无线通信原理与应用英lecture2 The Cellular Concept-System Design Fundamentals

1、Chapter2: The Cellular Concept-System Design FundamentalsLecture 27/20/202212.1 The Concept :Cellular Networks 7/20/20222geometric shapes Circle: Equilateral triangle:Square:Hexagon:7/20/20223Cellular coverage representation7/20/20224Macrocell large, covering a wide are range of several hundred kilo

2、metres (km) to ten km mostly deployed in rural and sparsely populated areasMicrocell medium cell, coverage are a smaller than in macro cells range of several hundred meters to a couple of metres deployed mostly in crowded areas, stadiums, shopping mallsPicocell small, covering a very small area rang

3、e of several tens of metres low power antennas, indoorThe size of a cell7/20/20225Three different kinds of cells7/20/20226Base station transmittersIn hexagonal cells, base stations transmitters are either:centre-excited, base station is at the centre of the cell edge-excited, base station at 3 of th

4、e 6 cell verticescentre-excitededge-excited7/20/20227C can be used as a measure of capacity 2.2 Frequency Reuse and Capacity 7/20/20228A mobile communication system uses a frequency reuse factor of 1/7 and 416 channels available. If 21 channels are allocated as control channels, compute its system c

5、apacity. Assume a channel supports 8 usersChannels available for allocation = 416 - 21 = 395Channel Number of a cell = 395 / 7 = 57Number of simultaneous users per cell = 8 x 57 = 456Number of simultaneous users in system = 7 x 456 = 3192Example 17/20/20229Method of locating co-channel cells7/20/202

6、210Channel Assignment Strategies:Two assignment approachesFixed and static (most common)DynamicFixed channel assignmentall channels in a cell could be in use all the timenew calls are then blocked (no channels left)may be solved by borrowing spare channels from nearby cells2.3 Channel Assignment and

7、handoff7/20/202211Dynamic channel assignmentMSC allocates frequencies when a call is madeProvides high channel utilizationTo do this it needs real-time information onchannel occupancy traffic distributionradio signal strength indication (RSSI)high computational load and increased storage7/20/202212H

8、andover (Handoff)Provides continuity of communication across cellsDifficultydropping a call before reconnecting is unacceptabledifferent cells use different frequenciesmobile phone users usually move from place to place and very quickly 7/20/202213Handover ProcessReceived signal weakens as mobile mo

9、ves out of cellCell site at some point requests handover to cell with stronger signal strengthMSC switches call to new cell after allocating channels.7/20/202214Illustration of a handoff scenario at cell boundary7/20/202215The basic concept of handover strategiesHandover must not be too frequent or

10、system is kept busy servicing handover requestshandover threshold is set slightly strongerMinimum usable signal level is normally set to be between -90 dBm and -100 dBm7/20/202216Choosing Handover MarginsHandover margin = P r handover - Pr minimum usable If is too large unnecessary handover will occ

11、ur, burdening the MSCIf is too small, there maybe insufficient time to complete the handover before a call is lost due to weak signalsTherefore is chosen carefully7/20/202217Setting Handover Thresholds7/20/202218Handoff in different system1G signal strength measurements are made by the base stations

12、 and supervised by the MSC GSM Handover based upon information provided to the BTS by the MS is commonly referred to as a mobile assisted handover (MAHO).CDMA pilot channel, soft handoff7/20/202219Interference and System Capacity:2.4 Interference7/20/2022207-cell reuse pattern7/20/202221Six Effectiv

13、e Interfering Cells of Cell 1 7/20/202222Co-channel reuse ratio Q 7/20/20222312-cell reuse pattern 7/20/20222419-cell reuse pattern 7/20/202225Co-channel reuse ratio vs. frequency reuse pattern 7/20/202226Tradeoff 7/20/202227Propagation model7/20/202228Co-channel interference ratio 7/20/202229Exampl

14、e 27/20/202230More exact geometry for the co-channel interferences7/20/202231S/I is approximated as （3.10），Using，7/20/2022322.5 Trunking and Grade of Service (GOS) Trunking: large number of users share a relatively small number of channelsEach user is allocated a channel on per call basisAll channel

15、 in use: new user is blocked or has toTo wait in a queueTrunking theory: A.K. Erlang(1917)7/20/202233Grade of Service (GOS)Grade of Service (GOS): measure of the ability of a user to access a trunked system during the busiest hour.Prob. call is blocked or Prob.delayTMeasure of traffic intensity: Erl

16、ang1 Erlang=amount of traffic intensity carried by a channel that is Completely occupied7/20/202234About ErlangOn average, during the busy hour, a company makes 120 outgoing calls of average duration 2 minutes. It receives 200 incoming calls of average duration 3 minutes. Find (1)the outgoing traffi

17、c,(2)the incoming traffic,(3)the total traffic.The outgoing traffic is The incoming traffic is The total traffic is During the busy hour, on average, a customer with a single telephone line makes three calls and receives three calls. The average call duration is 2 minutes. What is the probability th

18、at a call finds the line engaged?Occupancy of line7/20/202235Erlang B formulaErlangs, the total offered traffic intensity and the traffic intensity per channel, respectively,isThe probability of blocked call is, 7/20/202236Trunked mobile network provide cellular service in this area. System has 394

19、cells with 19 channels each. Find the number of users that can be supported at 2% blocking if each user averages two calls per hour at an average call duration of three minutes. Compute the total number of subscribers that can be supported by this system.Probability of blocking = 2% =0.02Number of c

20、hannels per cell used in the system, C=19Traffic intensity per user, Au = H= 2 x (3/60) = 0.1 Erlangsfrom the Erlang B chart, the total carried traffic, A, is obtained as 12 Erlangs.Therefore, the number of users that can be supported per cell isU=AIAu= 12/0.1 = 120Since there are 394 cells, the tot

21、al number of subscribers that can be supported by System A is equal to 120 x 394 = 47280Example 37/20/202237Often large cells need to be split into smaller ones because the population of users in the big cell has increased beyond what it can supportCell splitting increases system capacity Is used in

22、 high density subscriber areas Results to increased costs (eg. new base stations)2.6 Improving Capacity in Cellular System 7/20/202238Illustration of Cell splitting 7/20/202239(to be continued)7/20/202240Scalable Cell Splitting7/20/202241Sectoring reduces interference7/20/202242Example 4A cellular s

23、ystem: an average call lasts two minutes, and the probability of blocking is to be no more than 1 %. Every subscriber makes one call per hour, on average. If there are a total of 395 traffic channels for a seven-cell reuse system, there will be about 57 traffic channels per cell. Assume that blocked

24、 calls are cleared so the blocking is described by the Erlang B distribution. From the Erlang B distribution, it can be found that the unsectored system may handle 44.2 Erlangs or 1326 calls per hour.If employing 120 sectoring, there are only 19 channels per antenna sector (57/3 antennas). For the same probability of blocking and average call length, it can be found from the Erlang