王延 英文翻譯.docx

畢業(yè)（設計）論文外文翻譯外文題目 AN INTRODUCTION TO SPEREAD-SPECTRUM COMMUNICATION 譯文題目 擴頻通信系統的介紹 系 別 機電工程系 專 業(yè) 測控技術與儀器 班 級 162902 學生姓名 王 延 學 號 092151 指導教師 吳鵬飛 報告日期 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.擴頻通信系統的介紹簡介擴頻技術越來越受歡迎，就連這一領域以外的電器工程師都渴望能夠深入理解這一技術。很多書和網站上都有關于這方面的書，但是，很多都很難理解或描述的不夠詳盡。（例如，直接序列擴頻技術廣泛關注的是偽隨機碼的產生）。下面討論擴頻技術（雙關語意）。簡史一名女演員和一名音樂家首次以書面形式描述了擴頻通信技術。1941年，好萊塢女星Hedy Lamarr和鋼琴家George Antheil描述一個安全的無線鏈路來控制魚雷。他們獲得了美國專利#2.292.387。但這一技術被遺忘了，沒有在當時受到美軍的重視，直到20世紀80年代它才開始活躍起來。從那時起，這一技術在有關惡劣環(huán)境中的收音機鏈接方面越來越受歡迎。最典型的擴頻技術應用是數據收發(fā)器包括衛(wèi)星定位系統（GPS）、3G移動通信、無限局域網(符合IEEE802.11a,IEEE 802.11b,IEEE 802.11g標準)，還有藍牙技術也幫助了那些通訊落后和無線電通信條件有限的地方，因此，它是一種昂貴的資源。擴頻通信的原理擴頻是香農定理的典型：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),意指單位時間內信道中無差錯傳輸的最大信息量。B為信號頻帶寬度，單位是Hz，S/N為信噪比。也就是說，C為信道允許通過的信息量，也代表了擴頻的性能。帶寬(B)是代價，因為頻率是一個有限的資源。信噪比體現了環(huán)境條件或物理特性（如障礙、干擾器、干擾等）。上式說明，的情況下，在無差錯傳輸的信息速率C不變時，如果信噪比很低，則可以用足夠寬的帶寬來傳輸信號，即使信號功率密度低于噪音水平。（公式可用！）改變公式（1）中對數的底數，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MacLaurin擴展公式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）在擴頻應用中，通常S/N很低。（正如剛才提到的，信號功率密度甚至低于噪音水平。）假定噪音水平即S/N1，香農公式可簡單表示為：C/B1.443S/N 公式（4）簡化為：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)送錯誤的信息，只要執(zhí)行基本擴頻信號的傳播操作：增加傳輸帶寬。盡管這一原則看起來很簡單明確，但實現她卻很復雜，主要是因為展寬基帶的電子設備必須同時存在展寬和解擴的操作過程。定義不同的擴頻技術都有一個共同之處：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確地定義擴頻技術，術語“頻譜擴展”是指擴頻信號的幾個數量級的帶寬在有密鑰的傳輸信道中的擴展。以傳統的方式定義擴頻更為精確：在射頻通信系統中，將基帶信號擴展為比原有信號的帶寬寬得多的高頻信號（如圖1）。在此過程中，傳輸寬帶信號產生的損耗，表現為噪聲。擴頻信號帶寬與信息帶寬之比稱為處理增益。擴頻過程的處理增益大都在10dB 到60dB之間。要應用擴頻技術，只需在天線（接收器）之前加入相應的擴頻碼。相反，你可以刪除一個點的擴頻碼（稱為解擴操作）接收發(fā)射鏈路數據恢復。解擴過程是重新恢復原始帶寬的過程。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，在調制和解調的過程中代碼應該是知道的）。 輸電鏈擴頻代碼接收鏈擴頻代碼數據輸入射頻輸出射頻輸入RF IN射頻連接相同的配置序列數據輸出圖1.擴頻通信系統傳播工作帶寬的影響圖2說明了信號帶寬的通信鏈路評估輸入的擴頻碼頻率數據的處理增益數據輸入寬度擴頻調制數據輸入能量能量PF載體圖2.擴頻操作遍及一個更寬的頻率帶寬的信息能量擴頻調制是一種適用于如BPSK或直接轉換。傳統的調制可以證明所有其他信號接收不到擴頻代碼將保持它們原有的信息，極沒有被擴展。解擴過程中帶寬的影響同樣，解擴過程如圖3。能量數據輸入寬度數據輸入解擴調制能量輸入的擴頻碼數據的處理增益PF載體頻率圖3,在解擴過程中恢復的原有信號 在這里，解擴調制已經取得了正常解調操作，也表明了干擾或干擾信號在解擴傳輸過程中被擴展！由于帶寬的浪費抵消了傳播的多用戶,擴頻結果直接在一個更寬的頻帶使用，完全對應之前的“處理增益”。 因此擴頻并沒有節(jié)約有限的頻率資源。過度的使用雖然得到了補償，但是可能有很多用戶共享這一擴大頻率波段（如圖4）。用戶1+用戶2+用戶3+用戶N數據輸入獲得的擴頻增益圖4.在相同的頻帶多個用戶共享擴頻技術。擴頻是寬帶技術,相對于常規(guī)窄帶技術，擴頻過程是一種寬帶技術。例如，W - CDMA和UMTS都是寬帶技術，與窄帶廣播相比，它需要一個比較大的頻率帶寬。擴頻的優(yōu)點、抗干擾性能和抗干擾的影響擴頻技術有很多優(yōu)點。抗干擾性是最重要的一個優(yōu)點。有意或無意的干擾和干擾信號都是不希望存在的因為它們不包含擴頻密鑰。只有期望信號才有密鑰，在解擴過程中才會被接收器接收，如圖5。輸電鏈擴頻代碼接收鏈擴頻代碼數據輸入射頻輸出射頻輸入RF IN射頻連接數據輸出數據 干擾數據擴展和干擾擴展數據擴展數據擴展和干擾圖5.擴頻通信系統。注意，解擴鏈路中數據信號被傳輸的同時干擾能源也被傳輸。無論在窄帶或寬帶中，如果它不涉及解擴過程，你幾乎可以忽略干擾。這種抑制反應也適用于其他沒有正確密鑰的擴頻信號。因此不同的擴頻通信系統可以工作在同一頻段，例如CDMA。值得注意的是，擴頻是寬帶技術，但反之則不然：寬帶技術不涉及擴頻技術?？菇孬@抗截獲是擴頻通信技術的第二個優(yōu)勢。由于非法的聽眾沒有密鑰用于原始信號傳播，這些聽眾無法解碼。沒有合適的鑰匙，擴頻信號會出現噪音或干擾。（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，信號電平可以低于噪聲水平，因為擴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。）因此信息是無形的，這一