无线通信系统第2章.ppt

1、2020/7/28,1,The Cellular Concept： System Design Fundamentals,2020/7/28,2,Cellular Networks,2020/7/28,3,Cell shape,Circle: Equilateral triangle: Square: Hexagon:,2020/7/28,4,Cellular coverage representation,2020/7/28,5,Macrocell large, covering a wide are range of several hundred kilometres (km) to t

2、en km mostly deployed in rural and sparsely populated areas Microcell medium cell, coverage are a smaller than in macro cells range of several hundred metres to a couple of metres deployed mostly in crowded areas, stadiums, shopping malls Picocell small, covering a very small area range of several t

3、ens of metres low power antennas, indoor,The size of a cell,2020/7/28,6,2020/7/28,7,Base station transmitters,In 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 the 6 cell vertices,centre-excited,edg

4、e-excited,2020/7/28,8,Cellular coverage representation,C can be used as a measure of capacity,2020/7/28,9,A 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 capacity. Assume a channel sup

5、ports 8 users Channels available for allocation = 416 - 21 = 395 Channel Number of a cell = 395 / 7 = 57 Number of simultaneous users per cell = 8 x 57 = 456 Number of simultaneous users in system = 7 x 456 = 3192,Example 1,2020/7/28,10,If a total of 33 MHz of bandwidth is allocated to a particular

6、FDD cellular telephone system which uses two 25 kHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses four-cell reuse. If 1 MHz of the allocated spectrum is dedicated to control channels, determine an equitable dist

7、ribution of control channels and traffic channels in each cell for each of the system.,Example 2,2020/7/28,11,Method of locating co-channel cells,2020/7/28,12,Channel Assignment Strategies,Three assignment approaches Fixed and static (most common) Dynamic Hybrid Fixed channel assignment all channels

8、 in a cell could be in use all the time new calls are then blocked (no channels left) may be solved by borrowing spare channels from nearby cells,2020/7/28,13,Dynamic channel assignment MSC allocates frequencies when a call is made Provides high channel utilisation To do this it needs real-time info

9、rmation on channel occupancy traffic distribution and radio signal strength indication (RSSI) high computational load and increased storage,2020/7/28,14,Handover (Handoff),Provides continuity of communication across cells Difficulty dropping a call before reconnecting is unacceptable different cells

10、 use different frequencies mobile phone users usually move from place to place and very quickly too,2020/7/28,15,Handover Process,Parties in communication share the same channels Received signal weakens as mobile moves out of cell Cell site at some point requests handover to cell with stronger signa

11、l strength MSC switches call to new cell after allocating channels.,2020/7/28,16,Handover Process,2020/7/28,17,Handover Process,Handover must not be too frequent or system is kept busy servicing handover requests handover threshold is set slightly stronger Minimum usable signal level is normally set

12、 to be between -90 dBm and -100 dBm,2020/7/28,18,Choosing Handover Margins,Handover margin = P r handover - Pr minimum usable If is too large unnecessary handover will occur, burdening the MSC If is too small, there maybe insufficient time to complete the handover before a call is lost due to weak s

13、ignals Therefore is chosen carefully,2020/7/28,19,Setting Handover Thresholds,2020/7/28,20,Handoff in different system,1G signal strength measurements are made by the base stations and supervised by the MSC GSM MAHO handoff decisions are mobile assisted CDMA pilot channel, soft handoff,2020/7/28,21,

14、Interference and System Capacity,2020/7/28,22,Six Effective Interfering Cells of Cell 1,2020/7/28,23,Co-channel reuse ratio Q,2020/7/28,24,Co-channel reuse ratio vs. frequency reuse pattern,2020/7/28,25,7-cell reuse pattern,2020/7/28,26,12-cell reuse pattern,2020/7/28,27,19-cell reuse pattern,2020/7

15、/28,28,Co-channel reuse ratio vs. frequency reuse pattern,2020/7/28,29,Propagation model,2020/7/28,30,Co-channel interference ratio,2020/7/28,31,Example 3,2020/7/28,32,More exact geometry layout,2020/7/28,33,Closely approximately as,2020/7/28,34,Trunking and Grade of Service (GOS),2020/7/28,35,Grade

16、 of Service (GOS),2020/7/28,36,About Erlang,2020/7/28,37,Erlang B formula,where C is the number of trunked channels offered by a trunked radio system and A is the total offered traffic.,2020/7/28,38,Trunked mobile network provide cellular service in this area. System has 394 cells with 19 channels e

17、ach. 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% =0.02 Number of channels per cell used

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

19、ribers that can be supported by System A is equal to 120 x 394 = 47280,Example 4,2020/7/28,39,Often large cells need to be split into smaller ones because the population of users in the big cell has increased beyond what it can support Cell splitting increases system capacity Is used in high density

20、 subscriber areas Results to increased costs (eg. new base stations),Improving Capacity in Cellular System,2020/7/28,40,2020/7/28,41,2020/7/28,42,Cell Splitting,2020/7/28,43,Sectoring,2020/7/28,44,A cellular system:an average call lasts two minutes, and the probability of blocking is to be no more t

21、han 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 calls are cleared so the blocking is described by the Erlang B distribution. From the Erlan

22、g 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 B dist

23、ribution that each sector can handle 11.2 Erlangs or 336 calls per hour. Since each cell consists of three sectors, this provides a cell capacity of 3 x 336 = 1008 calls per hour. Thus, sectoring decreases the trunking efficiency while improving the S/I for each user in the system.,Example 5,2020/7/

24、28,45,1.Wireless has rapidly emerged in the communication field. Please use library or internet to find the answers of the following questions. (a)Describe at least one cellular system that being used today, the carrier frequency, the data rate, the RF bandwidth or more. (b)Explain what is WLAN?(IEEE 802.11,wi-fi,etc.) 2.If 20 MHz o