通信系统原理第一章概论

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,通信系统原理,任课老师：单志龙,前期课程,信号与系统,数字电子技术,高等数学,会喝酒,。,第一章 概论,1.1 现代通信技术及其发展,1.2 通信系统的构成,1.3 通信系统分类,1.4 通信系统的性能度量,1.1 现代通信技术及其发展,什么是通信？日常生活中的通信有哪些？,通信,（,communication,）,的目的是实现信息的传递。,The fundamental purpose of an electronic communications system is to,transfer information from one place to another,。,现代通信技术是指用现代科学技术手段如电子、光技术实现信息传递的一门技术学科。现代通信技术以电子技术为其基本手段，实际上应称为,电通信,，简称为,通信,。,为什么要学这门课程？,你想进中国移动吗？,想！那就要好好学这门课程！,不想！也要好好学习这门课程！因为,计算机网与通信网（包括有线、无线网络）的结合是一个趋势,物联网,1838年 摩尔斯发明有线电报并于1844年建立了第一条试验型电报线路。,1876年,贝尔,发明有线电话。,1898年 发明无线电报技术。,1906年 真空电子管出现并首先应用于通信。,1918年 出现无线电广播。,1938年 电视广播开始。,1940年 研制出雷达与微波通信系统。,通信发展历程,1946年 第一台电子管计算机诞生。,1948年 香农提出信息概念，奠定了经典信息论基础。,1948年 发明了晶体管，这是电子器件领域内的重大革命。,1956年 第一根越洋（大西洋）海缆建成。,1958年 第一颗通信卫星上天。,1961年 发明集成电路，它的问世对通信技术的发展产生巨大的影响。,通信发展历程（续）,1962年 第一颗同步通信卫星上天，1965年第一颗国际通信卫星进入商业运行，并在70年代开始广泛地进入商业应用。,1960 年激光器问世，1977年底第一个光纤通信系统投入使用。,70年代以来，大规模及超大规模集成电路在通信领域内应用，计算机技术与通信技术的结合。,80年代到90年代，通信网现代化,网络程控化、数字化、智能化以及各种网络技术,如ISDN网Internet（IP网）、移动通信网、宽带接入网等。,通信发展历程（续）,Alexander Graham Bell (1876),Mr.Watson, come here, I want you.,1.2 通信系统的构成,一个完整的点对点通信系统如下图所示：,通信的,目的,是为了传递消息，消息可以是语言、文字、符号、数据或是图像这些表征媒体的集合。,消息在通信系统中是被转换成电信号的形式传送的。将这些消息转变成电信号的装置，我们称之为,信源,。,在接收端完成相反功能的装置则称之为,信宿,。,信源与信宿,信道,信号传输需要通过信道。狭义的信道指的是传输媒介，如电缆、光纤、无线电波传播的空间等。,为了使信号能够适应信道的特性，在发送端及接收端均需要有相应的发送和接收设备。,针对不同的信道特性，相应的收发设备的技术特点及实现手段是不同的。这样便形成了各种不同的通信系统及不同的技术体制。,要传送的消息信号之外的所有干扰信号，统称为,噪声,。噪声方框是集中加在信道中的。,模拟通信系统是按照模拟信号的特点设计的，主要用来传输模拟信号。其,基本特点,是强调发送的信号波形在收端无失真地恢复。,数字通信系统传输的是数字信号，其关注的是数字符号状态的正确识别。,模拟与数字通信系统,模拟通信系统组成方框图,采用什么样的调制解调技术将对系统性能产生重要影响，是通信原理课程研究的重点内容之一。,数字通信系统组成方框图,数字通信系统一般具有信源编码、信道编码及相应的解码部分。,信源编码,的主要功能是将模拟信号转换为数字编码信号。,所谓,信道编码,，是指数字信号为了适应信道的传输特性，达到高效可靠的传输而进行的相应的信号处理过程。如数字信号的加密处理、差错控制编码及扩频编码等。,实际的数字通信系统中，上述各部分不一定全部具备。,数字通信系统组成方框图,1.2.3 数字通信系统的主要优点,可通过信道编码进一步改善传输质量；,数字信号便于进行信号处理，有利于新技术的实现，可提高通信系统的综合性能,；,数字信号便于不同种类信号的综合，适合各种新型业务的拓展,；,抗噪声性能好，可避免,噪声积累,；,数字信号波形的再生,数字通信系统的主要缺点,技术较为复杂，数字通信系统还需要同步技术来保证，这也增加了系统的复杂性。,数字信号一般都要占用较宽的带宽。,随着大规模、超大规模电路技术的发展，计算机技术的广泛应用，上述的缺点也不再成为问题。,1.3 通信系统分类,1.3.2 通信方式,1.3.2.1 按通信的方向及时间划分：,单向通信-也称为单工通信,双向通信：,半双工通信,全双工通信,Simplex (单工),e.g. Television,Half Duplex (半双工),Transmission in one direction only,e.g. police radio,Simplex (单工),Full Duplex (全双工),Transmission in one direction only,Transmission in both directions at the same time,e.g. telephone,Half Duplex (半双工),Simplex (单工),1.3.2.2 并行通信和串行通信,Two basic ways to transfer binary information from one place to another in data transmission.,Parallel（并行）,Serial（串行）,Parallel multiple wires allow one bit on each wire,Serial bits travel on the wire one after another,Parallel Transmission,In parallel data transmission, the bits of a code word are transmitted,simultaneously,.,Parallel data transmission is extremely fast since all the bits of the data word are transmitted simultaneously.,Parallel data transmission is not practical for long distance communication,Parallel transmission,Data transfers in communication systems are made serially. Each bit of the word is transmitted one after the another.,Because of the sequential nature of serial data transmission, naturally it takes longer time to transmit the