【正文】 于傳統(tǒng)加工難以切削的硬脆材料。激光加工不會(huì)掛渣，沒(méi)有毛邊，可以精確控制幾何精度。激光切割的邊緣光滑且潔凈，無(wú)須進(jìn)一步處理。激光加工中的熱影響區(qū)相對(duì)較窄，其重鑄層只有幾微米。采用這種方法可以節(jié)省材料，這對(duì)于貴重材料或微加工中的精密結(jié)構(gòu)而言非常重要。激光加工（）可以實(shí)現(xiàn)局部的非接觸加工，而且對(duì)加工件幾乎沒(méi)有作用力。雖然激光在某些場(chǎng)合可用來(lái)作為放大器，但它的主要用途是光激射振蕩器，或者是作為將電能轉(zhuǎn)換為具有高度準(zhǔn)直性光束的換能器。特別是在電化學(xué)加工中，材料去除率與被加工件的硬度、韌性及其他特性無(wú)關(guān)。通過(guò)將工具電極向去除工件表面材料的方向進(jìn)給， mm范圍內(nèi)，～20 mm/min左右。金屬的去除由一個(gè)合適形狀的工具電極來(lái)完成，最終加工出來(lái)的零件具有給定的形狀、尺寸和表面光潔度。這個(gè)加工過(guò)程一般用于在高強(qiáng)度材料上加工復(fù)雜形腔和形狀，特別是在航空工業(yè)中如渦輪機(jī)葉片、噴氣發(fā)動(dòng)機(jī)零件和噴嘴，以及在汽車(chē)業(yè)（發(fā)動(dòng)機(jī)鑄件和齒輪）和醫(yī)療衛(wèi)生業(yè)中。通過(guò)改變蝕刻劑和控制工件加工環(huán)境，這種尺寸改變可以減小到最小。在化學(xué)造型中最典型的公差范圍可保持在材料厚度的177。而被稱(chēng)為保護(hù)層的特殊涂層所保護(hù)下的區(qū)域中的材料則不會(huì)被去除?；瘜W(xué)加工化學(xué)加工是眾所周知的特種加工工藝之一，它將工件浸入化學(xué)溶液通過(guò)腐蝕溶解作用將多余材料從工件上去除掉。這就會(huì)使懸浮在電介質(zhì)中的導(dǎo)電粒子聚集在電場(chǎng)最強(qiáng)處。因此，材料的硬度不再是電火花加工中的關(guān)鍵因素。常見(jiàn)的特種加工工藝描述如下?；谝韵赂鞣N特殊理由，特種加工工藝或稱(chēng)為先進(jìn)制造工藝，可以應(yīng)用于采用傳統(tǒng)加工方法不可行，不令人滿(mǎn)意或者不經(jīng)濟(jì)的場(chǎng)合：； ； ；。傳統(tǒng)加工如車(chē)削、鉆削、刨削、銑削和磨削，都難以加工特別硬的或脆性材料。另一種實(shí)現(xiàn)形式的OFDM系統(tǒng)的多樣性是由前向糾錯(cuò)信道編碼，在這里，每個(gè)數(shù)據(jù)位的信息分散在幾個(gè)代碼位。沒(méi)有其他的分集技術(shù)被應(yīng)用。 GHz或24千兆赫。三種類(lèi)型的通道模型定義。207信道的成本模型是基于一個(gè)810兆赫的2G，如GSM系統(tǒng)中使用的900兆赫頻段信道帶寬的測(cè)量。在這種傳播環(huán)境的幾個(gè)表中的相應(yīng)路徑延遲和電源配置給出。一種廣泛使用的離散多徑信道模型概述于下。不過(guò)，如果多載波系統(tǒng)的設(shè)計(jì)選擇了這樣的多普勒展寬在子載波間隔或更高，秩序是在頻率RAKE接收機(jī)域名可以使用[22]。只要所有子載波只要是一個(gè)共同的多普勒頻移金融衍生工具的影響，這可以補(bǔ)償多普勒頻移在接收器和ICI是可以避免的。這種效果是利用多載波傳輸?shù)牡胤?，每發(fā)送符號(hào)的增加與子載波數(shù)控?cái)?shù)目，因此，ISI的金額減少的持續(xù)時(shí)間。根據(jù)信道脈沖響應(yīng)的假設(shè)一個(gè)復(fù)雜的值高斯過(guò)程，其大小通道的傳遞函數(shù)A的水稻分布給出賴(lài)斯因素KRice是由占主導(dǎo)地位的路徑權(quán)力的威力比分散的路徑。在對(duì)視線（LOS）或線的主要組成部分的情況下，這個(gè)過(guò)程是零的意思。多徑信道頻率分散性能是最常見(jiàn)的量化發(fā)生的多普勒頻率和多普勒f(shuō)Dmax蔓延fDspread最大。多普勒頻率取決于終端站，光速c，載波頻率fc的速度和發(fā)病路徑分配給速度v波αp角度頁(yè)具有相應(yīng)通道傳輸信道沖激響應(yīng)函數(shù)圖12所示。Some parents just buy whatever their children want.第二篇：中英文翻譯Fundamentals This chapter describes the fundamentals of today’s wireless a detailed description of the radio channel and its modeling are presented, followed by the introduction of the principle of OFDM multicarrier addition, a general overview of the spread spectrum technique, especially DSCDMA, is given and examples of potential applications for OFDM and DSCDMA are introduction is essential for a better understanding of the idea behind the bination of OFDM with the spread spectrum technique, which is briefly introduced in the last part of this Radio Channel Characteristics Understanding the characteristics of the munications medium is crucial for the appropriate selection of transmission system architecture, dimensioning of its ponents, and optimizing system parameters, especially since mobile radio channels are considered to be the most difficult channels, since they suffer from many imperfections like multipath fading, interference, Doppler shift, and choice of system ponents is totally different if, for instance, multipath propagation with long echoes dominates the radio , an accurate channel model describing the behavior of radio wave propagation in different environments such as mobile/fixed and indoor/outdoor is may allow one, through simulations, to estimate and validate