Indoor Wireless Infrared Link with a Holographic Multiple-Spot Diffuser

Abstract

The emergence of portable terminals is expected to accelerate the introduction of wireless Local Area Networks (LANs). Portable devices require low power consumption, small size and weight. The demand for inexpensive, high-speed links satisfying these requirements has motivated the research on infrared (IR) wireless communications. Indoor wireless optical links can be implemented in different configurations¹. High data rates IR systems with directed line-of-sight (LOS) configurations have been reported²⁴. Directed LOS, however, is not suitable for portable terminals, alignment is crucial and full mobility cannot be achieved. A proper choice for portable communications is a non-directed configuration non-LOS, often referred to as a diffuse configuration. Gfeller and Bapst⁵ have pioneered the wireless IR communications via diffuse radiation. This configuration has also been realized and investigated by other research groups⁶⁸. The system is tolerant to shadowing and no alignment requirements are imposed on the transceivers. The IR wireless transmission relies on the scattering properties of the surrounding surfaces, i.e., walls and ceiling, which act as distributed optical sources. In the near IR, most of the materials have a reflectivity of 40 to 90%. To achieve an approximately constant received signal power over the entire office space, the IR source radiation has to be distributed on the diffusely reflecting surfaces as homogeneous as possible. Light-emitting diodes produce diffuse radiation, however, a laser diode is required for a wider modulation bandwidth and to meet the power budget at high data rates. The receiver detects attenuated signals after multiple reflections from the ceiling and the walls.