工业自动化(自考)毕业论文 扩频通信技术(罗京京)译文.doc

吉林电子信息职业技术学院毕业设计译文题 目： 扩频通信技术译文 姓 名： 罗京京 专业班级： 工业自动化 指导教师： 常兆盛 讲师 完成时间： 2012年6月 A spread spectrum communication technology1.1 spread spectrum communication system overviewSpread spectrum communication, namely the spread spectrum communication ( Spread Spectrum Communication ), and optical fiber communication, satellite communication, together known as entering the information age of three big high-tech communication transmission method. Spread spectrum communication is to transmit information data by the pseudo random code ( spread spectrum sequence: Spread Sequence ) modulation, realization of spread spectrum and transmission; receiving end uses the same code demodulator and related treatment, recovery of the original information data. This communication mode and the conventional narrow communication mode are different: one is the information spread spectrum after the formation of broadband transmission; two is related to recovery after treatment into narrowband information data. It is due to the two holding point, make the spread spectrum communication has the following advantages: high anti jamming noise anti multipath fading with the confidentiality of the power spectral density is low, has the nature of concealment and low probability of intercept can be multiple access multiplexing and arbitrary location of high precision measurementBecause of the spread spectrum communication technology has the advantages of the United States Army, from 50 time metaphase begins study convenient for military communications, has been the exclusive, widely used in military communication, electronic countermeasure and navigation, measurement and other fields. Until the early 80s before being used in civil communication field. In order to meet the growing needs of the civilian communication capacity and the efficient use of spectrum resources, all countries have made in digital cellular mobile communications, mobile satellite communications and future personal communication using spread spectrum technology, spread spectrum technology has been widely used in cellular phones, cordless telephone, microwave communications, wireless data communication, remote sensing, monitoring, alarm systemTwo spread spectrum communication the basic concept2.1spread spectrum communicationsThe spread spectrum communication, can be simply expressed as follows: spread spectrum communication technique is a method for transmitting informationType, its signal the band width is far greater than the transmitted information required minimum bandwidth; bandwidth expansion is through an independent code sequence to complete, with the coding and modulation method, and the transmitted information data independence; at the receiving end with the same code synchronization, correlation despread and recovery the transmitted information data.2.2signal spectrum is broadened.We know, transmission of any information requires a certain amount of bandwidth, called information bandwidth. For example, in human speech information bandwidth of 300Hz -3400Hz, television image information bandwidth for the number MHz. In order to make full use of frequency resources, are usually used as roughly equivalent to that of the bandwidth of the signal to transmit information. In a radio communication in RF signal bandwidth and the transmitted information bandwidth is compared. As with amplitude modulation signals to transmit voice information, the voice information bandwidth bandwidth for two times; television broadcasting RF signal bandwidth is only the video signal bandwidth doubling. These all belong to the narrow band communication.The FM signal, or pulse code modulated signals, their bandwidth and information bandwidth ratio is also only a few ten. Spread spectrum communication signal bandwidth and bandwidth ratio is as high as 100-1000, belonging to the broadband communication.Why use such wide band signal to transmit information? Wouldnt that is a waste of valuable frequency resources?2.3 using the spread spectrum code sequence modulation way to stretch the signal spectrum.We know, in terms of time limited signal, the spectrum is infinite. For example, a very narrow pulse signal, the spectrum is very wide. Signal frequency band width and its duration is approximately inversely proportional to. 1 microsecond pulse bandwidth is about 1MHz. Therefore, if with limited narrow pulse sequence is transmitted information modulation, can produce very wide band signal. As following the presentation of the direct sequence spread spectrum system is obtained with such a method of spread spectrum signal. This very narrow pulse code sequence, the code rate is very high, known as the spreading code sequences. There needs to be explained is the spreading code sequences and the transmitted information data is irrelevant, that is to say it with a sinusoidal carrier signal, does not affect the transmission of information transparency. Spread spectrum code sequence is only extended signal spectrum function.2.4on the receiving end of correlation demodulation despreadingAs in the general communication, modulated signal at the receiving end are demodulated to recover the message. In the spread spectrum communication receiver and transmitter using the same spreading code sequence and the received spread spectrum signal correlation demodulation, restore message. In other words, this correlation demodulation despreading role play. That expansion signal back into the original message. At the beginning of the narrowband information that extended into the broadband signal, and the receiving end and the despread into narrowband information processing, can bring a series of benefits. Clear spread spectrum processing mechanism, is to understand the nature of the spread spectrum communication key.2.5 spread spectrum communication theoryLong-term