【正文】 隨機出現(xiàn)的，盡可能地顯示為“噪音像”。在任何情況下，代碼必須確保是可再生的?；蛘呓邮掌鞑荒芴崛∫寻l(fā)出去的消息。因此，該序列是幾乎是隨機的 。這樣的代碼被稱為偽隨機數(shù)（PRN）或序列。最常用的方法來產(chǎn)生偽隨機是基于反饋移位寄存器的。許多書籍都在介紹偽隨機碼的發(fā)展與特征，但是，實際的發(fā)展已超出了這些教材所敘述的。注意的是，建立或選擇適當?shù)男蛄谢蛐蛄屑⒉皇俏⒉蛔愕赖?。為了保證有效的擴頻通信，偽隨機序列必須尊重一定的規(guī)律如長度、自相關、互相關、正交。比較受歡迎偽隨機序列有Barker碼，M序列碼，Gold碼，Walsh碼等?？紤]到存在更復雜的序列集，給它提供了一個更強大的擴展頻譜鏈路。但是這產(chǎn)生了成本問題：擴頻和解擴都需要在速度和性能都更復雜的電子產(chǎn)品，數(shù)字擴頻解擴芯片包含幾百萬個等效的2輸入與非門在幾十兆赫間切換。An Introduction to SpreadSpectrum CommunicationsAbstract:This application note is a tutorial overview of spreadspectrum discussion covers both directsequence and fasthopping equations are given to allow performance directsequence spreadspectrum(DSSS) and frequencyhopping spreadspectrum(FHSS) methods.Introduction As spreadspectrum techmiques bee increasingly popular,electrical engineers outside the field are eager for understandable explanations of the are books and websites on the subject,but many are hard to understand or describe some aspects while ignoring others(.,the DSSS technique with extensive focus on PRNcode generation).The following discussion covers the full spectrum(pun intended).A Short HistorySpreadspectrum munications 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 received technology was not taken seriously at that time by the and was forgotten until the 1980s,when it became then the technology has bee increasingly popular for application that involve radio links in hostile environments.Typical applications for the resulting shortrange data transceivers include satellitepositioning systemsGPS，3G mobile telemunications,WLAN(IEEE174。,IEEE ,IEEE ),and Bluetooth174。.Spreadspectrum techniques also aid in the endless race between munication needs and radiofrequency availabilitysituations where the radio spectrum is limited and is,therefore,an expensive resource.Theoretical Justification for Spread Spectrum Spreadspectrum is apparent in the Shannon and Hartley channelcapacity theorem:C=Blog2(1+S/N) ()In this equation,C is the channel capacity in bits per second(bps),which is the maximum data rate for a theoretical biterror rate(BER).B is the required channel bandwidth in Hz,and S/N is the signaltonosie power be more explicit,one assumes that C,which represents the amount of information allowed by the munication channel,also represents the desired (B) is the price to be paid,bacause frequency is a limited S/N ratio expresses the environmental conditions or the physical characteristics (., 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 approach says that one can maintain or even increase munication 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 Napierian number) and by noting that In=loge.Therefore:C/B=(1/ln2)ln(1+S/N)=ln(1+S/N) ()Applying the MacLaurin series development forln(1+x)=xx2/2+x3/3x4/4+…+(1)k+1xk/k+…:C/B=(S/N1/2(S/N)2+1/3(S/N)3…) ()S/N is usually low for spreadspectrum applications. (As just mentioned, the signal power density can even be below the noise level.) Assuming a noise level such that S/N 1,Shannon39。s expression bees simply:C/B≈S/N ()Very roughly:C/N≈S/N ()Or:N/S≈B/C ()To send errorfree information for a given noisetosignal ratio in the channel,therefore,one need only perform the fundamental spreadspectrum signalspreading operation:increase the transmitted principle seems simple and ,implementation is plex,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 despreading operations necessary.DefinitionsDifferent spreadspectrum techniques are available,but all have one idea in mon:the key (also called the code or sequence) attached to the munication manner of inserting this code defines precisely the spreadspectrum 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 munication channel.The formal definition of spread spectrum is more precise:an RF munications 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 ratio (in dB) between the spread baseband and the original signal is called processing spreadspectrum processing gains run from 10dB to 60dB.To apply a spreadspectrum technique,simply inject the corresponding spreadspectrum code somewhere in the transmitting chain before the antenna (receiver).Conversely,you can remove the spreadspectrum code (called a despreading operation) at a point in the receive chain before data despreading operation reconstitutes the information into its original ,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:Spreadspectrum munication systemBandwidth Effects of the Spreading OperationFigure 2 illustrates the evaluation of signal bandwidths in a munication link.Figure 2:Spreading operation spreads the signal energy over