通信工程毕业设计（论文）外文翻译-扩频通信系统及MATLAB仿真

【实例简介】
通信类 外文翻译 扩频通信系统及MATLAB仿真
change. Combination of a number of ways of hybrid systems are often applied The most important measure of 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. 2 The purpose of spread spectrum communication studies and significance Spread spectrum communication is an important branch of communication and channel the direction of development of communication systems. Spread-spectrum technology, with strong anti-interference ability, confidentiality, and is easy to realize the advantages of multiple-access communications, so the technology is receiving increasing attention In recent years, with very large scale integrated circuit technology, the rapid development of microprocessor technology, as well as the application of some new components, spread spectrum communications is technically feasible to enter a new level, not only in an important military communications status, and are quickly penetrated into the personal communications and computer communications and other civilian areas, to become the new century, the most potential of communication technologies. The study is spread spectrum communication with far-reaching significance, I through the thesis the design, conduct in-depth study of the spread spectrum communication technology and its applications for simulation, knowledge will be summarized and summed in order to consolidate the basic knowledge of communications for the after the individual to lay a foundation to study and work 2 The basic principles of spread spectrum communication om 2. 1 The definition of spread spectrum communication The so-called spread spectrum communications, can be simply expressed as follows: Spread spectrum communication technology is a means of information transmission, Share some of its signal bandwidth is much larger than the minimum mass of information necessary bandwidth; The expansion of the band through a separate code sequence to complete, using the method of coding and modulation to achieve, with the mass of Information has ne othing to do; Receiving end in the same synchronization code related to receiving, despreading and recovery of the mass data. "This definition includes the meaning of the following three aspec I)The signal spectrum by a broadening We know that the transmission of any information needs to a certain bandwidth, as the information bandwidth. For example, the human voice information bandwidth of 300Hz 3400HZ, television image information bandwidth of Mhz. In order to make full use of frequency resources, which are roughly the same as far as possible the signal bandwidth to transmit information. In the radio communication bandwidth Rf signals with the mass of information is comparable to the bandwidth. Such as the AM signal used to transmit voice messages, voice messages of their bandwidth for twice the bandwidth; Television broadcasting RF signal bandwidth is only the video signal bandwidth more than doubled These are among the narrow-band communications. General FM signal, or pulse code modulation signals, their bandwidth and the information ratio of bandwidth to only a few dozen, Spread spectrum communication signals and information bandwidth than the bandwidth of up to 100---1000 belonging to broadband communications. Why use such a wide-band signal to transmit information? Do not waste valuable resources in the frequency of it? 2)The use of spread-spectrum code sequence modulation signal approach to broadening the spectrum. We know that in time limited signal, its spectrum is infinite. For example, very narrow pulses, the spectrum is very wide Signal bandwidth is inversely proportional to its approximate duration. I microsecond pulse bandwidth is about IMHz. Therefore, if the use of limited and narrow pulse sequence has been modulated by the mass of information it may give rise to very broad-band signal. As described in the following direct sequence spread spectrum system is the method used to obtain spread spectrum signals This very narrow pulse sequences, the code rate is very high, as the spreading code sequence Should be noted here that the spreading codes used by the mass and sequence data is not relevant, that is to say it and in general the same sinusoidal carrier signal, will not affect the transparency of information transmission Sequence spread-spectrum code signals only from the expansion of the role of the spectrum. 3)In the receiving end to use of the relevant despreading demodulator. As the narrow-band communications in general have been transferred signal demodulation receiver have to be to restore the mass of information, In spread spectrum communications receiver and transmitter used the same spread spectrum code sequence spread spectrum signals received by the relevant demodulation, the restoration of the mass of information. In other words, this correlation despreading demodulation role to play. That is, to expand beyond the original signal is restored by the mass of information This narrow-band information in the originator to expand into the broadband signal, and in the close-end solution will expand its narrow-band information into the process, will bring a number of advantages. Spread-spectrum extended to clarify the process of reconciliation mechanism is to understand the nature of spread spectrum communication key. 2.2 The theoretical basis for spread spectrum communication A long period of time, people have the idea of the signal as far as possible the occupation of the narrow spectrum in order to take full advantage of very valuable spectrum resources. Why uIse such a broad-band signal to transmit infornation? The short answer is mainly to secure communication The basic characteristics of spread spectrum commmunication is the transmission signal bandwidth occupied by (W) is much larger than the original information to their actual minimum requirements (effective) bandwidth (DF), the ratio referred to as processing gain Gp: G。