《通信原理与实验 Principles and Experiments of Communications》课程参考书：John G. Proakis Masoud Salehi《Fundamentals of Communication Systems》（Second Edition）.pdf_P26-P30

12 Introduction Chapter 1 1.3 COMMUNICATION CHANNELS AND THEIR CHARACTERISTICS As indicated in our preceding discussion,the communication channel provides the con- nection between the transmitter and the receiver.The physical channel may be a pair of wires that carry the electrical signal,or an optical fiber that carries the information on a modulated light beam,or an underwater ocean channel in which the information is trans- mitted acoustically,or free space over which the information-bearing signal is radiated by use of an antenna.Other media that can be characterized as communication channels are data storage media,such as magnetic tape,magnetic disks,and optical disks. One common problem in signal transmission through any channel is additive noise. In general,additive noise is generated internally by components,such as resistors and solid-state devices,used to implement system.This type of noise is usually called thermal noise.Other sources of noise and interference may arise externally to the system,such as interference from other users of the channel.When such noise and interference occupy the same frequency band as the desired signal,the effect can be minimized by proper design of the transmitted signal and the demodulator at the receiver.Other types of signal degradation may be encountered in transmission over the channel,such as signal attenuation,amplitude and phase distortion,and multipath distortion. The effects of noise may be minimized by increasing the power in the transmitted signal.However,equipment and other practical constraints limit the power level in the transmitted signal.Another basic limitation is the available channel bandwidth.A band- width constraint is usually due to the physical limitations of the medium and the electronic components used to implement the transmitter and the receiver.These two limitations result in constraining the amount of data that can be transmitted reliably over any communication channel. Next,we describe some of the important characteristics of several communication channels. Wireline Channels.The telephone network makes extensive use of wirelines for voice signal transmission,as well as data and video transmission.Twisted-pair wirelines and coaxial cable are basically guided electromagnetic channels that provide relatively modest bandwidths.Telephone wire generally used to connect a customer to a central office has a bandwidth of several hundred kilohertz(kHz).On the other hand,coaxial cable has a usable bandwidth of several megahertz(MHz).Figure 1.3 illustrates the frequency range of guided electromagnetic channels,which include waveguides and optical fibers. Signals transmitted through such channels are distorted in both amplitude and phase, and they are further corrupted by additive noise.Twisted-pair wireline channels are also prone to crosstalk interference from physically adjacent channels.Because wireline chan- nels carry a large percentage of our daily communications around the country and the world,much research has been performed on the characterization of their transmission properties and on methods for mitigating the amplitude and phase distortion encountered in signal transmission.In Chapter 10,we describe methods for designing optimum trans- mitted signals and their demodulation,including the design of channel equalizers that com- pensate for amplitude and phase distortion

12 Introduction Chapter 1 1 .3 COMMUNICATION CHANNELS AND THEIR CHARACTERISTICS As indicated in our preceding discussion, the communication channel provides the connection between the transmitter and the receiver. The physical channel may be a pair of wires that carry the electrical signal, or an optical fiber that carries the information on a modulated light beam, or an underwater ocean channel in which the information is transmitted acoustically, or free space over which the information-bearing signal is radiated by use of an antenna. Other media that can be characterized as communication channels are data storage media, such as magnetic tape, magnetic disks, and optical disks. One common problem in signal transmission through any channel is additive noise. In general, additive noise is generated internally by components, such as resistors and solid-state devices, used to implement system. This type of noise is usually called thermal noise. Other sources of noise and interference may arise externally to the system, such as interference from other users of the channel. When such noise and interference occupy the same frequency band as the desired signal, the effect can be minimized by proper design of the transmitted signal and the demodulator at the receiver. Other types of signal degradation may be encountered in transmission over the channel, such as signal attenuation, amplitude and phase distortion, and multipath distmtion. The effects of noise may be minimized