外文翻译外文文献英文文献扩频通信系统的介绍
【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
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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。
浅析扩频通信技术的应用及其系统的工作原理
第34卷 第2期 2012-2(下) 【59】收稿日期：2011-09-20作者简介：胡煜（1979－），男，湖北英山人，讲师，硕士，研究方向为通信工程技术。
1 扩频通信简介扩频通信 (Spread Spectrum Communication) 是将待传的信息数据被扩频序列（Spread Sequence 又称 Pseudo Noise Code ，即 PN 码）编码调制，实现频谱扩展后再传输；接收端采用同样的编码进行解调及相关处理，恢复原始数据。
扩频通信系统发端简化为调制和扩频，收端简化为解和解调，收发端两侧各有一个完全相同的伪随机码发生器，要求收发端的 PN 码精确同步，系统的其他环节合并到信道中。
扩频通信从电磁波角度来看，是与一般现有的常规方法完全不同的。
常规的通信是在频段上细分（频分多址）或时间上细分（时分多址）给通信用户，彼此互不干扰的分别使用。
而扩频通信用伪随机编码把基带信号的频谱进行扩展，形成相当带宽的低功率谱密度信号发射。
使用不同的伪随机编码，不同通信用户可在同一频段、同一时间工作，互相影响极小。
因此，扩频通信在调制、解调上也与众不同。
信息数据经通常的数据调制后变成带宽为 B1 的信号（B1 为数字频带信号带宽），用扩频序列发生器产生的伪随机编码去对数字频带信号作扩频调制，形成带宽为 B2(B2>>B1),功率谱密度极低的扩频信号发射。
众多的通信用户，使用各自不同的伪随机编码，可以同时使用带宽为 B2 的同一频带。
在接收端，首先使用与扩频信号发送者相同的伪随机编码作扩频解调处理，把带宽信号恢复成通常的数字频带信号，再使用通常的通信处理手段解调出发送来的信息数据。
显然，若接收端不知道发送的扩频信号所使用的伪随机编码时，要进行扩频调制是非常困难的甚至是不可能的。
这就实现了信息数据的保密通信。
如果接收端用某一伪随机编码在接收某一发送来的信号时，通信信道中的另一些伪随机编码调制的扩频信号不能在该接收端的扩频解调处理器中形成明显的信号输出，即不会对接收端的扩频处理形成干扰或干扰极小。
扩频通信
扩展频谱技术作者杨昭春（1.东北电力大学2.吉林省吉林市 132012 ）摘要:人类社会进入到了信息社会，通信现代化是人类社会进入信息时代的重要标志。
怎样在恶劣的环境条件下保证通信有效地、准确地、迅速地进行，是当今通信工作者面临的一大课题。
扩频通信技术是现代通信系统中的一种新兴的通信方式，其较强的抗干扰、抗衰落和抗多径性能以及频谱利用率高、多址通信等诸多优点越来越为人们所认识，并被广泛地应用于军事通信和民用通信的各个领域，从而推动了通信事业的发展。
关键词：扩展频谱通信；抗多径；频谱1 扩频通信系统发展概述扩频通信(spread spectrum communication)是近几年内迅速发展起来的一种通信技术。
在早期研究这种技术的主要目的是为提高军事通信的保密和抗干扰的性能，因此这种技术的开发和应用一直是处于保密状态。
美国在20世纪50 年代中期，就开始了对扩频通信的研究，当时主要侧重在空间探测、卫星侦察和军用通信等方面。
以后，随着民用通信的频带拥挤日益严重，又由于近代微电子技术、信号处理技术、大规模集成电路和计算机技术的快速发展，与扩频通信有关的器件的成本大大地降低，从而进一步推动了扩频通信在民用领域的发展金额应用，而且也使扩频通信的理论和技术也得到了进一步的发展。
军事通信抗干扰的驱动以及个人通信业务的驱动使得扩频技术的抗干扰性能和码分多址能力得到最大限度的挖掘。
时至今日，第四代移动通信系统(4G)的驱动无疑使扩频技术传输高速数据的能力得到更大的拓展。
相继的第五代移动通信系统正处于试验阶段，近期也将被广泛应用。
2扩频通信技术简介2.1 概念介绍扩展频谱通信（Spread Spectrum Communication)，扩频通信技术是一种信息传输方式，其信号所占有的频带宽度远大于所传信息必需的最小带宽。
频带的扩展是通过一个独立的码序列来完成，用编码及调制的方法来实现的，与所传信息数据无关；在接收端则用样的码进行相关同步接收、解扩及恢复所传信息数据。
外文翻译
DIRECT-SEQUENCE SPREAD-SPECTRUM SYSTEMJames P. StephensElectronic Warfare DivisionAvionics DirectorateWright LaboratoryWright-Patterson Air Force Base, Ohio 45433Lt. Col David M. NormanDepartment of Electrical and Computer EngineeringAir Force Institute of TechnologyWright-Patterson Air Force Base, Ohio 45433AbstractThis paper discusses the construction and evaluation of the performance of a direct-sequence spread spectrum (DSSS) system. The system, based upon a previously published design, uses a special type of injection-locked oscillator called a "synchronous oscillator" for code synchronization. The DSSS system was constructed in a manner that provides a test-bed for demonstrating spread spectrum principles and allows researchers to evaluate their own subsystem design concepts. The DSSS system, designed as a one-way link in the 420-450 MHz band, is constructed to operate in accordance with Federal Communications Commission rules and regulations pertaining to the Amateur Radio Service (Part 97). Unlike conventional DSSS systems which combine digital data with a pseudorandom (PN) code sequence, the system described here directly modulates an FM modulated carrier with the PN code sequence. The criteria used for evaluation are synchronization time, processing gain, and probability of bit-error rate. Because the DSSS receiver uses an inexpensive and practical direct conversion process before despreading, the receiver lacks good sensitivity. In spite of the limited range of the system, fundamental concepts of DSSS were evaluated and the measurements agreed favorably with theoretical values.IntroductionThere are several techniques by which spread spectrum can be implemented. One technique is called "direct-sequence." Direct-sequence spreadspectrum is achieved in this paper by directly modulating a conventional narrowband frequency-modulated (FM) carrier with a high rate digital code. The direct modulation is binary phase-shift keying (BPSK). The high rate digital code is referred to as a PN (pseudo-noise) code because it exhibits random-like properties which are necessary for providing good spectral characteristics and security.This paper deals with the construction and evaluation of the performance of one particular class of DSSS system that uses a synchronous oscillator for code synchronization. The system, Figure 1, was constructed in a manner that allows a variety of DSSS concepts to be observed, studied, and evaluated. By using modular construction techniques, the