【正文】 e 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。s , the processing gain G reflects the spread spectrum 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。Television broadcasting RF signal bandwidth is only the video signal bandwidth more than are among the narrowband FM signal, or pulse code modulation signals, their bandwidth and the information ratio of bandwidth to only a few spectrum munication signals and information bandwidth than the bandwidth of up to 100 1000, belonging to broadband use such a wideband signal to transmit information? Do not waste valuable resources in the frequency of it? 2)The use of spreadspectrum code sequence modulation signal approach to broadening the spectrum. We know that in time limited signal, its spectrum is example, very narrow pulses, the spectrum is very bandwidth is inversely proportional to its approximate microsecond pulse bandwidth is about , if the use of limited and narrow pulse sequence has been modulated by the mass of information it may give rise to very broadband 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 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 spreadspectrum 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 narrowband munications in general have been transferred signal demodulation receiver have to be to restore the mass of spread spectrum munications receiver and transmitter used the same spread spectrum code sequence spread spectrum signals received by the relevant demodulation, the restoration of the mass of other words, this correlation despreading demodulation role to is, to expand beyond the original signal is restored by the mass of information. This narrowband information in the originator to expand into the broadband signal, and in 4 the closeend solution will expand its narrowband information into the process, will bring a number of extended to clarify the process of reconciliation mechanism is to understand the nature of spread spectrum munication key. The theoretical basis for spread spectrum munication 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 use such a broadband signal to transmit information? The short answer is mainly to secure munication .The basic characteristics of spread spectrum munication 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: FWGP ?? / (1) As we all know, any information needed for effective transmission of the frequency of a certain width, such as the voice for the , television images were a few MHz 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 fiberoptic, etc.), and measures such as pression 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 narrowband munications.Gp of the spread spectrum munication value, up to hundreds, thousands, known as broadband munications.The feasibility of spread spectrum munication, information theory and antijamming from the basic formula of the theory of extension on the Shannon information capacity (Shannon) formula is: ）（ NPWLC /1og 2 ?? (2) （ 2） Where: C Channel capacity (with transmission rate measurement) W Signal bandwidth P Signal power N White noise power 5 Type (2) shows that： In a given transmission rate C under the same conditions, the bandwidth W and the signaltonoise level NP/ are increasing the bandwidth to the method, at a relatively low signal to noise ratio NP/ ( NS/ ), the transmission of spectrum signal to noise ratio requirements for the reduction is an important feature of spread spectrum munication, and thus the application of spread spectrum munication for the foundation. Spread spectrum munication based on the feasibility of another theory for the transmission of information Baryshnikov error p