双工无线语音数据传输系统外文翻译.doc

双工无线语音数据传输系统的设计Triple wireless voice data transmission system designStudent : Liu figure, physics and information engineering collegesInstructor : Xiao Yun hong, Jianghan University Every day, in our work and in our leisure time, we come in contact with and use a variety of modern communication media, the most common being the telephone, radio, television, and the Internet. Though these media we are able to communicate (nearly) instantaneously with people on diffident continents, transact our daily business, and receive information about various developments and events of note that occur all around the world. Electronic mail and facsimile transmission have made it possible to rapidly communicate written message across great distances. Wireless communications. The development of wireless communications stems from the works of Oersted, Faraday, Gauss, Maxwell, and Hertz. In1820, Oersted demonstrated that an electric current produces a magnetic field. On August 29,1831,Michael Faraday showed that an induced current is produced by moving a magnet in the vicinity of a conductor. Thus, he demonstrated that a changing magnetic field produces an electric field. With this early work as background, James C. Maxwell in 1864 predicted the existence of electromagnetic radiation and formulated the basic theory that has been in use for over a century. Maxwells theory was verified experimentally by Hertz in 1887. In 1894, a sensitive device that could device that could detect radio signals, called the coherer, was used by its inventor Oliver Lodge to demonstrate wireless communication over a distance of 150 yards at Oxford, England. Guglielmo Marconi is credited with the development of wireless telegraphy. Marconi demonstrated the transmission of radio signals at a distance of approximately 2 kilometers in 1895. Two years later, in 1897 , he patented a radio telegraph system and established the Wireless Telegraph and Signal Company. On December 12, 1901, Marconi received a radio signal at Signal Hill in Newfoundland, which was transmitted from Cornwall, England, a distance of about 1700 miles. The invention of the vacuum tube was especially instrumental in the development of radio communication system .The vacuum diode was invented by Fleming in 1904 and the vacuum triode amplifier was invented by De Forest in 1906, as previously indicated. The invention of the triode made radio broadcast possible in the early part of the twentieth century. Amplitude modulation (AM) broadcast was initiated in 1920 when radio station KDKA, Pittsburgh, went on the air. From that date, AM radio broadcasting grew rapidly across the country and around the world. The super heterodyne AM radio receiver, as we know it today, was invented by Edwin Armstrong during World War I. Another significant development in radio communications was the invention of Frequency modulation (FM), also by Armstrong. In 1933, Armstrong built and demonstrated the first FM communication system. However, the use of FM was slow to develop compared with AM broadcast. It was not until the end of World War II that FM broadcast gained in popularity and developed commercially. The first television system was built in the United States by V. K. Zworykin and demonstrated in 1929. Commercial television broadcasting began in London in 1936 by the British Broadcasting Corporation(BBC) . Five years later the Federal Communications Commission(FCC) authorized television broadcasting in the United States. ELEMENTS OF AN ELECTRICAL COMMUNICA SYSTEM Electrical communication systems are designed to send messages or information from a source that generates the message to one more destinations. In general, a communication system can be represented by the functional block diagram shown . The information generated by the source may be of the form of voice (speech source), a picture (image source), or plain text in some particular language, such as English , Japanese, German , French, etc. An essential feature of any source that generates information is that its output is described in probabilistic terms; i.e., the output of a source is not deterministic. Otherwise, there would be no need to transmit the message. A transducer is usually required to convert the output of a source into an electrical signal that is suitable for transmission. For