data than parallel data transmission .,Serial Transmission,Serial transmission,In most computers and other digital equipments, the binary data is transferred between circuits(电路) in parallel format.,When it is transmitted over long distances, it is sent in serial format,Serial and Parallel,1.3.2.3 按通信对象连接划分,Line configuration refers to the way two or more communication devices attach to a link.,A link is the physical communication pathway that transfers data from one device to another.,There are two line configurations:,Point-to-Point (,点对点,),Multipoint,(,多点,),1.3.2.3.1 Point-to-Point,A point-to-point line configuration provides a dedicated link (专线) between two devices.,The entire capacity of the channel is reserved（保留） for transmission between those two devices.,Point-to-Point Connection,.2,Multipoint,A multipoint is one in which more than two specific devices share a single link.,In a multipoint environment, the capacity of the channel is shared, either spatially(空间) or temporally（时间）.,If several devices can use the link simultaneously, it is a spatially shared line configuration.,If users must take turns, it is a time-shared line configuration.,Multipoint Line Configuration,The topology of a network is the geometric representation (几何表示) of the relationship of all links and linking devices to one other.,Two or more devices connect to a link; two or more links form a topology.,Topology (拓扑),There are five basic topologies:,Mesh (,网形,),Star (,星形,),Tree (,树形,),Bus (,总线形,),Ring (,环形,),Topology (拓扑),Mesh Topology,In a mesh topology, every device has a dedicated point-to-point link to every other device.,The term dedicated means that the link carries traffic only between the two devices it connects.,Mesh Topology,The use of a dedicated links guarantees（保证） that each connection can carry its own load, thus eliminating（消除） the traffic problems that can occur when links must be shared by multiple devices.,A mesh topology is robust（鲁棒）. If one link becomes unusable, it does not incapacitate (不能胜任) the entire system.,Advantages,Advantages,Privacy (隐私). When every message sent travels along a dedicated link, only the intended recipient(接收者) sees it. Physical boundaries prevent other users from gaining access to messages.,Point-to Point links make fault identification (鉴别) and fault isolation (隔离) easy.,A big amount of cabling and a big number of I/O ports required, i.e, the hardware required can be very expensive.,Because every device must be connected to every other device, installation and reconfiguration are difficult.,Disadvantages,For these reasons a mesh topology is usually implemented in a limited fashion.,For example, as a backbone connecting the main computers to a,hybrid network,(混合网络) that can include several other topologies.,Disadvantages,Star Topology,In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub (集线器).,A star topology does not allow direct traffic between devices.,If one device wants to send data to another, it sends the data to the controller, which then relays (转播) the data to the other connected device.,Star Topology,Advantages,It is easy to install and