system provides a flexible test-bed that is useful for evaluating sub-system design concepts.Figure 1. The DSSS SystemRegulatory AspectsThe operation of a radio transmitter, with a few exceptions, requires a license for the transmitter and usually the operator. In developing this spread spectrum system it was important that the final result be capable of actually radiating so that field tests could be performed as part of the system evaluation. Transmitter radiation in free space leads to the problem of insuring that the system is operating legally.US government agencies, including the military, generally do not require the licensing of radio transmitters provided they are used for official activities. Government and military transmitters do, however, require some type of authorization from a regulating authority. This authorization is necessary to insurethe systematic operation of radio transmitters in a manner which will not cause interference to other stations. To accomplish this, there are governmental committees or military departments that establish frequency allocation standards and other guidelines of operation. These organizations coordinate their activities with the State Department, or in the case of non-governmental operations, the Federal Communications Commission (FCC). The problem of acquiring military or governmental authorization to operate a spread spectrum transmitter was considered arduous for a research activity of this nature. Therefore, the decision was made to provide for transmitter operation in the Amateur Radio Service.The Amateur Radio Service is ideally suited for experimental transmitter operation of this nature. Amateur Radio is a service regulated by the FCC for US citizens to operate radio transmitters for hobby purposes, without business interests, to further the art of radio communications. Radio experimentation, such as this research project provides, is fundamental to the nature of the Amateur Radio ServiceBasic System ConfigurationConstruction of the DSSS system is based upon an article appearing in OST Magazine, May 1989, by Andre' Kestekoot. The system as described by Kesteloot is intended for use in the Amateur Radio Service. For this study, several design modifications were required. The resulting DSSS system, shown in Figure 2, consists of a transmitter unit and a receiver unit. The transmitter consists of an FM exciter, a PN code generator, a double-balanced mixer (DBM), an RF amplifier, a band-pass fitter, and an antenna. The receiver consists of an antenna, an RF preamplifier, a DBM, a PN code generator, a synchronous oscillator, and a narrowband FM receiver.The DSSS system operates as a one-way link in the 420-450 MHz band with an output power of approximately 100 mw. It uses a process called the "stored reference" design approach, in which a replica of the PN code used for spreading the signal is contained within the receiver. The process of demodulation requiresthat the spreading PN code must be synchronized to the stored reference PN code. The process of synchronization is accomplished by a special circuit called a synchronous oscillator. The capability of the synchronous oscillator to perform spread spectrum synchronization is evaluated in this paper.Figure 2a. TransmitterFigure 2b. ReceiverThe PN code generator provides the pseudorandom digital waveform that is used to spread the frequency modulated carrier to produce a DSSS signal. The PN code generator is designed to be programmable from the transmitter and the receiver front panel. The PN code generator can output linear recursive sequences (LRS) for any polynomial up to, and including, degree-12. This class of PN codes is generated by digital feedback shift-registers. A digital shift-register of degree-n is a device consisting of n consecutive binary storage elements. The PN code generator for this system is clocked at the rate of 2.7875 MHz as derived from the FM exciter.The PN code sequence used in this analysis has the following parameters: rc = chip rate = 2.7875 MHzTc = chip time = 362.5 nsn = code sequence polynomial degree = 7N = sequence length = 127 bitsTN = code period = NTc = 46.04 nsSynchronizationThe problem of synchronization in DSSS systems is that of aligning the receiver's locally generated PN code sequence with the spreading modulation superimposed on the incoming signal. This process is often accomplished in two steps. The first