by increasing the power in the transmitted signal. However, equipment and other practical constraints limit the power level in the transmitted signal. Another basic limitation is the available channel bandwidth. A bandwidth constraint is usually due to the physical limitations of the medium and the electronic components used to implement the transmitter and the receiver. These two limitations result in constraining the amount of data that can be transmitted reliably over any communication channel. Next, we describe some of the important characteristics of several communication channels. Wireline Channels. The telephone network makes extensive use of wirelines for voice signal transmission, as well as data and video transmission. Twisted-pair wirelines and coaxial cable are basically guided electromagnetic channels that provide relatively modest bandwidths. Telephone wire generally used to connect a customer to a central office has a bandwidth of several hundred kilohertz (kHz). On the other hand, coaxial cable has a usable bandwidth of several megahertz (MHz). Figure 1 .3 illustrates the frequency range of guided electromagnetic channels, which include waveguides and optical fibers. Signals transmitted through such channels are distorted in both amplitude and phase, and they are further corrupted by additive noise. Twisted-pair wireline channels are also prone to crosstalk interference from physically adjacent channels. Because wireline channels carry a large percentage of our daily communications around the country and the world, much research has been performed on the characterization of their transmission properties and on methods for mitigating the amplitude and phase distortion encountered in signal transmission. In Chapter 1 0, we describe methods for designing optimum transmitted signals and their demodulation, including the design of channel equalizers that compensate for amplitude and phase distortion

14 Introduction Chapter 1 The transmitter or modulator in a fiber-optic communication system is a light source, either a light-emitting diode(LED)or a laser.Information is transmitted by varying(mod- ulating)the intensity of the light source with the message signal.The light propagates through the fiber as a light wave and is amplified periodically(in the case of digital trans- mission,it is detected and regenerated by repeaters)along the transmission path to compen- sate for signal attenuation.At the receiver,the light intensity is detected by a photodiode, whose output is an electrical signal that varies in direct proportion to the power of the light impinging on the photodiode. It is envisioned that fiber optic channels will replace nearly all wireline channels in the telephone network in the next few years. Wireless Electromagnetic Channels.In radio communication systems,elec- tromagnetic energy is coupled to the propagation medium by an antenna,which serves as the radiator.The physical size and the configuration of the antenna depend primar- ily on the frequency of operation.To obtain efficient radiation of electromagnetic energy, the antenna must be longer than 1/10 of the wavelength.Consequently,a radio station transmitting in the AM frequency band,say,at 1 MHz(corresponding to a wavelength of A=c/fe 300 m)requires an antenna of at least 30 meters. Figure 1.4 illustrates the various frequency bands of the electromagnetic spectrum. The mode of propagation of electromagnetic waves in the atmosphere and in free space may be subdivided into three categories,namely,ground-wave propagation,sky-wave propagation,and line-of-sight (LOS)propagation.In the very low frequency (VLF)and extremely low frequency bands where the wavelengths exceed 10 kilometers,the earth and the ionosphere act as a waveguide for electromagnetic wave propagation.In these fre- quency ranges,communication signals practically propagate around the globe.For this reason,these frequency bands are primarily used to provide navigational aids from shore to ships around the world.The channel bandwidths available in these frequency bands are relatively small(usually 1%-10%of the center frequency);hence,the information that is transmitted through these channels is relatively of slow speed and generally confined to digital transmission.A dominant type of noise at these frequencies is generated from thun- derstorm activity around the globe,especially in tropical regions.Interference results from the many users of these frequency bands. Ground-wave propagation,illustrated in Figure 1.5,is the dominant mode of prop- agation for frequencies in the medium frequency(MF)band(0.3-3 MHz).This is the frequency band used for AM broadcasting and maritime radio broadcasting.In AM broad- cast,ground-wave propagation limits the range of even the most powerful radio stations to about 100 miles.Atmospheric noise,man-made noise,and thermal noise from electronic components at the receiver are dominant disturbances for signal transmission at MF. Sky-wave propagation,as illustrated in Figure 1.6,results from transmitted signals being reflected (bent or refracted)from the ionosphere,which consists of several layers of charged particles ranging in altitude from 30 to 250 miles above the surface of the earth.During the daytime hours,the heating of the lower atmosphere by the sun causes the formation of the lower layers at altitudes below 75 miles.These lower layers,especially the D-layer,absorb frequencies below 2 MHz;thus,they severely limit sky-wave propagation of AM radio broadcast.However,during the nighttime hours,the electron density in the