reconfigure,The star topology is robust. If one link fails, only that link is affected.,This factor also lends itself to easy fault identification (鉴别) and fault isolation (隔离).,A big amount of cabling and a big number of I/O ports required, i.e, the hardware required can be very expensive.,Note,: Star topology requires far less cable than Mesh topology.,Disadvantages,Tree Topology,A tree topology is a variation（变种）of a star.,As in the star, nodes in a tree topology are linked to a central hub that controls the traffic to the network.,The majority of devices connect to a secondary hub that in turn is connected to a central hub.,Tree Topology,Tree Topology,The advantages and disadvantages of a tree topology are generally the same as those of a star.,A good example of tree topology can be seen in,cable TV technology,where the main cable from the main office is divided into main branches and each branch is divided into smaller branches and so on. The hubs are used when a cable is divided.,Bus Topology,In a bus topology, one long cable acts as a backbone to link all the devices in the network.,Nodes are connected to the bus cable by drop lines and taps (分接头).,A drop line is a connection running between the device and the main cable.,Bus Topology,When a signal travels along the backbone, some of its energy is transformed into heat.,Therefore, it becomes weaker and weaker the farther it has to travel.,For this reason there is a limit on the number of taps a bus can support and on the distance between those taps.,Bus Topology,Advantages,Easy to install,Bus topology uses less cabling than mesh, star, or tree topologies.,Disadvantages,Difficult reconfiguration and fault isolation.,A bus is usually designed to be optimally efficient at installation. It can therefore be,difficult to add a new devices,.,A fault or break in the bus cable stops all transmission,Ring Topology,In a ring topology, each device has a dedicated point-to-point line configuration only with the two devices on either side of it.,Ring Topology,Advantages,Easy to install and reconfigure. To add or delete a device require only two connections.,Easy fault isolation,Disadvantages,A break in the ring (such as a disabled station) can disable the entire network.,Hybrid Topologies,Often a network combines several topologies as sub networks linked together in a larger topology.,1.3.3 通信使用的频段,通信系统中，信号的传输是工作在一定的频段的。,在有线通信中，每一种传输媒介都有自己特定的频率特性，这主要取决于该媒介的材质、物理尺寸、生产工艺等。,在无线通信中，信号是以电磁波的形式通过空间传播的，不同的频段，其传输特性也是不尽相同。适用于不同工作频段的通信应用系统。,Frequency,(,频率,) is simply the number of times a periodic motion, such as a sine wave of voltage or current, occurs in a given period of time.,Each complete alternation of the waveform is called a,cycle,(,循环,).,Frequency,The basic unit of frequency is,hertz,(,赫兹,Hz) and one hertz equals one cycle-per-second (1Hz=1 cps).,It is common to use metric prefixes (,前缀,) to represent higher frequencies.,E.g. kHz (kilohertz) is used for thousands of hertz, MHz (megahertz) is used for millions of hertz.,Frequency,Spectrum (频谱),The,electromagnetic spectrum,(usually just,spectrum,频谱,) is the range of all possible electromagnetic radiation that it emits, reflects (,反射,), or transmits.,Spectrum,is the,range of frequencies contained in a signal.,频谱用来表示一个信号的各频率分量的分布规律,。