外文翻译外文文献英文文献扩频通信系统的介绍

合集下载

【VIP专享】扩频通信系统的介绍

An Introduction to Spread-Spectrum CommunicationsAbstract:This application note is a tutorial overview of spread-spectrum principles.The discussion covers both direct-sequence and fast-hopping methods.Theoretical equations are given to allow performance estimates.The following discussion of direct-sequence spread-spectrum(DSSS) and frequency-hopping spread-spectrum(FHSS) methods.As spread-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 with extensive focus on PRN-code generation).The following discussion covers the full spectrum.1.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 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 radio 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 endless race between communication needs and radio-frequency availability-situations where the radio spectrum is limited and is,therefore,an expensive resource.2.Theoretical Justification for Spread SpectrumSpread-spectrum is apparent in the Shannon and Hartley channel-capacity theorem: C=B×log2(1+S/N) (Eq.1)I n 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 of 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 ,interferences,etc.).There is an elegant interpretation of this equation,applicable for difficult environments,forexample,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 Napierian number) and by noting that In=loge.Therefore:C/B=(1/ln2)×ln(1+S/N)=1.443×ln(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,Shannon's expression becomes simply:C/B≈1.443×S/N (Eq.4)Very roughly:C/N≈S/N (Eq.5)Or:N/S≈B/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 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 necessary.3.Spread Spectrum 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 thespread 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 despreading operation) at a point in the receive chain before data retrieval.A despreading 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).Therefore, the impact caused by the bandwidth of the following:Figure 1.Spread-spectrum communication system（1）Bandwidth 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.（2）Bandwidth Effects of the Despreading OperationSimilarly,despreading can be seen in Figure 3.Figure 3. The despreading 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 despreading operation!（3）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 multipleusers with spread-spectrum techniques.4.Spread Spectrum Is a Wideband TechnologyIn contrast to regular narrowband technology,the spread-spectrum process is a wideband technology.For example，W-CDMA and UMTS,are wideband technologies that require a relatively large frequency bandwidth, compared to narrowband radio.Benefits of Spread Spectrum：(1) Resistance to Interference and Antijamming 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 despreading operation is exercised.See Figure 5.Figure 5. A spread-spectrum communication system.Note that the interferer’s energyis spread while the data signal is despread 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.(2) Resistance to InterceptionResistance to interception is the second advantage provided by spread-spectrum techniques.Because nonauthorized 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.(3) 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 multipaths can be caused byatmospheric 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.5.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.a.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.b.TDMA-Time Division Multiple AccessWith 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.c.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 access methods.GSM,for instance,combines TDMA and FDMA.GSM defines the topological areas (cells) with different carrier frequencies,and sets time slots within each cell.6.Spread Spectrum and coding “Keys”At this point,it is worth restating that the main characteristic of spread spectrum is the presence of a code or key,which must be known in advance by the transmitter and receiver (s).In modern communications the codes are digital sequences that must be as long and as random as possible to appear as “noise-like” as possible.But in any case,the codes must remain reproducible.or the receiver cannot extract the message that has been sent.Thus,the sequence is “nearly random”.Such a code is called a pseudo-random number (PRN) or sequence.The method most frequently used to generate pseudo-random codes is based on a feedback shift register.Many books are available on the generation of PRNs and their characteristics,but that development is outside the scope of this basic tutorial.Simply note that the construction orselection of proper sequences,or sets of sequences,is not trivial.To guarantee efficient spread-spectrum communications,the PRN sequences must respect certain rules,such as length, autocorrelation,cross-correlation,orthogonality,and bits balancing.The more popular PRN sequences have names:Barker,M-Sequence,Gold,Hadamard-Walsh,etc.Keep in mind that a more complex sequence set provides a more robust spread-spectrum link.But there is a cost to this: more complex electronics both in speed and behavior,mainly for the spread-spectrum despreading operations.Purely digital spread-spectrum despreading chips can contain more than several million equivalent 2-input NAND gates,switching at several tens of megahertz.。