14 Introduction Chapter 1 The transmitter or modulator in a fiber-optic communication system is a light source, either a light-emitting diode (LED) or a laser. Information is transmitted by varying (modulating) the intensity of the light source with the message signal. The light propagates through the fiber as a light wave and is amplified periodically (in the case of digital transmission, it is detected and regenerated by repeaters) along the transmission path to compensate for signal attenuation. At the receiver, the light intensity is detected by a photodiode, whose output is an electrical signal that varies in direct proportion to the power of the light impinging on the photodiode. It is envisioned that fiber optic channels will replace nearly all wireline channels in the telephone network in the next few years. Wireless Electromagnetic Channels. In radio communication systems, electromagnetic energy is coupled to the propagation medium by an antenna, which serves as the radiator. The physical size and the configuration of the antenna depend primarily on the frequency of operation. To obtain efficient radiation of electromagnetic energy, the antenna must be longer than 1/ 10 of the wavelength. Consequently, a radio station transmitting in the AM frequency band, say, at 1 MHz (corresponding to a wavelength of A = c / fc = 300 m) requires an antenna of at least 30 meters. Figure 1.4 illustrates the various frequency bands of the electromagnetic spectrum. The mode of propagation of electromagnetic waves in the atmosphere and in free space may be subdivided into three categories, namely, ground-wave propagation, sky-wave propagation, and line-of-sight (LOS) propagation. In the very low frequency (VLF) and extremely low frequency bands where the wavelengths exceed 10 kilometers, the earth and the ionosphere act as a waveguide for electromagnetic wave propagation. In these frequency ranges, communication signals practically propagate around the globe. For this reason, these frequency bands are primarily used to provide navigational aids from shore to ships around the world. The channel bandwidths available in these frequency bands are relatively small (usually 1 %-10% of the center frequency); hence, the information that is transmitted through these channels is relatively of slow speed and generally confined to digital transmission. A dominant type of noise at these frequencies is generated from thunderstorm activity around the globe, especially in tropical regions. Interference results from the many users of these frequency bands. Ground-wave propagation, illustrated in Figure 1.5, is the dominant mode of propagation for frequencies in the medium frequency (MF) band (0.3-3 MHz). This is the frequency band used for AM broadcasting and maritime radio broadcasting. In AM broadcast, ground-wave propagation limits the range of even the most powerful radio stations to about 100 miles. Atmospheric noise, man-made noise, and thermal noise from electronic components at the receiver are dominant disturbances for signal transmission at MF. Sky-wave propagation, as illustrated in Figure 1.6, results from transmitted signals being reflected (bent or refracted) from the ionosphere, which consists of several layers of charged particles ranging in altitude from 30 to 250 miles above the surface of the earth. During the daytime hours, the heating of the lower atmosphere by the sun causes the formation of the lower layers at altitudes below 75 miles. These lower layers, especially the D-layer, absorb frequencies below 2 MHz; thus, they severely limit sky-wave propagation of AM radio broadcast. However, during the nighttime hours, the electron density in the

16 Introduction Chapter 1 Ionosphere Figure 1.6 Illustration of sky-wave propagation. lower layers of the ionosphere drops sharply and the frequency absorption that occurs during the day is significantly reduced.As a consequence,powerful AM radio broadcast stations can propagate over large distances via sky wave over the F-layer of the ionosphere, which ranges from 90 miles to 250 miles above the surface of the earth. A common problem with electromagnetic wave propagation via sky wave in the high frequency (HF)range is signal multipath.Signal multipath occurs when the transmit- ted signal arrives at the receiver via multiple propagation paths at different delays.Signal multipath generally results in intersymbol interference in a digital communication system. Moreover,the signal components