扩频通信外文翻译

1、扩频通信系统介绍摘要：本应用笔记概述了扩频技术原理，讨论了涵盖直接序列和快速跳频方法。 相关理论方程性能估算。以及讨论直接序列扩频（DSSS）和跳频（FHSS）这两种扩 频方式。简介扩频技术越来越受欢迎，就连这一领域以外电器工程师都渴望能够深入理解这 一技术。很多书和网站上都有关于这方面书，但是，很多都很难理解或描述不够详 尽。（例如，直接序列扩频技术广泛关注是伪随机码产生）。下面讨论扩频技术（双关语意）。简史一名女演员和一名音乐家首次以书面形式描述了扩频通信技术。1941年，好莱 坞女星Hedy Lamarr和钢琴家George Antheil描述一个安全无线链路来控制鱼雷。 他们获得了美国

2、专利#2. 292. 387。但这一技术被遗忘了，没有在当时受到美军重视, 直到20世纪80年代它才开始活跃起来。从那时起，这一技术在有关恶劣环境中收 音机链接方面越来越受欢迎。最典型扩频技术应用是数据收发器包括卫星定位系统（GPS）、3G移动通信、无 限局域网（符合IEEE®802. Ila, IEEE 802. 11b, IEEE 802. 11g标准），还有蓝牙技术 也帮助了那些通讯落后和无线电通信条件有限地方，因此，它是一种昂贵资源。扩频通信原理扩频是香农定理典型：C=BXlog2（l+S/N）公式（1）在公式中，C为信道容限，单位是比特/秒（bps）,意指单位时间内信道中无

3、差错 传输最大信息量。B为信号频带宽度，单位是Hz, S/N为信噪比。也就是说，C为 信道允许通过信息量，也代表了扩频性能。带宽（B）是代价，因为频率是一个有限 资源。信噪比体现了环境条件或物理特性（如障碍、干扰器、干扰等）。上式说明，情况下，在无差错传输信息速率C不变时，如果信噪比很低，则可 以用足够宽带宽来传输信号，即使信号功率密度低于噪音水平。（公式可用！）改变公式（1）中对数底数，2改为e,则为In二loge。因此，C/B= （l/ln2） X In（1+S/N） =1. 443X In（1+S/N）公式（2）根据MacLaurin扩展公式In （1+x） =x-x2/2+x3/3-x

4、4/4+ （-1） k+lxk/k+：C/B=l. 443X （S/N-l/2X （S/N）2+1/3X （S/N）3-）公式（3）在扩频应用中，通常S/N很低。（正如刚才提到，信号功率密度共至低于噪音水 平。）假定噪音水平即S/NG1,香农公式可简单表示为：C/B1.443XS/N公式（4）简化为： C/NS/N公式（5）或者： N/SB/C公式（6）向固定了信噪比信道发送错误信息，只要执行基本扩频信号传播操作：增加传 输带宽。尽管这一原则看起来很简单明确，但实现她却很复杂，主要是因为展宽基 带电子设备必须同时存在展宽和解扩操作过程。定义不同扩频技术都有一个共同之处：密钥（也称为代码或序列）

5、依附于传输信道。 以插入代码形式准确地定义扩频技术，术语“频谱扩展”是指扩频信号儿个数量级 带宽在有密钥传输信道中扩展。以传统方式定义扩频更为精确：在射频通信系统中，将基带信号扩展为比原有 信号带宽宽得多高频信号（如图1）。在此过程中，传输宽带信号产生损耗，表现为 噪声。扩频信号带宽及信息带宽之比称为处理增益。扩频过程处理增益大都在10dB 到60dB之间。要应用扩频技术，只需在天线（接收器）之前加入相应扩频码。相反，你可以 删除一个点扩频码（称为解扩操作）接收发射链路数据恢复。解扩过程是重新恢复 原始带宽过程。很明显，同样代码必须在事先知道在传输通道两端信息。（在某些 情况下，在调制和解调过

