Developing Tendency Research Of Communication Technology In New Century

In this dissertation, we mainly introduce the developing trend of communication technology in 21st century, and we also make some deeply discussion about the hotspot problem in this field. Spread Spectrum Communication is an important information transmission way in mobile communication. This method spreads the frequency spectrum to get better performance in information transmission. First, the information inputted at the emission end become digital signal by means of information modulation; second, the spread spectrum code sequence produced by spread spectrum code generator modulates digital signal and spreads the signal spectrum; third, the spreading signal dispatch through the radio spectrum modulation. At the receiving end, the spreading signal pass the radio spectrum demodulation and the spread spectrum untie to resume the original information. According to the spreading way of spread spectrum, spread spectrum communication system is divided into Direct Sequence Spread Spectrum, Frequency Hopping, Time Hopping, Chirp, and the combination of these above-mentioned modes. Direct Sequence Spread Spectrum and Frequency Hopping Spread Spectrum are used frequently. Direct Sequence Spread Spectrum uses high code rate spread spectrum code sequence at the emission end, and uses the same spread spectrum code sequence demodulation to restore the original information at the receiving end. It is the most popular spread spectrum system at present. The modulation ways of spread system are BPSK, QPSK, etc. The working principle of Frequency Hopping Spread Spectrum is essentially similar with Direct Sequence Spread Spectrum. Carrier wave hops from a frequency to another in a very wide waveband according to the definition of pseudo-random code. Hopping rate decides from the data rate of original information. According to hopping rate, Frequency Hopping Spread Spectrum system can be divided into three systems, including the high one, the middle one and the low one. The low rate Frequency Hopping Spread Spectrum is popular, and several sequential data bits are used to modulate the same frequency in this system. Spread Spectrum Communication has many advantages: strong interference rejection, low mistake code rate; strong concealment, little disturbance for other equipments; good rejection of multichannel disturbance; high utilization ratio of frequency band, and so on. Spread spectrum technology has applied widely in many fields, such as mobile communication, satellite communication, and wireless LAN. In 21st century, spread spectrum technology may get development in the following aspects: Ultra-wide Band, Multi-carrier Modulation, and Software Defined Radio, and so on. Increasing with people's demand, more and more data need to transmit; higher and higher speed is required; larger and larger bandwidth is needed. In this dissertation, we introduce several major broadband access technologies: XDSL technology, which uses telephone copper cable as access way; HFC technology, which uses coaxial cable as access way; MMDS, LMDS and WLAN technology, which uses wireless as physical medium; broadband PON technology, which uses optical cable as access way; Ethernet access technology, which uses twisted-pair as access way. XDSL technology mainly includes ADSL, VDSL and SHDSL. ADSL and VDSL both use ordinary telephone copper cable, and offer high speed data transmission for user under the condition that does not affect narrowband voice business. Since China has rich copper cable resource, ADSL and VDSL are major broadband access technology at present. While SHDSL offers high speed up and down symmetrical data transmission by means of telephone coppercable, mainly for special telephone line of some enterprises. At present, ADSL has been widely applied as major broadband access technology. HFC access technology is based on the present CATV network, by means of simulated frequency technology, combining with simulated and digital transmission technology, radio frequency technology, and computer technology. The coaxial cable of this broadband access technology adopts tree type structure and connects with terminal user through embranchment. From ODU to HE, star type structure is used; AN-SCM technology is adopted to transmit signal through cable; all users connected with ODU share one optical cable line. The major function of wireless access technology is to offer immobile or mobile terminal to user by using wireless technology as transmission medium. Wireless access technology can be divided into mobile access and wireless access. The user terminal of mobile access can move in greater scope. For example, group mobile phone system comes from one to one interphone, and has been multi-user auto-dial system linked with telephone net now. The representative of beehive mobile phone system is GSM mobile communication system, which has been widely applied at present. Broadband PON technology uses optical cable as access way. It transmit information between local switch/far end module and user by means of whole or partial optical cable communication system. Usually, OAN is an access transmission system, which uses base band digital transmission technology to transmit double action mutual business. In the future, OAN will use digital or simulated technology to transmit broadband broadcasting business and mutual business. Broadband access network based on Ethernet technology is composed by local end equipment and user end equipment. Local end equipment normally locates in sub-districts or commercial buildings, and user end equipment normally locates in resident building; Local end equipment offer the interface...