High-rate digital acoustic communications in a shallow water channel
Date
2018-01-12
Authors
Yoon, Young-Hoon
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Abstract
The subject of this dissertation is coherent digital acoustic communication in the underwater environment. The objective of the research is to develop algorithms for reliable communication in the shallow underwater channel. Investigation is focused on channel depth less than 100 m and distances between transmitter and receiver from 5 km to 50 km.
Based on the characteristics of the underwater acoustic channel and using a conventional approach, the achievable transmission range and the required acoustic power is determined for given channel conditions and system parameters.
A channel model suitable for the investigation of shallow water communication is developed which takes into account transmitter-receiver geometry, environmental conditions and system parameters. The model is based on multiple reflections in the channel with weightings according to signal attenuation due to spreading, reflection losses and absorption. Time-variability is introduced by incorporating Doppler frequency shifts due to transmitter/receiver motion.
A new method of evaluating performance of a system operating in such multipath conditions is proposed by introduction of a signal-to-multipath ratio (SMR), which is a measure of intersymbol interference (ISI) caused by the multipath. The SMR allows assessment of system performance for various receiver/transmitter positions and channel parameters. It can be used, for instance, to find the transmitter/receiver depth for optimum transmission. A suitable equalizer can improve a SMR. For example, a decision feedback equalizer (DFE) using a least mean square (LMS) and fast optimized LMS criterion is effective in coping with ISI as demonstrated by computer simulations. Hardware complexities of several equalizer algorithms are investigated for a selected channel. The performance degradation due to the presence of Gaussian noise in addition to multipath is analyzed by simulation.
A novel structure of an equalizer suitable for the time-variant underwater acoustic channel is proposed. By adaptively adjusting the number of equalizer taps depending on the channel condition, the proposed structure offers reduced hardware complexity. Computer simulations demonstrate the effectiveness of this approach.
It is anticipated that the results of this work will find application in the design of high data rate transmission systems for ocean bottom instrumentation, the design of telemetry for autonomous underwater vehicles, and others.
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Keywords
Digital communications, Signal processing, Digital techniques