王延 英文翻譯.docx

畢業(yè)（設(shè)計(jì)）論文外文翻譯外文題目 AN INTRODUCTION TO SPEREAD-SPECTRUM COMMUNICATION 譯文題目 擴(kuò)頻通信系統(tǒng)的介紹 系 別 機(jī)電工程系 專 業(yè) 測(cè)控技術(shù)與儀器 班 級(jí) 162902 學(xué)生姓名 王 延 學(xué) 號(hào) 092151 指導(dǎo)教師 吳鵬飛 報(bào)告日期 2012.3.18 AN INTRODUCTION TO SPEREAD-SPECTRUM COMMUNICATION Introduction As spread-spectrum techniques become increasingly popular, electrical engineers outside the field are eager for understandable explanations of the technology. There are books and websites on the subject, but many are hard to understand or describe some aspects while ignoring others(e.g., the DSSS technique with extensive focus on PRN-code generation).The following discussion covers the full spectrum(pun intended).A Short HistorySpread-spectrum communications technology was first described on paper by an actress and a musician! In 1941 Hollywood actress Hedy Lamarr and pianist George Antheil described a secure radio link to control torpedoes. They received U.S. Patent #2.292.387.The technology was not taken seriously at that time by the U.S. Army and was forgotten until the 1980s, when it became active. Since then the technology has become increasingly popular for application that involve radio links in hostile environments.Typical applications for the resulting short-range data transceivers include satellite-positioning systems GPS，3G mobile telecommunications, W-LAN(IEEE802.11a,IEEE 802.11b,IEEE 802.11g),and Bluetooth. Spread-spectrum techniques also aid in the endless race between communication needs and radio-frequency availability-situations where the radio spectrum is limited and is, therefore, an expensive resource.Theoretical Justification for Spread Spectrum Spread-spectrum is apparent in the Shannon and Hartley channel-capacity theorem: C=Blog2(1+S/N) (Eq.1)In this equation, C is the channel capacity in bits per second (bps), which is the maximum data rate for a theoretical bit-error rate (BER). B is the required channel bandwidth in Hz, and S/N is the signal-to-noise power ratio. To be more explicit, one assumes that C, which represents the amount of information allowed by the communication channel, also represents the desired performance. Bandwidth (B) is the price to be paid, because frequency is a limited resource. The S/N ratio expresses the environmental conditions or the physical characteristics (i.e., obstacles ,presence of jammers ,interferences, etc.).There is an elegant interpretation of this equation, applicable for difficult environments, for example, when a low S/N ratio is caused by noise and interference. This approach says that one can maintain or even increase communication performance (high C) by allowing or injecting more bandwidth (high B), even when signal power is below the noise floor. (The equation does not forbid that condition!)Modify Equation 1 by changing the log base from 2 to e (the Napier Ian number) and by noting that in=loge.Therefore:C/B= (1/ln2) ln(1+S/N)=1.443ln(1+S/N) (Eq.2)Applying the MacLaurin series development forln(1+x)=x-x2/2+x3/3-x4/4+(-1)k+1xk/k+:C/B=1.443(S/N-1/2(S/N)2+1/3(S/N)3-) (Eq.3)S/N is usually low for