6、程中代码应该是知道）。数据愉入数据输出射频输出 平、/射频输入 射频连接 77T输电链 L 接收链扩频代码相同的配置序列扩频代码图1.扩频通信系统传播工作带宽影响图2说明了信号带宽通信链路评估图2.扩频操作遍及一个更宽频率带宽信息能量扩频调制是一种适用于如BPSK或直接转换。传统调制可以证明所有其他信号接 收不到扩频代码将保持它们原有信息，极没有被扩展。解扩过程中带宽影响同样，解扩过程如图3。图3,在解扩过程中恢复原有信号在这里，解扩调制已经取得了正常解调操作，也表明了干扰或干扰信号在解扩 传输过程中被扩展！由于带宽浪费抵消了传播多用户扩频结果直接在一个更宽频带使用，完全对应之前“处理增益”。

7、因此扩频并 没有节约有限频率资源。过度使用虽然得到了补偿，但是可能有很多用户共享这一 扩大频率波段（如图4）。用户1+用户2+用户3+用户N数据输入获得的扩频增益图4.在相同频带多个用户共享扩频技术。扩频是宽带技术相对于常规窄带技术，扩频过程是一种宽带技术。例如，W - CDMA和UMTS都是 宽带技术，及窄带广播相比，它需要一个比较大频率带宽。扩频优点 抗干扰性能和抗干扰影响扩频技术有很多优点。.抗干扰性是最重要一个优点。有意或无意干扰和干扰信 号都是不希望存在因为它们不包含扩频密钥。只有期望信号才有密钥，在解扩过程中才会被接收器接收，如图5。扩频代码数据数据扩展数据输出数据扩展和 干扰扩展

8、图5.扩频通信系统。注意，解扩链路中数据信号被传输同时干扰能源也被传输。无论在窄带或宽带中，如果它不涉及解扩过程，你几乎可以忽略干扰。这种抑 制反应也适用于其他没有正确密钥扩频信号。因此不同扩频通信系统可以工作在同 一频段，例如CDMA。值得注意是，扩频是宽带技术，但反之则不然：宽带技术不涉 及扩频技术。抗截获抗截获是扩频通信技术第二个优势。由于非法听众没有密钥用于原始信号传播, 这些听众无法解码。没有合适钥匙，扩频信号会出现噪音或干扰。（扫描方法可以 打破这些密钥，但是密钥是短暂。）甚至更好，信号电平可以低于噪声水平，因为 扩频传输降低了频谱密度，如图6o （总能量是相同，但它是广泛存在于频

9、率。）因 此信息是无形，这一影响在直接序列扩频（DSSS）技术上有充分体现。（在下文DSSS 作更详细说明。）其他接收机无法“看到”这种传输，它们只能出现在整体噪音水 平略有增加情况下！噪声基准口数据传播之前噪声基准1 扩展后的数据土图6.在被噪音水平之下扩频频谱信号。在没有正确扩频传输密钥情况下，接 收器不能“看到”传输过程。抗衰落（多径效应）无线信道通常具有多径传播，即有一个以上信号从发射机传到接收器（如图7）o 这种多路径可以通过空气反射或折射以及从地面反射或物体如这些路径建筑物引 起。图7.信号是如何通过多个路径到达接收器。这种反射路径（R）可干扰直接路径（D）现象称为解扩过程同步衰落

10、。因为解扩过 程使信号D及信号R同步被拒绝，即使它们包含了相同密钥。将反射路径信号应用 于解扩是个有用方法。扩频技术在CDMA应用请注意，扩展频谱不是一个扩频调制方案，不应及其他调制方式相混淆。例如 我们可以使用扩频技术发射一个由PSK或BPSK已调信号。.感谢调制信号编码基础, 使扩频频谱也可用于其他类型多址实现（即可以同时进行多个通讯联系和实际或表 面上相同物理介质共存）。到目前为止，有三个主要方法可用。FDMA-频分多址FDMA分配一个特定载波频率给通信信道。不同用户使用频谱切片数是受到限制 （如图8）o在已有三种多路存取方法中，FDMA在频带利用方面是效率最低。FDMA 方法包括Met

