2 Introduction Chapter 1 TABLE 1.1 THE MORSE CODE A N B 0 P D Q 1 E R 3 ，。 3 G 入 H 4 U V 6 J W 7 K X 8 。。 Y 9 M Z 0 (a)Letters (b)Numbers Period(.) Wait sign (AS) Comma(,) Double dash (break) Interrogation(?) Error sign 。。。44.4 Quotation Mark (" 一+”一” Fraction bar(/) Colon (: End of message(AR) Semicolon(:) End of transmission(SK) Parenthesis() (c)Punctuation and special characters Nearly 40 years later,in 1875,Emile Baudot developed a code for telegraphy in which each letter was encoded into fixed-length binary code words of length 5.In the Baudot code,the binary code elements have equal length and are designated as mark and space. An important milestone in telegraphy was the installation of the first transatlantic cable that linked the United States and Europe in 1858.This cable failed after about four weeks of operation.A second cable was laid a few years later and became operational in July 1866. Telephony came into being with the invention of the telephone in the 1870s.Alexan- der Graham Bell patented his invention of the telephone in 1876;in 1877,established the Bell Telephone Company.Early versions of telephone communication systems were rela- tively simple and provided service over several hundred miles.Significant advances in the quality and range of service during the first two decades of the twentieth century resulted from the invention of the carbon microphone and the induction coil
2 TABLE 1.1 THE MORSE CODE A B c D E F G H I J · --- K L M Period (.) Comma (,) Interrogation (?) Quotation Mark (") Colon (:) Semicolon (;) Parenthesis ( ) N 0 p Q R s T u v w x y z (a) Letters - -- - -· - 2 3 4 5 6 7 8 -·- - 9 --· · 0 Wait sign (AS) Double dash (break) Error sign Fraction bar (/) Introduction ·---- · ·-- - ···-- --· ·· ---· · ----· ----- (b) Numbers End of message (AR) End of transmission (SK) (c) Punctuation and special characters Chapter 1 Nearly 40 years later, in 1875, Emile Baudot developed a code for telegraphy in which each letter was encoded into fixed-length binary code words of length 5. In the Baudot code, the binary code elements have equal length and are designated as mark and space. An important milestone in telegraphy was the installation of the first transatlantic cable that linked the United States and Europe in 1858. This cable failed after about four weeks of operation. A second cable was laid a few years later and became operational in July 1866. Telephony came into being with the invention of the telephone in the 1870s. Alexander Graham Bell patented his invention of the telephone in 1876; in 1877, established the Bell Telephone Company. Early versions of telephone communication systems were relatively simple and provided service over several hundred miles. Significant advances in the quality and range of service during the first two decades of the twentieth century resulted from the invention of the carbon microphone and the induction coil
Section 1.1 Historical Review In 1906,the invention of the triode amplifier by Lee DeForest made it possible to introduce signal amplification in telephone communication systems and,thus,to allow for telephone signal transmission over great distances.For example,transcontinental telephone transmission became operational in 1915. The two world wars and the Great Depression during the 1930s must have been a deterrent to the establishment of transatlantic telephone service.It was not until 1953,when the first transatlantic cable was laid,that telephone service became available between the United States and Europe. Automatic switching was another important advance in the development of tele- phony.The first automatic switch,developed by Strowger in 1897,was an electrome- chanical step-by-step switch.This type of switch was used for several decades.With the invention of the transistor,electronic (digital)switching became economically feasible. After several years of development at the Bell Telephone Laboratories,a digital switch was placed in service in Illinois in June 1960. During the past 50 years,there have been significant advances in telephone commu- nications.Fiber optic cables are rapidly replacing copper wire in the telephone plant,and electronic switches have replaced the old electromechanical systems. Wireless Communications.The development of wirelesscommunications stems from the works of Oersted,Faraday,Gauss,Maxwell,and Hertz during the nineteenth cen- tury.In 1820,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.Maxwell's theory was verified experimentally by Hertz in 1887. In 1894,a sensitive 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 in Oxford,England.Guglialmo Marconi is credited with the development of wireless telegraphy.In 1895,Marconi demonstrated the transmission of radio signals at a distance of approximately 2 km.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;this signal was transmitted from Cornwall,England,a city located about 1700 miles away. The invention of the vacuum tube was especially instrumental in the development of radio communication systems.The vacuum diode was invented by Fleming in 1904, and the vacuum triode amplifier was invented by DeForest in 1906,as previously indi- cated.In the early part of the twentieth century,the invention of the triode made radio broadcast possible.The AM (amplitude modulation)broadcast was initiated in 1920 when the radio station KDKA,Pittsburgh,went on the air.From that date,AM radio broad- casting grew very rapidly across the country and around the world.The superheterodyne 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 FM (frequency modulation),also by Armstrong.In 1933,Armstrong built and demonstrated
