Doppler Compensated Front – End Receiver Design for Underwater Acoustic Channels Using Mimo-Ofdm

Over the past three decades underwater acoustic communications has grown in a spectaculous manner. Continued research over the years has resulted in improved performance and robustness as compared to the initial single-carrier communication systems. Multicarrier modulation in the form of orthogonal frequency division multiplexing (OFDM) has emerged as a promising modulation scheme for underwater acoustic (UWA) communications. Multiple-input multiple-output (MIMO) techniques have been actively pursued in UWA communications recently to increase the system performance over the bandwidth-limited channels. However, UWA channels are far more challenging than their radio counterparts. Due to their limited bandwidth, even though UWA channels are wideband in nature due to the small ratio of the carrier frequency to the signal bandwidth, which introduces Doppler shift that destroys the orthogonality among OFDM subcarriers. To mitigate the effect of Doppler shifts in OFDM transmissions over UWA channels, Doppler compensation is done through multiple resampling at front-end receivers. In this paper, we treat the channel as having a common Doppler scaling factor on all propagation paths, and propose linear minimum mean square error (LMMSE) estimation algorithm. Comparing with the conventional MMSE, this algorithm has the advantage of low complexity. The results suggest that MIMO-OFDM is an appealing solution for Doppler compensations for data transmissions over underwater acoustic channels.