【導讀】在本書中,我們將介紹作為數(shù)字通信系統(tǒng)分析和設計基礎的基本原理。研究主題包括數(shù)字形式的信息從產(chǎn)生該信息的信源到一個或多個目的地的傳輸問題。圖1-1-1顯示了一個數(shù)字通信系統(tǒng)的功能性框圖和基本組成部分。上是離散的，并且只有有限個輸出字符。在數(shù)字通信系統(tǒng)屮，由信源產(chǎn)生的消息變換。理論上，應當用盡可能少的二進制數(shù)字表示信源輸出（消息）。由信源編碼器輸出的二進制數(shù)字序列稱為信息序列，它被傳送到信道編碼器。中用來克服信號在信道中傳輸時所遭受的噪聲和干擾的影響。因此，所增加的冗余是。用來提高接收數(shù)據(jù)的可靠性以及改善接收信號的逼真度的。余有助于接收機譯出期望的信息序列。編碼就是將每個二進制數(shù)字簡單重復m次.這里m為一個正整數(shù)。的）編碼涉及到一次取k個信息比特,并將毎個k比特序列映射成惟一的n比特序列，的設計,它是用來補償信道的幅度和相位失真的。發(fā)出具有較低倌號衰減的光纜，以及用于信號和信號檢測的可靠性光子器件。

　　

【正文】 n the range 3060 MHz, resulting from signal scattering from the lower ionosphere. It is also possible to municate over distances of several hundred miles by use of tropospheric scattering at frequencies in the range 40300 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 extraterrestrial munications possible. Hence, at frequencies in the VHF band and higher, the dominant mode of electromagic propagation is lincofsight (LOS) propagation. For terrestrial munication 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 VHF and UHF frequency bands mount their antennas on high towers 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 m above the surface of the earth, the distance to the radio horizon, assuming no physical obstructions such as mountains, is approximately d VlSh km. For example, a TV antenna mounted on a tower of 300 m in height provides a coverage of approximately 67 km. As another example, microwave radio relay systems used extensively for telephone and video transmission at frequencies above I GHz have antennas mounted on tall towers or on the top of tall buildings. The dominant noise limiting the performance of a munication system in VHF and UHF frequency ranges is thermal noise generated in the receiver front end and cosmic noise picked up by the antenna. At frequencies in the SHF band above 10 GHz, atmospheric conditions play a major role in signal propagation. For example, at 10 GHz, the attenuation ranges from about dB/km in light rain to about dB/km in heavy rain. At 100 GHz, the attenuation ranges from about dB/km in light rain to about 6 dB/km in heavy rain. Hence, in this frequency range, heavy rain introduces extremely high propagation losses that can result in service outages (total breakdown in the munication system). At frequencies above the EHF (extremely high frequency) band, we have the infrared and visible light regions of the electromagic spectrum, which can be used to provide LOS optical munication in free space. To date,these frequency bands have been used in experimental munication systems, such as satellitetosatellite links. Underwater Acoustic Channels Over the past few decades, ocean exploration activity has been steadily increasing. Coupled with this increase is the need to transmit data, collected by sensors placed under water, to the surface of the ocean. From there, it is possible to relay the data via a satellite to a data collection center. Electromagic waves do not propagate over long distances under water except at extremely low frequencies. However, the transmission of signals at such low frequencies is prohibitively expensive because of the large and powerful transmitters required. The attenuation of electromagic waves in water can be expressed in terms of the skin depth, which is the distance a signal is attenuated by 1/r. For sea water, the skin depth 250/v7, where f is expressed in Hz and 8 is in m. For example, at 10 kHz. the skin depth is contrast, acoustic signals propagate over distances of tens and even hundreds of kilometers. An underwater acoustic channel is characterized as a multipath channel due to signal reflections from the surface and the bottom of the sea. Because of wave motion, the signal multipath ponents undergo timevarying propagation delays that result in signal fading. In addition, there is frequencydependent attenuation, which is approximately proportional to the square of the signal frequency. The sound velocity is nominally about 1500 m/s, but the actual value will vary either above or below the nominal value depending on the depth at which the signal propagates. Ambient ocean acoustic noise is caused by shrimp, fish, and various mammals. Near harbors, there is also manmade acoustic noise in addition to the ambient noise. In spite of this hostile environment, it is possible to design and implement efficient and highly reliable underwater acoustic munication systems for transmitting digital signals over large distances. Storage Channels Information storage and retrieval systems constitute a very significant part of datahandling activities on a daily basis. Magic (ape, including digital audio tape and video tape, magic disks used for storing large amounts of puter data, optical disks used for puter data storage, and pact disks are examples of data storage systems that can be characterized as munication channels. The process of storing data on a magic tape or a magic or optical disk is equivalent to transmitting a signal over a telephone or a radio channel. The rcadback process and the signal processing involved in storage systems to recover the stored information are equivalent to the functions performed by a receiver in a telephone or radio munication system to recover the transmitted information. Additive noise generated by the electronic ponents and interference from adjacent tracks is generally present in the readback signal of a storage system, just as is the case in a telephone or a radio munication system. The amount of data that can be stored is generally limited by the size of the disk or tape and the density (number of bits stored per square inch) that can be achieved by the write/read electronic systems and heads. For example, a packing