since, people always thought that the occupation of signal spectrum as narrow as possible, in order to make full use of spectrum resources is very valuable. Why use such broadband signals to transmit information? The simple answer is mainly for communication security.Spread spectrum communication the basic characteristics of signal transmission, is occupied by the band width ( W ) is much larger than the original information itself actual required minimum ( effective ) bandwidth (DF ), the ratio is called the processing gain Gp: Gp = W/DF . (1) as everyone knows, any effective information transmission requires certain frequency width,1.7-3.1kHz such as voice, television image width to several mhz. In order to make full use of the limited frequency resource, increase the channel number, widely different modulation modes, using the broadband channel ( coaxial cable, microwave and optical fiber ), and compression bands and other measures, and strive to make transmission medium transmission signals to occupy as narrow bandwidth. Because of the present use of telephone, radio systems, either using amplitude modulation, frequency modulation and pulse code modulation, the Gp value generally in more than 10times range, collectively referred to as the narrow band communication. The spread spectrum communication Gp value, up to hundreds of, thousands of, referred to as broadband communications.Spread spectrum communication is discussed, from information theory and anti interference theory the basic formula of extension to. Information theory of information capacity of the Shannon ( Shannon ) formula is: C = WLog2(1ten P/N ) . (2) type : C - channel capacity ( with a transmission rate of W signal frequency band width measurement ) - P - N - white noise signal power power type (2), at a given transmission rate of C under the same conditions, the band width of W and the signal to noise ratio of P / N are interchangeable. Can increase bandwidth method, the low signal-to-noise ratio of P / N ( S / N ) cases, transmission of information. Spread spectrum change to win the trust of noise ratio is reduced, it is the important characteristic of spread spectrum communication, and thus lays a foundation for the application of spread spectrum communication.Spread spectrum communication feasibility of another theoretical basis, as Kejieer Metchnikoff on information transmission error probability formula: Powj f ( E/N. ) (3): Powj - error probability in E - N signal energy. - noise power spectral density because of the signal power, P = E / T ( T information for the duration of the noise power of N = WN ). ( W signal bandwidth ) information bandwidth D F = L / T type (3) can be reduced to: Powj, f ( TW.P/N ) = f ( P/N.W/D F ) (4) type (4), for a certain DF bandwidth information, with the larger Gp value broadband signal transmission, can be improve communication anti jamming ability, strong interference conditions, the safe and reliable communication. Which type (4) and type (2), description of signal-to-noise ratio and bandwidth are interchangeable.In short, we use the information bandwidth of100 times, or 1000 times more broadband signals to transmit information, is to improve communication anti-jamming ability, namely under strong interference to ensure the reliable and safe communication. This is the spread spectrum communication basic idea and theory basis.2.6 spread spectrum communication main performance indexProcessing gain and the anti interference tolerance is spread spectrum communication system are two important performance indexes. Processing gain of G also known as spread spectrum gain ( Spreading Gain ) define it as a spread spectrum information before the DF bandwidth and bandwidth expansion after the signal bandwidth of W is G = W / DF in spread spectrum communication system receiver for spread spectrum demodulation. After, only extraction of pseudo random code correlation processing after the bandwidth of DF information, to the exclusion of out of band W in the external interference, noise and its user communication effect. Therefore, the processing gain of G reflects the spread spectrum communication system signal to noise ratio and the degree of improvement.Anti interference tolerance refers to the spread spectrum communication system can be in how old the interference environment under normal working ability, defined as: Mj = G ( S/N ) out + Ls: Mj - anti interference capacity G - processing gain ( S / N ) out - information data to be correctly demodulated and minimum output signal noise ratio Ls - receiving system such as job loss, a spread spectrum processing gain of the system requirements for35dB. bit error rate less than l0 - 5information data demodulating the minimum output signal-to-noise ratio ( S/N ) out 10dB, Ls = 3dB system loss, then the interference tolerance Mj = 35(10 +3) = 22dB this description, the system can input power level ratio in interference of spread spectrum signal power level high 22dB within the range of normal work, namely the system is able to receive the input SNR is greater than or equal to22dB environment under normal operation.Three major characteristics of spread spectrum communicationThe spread spectrum communication can greatly expand the frequency spectrum of the signal, originating with spread spectrum code sequence spread spectrum modulation,And at the receiving end by correlation demodulation technique, which has many narrow band communication is difficult to replace the excellent performance,In army turn civilian, quickly extended to various public and private communication network, basically have the followingCharacteristic:3.1 easy to reuse of frequencies, improves the spectrum utilization rateWireless spectrum is very valuable, although from the wave to the microwave have been developed using, still can not meet the demand of society. In a narrow