11、hods包括无线电广播，电视，高级移动电话系统AMPS等。Fcl Fc2 Fc3用户1用户2用户3 用户NFcN 频率 (kHz.MHz,GHz)图8. FDMA系统中不同用户载波频率分配。TDMA-时分多址TDMA不同用户彼此间发言和听取信息时，是根据定义时隙分配来处理（如图9）。 不同通信信道可以建立一个唯一载波频率。TDMA例子有全球移动通信系统GSM, DECT,CDMA-码分多址CDMA传播是由密钥或代码决定（如图10）o在这个意义上说，扩频就是一种 CDMA。在发射器和接收器密钥必须提前被定义和确定。它例子有IS-95 （DS）, IS- 98,蓝牙和无线局域网。图10. CDMA

12、系统中相同频带有独特钥匙或代码。当然，人们可以结合上述存取方法，例如，全球移动通信系统GSM结合了 TDMA 和FDMA。GSM定义了不同载波频率（细胞）拓扑领域，并设定时段内每一个细胞。扩频和（）编码密钥在这一点上，值得重申是扩频主要特点是一个代码或密钥必须在发射器和接收 器之前就是已知。现代通讯代码是数字序列必须长期存在和随机出现，尽可能地显 示为“噪音像”。在任何情况下，代码必须确保是可再生。或者接收港不能提取己 发出去消息。因此，该序列是几乎是随机。这样代码被称为伪随机数（PRN）或序 列。最常用方法来产生伪随机是基于反馈移位寄存器。许多书籍都在介绍伪随机码发展及特征，但是，实际发展已

13、超出了这些教材所 叙述。注意是，建立或选择适当序列或序列集并不是微不足道。为了保证有效扩频 通信，伪随机序列必须尊重一定规律如长度、自相关、互相关、正交。比较受欢迎 伪随机序列有Barker码，M序列码，Gold码，Walsh码等。考虑到存在更复杂序列 集，给它提供了一个更强大扩展频谱链路。但是这产生了成本问题：扩频和解扩都 需要在速度和性能都更复杂电子产品，数字扩频解扩芯片包含几百万个等效2输入 及非门在儿十兆赫间切换。An Introduction to Spread-Spectrum CommunicationsAbstract：This application note is a tu

14、torial overview of spread-spectrum principles. The discussion covers both direct-sequence and fast-hopping methods. Theoretical equations are given to allow performance estimates. Relation direct-sequence spread-spectrum(DSSS) and frequency-hopping spread-spectrum(FHSS) methods.IntroductionAs spread

15、-spectrum techmiques 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

16、 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 Anthei1 described

17、a secure radio link to control torpedos.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 radi

18、o links in hostile environments.Typical applications for the resulting short-range data transceivers include satellite-positioning systemsGPS , 3G mobile telecommunications, W-LAN(IEEE®802. 11a, IEEE 802. 11b,IEEE 802. 11g), and Bluetooth®. Spread-spectrum techniques also aid in the endles

19、s race between communication needs and radio-frequency availabi1ity-situations where the radio spectrum is limited and is, therefore, an expensive resource.Theoretical Justification for Spread SpectrumSpread-spectrum is apparent in the Shannon and Hartley channel-capacity theorem：C=BXlog2(l+S/N)(Eq.

