第二章蜂窝概念系统设计基础

1、Chapter 2 The Cellular Concept System Design FundamentalsBasic Concepts for Wireless Communications System Base Station (BS) 基站 A fixed station in a mobile system used for radio communication with mobile stations. Mobile Station (MS)移动台 A station in the cellular radio service intended for use while

2、in motion at unspecified locations.Subscriber & Transceiver Subscriber用户 A user who pays subscription charges for using a mobile communications system. Transceiver (Transmitter+ Receiver)收发信机 A device Capable of simultaneous transmitting and receiving radio signals. MSC & MTSO MTSO (Mobile T

3、elephone Switching Office)移动电话交换局 MSC (Mobile Switching Center)移动交换中心 Switching center which coordinate the routing of calls in a large service area.(交换中心用来在一个大的服务区协调呼叫路由) The MSC coordinate the activities of all of the base stations and connects the entire cellular system to the PSTN.(同时交换中心还用来协调所有

4、基站的活动并连接公用电话网) A typical MSC handles 100,000 cellular subscribers and 5,000 conversations at a time, and accommodates all billing and system maintenance functions . (一个典型的移动交换中心可以管理100000个用户,并同时处理5000个通话. 同时还提供所帐单和系统维护功能.)分分 类类频频 段段移动台发移动台发基站收基站收基站发基站发移动台收移动台收GSM900/1800频段频段900MHz频段频段890-915MHz935-9

5、60MHz1800MHz频段频段1710-1785MHz1805-1880MHzGSM Spectrum900MHz频段频段1800MHz频段频段频道间隔频道间隔相邻频道间隔为相邻频道间隔为200KHz每个频道采用时分多址接入每个频道采用时分多址接入(TDMA)方式分为方式分为8个时隙，即个时隙，即8个信道个信道 双工收发双工收发间隔间隔45MHz95MHz等等间间隔隔频频道道配配置置频道序号频道序号1 124512 885频道数频道数124374频道序号频道序号和频道标和频道标称中心频称中心频率的关系率的关系F1(n)=890.2+(n-1)*0.2 上行上行 F2(n)=f1(n)+45M

6、Hz 下行下行其中，其中，n=1,.,124F1(n)=1710.2+(n-512)*0.2 上行上行F2(n)=f1(n)+95MHz 下行下行其中，其中，n=512,513,885GSM ChannelsControl / Forward /Reverse Channel Control Channel(控制信道) Radio channel used for transmission of call setup,call request,call initiation, and other beacon or control purpose. Forward Channel (Down

7、Link）前向信道 Radio channel used for transmission of information from base station to mobile station. Reverse Channel (Up Link）反向信道 Radio channel used for transmission of information from mobile station to base station.Cellular Concept History 1947, Idea for large capacity mobile communications systems.

8、 1940S1950S, TV age for America. 1964, FCC refuse again to allocate spectrum for mobile telephone service. 19681970, assign 40 M bandwidth. Bell mobile system in New York city in 1970s could only support a maximum of twelve simultaneous calls over a thousand square miles. Frequency Reuse/Planning频率复

9、用频率复用/频率规划频率规划 Frequency Reuse(频率复用) The design process of selecting and allocating channels groups for all of the cellular base stations with a system (基站信道分配) Base stations in adjacent cells are assigned channel groups which contain completely different channels than neighboring cells.(为相邻小区基站分配完全

10、不同的信道) By limiting the coverage area to within the boundaries of a cell, the same group of channels may be used to cover different cells that are separated from one another by distance large enough to keep interference levels within tolerable limits.(相同的信道分组可以分配给相隔足够远的基站)Strip coverage region The us

11、er of the sytem is distributed in a strip area, such as river, railway, highwayFace coverage regionFigure 3.1 Comments Figure 3.1 illustrates the concept of cellular reuse, which cells labeled with the same letter use the same group of channels.(相同字母的小区使用相同的信道组) The hexagonal cell shape shown in Fig

12、ure 3.1 is conceptual and a simplistic model of the radio coverage for each base station, but it has been universally adopted since the hexagon permits easy and manageable analysis of a cellular system.(用六边形表示的小区是基站无线覆盖的简单模型) The actual radio coverage is known as the footprint and it is determined f

13、rom field measurement or propagation predication model. (实际的无线覆盖区域应从实际场地测量和传播预测模型中来)Why hexagon?Why not circle? While it might seem natural to choose a circle to present the coverage area of a base station, adjacent circles cannot be overlaid upon on a map without leaving gaps or cerate overlapping

14、regions.( .(圆型无法做到无缝且无重复覆盖圆型无法做到无缝且无重复覆盖) ) When considering geometric shapes which cover an entire region without overlapping and with equal area, there are three sensible choices:a square, an equilateral triangle, and a hexagon.(要进行完全的无缝及无重复覆盖一相同区要进行完全的无缝及无重复覆盖一相同区域可以有三种方式域可以有三种方式:正方形正方形,等边三角形和六边形

15、等边三角形和六边形) A cell must be designed to serve the weakest mobiles within the footprint, and these are typical located at the edge of the cell. For a given distance between the center of a polygon and its farthest perimeter points, the hexagon has the largest area of the three.(从中心位置到给定的最远距离从中心位置到给定的最远

16、距离,六边形是三种六边形是三种选择中具有最大覆盖面积的选择中具有最大覆盖面积的.)Cluster(簇) The N cells which collectively use the complete set of available frequencies is called Cluster.( .(使用能够得到的完整频率使用能够得到的完整频率集的集的N N个小区叫一个簇个小区叫一个簇) ) If each cell is allocated a group k channels, and if S channels are divided among N cells into unique