example, a microphone serves as the transducer that converts an acoustic speech signal. At the destination, a similar transducer is required to convert the electrical signals that are received into a form that is suitable for the user; e.g., acoustic signals, images, etc. The heart of the communication system consists of three basic parts, namely, the transmitter, the channel, and the receiver. The functions performed by these three elements are described next.The Transmitter. The Transmitter converts the electrical signal into a form that is suitable for transmission though the physical channel or transmission medium. For example, in radio and TV broadcast, the Federal Communications Commission (FCC) specifies the frequency range for each transmitting station. Hence, the transmitter must translate the information signal to be transmitted into the appropriate The Transmitter range that matches the frequency allocation assigned to the transmitter. Thus, signal transmitted by multiple radio station do not interfere with one another. Similar functions are performed in telephone communication systems where the electrical speech signals from many users are transmitted over the same wire. In general, the transmitter performs the matching of the message signal to the channel by a process called modulation. Usually, modulation involves the use of the information signal to systematically vary either the amplitude, frequency, or phase of a sinusoidal carrier. For example, in AM radio broadcast, the information signal that is transmitted is contained in the amplitude variations of the sinusoidal carrier, which is the center frequency in the amplitude modulation. In FM radio broadcast., the information signal that is transmitted is contained in the frequency variations of the sinusoidal carrier. This is an example of frequency modulation. Phase modulation (PM) is yet a third method for impressing the information signal on a sinusoidal carrier.In general, carrier modulation such as AM, FM, and PM is performed at the transmitter, as indicated above, to convert the information signal to a form that matches the characteristics of the channel. Thus, though the process of modulation, the choice of the type of modulated in frequency to match the allocation of the channel. The choice of the type of modulation is based on several factors, such as the amount of bandwidth over the channel, the type of noise and the interference that the signal encounters in transmission. In any case, the modulation process makes it possible to accommodate the transmission of multiple messages from many users over the same physical channel.In addition to modulation, other functions that are usually performed at the transmitter are filtering of the information-bearing signal , amplification of the modulated signal, and in case of wireless transmission, radiation of the signal by means of a transmitting antenna.The channel. The communications channel is the physical medium that is used to send the signal from the transmitter to the receiver. In wireless transmission, the channel is usually the atmosphere (free space). On the other hand, telephone channels usually employ a variety of physical media, including wirelines, optical fiber cables, and wireless (microwave radio). Whatever the physical medium for signal transmission, the essential feature is that the transmitted signal is corrupted in a random manner by a variety of possible mechanisms. The most common from of signal degradation comes in the form of additive noise ,which is generated at the front end of the receiver, where signal amplification is performed. This noise is often called thermal noise. In wire less transmission, additional additive disturbances are man-made noise, and atmospheric noise picked up by a receiving antenna. Automovile ignition noise is an example of man-made noise, and electrical lightning discharges from thunderstorms is an example of atmospheric noise. Interference from other users of the channel is another form of additive noise that often arises in both wireless and wire line communication systems .In some radio communication channels, such as the ionospheric channel that is used for long range ,short-wave radio transmission, another form of signal degradation is multipath propagation. Such signal distortion is characterized as a