20、 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-nosie power ratio. To be more explicit, one assumes that C, which represents the amount o

21、f information allowed by the communication channel, also represents the desired performance. Bandwidth (B) is the price to be paid, bacause frequency is a limited resource. The S/N ratio expresses the environmental conditions or the physical characteristics (i.e. , obstacles ,presence of jammers ,in

22、terferences, 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 m

23、ore 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=(l/ln2) X In(1+S/N)=1. 443 X ln(l+S/N)(Eq. 2)Applying the MacL

24、aurin series development forln(l +x) =x-x2/2+x3/3-x4/4+ (T) k+1 xk/k+：C/B=l. 443 X (S/N-1/2 X (S/N) 2+1/3 X (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 

25、1;1, Shannon，s expression becomes simply：C/B1.443XS/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 perform the fundamental spread-spectrum signal-spreading operation：increase the transmitted

26、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 despreading operations

27、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&

28、quot; 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 intentional

29、ly 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

30、. Typical spread-spectrum processing gains run from lOdB 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 despreading ope

31、ration) at a point in the receive chain before data retrieval. A despreading operation reconstitutes the information into itsoriginal 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

32、two parties.)Bandwidth Effects of the Spreading OperationFigure 2 illustrates the evaluation of signal bandwidths in acommunication 1ink.ENERGYDATA IN BBENERGY(DATA IN) SPREAD-SPECTRUM CODE! DATAIMBBX ! FREQUENCY PROCESSING GAINSPREADING ANDMODULATION OPERATIONS7DATA INRF CARRIERFigure 2. Spreading

33、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, unsprea

34、d.Bandwidth Effects of the Despreading OperationSimilarly, despreading can be seen in Figure 3.ENERGYENERGYDESPREADING ANDDEMODUSTIQN OPERATIONSDATAIN! DATAINB8X ! FREQUENCY PROCESSING GAINSPREAD-SPECTRUMCODERF CARRIERDATA IN BBFigure 3. The despreading operation recovers the original signal.Here a

35、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 despreading operation!Waste of Bandwidth Due to Spreading Is Offset by Multiple Users

36、Spreading 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 s

37、hare the enlarged frequency band (Figure 4).USER 1 + USER 2 USER 3 USER N |DATA IN BB X PROCESSING GAJNFigure 4. The same frequency band can be shared by multiple users with spread-spectrum techniques.Spread Spectrum Is a Wideband TechnologyIn contrast to regular narrowband technology, the spread-sp

38、ectrum 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 SpectrumResistance to Interference and Antijamming EffectsThere are many benefits to spread-spectrum t

39、echnology. 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 despreading operati

40、on is exercised. See Figure 5.0KA DATA SPREAD DATA SPREAD AN 0DATA SPREAD ANDIMTERFERER INTERFERER SPREADFigure 5. A spread-spectrum communication system. Note that the interferers energyis spread while the data signal is despread in the receive chain.You can practically ignore the interference, nar

41、rowband 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 spre

42、ad-spectrum is a wideband technology, but the reverse is not true:wideband techniques need not involve spread-spectrum technology.Resistance to InterceptionResistance to interception is the second advantage provided by spread-spectrum techniques. Because nonauthorized listeners do not have the key u

43、sed 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 sprea

44、ding 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

45、. ) Other receivers cannot “see" the transmission;they only register a slight increase in the overall noise level!0DATA BEFORE SPREADFigure 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

46、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.NOISE FLOOR | DATA SPREAD,Figure 6. Spread-s

47、pectrum signal is buried under noise level. The receiver cannot seethe 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 that one path from the transmitter to the receiver

48、 (Figure 7).Such multipaths can be caused by atmospheric reflection or refraction, and by reflection from the ground or from objects such as buildings.Spread Spectrum Allows CDMANote that spread spectrum is not a modulation scheme, and should not be confused with other types of modulation. One can,

49、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 sa

50、me 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

51、 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 AccessWith TDMA the different

52、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

53、system.CDMA-Code Division Multiple AccessCDMA access to the air is determined by a key or code (Figure 10). In that sence,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.Figure 10. CDMA systems access the same frequency band with unique keys or codes.One can,of course, combine the above a