*Wireless NetworksLecture 14Fundamentals of Cellular Networks (Part IV)

Dr. Ghalib A. Shah

*OutlinesTrunking and Grade of ServiceMeasuring Traffic IntensityTrunked SystemsBlocked Calls ClearedBlocked Calls DelayedErlang ChartsImproving Coverage and CapacityCell SplittingSectoringRepeaters for Range ExtensionMicrocell Zone Concept

*Last lecture reviewInterference and system capacityCo-channel interference and capacityAdjacent channel interference and capacityChannel Planning for Wireless System

*TrunkingAllows a large number of users to share a small number of channelsChannel allocated per call basis from a pool of available channelsRelies on statistical behavior of users so that a fixed number of channels (circuits) may accommodate a large random user communityTrunking theory is used to determine number of channels for particular area (users)Tradeoff between the number of available channels and likelihood of call blocking during peak calling hours

*Trunking TheoryDeveloped by Erlang, Danish Mathematician, how a large population can be accommodated by a limited number of servers, in late 19th centuryToday, used to measure traffic intensity1 Erlang represents the amount of traffic intensity carried by a completely occupied channeli.e. one call-hour per hour or one call-minute per minute0.5 Erlang: Radio channel occupied 30 minutes during 1 hour

*Grade of ServiceGOS is a benchmark used to define performance of a particular trunked systemMeasure of the ability of a user to access trunked system during the busiest hour.Busy hour is based on the demands in an hour during a week, month or year. Typically occur during rush hours between 4 pm to 6 pm.GOS is typically given as likelihood of call blocking or delay experienced greater than certain queue time

*Traffic intensityTraffic intensity is measured as call request rate multiplied by call holding timeUser traffic intensity of Au Erlang is(1)Au= HWhere H is average call duration or holding time and is average number of call requests.For system of U users and unspecified channels, the total offered traffic intensity A is(2)A = UAuIn a C channel trunked system, traffic equally distributed, traffic intensity per channel Ac is(3)Ac= UAu/C

*Note that traffic is not necessarily the carried traffic but offered to the trunked systemIf offered load increases the system capacity, the carried traffic becomes limitedIn Erlang, max possible carried traffic is the number of channels CAMPS is designed for a GOS of 2% blockingi.e. 2 out of 100 calls will be blocked due to channel occupancyThere are two types of commonly used trunked systemsBlocked Calls ClearedBlocked Calls Delayed

*Block Calls ClearedUser is given immediate request if a channel is available.If no channel available, the requesting user is blocked and free to try laterAssume call arrivals as Poisson Distributionthe Erlang B formula determines the probability that call is blocked with no queuing, is a measure of GOS for trunked system

*Erlang B Trunking GOSCapacity of an Erlang B System

Number of Channels CCapacity (Erlangs) For GOS= 0.01= 0.005= 0.002= 0.00120.1530.1050.0650.04640.8690.7010.5350.43951.361.130.9000.762104.463.963.433.092012.011.110.19.412415.314.213.012.24029.027.325.724.57056.153.751.049.210084.180.977.475.2

*Erlang B

*Block Calls DelayedQueue is provided to hold blocked calls.Call request may be delayed until a channel becomes availableIts measure of GOS is defined as the probability that a call is blocked after waiting specific length of time in the queueThe likelihood of a call not having immediate access is determined by Erlang C formula

*Erlang C

*if no channels are available immediately, the call is delayed, probability that call is forced to wait more than t seconds is

Average delay D in all calls in queued system is

*Trunking EfficiencyA measure of the number of users which can be offered a particular GOS with particular configuration of channelsThe way channels are grouped can alter the number of users handledFor example, From table10 trunked channels at GOS of 0.01 can support 4.46 Erlang of trafficWhereas 2 groups of 5 channels can support 2x1.36=2.72 Erlangs of traffic, 60% lesser

*Improving Coverage and CapacityAs demand increases, number of channels per cell become insufficientCellular design techniques needed to provide more channels per unit coverage areaVarious techniques developed to expand the capacity of systemCell splittingSectoringMicro cell zone concept

*Cell SplittingAchieve capacity improvement by decreasing R and keeping D/R (cell reuse ratio) unchangedDivide the congested cells into smaller cellsSmaller cells are called micro cellsIf radius of cell is cut to half, approximately four cells would be requiredIncreased number of cells would increase the number of clusters, which in turn increase the capacityAllows a system to grow by replacing larger cells with smaller cells without upsetting the allocation scheme

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*For new cells to be smaller in size, tx power must be reduced. By which factor?

If n = 4 then the received powers equal to each other becomes

Power must be reduced by 12 dB in order to maintain S/I requirements

*Thus low speed and high speed users can simultaneously handledChannels in old cell must be broken down into two groups corresponding to smaller and larger cellsAt beginning of cell splitting, fewer channels to smaller power groups.As demand grows, more channels will be required and thus more micro cellsIn the end, the whole system will be replaced with micro cells

*SectoringKeep cell radius unchanged and decrease D/RIncreases SIR so that cluster size may be reducedSIR is improved using directional antennasHence increasing frequency reuse without changing transmission powerCell is partitioned into 3 120o sectors or 6 60o sectors as shown in Fig

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*Instead of interference from 6 cells, only 2 sectors interferethus S/I can be found to be 24.2 dB, where it is 17 dB in worst case presented beforeThis S/I improvements allow designers to decrease cluster size N and hence enhances capacityDrawbacksIncreased number of handoffs

*Microcell Zone ConceptA cell is divided into zones with a single BS sharing the same radio equipmentZones are connected through coaxial cable, fiber optics or microwave links to the BSSuperior to sectoring since antennas are placed at outer edges of the cells and any channel may be assigned to any zone by BSAs mobile travels from one zone to other, it retains same channel, BS simply switches the channel to a different zone.Co-channel interference is minimized becuaseLarge BS is replaced by several low powered txImproves S/I

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*B Erlang is more conservative approach*Fig. 2.8*Fig. 2.9