step, called "acquisition', consists of bringing the two spreading signals into coarse alignment. Once acquisition has occurred, the second step, called tracking" takes over and continuously maintains fine alignment, usually by means of a feedback loop.The synchronization technique used in the system described here uses a device called a …'synchronous oscillator" (SO). Kesteloot states that the SO provides the function of a phased-locked loop (PLL) while offering several advantages: simpler to implement and better noise immunity. Uzunoglu and White, co-developers of the patented SO, have provided theoretical and experimental results of its performance.The synchronous oscillator is a synchronization network used for both acquisition and tracking. Fundamentally, the SO is a free-running oscillator in the absence of an externally applied signal. In the presence of a signal, the oscillator acquires and tracks the input waveform. An important feature of the SO is its ability to lock and track an input signal whose frequency is an integer multiple or a rational integer number such as 4,114 and 3/2. The SO possesses constant output signal amplitude in the tracking region and an adaptive tracking bandwidth proportional to the input signal level. A phase shift exists between the input and the output of the SO when it is synchronized that depends on the difference between the free-running frequency of the SO and the injected frequency.At the transmitter, the PN code generator is clocked (chip rate) from asource coherently derived from the TA451 exciter. The exciter signal, at 9 times the crystal frequency, is divided by 40 to produce the 2.7875 MHz chip rate. This chip rate is precisely 11160 of the carrier frequency. At the receiver, the output of the SO, which free-runs at approximately 111.5 MHz, is divided by 40 to re-create the transmitted chip rate. The transmitter and receiver chip rates will not be precisely the same because of variations such as oscillator instabilities, doppler, and multipath. Furthermore, the starting point of their respective sequences will not be identical.The output of the receiver DBM, at 446 MHz, is input to the SO completing the feedback path and allowing synchronization. When this occurs, the SO feature of division by an integer is implemented. The input to the SO at 446 MHz is divided by 4 and tracked by the 111.5 MHz oscillator. The SO output is divided by 40, reconditioned, and passed on to drive the receiver PN code generator. The output of PN code generator is applied to the LO port of the DBM to despead the signal.The rate difference between the transmitted sequence and the receiver's locally generated sequence establishes a "sliding correlator" process. When correlation occurs, the signal power over a large bandwidth collapses into a narrowband signal having much greater power per unit bandwidth. The despread DBM output at 446 MHz is instantaneously injected into the synchronous oscillator where acquisition and tracking begin. Provided the rate difference of the two PN code sequences is not outside the tracking ranged of the SO, the oscillator will become "locked‟'.Performance Testing and ResultsTests of synchronization time, processing gain, and bit-error rate were conducted in the laboratory. Tests regarding the ease of synchronization and the communications range were field tested. The effects of interference were conducted in the laboratory. Synchronization time was measured with instrumentation configured as shown in Figure 3. A received signal level of 0 dbm was used to ensure a large SNR. The FSK demodulator was adjusted to keep theHP5326A Interval Timer enabled until it sensed an audio tone of approximately 2 kHz from the receiver. When the DSSS transmitter was keyed, the SYNC pulse was generated from the PN code generator at the code period rate of 1/TN". The SYNC pulse will not be generated until the transmitter is keyed. At the first SYNC pulse, the HP5326A is enabled and disabled when the 2 kHz tone is received by the FSK demodulator. The time difference between the start and stop signals is the total synchronization time. The additional propagation times resulting from the FSK demodulator, the FM narrowband receiver, and the propagation path are small by comparison and are neglected. In fact, these delays are somewhat offset by the fact that the fit SYNC start pulse is actually delayed by one code period equal to 46.04 