,The frequency spectrum is further divided into subsections or bands. Each band has a name and boundaries.,Electromagnetic Spectrum,Transmission Frequencies,ELF（极低频）：30Hz-300Hz,VF（话音频率）：300Hz-3000Hz,VLF（甚低频 ）：3KHz-30KHz Specialized government and military systems,LF（低频 ）：30KHz-300KHz,Marine (海运) and aeronautical (航空) navigation,MF（中频,）：,300KHz-3MHz,Commercial AM radio broadcasting,HF（高频,）：,3MHz-30MHz,Most two-way radio communications,Transmission Frequencies,VHF（甚高频,）：,30MHz-300MHz,Mobile radio, marine and aeronautical, FM radio, commercial TV,VHF（特高频,）：,300MHz-3GHz,Cellular phone, radar, microwave and satellite radio systems,SHF（超高频,）：,3GHz-30GHz,microwave and satellite radio systems,EHF（极高频,）：,30GHz-300GHz,Infrared（红外）：0.3THz-300THz,Visible light（可见光）：0.3PHz-3PHz,Wavelength (波长),Distance occupied by one cycle,Distance between two points of corresponding phase in two consecutive cycles,Assuming signal velocity,v, =,cT,f,=,c,c,3*10,8,ms,-1,(speed of light in free space,299792458 m/s,),Electromagnetic Spectrum,Example,Example 1-1:,Determine the wavelength in meters for the following frequencies: 1kHz，100kHz和10kHz。,Solution:,对一个通信系统，从研究消息的传输来说，,有效性和可靠性,将是主要的两个指标，这也是通信技术讨论的重点。,通信系统的有效性和可靠性，是一对矛盾。一般情况下，要增加系统的有效性，就得降低可靠性，反之亦然。,在实际中，常常依据实际系统要求采取相对统一的办法，即在满足一定可靠性指标下，尽量提高消息的传输速率，即有效性；或者，在维持一定有效性条件下，尽可能提高系统的可靠性。,1.4 通信系统的性能度量,通信系统的性能度量（续）,对于模拟通信来说，系统的有效性和可靠性具体可用系统有效带宽和输出,信噪比,（或均方误差）来衡量。模拟系统的有效传输带宽越大，系统同时传输的话路数也就越多，有效性就越好。,对于数字通信系统而言，系统的有效性和可靠性具体可用传输速率和误码率来衡量。,数字通信系统的主要性能指标,1,.,有效性指标,码元传输速率,：,码元传输速率通常又可称为码率，它指的是指单位时间（每秒钟）内传输码元的数目，单位为波特（,Baud,），常用符号“,B”表示（注意，不能用小写）,数字信号一般有二进制与多进制之分，但码元速率,与信号的进制数无关，只与码元宽度 有关。,有效性指标,信息传输速率,信息传输速率简称信息速率，又可称为传信率、比特率等。,信息传输速率用符号,R,b,表示，指单位时间（每秒钟）内传送的信息量。,单位为比特,/,秒（,bit/s,），简记为,b/s,或,bps,。,是一比特单位时间的倒数，,Date rate = 1 / T,b,Bit Rate and Baud,Illustration（说明）,Assume a system that represents 2 bits of data as different voltage levels. So there are four possible combination of 2 bits exists,00 0 volts 10 2volts,01 1 volts 11 3volts,数字通信系统的主要性能指标,比特,R,b,与波特,R,B,之间可以互换,在二进制中，码元速率,R,B,同信息速率,R,b,的关系在数值上相等，但单位不同。,在多进制中，码元速率,R,B,同信息速率,R,b,的关系在数值上和单位上都不同。,它们之间在数值上有如下关系式,数字通信系统的主要性能指标,频带利用率,在比较不同通信系统的效率时，只看它们的传输速率是不够的，还应看在这样传输速率下所占的信道的频带宽度。,因为传输速率越高，所占用的信道频带越宽，因此，能够真正体现出信息的传输效率的指标应该是频带利用率，即单位频带内的传输速率。,= R,B,/B （Baud/Hz）,对二进制传输可表示为,= R,b,/B （bps/Hz）,数字通信系统的主要性能指标,2,.可靠性指标,码元差错率,简称误码率，它是指接收错误的码元数在传送总码元数中所占的比例。,更确切地说，误码率就是码元在传输系统中被传错的概率。用表达式可表示成,数字通信系统的主要性能指标,2,.可靠性指标,信息差错率,简称误信率，或误比特率，它是指接收错误的信息量在传送信息总量中所占的比例.,或者说，它是码元的信息量在传输系统中被丢失的概率。用表达式可表示成,Logarithmic Function for S/N,The S/N is often expressed as a logarithmic function with the decibel (分贝) unit.,S/N can also be expressed in terms of voltages and resistances (,电阻,).,dB,dB是一个表征相对值的值，当考虑甲的功率相比于乙功率大或小多少个dB时，按下面计算公式：,10lg (甲功率/乙功率),Eg. 3 甲功率比乙功率大一倍，那么10lg（甲功率/乙功率）=10lg2=3dB。也就是说，甲的功率比乙的功率大3 dB。,Eg. 4 如果甲的功率为46dBm，乙的功率为40dBm，则可以说，甲比乙大6 dB。,dBm,dBm是一个考征功率绝对值的值，计算公式为：10lgP (功率值/1mw)。,Eg. 1 如果发射功率P为1mw，折算为dBm后为0dBm。,Eg. 2 对于40W的功率，按dBm单位进行折算后的值应为：,10lg (40W/1mw)=10lg (40000),=10lg4+10lg10000=46dBm。,