Section 1.1 Historical Review 3 In 1906, the invention of the triode amplifier by Lee DeForest made it possible to introduce signal amplification in telephone communication systems and, thus, to allow for telephone signal transmission over great distances. For example, transcontinental telephone transmission became operational in 1915. The two world wars and the Great Depression during the 1930s must have been a deteITent to the establishment of transatlantic telephone service. It was not until 1953, when the first transatlantic cable was laid, that telephone service became available between the United States and Europe. Automatic switching was another important advance in the development of telephony. The first automatic switch, developed by Strowger in 1897, was an electromechanical step-by-step switch. This type of switch was used for several decades. With the invention of the transistor, electronic (digital) switching became economically feasible. After several years of development at the Bell Telephone Laboratories, a digital switch was placed in service in Illinois in June 1960. During the past 50 years, there have been significant advances in telephone communications. Fiber optic cables are rapidly replacing copper wire in the telephone plant, and electronic switches have replaced the old electromechanical systems. Wireless Communications. The development of wireless communications stems from the works of Oersted, Faraday, Gauss, Maxwell, and Hertz during the nineteenth century. In 1820, Oersted demonstrated that an electric cuITent produces a magnetic field. On August 29, 1831� Michael Faraday showed that an induced cuITent 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. Maxwell's theory was verified experimentally by Hertz in 1887. In 1894, a sensitive 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 in Oxford, England. Guglialmo Marconi is credited with the development of wireless telegraphy. In 1895, Marconi demonstrated the transmission of radio signals at a distance of approximately 2 km. 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; this signal was transmitted from Cornwall, England, a city located about 1700 miles away. The invention of the vacuum tube was especially instrumental in the development of radio communication systems. The vacuum diode was invented by Fleming in 1904, and the vacuum triode amplifier was invented by DeForest in 1906, as previously indicated. In the early part of the twentieth century, the invention of the triode made radio broadcast possible. The AM (amplitude modulation) broadcast was initiated in 1920 when the radio station KDKA, Pittsburgh, went on the air. From that date, AM radio broadcasting grew very rapidly across the country and around the world. The superheterodyne 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 FM (frequency modulation), also by Armstrong. In 1933, Armstrong built and demonstrated
4 Introduction Chapter 1 the first FM communication system.However,the use of FM was developed more slowly than the use of 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 was initiated in London in 1936 by the British Broadcasting Corporation(BBC).Five years later,the Federal Communica- tions Commission(FCC)authorized television broadcasting in the United States. The Past 60 Years.The growth in communication services over the past 60 years has been phenomenal.Significant achievements include the invention of the tran- sistor in 1947 by Walter Brattain,John Bardeen,and William Shockley;the integrated circuit in 1958 by Jack Kilby and Robert Noyce;and the laser in 1958 by Townes and Schawlow.These inventions have made possible the development of small-size,low-power, low-weight,and high-speed electronic circuits that are used in the construction of satellite communication systems,wideband microwave radio systems,cellular communication sys- tems,and light-wave communication systems using fiber optic cables.A satellite named Telstar I was launched in 1962 and used to relay TV signals between Europe and the United States.Commercial satellite communication services began in 1965 with the launching of the Early Bird satellite. Currently,most of the wireline communication systems are being replaced by fiber optic cables,which provide extremely high bandwidth and make possible the transmis- sion of a wide variety of information sources,including voice,data,and video.Cellular radio has been developed to provide telephone service to people in automobiles,buses,and trains.High-speed communication networks link computers and a variety of peripheral devices,literally around the world. Today,we are witnessing a significant growth in the introduction and use of personal communication services,including voice,data,and video transmission.Satellite and fiber optic networks provide high-speed communication services around the world.Indeed,this is the dawn of the modern telecommunications era. There are several historical treatments in the development of radio and telecommuni- cations covering the past century.We cite the books by McMahon,entitled The Making of a Profession-A Century of Electrical Engineering in America (IEEE Press,1984);Ryder and Fink,entitled Engineers and Electronics (IEEE Press,1984);and S.Millman,Edi- tor,entitled A History of Engineering and Science in the Bell System-Communications Sciences(1925-1980)(AT&T Bell Laboratories,1984). 1.2 ELEMENTS OF AN ELECTRICAL COMMUNICATION SYSTEM Electrical communication systems are designed to send messages or information from a source that generates the messages to one or more destinations.In general,a communica- tion system can be represented by the functional block diagram shown in Figure 1.1.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,Ger- man,and French.An essential feature of any source that generates information is that its
