Underwater acoustic communication (UWA) is a reliable way for data transmission in underwater environment, however, it is quite challenging due to its doubly spread characteristics as delay spread and Doppler spread. In this paper, a discrete-time channel input-output relationship for multipleinput-multiple-output (MIMO) single-carrier block transmissions is derived based on parameterization for amplitude and delay variations with polynomial fitting, where the mathematical model is characterized by a few parameter sets, transforming the estimation problem into estimation of the low-dimensional parameter sets. Then a two-stage sparse approach for delay-spread and Doppler-scale estimation of MIMO UWA channel is proposed, using orthogonal matching pursuit (OMP) algorithm to search on the delay-Doppler scale grid. The results of the proposed approach are further evaluated by simulations.
In dispersive underwater channels where impulse responses commonly last tens of milliseconds, large symbol durations and guard intervals are needed for orthogonal frequency-division multiplexing (OFDM) acoustic communications, which could introduce severe inter-carrier interferences and reduces effective data rate. This paper presents a scheme of OFDM transmission combined with passive time reversal processing, which has been demonstrated as a promising self-adaptive technique to compensate for multipath distortion explicitly due to its spatial focusing and temporal compressing characteristics. Using time reversal as a preprocessing step prior to OFDM, the equivalent channel impulse response is greatly shortened; moderate symbol durations and guard intervals can thus be used the same way as in conventional OFDM schemes. Moreover, to improve the robustness in harsh-environment applications, Reed Solomon channel coding is exploited for its good performance against burst errors caused by channel fading and ambient burst noise. Tradeoffs between data rate and robustness are discussed along with the transmission scheme. Finally some field experimental results are presented, which demonstrate the effectiveness of the developed approach. [Work supported by Chinese 863 high-tech program under Grant 2009AA093601]
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