=W/△F As we all know, any information needed for effective transmission of the frequency of a certain width, such as the voice for the 1.7---3. 1kHz, television images were a few mhz wide. In order to make full use of limited frequency resources, increasing the number of channels. it is a wide selection of different modulation methods the use of broadband channel (coaxial cable, microwave and fiber-optic, etc. ) and measures such as compression band, at the same time strive to make the transmission of the signal transmission media occupy a narrow bandwidth as much as possible Due to the current use of the telephone, radio system, whether it is using am, fM, or pulse code modulation format, Gp values are within the scope of more than 10 times, collectively referred to as"narrow-band communications. Gp of the spread spectrum communication value up to hundreds, thousands, known as"broadband communications. The feasibility of spread spectrum communication, information theory and anti-jamming from the basic formula of the theory of extension from Information on the Shannon information capacity(Shannon) formula C=WL og, (1+P/N) (2) (2) Where C-- Channel capacity( with transmission rate measurement) W---Signal bandwidth P---Signal power N--- White noise power Type(2) shows that: In a given transmission rate C under the same conditions, the bandwidth w and the signal-to-noise level P/N are interchangeable. By increasing the bandwidth to the method, at a relatively low signal to noise ratio PiN (S/N), the transmission of information Spread spectrum signal to noise ratio requirements for the eduction is an important feature of spread spectrum communication, and thus the application of ad spectrum communication for the foundation Spread spectrum communication based on the feasibility of another theory for the transmission of information Baryshnikov error probability formula Pow,≈f(E/N) (3) Where Pow: Error Probability E-- Signal g No--- Noise power Signal power P=E/T (T for the duration of information) Noise Power N=WNo(W for the signal band width Information bandwidth DF-1/T 丁 The type(3)can be translated into Pow≈f(TWP/M)=f(P/N.W/△F) (4)shows that for a certain bandwidth of the information in terms of DF, with Gp values greater broadband signal transmission, anti-interference ability can improve communications to ensure that under conditions of strong interference, safe and reliable communications.That is, (4)and type(2) the same signal to noise ratio and bandwidth that can be interchangeable In short, we use the information bandwidth of 100 times, or even 1000 times higher than the broadband signal to transmit the message that in order to improve the communication of anti-interference ability, that is in strong interference conditions to ensure safe and reliable communication.This is the spread-spectrum communications based on the basic ideas and theories 2.3 Spread Spectrum Communications main performance indicators )The processing gain is the tolerance and anti-jamming spread-spectrum communication systems are two important performance indicators. G, also known as processing gain spread spectrum gain( Spreading Gain) it is defined as the information before spread spectrum bandwidth expansion band DF and the signal bandwidth w ratio Gp=W/△F (5) In spread spectrum communication system. Demodulation for spread-spectrum receiver, the only pseudo-random code extract treatment, the bandwidth related to the information DF, and ruled out the external w-wideband interference, noise and its impact to the users communications.Therefore, the processing gain G reflects the spread spectrum communication system to improve the level of signal to noise ratio 2)interference tolerance Spread spectrum communication system refers to how much interference in the environment's ability to work, is defined as M,=G-[(S/N)out+LsI Of which Anti-interference capacity G---Processing gain S/N)out---Information data is correct and the requirements of the smallest demodulator output signal to noise ratio Ls -- The work of the receiving system loss 2.4 Spread Spectrum Communication System working principl Digital Spread Spectrum Communication general working principle as shown in Figure 1 messag Deta EsqueR Channe Channel output Modulator Channel 1盘t encoder decoder cuo can 出2tor Generator Figure 1 the number of the basic block diagram of spread spectrum communication system Shown in Figure 1 for a digital spread spectrum communication system, the basic block diagram. which channel encoder channel decoder. modulator and demodulator is a traditional digital communication systems constitute the basic unit. Spread spectrum communication system in the removal of these units, should use the same pseudo-random sequence generator, sending the front-end, respectively, the role of the modulator and demodulator to receive the front end. The two pseudo-random sequence generator noise(PN) binary sequence, the client will send the signal modulation in the frequency domain for the expansion, in the end demodulation despreading the spread spectrum signa 3 Characteristics of Spread Spectrum Communication System Spread spectrum signal is unpredictable pseudo-random broadband signal bandwidth is much larger than its wish to transfer data (information) bandwidth, at the same time, the receiver must be synchronized with a copy of broadband carrier. Spread-spectrum signal as a result of these characteristics, spread spectrum system has the following characteristics 3. 