical Handoff Considerations Umbrella Cells

Practical Handoff considerationsThe umbrella cell approach ensures that the number of handoffs is minimized for high speed users and provides additional microcell channels for pedestrian usersThe speed of vehicle may be evaluated by MSC,by calculating how rapidly received signal strength varies for each reverse voice channelIf a high speed user is approaching base station , and its speed is decreasing , the base station will handover this user to microcell , without consulting MSc Practical Handoff Considerations Cell DraggingIf a uses travels away from the base station at a slow speed , the average signal strength does not decay rapidly. Even when the user has traveled well beyond the designed range of the cell, the received signal at the base station may be above the D, thus the HO may not be made.It Creates potential Interference and traffic management problems because user has travelled down into neighboring cell.To avoid cell dragging ,handoff thresholds and radio coverage parameters must be adjusted carefully.Cont..In 1G cellular systems , the typical time to make a handoff is about 10 sec.However in digital cellular systems such as GSM, mobile assists with the handoff procedure by determining the best handoff candidates and typically require 1 to 2 secThe faster handoff process supports a much greater range of options for handling high speed and low speed usersIt provides the MSC with substantial time to rescue a call that is in need of handoff

InterferenceInterference in voice channels can cause Cross talkInterference in control channels leads to missed and blocked callsInterference is major bottleneck in increasing capacity and is often responsible for dropped callsInterference is of two types:Co-channel interferenceAdjacent channel interference

InterferenceSources of interference :Another mobile in the same cellA call in progress in a neighboring cellOther base stations operating in the same frequency bandAny non-cellular system which leaks energy into cellular frequency band

Interference(Cont)The interference created by out of band users is very difficult to controlThe transmitters from competing cellular carriers are often a significant source of out-of band interference because they often locate their base stations in close proximity to one another in order to provide comparable coverageCo-Channel InterferenceCells which use the same set of frequencies are called as co-channel cellsThe interference between signals from these cells is called as co-channel interferenceThe co-channel interference cannot be combated by increasing the carrier power of transmitter (as we do in case of thermal noise)To reduce this, co-channel cells must be physically separated by minimum distance to provide isolationCo-channel Reuse RatioThe parameter Q, called as co-channel re-use ratio is defined as :Q=D/R where D=distance between centers of the nearest co-channel cellsR=radius of the cell By increasing the D/R, the spatial separation between co-channel cells relative to the coverage distance of a cell is increasedExampleA total of 33MHz bandwidth is allocated to a particular FDD cellular phone system. If the simplex voice/control channel bandwidth s 25kHz,find the total number of channels available per cell if the system uses (a)4-cell frequency reuse(b)7-cel frequency reuse planIf 1MHz out of total allocated bandwidth is used for control channels, determine an equitable distribution of the control and voice channels in each cell in case of frequency reuse plan33Mhz allocated to a particular FDD cellular phone system. If the simplex voice/control channel bandwidth s 50kHz,find the total number of channels available per cell if the system uses (a)4-cell frequency reuse(b)7-cel frequency reuse planIf 1MHz out of total allocated bandwidth is used for control channels, determine an equitable distribution of the control and voice channels in each cell in case of frequency reuse plan