17、and disjoint channel groups which have same number of channels, the total number of available radio channels S can be expressed as: S=kN(N(N个小区个小区, ,每个小区分配每个小区分配k k个信道个信道, ,则共有则共有S S个信道个信道) ) If a cluster is replicated M times within the system, the total number of channels C, can be used as a measu

18、re of capacity and is given by C=MkN=MS(如果系统中一个簇重复M次,则总的信道数C=MS=MkN)frequency reuse factor(频率复用因子) The capacity of cellular system is directly proportional to the number of times of a cluster is replicated in a fixed service area.( .(系统容量与簇的复用次数成正比系统容量与簇的复用次数成正比) ) The factor N is called the cluster

19、 size ( (簇大小簇大小) ) If the cluster size is reduced while the cell size is kept constant, more clusters are required to cover a given area, hence more capacity (a larger value of C) is achieved.(小小区大小不变簇大小减少区大小不变簇大小减少, ,则覆盖一给定区域要则覆盖一给定区域要求更多的簇求更多的簇, ,系统容量增加系统容量增加) ) A larger cluster size leads to weak

20、er co-channel interference . The frequency reuse factor is given by 1/N CDMA system frequency reuse factor is 1/1.Computing the value of NCluster Size N (1)22NiijjijjiNRNDjRJiRIRHHjHJiHIIJJIIJJID2222222:33323: )(2,2120cos2所以有有为小区中心到边的距离令Cluster Size N (2) i=1, j=0, N=1 i=1, j=1, N=3 i=2, j=0, N=4 i=

21、2, j=1, N=7 i=2, j=2, N=12 i=3, j=0, N=9 i=3, j=1, N=13 i=3, j=2, N=19Some examples of cluster19-cell reuse example (N=19)Figure 3.2 Method of locating co-channel cells in a cellular system. In this example, N = 19 (i.e., I = 3, j = 2). (Adapted from Oet83 IEEE.)Example P61 Example3.1 If a total of

22、33MHz of bandwidth is allocated to a particular FDD cellular telephone system which uses two 25kHz simplex channels to provide full duplex voice and control channels, compute the number of channels available per cell if a system uses (a) four-cell reuse, (b) seven-cell reuse, and (c) 12-cell reuse.I

23、f 1MHz of the allocated spectrum is dedicated to control channels, determine an equitable distribution of control channels and voice channels in each cell for each of the three systems.Channel Assignment strategies(信道分配策略) Fixed channel assignment(固定分配) Dynamic channel assignment (动态分配) Dynamic chan

24、nel assignment reduce the likelihood of blocking , which increase the trunking capacity of the system. Dynamic channel assignment strategies require the MSC to collect real-time data on channel occupancy,traffic distribution, and radio (receive) signal strength indications (RSSI) .Handoff/Handover切换

25、切换 Handoff/Handover The process of transferring a mobile station from one channel or base station to another(将移动台从一个信道或一个基站转换到另一个信道或基站的过程) Request: 很多系统中，切换信道分配比初始呼叫信道分配具有更高优先级； 切换应尽可能成功并尽可能少地发生； 对用户透明。 In order to meet the requirements listed above, an optimum signal level at which to initiate a ha

26、ndoff is specified. 基站接收机可接受话音质量的最小可用信号强度：Prminimumusable 启动切换的最恰当信号强度：Prhandoff Delta= Prhandoff -Prminimumusable 当Delta太大，增加MSC的负担； 当Delta太小，可能会掉话。Handoffs the basicsHandoff decisions Mobile Control Handoff 欧洲的DECT和北美的PACS Network Control Handoff TACS和AMPS Mobile Assisted Handoff GSM和CDMAMethods o

27、f Handoff Hard Handoff Soft HandoffChannel assignment in Handoff Prioritizing Handoffs A fraction of the total available channels in a cell is reserved exclusively for handoff requests from ongoing calls which may be handed off into the cell. Roamer(漫游) Roamer A mobile station which operate in a ser

28、vice area (market) other than that from which service has been subscribed.(当移动台不是在注册地使用服务时,移动台就成为漫游者) RoamingInterference and system capacity(干扰与系统容量) Interference is the major limiting factor in the performance of cellular radio system. Interference: Noise(噪声) MAI (Multiple Access Interference)(多址干

29、扰) ACI (Adjacent Channel Interference, Adjacent Cell/sector Interference)(邻道干扰) ISI ( Inter-Symbol Interference)(符号间串扰) Co-channel Interference(同道干扰)Co-channel Interference and Capacity (同道干扰与容量) Co-channel Interference reuse ratio Q(同道干扰复用比) Where R is the cell radius and D is the distance between

30、centre of the nearest co-channel cells 3DQNRCo-channel Interference and Capacity0P0nrdPd0P ()() 10 log0rddBmP dBmndCo-channel Interference and Capacity Signal-to-interference ratio (S/I or SIR) Signal-to-noise ratio SNR SINRCo-channel Interference and Capacity01iiiSSII01ininiiSRIDCo-channel Interfer

31、ence and Capacity003/nnND RSIiiCo-channel cells for 7-cell reuseCo-channel Interference and Capacity Two useful approximate equation : See PP.70 3.10 and 3.11Smaller N is greater capacityExample P72 3.2If a signal-to-interference ratio of 15dB is required for satisfactory forward channel performance

32、 of a cellular system, what is the frequency reuse factor and cluster size that should be used for maximum capacity if the path loss exponent is (a) n=4,(b)n=3? Assume that there are six co-channel cells in the first tier, and all of them are at the same distance form the mobile. Use suitable approximations. Adjacent Channel Interference Interference resulting from signals which are adjacent in frequency to the desired signal is called adjacent channel interference. See Page 74-76Trunking and Grade of Service中继和服务等级 GOS: Grade of Service QoS: Quality of Service