Investigation of Underwater Acoustic Multi-path Doppler and Delay Spreading in a Shallow Marine Environment

INTRODUCTION The choice for the wireless transmission of data underwater is between electromagnetic waves (e.g. light or radio) or sound waves. Light and radio waves are valuable for high-rate data transmission through water over short ranges of the order of a few metres. When transmission is required over longer distances through water, sound waves are the only viable wireless option. Underwater acoustic data transmission is not without signifi cant constraints intrinsic to the marine environment. The underwater acoustic environment is highly reverberant, resulting in multiple refl ected copies of any transmitted signal arriving at the receiver at delayed intervals (delay spreading), and with the relative delays generally changing with time. The frequency of transmitted signals is also signifi cantly distorted by transient Doppler effects generated by elongation and contraction of surface refl ected transmission paths (Doppler frequency spreading), or Doppler frequency shifts from movement of either (or both) the transmitter and receiver. Transient delay spreading and Doppler spreading of the received signal present signifi cant challenges to the decoding of incoming data symbols by a communications receiver, with the problem becoming more diffi cult as the rate of data transmission increases. In 2011 the Department of Electrical and Computer Engineering and the Centre for Marine Science and Technology (CMST) at Curtin University commenced a project to develop high-rate underwater acoustic communications to support developing ocean-based industries in Australia [1]. The authors’ role is to develop an underwater acoustic communication channel simulator to support this project. The purpose of the simulator is to simulate the way that the real ocean produces transient distortion of acoustic communication signals between a transmitter and a receiver, including interference effects from highly variable natural and anthropogenic noise. The simulator provides a confi gurable analogue of the real ocean that can be used to improve understanding of the infl uence of the marine acoustic environment on communications signals, and assists the development of underwater communication modulation and demodulation algorithms and hardware. Transient phenomena that are key to the development of an acoustic channel simulator for high-rate data communications are the transient delay and Doppler frequency spreading of the received signal imparted by the moving sea-surface, shown schematically in Fig. 1, and the transient Doppler imparted by moving transmitter and/or receiver platforms [2,3].