4 Introduction Chapter 1 the first FM communication system. However, the use of FM was developed more slowly than the use of 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 was initiated 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. The Past 60 Years. The growth in communication services over the past 60 years has been phenomenal. Significant achievements include the invention of the transistor in 1947 by Walter Brattain, John Bardeen, and William Shockley; the integrated circuit in 1958 by Jack Kilby and Robert Noyce; and the laser in 1958 by Townes and Schawlow. These inventions have made possible the development of small-size, low-power, low-weight, and high-speed electronic circuits that are used in the construction of satellite communication systems, wideband microwave radio systems, cellular communication systems, and light-wave communication systems using fiber optic cables. A satellite named Telstar I was launched in 1962 and used to relay TV signals between Europe and the United States. Commercial satellite communication services began in 1965 with the launching of the Early Bird satellite. Currently, most of the wireline communication systems are being replaced by fiber optic cables, which provide extremely high bandwidth and make possible the transmission of a wide variety of information sources, including voice, data, and video. Cellular radio has been developed to provide telephone service to people in automobiles, buses, and trains. High-speed communication networks link computers and a variety of peripheral devices, literally around the world. Today, we are witnessing a significant growth in the introduction and use of personal communication services, including voice, data, and video transmission. Satellite and fiber optic networks provide high-speed communication services around the world. Indeed, this is the dawn of the modern telecommunications era. There are several historical treatments in the development of radio and telecommunications covering the past century. We cite the books by McMahon, entitled The Making of a Profession-A Century of Electrical Engineering in America (IEEE Press, 1984); Ryder and Fink, entitled Engineers and Electronics (IEEE Press, 1984); and S. Millman, Editor, entitled A Histmy of Engineering and Science in the Bell System-Communications Sciences ( 1925-1980) (AT&T Bell Laboratories, 1984). 1.2 ELEMENTS OF AN ELECTRICAL COMMUNICATION SYSTEM Electrical communication systems are designed to send messages or information from a source that generates the messages to one or more destinations. In general, a communication system can be represented by the functional block diagram shown in Figure 1.1. 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, and French. An essential feature of any source that generates information is that its
Section 1.2 Elements of an Electrical Communication System 5 Information source and Transmitter input transducer Channel Output Output transducer Receiver signal Figure 1.1 Functional diagram of a communication system. 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 trans- ducer that converts an acoustic speech signal into an electrical signal,and a video camera that converts an image into an electrical 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 and images. 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 through the physical channel or transmission medium.For example,in radio and TV broadcasts,the FCC specifies the frequency range for each transmitting station.Hence,the transmitter must translate the outgoing information sig- nal into the appropriate frequency range that matches the frequency allocation assigned to the transmitter.Thus,signals transmitted by multiple radio stations 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 matches the message signal to the channel via a process called modulation.Usually,modulation involves the use of the information signal to sys- tematically vary either the amplitude or the frequency or the 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 frequency band allocated to the radio transmitting station.This is an example of amplitude modulation.In an FM radio broadcast,the information signal that is transmitted is con- tained 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