the performance of a given transmission scheme in its several design Understanding Radio Channels In mobile radio channels(see Figure 11), the transmitted signal suffers from different effects, which are characterized as follows: Multipath propagation occurs as a consequence of reflections, scattering, and diffraction of the transmitted electromagnetic wave at natural and manmade , at the receiver antenna, a multitude of waves arrives from many different directions with different delays, attenuations, and superposition of these waves results in amplitude and phase variations of the posite received spread is caused by moving objects in the mobile radio in the phases and amplitudes of the arriving waves occur which lead to timevariant multipath small movements on the order of the wavelength may result in a totally different wave varying signal strength due to timevariant multipath propagation is referred to as fast is caused by obstruction of the transmitted waves by, ., hills, buildings, walls, and trees, which results in more or less strong attenuation of the signal to fast fading, longer distances have to be covered to significantly change the shadowing varying signal strength due to shadowing is called slow fading and can be described by a lognormal distribution [36].Path loss indicates how the mean signal power decays with distance between transmitter and free space, the mean signal power decreases with the square of the distance between base station(BS)and terminal station(TS).In a mobile radio channel, where often no line of sight(LOS)path exists, signal power decreases with a power higher than two and is typically in the order of three to of the received power due to shadowing and path loss can be efficiently counteracted by power the following, the mobile radio channel is described with respect to its fast fading Channel Modeling The mobile radio channel can be characterized by the timevariant channel impulse response h(τ , t)or by the timevariant channel transfer function H(f, t), which is the Fourier transform of h(τ , t).The channel impulse response represents the response of the channel at time t due to an impulse applied at time t ? mobile radio channel is assumed to be a widesense stationary random process, ., the channel has a fading statistic that remains constant over short periods of time or small spatial environments with multipath propagation, the channel impulse response is posed of a large number of scattered impulses received over Np different paths，Whereand ap, fD,p, ?p, and τp are the amplitude, the Doppler frequency, the phase, and the propagation delay, respectively, associated with path p, p = 0,..., Np ? assigned channel transfer function isThe delays are measured relative to the first detectable path at the Doppler Frequencydepends on the velocity v of the terminal station, the speed of light c, the carrier frequency fc, and the angle of incidence αp of a wave assigned to path channel impulse response with corresponding channel transfer function is illustrated in Figure delay power density spectrum ρ(τ)that characterizes the frequency selectivity of the mobile radio channel gives the average power of the channel output as a function of the delay mean delay τ , the root mean square(RMS)delay spread