Multi-frequency digital communications in underwater channels

Abstract

Strong multipath conditions which exist particularly in a shallow water acoustic channel make conventional coherent transmission schemes ineffective. To this end many non-coherent detection schemes employing frequency diversity have been proposed to improve transmission reliability. -- In this thesis, a novel transmission scheme based on the Sliding Discrete Fourier Transform (SDFT) is proposed for communicating in multipath acoustic channels and it's performance is evaluated analytically and by computer simulations. The system described uses constant envelope multi-frequency signals. Since the envelope of the received signal varies depending on the intensities of the direct path and multipath signals, it is assumed that an Automatic Gain Controller (AGC) stabilizes the received signal before carrying out the demodulation. Due to the non-linearity effect of the AGC process, the analytical evaluation is complicated for the case with AGC. Hence the analytical evaluation is carried out only for the case with no AGC at the receiver. -- First, the variation of eye opening with Direct to Multipath Ratio (DMR) and the minimum DMR required for correct detection are determined. Then the effect of the synchronization error at the receiver on the eye opening is obtained. Finally, the error performance of the signal in the presence of Gaussian noise is evaluated for various DMR values. -- A computer simulation is carried out for the case with an AGC at the receiver. By generating sequences of random data for transmission, the eye patterns of the received signals are obtained. The variation of DMR with the distance between the transmitter and the receiver is also plotted for various transmitter-receiver locations and configurations. -- Based on the results obtained, the proposed system is shown to be a reliable communication scheme in underwater channels.