arriving via different propagation paths may add destruc- tively,resulting in a phenomenon called signal fading.Most people have experienced this phenomenon when listening to a distant radio station at night,when sky wave is the dom- inant propagation mode.Additive noise at HF is a combination of atmospheric noise and thermal noise. Sky-wave ionospheric propagation ceases to exist at frequencies above approxi- mately 30 MHz,which is the end of the HF band.However,it is possible to have iono- spheric scatter propagation at frequencies in the range of 30-60 MHz;this is a result of signal scattering from the lower ionosphere.It is also possible to communicate over dis- tances of several hundred miles using tropospheric scattering at frequencies in the range of 40-300 MHz.Troposcatter results from signal scattering due to particles in the atmo- sphere at altitudes of 10 miles or less.Generally,ionospheric scatter and tropospheric scat- ter involve large signal propagation losses and require a large amount of transmitter power and relatively large antennas. Frequencies above 30 MHz propagate through the ionosphere with relatively little loss and make satellite and extraterrestrial communications possible.Hence,at frequencies in the VHF band and higher,the dominant mode of electromagnetic propagation is LOS propagation.For terrestrial communication systems,this means that the transmitter and receiver antennas must be in direct LOS with relatively little or no obstruction.For this reason,television stations transmitting in the very high frequency(VHF)and ultra high frequency (UHF)bands mount their antennas on high towers in order to achieve a broad coverage area. In general,the coverage area for LOS propagation is limited by the curvature of the earth.If the transmitting antenna is mounted at a height h feet above the surface of the earth,the distance to the radio horizon is approximately d =v2h miles (assuming no physical obstructions such as a mountain).For example,a TV antenna mounted on a tower of 1000 feet in height provides a coverage of approximately 50 miles.As another

16 Introduction Chapter 1 Ionosphere \ ____ _ Figure 1.6 Illustration of sky-wave propagation. lower layers of the ionosphere drops sharply and the frequency absorption that occurs during the day is significantly reduced. As a consequence, powerful AM radio broadcast stations can propagate over large distances via sky wave over the F-layer of the ionosphere, which ranges from 90 miles to 250 miles above the surface of the earth. A common problem with electromagnetic wave propagation via sky wave in the high frequency (HF) range is signal multipath. Signal multipath occurs when the transmitted signal an-ives at the receiver via multiple propagation paths at different delays. Signal multipath generally results in intersymbol interference in a digital communication system. Moreover, the signal components an-iving via different propagation paths may add destructively, resulting in a phenomenon called signal fading. Most people have experienced this phenomenon when listening to a distant radio station at night, when sky wave is the dominant propagation mode. Additive noise at HF is a combination of atmospheric noise and thermal noise. Sky-wave ionospheric propagation ceases to exist at frequencies above approximately 30 MHz, which is the end of the HF band. However, it is possible to have ionospheric scatter propagation at frequencies in the range of 30-60 MHz; this is a result of signal scattering from the lower ionosphere. It is also possible to communicate over distances of several hundred miles using tropospheric scattering at frequencies in the range of 40-300 MHz. Troposcatter results from signal scattering due to particles in the atmosphere at altitudes of 10 miles or less. Generally, ionospheric scatter and tropospheric scatter involve large signal propagation losses and require a large amount of transmitter power and relatively large antennas. Frequencies above 30 MHz propagate through the ionosphere with relatively little loss and make satellite and extraten-estrial communications possible. Hence, at frequencies in the VHF band and higher, the dominant mode of electromagnetic propagation is LOS propagation. For ten-estrial communication systems, this means that the transmitter and receiver antennas must be in direct LOS with relatively little or no obstruction. For this reason, television stations transmitting in the very high frequency (VHF) and ultra high frequency (UHF) bands mount their antennas· on high towers in order to achieve a broad coverage area. In general, the coverage area for LOS propagation is limited by the curvature of the earth. If the transmitting antenna is mounted at a height h feet above the surface of the earth, the distance to the radio horizon is approximately d = -/2ii miles (assuming no physical obstructions such as a mountain). For example, a TV antenna mounted on a tower of 1 000 feet in height provides a coverage of approximately 50 miles. As another