ps. The mean acquisition time was measured to be 164.1 ms.Figure 3. Block Diagram for Sync-Time MeasurementThe theoretical value of the processing gain was compared to a measured value. By definition, processing gain Gp isGp = Bss / BD (1) Where,Bss = bandwidth of the DSSSBD = minimum bandwidth necessary to send the information in Hz.A null-to-null bandwidth is assumed for the spread spectrum signal and a bounded power spectral density bandwidth (4 kHz) is assumed for the analog voice channel. The measured Gp was determined by the expression.Gp = output SNR / input SNR (2) The theoretical processing gain is 31.4 dB and the measured value was 27.0dB, a difference of only 4.4 dB. This is reasonable allowing for conversion losses and bandwidth approximations.The bit-error rate for audio FSK at 300 bps was measured with instrumentation configured as shown in Figure 4. The DSSS transmitter and receiver were connected directly through a stepped attenuator from 0 to 110 dB in 1 dB increments. The SYNC outputs were connected to the HP1746A oscilloscope to observe that the transmitter and receiver were synchronized.The HP1746A was set to the dual trace (chop) mode to trigger on the transmitter SYNC pulse. The receiver synchronous oscillator TUNE control was adjusted, when needed, to maintain code rate and epoch alignment with the transmitter PN code. The FSK modulator was configured to output 300 bps FSK with 2025 Hz and 2225 Hz tones. The simulated data was generated by an HP8006A word generator.Test data consisted of a PN sequence of length 65535 bits (216 - 1) in order to assure randomness over the measurement interval. However, a time delay of greater than 1 ms was present between the waveform at input A (transmitted waveform) and the waveform at input B (received waveform). This caused excessive errors to be falsely recorded by the counter. To correct this problem, a digital time-delay device was inserted in the measurement system. The digital time-delay device delayed the transmitted waveform by an adjusted amount so that the waveforms were aligned before comparison. In Figure 5, measured data is compared to a theoretical plot of non-coherent FSK for the probability-of-error versus SNR (Eb / No = 2SNR). The large difference between the measured BER and the theoretical curve is attributed to poor receiver sensitivity.Before field tests were conducted, receiver sensitivity was measured in the laboratory to determine the maximum path loss that could be tolerated before the DSSS system lost synchronization. The maximum path loss was measured by connecting the DSSS transmitter, through a stepped attenuator, into the DSSS receiver. Problems with synchronization began at 40 dB of attenuation. Using theexpression for free-space path loss, a predicted range of only 5 to 10 meters was expected.Figure 4. Block Diagram for BER MeasurementFigure 5. BER vs SNRThe effect of co-channel, narrowband interference on the performance of the DSSS system was tested in the laboratory because of the limited range over which the system could operate. The laboratory interference tests were not exhaustive. A CW carrier was input into the DSSS receiver and slowly varied around the receiver frequency of 446 MHz. The DSSS transmitter was keyed and the effects of the CW carrier were observed. The CW carrier and the DSSS signal were initially at equal power levels. As the CW carrier power was increased, the DSSS signal was noticeably affected when the carrier-to-noise ratio was -30 dB. This is consistent with the measured processing gain of 31.4 dB.SummaryThe requirements for a spread spectrum synchronization technique depend largely on the system application. In military applications where anti-jam (AJ) or low-probability-of-intercept (LPI) performance is major factors, synchronizationmust be capable of operating under adverse conditions. Such a synchronization technique must be carefully designed, thereby becoming complex, costly, and difficult to implement. In civilian applications, such as the Amateur Radio Service, spread spectrum is not used to provide AJ or LPI, although resistance to interference may be an important consideration. In these cases, a more practical synchronization technique is desirable; one, that is simple, reliable, and sufficiently fast to support push-to-talk (PTT) operation.The synchronous oscillator used in this system provided a fast and easily implemented means of