信息科学与电子工程专业英语Lecture06

1、EE141 电子信息科学概论MTI 2011 Aifeng Ren电子信息科学概论11任爱锋任爱锋电子工程学院电子工程学院 E-mail: Office: 新科技楼A901 Phone: 88202830-607 Mobile:E141 2011 Aifeng Ren电子信息科学概论2 电子信息科学概论MTI2 Unit 6-1 移动通信移动通信EE141 2011 Aifeng Ren电子信息科学概论3 电子信息科学概论MTI3pager 寻呼机 cellular 蜂窝状的 cordless phone 无绳电话 mobility 移动性 transceiver 收

2、发器 infrastructure 基础设施 carrier 载波 hexagon 六边形 handoff 切换，移交 roam 漫游 threshold 阈值，门限 power 幂 multipath fading 多径衰落 fluctuation 波动, 起伏 irregular 不规则的, 无规律的 diversity 分集，多样性 redundancy 冗余 fraction 小部分，分数 EE141 2011 Aifeng Ren电子信息科学概论4 电子信息科学概论MTI4delay spread 延迟扩展 attenuation 衰减 reflective 反射的 refracti

3、ve 折射的 RMS (root mean square) 均方根 alleviate 减轻 replica 复制品 interleaving 交叉，交织 scramble 打乱，使混杂 overhead 开销 entail 需要，使必须 latency 等待时间，时间延迟 EE141 2011 Aifeng Ren电子信息科学概论5 电子信息科学概论MTI5A mobile system is one in which users can physically move while communicating with one another. Examples include pagers

4、, cellular phones, and cordless phones. It is the mobility that has made RF communications powerful and popular. The transceiver carried by the user is called the “mobile unit” (or simply the “mobile”), the “terminal,” or the “hand-held unit.” Unit 6-1 移动通信EE141 2011 Aifeng Ren电子信息科学概论6 电子信息科学概论MTI6

5、The complexity of the wireless infrastructure often demands that the mobiles communicate only through a fixed, relatively expensive unit called the “base station.” Each mobile receives and transmits information from and to the base station via two RF channels called the “forward channel” or “downlin

6、k” and the “reverse channel” or “uplink,” respectively. Unit 6-1 移动通信EE141 2011 Aifeng Ren电子信息科学概论7 电子信息科学概论MTI7Most of our treatment relates to the mobile unit because, compared to the base station, hand-held units constitute a much larger portion of the market and their design is much more similar

7、 to other types of RF systems. Unit 6-1 移动通信EE141 2011 Aifeng Ren电子信息科学概论8 电子信息科学概论MTI8With the limited available spectrum (e.g., 25 MHz around 900 MHz), how do hundreds of thousands of people communicate in a crowded metropolitan area? To answer this question, we first consider a simpler case: thou

8、sands of FM radio broadcasting stations may operate in a country in the 88-108 MHz band. Unit 6-1 移动通信 蜂窝系统 (Cellular system)EE141 2011 Aifeng Ren电子信息科学概论9 电子信息科学概论MTI9This is possible because stations that are physically far enough from each other can use the same carrier frequency (frequency reuse

9、) with negligible mutual interference, except at some point in the middle where the stations are received with comparable signal levels.1 The minimum distance between two stations that can employ equal carrier frequencies depends on the signal power produced by each. Unit 6-1 移动通信 蜂窝系统 (Cellular sys

10、tem)EE141 2011 Aifeng Ren电子信息科学概论10 电子信息科学概论MTI10In mobile communications, the concept of frequency reuse is implemented in a “cellular” structure, where each cell is configured as a hexagon and surrounded by six other cells as shown in Figure 6.1(a). Unit 6-1 移动通信 蜂窝系统 (Cellular system)f1f1f1f1f1f1

11、f1f1f2f1f2EE141 2011 Aifeng Ren电子信息科学概论11 电子信息科学概论MTI11The idea is that, if the center cell uses a frequency f1 for communication, the six neighboring cells cannot utilize this frequency but the cells beyond the immediate neighbors may.2 In practice, more efficient frequency assignment leads to the

