【正文】 , 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 probability formula: ）（ 0j /fow NEP ? （ 3） Where: jPow Error Probability E Signal energy 0N Noise power spectral density Signal power TEP /? (T for the duration of information) Noise Power 0WNN? (W for the signal band width) Information bandwidth TDF /1? The type (3) can be translated into: )//(/ow FWNPfNPTWfP ????? ）（ （ 4） (4) shows that for a certain bandwidth of the information in terms of DF , with PG values greater broadband signal transmission, antiinterference ability can improve munications to ensure that under conditions of strong interference, safe and reliable 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 munication of antiinterference ability, that is in strong interference conditions to ensure safe and reliable is the spreadspectrum munications based on the basic ideas and theories. Spread Spectrum Communication39。s main performance indicators 1) The processing gain is the tolerance and antijamming spreadspectrum munication systems are two important performance , also known as processing gain spread spectrum gain (Spreading Gain) it is defined as the information before spread spectrum 6 bandwidth expansion band DF and the signal bandwidth W ratio: FWGP ?? / （ 5） In spread spectrum munication system. Demodulation for spreadspectrum receiver, the only pseudorandom code extract treatment, the bandwidth related to the information DF, and ruled out the external Wwideband interference, noise and its impact to the user39。s , the processing gain G reflects the spread spect