自动化专业-外文文献-英文文献-外文翻译-plc方面

1、外文原文（复印件)A: Fundamentals of Single-chip MicrocomputerTh e si ng le-ch i p mi cr oc om pu ter is t he c ul mi nat i on o f bo th t h e d ev el op me nt o f th e d ig it al com p ut er an d t he int e gr at ed ci rc ui ta r gu ab ly th e t ow m os t s i gn if ic ant i nv en ti on s o f t h e 20t h c en tu ry[1].Th es e to w t ype s o f a rc hi te ct ur e a re fo un d i n s i ng le—ch ip m i cr oc om pu te r。

S o me em pl oy th e s p li t p ro gr am/d at a me mo ry of t he H a rv ar d ar ch it ect u re, sh ow n in Fi g.3-5A—1，ot he r s fo ll ow t hep h il os op hy, wi del y a da pt ed f or ge n er al—pu rp os e c o mp ut er s an dm i cr op ro ce ss or s, of ma ki ng no lo gi c al di st in ct io n be tw ee n p ro gr am a n d da ta m em or y a s i n th e Pr in cet o n ar ch it ec tu re,sh ow n in F ig。

3-5A-2.In g en er al te r ms a s in gl e—ch i p mi cr oc om pu ter isc h ar ac te ri zed b y the i nc or po ra tio n of al l t he uni t s o f a co mp ut er i n to a s in gl e de v i ce，as s ho wn i n F ig3—5A—3。

计算机网络中英文对照外文翻译文献

中英文资料外文翻译计算机网络计算机网络，通常简单的被称作是一种网络，是一家集电脑和设备为一体的沟通渠道，便于用户之间的沟通交流和资源共享。

网络可以根据其多种特点来分类。

计算机网络允许资源和信息在互联设备中共享。

一．历史早期的计算机网络通信始于20世纪50年代末，包括军事雷达系统、半自动地面防空系统及其相关的商业航空订票系统、半自动商业研究环境。

1957年俄罗斯向太空发射人造卫星。

十八个月后，美国开始设立高级研究计划局(ARPA)并第一次发射人造卫星。

然后用阿帕网上的另外一台计算机分享了这个信息。

这一切的负责者是美国博士莱德里尔克。

阿帕网于来于自印度，1969年印度将其名字改为因特网。

上世纪60年代，高级研究计划局(ARPA)开始为美国国防部资助并设计高级研究计划局网(阿帕网)。

因特网的发展始于1969年，20世纪60年代起开始在此基础上设计开发，由此，阿帕网演变成现代互联网。

二．目的计算机网络可以被用于各种用途:为通信提供便利:使用网络，人们很容易通过电子邮件、即时信息、聊天室、电话、视频电话和视频会议来进行沟通和交流。

共享硬件:在网络环境下，每台计算机可以获取和使用网络硬件资源，例如打印一份文件可以通过网络打印机。

共享文件：数据和信息: 在网络环境中，授权用户可以访问存储在其他计算机上的网络数据和信息。

提供进入数据和信息共享存储设备的能力是许多网络的一个重要特征。

共享软件:用户可以连接到远程计算机的网络应用程序。

信息保存。

安全保证。

三．网络分类下面的列表显示用于网络分类：3．1连接方式计算机网络可以据硬件和软件技术分为用来连接个人设备的网络，如：光纤、局域网、无线局域网、家用网络设备、电缆通讯和G.hn（有线家庭网络标准）等等。