achieving direct-sequence spread spectrum synchronization for civilian applications where short PN codes may be used. The measured system processing gain agreed favorably with the theoretical value. FSK was implemented at 300 bps and, allowing for poor receiver sensitivity, produced reasonably expected bit-error rate versus SNR performance. The range of the DSSS system was extremely limited; however the range was predicted and verified prior to the field tests. The DSSS system was found to be unaffected by the presence of a CW carrier at the same input power and center frequency of the spread spectrum signal.Although the limited range was disappointing, this does not detract from the research effort. A direct conversion receiver, such as used by the DSSS receiver, is simple, inexpensive, and easy to build. Commercial receivers requiring good sensitivity use a superheterodyne technique. Reference 5 provides additional details regarding this effort.References[1] Federal Communications Commission. Part 97-Amateur Radio Service.Title 47 CFR Ch. I, Section 97.71. Washington: Government Printing Office; 1 Oct 1988[2] Kesteloot, Andre`. “A Practical Direct-Sequence Spread-Spectrum UHFLink.” QST Magazine. 5:14-21(May 89).[3] Uzunoglu, Vasil and Marvin H. White. “The Synchronous Oscillator: ASynchronization and Tracking Network.” IEEE Journal of Solid-State-Circuit. Vol. SC-20. No.6: 1214-1225 (December 1985).[4] Uzunoglu, Vasil and Marvin H. White. “Some Important Properties of Synchronous Oscillators.” Proceedings of IEEE. Vol.74.[5] Stephens. James P. “Direct-Sequence Spread-Spectrum System.”Masters Thesis, Air Force Institute of Technology, 1 June 90.直接序列扩频系统James P. StephensElectronic Warfare DivisionAvionics DirectorateWright LaboratoryWright-Patterson Air Force Base, Ohio 45433Lt. Col David M. NormanDepartment of Electrical and Computer EngineeringAir Force Institute of TechnologyWright-Patterson Air Force Base, Ohio 45433摘要这篇论文讨论了直接序列扩频系统（DSSS系统）的结构和性能评估。
跳频扩频技术-毕业论文外文翻译
跳频扩频技术跳频扩频（FHSS）的传输无线电信号，通过快速切换方法的载波频率在许多渠道，使用伪随机序列发射器和接收器。
它是利用作为多址接入方法在跳频码分多址接入（FH-CDMA）的计划。
扩频传输提供一个固定的频率传输的三个主要优点：1.扩频信号是高抗窄带干扰。
重新收集传播信号的过程中展开的干扰信号，使其回落到后台。
2.扩频信号是难以拦截。
一个跳频信号只出现在窄带接收机的背景噪声的增加。
窃听者只能够拦截传输，如果被称为伪随机序列。
3.扩频传输，可以与许多传统的传输类型的频带，以最小的干扰。
扩频信号加噪音极小狭窄的高频通信，反之亦然。
作为一个结果，可以更有效地利用带宽。
历史跳频的概念跳频首次提到在1903年美国专利723188 美国专利725605 特斯拉在1900年7月。
特斯拉来到展示了世界上第一个无线电遥控潜水船在1898年，当它成为明显的控制无线信号的船需要的是从安全“受到干扰，拦截，或以任何方式干预后的想法。
”他的专利涉及两个根本不同的技术实现的抗干扰能力，这两个的作用，通过改变载波的频率或其他专属特性。
首先有一个发射器，同时在两个或两个以上的不同频率和一个接收器，在每一个人的传播频率进行调整，为了控制电路响应，工作。
第二种方法使用可变频率的发射器，由一个编码轮，在预定的方式改变发射频率控制。
这些专利描述跳频频率的基本原则和频分复用，电子与门的逻辑电路。
跳频无线电先驱乔纳森Zenneck的书无线电报（德国，1908年，英文翻译McGraw Hill出版社，1915）也提到，虽然Zenneck自己指出，德律风根已经尝试过了几年前。
zenneck的书是一个时间领先的文本，它是可能的，许多后来的工程师们意识到这一点。
德国军队在第一次世界大战中，英国的力量，没有技术，按照顺序，以防止窃听有限使用固定指挥点之间的通信跳频。
一位波兰工程师，伦纳德Danilewicz ，来到了在1929年的想法。
在20世纪30年代被其他几个专利，包括一个由威廉Broertjes（德国1929年，美国专利1869695 ，1932年）。
通信 外文翻译 外文文献 英文文献 扩频通信系统的介绍
外文出处：/en/app-notes/index.mvp/id/1890/扩频通信系统的介绍一、扩频通信的原理扩频是香农定理的典型：C=B×log2(1+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差错传输的最大信息量。
B为信号频带宽度，单位是Hz，S/N为信噪比。
也就是说，C为信道允许通过的信息量，也代表了扩频的性能。
带宽(B)是代价，因为频率是一个有限的资源。
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，的情况下，在无差错传输的信息速率C不变时，如果信噪比很低，则可以用足够宽的带宽来传输信号，即使信号功率密度低于噪音水平。
（公式可用！）改变公式（1）中对数的底数，2改为e，则为In=loge。
因此，C/B=(1/ln2)×ln(1+S/N)=1.443×ln(1+S/N)公式（2）根据MacLaurin扩展公式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）在扩频应用中，通常S/N很低。
（正如刚才提到的，信号功率密度甚至低于噪音水平。
）假定噪音水平即S/N<<1，香农公式可简单表示为：C/B≈1.443×S/N公式（4）简化为：C/N≈S/N 公式（5）或者：N/S≈B/C 公式（6）向固定了信噪比的信道发送错误的信息，只要执行基本扩频信号的传播操作：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的电子设备必须同时存在展宽和解扩的操作过程。
二、定义不同的扩频技术都有一个共同之处：密钥（也称为代码或序列）依附于传输信道。
以插入代码的形式准确地定义扩频技术，术语“频谱扩展”是指扩频信号的几个数量级的带宽在有密钥的传输信道中的扩展。
扩频通信系统概述
第一讲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，一同被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编码(扩频序列：Spread Sequence)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。
这种通信方式与常规的窄道通信方式是有区别的：一是信息的频谱扩展后形成宽带传输；二是相关处理后恢复成窄带信息数据。
正是由于这两大持点，使扩频通信有如下的优点：抗干扰抗噪音抗多径衰落具有保密性功率谱密度低，具有隐蔽性和低的截获概率可多址复用和任意选址高精度测量等正是由于扩频通信技术具有上述优点，自50年代中期美国军方便开始研究，一直为军事通信所独占，广泛应用于军事通信、电子对抗以及导航、测量等各个领域。
直到80年代初才被应用于民用通信领域。
为了满足日益增长的民用通信容量的需求和有效地利用频谱资源，各国都纷纷提出在数字峰窝移动通信、卫星移动通信和未来的个人通信中采用扩频技术，扩频技术已广泛应用于蜂窝电话、无绳电话、微波通信、无线数据通信、遥测、监控、报警等系统中。
第二讲扩展频谱通信的基本概念2.1 扩展频谱通信的定义所谓扩展频谱通信，可简单表述如下：“扩频通信技术是一种信息传输方式，其信号所占有的频带宽度远大于所传信息必需的最小带宽；频带的扩展是通过一个独立的码序列来完成，用编码及调制的方法来实现的，与所传信息数据无关；在接收端则用同样的码进行相关同步接收、解扩及恢复所传信息数据”。
