Research at the Li-Fi Centre, University of Edinburgh
In typical large indoor environment such as a large office or conference room, multiple light emitting diode (LED)
lighting fixtures meet the need for lighting. This brings an opportunity of embedding a networked VLC system into an existing lighting network. A potential VLC network system functions as a conventional cellular system such that each lighting fixture works as a base station or an access point (AP) serving the nearby wireless users. The area in the room is divided into multiple cells. Because of its extremely small cell size, we name this small VLC cellular network as an optical attocell network [1].
A single VLC AP offers limited coverage area, while multiple APs can improve the coverage capability in large indoor environment. In addition, communication resources can be reused by different APs. In this way, the spectral efficiency of the VLC can be improved.
An optical attocell network has the potential to achieve a system with robust wireless connection. One of the critical problems in a VLC system is that shadowing and blockage of the transmission paths may lead to link disconnection. However, in an optical attocell system, multiple line-of-sight transmission paths can be established between one user and multiple nearby APs. If one of the paths is blocked, an alternative transmission path can be used instead. In addition, multipath fading doesn’t exist in VLC systems with intensity modulation and direct detection (IM and DD).
The experiments undertaken in the Li-Fi Research and Development Centre have shown that robust connection can be achieved with reflection paths. Therefore, optical signals bounced by walls, human bodies and other objects can also be used to improve the connection reliability. In a fully illuminated room, the chance not being able to read a magazine due to lack of light reaching the pages is very unlikely. Similarly, the outage probability of a wireless user should also be low.
An optical attocell network has the potential to be a flexible system. Depending on the load condition, the system can work in different modes. For example, if the number of users is large in the network, the system can function in a cellular mode, which allows each AP to serve multiple users. In the case that there are only one or two users in the network, multiple APs can serve the users jointly. In this condition, MIMO techniques can be used to either boost the user data rate or improve the energy efficiency.
[1] H. Haas, “High-speed wireless networking using visible light,” SPIE Newsroom, Apr.19 2013.
Cheng Chen
Li-Fi PhD Research Student