Study On Multiuser OFDM-based Underwater Acoustic Communication

The characteristics of the underwater acoustic communication channel,such as large transmission loss,large propagation delay,limited communication bandwidth,severe Doppler effects,large multipath delay spread and so on,pose a big challenge to the underwater acoustic network and multiuser communication at the physical layer.Multiuser channel estimation,asynchronous multiuser reception,non-uniform Doppler scale estimation and downlink non-orthogonal multiple access with high spectral efficiency are considered in this paper,based on orthogonal frequency-division multiplexing(OFDM)modulation.Firstly,a method for sparse channel estimation and pilot optimization is proposed in orthogonal frequency division multiple access(OFDMA)system,in order to improve the performance of multiuser channel estimation for uplink underwater acoustic communication.A compressed sensing(CS)based algorithm is utilized for sparse channel impulse response estimation.The pilots' pattern and power joint optimization algorithm based on random search technique is proposed according to the minimum mutual coherence criterion in CS theory,which further improves the performance of CS estimation algorithm.Simulation and experimental results in pool show that the proposed method outperforms the conventional least-squares channel estimator based on linear interpolation,CS channel estimator without pilot optimization and CS channel estimator merely with pilots' pattern optimization.Secondly,a sliding iteration based asynchronous multiuser reception approach is proposed for underwater multiple-input-multiple-output(MIMO)OFDM system.Data detection algorithms of synchronous multiuser MIMO system are studied first,including zero-forcing(ZF),minimum mean square error(MMSE)and ordered successive interference cancellation(OSIC).The sliding iteration based asynchronous multiuser reception approach consists of overlapped truncation,interference cancellation,frequency-domain oversampling,multiuser channel estimation and data decoding,and interference reconstruction,which works in a sliding iteration fashion to eliminate the interference from the preceding and succeeding processing units.Simulation and experimental results in pool demonstrate the effectiveness of the proposed method.Thirdly,a combined non-uniform Doppler scale estimation scheme is proposed for underwater point-to-point communication or downlink multiuser communication.Two identical short OFDM symbols preceded by a cyclic prefix(CP)are transmitted as a preamble prior to the data transmission for initial Doppler scale estimation.Then fine synchronization and Doppler scale estimation can be jointly obtained based on the CP of each CP-OFDM block.Simulation results and experimental results from the lake test and the sea test show that the proposed scheme is able to track the Doppler scale of each CP-OFDM block.Bit-error-rate performance can be improved by applying time-domain resampling methodology for Doppler compensation.Finally,an OFDM modulated superposition coding scheme for downlink underwater acoustic communications,where the transmitter splits the power between two users based on statistical channel state information.The expressions to characterize the boundary of the ergodic rate region achievable by the proposed scheme over long codewords are presented first,followed by the analysis of outage probability when coding is applied within one OFDM block.Then we examine the performance of superposition coding in an OFDM modulated system with practical coding and modulation pairs.Simulations results show that the OFDM modulated superposition coding scheme outperforms OFDMA in performance of both block-error-rate(BLER)and spectral efficiencies under different data rate pairs when both users with disparate channels have nonzero data rates.Recorded data from both medium-range and short-range sea tests verify that channel statistics are stable over a long period of time and can be used to assist resource allocation for the proposed scheme.Compared with OFDMA,considerable increase of spectral efficiencies has been found for superposition coding based on experimental data when both users with disparate channels have nonzero data rates.