nonadditive signal disturbance which manifests itself as time variations in the signal amplitude, usually called fading .Both additive and nonadditive signal distortions are usually characterized as random phenomena and described in statistical terms. The effect of these signal distortions must be taken into account on the design of the communication system. In the design of a communication system, the system, the system designer works with mathematical models that statistically characterize he signal distortion encountered on physical channels. Often, the statistical description that is used in mathematical model is a result of actual empirical measurements obtained from experiments involving signal transmission over such channels .In such cases , there is a physical justification for the mathematical model used in the design of communication systems. On the other hand, in some communication system designs ,the statistical characteristics of the channel may vary significantly with time. In such cases, the system design may designer may design a communication system that is robust to the variety of signal distortions. This can be accomplished by having the system adapt some of its parameters to the channel distortion encountered. The receiver. The function of the receiver is to recover the message signal contained in the received signal. If the message signal is transmitted by carrier modulation, the receiver performs carrier demodulation in order to extract the message from the sinusoidal carrier. Since the signal demodulation is performed in the presence of additive noise and possibly other signal distortion, the demodulated message signal is generally degraded to some extent by the presence of these distortions in the received signal. As we shall see, the fidelity of the additive noise, the type and strength of any other additive interference, and the type of any nonadditive interference. Besides performing the primary function of signal demodulation, the receiver also performs a number of peripheral functions, including signal filtering and noise suppression.Digital Communication SystemAn electrical communication system in rather broad terms based on the implicit assumption that message signal is a continuous timevarying waveform. We refer to such continuous-time signal waveforms as analog sources. Analog signal can be transmitted directly via modulation over the communication channel and demodulated accordingly at the receiver. We call such a i communication system an analog communication system.Alternatively, an analog source output may be converted into a digital form and the message can be transmitted via digital modulation as a digital signal at the receiver. There are some potential advantage to transmitting an analog signal by means of digital modulation. The most important reason is that signal fidelity is better controlled though digital transmission than analog transmission. In particular, digital transmission allows us to regenerate the digital signal in long-distance transmission, thus eliminating effects of noise at each regeneration point. In contrast, the noise added in analog transmission is amplified along with the signal when amplifiers are used periodically to boost the signal level in long-distance transmission. Another reason for choosing digital transmission over analog is that the analog message signal may be highly redundant. With digital processing, redundancy may be removed prior to modulation, thus conserving channel bandwidth. Yet a third reason may be that digital communication systems are often cheaper to implement.In some applications, the information to be transmitted is inherently digital; e.g., in the form of English text, computer data, etc. In such cases, the information source that generates the data is called a discrete (digital)source. In a digital communication systems , the some applications, the functional operations performed at the transmitter and receiver must be expanded to include message signal discrimination at the transmitter and message signal synthesis or interpolation at the receiver. Additional functions include redundancy removal, and channel coding and decoding. The source output may be either an analog signal, such as audio or video signal, or a digital signal , such as the output of a computer which is discrete in