Section 1.2 Elements of an Electrical Communication System Information source and input transducer Transmitter Output signal Output transducer Figure 1.1 Functional diagram of a communication system. Receiver 5 Channel 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 into an electrical signal, and a video camera that converts an image into an electrical 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 and images. 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 through the physical channel or transmission medium. For example, in radio and TV broadcasts, the FCC specifies the frequency range for each transmitting station. Hence, the transmitter must translate the outgoing information signal into the appropriate frequency range that matches the frequency allocation assigned to the transmitter. Thus, signals transmitted by multiple radio stations 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 matches the message signal to the channel via a process called modulation. Usually, modulation involves the use of the information signal to systematically vary either the amplitude or the frequency or the 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 frequency band allocated to the radio transmitting station. This is an example of amplitude modulation. In an 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
6 Introduction Chapter 1 In general,carrier modulation such as AM,FM,and PM is performed at the trans- mitter,as previously indicated,to convert the information signal to a form that matches the characteristics of the channel.Thus,through the process of modulation,the informa- tion signal is translated 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 allocated,the types of noise and interference that the signal encounters in transmission over the channel,and the electronic devices that are available for signal amplification prior to transmission.In any case,the modulation process makes it possible to accom- modate 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 trans- mitter are filtering of the information-bearing signal,amplification of the modulated signal and,in the case of wireless transmission,radiation of the signal by means of a transmitting antenna. The Channel.The communication channel is the physical medium that is used to send the signal from the transmitter to the receiver.In wireless transmission,the chan- nel is usually the atmosphere(free space).On the other hand,telephone channels usually employ a variety of physical media,including wirelines,fiber optic 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 possi- ble mechanisms.The most common form of signal degradation comes in the form of addi- tive noise,which is generated at the front end of the receiver,where signal amplification is performed.This noise is often called thermal noise.In wireless transmission,additional additive disturbances are man-made noise and atmospheric noise picked up by a receiv- ing antenna.Automobile 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 wireline 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 mul- tipath propagation.Such signal distortion is characterized as a nonadditive signal distur- bance that manifests itself as time variations in the signal amplitude,usually called fading. This phenomenon is described in more detail in Section 1.3. 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 considered in the design of the communication system. In the design of a communication system,the system designer works with mathe- matical models that statistically characterize the signal distortion encountered on physical channels.Often,the statistical description that is used in a mathematical model is a result of actual empirical measurements obtained from experiments involving signal transmis- sion 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 commu- nication system designs,the statistical characteristics of the channel may vary significantly with time.In such cases,the system designer may design a communication system that is
6 Introduction Chapter 1 In general, carrier modulation such as AM, FM, and PM is performed at the transmitter, as previously indicated, to convert the information signal to a form that matches the characteristics of the channel. Thus, through the process of modulation, the information signal is translated 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 allocated, the types of noise and interference that the signal encounters in transmission over the channel, and the electronic devices that are available for signal amplification prior to 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 the case of wireless transmission, radiation of the signal by means of a transmitting antenna. The Channel. The communication 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, fiber optic 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 form 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 wireless transmission, additional additive disturbances are man-made noise and atmospheric noise picked up by a receiving antenna. Automobile 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 wireline 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 that manifests itself as time variations in the signal amplitude, usually called fading. This phenomenon is described in more detail in Section 1.3. 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 considered in the design of the communication system. In the design of a communication system, the system designer works with mathematical models that statistically characterize the signal distortion encountered on physical channels. Often, the statistical description that is used in a 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 designer may design a communication system that is