以太网的定义，它是由IEEE 802标准，并利用各种媒介，使设备之间进行通信的网络。

经常部署的设备包括网络集线器、交换机、网桥、路由器。

无线局域网技术是使用无线设备进行连接的。

扩频通信

对于单频或多频载波信号的干扰、其他伪随机调制信号的干扰以及脉冲正弦信号的干扰等，扩频系统都有抑制干扰、提高输出信噪比的作用。扩频通信系统抗干扰能力强是扩频通信系统最突出的优点。
（2）保密性好
由于扩频信号在很宽的频带上被扩展了，单位频带内的功率就很小，即信号的功率谱密度很低。所以，应用扩频码序列扩展频谱的直接序列扩频系统，可在信道噪声和热噪声的背景下，在很低的信号功率谱密度上进行通信。信号既然被淹没在噪声里，敌方就很不容易发现有信号存在，而想进一步检测出信号的参数就更加困难了。这在军事通信上是十分有用的，即可进行隐蔽通信。
扩频通信系统与普通的数字通信系统相较，就是多了扩频调制和解扩部分。正如在一般的窄带通信中，已调信号在接收端都要进行解调来恢复发送端所传的信息。在扩频通信中，接收端则要用与发送端完全相同的扩频码序列与收到的扩频信号进行相关解扩，以恢复所传输的原始信号。
扩频通信系统原理图
扩频通信的基本特征就是扩展频谱，具体做法是使用比发送的信息数据速率高许多倍的伪随机码把载有信息数据的基带信号的频谱进行扩展，形成宽带的低功率谱密度的信号来通信。发射端，在天线之前某处链路注入扩频码，这个过程称为扩频处理，经扩频处理后原数据信息能量被扩散到一个很宽的频带内；接收端，相应链路中移去扩频码，恢复数据，此过程称为解扩。显然，收发两端需要预先知道扩频码。
扩频通信，是扩展频谱通信的简称。
扩频通信的定义简述为: 扩频通信技术是一种信息传输方式，在发送端采用扩频码调制，使信号所占的频带宽度大于所传信息必需的带宽，在接收端采用同样的扩频码进行解扩以恢复所传原始信息数据。
扩频码序列：指的是很窄的脉冲码序列
需要说明的是：扩频技术所采用的扩频码序列与所传信息数据时无关的，也就是说它与一般的正弦载波信号是类似的，丝毫不影响信息传输的透明性。扩频码序列仅仅起扩展信号频谱的作用。

有关扩频通信毕业设计的外文资料(中+英)

