【正文】 于傳統(tǒng)加工難以切削的硬脆材料。激光加工不會掛渣，沒有毛邊，可以精確控制幾何精度。激光切割的邊緣光滑且潔凈，無須進一步處理。激光加工中的熱影響區(qū)相對較窄，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（）可以實現(xiàn)局部的非接觸加工，而且對加工件幾乎沒有作用力。雖然激光在某些場合可用來作為放大器，但它的主要用途是光激射振蕩器，或者是作為將電能轉(zhuǎn)換為具有高度準直性光束的換能器。特別是在電化學加工中，材料去除率與被加工件的硬度、韌性及其他特性無關。通過將工具電極向去除工件表面材料的方向進給， mm范圍內(nèi)，～20 mm/min左右。金屬的去除由一個合適形狀的工具電極來完成，最終加工出來的零件具有給定的形狀、尺寸和表面光潔度。這個加工過程一般用于在高強度材料上加工復雜形腔和形狀，特別是在航空工業(yè)中如渦輪機葉片、噴氣發(fā)動機零件和噴嘴，以及在汽車業(yè)（發(fā)動機鑄件和齒輪）和醫(yī)療衛(wèi)生業(yè)中。通過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。在化學造型中最典型的公差范圍可保持在材料厚度的177。而被稱為保護層的特殊涂層所保護下的區(qū)域中的材料則不會被去除?；瘜W加工化學加工是眾所周知的特種加工工藝之一，它將工件浸入化學溶液通過腐蝕溶解作用將多余材料從工件上去除掉。這就會使懸浮在電介質(zhì)中的導電粒子聚集在電場最強處。因此，材料的硬度不再是電火花加工中的關鍵因素。常見的特種加工工藝描述如下?；谝韵赂鞣N特殊理由，特種加工工藝或稱為先進制造工藝，可以應用于采用傳統(tǒng)加工方法不可行，不令人滿意或者不經(jīng)濟的場合：； ； ；。傳統(tǒng)加工如車削、鉆削、刨削、銑削和磨削，都難以加工特別硬的或脆性材料。另一種實現(xiàn)形式的OFDM系統(tǒng)的多樣性是由前向糾錯信道編碼，在這里，每個數(shù)據(jù)位的信息分散在幾個代碼位。沒有其他的分集技術被應用。 GHz或24千兆赫。三種類型的通道模型定義。207信道的成本模型是基于一個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量。在這種傳播環(huán)境的幾個表中的相應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，如果多載波系統(tǒng)的設計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接收機域名可以使用[22]。只要所有子載波只要是一個共同的多普勒頻移金融衍生工具的影響，這可以補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)送符號的增加與子載波數(shù)控數(shù)目，因此，ISI的金額減少的持續(xù)時間。根據(jù)信道脈沖響應的假設一個復雜的值高斯過程，其大小通道的傳遞函數(shù)A的水稻分布給出賴斯因素KRice是由占主導地位的路徑權力的威力比分散的路徑。在對視線（LOS）或線的主要組成部分的情況下，這個過程是零的意思。多徑信道頻率分散性能是最常見的量化發(fā)生的多普勒頻率和多普勒fDmax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角度頁具有相應通道傳輸信道沖激響應函數(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