band communication, mainly rely on the channel to prevent interference between the division of channel. Therefore, all the countries in the world have established frequency management mechanism, the user can only use approved frequency.Spread spectrum communication transmission power is very low (1-650mW ), the correlation receiving this high technology, and can work in the channel noise and thermal noise in the background, is easy to be in the same area of the repeated use of the same frequency, and may also present a variety of narrow communication sharing the same frequency resource. Therefore, in the United States and most of the countries in the world, spread spectrum communication need not apply for any frequency, individual and unit can be without a license to use.3.2strong anti-interference, low bit error rateSpread spectrum communication in the space occupied by the transmission bandwidth is relatively wide, and the receiving end and the correlation detection approach to despread, make useful wideband information signal into the narrow band signal, and the desired signal spread into wideband signal, and then through the narrow band filtering technique to extract useful signal. This auspicious, for various kinds of interference signal at the receiving end, because of its non correlation, de-spread narrowband signal only very weak component, signal to noise ratio is very high, so the strong interference immunity. As the above examples ( second ), when Gp =35dB, Mj = 22dB interference tolerance, namely in the negative signal to noise ratio (22dB ) condition, the signal can be extracted from the noise of the annihilation. In the current commercial communication systems, spread spectrum communication is only able to work in a negative SNR communication mode.For broadband interference and pulse interference in spread spectrum apparatus to inhibit physical process, for various forms of human ( such as electronic warfare ) interference or other narrowband or broadband ( DSSS) system disturbance, as long as the waveform, time and symbol is slightly different, de-spread still maintains its broad band, and the useful signal will be compressed, the pulse interference, bandwidth will be expanded to B, and the useful signal recovery ( compression ), ensure higher interference, because of the excellent performance of spread spectrum system, the bit error rate is very low, normal conditions can be as low as l0-10, the worst conditions about 10-6, can fully meet the domestic system of channel transmission quality requirement.As a result of spread spectrum signal in a relatively wide frequency band was expanded, the unit within the frequency band of the power is very small, signal submerged in noise, are generally not easily found, and would like to further detection signal parameters ( such as pseudorandom coded sequence ) is more difficult, so that its concealment. Furthermore, due to the spread spectrum signal has very low power spectral density, the current use of the narrowband communication interference is very small.3.3 realize the code division multiple accessSpread spectrum communication improves the anti-interference performance, but paid the occupied bandwidth costs, if many users share the bandwidth, can greatly improve the utilization rate of frequency band. Due to the spread spectrum communication in presence of the spreading code sequence spread spectrum modulation, make full use of all sorts of different type of code spreading code sequences between excellent auto-correlation and cross-correlation properties, at the receiving end using correlation detection technology for despreading, are assigned to different user code case can distinguish between different user signal, extraction the useful signal. Thus, in a wide frequency band is many user can also call and no mutual interference.3.4anti multipath interferenceIn a wireless communication in various frequency bands, long-term since, multipath interference is always a difficult problem to solve. In the past the narrow band communication, using two methods to improve the ability of anti multipath interference : one is to put the most useful signal is separated, the exclusion of other paths of the interference signal, which uses diversity / receiving technology; two is to try to put different paths to different delay times, different phase signal at the receiving end in time domain aligned together, combined into a strong signal, which adopts the comb filter method.The two techniques in spread spectrum communication is easy to realize. Using a spreading code auto-correlation properties, at the receiving end from the multipath signal extracting and separating out the strongest of the useful signal, or the multiple paths to the same code sequence waveform additive synthesis, which is equivalent to the comb filter effect. In addition, using frequency hopping spread spectrum modulation spread spectrum system, by using multiple frequency signal is transmitted with a message, actually plays a role of frequency diversity.3.5precise timing and rangingWe know that the electromagnetic wave propagation in the space velocity is constant speed of light. It is natural to wonder if capable of accurately measuring the electromagnetic wave in the two objects are transmitted between the time, also be equal to measure the distance between two objects.In the spread spectrum communication if spread spectrum is very wide, it means the code rate is very high, each chip time is very short. When the transmitted spread spectrum signal reflected back upon the object to be measured, in the receiver demodulates the spread spectrum code sequence, and then compare the transceiver two code sequence phase difference, can accurately detect the spread spectrum signal and time difference, so as to calculate the distance between two people. Measurement accuracy de