【正文】 tronic ponents at the receiver are dominant disturbances for signal transmission in the MF propagation, as illustrated in results from transmitted signals being reflected(bent or refracted)from the ionosphere, which consists of several layers of charged particles ranging in altitude from 50 to 400 km above the surface of the the daytime hours, the heating of the lower atmosphere by the sun causes the formation of the lower layers at altitudes below 120 lower layers, especially the Dlayer, serve to absorb frequencies below 2 MHz, thus severely limiting skywave propagation of AM radio , during the nighttime hours, the electron density in the lower layers of the ionosphere drops sharply and the frequency absorption that occurs during the daytime is significantly a consequence, powerful AM radio broadcast stations can propagate over large distances via sky wave over the Flayer of the ionosphere, which ranges from 140 to 400 km above the surface of the frequently occurring problem with electromagnetic wave propagation via sky wave in the HF frequency range is signal multipath occurs when the transmitted signal arrives at the receiver via multiple propagation paths at different delays, tt generally results in intersymbol interference in a digital munication , the signal ponents arriving via different propagation paths may add destructively, resulting in a phenomenon called signal fading, which most people have experienced when listening to a distant radio station at night when sky wave is the dominant propagation noise at HF is a bination of atmospheric noise and thermal ionospheric propagation ceases to exist at frequencies above approximately 30 MHz, which is the end of the HF , it is possible to have ionospheric scatter propagation at frequencies in the range 3060 MHz, resulting from signal scattering from the lower is also possible to municate over distances of several hundred miles 黃河科技學(xué)院畢業(yè)論文（文獻翻譯）by use of tropospheric scattering at frequencies in the range 40300 results from signal scattering due to particles in the atmosphere at altitudes of 10 miles or , ionospheric scatter and tropospheric scatter involve large signal propagation losses and require a large amount of transmitter power and relatively large above 30 MHz propagate through the ionosphere with relatively little loss and make satellite and extraterrestrial munications , at frequencies in the VHF band and higher, the dominant mode of electromagnetic propagation is lincofsight(LOS) terrestrial munication systems, this means that the transmitter and receiver antennas must be in direct LOS with relatively little or no this reason, television stations transmitting in the VHF and UHF frequency bands mount their antennas on high towers to achieve a broad coverage general, the coverage area for LOS propagation is limited by the curvature of the 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 dr Thus,r represents the “age”(elapsedtime) three mathematical models described above adequately characterize the great majority of the physical channels encountered in three channel models are used in this text for the analysis and design of munication A HISTORICAL PERSPECTIVE IN THE DEVELOPMENT OF DIGITAL COMMUNICATIONS It is remarkable that the earliest form of electrical munication, namely telegraphy, 黃河科技學(xué)院畢業(yè)論文（文獻翻譯）was a digital munication electric telegraph was developed by Samuel Morse and was demonstrated in devised the variablelength binary code in which letters of the English alphabet are represented by a sequence of dots and dashes(code words).In this code, more frequently occurring letters are represented by short code words, while letters occurring less frequently are represented by longer code , the Morse code*was the precursor of the variablelength source coding methods described in Chapter 40 years later, in 1875, Emile Baudot devised a code for telegraphy in which every letter was encoded into fixedlength binary code words of length the Baudot code, binary code elements are of equal length and designated as mark and Morse is responsible for the development of the first electrical digital munication system(telegraphy), the beginnings of what we now regard as modern digital munications stem from the work of Nyquist(1924), who investigated the problem of determining the maximum signaling rate that can be used over a telegraph channel of a given bandwidth without intersymbol formulated a model of a telegraph system in which a transmitted signal has the general form where g（t）represents a basic pulse shape and is the binary data sequence of {177。1} transmitted at a rate of 1/Tbits/ set out to determine the optimum pulse shape that was bandlimited to W Hz and maximized the bit rate under the constraint that the pulse caused no intersymbol interference at the sampling time =0, 177。1, 177。2 ……His studies led him to concludc that the maximum pulse rate is 2W pulses/ rate is now called Nyquist 第四篇：文獻翻譯（推薦）關(guān)于ITSW對海關(guān)效率的影響：墨西哥案例研究作者：Am233。rica , C233。sar NavarroCh225。vez, Oscar Hugo PedrazaRend243。n 摘要國際貿(mào)易的演化和發(fā)展使得分配給海關(guān)服務(wù)的任務(wù)更加重要和復(fù)雜?，F(xiàn)在一天的海關(guān)管理是國際貿(mào)易中相關(guān)監(jiān)管操作者相當于貿(mào)易的調(diào)解人和加速器。然而，在公共部門的效率研究比私營，利潤為導(dǎo)向的行業(yè)的效率研究更加復(fù)雜。因為事實是公共部門的效率可以被定義為實現(xiàn)目標，而不是感知利潤的能力。這項研究的目的是研究那些從電子服務(wù)體系國際貿(mào)易單一窗口（ITSW）的實施取得最大的成本/效率的提高的地區(qū)，并注意作為一個研究案例出現(xiàn)在墨西哥的實施ITSW的障礙或缺點。研究結(jié)果指出，根據(jù)ITSW實施的第一個結(jié)果，墨西哥取得了良好的進展關(guān)系到國際貿(mào)易程序的效率，簡化在商人和海關(guān)總署之間的信息流動，降低了20％的通關(guān)時間。關(guān)鍵詞：效率。海關(guān)。公共服務(wù)。電子系統(tǒng)。ITSW。國際貿(mào)易。墨西哥介紹在過去的二十年中，國際貿(mào)易增長比GDP快兩倍。許多經(jīng)濟體確保國際貿(mào)易在經(jīng)濟增長中的重要作用。國際貿(mào)易涉及的貨物是為了什么過境是需要一些隱蔽的程序。其中的一些程序，涉及到安全和標準問題，而另一些則要應(yīng)對海關(guān)。通關(guān)對一個國家的競爭力的影響是上促使貿(mào)易商和政治領(lǐng)導(dǎo)人去尋求方法，使他們的海關(guān)組織更加有效和高效的國際貿(mào)易的有力的一步。然而，這并不是一個簡單的任務(wù)，由于不斷變化和充滿挑戰(zhàn)的全球貿(mào)易已變得越來越困難，貿(mào)易政策也由于區(qū)域和國際協(xié)定的擴散不斷增長它的復(fù)雜性，更大的復(fù)雜交易，并強加海關(guān)的多重轉(zhuǎn)移目標。在另一方面，在政府的審查已經(jīng)要求公共機構(gòu)利用他們所管理的資源提高效率。許多國家，包括墨西哥，實施電子系統(tǒng)國際貿(mào)易單一窗口（ITSW）以提高與國際貿(mào)易有關(guān)海關(guān)的效率。單一窗口是由世界海關(guān)組織（WCO）和世界貿(mào)易組織（WTO）定義為“跨界智能設(shè)備，允許從事貿(mào)易和運輸方提出標準化的信息和文件用單一入口點來滿足所有的進口，出港和過境的相關(guān)監(jiān)管要求“或”讓交易者一次只能通過一