12、“7-cell” reuse pattern shown in Figure 6.1(b). Note that in reality each cell utilizes a group of frequencies.Unit 6-1 移动通信 蜂窝系统 (Cellular system)EE141 2011 Aifeng Ren电子信息科学概论12 电子信息科学概论MTI12The mobile units in each cell of Figure 6.1(b) are served by a base station, and all of the base stations are

13、 controlled by a “mobile telephone switching office” (MTSO).Unit 6-1 移动通信 蜂窝系统 (Cellular system)f1f1f1f1f1f1f1f1f2f1f2EE141 2011 Aifeng Ren电子信息科学概论13 电子信息科学概论MTI13An important issue in a cellular system is how much two cells that use the same frequency interfere with each other. Called co-channel in

14、terference (CCI), this effect depends on the ratio of the distance between two co-channel cells to the cell radius and is independent of the transmitted power. Unit 6-1 移动通信 同信道干扰(Co-channel interference)EE141 2011 Aifeng Ren电子信息科学概论14 电子信息科学概论MTI14Given by the frequency reuse plan, this ratio is ap

15、proximately equal to 4.6 for the 7-cell pattern of Figure 6.1(b). It can be shown that this value yields a signal-to-co-channel interference ratio of 18 dB.Unit 6-1 移动通信 同信道干扰(Co-channel interference)EE141 2011 Aifeng Ren电子信息科学概论15 电子信息科学概论MTI15What happens when a mobile unit “roams” from cell A to

16、cell B? Since the power level received from the base station in cell A is insufficient to maintain communication, the mobile must change its server to the base station in cell B. Furthermore, since adjacent cells do not use the same group of frequencies, the channel must also change. Called “handoff

17、,” this process is performed by the MTSO. Unit 6-1 移动通信 切换(Handoff )EE141 2011 Aifeng Ren电子信息科学概论16 电子信息科学概论MTI16Once the level received by the base station in cell A drops below a threshold, the MTSO hands off the mobile to the base station in cell B, hoping that the latter is close enough. This st

18、rategy fails with relatively high probability, resulting in dropped calls.Unit 6-1 移动通信 切换(Handoff )EE141 2011 Aifeng Ren电子信息科学概论17 电子信息科学概论MTI17To improve the handoff process, second-generation cellular systems allow the mobile unit to measure the received signal level from different base stations,

19、 thus performing handoff when the path to the second base station has sufficiently low loss.Unit 6-1 移动通信 切换(Handoff )EE141 2011 Aifeng Ren电子信息科学概论18 电子信息科学概论MTI18Propagation of signals in a mobile communication environment is quite complex. We briefly describe some of the important concepts here. S

20、ignals propagating through free space experience a power loss proportional to the square of the distance, d, from the source.3Unit 6-1 移动通信 路径损耗和多径衰落EE141 2011 Aifeng Ren电子信息科学概论19 电子信息科学概论MTI19In reality, however, the signal travels through both a direct path and an indirect, reflective path (Figur

21、e 6.2).Unit 6-1 移动通信 路径损耗和多径衰落41dlog (loss)Direct PathReflective PathEE141 2011 Aifeng Ren电子信息科学概论20 电子信息科学概论MTI20It can be shown that in this case, the loss increases with the fourth power of the distance. In crowded areas, the actual loss profile maybe proportional to d2 for some distance and d4 f

22、or another.Unit 6-1 移动通信 路径损耗和多径衰落EE141 2011 Aifeng Ren电子信息科学概论21 电子信息科学概论MTI21In addition to the overall loss profile depicted in Figure 6.2, another mechanism gives rise to fluctuations in the received signal level as a function of distance. Since the two signals shown in Figure 6.2 generally expe

23、rience different phase shifts, possibly arriving at the receiver with opposite phases and roughly equal amplitudes, the net received signal may be very small.4 Unit 6-1 移动通信 路径损耗和多径衰落EE141 2011 Aifeng Ren电子信息科学概论22 电子信息科学概论MTI22Called “multipath fading,” this phenomenon introduces enormous variation

24、s in the signal level as the receiver moves by a fraction of the wavelength. In reality, since the transmitted signal is reflected by many buildings and moving cars, the fluctuations are quite irregular.Unit 6-1 移动通信 路径损耗和多径衰落EE141 2011 Aifeng Ren电子信息科学概论23 电子信息科学概论MTI23The effect of fading can be l