外文资料Pseudorandom Noise SequencesDirect sequence(DS). Direct-sequence spread spectrum(DS-SS) is produced when a bipolar data-modulated signal is linearly multiplied by the spreading signal in a special balanced modulator called a spreading correlator .The spreading code rate R cw=1/T c,where T c is the duration of a single bipolar pulse(i,e., the chip). Chip rates are 100 to 1000 times faster than the data message,therefore,chip times are 100 to 1000 times shorter in duration than the time of a single data bit. As a result, the transmitted output frequency spectrum using spread spectrum is 100 to 1000 times wider than the bandwidth of the initial PSK data-modulated signal.The spreading codes used in spread-spectrum systems are either maximal-length sequence codes, sometimes called m-sequence codes, or Gold codes. Gold codes are combinations of maximal-length codes invented by Magnavox Corporation in 1967 especially for multiple-access CDMA applications .There is a relatively large set of Glod codes available with minimal correlation between chip codes.For a reasonable number of satellite users,it is impossible to achieve perfectly orthogonal codes.You can only design for a minimum cross correlation among chips.One of the advantages of CDMA was that the entire bandwidth of a satellite channel or system may be used for each transmission from every earth station. For our example, the chip rate was six times the original bit rate. Consequently, the actual transmission rate of information was one-sixth of the PSK modulation rate,and the bandwidth required is six times that required to simply transmit the original data as binary. Because of the coding inefficiency resulting from transmitting chips for bits, the advantage of more bandwidth is partially offset and is, thus, less of an advantage. Also, if the transmission of chips from various earth station must be synchronized, precise timing is required for the system to work. Therefore, the disadvantage of requiring time synchronization in TDMA systems is also present with CDMA. In short, CDMA is not all that it is cracked up to be.The most significant advantage of CDMA is immunity to interference, which makes CDMA ideally suited for military applications Pseudorandom Noise SequencesIn CDMA systems, PN sequences are used toSpread the bandwidth of the modulated signal to the larger transmissionbandwidthDistinguish between the different user signals by utilizing the same transmission bandwidth in the multiple access scheme.PN squences are not random; they are deterministic, periodic sequences. The following are the three key properties of an ideal PN sequence:1.The relative frequencies of 0 and 1 are each 1/2.2.The run length(of 0s or 1s)are: 1/2 of all run lengths are of length 1; 1/4are of length 2;1/8 are of length 3; and so on.3.If a PN sequence is shifted by any nonzero number of elements, theresulting sequence will have an equal number of agreements and disagreements with respect to the original sequence.PN sequence are generated by combining the outputs of feedback shift registers. A feedback shift register consists of consecutive two-stage memory or storage stages and feedback lobic. Binary sequences are shifted register in response to clock pulses. The contents of the stages are olgically combined to produce the input to the first stage. The initial contents of the stages and feedback olgic determine the successive contents of the stages. A feedback shift register and its output are called linear when the feedback logic consists entirely of modulo-2 adders.To demonstrate the properties of a PN a binary sequence, we consider a linear feedback shift register(see Fig. 1) that has a four-stage register for storage and shifting, a modulo-2 adder, and a feedback path from adder to the input of the register.The operation of the shift register is controlled by a sequence of clock pulses. At each clock pulse the contents of each stage in the register is shifted by one stage to the right. Also, at each clock pulse the contents of stages x3 and x4 are modulo-2 added, and the result is fed back to stage x1. The shift register sequence is defined to be the output of stage x4. W assume that stage x1 is initially filled with a 0 and the other remaining stages are filled with 0, 0, and 1; i.e., the initial state of the register is 0 0 0 1. Next, we perform the shifting, adding , and feeding operations, where we obtain the results after each cycle that is shown in Table 1.We notice that the contents of the registers repeat after 24-1=15 cycles. The output sequence is given as 0 0 0 1 0 0 1 1 0 1 0 1 1 1 1 ,where the left-most bit is the earliest bie. In the output sequence, the total number of 0s is 7 and total number of 1s is 8; the numbers differ by 1.If a linear feedback shift register reached the 0 state an some time, it would always remain in the 0 state and the output sequence would subsequently be all 0s. Since there are exactly 2n-1 nonzero states, the period of a linear n-stage shift register output sequence can not exceed 2n-1.The output sequences are classified as either maximal length or nonmaximal length. Maximal-length sequences are the longest sequences that can be generated by a given shift register of a given length. In the binary shift register sequence generators, the maximal length sequence is 2n-1 chips, where n is the number of stages in the shift registers. Maximal-length sequences have this property for an n-stage linear feedback shift register: the sequence repetition period in clock pulses is T0=2n-1. If a linear feedback shift register generates a maximal sequence, then all of its nonzero output sequences are maximal, regardless of the initial stage. A maximal sequence contains(2n-1-1) 0s and (2n-1) 1s per period.Figure1 Four-Stage Linear Feedback Shift Register二、译文伪随机序列直接序列(DS)。

