A New Solution to Long Distance Communication
The concept of Li-Fi, first introduced to the general public by Professor Harald Haas, Chair of Mobile Communications at theUniversity of Edinburgh, in a TED Talk in 2011, is currently attracting a great deal of interest and many researches have stepped into the field by proposing new devices as potential transmitters and receivers for Li-Fi systems.
There has been significant progress towards the realisation of optical receivers fully integrated with the standard digital CMOS technology. Recent trends towards integrated CMOS high-speed optical receivers have specially employed avalanche photodiodes (APDs). However, the maximum achievable gain of an APD is limited due to low sensitivity and the gain-dependent excess noise. This necessitates the use of intricate high gain transimpedance amplifiers (TIAs), limiting amplifiers and adaptive equalisation.
These challenges can be tackled by operating the APD in Geiger-mode as a single photon avalanche diode (SPAD). Due to the high electric field, avalanche multiplication leads to a large internal gain, creating a density modulated pulse train and the additional noise source can then be avoided .
The high sensitivity and time resolution of SPADs have highlighted the potential of employing SPADs as photon counting receivers for Li-Fi systems. They can be used with the long term aim of power efficient, high sensitivity receivers and are particularly attractive because they are able to closely approach quantum-limited sensitivity in the detection of weak optical signals in long distance communications, such as in the gas extraction industry, or in downhole monitoring communication systems .
Despite advances, the maximum achieved data rate by a SPAD receiver is still limited to a few MHz . Currently at the Li-Fi Centre at the University of Edinburgh, we are conducting research on these state of the art receivers and trying to provide solutions to possible challenges by investigating alternative approaches.
Li-Fi Postgraduate Research Student Chitnis, D. and Collins, S. “A SPAD-Based Photon Detecting System for Optical Communications,” Journal of Lightwave Technology, vol. 32, no. 10, pp. 2028–2034, May 2014.  Li, Y., Videv, S., Abdallah, M. , Qaraqe, K., Uysal, M., Haas, H. “Single Photon Avalanche Diode (SPAD) VLC System and Application to Downhole Monitoring,” in IEEE Global Communications Conference (GLOBECOM), Austin, Texas, USA, Dec. 8–12 2014, pp. 2108–2113.  Fisher, E., I. Underwood, I., and Henderson, R. “A Reconfigurable Single- Photon-Counting Integrating Receiver for Optical Communications,” IEEE Journal of Solid-State Circuits, vol. 48, no. 7, pp. 1638–1650, Jul. 2013