1 The strong anti-jamming Due to the unpredictability of spread-spectrum signal, the spread spectrum system has a high anti-interference ability. It difficult because of interference by observing interference to improve their performance, and can only be fired with the interference of the interference signal does not match the technology, therefore, would not contribute significantly interfere; As the spread spectrum communication system in the transmission signal bandwidth expansion, so even if the signal to noise ratio is very low, even lower than the useful signal power of interference signal power is still the case to high-quality uninterrupted communication, the expansion of the spectrum The more width, the stronger the anti-interference 3.2 LPI of Spread spectrum signal is equivalent to the power being distributed evenly over a wide frequency band, so that transmission signals are very low power density, so that it is difficult to monitor to detect the receiver. Therefore, the spread spectrum communication system has a low probability of interception 3.3 Anti-interference performance of multi-path Multi-path interference is due to radio propagation encountered in the course of a variety of non-expected reflector(such as the ionosphere, the mountains, buildings, etc. ) caused by reflection or scattering Reflection or scattering of these signals with the direct path signal interference in the receiver interfere with each other. Multi-path interference in radar and communication are severely affected. Since the spread spectrum communication system in an increase of spread-spectrum modulation and de-spread process, This spread-spectrum sequences using the correlation between the characteristics of Despreading at the receiving end when the relevant technology from multi-path signals to isolate the most useful signals, or multipath signals of the same code sequence signal superposition, so that you can effectively eliminate the wireless communications in multi-path interference caused by signals decline and, therefore spread spectrum communication system has good properties of anti-multipath fading 3.4 Good Privacy Transmitting power in certain circumstances, due to the distribution of spread-spectrum signals over a wide frequency band, Useful signal channel power spectral density is extremely low, so the signal can be in strong background noise, or even a useful signal to be the case of noise drowned reliable communications, Intercepted by the outside world it is very difficult to send the information, would like to detect the characterist ic parameters of the signal even harder. Therefore, the spread spectrum communication system can be concealed; At the same time, different users use different code, others can not be tapping their communications, thus spread-spectrum system with high confidentiality. 4 Spread Spectrum Communication in the MATLaB simulation 4. 1 Direct Sequence Spread Spectrum system BER simulation According to Shannon theorem, we can see: C=WLog (1+S/N) (7) Where: W for the transmission bandwidth; C for more information. (7)means that in a given signal power S and noise power n of the conditions, using a co oding system to a very small bit error rate of close to C transmission of information transfer rate. Baryshnikov in accordance with theory, we can see Pe=F(/NO (8) Where:P For the bit error rate; F For the signal frequency; E For the signal energy; No For the noise power spectral density. The transmission bandwidth W, Signal power S=E/T, Noise Power N=WNo, Signal duration T, Information signal bandwidth AF=1/T, Into the type( 8), a e STw s w N△F Style(9)Description, Pe By---Decision, In other words, a certain bandwidth N△F transmission AF Information, SNR And bandwidth can be w Exchange N Binary sequences for spread spectrum communication of information, assuming that the information rate of r, Symbol interval is Tb=1/Re,Effective bandwidth to transmit information to B(BmR). In the Ds system, the use of Matlab toolbox function of the pseudo-random sequence to generate pseudo-random code sequence. There is the pn code is a bit more than 0, 1 code sequences, which undergo a rigorous selection process, and has a good auto-correlation, but similar to each other but are mutually orthogonal, linearly independent and in many key type, they can easily identify with their own code the same type, and not easily confused with other code-based. The use of pn sequences in the transmitter, the message sequence will be the expansion of narrow-band wideband signal for transmission into the hannel, the receiver will receive the broadband signal synchronized with the transmitter pN sequence multiplied, the narrow-band de-spread the message sequence, and the types of interference signals are multiplied with the PN sequence to the expansion of the form for broadband, Will eventually weaken the energy of interference signal W!R times. In the modulator, the message sequence for the bandwidth expansion W=Br, Prevalent as a result of transmission interference. and therefore must be taken into account in the simulation of the interference signal of the decision ruling the impact of device. Assuming that the process of signal transmission by the cosine form of interference with the role of signal, the signal current f ()=A,cos(2可f) (10 ype(10): In order to send the signal interference within the frequency handwidth i(t)and c(t) Multiplied so that cosine form of interference into the broadband interference Its energy density spectrum Jo=P/W, where: P,=Af/2 for the average energy of interference signaLIn order to more intuitive analysis of DS-sinusoidal interference signal suppression effects, The establishment of a Monte Carlo simulation model reacTOr siganl Fenta% TefLon devIcs ETtur Figure 2 DS Monte Carlo simulation model system 9 【实例截图】
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