Proceedings Article | 9 March 2020
Zizheng Cao, Xuebing Zhang, Gerwin Osnabrugge, Juhao Li, Ivo Vellekoop, A. Koonen
KEYWORDS: Telecommunications, Wireless communications, Optical communications, Receivers, Transmitters, Non-line-of-sight propagation, Light scattering, Diffusion, Reflection, Mirrors
To meet the ever-growing demand for faster wireless communications, optical wireless communication (OWC) has been extensively developed, which can bring a huge improvement in communication capabilities, both in terms of ultra-high capacity per user and in terms of electromagnetic interference-free wireless communication [1-2]. However, one fundamental challenge for OWC arises when the direct pathway between transmitter and receiver is obstructed by an obstacle. When an optical beam is illuminated on a rough surface, the light is scattered to different directions, which results in the near-isotropic, speckled optical intensity distribution of the diffused light. Therefore, the intensity of the diffused light is inherently much lower than that of a collimated incident light arriving directly at the receiver. Usually, the proposed solutions do not address the diffusion mechanism itself but instead of focusing on the compensation of the diffusion-caused loss by increasing the system power, or they avoid diffuse reflection and/or scattering altogether, i.e., using a near-perfect mirror as a reflector. As a long-standing challenge, such diffuse loss critically hinders the application of OWC. Here, a novel non-line-of-sight (NLOS) beam reconfigurable optical wireless data transmission system for energy-efficient communication is proposed and experimentally verified. This is an overview of our previous work which is published on Light: Science&Applications [3]. By spatially modulating the light incident on a rough surface using a spatial light modulator (HOLOEYE PLUTO Phase Only SLM), the diffused light is focused on an optical wireless receiver, which breaks the NLOS limitation of OWC. A record-breaking 30-Gbit/s orthogonal frequency division multiplexing (OFDM) signal is transmitted over a diffused 110-mm optical wireless link with >17-dB gain, in an angular range of 20°. In this experiment, the 1550-nm laser source is used to match the well-established fibre-system. The OFDM signal is modulated onto the optical domain and amplified to the eye-safety power limit of 10 dBm. Then the light is collimated and delivered to an SLM. The angle between the incident beam and the reflected beam is 45°. To match the Gaussian beam (size), the central 1024-by-1024 pixels are activated, which are further grouped into segments of 64×64 pixels, yielding a total of 16×16 segments for data transmission. All pixels in a segment can be phase modulated from 0 to 2π in increments of π/8 individually. The phase-compensated beam is illuminated onto a rough barrier, which emulates the rough surface of ceilings or walls in an indoor scenario. Here, a Thorlabs polystyrene screen (EDU-VS1/M) and a sandblasted aluminium film are verified. To collect the diffused light to realize a large-capacity transmission, the light is coupled into a single-mode fibre using a collimator. Enabled by the feedback signal from a power meter, the received optical signal can be optimized by using the wavefront shaping [4]. The data-rate of 30 Gbit/s with blocked sightlines is achieved using the stepwise sequential algorithm [4].