王延英文翻译

毕业（设计）论文外文翻译外文题目AN INTRODUCTION TO SPEREAD-SPECTRUMCOMMUNICATION译文题目扩频通信系统的介绍系别机电工程系专业测控技术与仪器班级 162902学生姓名王延学号 092151指导教师吴鹏飞报告日期 2012.3.18AN INTRODUCTION TO SPEREAD-SPECTRUMCOMMUNICATIONIntroductionAs 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 systemsGPS，3Gmobile telecommunications,W-LAN(IEEE®802.11a,IEEE 802.11b,IEEE 802.11g),and Bluetooth®.Spread-spectrum techniquesalso 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 SpectrumSpread-spectrum is apparent in the Shannon and Hartley channel-capacitytheorem:C=B×log2(1+S/N) (Eq.1)I n 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)=1.443×ln(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,Shannon's expression becomes simply:C/B≈1.443×S/N (Eq.4) Very roughly:C/N≈S/N (Eq.5) Or:N/S≈B/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 somecircumstances,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 frequencybandwidth.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 OperationSimilarly,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 Users 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,howeverby the possibility that many users will share the enlarged frequency band (Figure 4).Figure 4. The same frequency band can be shared by multipleUsers 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 SpectrumResistance 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 dispreadingoperation is exercised.See Figure 5.Figure 5. A spread-spectrum communication system.Note that the interferer’s 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 InterceptionResistance 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 i n the overall noise level!Figure 6.Spread-spectrum signal is buried under noise level.The receiver cannotseethetransmission 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 communicationlinks 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 AccessWith 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 thatsince,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.扩频通信系统的介绍简介扩频技术越来越受欢迎，就连这一领域以外的电器工程师都渴望能够深入理解这一技术。

外文翻译--通信系统简介

外文翻译--通信系统简介中文2040字Introduction to Communication SystemIt is often said that we are living in the information age. Communication technology is absolutely vital to the generation, storage, and transmission of this information.Any communication system moves information from a source to a destination through a channel. Figure 1 illustrates this very simple idea. The information from the source will generally not be in a form that can travel through the channel, so a device called a transmitter will be employed at one end and a receiver at the other.Figure 1 simple communication systemThe source or information signal can be analog or digital. Common examples are analog audio, video signals and digital data. Sources are often described in terms of the frequency range that they occupy. Telephone-quality analog voice signals, for instance, contain frequencies from 300Hz to 3kHz, while analog high-fidelity music needs a frequency range of approximately 20Hz to 20kHz.Digital sources can be derived from audio or video signals can have almost any bandwidth depending on the number of bits transmitted per second, and the method used to convert binary ones and zeros into electrical signals.A communication channel can be almost anything: a pair of conductors, an optical fiber or a free space that we live. Sometimes a channel can carry the information signal directly. For example, an audio signal can be carried directly by a twisted-pair telephone cable. On the other hand, a radio link through free space cannot be used directly for voice signals. Such situation require the use of a carrier wave will be altered, or modulated m, by the information signals in such a way that the information can be recovered at the destination. When a carrier is used, the information signal is also known as the modulating signals.Technology is at the core of many new and emerging digital information products and applications that support the information society. Such products and applications often require the collection, sometimes in real time. The ability of technology to handle real world signals digitally has made it possible to create affordable, innovative; and high quality products and applications for large consumer market for example: digital cellular mobile phone, digital television and video games. The impact of is also evident in many other areas, such as medicine and healthcare. For example: in patient monitors for intensive care, digital X-ray appliances, advanced cardiology and brain mapping systems and so on, digital audio, for example: CD players; audio mixers