【文章內容簡介】 rum munication system to improve the level of signal to noise ratio. 2) interference tolerance Spread spectrum munication system refers to how much interference in the environment39。s ability to work, is defined as: ]s)/[( outj LNSGM ??? （ 6） Of which: jM Antiinterference capacity G Processing gain out)/( NS Information data is correct and the requirements of the smallest demodulator output signal to noise ratio sL The work of the receiving system loss Spread Spectrum Communication System working principle Digital Spread Spectrum Communication general working principle as shown in Figure 1. Figure 1 the number of the basic block diagram of spread spectrum munication system Shown in Figure 1 for a digital spread spectrum munication system, the basic block channel encoder, channel decoder, modulator and demodulator is a traditional digital munication systems constitute the basic spectrum munication system in the removal of these units, should use the same pseudorandom sequence generator, sending the frontend, respectively, the role of the modulator and demodulator to receive the front two pseudorandom sequence generator noise (PN) binary sequence, the client 7 will send the signal modulation in the frequency domain for the expansion, in the end demodulation despreading the spread spectrum signal. 3 Characteristics of Spread Spectrum Communication System Spread spectrum signal is unpredictable pseudorandom 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 .Spreadspectrum signal as a result of these characteristics, spread spectrum system has the following characteristics: The strong antijamming Due to the unpredictability of spreadspectrum signal, the spread spectrum system has a high antiinterference 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 munication 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 highquality uninterrupted munication, the expansion of the spectrum The more width, the stronger the antiinterference. 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 , the spread spectrum munication system has a low probability of interception. Antiinterference performance of multipath Multipath interference is due to radio propagation encountered in the course of a variety of nonexpected reflector (such as the ionosphere, the mountains, buildings, etc.) caused by reflection or or scattering of these signals with the direct path signal interference in the receiver interfere with each interference in radar and munication are severely the spread spectrum munication system in an increase of spreadspectrum modulation and despread process,This spreadspectrum sequences using the correlation between the characteristics of,Despreading at the receiving end when the relevant technology from multipath 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 munications in multipath interference caused by signals decline and, therefore, 8 spread spectrum munication system has good properties of antimultipath fading. Good Privacy Transmitting power in certain circumstances, due to the distribution of spreadspectrum 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 munications,Intercepted by the outside world it is very difficult to send the information, would like to detect the characteristic parameters of the signal even , the spread spectrum munication system can be concealed。 At the same time, different users use different code, others can not be tapping their munications, thus spreadspectrum system with high confidentiality. 4 Spread Spectrum Communication in the MATLAB simulation Direct Sequence Spread Spectrum system BER simulation According to Shannon theorem, we can see: ）（ NSWLC /1og ?? （ 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 coding 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: )/( 0e NEFP? （ 8） Where: eP For the bit error rate。 F For the signal frequency。 E For the signal energy。 0N For the noise power spectral transmission bandwidth W ,Signal power TES /? ,Noise Power 0WNN? ,Signal duration T ,Information signal bandwidth TF /1?? ,Into the type(8) ,a： FWNSNST WNE ???0 (9) Style (9) Description, eP By FWNS? Decision,In other words, a certain bandwidth transmission F? Information,SNR NS And bandwidth can beW Exchange. Binary sequences for spread spectrum munication of information, assuming that the information rate of R , Symbol interval is SRT /1b? ,Effective bandwidth to transmit information to )m( RBB CC .In the DS system, the use of MatLab toolbox function of the