With the popularization of computer application technology and the development of national economic informatization, client / server computing, distributed processing, Internet, Intranet and other technologies are widely accepted and applied, and the demand for computer networking has expanded rapidly. Among many computer networking technologies, the wireless network (Wireless Network) has the advantages of no wiring, roaming in a certain area, and low operating costs. In many applications, it has an irreplaceable role for other networking technologies. The spread spectrum technology of computer wireless network has the characteristics of strong anti-interference ability, easy to implement code division multiple access, security and confidentiality, and no need to apply for frequency resources. In recent years, it has been widely used in various fields of computer networking.
Spread spectrum communication has unparalleled advantages in other ways, and it has been more and more widely used in wireless communication in recent years. Spread Spectrum Communication (Spread Spectrum CommunicaTIONs) began to appear in the Second World War, as the wireless secret communication technology used by the US military. Now spread spectrum technology is widely used in many fields including computer wireless networks. Spread-spectrum communication is a method of information transmission. The bandwidth of the frequency band occupied by the signal is much larger than the minimum bandwidth necessary for the transmitted information. The widening of the frequency band is achieved by the method of encoding and modulation, and has nothing to do with the transmitted information. The end uses the same spreading code for related demodulation to despread and recover the transmitted information.
Spread spectrum communication has the following characteristics:
1. Strong anti-interference:
Anti-interference is one of the main characteristics of spread spectrum communication. For example, the signal spreading width is 100 times, narrow-band interference basically does not work, and the intensity of broadband interference is reduced by 100 times. If you want to maintain the original interference intensity, you need to increase 100 Double the total power, which is essentially difficult to achieve. Signal reception requires spread spectrum coding for correlation despreading processing, so even if the same type of signal is used for interference, without knowing the signal's spreading code, due to the different correlation between different spreading codes, the interference Does not work either. Because of the anti-jamming nature of spread-spectrum technology, the US military has widely used spread-spectrum wireless bridges to connect computer networks distributed in different areas in the Gulf War.
2. Good concealment:
Because the signal is spread over a wide frequency band, the power per unit bandwidth is very small, that is, the signal power spectral density is very low, and the signal is submerged in white noise. Pick up useful signals, and the extremely low power spectral density rarely interferes with other telecommunications equipment.
3. Easy to implement code division multiple access (CDMA): Spread spectrum communication occupies broadband spectrum resource communication, improves the anti-interference ability, does it waste the frequency band? In fact, on the contrary, spread spectrum communication improves the frequency band utilization. It is precisely because spread spectrum communication uses spread spectrum coding for spread spectrum modulation transmission, and signal reception requires the same spread spectrum code for related despreading to obtain, which provides a basis for frequency multiplexing and multiple access communication. Make full use of the correlation characteristics between spreading codes of different code types. Different spreading codes assigned to different users can distinguish the signals of different users. As long as many users use their own spreading codes in pairs, they can Interferingly use the same frequency communication at the same time, thereby achieving frequency reuse and making full use of the crowded spectrum. The sender can use different spreading codes to send data to different receivers; similarly, the receivers can use different spreading codes to receive the data sent by different senders and realize multiple-access communication. A technical report from the National Aeronautics and Space Administration (NASA) states that the use of spread-spectrum communications has increased spectrum utilization. In addition, spread spectrum code division multiple access is also easy to solve the problem of adding new users at any time.
4. Anti-multipath interference:
In wireless communication, the anti-multipath problem has always been a difficult problem to solve. Taking advantage of the correlation characteristics between spreading codes, the receiving end can use correlation technology to extract and separate the strongest useful signal from the multipath signal. The waveforms of the same code sequence from multiple paths are added to make it strengthened, so as to achieve effective anti-multipath interference.
5. Other advantages of spread spectrum communication:
Spread spectrum communication rate is high (2M, 8M, 11Mbps), no need to apply for frequency resources, network construction is simple, network performance is good.
Compared with X.25 and DDN, the wireless spread spectrum network has a high rate (2Mbps, 8Mbps, 11Mbps), simple installation, low operating cost (no rental fee, only a small maintenance cost), no need to apply for frequency resources, easy to expand investment Less advantages. In addition, if X.25 or DDN is used as the link for interconnection between the network, routers, multiplexers and other devices should be used at both ends of the link, and wireless spread spectrum products include bridges, routers, modems, etc. Selection, saving equipment and investment, so wireless spread spectrum network has obvious advantages over X.25 and DDN networking within hundreds of kilometers.
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