수중 다중경로 시간확산채널에서 적응등화기와 직교주파수분할다중화 기법의 성능해석

Abstract

In recent years, underwater acoustic(UWA) communications are a rapidly growing field of research and engineering as the applications. Wireless UWA communications can be established by transmission of acoustic waves. The environment factors of UWA communication channel consist of loss, ambient noise, doppler effect, and multipath etc. Among these, multipath is principal factor determining the available bandwidth and signal to noise ratio. The channel frequency response and time dispersion due to multipath is characterized by the normalized delay spread T_(m) and coherence bandwidth. The delay spread T_(m)is a parameter as defining the time extent of multipath time spread. Simultaneously coherence bandwidth should be also noted which directly influence the system performance. In this paper, the multipath effect on UWA channel bandwidth and BFSK and BPSK bit error dependancy on channel bandwidth are analyzed. To mitigate the distortion caused by Inter Symbol Interference(ISI) due to multipath, adaptive equalizer based on LMS algorithm is employed in bandwidth efficiency BFSK and BPSK. Bit error characteristics on the channel bandwidth and the criteria of the multipath delay spread constant are found through the numerical simulation. Due to the UWA channels limited bandwidth Orthogonal Frequency Division Multiplexing(OFDM) scheme has been received attract. OFDM is more robustness to multipath effects which result in frequency selective fading. Conventional OFDM employs a cyclic extension of transmitted OFDM symbols to combat ISI caused by channel delay spread. However, the insertion of cyclic prefix(CP)is at the cost of reduced bandwidth efficiency. It is proposed that Multi Symbol Encapsulated-OFDM (MSE-OFDM) is employed in multipath channel. For MSE-OFDM scheme, a number of OFDM symbols are grouped together as a frame and one single CP is appended to the beginning of each frame symbols. The results prove MSE-OFDM using only one CP during one frame can improve bandwidth efficiency. The Pulse shaping function is also proposed to reduce the OFDM systems sensitivity to frequency offset which leads to Inter Carrier Interference(ICI). The simulation results demonstrate that pulse shaped OFDM signals can reduce ICI and achieve good performance. The results demonstrate that adaptive equalizer and OFDM is an efficient way to achieve high transmission data rate in the shallow water channel.