ELEC5508-LECTURE5 无线通信, 无线工程

Wireless EngineeringChapter 5Capacity of Wireless SystemsChapter 3 pp. 77-96Wireless EngineeringTraffic and CapacityTrunking and Grade of ServiceCellular radio systems rely on trunking to accommodate a large number of random users in a limited radio spectrum (fixed number of channels or circuits).Trunking theory exploits the statistical behavior of users. It was first used by telephone companies to determine the number of telephone circuits that need to be allocated for buildings.The fundamentals of trunking theory were developed by Erlang.The measure of traffic intensity has the unit of Erlang.One Erlang represents the amount of traffic intensity carried by a channel that is completely occupied.Wireless EngineeringTrunking and Grade of ServiceThe grade of service (GOS) is a measure of the ability of a user to access a trunked system during the busiest hour.The GOS is an important benchmark used to define the desired performance ofa particular trunked system.It is the wireless designer’s job to estimate the maximum required capacity and to allocate the proper number of channels to meet the GOS.GOS is typically given as the likelihood that a called is blocked, or the likelihood of a call experiencing a delay greater than a certain queuing time.Wireless EngineeringTrunkingand Grade of ServiceWireless EngineeringWireless EngineeringTrunking and Grade of ServiceThe traffic intensity offered by each user in ErlangsThe total offered traffic for a system containing U usersTraffic intensity per channel of a trunked system with C channels (assuming traffic is equally distributed among the channels)The maximum possible carried traffic is the total number of channels, C ,in Erlangs.HA u λ=HU UA A u λ==CUA A u c =Wireless EngineeringL,2,1,0!}Pr{==−k k e k k λλ A discrete distribution that expresses the probability of a number of events occurring in a fixed period of time if these events occur with a known average rate, and are independent of the time since the last event.Poisson DistributionMean value:Variance: λσ=2λµ=Poisson Distribution, 10=λk : the number of occurrence of an event the expected number of occurrences that occurduring the given interval.:λWireless Engineering The Blocked Calls Cleared SystemThe blocked calls cleared system (Erlang-B system)--No queuing for call requests (no setup time, and immediate access to channel if available.--Arrived calls follow a Poisson distribution.--An infinite number of users, a finite number of channels--Arrivals of requests are memoryless--The probability of a user occupying follows an exponential distribution The probability that a call is blocked (Erlang-B formula , Erlang loss function )GOS k A C A A E C k k CC ===∑=0!!)(]blocked is call Pr[C : the number of trunked channels A : the total offered trafficWireless EngineeringThe Blocked Calls Cleared SystemThe Erlang loss function is best calculated iteratively by:withErlang B formula provides a conservative estimate of the GOS, as the finite number of users always predict a smaller likelihood of blocking.)(11)(11A E A C A E C C −+=1)(0=A EWireless EngineeringThe Blocked Calls Cleared SystemLarge groups are more efficient than small ones.Wireless EngineeringThe Blocked Calls Cleared SystemWireless EngineeringThe Blocked Calls Cleared SystemTrunking efficiency is a measure of the number of users which can be offered a particular GOS with a particular configuration of fixed channels.The way in which channels are grouped can substantially alter the number of users handled by a trunked system.ExampleHow many users could be supported for 0.5% blocking probability for 5 trunked channels? Assume that each user generates 0.1 Erlangs of traffic.Solution: C =5. From Fig. 3.6, we have traffic intensity A =1.13.3.111.0/13.1/===u A A U Therefore, total number of users that can be supported is 11.Improving Coverage and Capacity inCellular SystemsCellular design techniques are needed to provide more channels per unit coverage area if GOS degrades beyond a threshold.Major techniques to expand the capacity:---Cell Splitting: Allows an orderly growth of the cellular system.---Sectoring: Uses directional antennas to further control interference andfrequency reuse.---Zone microcells : Distributes the coverage of a cell and extends the cell boundary to hard-to-reach places.Cell splitting increase capacity by increasing the number of base stationsSectoring and zone microcells rely on base station antenna placements to improve capacity by reducing co-channel interference.Wireless EngineeringWireless Engineering Cell Basics (Review)N RD Q 3==S duplex channels available for use.Each cell is allocated a group of k channelsCluster size N If a cluster is replicated M times within the system, the capacity C=MkN=MSThe co-channel reuse ratio:The SIR ()003i N i Q I S nn ==Cell SplittingCell splitting is the process of subdividing a congested cell into smaller cells (microcells), each with its own base station and a corresponding reduction in antenna height and transmitter power.By decreasing the cell radius R and keeping the co-channel reuse ratio D/R unchanged, cell splitting increases the number of the channels per unit area.Wireless EngineeringWireless EngineeringCell SplittingHandoff issues must be addressed so that high speed and low speed traffic canbe accommodated simultaneously.Wireless EngineeringCell SplittingCell splitting will not upset thechannel allocationscheme requiredto maintain theminimum co-channel reuse ratioQ .Wireless EngineeringCell SplittingFor the new cells to be smaller in size, the transmit power of these cells must be reduced.If the radius of each new microcell is half that of the original cell, []nt r R P P −∝1boundary cell old at []nt r R P P −∝)2/(boundary cell new at 2 If we take n =4, and set the received powers equal to each other, then1612t t P P =Cell SplittingIn practice, not all cells are split at the same time. Therefore, different cell sizes might exist simultaneously.Channels in the old cell must be broken down into two channel groups, corresponding to the smaller and larger cell reuse requirements.Antenna downtilting is often used to limit the radio coverage of newly formed microcells.Cell splitting achieves capacity improvement by essentially rescaling the system. By decreasing the cell radius R and keeping the co-channel reuse ratio D/R unchanged, cell splitting increases the number of channels per unit area.Wireless EngineeringSectoringSectoring keeps the cell radius unchanged and seek method to reduce Q.The channels used in a particular cell are broken down into sectored groups and are used only within a particular sector.Several directional antennas are used at the base station.This technique for decreasing co-channel interference and thus increasing system performance is called sectoring .Wireless EngineeringWireless EngineeringSectoringWireless EngineeringSectoringThe signal-to-interference ratio (SIR) for a mobile receiverAssume n =4 and cluster size N=7.If there is no sectoring and omnidirectional antennas are used, S/I=18.6 dB.With sectoring, S/I=25.1 dB (24.2 dB in the textbook).()003i N i Q I S n n ==SectoringThe reduction in interference offered by sectoring enable planners to reduce the cluster size N, and provides an additional degree of freedom in assigned channels.Sectoring needs an increased number of antennas at each base station.Trunking efficiency is decreased due to channel sectoring at the base station.The number of handovers also increases, since sectoring reduces the coverage area of a particular group of channels.In urban areas, directional antenna patterns are somewhat ineffective in controlling radio propagation.Wireless EngineeringA Microcell Zone ConceptThe increased number of handoffs required when sectoring is employed results in an increased load on the switching and control link elements..In microcell zone approach,when a mobile travels fromone zone to another withinthe cell, it retains the samechannel.Any base station channelmay be assigned to any zoneby the BS.A handoff is not required atMSC when the mobiletravels between zones withina cell.Wireless EngineeringA Microcell Zone ConceptA given channel is activeonly in the particular zonein which the mobile istravelling, and the basestation radiation islocalized and interferenceis reduced.Particularly useful alonghighways or along urbantraffic corridorsWireless Engineering。