1、A spread spectrum communication technology 1.1 spread spectrum communication system overview Spread spectrum communication, namely the spread spectrum communication ( Spread Spectrum Communication ), and optical fiber communication, satellite communication, together known as entering the information
2、 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 demodulat
3、or 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
4、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 b
5、e multiple access multiplexing and arbitrary location of high precision measurement Because 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
6、 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 ha
7、ve 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
8、 sensing, monitoring, alarm system Two spread spectrum communication the basic concept 2.1spread spectrum communications The spread spectrum communication, can be simply expressed as follows: spread spectrum communication technique is a method for transmitting informationType, its - 2 - signal the b
9、and 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 synchroniza
10、tion, 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 im
11、age 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
12、. 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 modu
13、lated 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 o
14、f 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 durat
15、ion 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 obtai
16、ned 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 sinusoid
17、al 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 despreading As in the general communication, modulated signal at the receiving end are - 3 - d
18、emodulated 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 expansi
19、on 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 understan
20、d the nature of the spread spectrum communication key. 2.5 spread spectrum communication theory Long-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 tra
21、nsmit 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 ), th
22、e 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 numb
23、er, 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 syste
24、ms, 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 c
25、ommunications. 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 transmi
26、ssion 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