and electronic music and so on. And personal computer systems for example: disks for efficient data storage and error correction, moderns, sound cards and video conferencing and so on.Most of the major cities in the domestic bus stop artificial voice. Every one of the key points from the driver or attendant to stop by voice. But sometimes due to various factors such as weather, vehicle congestion,flight attendants are feeling the effects of the changes. There being given the station's reporting stations, especially for passengers not familiar with the topography of the city, causinga lot of unnecessary trouble. Well thus affect the image of a city construction window, then developed automatic stop system inevitable. As required before the docking system bus GPS information (latitude and longitude information, etc.), longitude and latitude information generated by the distance between bus stops with the message that this is going to experience the tedious, use the micro-controller difficult to achieve, and when using chips, the proper solution of this problem.Using radians per second in the mathematics dealing with modulation makes the equation simpler. Of course, frequency is usually given in hertz, rather than in radians per second, when practical devices are being discussed. It is easy to convert between the two systems per second, when practical devices are being discussed. It is easy to convert between the two systems by recalling from basic AC theory, ω=2πf.In modula tion, the parameters that can be changed are amplitude E, frequency ω,and phase θ. Combin ations are also possible. For example, many schemes for transmitting digital information use both amplitude and phase modulation.Multiplexing is the term used in communications to refer to the combining of two or more information signals. When the available frequency range is divided among the signals, the process is known as frequency-division multiplexing (FDM).Radio and television broadcasting, in which the available spectrum is divided among many signals, are everyday examples of FDM. There are limitations to the number of signals that can be crowded into a given frequency range because each requires a certain bandwidth, For example, a television channel only occupies s given bandwidth of 6MHz in 6~8MHz bandwidth of VHF.Parallel DSP chip to enhance the performance of a traditional improved through the use of multiply-add units and the Harvard structure, it goes far beyond the computational capabilities of the traditional microprocessor. A reasonable inference is: chip operations by increasing the number of modules and the corresponding number of bus linking computational modules. The chip can be doubled to enhance the overall operational capacity. Of course, such an inference two preconditions must be met : First, the memory bus bandwidth as necessary to meet the increase in the number of enhanced data throughput; In addition, various functional units involved in the parallel scheduling algorithm is its complexity can be achieved.An alternative method for using a single communication channel to send many signals is to use time-division multiplexing (TDM). Instead of dividing the available bandwidth of the channel among many signals, the entire bandwidth is used for each signal, but only for a small part of the time. A nonelectronic example is the division of the total available time on a television channel among the various programs transmitted. Each program uses the whole bandwidth of the channel, but only for part of the time.It is certainly possible to combine FDM and TDM, For example, the available bandwidth of acommunication satellite is divided among a number of transmitter-receiver combinations called transponders. This is an example of FDM. A single transponder can be used to carry a large number of digital signals using TDM.This course presents a top-down approach to communications system design. The course will cover communication theory, algorithms and implementation architectures for essential blocks in modern physical-layercommunication systems (coders and decoders, filters, multi-tone modulation, synchronization sub-systems). The course is hands-on, with a project component serving as a vehicle for study of different communication techniques, architectures and implementations. This year, the project is focused on WLAN transceivers. At the end of the course, students will have gone through the complete WLAN System-On-a-Chip design process, from communication theory, through algorithm and architecture all the way to the synthesized standard-cell RTL chip representation.通信系统简介人们常说我们正生活在一个信息时代，通信技术对信息的产生，存储与转换有着至关重要的作用。