spread-spectrum applications. (As just mentioned, the signal power density can even be below the noise level.) Assuming a noise level such that S/N 1, Shannons expression becomes simply:C/B1.443S/N (Eq.4)Very roughly:C/NS/N (Eq.5)Or:N/SB/C (Eq.6)To send error-free information for a given noise-to-signal ratio in the channel, therefore, one need only performs the fundamental spread-spectrum signal-spreading operation: increase the transmitted bandwidth. That principle seems simple and evident. Nonetheless, implementation is complex, mainly because spreading the baseband (by a factor that can be several orders of magnitude) forces the electronics to act and react accordingly, which, in turn, makes the spreading and dispreading operations necessary.DefinitionsDifferent spread-spectrum techniques are available, but all have one idea in common: the key (also called the code or sequence) attached to the communication channel. The manner of inserting this code defines precisely the spread-spectrum technique. The term spread spectrum refers to the expansion of signal bandwidth, by several orders of magnitude in some cases, which occurs when a key is attached to the communication channel.The formal definition of spread spectrum is more precise: an RF communications system in which the baseband signal bandwidth is intentionally spread over a larger bandwidth by injecting a higher frequency signal (Figure 1).As a direct consequence, energy used in transmitting the signal is spread over a wider bandwidth, and appears as noise. The ratio (in dB) between the spread baseband and the original signal is called processing gain. Typical spread-spectrum processing gains run from 10dB to 60dB.To apply a spread-spectrum technique, simply inject the corresponding spread-spectrum code somewhere in the transmitting chain before the antenna (receiver).Conversely, you can remove the spread-spectrum code (called a dispreading operation) at a point in the receive chain before data retrieval. A dispreading operation reconstitutes the information into its original bandwidth. Obviously, the same code must be known in advance at both ends of the transmission channel. (In some circumstances, the code should be known only by those two parties.)Figure 1.Spread-spectrum communication systemBandwidth Effects of the Spreading OperationFigure 2 illustrates the evaluation of signal bandwidths in a communication link.Figure 2.Spreading operation spreads the signal energy over a wider frequency bandwidth.Spread-spectrum modulation is applies on top of a conventional modulation such as BPSK or direct conversion. One can demonstrate that all other signals not receiving the spread-spectrum code will remain ad they are, that is, unspread.Bandwidth Effects of the Dispreading Operation Similarly, dispreading can be seen in Figure 3.Figure 3. The dispreading operation recovers the original signal.Here a spread-spectrum demodulation has been made on top of the normal demodulation operations. One can also demonstrate that signals such as an interferer or jammer added during the transmission will be spread during the