25、owered by adding redundancy to the transmission or reception of the signal. “Space diversity” or “antenna diversity” employs two or more antennas spaced apart by a significant fraction of the wavelength so as to achieve a higher probability of receiving a non-faded signal.5 Unit 6-1 移动通信 分集 (Diversi

26、ty)空间分集空间分集得到几个独立信号,（携带同一信息，不同衰落特性的信号）（不好的衰落特性变成好的）n利用多条路径传输同一信息源信号, 接收端对信号合并，降低多径衰落的影响，z.多个基站同时受到屏蔽效应屏蔽效应的机会减少 （选信号好的基站）路径 1路径 2发射分集处理数据流数据流 1数据流 2恢复数据流“宏分集宏分集” 影响， “微分集微分集”4.2分集方式 （按获得多路信号的方式）1）:空间独立， 2）(1)空间分集空间分集Diversity 3) 天线间距离D米 工程中D为10倍20倍波长。不相关空间的间距越大，多径传播的差异就越大，900M：3m 6m1800M：1.5m 3m1):不

27、同极化电磁波衰落独立, 2): 收发端用两位置位置很近很近但不同极化的天线, 3)：结构紧凑 (2)空间分集(极化极化分集) (Polarization)Diversity receptionEE141 2011 Aifeng Ren电子信息科学概论30 电子信息科学概论MTI30“Frequency diversity” refers to the case where multiple carrier frequencies are used, with the idea that fading is unlikely to occur simultaneously at two freq

28、uencies sufficiently far from each other. “Time diversity” is another technique whereby the data is transmitted or received more than once to overcome short-term fading.Unit 6-1 移动通信 分集 (Diversity) 10 dB-25-20-15-10-505 Time 发射机发射机TR（2）频率分集1):频率选择性2):不同载波3)减少接收天线（相应设备） 缺点:占用频谱多（2）频率分集1):频率选择性2):不同载波

29、3)减少接收天线（相应设备） 缺点:占用频谱多发射机发射机TR窄带系统大衰落发射信号接收到的衰落信号频率频率强度强度大衰落发射信号接收到的衰落信号频率频率强度强度宽带系统频率分集（扩频）频率分集（扩频）（3）时间分集1)时间间隔足够大，两样点衰落互不相关，2)信号隔一定时间重复发送m次， 缺点:占用时隙资源相干时间R分集技术分集技术空间分集空间分集极化分集极化分集频率分集频率分集时间分集时间分集User2跳频、用不同和在不同.得到从不同路径来的同一信号（独立）EE141 2011 Aifeng Ren电子信息科学概论36 电子信息科学概论MTI36Suppose two signals in

30、a multipath environment experience roughly equal attenuation but different delays. This is possible because the absorption coefficient and phase shift of reflective or refractive materials vary widely, making it likely for two paths to exhibit equal loss and unequal delays.Unit 6-1 移动通信 延迟扩展(Delay s

31、pread)EE141 2011 Aifeng Ren电子信息科学概论37 电子信息科学概论MTI37In a multipath environment, many signals arrive at the receiver with different delays, yielding RMS delay spreads as large as several microseconds and hence fading bandwidths of several hundreds of kilohertz.6 Thus, an entire communication channel m

32、ay be suppressed during such a fade.Unit 6-1 移动通信 延迟扩展(Delay spread)EE141 2011 Aifeng Ren电子信息科学概论38 电子信息科学概论MTI38Large delay spreads introduce another difficulty as well: if the delay spread is comparable with the bit period of the digital modulating waveform, then multiple replicas of the signal ar

33、e received with different delays, giving rise to considerable inter-symbol interference.Unit 6-1 移动通信 延迟扩展(Delay spread)EE141 2011 Aifeng Ren电子信息科学概论39 电子信息科学概论MTI39The nature of multipath fading and the signal processing techniques used to alleviate this issue is such that errors occur in clusters

34、of bits.7Unit 6-1 移动通信 交织(Interleaving )EE141 2011 Aifeng Ren电子信息科学概论40 电子信息科学概论MTI40In order to lower the effect of these errors, the baseband bit stream in the transmitter undergoes “interleaving” before modulation. An interleaver in essence scrambles the time order of the bits according to an alg

