천해 음향통신 채널이 수중음향통신 시스템에 미치는 영향

Abstract

Underwater acoustic communication systems are used widely in the fields of underwater network, autonomous underwater vehicle, environment monitoring, and etc. However, their design and performance are influenced very much by underwater acoustic channel fading such as a large scale fading and a small scale fading . The large scale fading which is characterized by transmission loss, limits a communication range. However the small scale fading which is characterized by channel parameters such as impulse response, delay spreading, scattering function, coherence bandwidth, frequency selective fading, coherence time and fading statistics, affect on design and performance of a modem, a channel equalizer or a channel coding. In this study, the small scale fading which is influenced by sound velocity profile, medium and platform motion and rough sea surface in shallow water, is mainly focused. Its effects on underwater acoustic system design and performance are analyzed as following order. Firstly, a channel model based on phasor, a channel simulator, measurement and analysis methods of channel parameters are given in a fixed source-to-receiver system. Secondly, the parameters are analyzed for two different source-to-receiver ranges in shallow water. Finally, effects on FSK(frequency shift keying), PSK(phase shift keying), CC(convolution code) and RS(Reed-Solomon code) are evaluated in a simulator and a real shallow water channel. It is found that the small scale fading in 50m depth of water is characterized by two or three multipath such as a direct, a surface reflection path with time variant scattering and a bottom reflection path. The -3dB coherence bandwidths of 300 and 600m source-to-receiver range are 67 and 77Hz, respectively. The coherence times of 300 and 600m source-to-receiver range are about 2.5s in both ranges. A constructive and a destructive interference are clearly observed. The fading statistics of envelope amplitude is Rice distribution with Rice factor K=2 in 300m and 4 in 600m. Rice factor also depends on constructive and destructive interference frequencies. The bit error rate of FSK and PSK depends on frequency correlation function or frequency selectivity and interference frequencies in water tank and 16m depth of a real ocean experiments. In addition, Reed-Solomon channel coding adapted in channel fading with burst errors, is found to be better than convolution coding adapted generally in channel fading with random errors. In conclusions, the phasor model proposed for the small scale channel fading in shallow water acoustic channel can be used to analyze and interpret the channel parameters such as delay spreading, scattering function and etc. can be analyzed and interpreted based on the phasor model. The experimental results for effects of the channel parameters on modem and channel coding performances show that both time and frequency domain diversity techniques are required in underwater acoustic communication system in shallow water acoustic channel.