Research On Performance Of Wireless Broadband Modulation Techniques In Tunnel Environment

Since coal is China's primary energy; safety of coal mine production plays an important role in the scientific development of China's modernization. But this year, many coal mine accidents happened, and serious casualties and economic losses are cost. As a result, the establishment of a scientific, reliable and effective communications system is of realistic significance to the protection of both coal miners and state property. While there are many more mature means of communication currently, the research on this particular confined space environment is still inadequate. MC-CDMA, which combines the advantages of CDMA and OFDM, multi-carrier spread-spectrum technology is considered to be a very promising one in the future 4G communication systems. This article studied in depth the performance of MC-CDMA, as well as a series of wide band communication systems under the special confined space of mine tunnel.In the beginning, wireless channel characteristics are introduced, with emphasis on analysis of the mine tunnel environment factors that affect the signal transmission. In the long and narrow tunnels operating mechanical work, the signals have smaller propagation delay, low velocity, and suffer from a variety kind of noises. This article studied these factors and modeled tunnel environment, which laid the foundation for the following research.Then, based on the analysis of narrow band and broad band systems, a close form of average BER expression in presented, in which modulated signals transmit in Rayleigh and Nakagami channels, and MRC is used in the receiver. After that, performance of MC-CDMA system suffering from small-scale fading under coal mine tunnels is analyzed thoroughly. A typical MC-CDMA model and a Modified MC-CDMA system model under coal mine environment are established, and their performance under impulsive noise and Doppler shift are discussed respectively.The article creativity presented a tunnel comprehensive channel model, in which both small-scale and large-scale fading are considered. With this model, the BER performance of broadband communication systems, MC-CDMA mainly, as well as OFDM, DS-CDMA, MC-DS-CDMA and MT-CDMA under large-scale fading in tunnels are studied. The simulation results showed that the CDMA communication system families represented by MC-CDMA have a great advantage in short distance propagation, while in the long-distance transmission OFDM achieved a much lower BER than the former.Finally, in order to improve the MC-CDMA performance in coal mine tunnels, the techniques of detection, sub-carrier power allocation and adaptive modulation are introduced. The results showed that after the adaptive modulation, sub-carrier power allocation, and MMSE detection are brought in, the coal mine MC-CDMA systems could transmit farther given same situation.Through this paper, we can come to the conclusion that CDMA systems represented by MC-CDMA perform better in close range underground, while OFDM suit for long-distance transmission, when improvement techniques are brought in, MC-CDMA system's effective transmission distance can be enlarged.