Cascaded Lagrangian Power Allocation to Optimize D-MIMO OWC Systems

Light mainly propagates along a line-of-sight without the rich multipath scattering that can be exploited in radio frequency (RF) multiple-input multiple-output (MIMO) systems. To ensure outage-free communication MIMO can nonetheless be used, provided that multiple ceiling nodes are spatially distributed. Angular-selective client nodes can to some extent, but not perfectly, separate spatial streams coming from different ceiling points. In addition, many optical wireless communication (OWC) links have a low-pass frequency response. Because of these properties, indoor distributed (D)-MIMO OWC differs in many aspects from RF links. This paper explores how OWC D-MIMO systems can best be operated. The optimization of how to spread power over spatial streams and frequencies comes to different settings than typically seen for radio. We evaluate algorithms by an analytical foundation and by simulation based on a commonly accepted channel model. We derive a simple criterion to select the number of spatial streams which can be used. This also can serve as a selector for switching between spatial diversity and spatial multiplexing transmission modes. Moreover, we derive a simple formula for how to best spread power among spatial streams. Besides theoretically optimum solutions, we propose a very-near-optimum algorithm that is highly efficient for practical systems in rapidly varying channels.