Location-based services are becoming increasingly important in indoor environments with the development of Internet-of-thing (IoT). The visible light positioning (VLP) system offers great potential because of its immunity to radio frequency (RF)-induced electromagnetic interference, free and unrestricted spectrum, and a much higher level of security. 

Line-of-sight (LOS) VLP in previous studies have been demonstrated with high accuracy at very low cost. However, for LOS VLP blocking, and shadowing is a major problem, and there is the requirement for large numbers of light-emitting diodes (LEDs). Few methods to solve this problem have been investigated. 

In a study published in IEEE Internet of Things Journal, Dr. LIN Bangjiang’s group from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences proposed a non-line-of sight (NLOS) VLP system based on a luminance distribution model for image sensor, which can realize 3D positioning by the reflected lights from the floor using a single LED and an image sensor. 

The researchers first gave a block diagram of the proposed system which consists of two function modules: a NLOS optical camera communication (OCC) module and a position estimation module. The former uses the diffuse reflections to receive the coordinate information of the LED. And the later estimates the camera’s position by resolving the 2D-3D projection equations of the pixel coordinates and the world coordinates of the two virtual LEDs that can be regarded as the projections of the LED. 

They then proposed a physically based luminance distribution model (LDM) for the primary reflected light, which is capable of calculating the gray value of every pixel in a picture and estimating the channel gain of an NLOS link for an NLOS OCC system.  

The researchers proved that the two highlights in the captured image can be viewed as the projections formed by the two virtual LEDs through LOS paths via the LDM, one of which is the symmetry point of the LED about the floor, and the other is the projection of the LED on the floor. As a result of geometrical optics and microfacets theory, the LDM is considered as the first theoretical study in the field to demonstrate how reflective surfaces affect the imaging process. 

In addition, the researchers designed an experimental testbed with a scale of 200X200X196cm³, and chose a STM32 microcontroller unit (MCU) operating at the frequency of 3.3 kHz to driver a LED. At the receiver, they used a CMOS camera to capture the reflected lights from the ground at two different exposure modes (one long and one short). They can gain the LED position by the NLOS OCC module using the short exposure image, and get the pixel coordinates of the projection of the LED reflected by the ground in the long exposure image. By all this information, they calculated the error between the estimation value and the measured value of the camera’s position.  

This study overcomes the challenge of the shadowing/blocking of the LOS links, realizes indoor positioning with the minimum number of beacons (only single LED), and greatly improves the practicability of VLP.