码分多址通信系统、扩频通信外文翻译

中文2800字毕业设计英文翻译专业电子信息工程班级2010级学生姓名学号课题码分多址通信系统的建模、仿真和设计——初始化模块、基站接收模块指导教师2014 年06 月10 日译文原文1.1 The basic concept of spread-spectrum communicationSpread spectrum communication’s basic characteristics, is used to transmit information to the signal bandwidth(W) is far greater than practical required minimum(effective) bandwidth (F∆),as the radio of processing gain P G.=/G P∆FWAs we well know,the ordinary AM,FM,or pulse code modulation,GP value in the area more than 10 times,collectively,the “narrow-band communication”,and spread-spectrum communication GP values as hundred or even thousands of times, can be called “broadband communication”.Due to the spread-spectrum signal,it is very low power transmitters,transmission space mostly drowned in the noise,it is difficult to intercepted by the other receiver ,only spreading codes with the same (or random PN code) receiver, Gain can be dealt with ,and despreading resume the original signal.1.2 The technology superiority of spread-spectrum communication.Strong anti-interference, bit error rate is low. As noted above, the spread spectrum communication system due to the expansion of the transmitter signal spectrum, the receiver despreading reduction signal produced spreading gain, thereby greatly enhancing its interference tolerance. Under the spreading gain, or even negative in the signal-to-noise ratio conditions, can also signal from the noise drowned out Extraction, in the current business communications systems, spread spectrum communications systems, spread spectrum communication is only able to work in a negative signal-to-noise ratio under the conditions of communication .Anti-multi-path interference capability, increase the reliability of system. Spread-spectrum systems as used in the PN has a good correlation, correlation is very weak. Different paths to the transmission signal can easily be separated and may intime and re-alignment phase, formation of several superimposed signal power, thereby improving the system’s performance to receive increased reliability of the system.Easy to use the same frequency, improving the wireless spectrum utilization. Wireless spectrum is very valuable,although long-wave microwave have to be exploited, and still can not meet the needs of community. To this end, countries around the world are designed spectrum management, users can only use the frequency applications,rely on the channel to prevent the division between the channel interference.Due to the use of spread-spectrum communication related receive this high-tech,low signal output power(“a W,as a general-100mW),and will work in the channel noise and thermal noise in the background,easy to duplicate in the same area using the same frequency,can now all share the same narrow-band frequency communication resources.Spread-spectrum communication is digital communication,particularly for digital voice and data transmission with their own encryption, only in the same PN code communication between users, is good for hiding and confidential in nature, facilitating communication business. Easy to use spread-spectrum CDMA communications, voice compression and many other new technologies, more applicable to computer networks and digitization of voice,image information transmission.Communication in the most digital circuits, equipment, highly integrated, easy installation, easy maintenance, but also very compact and reliable. The average failure rate no time was very long.1.3 Spread spectrum communication systemSpread spectrum communication,namely, spread spectrum communications (Spread spectrum communication), with fiber-optic communications,satellite communications,with access to the information age as the three major high-tech communications transmission. Spread spectrum communication is to send the information to be pseudo-random data is coded(Spread spectrum sequence: spread sequence) modulation, spread spectrum and then the realization of transmission; thereceiving end is using the same modem code and related processing, the restoration of the original data. Spread spectrum communication system has three main characteristics.(1) Carrier is an unpredictable, or so-called pseudo-random broadband signal.(2) Carrier data bandwidth than the modulation bandwidth is much wilder.(3) Receiving process is generated by local broadband carrier signal and receiving a copy of the signal to the broadband signal to achieve.The main way of spread spectrum are as follows: Direct Sequence Spread Spectrum(DSSS) using high-speed pseudo-random code on to the low-speed data transmission spread spectrum modulation; Frequency-hopping system using pseudo-random code to control the carrier frequency in a wider band of the change; TH is the data transmission time slot is a pseudo-random; chirp frequency system is a linear extension of the process of change. Combination of a number of ways of hybrid systems are often applied.The most important measure pf spread-spectrum system is an indicator of spreading gain, also known as processing gain. It is precisely because of the spread spectrum system itself with its performance characteristics with a series of advantages.1.4 Code division multiple accessCode division multiple access (CDMA) is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne, CDMA2000(the 3G evolution of cdmaOne) and WCDMA (the 3Gstandard used by GSM carrier), which are often referred to as simply CDMA, and use CDMA as an underlying channel access method.One of the concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a signal communication channel. This allows several users to share a band of frequencies (see