고속데이터 전송을 위한 수중 음향 통신 시스템에 관한 연구

Abstract

Recently, the necessity of underwater acoustic comunication increase, and demands for transmitting and receiving various data such as voice or high resolution data are increasing as well. However, the performance of underwater acoustic communication is influenced on underwater channel characteristic. Many obstacles exist in underwater acoustic communication. Especially, an important feature of the underwater acoustic communication is multipath propagation and Doppler spread cause by the surface and bottom reflections. Orthogonal Frequency Division Multiplexing(OFDM) technique for mitigating of multipath fading is central to improving the performance of underwater acoustic communication system.

Conventional OFDM system uses a fixed-length Cyclic Prefix(CP) to mitigate Inter-Symbol Interferences (ISI) caused by underwater channel delay spreading. CP length may cause considerable performance degradation when it is less than the channel Root-Mean-Square (RMS) delay spread or decrease the system power and spectrum efficiency when it is much larger than the channel RMS delay spread.

This thesis proposed to variable CP length to adapt variation of channel delay spread. Adaptive OFDM utilizes the preamble or pilot symbol of each OFDM packet to estimate the channel RMS delay spread and then uses a criterion to calculate the CP length.

In this thesis, the performance of the conventional OFDM and the adaptive OFDM system are compared in Underwater Acoustic Communication System. The thesis compared the original image data with demodulated image data. Futhermore, this thesis also compare the multi-carrier underwater acoustic communication system with single-carrier underwater acoustic communication system. As a result, this thesis shows that the improvement of bit rate is about 20% compared with conventional method.