Two-user NOMA based Indoor Optical Wireless Communication with Variable Cell Coverage using Beam Divergence Control

We experimentally demonstrate a two-user non-orthogonal multiple access (NOMA) based indoor optical wireless communication (OWC) system with variable cell coverage using a programmable liquid lens for beam divergence control. This is a novel approach to two-user NOMA-OWC to handle mobile users by varying the cell coverage area and the RF power allocation ratio to ensure uniform data transmission. The beam divergence control is characterized using optical power measurements and eye-diagrams from received on-off keying (OOK) modulated waveforms across different coverage area using variable liquid lens bias current. Communication performance is also characterized using NOMA encoded 4-QAM orthogonal frequency division multiplexing (OFDM) waveforms for five different cell coverage areas and decoding the received NOMA waveforms for the two receivers acting as the far and near users located at the cell edge and at the center along the optical axis respectively for a fixed link length of 2 meters. Overall data throughput of similar to 240 Mbps is maintained for the different cell coverage areas through appropriate selection of the RF modulation power allocation ratio for NOMA waveform construction.