time and has a finite number of output characters. In a digital communication system, the message produced by the source are usually converted into a sequence of binary digits as possible. In other words, we seek inefficient representation of the source output of either an analog or a digital source into a sequence of binary digits is called source encoding or date compression. The sequence of binary digits from the coerce encoder, which we call the information sequence is passed to the channel encoder. The purpose of the channel encoder is to introduce, in a controlled manner, some redundancy in binary information sequence which can be used at the receiver to overcome the effects of noise and interference encountered in the transmission of the signal though the channel. Thus the added redundancy serves to increase the reliability of the received data and improves the fidelity in decoding the deceived signal. In fact, redundancy serves in the information sequence aids the receiver in decoding the desired information sequence .The binary sequence at the output of the channel encoder is passed to the digital modulator, which servers as the interface to the communications channel. Since nearly all of the communication channels encountered in practice are capable of transmitting electrical signals (waveforms), the primary purpose of the digital modulator is to map the binary information sequence into signal waveforms.At the receiving end of a digital communication system, the digital demodulator processes the channel-corrupted transmitted waveform and reduces reduce each waveform to a signal number that represents an estimate of the transmitted data symbol (binary or Mary) . When there is no redundancy in the transmitted information, the demodulator must decide which of the M waveform was transmitted in any given time interval. A measure of how well the demodulator and encoder perform is the frequency with which errors occur in the decoded sequence. As a final step, when an analog output is desired, the source decoder accepts the output sequence from the channel and , from knowledge of the source-encoding method used, attempts to reconstruct the original signal from the source. 双工无线语音数据传输系统的设计学生：X X X，物理与信息工程学院指导老师：X X X，X X大学在日常的工作和生活中，人们每天都要接触和使用大量的现代通信系统和通信媒介，其中最常见的是电话，无线电广播，电视和因特网。 通过这些媒介，生活在地球各地的人们可可以随时进行通信，处理日常事务，通晓天下大事，了解世界发展。 而电子邮件和传真则使得文字信息的远距离快速传递成为现实。无线通信。无线通信的出现归功于噢斯特（Oersted ），法拉第（Faraday），高斯（Gauss），麦克斯韦（Maxwell）和 赫兹（Hertz）等人的研究工作。1820年噢斯特证明了电流能够产生磁场；1831年8月29日，法拉第演示了字一个导体的附近移动磁体产生出感应电流，随后他证明了电磁辐射的存在，他所建立的基本理论，已经使用了一个多世纪；麦克斯的理论由赫兹在1887年经实验所证明。1894年，噢利弗-洛奇（Oliver Lodge）发明了一种称为金属屑检测器的灵敏器件，用于检测无线信号，他在英国的牛津使用这种灵敏器件演示了在150码的距离上进行无线通信；马克尼（Marconi）的名字则与无线电报密切相关，他于1895年演示了在大约2英里距离上进行无线信号传输，1897年马可波罗又申请了无线电报系统的专利，并建立了无线电报和信号公司。1901年12月12日，马克尼在纽芬兰的信号山收到了发自英国康沃尔的无线信号，传输距离约1700英里。真空电子管的发明，对无线通信系统的发展所起的作用尤其突出。如前所述，弗莱明（Fleming）在1904年发明了真空二极管，德福雷斯特在1906年发明了真空三极管放大器。三极管的发明使得无线电广播在20世纪初期成为现实，1920年在匹兹堡的KDKA无线电台将调幅（AM）广播信号送进了空中，从那时起，调幅无线电广播在英国及世界各地迅速发展。今天我们所熟知的超外差式调幅无线接收机，则是第一次世界大战期间由埃德温-阿姆斯壮（Edwin Armstrong）发明的。无线通信的另一个重大进展是频率调制（FM）的出现，它也是阿姆斯壮最早提出的，1933年阿姆斯壮建立并演示了第一个FM通信系统。不过，与AM广播相比，FM的使用发展缓慢，直到二战结束时FM广播开始流行并进入商用系统。第一个电视系统由V.K.Z 于1929年在美国建立并进行演示试验，伦敦的英国广播公司（BBC）于1936年开始商业电视广播，五年后联邦通信委员会（FCC）批准了美国的电视广播。电通信系统的基本组成电通信系统的作用，是将产生消息的信源信息发送到一个或多个目的地。一般情况下，通信系统可以用功能框图进行表示。信源所产生的信息可以是声音（语音图像），图像（影像源），或以某些特殊语言如英语，日语，德语，法语等写成的纯文本。产生信息的任何一个信源，都有一个基本的特征，即它的输出是以概率参量描述的，也就是说一个信源的输出是不确定的，否则信息传输就失去了意义。变换器通常用于将信源的输出变换成适合传输的电信号。例如，用做变换器的话筒，可以将声频的语音信号变换成电信号，而摄像机则将图像信号变换成电信号。在接收端，使用一个类似的变换器将收到的电信号变换成适合用户的形式，如声响信号，图像等。通信系统的核心由三个部分构成，即发信机，信道和接收机。这三个基本组成部分完成的功能如下所述。发信机。发信机将电信号变换成适合物理信道或其他传输介质传输的形式。例如，在无线电和电视广播中，联邦通信委员会（FCC）指定的各个发射台的频率范围，因此发信机必须将信息信号转换到合适的频率范围来发送，以便与分配给此发信机的频率相匹配。这样，由多个无线电台发送的信号就不会彼此干扰。电话通信系统中也需要实现类似的功能，从而让来自许多用户的电语言信号在同一条电线上传输。一般而言，发信机通过一个所谓的调制过程来实现信号与信道的匹配。通常，调制需要使用信息信号来系统地改变正弦载波的振幅，频率或相位分量。例如，在AM无线电广播中，发送的信息包含正弦载波的振幅变换中，此载波是分配给无线电发射台的频带的中心频率，这是一个振幅调制的例子；在FM无线电广播中，发送的信息信号包含在正弦载波的频率变化中，而这是一个频率调制的例子；相位调制则是将信息信号记载在正弦载波上的第三种调制方法。如上所述，诸如AM，FM和PM之类的载波调制，通常是由发信机通过将信息信号变换成与信道特性相匹配的信号形式来完成的。这样通过调制过程，信息信号在频率上发生了改变（搬移），以便与所分配的信道相匹配。调制类型的选择主要基于几个因素，如所分配的带宽，信号在信道传输中遇到的噪声及干扰的类型，在发送前进行信号放大所能采用的电子器件等等。总之，调制过程使得来自许多用户的多重信息能够在同一物理信道上同时发送。除了调制，其他一些通常由发信机完成的功能还包括：信息信号的滤波，已调信号的放大，以及在无线电通信信号通过发射天线完成的辐射等等。信道。通信信道是一种物理介质，用于将来自发信机的信号发送到接收机。在无线传输中，信道通常是大气层（自由空间）；电话信道则可以采用多种物理介质，包括电线，光缆和无线电（微波）信道。不管哪一种用于信号传送的物理介质，都有一