Angular MIMO for Underwater Wireless Optical Communications: Link Modeling and Tracking

Angular imaging multiple-input--multiple-output (A-MIMO) is investigated for short-range, high-speed underwater wireless optical communications (UWOCs) where, unlike conventional imaging MIMO (C-MIMO), data are transmitted in an angle rather than in space. In this approach, the strict requirements of on-axis alignment and fixed channel length are relaxed. This technique also allows for simpler estimation of the relative misalignment between the transmitter and the receiver from the received image. For the first time, we derive a comprehensive model for the underwater A-MIMO link by taking into account link misalignment, background noise, as well as seawater absorption and scattering. Power distributions at the receiver are modeled by the angle of arrival of the received signal on the lens and its position of arrival on the focal plane of the detector. We further propose and model a tracked A-MIMO (TA-MIMO) system that maintains the alignment between the two ends of the link, for which the distribution of the residual tracking error is calculated. The UWOC channel capacity is then estimated for buoyed-to-fixed (B2F) (which has dominant angular misalignments) and mobile-to-fixed (M2F) (which has dominant off-axis misalignment) communication scenarios. Numerical results indicate that in the B2F scenario, A-MIMO is sensitive to angular misalignments; however, TA-MIMO outperforms C-MIMO. In the case of M2F links, A-MIMO greatly outperforms C-MIMO when off-axis misalignments are present. This work serves as a design guide to determine the selection of A-MIMO, TA-MIMO, or C-MIMO receivers depending on the misalignment conditions for a particular underwater application.