这一定义包含了以下三方面的意思：一、信号的频谱被展宽了。
我们知道，传输任何信息都需要一定的带宽，称为信息带宽。
例如人类的语音的信息带宽为300Hz --- 3400Hz，电视图像信息带宽为数MHz。
为了充分利用频率资源，通常都是尽量采用大体相当的带宽的信号来传输信息。
在无线电通信中射频信号的带宽与所传信息的带宽是相比拟的。
扩频技术外文翻译
毕业设计(论文)外文资料翻译学院通信与信息工程学院专业通信工程学生姓名班级学号外文出处Robert Clyde Dixon.Spread Spectrum Techniques[M].IEEE Press,1976附件：1.外文资料翻译译文；2.外文原文附件1：外文资料翻译译文扩频技术摘要扩频技术是信号（例如一个电气、电磁，或声信号）生成的特定带宽频率域中特意传播，从而导致更大带宽的信号的方法。
这些技术用于各种原因包括增加抗自然干扰和干扰，以防止检测，并限制功率流密度（如在卫星下行链路）的安全通信设立的。
跳频的历史：跳频的概念最早是归档在1903年美国专利723188和美国专利725605由尼古拉特斯拉在1900年7月提出的。
特斯拉想出了这个想法后，在1898年时展示了世界上第一个无线电遥控潜水船，却从“受到干扰，拦截，或者以任何方式干涉”发现无线信号控制船是安全的需要。
他的专利涉及两个实现抗干扰能力根本不同的技术，实现这两个功能通过改变载波频率或其他专用特征的干扰免疫。
第一次在为使控制电路发射机的工作，同时在两个或多个独立的频率和一个接收器，其中的每一个人发送频率调整，必须在作出回应。
第二个技术使用由预定的方式更改传输的频率的一个编码轮控制的变频发送器。
这些专利描述频率跳变和频分多路复用，以及电子与门逻辑电路的基本原则。
跳频在无线电报中也被无线电先驱约翰内斯Zenneck提及(1908，德语，英语翻译麦克劳希尔，1915年)，虽然Zenneck自己指出德律风根在早几年已经试过它。
Zenneck的书是当时领先的文本，很可能后来的许多工程师已经注意到这个问题。
一名波兰的工程师(Leonard Danilewicz)，在1929年提出了这个想法。
其他几个专利被带到了20世纪30年代包括威廉贝尔特耶斯（德国1929年，美国专利1869695，1932）。
在第二次世界大战中，美国陆军通信兵发明一种称为SIGSALY的通信系统，使得罗斯福和丘吉尔之间能相互通信，这种系统称为扩频，但由于其高的机密性，SIGSALY的存在直到20世纪80年代才知道。
通信工程专业外文翻译--CDMA蜂窝网介绍
外文原文Introduction to Cellular CDMA中文译文CDMA蜂窝网介绍扩频调制技术已经历了过去40多年来的演化。
扩频技术曾经广泛用于抗干扰和多径场合以及测距和跟踪。
扩频技术还被用于CDMA，以支持在大量群体用户之间同时进行数字通信的服务。
CDMA概念可简单地解释成基于扩频通信的调制和多址接入方案。
本文概要介绍了美国圣迭戈高通公司倡导的CDMA数字蜂窝系统。
在很多参与其中的通信公司和设备制造商(AT＆T，Motorola，North Telecom 和其他)的协作下，基于多址接入方案的数字蜂窝应用也取得了进展，CDMA系统作为候选标准(Is-95)完全符合蜂窝通信工业协会(CTIA)要求。
典型的数字蜂窝系统有GSM(欧洲1990年提出的方案)、NATDMA(北美1990年提出的IS-54方案)、PDC(日本1990年提出的标准方案)以及CDMA(美国1993年提出的IS-95方案)。
1982年6月，西欧提山了基于时分多址(TDMA)的GSM系统。
GSM 能够扩展多样的电信网络(例如ISDN)，并提供了对整个欧洲大陆的兼容性。
1992年，第一个商用GSM系统在德国设计成功。
GSM基于频分多址和时分多址的组合。
NA-TDMA系统和GSM相似，惟一差别在于该系统中仅仅存在一个公共无线接口。
PDC(个人数字蜂窝)是日本提出的TDMA蜂窝系统，工作在800 MHz和1.5GHz。
该系统在数字蜂窝网络之间提供了9个接口。
1.5GHzPDC于1994年公开投入运营。
除了数字多址接入系统，还有TDD无绳电话系统，如PHP，CT-2，DCT-900（或CT-3）以及DECT。
TDD(时分双工)系统都是数字系统，但只使用—个载波发送和接收信息。
PHP(个人便携式电话)是支持PCS(个人通信服务)的TDD无线通信系统。
PHP可以用于住宅无绳电话、私有无线PBX(专用分组交换机)、公众远程点和无线电话通信。
一点多址扩频通信系统的应用-毕业论文外文翻译
英文文献The Application of one point Multiple Access Spread SpectrumCommunication SystemLiu Jiangang, Nanyang City, Henan Province Electric Power Industry Bureau【ABSTRACT】Spread Spectrum Digital Microwave communication as a communication, because their excellent performance have been widely used. The article in Nanyang City Power Industry Bureau one point Multiple Access Spread Spectrum Communication System as an example. briefed the spread spectrum communications, the basic concept and characteristics of the power system communication applications.KEYWORDS：one point multiple access; Spread-spectrum communication; Attenuation Nanyang City in the outskirts of Central cloth 35 to 11 kv substation farm terminals, their operation management rights belong to the Council East, Rural Power Company west (the eastern suburb of agricultural management companies -- four, the western suburbs of Rural Power Company Management 7), Scheduling of the various stations of the means of communication to the original M-150 radio and telephone posts. 2002 With the transformation of rural network, the remote station equipment into operation and communication channels to put a higher demand .As PUC Dispatch Communication Building to the east and west of farmers -- the difference between a company linked to fiber, Therefore, if 11 substations and the establishment of a transfer Link Building links Point may be the data and voice were sent to two rural power companies dispatch room, Rural Network scheduling for the implementation of automation to create the necessary conditions.Given the status and power grid substation level, nature, taking into account the carrier and optical-fiber communications to conduct multiple forwarding, increasing the instability factor, considering the cost and conditions of the urban construction, Finally decided to adopt wireless spread-spectrum technology to establish that 11 farm terminal substation communication system. This paper describes the spread spectrum technology and the current system of the building.1.The basic concept of spread-spectrum communication.Spread Spectrum Communication's basic characteristics, is used to transmit information to the signal bandwidth (W) is far greater than the practical information required minimum (effective) bandwidth (△ F) , as the ratio of processing gain GP .G P = W/△FAs we all know, the ordinary AM, FM, or pulse code modulation communications, GP values in the area more than 10 times, collectively, the "narrow-band communication", and spread-spectrumcommunications GP values as high as hundreds or even thousands of times, can be called "broadband communications."