35、orithm known by the receiver.8 Interleaving can also be viewed as a type of time diversity with no overhead (although it entails some latency).Unit 6-1 移动通信 交织(Interleaving )EE141 2011 Aifeng Ren电子信息科学概论41 电子信息科学概论MTI41 Unit 6-2 第三代无线网络第三代无线网络EE141 2011 Aifeng Ren电子信息科学概论42 电子信息科学概论MTI42video confer

36、encing 电视会议reveal 披露，显示connectivity 互联性deployment 部署downfall 衰败，跌落paging 寻呼converge 收敛，汇集于一点in-building 建筑物内的tolerance 偏差，容差，宽容度pico-cell 微微蜂窝trade off 折衷，妥协undergo 经历substantial 实质性的hybrid 混合的aeronautical 航空的maritime 航海的roll out 展开virtually 事实上，几乎是EE141 2011 Aifeng Ren电子信息科学概论43 电子信息科学概论MTI43The ex

37、pansion of the use of digital networks has led to the need for the design of new higher capacity communications networks. The demand for cellular-type systems in Europe is predicted to be between 15 and 20 million users by the year 2000, and is already over 30 million (1995) in the U.S. Unit 6-2 第三代

38、无线网络EE141 2011 Aifeng Ren电子信息科学概论44 电子信息科学概论MTI44Wireless services have been growing at a rate greater than 50% per year, with the current second-generation European digital systems (GSM) being expected to be filled to capacity by the early 2000s.1 Unit 6-2 第三代无线网络EE141 2011 Aifeng Ren电子信息科学概论45 电子信

39、息科学概论MTI45The telecommunications industry is also changing, with a demand for a greater range of services such as video conferencing, Internet services, data networks, and multimedia. This demand for higher capacity networks has led to the development of third generation telecommunications systems.

40、Unit 6-2 第三代无线网络EE141 2011 Aifeng Ren电子信息科学概论46 电子信息科学概论MTI46One of the proposed third generation telecommunications systems is the Universal Mobile Telecommunications System (UMTS), with the aim of providing more flexibility, higher capacity, and a more tightly integrated service. This section focu

41、ses on the services and aims of the UMTS. Other systems around the world are being developed, however many of these technologies are expected to be combined into the UMTS. Unit 6-2 第三代无线网络EE141 2011 Aifeng Ren电子信息科学概论47 电子信息科学概论MTI47The World Wide Web (WWW) has become an important communications med

42、ia, as its use has increased dramatically over the last few years. This has resulted in an increased demand for computer networking services. In order to satisfy this, telecommunications systems are now being used for computer networking, Internet access and voice communications. Unit 6-2 第三代无线网络EE1

43、41 2011 Aifeng Ren电子信息科学概论48 电子信息科学概论MTI48A WWW survey revealed that more than 60% of users access the Internet from residential locations, where the bandwidth is often limited to 28.8kbps. This restricts the use of the Internet, preventing the use of real time audio and video capabilities. Higher s

44、peed services are available, such as integrated-services digital network (ISDN). These provide data rates up to five times as fast, but at a much increased access cost. Unit 6-2 第三代无线网络EE141 2011 Aifeng Ren电子信息科学概论49 电子信息科学概论MTI49This has led to the demand of a more integrated service, providing fas

45、ter data rates, and a more universal interface for a variety of services. The emphasis has shifted away from providing a fixed voice service to providing a general data connection that allows for a wide variety of applications, such as voice, Internet access, computer networking, etc. Unit 6-2 第三代无线

46、网络EE141 2011 Aifeng Ren电子信息科学概论50 电子信息科学概论MTI50The increased reliance on computer networking and the Internet has resulted in an increased demand for connectivity to be provided “any where, any time”, leading to an increase in the demand for wireless systems. This demand has driven the need to devel

47、op new higher capacity, high reliability wireless telecommunications systems. Unit 6-2 第三代无线网络EE141 2011 Aifeng Ren电子信息科学概论51 电子信息科学概论MTI51The development and deployment of third generation telecommunication systems aim to overcome some of the downfalls of current wireless systems by providing a hig

