Title

Multicarrier modulation for underwater acoustic communications

Date of Completion

January 2009

Keywords

Engineering, Electronics and Electrical

Degree

Ph.D.

Abstract

Underwater acoustic (UWA) communication is getting increased attention in a wide range of applications, such as coastal surveillance systems, environmental research and autonomous underwater vehicle (AUV) operation. High-speed, flexible, and robust UWA transmission is a key element for the development of practical underwater networked systems. However, UWA channels pose grand challenges for effective communication, in that the channels are widely spread over time and frequency, and change at a rapid rate.^ Existing coherent underwater acoustic communication systems rely on single carrier transmission and adaptive decision feedback equalization. As the data rates increase, the symbol durations decrease, and thus the same physical underwater channel contains more channel taps in the baseband discrete-time model. This poses great challenges for the channel equalizer. Receiver comlexity will prevent any substantial rate improvement with existing approaches. In this dissertation, we suggest a paradigm shift from single carrier modulation to multicarrier modulation. Multicarrier modulation in the form of orthogonal frequency division multiplexing (OFDM) converts a frequency selective channel into a set of parallel frequency-flat subchannels, thus greatly simplifying receiver equalization. Our contributions in the dissertation include three main aspects. ^ First, we propose an effective transceiver algorithm based on OFDM with a single transmitter. UWA channels are wideband due to the small ratio of the carrier frequency to the signal bandwidth, which introduces frequency-dependent Doppler shifts. We treat the channel as having a common Doppler scaling factor on all propagation paths, and propose an effective two-step approach to mitigating the Doppler effect. The data from two shallow water experiments near Woods Hole, MA, are used to demonstrate the receiver performance, in which the transmitter moved at a speed of up to 10 knots relative to the receiver. ^ Second, we propose a system design that incorporates multiple-input multiple-output (MIMO) techniques, where spatial multiplexing is applied with OFDM signals. Exploiting the spatial dimension, MIMO-OFDM can greatly increase the spectral efficiency, which is extremely important for the bandwidth-limited UWA channels. The proposed design has been tested using data recorded from two different experiments, in which a spectral efficiency of nearly 3.5 bits/sec/Hz was obtained with different configurations. ^ Finally, we expound on a desirable property of OFDM: one signal design can be easily scaled to fit into different transmission bandwidths with negligible changes to the receiver. Using a large bandwidth together with MIMO techniques, very high data rate can be achieved. In one experiment, a data rate of 125.7 kb/s was achieved with two transmitters, 16-QAM modulation, rate 1/2 coding, and a bandwidth of 62.5 kHz. ^ Extensive experimental results in this dissertation demonstrate that our proposed algorithms work very well in various shallow water environments. The work in this dissertation helps to advance the state-of-the-art in underwater telemetry. ^