Multi-User Sunlight-Based Modulation Downlink System Using Dual-Cell Liquid Crystal Shutter

Sunlight is commonly used for indoor natural illumination in modern buildings, but it also holds potential as an information carrier in indoor optical wireless communication (OWC) systems. This paper introduces a multi-user, sunlightbased modulation downlink multiple-input single-output (MISO) system employing a recently developed dual-cell liquid crystal shutter (DLS) as the modulator, allowing building facades to simultaneously provide communication and illumination based on user needs. We first outline the system framework, detailing the transmitter and receiver structures. To assess channel performance, we derive an end-to-end transmission model by examining the physical mechanisms of the DLS. A preprocessing linearization method is applied to mitigate nonlinear behavior, enabling us to derive a theoretical lower bound for the system's channel capacity, facilitated by time division multiplexing (TDM). Additionally, we establish an illumination model that accounts for the contributions of active and inactive DLS cells. An optimization problem is formulated to maximize total channel capacity across multiple users, subject to minimum illumination requirements and boundary constraints. Simulation results show that the proposed system achieves high communication throughput by dynamically adjusting the ratio of active to inactive DLS cells. This study establishes a basis for high-speed indoor OWC systems powered by natural sunlight.