dispreading operation!Waste of Bandwidth Due to Spreading Is Offset by Multiple UsersSpreading results directly in the use of a wider frequency band by a factor that corresponds exactly to the processing gain mentioned earlier. Therefore spreading does not spare the limited frequency resource. That overuse is well compensated, however by the possibility that many users will share the enlarged frequency band (Figure 4).Figure 4. The same frequency band can be shared by multiple Users with spread-spectrum techniques.Spread Spectrum Is a Wideband Technology In contrast to regular narrowband technology, the spread-spectrum process is a wideband technology. W-CDMA and UMTS, for example, are wideband technologies that require a relatively large frequency bandwidth, compared to narrowband radio.Benefits of Spread Spectrum Resistance to Interference and Ant jamming EffectsThere are many benefits to spread-spectrum technology. Resistance to interference is the most important advantage. Intentional or unintentional interference and jamming signals are rejected because they do not contain the spread-spectrum key. Only the desired signal, which has the key, will be seen at the receiver when the dispreading operation is exercised. See Figure 5.Figure 5. A spread-spectrum communication system. Note that the interferers energy is spread while the data signal is dispread in the receive chain.You can practically ignore the interference, narrowband or wideband, if it does not include the key used in the dispreading operation. That rejection also applies to other spread-spectrum signals that do not have the right key. Thus different spread-spectrum communications can be active simultaneously in the same band, such as CDMA. Note that spread-spectrum is a wideband technology, but the reverse is not true: wideband techniques need not involve spread-spectrum technology.Resistance to Interception Resistance to interception is the second advantage provided by spread-spectrum techniques. Because no authorized listeners do not have the key used to spread the original signal, those listeners cannot decode it. Without the right key, the spread-spectrum signal appears as noise or as an interferer. Scanning methods can break the code, however, if the key is short.) Even better, signal levels can be below the noise floor, because the spreading operation reduces the spectral density. See Figure 6. (Total energy is the same, but it is widely spread in frequency.) The message is thus made invisible, an effect that is particularly strong with the direct-sequence spread-spectrum (DSSS) technique. (DSSS is discussed in greater detail below.) Other receivers cannot “see” the transmission; they only register a slight increase in the overall noise level!Figure 6.Spread-spectrum signal is buried under noise level. The receiver cannot see the transmission without the right spread-spectrum keys.Resistance to Fading (Multipath Effects)Wireless channels often include multiple-path propagation in which the signal has more than one path from the transmitter to the receiver (Figure 7).Such multipath can be caused