Optimum arrangement of LEDs in base station of optical wireless LANs

Abstract

A commercial optical wireless LAN system has a base station attached to a ceiling, and a number of LEDs therein broadcast optical beams to movable stations below. The movable stations always communicate with the base station when they are in a service area. The LEDs are arranged so that their combined optical beams effectively irradiate the service area. However, studies concerning such designs have not been reported and thus LEDs in base stations have been arranged empirically. The minimum optical intensity in the service area is a key parameter in designing systems because it determines the optical power margin of the system and should be increased as much as possible. We examined the optimum arrangement of LEDs with which the minimum optical intensity in the service area is maximized, assuming the total number and power of LEDs are fixed. Referring to the commercial systems, we assumed LEDs are aligned on coaxial circles in a rotationally symmetric scheme and thus a disk-shaped service area is implemented. We assumed LEDs have the same beam profile, but each group aligned on a different circle has its own number of elements and inclination angle with respect to the vertical axis. We compared numerical results with our experimental results. This study will contribute to designing the base stations of optical wireless LANs.