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.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 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 the 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 in time 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 the 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-100 mW), 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 communications resources.Spread spectrum communication is digital communications, particularly for digital voice and data transmission while, spread spectrum communication with their own encryption, only in the same PN code communication between users, is good for hiding and confidential in nature, facilitating communications 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 is 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.We have decided to adopt the spread-spectrum communication technology construction of 11 farm terminal substation communications system, Due to the spread-spectrum communication by the line-of-sight transmission distance restrictions, has become unstoppable system design premise.If the PUC scheduling Building and 11 substations have stopped, and the problem becomes more complicated, use spread spectrum system on the feasibility greatly reduced. Therefore, we look at the city Aerial topographical map, initially identified has not stopped to consider systems design, and requests the companies used this equipment Spread Spectrum 11 points transmission routing of the measured and the results have been satisfactory.Then spread spectrum wireless equipment market supply of cash, Initially, we selected a series of Spread Spectrum Comlink third generation products. Because most of the point-to-point mode, Merit function of the spread-spectrum equipment in a point-to-multipoint application environments encountered many problems : First is the issue of frequency resources. Even a minimum of 64 kbit / s data rate radio, space also occupied bandwidth 5 MHz, Because 32 of the PN code isolation is only about 15 dBm, the project had to use frequency division multiple access 35 db to get around the theoretical isolation. 11 stations will use 11 frequency, frequency greater waste of resources. Comlink and Spread Spectrum products in the same frequency to achieve a point-to-multipoint communications.Second antenna erection problems, point-to-point equipment for the main radio station, the main station need to set up a number of terminal antennas, the vast majority of domestic engineering companies used by the U.S. Conifer 24 dBi parabolic semi-cast magnesium grid directional antenna. vertical polarization - 1 m wide, it is difficult to top the layout and avoid flap flap and the mutual interference, Although the project can be set up to take stratified, or through cooperation and on the road to one or more omnidirectional antenna launch, However, as construction of a road and the signal attenuation, transmission result is not satisfactory.In addition, the RF cable laying, The application of network management software such factors we have also decided to adopt the final 1:00 Comlink Multiple Access Spread Spectrum products. Its system configuration, as shown in Figure 2:3.Routing AnalysisCombining visual distance access and use the radio and antenna gain, cable attenuation and environmental factors, and testing the design is reasonable, determine the attenuation affluent channel capacity. Spread spectrum microwave link attenuation depends on the reliability margin. Attenuation margin calculation formula : F G= G SG + G ANT - L GL - L PLF G——Attenuation margin ；G SG——System Gain (dB）；G ANT——Antenna Gain (dBi）；L GL——Connectors and cables attenuation (dB)；L pL—— Channel attenuation (dB）。
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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。
相关理论方程的性能估算。
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，就连这一领域以外的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的书，但是，很多都很难理解或描述的不够详尽。
（例如，直接序列扩频技术广泛关注的是伪随机码的产生）。
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描述一个安全的无线链路来控制鱼雷。
他们获得了美国专利#2.292.387。
但这一技术被遗忘了，没有在当时受到美军的重视，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的收音机链接方面越来越受欢迎。