48、h capacity, integrated wireless network. There are currently several third generation wireless standards, including UMTS, cdmaOne, IMT 2000, and IS-95. Unit 6-2 第三代无线网络EE141 2011 Aifeng Ren电子信息科学概论52 电子信息科学概论MTI52Many mobile radio standards have been developed for wireless systems throughout the wor

49、ld, with more standards likely to emerge. Unit 6-2 第三代无线网络 电信系统的发展（Evolution of telecommunication systems ）EE141 2011 Aifeng Ren电子信息科学概论53 电子信息科学概论MTI53Most first generation systems were introduced in the mid 1980s, and can be characterized by the use of analog transmission techniques, and the use o

50、f simple multiplex access techniques such as Frequency Division Multiple Access (FDMA).2 Unit 6-2 第三代无线网络 电信系统的发展（Evolution of telecommunication systems ）EE141 2011 Aifeng Ren电子信息科学概论54 电子信息科学概论MTI54First generation telecommunications systems such as Advanced Mobile Phone Service (AMPS) only provide

51、d voice communications. They also suffered from a low user capacity, and security problems due to the simple radio interface used. Unit 6-2 第三代无线网络 电信系统的发展（Evolution of telecommunication systems ）EE141 2011 Aifeng Ren电子信息科学概论55 电子信息科学概论MTI55Second generation systems were introduced in the early 1990

52、s, and all use digital technology. This provided an increase in the user capacity of around three times. This was achieved by compressing the voice waveforms before transmission.Unit 6-2 第三代无线网络 电信系统的发展（Evolution of telecommunication systems ）EE141 2011 Aifeng Ren电子信息科学概论56 电子信息科学概论MTI56Third genera

53、tion systems are an extension on the complexity of second generation systems and are expected to be introduced after the year 2000. The system capacity is expected to be increased to over ten times original first generation systems. This is going to be achieved by using complex multiplex access tech

54、niques such as Code Division Multiplex Access (CDMA), or an extension of TDMA, and by improving flexibility of services available. Unit 6-2 第三代无线网络 电信系统的发展（Evolution of telecommunication systems ）EE141 2011 Aifeng Ren电子信息科学概论57 电子信息科学概论MTI57Table 6.1 and Table 6.2 show some of the major cellular mob

55、ile phone standards in North America and Europe. Figure 6.3 shows the evolution of current services and networks to the aim of combining them into a unified third generation network. Unit 6-2 第三代无线网络 电信系统的发展（Evolution of telecommunication systems ）Analog CellularRadio PagingDigital CellularPrivate R

56、adioPCNsMobile satelliteCordless TelephonyThird Generation Wireless NetworkEE141 2011 Aifeng Ren电子信息科学概论58 电子信息科学概论MTI58Many currently separate systems and services such as radio paging, cordless telephony, satellite phones, private radio systems for companies, etc., will be combined so that all the

57、se services will be provided by third generation telecommunications systems.3Unit 6-2 第三代无线网络 电信系统的发展（Evolution of telecommunication systems ）5960EE141 2011 Aifeng Ren电子信息科学概论61 电子信息科学概论MTI61The main aims of the Universal Mobile Telecommunications System is to provide a more unified high capacity ne

58、twork, in wireless and wired environments. UMTS will enable fixed and wireless services to converge. They are to be three main channel capacity connections: a mobile rate of 144kbps, a portable rate of 384kbps and an in-building rate of 2Mbps. Unit 6-2 第三代无线网络 UMTS的总体目标 （Overall aims of UMTS ）EE141

59、2011 Aifeng Ren电子信息科学概论62 电子信息科学概论MTI62It will have the capacity to provide services and features requiring less than 2Mbps that would otherwise have been provided with a fixed network.4Unit 6-2 第三代无线网络 UMTS的总体目标 （Overall aims of UMTS ）EE141 2011 Aifeng Ren电子信息科学概论63 电子信息科学概论MTI63UMTS must therefore

60、 provide on-demand, variable bandwidth allocation. It will also combine a range of applications including cordless phones, cellular phones, mobile data networking, for personal, business and residential uses.5 Unit 6-2 第三代无线网络 UMTS的总体目标 （Overall aims of UMTS ）EE141 2011 Aifeng Ren电子信息科学概论64 电子信息科学概论