bandwidth). This concept is called multiple access. CDMA employs spread-spectrum technology and a special coding scheme( where each transmitter is assigned a code) to allow multiple user to be multiplexed over the same physical channel. By contrast, time division multipleaccess (FDMA) divides it by frequency. CDMA is a form of spread-spectrum signaling, since the modulated coded signal has a much higher data bandwidth than the data being communicated.1.5 Spread-spectrum characteristic of CDMAMost modulation schemes try to minimize the bandwidth of this signal since bandwidth is a limited resource. However, spread spectrum use a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth. One of the initial reasons for doing this was military applications including guidance and communication systems. These system were designed using spread spectrum because if its security and resistance to jamming. Asynchronous CDMA has some level of privacy built in because the signal is spread using a pseudo-random code; this code makes the spread spectrum signals appear random or have noise-like properties. A receiver cannot demodulate this transmission without knowledge of the pseudo-random sequence used to encode the data. CDMA also resistant to jamming. A jamming signal only has a finite amount of power available to jam the signal. The jammer can either spread its energy over the entire bandwidth of the signal or jam only part of the entire signal.CDMA can also effectively reject narrow band interference. Since narrow band interference affects only a small portion of the spread spectrum signal, it can easily be removed through notch filtering without much loss of information. Convolution encoding and interleaving can be used to assist in recovering this lost data. CDMA signal are also resistant to multipath fading. Since the spread spectrum signal occupies a large bandwidth only a small portion of this will undergo fading due to multipath at any give time. Like the narrow band interference this will result in only a small loss of data and can be overcome.Another reason CDMA is resistant to multipath interference is because the delayed versions of the transmitted pseudo-random code, and will thus appear as another user, which is ignored at the receiver. In other words, as long as the multipath channel induces at least one chip of delay, 天the multipath channel induces at least one chip of delay,the multipath signals will arrive at the receiver.in other words, as long as the multipath channel induces at least one chip of delay, the multipath signalswill arrive at the receiver such that they are shifted in time by at least one chip from the intended signal. The correlation properties of the pseudo-random codes are such that this slight delay causes the multipath to appear uncorrelated with the intended signal, and it is thus ignored.Some CDMA devices use a rake receiver, which exploits multipath delay components to improve the performance of the system. A rake receiver combines the information from several correlators, each one tuned to a different path delay, producing a stronger version of the signal than a simple receiver with a signal correlation tuned to the path delay of the strongest signal.Frequency reuse is the ability to reuse the same radio channel frequency at other cell sites within a cellular system. In the FDMA and TDMA systems frequency planning is and important consideration. The frequencies used in different cells must be planned carefully to ensure signals from different cells do not interfere with each other. In a CDMA system, the same frequency can be used in every cell, because channelization is done using the pseudo-random codes. Reusing the same frequency in every cell eliminates the need for frequency planning in a CDMA system; however, planning of the different pseudo-random sequences must be done to ensure that the received signal from one cell does not correlate with the signal from a nearby cell.Since adjacent cell use the same frequencies, CDMA systems have the ability to perform soft handoffs. Soft handoffs allow the mobile telephone to communication simultaneously with two or more cells. The best signal quality in selected until the handoff is complete. This is different from hard handoffs utilized in other cellular systems. In a hard handoff situation, as the mobile telephone approaches a handoff, signal strength may vary abruptly. In contrast, CDMA systems use the soft handoff, which is undetectable and provides a more reliable and higher quality signal.Concluding remarksspread-spectrum technology in the initial stages of development, it has become a theory and a major technological breakthrough. Later in the development process is the improvement and hardware performance improved. Development to thepresent,spread-spectrum technology and the theory has been almost perfect,mainly from the point of view of overall performance, and the other new technology applications. Therefore, the application has been driven by the development of spread-spectrum technology is a power driving force, the future wireless communication systems, such as mobile communication. Wireless LAN, global personal communications, spread-spectrum technology will certainly play an important role.译文正文1.扩频通信系统概述扩频通信，即扩展频谱通信（Spread spectrum communication)，它与光纤通信、卫星通信，一同誉为进入信息时代的三大高技术通信传输方式，扩频通信是将待传送的信息数据被伪随机码调制，实现频谱扩展后再传输；接收端则采用相同的编码进行解调及相关处理，恢复原始信息数据。

1、下载文档前请自行甄别文档内容的完整性，平台不提供额外的编辑、内容补充、找答案等附加服务。
2、"仅部分预览"的文档,不可在线预览部分如存在完整性等问题,可反馈申请退款(可完整预览的文档不适用该条件!)。
3、如文档侵犯您的权益，请联系客服反馈,我们会尽快为您处理(人工客服工作时间：9:00-18:30)。

外文翻译外文文献英文文献扩频通信系统的介绍扩频通信系统的介绍摘要：本应用笔记概述了扩频技术的原理，讨论了涵盖直接序列和快速跳频的方法。