最典型的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?802.11a,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的地方，因此，它是一种昂贵的资源。
扩频通信的原理扩频是香农定理的典型：C=B×log2(1+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差错传输的最大信息量。
B为信号频带宽度，单位是Hz，S/N为信噪比。
也就是说，C为信道允许通过的信息量，也代表了扩频的性能。
带宽(B)是代价，因为频率是一个有限的资源。
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，的情况下，在无差错传输的信息速率C不变时，如果信噪比很低，则可以用足够宽的带宽来传输信号，即使信号功率密度低于噪音水平。
（公式可用！）改变公式（1）中对数的底数，2改为e，则为In=loge。
因此，C/B=(1/ln2)×ln(1+S/N)=1.443×ln(1+S/N)公式（2）根据MacLaurin扩展公式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）在扩频应用中，通常S/N很低。
（正如刚才提到的，信号功率密度甚至低于噪音水平。
）假定噪音水平即S/N<<1，香农公式可简单表示为：C/B≈1.443×S/N公式（4）简化为：C/N≈S/N 公式（5）或者：N/S≈B/C 公式（6）向固定了信噪比的信道发送错误的信息，只要执行基本扩频信号的传播操作：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的电子设备必须同时存在展宽和解扩的操作过程。
定义不同的扩频技术都有一个共同之处：密钥（也称为代码或序列）依附于传输信道。
以插入代码的形式准确地定义扩频技术，术语“频谱扩展”是指扩频信号的几个数量级的带宽在有密钥的传输信道中的扩展。
以传统的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的带宽宽得多的高频信号（如图1）。
在此过程中，传输宽带信号产生的损耗，表现为噪声。
扩频信号带宽与信息带宽之比称为处理增益。
扩频过程的处理增益大都在10dB 到60dB 之间。
要应用扩频技术，只需在天线（接收器）之前加入相应的扩频码。
相反，你可以删除一个点的扩频码（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的过程。
很明显，同样的代码必须在事先知道在传输通道两端的信息。
（在某些情况下，在调制和解调的过程中代码应该是知道的）。
图1.扩频通信系统传播工作带宽的影响图2说明了信号带宽的通信链路评估图2.扩频操作遍及一个更宽的频率带宽的信息能量扩频调制是一种适用于如BPSK 或直接转换。
传统的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的信息，极没有被扩展。
输入的扩频码频率数据的处理增益数据输入宽度扩频调制数据输入能量能量PF 载体输电链扩频代码接收链扩频代码数据输入射频输出射频输入射频连接相同的配置序列数据输出解扩过程中带宽的影响同样，解扩过程如图3。
图3,在解扩过程中恢复的原有信号在这里，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！由于带宽的浪费抵消了传播的多用户扩频结果直接在一个更宽的频带使用，完全对应之前的“处理增益”。
因此扩频并没有节约有限的频率资源。
过度的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）。
图4.在相同的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，与窄带广播相比，它需要一个比较大的频率带宽。
扩频的优点抗干扰性能和抗干扰的影响扩频技术有很多优点。
.抗干扰性是最重要的一个优点。
有意或无意的干扰和用户1+用户2+用户3+…+用户N数据输入获得的扩频增益能量数据输入宽度数据输入解扩调制能量输入的扩频码数据的处理增益PF 载体频率干扰信号都是不希望存在的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。
图5.扩频通信系统。
注意，解扩链路中数据信号被传输的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的扩频信号。
因此不同的扩频通信系统可以工作在同一频段，例如CDMA 。
值得注意的是，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的第二个优势。
由于非法的听众没有密钥用于原始信号传播，这些听众无法解码。
没有合适的钥匙，扩频信号会出现噪音或干扰。
（扫描方法可以打破的这些密钥，但是密钥是短暂的。
）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6。
（总能量是相同的，但它是广泛存在于频率的。
）因此信息是无形的，这一影响在直接序列扩频（DSSS ）技术上有充分的体现。
（在下文的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的情况下！图6.在被噪音水平之下的扩频频谱信号。
在没有正确的扩频传输密钥的情况下，接收器不能“看到”传输过程。
抗衰落（多径效应）噪声基准扩展后的数据噪声基准数据传播之前输电链扩频代码接收链扩频代码数据输入射频输出射频输入射频连接数据输出数据干扰数据扩展和干扰扩展数据扩展数据扩展和干扰无线信道通常具有多径传播，即有一个以上的信号从发射机传到接收器（如图7）。
这种多路径可以通过空气的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的。
这种反射路径（R ）可干扰直接路径（D ）的现象称为解扩过程的同步衰落。
因为解扩过程使信号D 与信号R 的同步被拒绝，即使它们包含了相同的密钥。
将反射路径的信号应用于解扩是个有用的方法。
扩频技术在CDMA 的应用请注意，扩展频谱不是一个扩频调制方案，不应与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 的已调信号。
.感谢调制的信号的编码基础，使扩频频谱也可用于其他类型的多址实现（即可以同时进行多个通讯联系和实际或表面上相同的物理介质共存）。
到目前为止，有三个主要的方法可用。
FDMA-频分多址FDMA 分配一个特定的载波频率给通信信道。
不同用户使用频谱的切片数是受到限制的（如图8）。
在已有的三种多路存取方法中，FDMA 在频带利用方面是效率最低的。
FDMA 的方法包括Methods 包括无线电广播，电视，高级移动电话系统AMPS 等。
图8. FDMA 系统中不同的用户的载波频率分配。
TDMA-时分多址用户1 用户2 用户3 用户N频率(kHz,MHz,GHz)Fc1 Fc2 Fc3 FcNRxRDTxTDMA 的不同用户彼此间发言和听取信息时，是根据定义的时隙分配来处理的（如图9）。
不同的通信信道可以建立一个唯一的载波频率。
TDMA 的例子有全球移动通信系统GSM ，DECT ，TETRA 和IS - 136。
图9.在TDMA 系统中不同用户的时隙分配。
CDMA-码分多址CDMA 的传播是由密钥或代码决定的（如图10）。
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。
它的例子有IS - 95(DS)，IS- 98，蓝牙和无线局域网。
图10.CDMA 系统中相同频带有独特的钥匙或代码。
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 定义了不同的载波频率（细胞）的拓扑领域，并设定时段内每一个细胞。
扩频和（的）编码密钥在这一点上，值得重申的是扩频的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的。
现代通讯的代码是数字序列必须长期存在和随机出现的，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的。
或者接收器不能提取已发出去的消息。
因此，该序列是几乎是随机的。
这样的代码被称为伪随机数（PRN ）或序列。
最常用的方法来产生伪随机是基于反馈移位用户1用户5用户4用户3 用户2用户1 用户2 用户3 用户N 用户1 用户2 用户3 用户N时间段时间段时间(ms,us)寄存器的。
许多书籍都在介绍伪随机码的发展与特征，但是，实际的发展已超出了这些教材所叙述的。
注意的是，建立或选择适当的序列或序列集并不是微不足道的。
为了保证有效的扩频通信，伪随机序列必须尊重一定的规律如长度、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。
考虑到存在更复杂的序列集，给它提供了一个更强大的扩展频谱链路。
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的电子产品，数字扩频解扩芯片包含几百万个等效的2输入与非门在几十兆赫间切换。
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.Relation direct-sequence spread-spectrum(DSSS) and frequency-hopping spread-spectrum(FHSS) methods.IntroductionAs 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(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 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.Theoretical Justification for Spread SpectrumSpread-spectrum is apparent in the Shannon and Hartley channel-capacity。