  Future smart ocean applications require long distance and reliable
communications to connect underwater sensors/robots with remote surface base
stations. It is challenging to achieve such goal due to the harsh and dynamic
underwater acoustic channel. While Multiple-Input and Multiple-Output (MIMO)
technique can enhance reliability and transmission range, it is difficult to
place multiple acoustic transducers on one single underwater device due to the
large wavelength. Although the cooperative MIMO technique that let multiple
underwater devices form a virtual MIMO system could solve the issue, it was
impossible to synchronize the distributed underwater devices due to the
extremely large and dynamic propagation delay of acoustic waves. To this end,
this paper proposes 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 perfectly solved by using the MI
technique so that the distributed acoustic transducers can cooperatively form
narrow beams for long distance underwater communications. The synchronization
time and errors are significantly reduced since MI has negligible signal
propagation delays. To quantitatively analyze the improvement, the closed-form
expressions of the synchronization error, signal-to-noise ratio (SNR),
effective communication time, and throughput of the proposed system are
rigorously derived. The proposed hybrid system is implemented in a
software-defined testbed under the beamforming and space-time coding scheme.
Through both numerical analysis and real-world experiments, the paper shows
that the proposed hybrid cooperative MIMO mechanism achieves much lower bit
error rate and synchronization error than the conventional acoustic systems.
