Research at the Li-Fi Centre, University of Edinburgh

A compound eye consists of thousands of individual photo receptors
A compound eye consists of thousands of individual photo receptors

Have you ever seen the compound eye of insects? If so, you must be astonished by its sophistication. In general, a compound eye consists of thousands of individual photo receptor units which are located on a convex surface and thus pointing to a slightly different direction. When the input of all photoreceptor units are combined, the image of an object can be accurately perceived. Compared with a simple eye, acompound eye has a very large field of view angle, and can detect fast movement and even the polarization of light. Compound eye is the result of natural selection. People gain inspiration from the insect world and turn it into reality. One well-known implementation of compound eye is ‘theInsect-eye camera’’[1]. This type of camera offers an extremely wide angle which could help flying drones to see into every corner of a battlefield.

Is it possible to use the powerful ‘compound eye’ in Li-Fi system?

In a Li-Fi system, light signal is captured by an optical receiver with only one photodiode. In order to fulfil the coverage requirement, the field of view angle of the photodiode needs to be considerably wide. Butthere are two drawbacks:: firstly, it collects unwanted ambient light along with the desired light signal. Ambient light sources such as the sun, incandescent light bulb and other modulated LED source cause extra noise.  Secondly, the wide field of view of a photodiode collects both the light signal from the desired source and also the delayed signals which have undergone two or more reflections. These delayed signals result in multipath distortion which degrades the system performance. In order to address these issues, we replace the conventional single-photodiode receiver by an angle diversity receiver, and the inspiration for this was the compound eye.

An angle diversity receiver consists of multiple photodiodes which are located on a semi-sphere base. These photodiodes have a narrow field of view which, when combined, result in a large field of view angle and coverage area. Not only that, with these independent photodiodes, it is easy to implement smart signal processing schemes on an angle diversity receiver. When these narrow field of view photodiodes are properly selected and combined, an angle diversity receiver can significantly minimize the effect of ambient light, reflection light and even the interfering signals from other light sources.

Not only as an optical receiver, a ‘compound eye’ can also be implemented as an optical transmitter, and this is termed an angle diversity transmitter. Similar to an angle diversity receiver, an angle diversity transmitter consists of multiple LED elements. Each LED element can generate a narrow-beam light with a specific direction. Using an angle diversity transmitter, it is possible to achieve space division multiple access (SDMA) in a multi-user scenario, where an angle diversity transmitter can serve multiple users simultaneously within one time slot without significant mutual interference.

[1] Song, Y.M. et al. “Digital cameras with designs inspired by the arthropod eye.” Nature 497, 95–99 (2013).

Zhe Chen

Li-Fi PhD Research Student