Achievable rate of LED-based distributed MIMO OWC systems under a per-LED power constraint

Multiple-input multiple-output (MIMO) systems have been widely studied for improving throughput and robustness of optical wireless communication (OWC) systems. Indeed, with MIMO, the likelihood of simultaneous interruption of all optical beams is greatly reduced. For throughput improvement, many power loading strategies are obtained for a total power constraint while allowing power to be freely exchanged among light-emitting diodes (LEDs). In scenarios with unequal distances between LEDs and the user terminal, more power is allocated to the LED that has a better channel than to others. In practice, every optical front-end has its own power limitation and a per-LED power constraint is more meaningful than a total power constraint. This paper studies to what extent this has an impact on the performance of LED-based distributed MIMO (D-MIMO) OWC links. We formulate the achievable rate optimization problem for both cases, when the total power is constrained or when under a per-LED power constraint. Results show that, the achievable rate under a per-LED power constraint is lower than under a total power constraint, but it is higher than if the power is uniformly spread over the entire bandwidth. It also allows for transmit power saving, at a cost of higher computational complexity. In addition, under a per-LED power constraint, LEDs with better channel condition tends to use a modulation bandwidth far above the 3-dB bandwidth.