【正文】 and the simple reproducibility of the simulation The coherence bandwidth of a mobile radio channel is the bandwidth over which the signal propagation characteristics are correlated and it can be approximated byThe channel is frequencyselective if the signal bandwidth B is larger than the coherence the other hand, if B is smaller than , the channel is frequency nonselective or coherence bandwidth of the channel is of importance for evaluating the performance of spreading and frequency interleaving techniques that try to exploit the inherent frequency diversity Df of the mobile radio the case of multicarrier transmission, frequency diversity is exploited if the separation of subcarriers transmitting the same information exceeds the coherence maximum achievable frequency diversity Df is given by the ratio between the signal bandwidth B and the coherence bandwidth，The coherence time of the channel is the duration over which the channel characteristics can be considered as timeinvariant and can be approximated byIf the duration of the transmitted symbol is larger than the coherence time, the channel is the other hand, if the symbol duration is smaller than , the channel is time nonselective during one symbol coherence time of the channel is of importance for evaluating the performance of coding and interleaving techniques that try to exploit the inherent time diversity DO of the mobile radio diversity can be exploited if the separation between time slots carrying the same information exceeds the coherence number of Ns successive time slots create a time frame of duration maximum time diversity Dt achievable in one time frame is given by the ratio between the duration of a time frame and the coherence time, A system exploiting frequency and time diversity can achieve the overall diversityThe system design should allow one to optimally exploit the available diversity instance, in systems with multicarrier transmission the same information should be transmitted on different subcarriers and in different time slots, achieving uncorrelated faded replicas of the information in both multicarrier systems with flat fading per subchannel and timeinvariance during one symbol cannot exploit diversity and have a poor performance in time and frequency selective fading methods have to be applied to exploit approach is the use of data spreading where each data symbol is spread by a spreading code of length , in bination with interleaving, can achieve performance results which are given forby the closedform solution for the BER for diversity reception in Rayleigh fading channels according to [40]Whererepresents the binatory function，and σ2 is the variance of the soon as the interleaving is not perfect or the diversity offered by the channel is smaller than the spreading code length L, or MCCDMA with multiple access interference is applied,()is a lower L = 1, the performance of an OFDM system without forward error correction(FEC)is obtained, 9which cannot exploit any BER according to()of an OFDM(OFDMA, MCTDMA)system and a multicarrier spread spectrum(MCSS)system with different spreading code lengths L is shown in Figure other diversity techniques are modulation is used for symbol mobile radio channel is modeled as uncorrelated Rayleigh fading channel(see Section ).As these curves show, for large values of L, the performance of MCSS systems approaches that of an AWGN form of achieving diversity in OFDM systems is channel coding by FEC, where the information of each data bit is spread over several code to the diversity gain in fading channels, a coding gain can be obtained due to the selection of appropriate coding and decoding 1基本原理這章描述今日的基本面的無(wú)線(xiàn)通信。但日子在繼續(xù)。，慶祝我的生日。However, if we do not have enough courage, the plan may prove to be a plete ，不要事事依靠父母。She wants to further her study abroad after do 。Whether she is telling the truth or not ,nobody really 。Whether the research team will acplish the task ro not, nobody really ，沒(méi)有人會(huì)真的知道。我也想。He will fall asleep as soon as his head touches the ，如果我們沒(méi)有足夠的勇氣，這個(gè)計(jì)劃將會(huì)徹底失敗。Please remain seated an keep the windows would like to ask my supervisor to write a reference letter for 。Help yourself to the it while it’s live and die but life ，但生活仍在繼續(xù)。Otherwise, it is likely that the customers will lose their interest in our 。平均時(shí)延特性參數(shù)為有圖12時(shí)變信道沖激響應(yīng)和通道傳遞函數(shù)頻率選擇性衰落是權(quán)力頁(yè)的路徑均方根時(shí)延擴(kuò)展的定義為 同樣，多普勒頻譜的功率密度（FD）的特點(diǎn)可以定義在移動(dòng)時(shí)變無(wú)線(xiàn)信道，并給出了作為一種金融衍生工具功能的多普勒頻率通道輸出的平均功率。在案件的多通道包含洛杉磯的或主要組件除了隨機(jī)移動(dòng)散射，通道脈沖響應(yīng)可以不再被建模為均值為零。對(duì)于多載波調(diào)制系統(tǒng)，子通道間距FS可以變得非常小，這樣可以造成嚴(yán)重的多普勒效應(yīng)ICI的。這些通道模型定義的離散傳播路徑的統(tǒng)計(jì)信息?；蛘?，不同的多普勒譜定義在[8]個(gè)人頻道。細(xì)胞類(lèi)型代表的Pico與細(xì)胞體積小于100工業(yè)建筑物和辦公室中的10 m階米室內(nèi)信道模型。由于這些曲線(xiàn)顯示，辦法，AWGN信道的一對(duì)L時(shí)，對(duì)MCSS系統(tǒng)性能有很大價(jià)值。由于在加工過(guò)程中會(huì)產(chǎn)生殘余應(yīng)力，傳統(tǒng)加工方法還會(huì)造成刀具磨損，損壞產(chǎn)品質(zhì)量。傳統(tǒng)加工工藝依靠硬質(zhì)刀具或磨料去除較軟的材料，而特種加工工藝如電火花加工，則是利用電火花或熱能來(lái)電蝕除余料，以獲得所需的零件形狀。電極之間的間隙只有千分之幾英寸，通過(guò)伺服機(jī)構(gòu)驅(qū)動(dòng)和控制工具電極的進(jìn)給使該值保持常量。由于蝕刻劑沿垂直和水平方向開(kāi)始蝕除材料，鉆蝕（又稱(chēng)為淘蝕）量進(jìn)一步加大。利用陽(yáng)極溶解，電化學(xué)加工可以去除具有導(dǎo)電性質(zhì)工件的材料，而無(wú)須機(jī)械能和熱能。為了獲得電化學(xué)過(guò)程形狀復(fù)制的高精度和高的材料去除率，需要采用高的電流密度（范圍為10～100 A/cm2)和低電壓（范圍為8～30V）。激光束加工LASER是英文Light Amplification by Stimulated Emission of Radiation 各單詞頭一個(gè)字母所組成的縮寫(xiě)詞。激光打孔深度可以控制到每個(gè)激光脈沖不超過(guò)一微米，且可以根據(jù)加工要求很靈活地留下非常淺的永久性標(biāo)記。采用傳統(tǒng)加工方法，非常難以加工硬脆材料如陶瓷等，而激光加工是解決此類(lèi)問(wèn)題的最好選擇。這種加工過(guò)程不產(chǎn)生熱量、無(wú)化學(xué)反應(yīng)，加工出的零件在微結(jié)構(gòu)、化學(xué)和物理特性方面都不發(fā)生變化，可以獲得無(wú)應(yīng)力加工表面。音極是暴露在超聲波振動(dòng)中的一小塊金屬或工具，它將振動(dòng)能傳給某個(gè)元件，從而激勵(lì)漿料中的磨粒。Nontraditional Machining Processes IntroductionTraditional or conventional machining, such as turning, milling, and grinding etc., uses mechanical energy to shear metal against another substance to create holes or remove machining processes are defined as a group of processes that remove excess material by various techniques involving mechanical, thermal, electrical or chemical energy or binations of these energies but do not use a sharp cutting tool as it is used in traditional manufacturing hard and brittle materials are difficult to be machined by traditional machining traditional methods to machine such materials means increased demand for time and energy and therefore increases in costs