by atmospheric reflection or refraction, and by reflection from the ground or from objects such as buildings.Figure 7.Illustration of how the signal can reach the receiver over multiple paths.The reflected path (R) can interfere with the direct path (D) in a phenomenon called fading. Because the dispreading process synchronizes to signal D, signal R is rejected even though it contains the same key. Methods are available to use the reflected-path signals by dispreading them and adding the extracted results to the main one.Spread Spectrum Allows CDMANote that spread spectrum is not a modulation scheme, and should not be confused with other types of modulation. One can, for example, use spread-spectrum techniques to transmit a signal modulated by PSK or BPSK. Thanks to the coding basis, spread spectrum can also be used as another method for implementing multiple access (i.e., the real or apparent coexistence of multiple and simultaneous communication links on the same physical media).So far, three main methods are available.FDMA-Frequency Division Multiple AccessFDMA allocates a specific carrier frequency to a communication channel. The number of different users is limited to the number of “slices” in the frequency spectrum (Figure 8).Of the three methods for enabling multiple access, FDMA is the least efficient in term of frequency-band usage. Methods of FDMA access include radio broadcasting, TV, AMPS, and TETRAPOLE. Figure 8.Carrier-frequency allocations among different users in a FDMA system.TDMA-Time Division Multiple Access With TDMA the different users speak and listen to each other according to a defined allocation of time slots (Figure 9).Different communication channels can then be established for a unique carrier frequency. Examples of TDMA are GSM, DECT, TETRA, and IS-136.Figure 9. Time-slot allocations among different users in a TDMA system.CDMA-Code Division Multiple AccessCDMA access to the air is determined by a key or code (Figure 10).In that since, spread spectrum is a CDMA access. The key must be defined and known in advance at the transmitter and receiver ends. Growing examples are IS-95 (DS), IS-98, Bluetooth, and WLAN.擴(kuò)頻通信系統(tǒng)的介紹簡(jiǎn)介擴(kuò)頻技術(shù)越來(lái)越受歡迎，就連這一領(lǐng)域以外的電器工程師都渴望能夠深入理解這一技術(shù)。很多書和網(wǎng)站上都有關(guān)于這方面的書，但是，很多都很難理解或描述的不夠詳盡。（例如，直接序列擴(kuò)頻技術(shù)廣泛關(guān)注的是偽隨機(jī)碼的產(chǎn)生）。下面討論擴(kuò)頻技術(shù)（雙關(guān)語(yǔ)意）。簡(jiǎn)史一名女演員和一名音樂(lè)家首次以書面形式描述了擴(kuò)頻通信技術(shù)。1941年，好萊塢女星Hedy Lamarr和鋼琴家George Antheil描述一個(gè)安全的無(wú)線鏈路來(lái)控制魚雷。他們獲得了美國(guó)專利#2.292.387。但這一技術(shù)被遺忘了，沒(méi)有在當(dāng)時(shí)受到美軍的重視，直到20世紀(jì)80年代它才開(kāi)始活躍起來(lái)。從那時(shí)起，這一技術(shù)在有關(guān)惡劣環(huán)境中的收音機(jī)鏈接方面越來(lái)越受歡迎。最典型的擴(kuò)頻技術(shù)應(yīng)用是數(shù)據(jù)收發(fā)器包括衛(wèi)星定位系統(tǒng)（GPS）、3G移動(dòng)通信、無(wú)限局域網(wǎng)(符合IEEE802.11a,IEEE 802.11b,IEEE 802.11g標(biāo)準(zhǔn))，還有藍(lán)牙技術(shù)也幫助了那些通訊落后和無(wú)線電通信條件有限的地方，因此，它是一種昂貴的資源。擴(kuò)頻通信的原理擴(kuò)頻是香農(nóng)定理的典型：C=Blog2(1+S/N) 公式（1）在公式中，C為信道容限，單位是比特/秒(bps),意指單位時(shí)間內(nèi)信道中無(wú)差錯(cuò)傳輸?shù)淖畲笮畔⒘?。B為信號(hào)頻帶寬度，單位是Hz，S/N為信噪比。也就是說(shuō)，C為信道允許通過(guò)的信息量，也代表了擴(kuò)頻的性能。帶寬(B)是代價(jià)，因?yàn)轭l率是一個(gè)有限的資源。