Underwater cooperative MIMO communications using hybrid acoustic and magnetic induction technique

Future smart ocean applications require reliable communication networks connecting underwater sensors/robots and remote base stations, which is challenging to achieve due to the harsh and dynamic underwater channels. While Multiple-Input and Multiple-Output (MIMO) acoustic technique can address this problem, it is difficult to place multiple transducers on a single device due to the large wavelength. Although the cooperative MIMO technique could solve this issue, it was impossible to synchronize the distributed transmitters due to the extremely large and dynamic propagation delay of acoustic waves. In this paper, we propose an underwater cooperative MIMO communication mechanism, which is based on a hybrid acoustic and Magnetic Induction (MI) technique. The inter-node synchronization problem can be solved by using the MI technique so that the distributed transducers can cooperatively form narrow beams for long distance communications. The synchronization time and errors are significantly reduced since MI has negligible propagation delays. To quantitatively analyze the improvement, the closed-form of the synchronization error, signal-to-noise ratio (SNR), effective communication time, and throughput of the proposed system are rigorously derived. The proposed system is implemented in a software-defined testbed under the beamforming and space-time coding scheme. Through both numerical and experimental analysis, our hybrid cooperative MIMO mechanism can achieve much lower bit error rate and synchronization error than the conventional acoustic MIMO systems.