Research On ICI Suppression For OFDM Over Time-varying Underwater Acoustic Channels

Orthogonal frequency division multiplexing (OFDM) technology has become one of the significant research topics in high-speed underwater acoustic communications. The orthogonality between sub-carriers on one hand benefits OFDM with high spectrum efficiency, anti-multipath ability, simple implement, and so on. On the other hand it also makes OFDM systems sensitive to time variations of the channel. However, the relative movements between nodes as well as the random time variations of underwater acoustic channels can destroy the orthogonality between sub-carriers,and finally introduce inter-carrier interference (ICI). Traditional channel equalization methods always assume an invariant channel within the duration of an OFDM symbol, which makes them fail to deal with time-varying underwater acoustic environments. Consequently, channel estimation and equalization,ICI suppression technology over time-varying channels has become one of the key technical issues in underwater acoustic communications, which has been increasingly attracting researchers all over the world.This paper studies on ICI suppression technology for OFDM over time-varying underwater acoustic channels. The author's works are listed as follows:1. The existing inter-carrier interference suppression techniques are investigated. The strengths and shortcomings of OFDM in underwater acoustic communications are also analyzed. Moreover, time-varying channel modeling and simulation is provided.2. The various causes of ICI and its mathematical models in underwater acoustic communications are carefully investigated. Theoretical analysis and computer simulations are provided to explore the impacts of ICI on OFDM systems, especially ICI caused by time variations. Common Doppler estimation and compensation techniques are then introduced.3. Time-varying channel estimation based parallel iterative ICI cancellation algorithm is studied where the theoretical basis and specific application of piecewise linear approximation channel model are discussed. Performance comparison among the proposed ICI cancellation algorithm and linear equalizers, such as LS, MMSE, etc., over the time-varying underwater acoustic channels is conducted through simulation and out-field test.4. Decision twice equalization and differential coding based ICI self-cancellation algorithm are respectively used to improve the performance of the proposed parallel iterative ICI cancellation algorithm mentioned above. The principles and implementation are presented.Finally, simulations are done to test the performance of those ICI cancellation algorithms.