信噪比體現(xiàn)了環(huán)境條件或物理特性（如障礙、干擾器、干擾等）。上式說(shuō)明，的情況下，在無(wú)差錯(cuò)傳輸?shù)男畔⑺俾蔆不變時(shí)，如果信噪比很低，則可以用足夠?qū)挼膸拋?lái)傳輸信號(hào)，即使信號(hào)功率密度低于噪音水平。（公式可用?。└淖児剑?）中對(duì)數(shù)的底數(shù)，2改為e，則為In=loge。因此，C/B=(1/ln2)ln(1+S/N)=1.443ln(1+S/N) 公式（2） 根據(jù)MacLaurin擴(kuò)展公式ln(1+x)=x-x2/2+x3/3-x4/4+(-1)k+1xk/k+:C/B=1.443(S/N-1/2(S/N)2+1/3(S/N)3-) 公式（3）在擴(kuò)頻應(yīng)用中，通常S/N很低。（正如剛才提到的，信號(hào)功率密度甚至低于噪音水平。）假定噪音水平即S/N1，香農(nóng)公式可簡(jiǎn)單表示為：C/B1.443S/N 公式（4）簡(jiǎn)化為：C/NS/N 公式（5）或者：N/SB/C 公式（6）向固定了信噪比的信道發(fā)送錯(cuò)誤的信息，只要執(zhí)行基本擴(kuò)頻信號(hào)的傳播操作：增加傳輸帶寬。盡管這一原則看起來(lái)很簡(jiǎn)單明確，但實(shí)現(xiàn)她卻很復(fù)雜，主要是因?yàn)檎箤捇鶐У碾娮釉O(shè)備必須同時(shí)存在展寬和解擴(kuò)的操作過(guò)程。定義不同的擴(kuò)頻技術(shù)都有一個(gè)共同之處：密鑰（也稱為代碼或序列）依附于傳輸信道。以插入代碼的形式準(zhǔn)確地定義擴(kuò)頻技術(shù)，術(shù)語(yǔ)“頻譜擴(kuò)展”是指擴(kuò)頻信號(hào)的幾個(gè)數(shù)量級(jí)的帶寬在有密鑰的傳輸信道中的擴(kuò)展。以傳統(tǒng)的方式定義擴(kuò)頻更為精確：在射頻通信系統(tǒng)中，將基帶信號(hào)擴(kuò)展為比原有信號(hào)的帶寬寬得多的高頻信號(hào)（如圖1）。在此過(guò)程中，傳輸寬帶信號(hào)產(chǎn)生的損耗，表現(xiàn)為噪聲。擴(kuò)頻信號(hào)帶寬與信息帶寬之比稱為處理增益。擴(kuò)頻過(guò)程的處理增益大都在10dB 到60dB之間。要應(yīng)用擴(kuò)頻技術(shù)，只需在天線（接收器）之前加入相應(yīng)的擴(kuò)頻碼。相反，你可以刪除一個(gè)點(diǎn)的擴(kuò)頻碼（稱為解擴(kuò)操作）接收發(fā)射鏈路數(shù)據(jù)恢復(fù)。解擴(kuò)過(guò)程是重新恢復(fù)原始帶寬的過(guò)程。很明顯，同樣的代碼必須在事先知道在傳輸通道兩端的信息。（在某些情況下，在調(diào)制和解調(diào)的過(guò)程中代碼應(yīng)該是知道的）。 輸電鏈擴(kuò)頻代碼接收鏈擴(kuò)頻代碼數(shù)據(jù)輸入射頻輸出射頻輸入RF IN射頻連接相同的配置序列數(shù)據(jù)輸出圖1.擴(kuò)頻通信系統(tǒng)傳播工作帶寬的影響圖2說(shuō)明了信號(hào)帶寬的通信鏈路評(píng)估輸入的擴(kuò)頻碼頻率數(shù)據(jù)的處理增益數(shù)據(jù)輸入寬度擴(kuò)頻調(diào)制數(shù)據(jù)輸入能量能量PF載體圖2.擴(kuò)頻操作遍及一個(gè)更寬的頻率帶寬的信息能量擴(kuò)頻調(diào)制是一種適用于如BPSK或直接轉(zhuǎn)換。傳統(tǒng)的調(diào)制可以證明所有其他信號(hào)接收不到擴(kuò)頻代碼將保持它們?cè)械男畔ⅲ瑯O沒(méi)有被擴(kuò)展。解擴(kuò)過(guò)程中帶寬的影響同樣，解擴(kuò)過(guò)程如圖3。能量數(shù)據(jù)輸入寬度數(shù)據(jù)輸入解擴(kuò)調(diào)制能量輸入的擴(kuò)頻碼數(shù)據(jù)的處理增益PF載體頻率圖3,在解擴(kuò)過(guò)程中恢復(fù)的原有信號(hào) 在這里，解擴(kuò)調(diào)制已經(jīng)取得了正常解調(diào)操作，也表明了干擾或干擾信號(hào)在解擴(kuò)傳輸過(guò)程中被擴(kuò)展！由于帶寬的浪費(fèi)抵消了傳播的多用戶,擴(kuò)頻結(jié)果直接在一個(gè)更寬的頻帶使用，完全對(duì)應(yīng)之前的“處理增益”。 因此擴(kuò)頻并沒(méi)有節(jié)約有限的頻率資源。過(guò)度的使用雖然得到了補(bǔ)償，但是可能有很多用戶共享這一擴(kuò)大頻率波段（如圖4）。用戶1+用戶2+用戶3+用戶N數(shù)據(jù)輸入獲得的擴(kuò)頻增益圖4.在相同的頻帶多個(gè)用戶共享擴(kuò)頻技術(shù)。擴(kuò)頻是寬帶技術(shù),相對(duì)于常規(guī)窄帶技術(shù)，擴(kuò)頻過(guò)程是一種寬帶技術(shù)。例如，W - CDMA和UMTS都是寬帶技術(shù)，與窄帶廣播相比，它需要一個(gè)比較大的頻率帶寬。擴(kuò)頻的優(yōu)點(diǎn)、抗干擾性能和抗干擾的影響擴(kuò)頻技術(shù)有很多優(yōu)點(diǎn)。抗干擾性是最重要的一個(gè)優(yōu)點(diǎn)。有意或無(wú)意的干擾和干擾信號(hào)都是不希望存在的因?yàn)樗鼈儾话瑪U(kuò)頻密鑰。只有期望信號(hào)才有密鑰，在解擴(kuò)過(guò)程中才會(huì)被接收器接收，如圖5。輸電鏈擴(kuò)頻代碼接收鏈擴(kuò)頻代碼數(shù)據(jù)輸入射頻輸出射頻輸入RF IN射頻連接數(shù)據(jù)輸出數(shù)據(jù) 干擾數(shù)據(jù)擴(kuò)展和干擾擴(kuò)展數(shù)據(jù)擴(kuò)展數(shù)據(jù)擴(kuò)展和干擾圖5.擴(kuò)頻通信系統(tǒng)。注意，解擴(kuò)鏈路中數(shù)據(jù)信號(hào)被傳輸?shù)耐瑫r(shí)干擾能源也被傳輸。無(wú)論在窄帶或?qū)拵е?，如果它不涉及解擴(kuò)過(guò)程，你幾乎可以忽略干擾。這種抑制反應(yīng)也適用于其他沒(méi)有正確密鑰的擴(kuò)頻信號(hào)。因此不同的擴(kuò)頻通信系統(tǒng)可以工作在同一頻段，例如CDMA。值得注意的是，擴(kuò)頻是寬帶技術(shù)，但反之則不然：寬帶技術(shù)不涉及擴(kuò)頻技術(shù)?？菇孬@抗截獲是擴(kuò)頻通信技術(shù)的第二個(gè)優(yōu)勢(shì)。由于非法的聽(tīng)眾沒(méi)有密鑰用于原始信號(hào)傳播，這些聽(tīng)眾無(wú)法解碼。沒(méi)有合適的鑰匙，擴(kuò)頻信號(hào)會(huì)出現(xiàn)噪音或干擾。（掃描方法可以打破的這些密鑰，但是密鑰是短暫的。）甚至更好，信號(hào)電平可以低于噪聲水平，因?yàn)閿U(kuò)頻傳輸降低了頻譜密度，如圖6。（總能量是相同的，但它是廣泛存在于頻率的。）因此信息是無(wú)形的，這一