Augmenting WiFi Infrastructure with Visible Light Communications

This proposed project concerns Visible Light Communications (VLC), and in particular the combination of Polymer Optical Fibre (POF) and free space low power VLC to create inexpensive communication links to augment existing WiFi infrastructure. This project falls within the EPSRC Optical Communications research area.

There is a growing issue of high-density traffic saturation on WiFi. While an Access Point (AP) may
have a high capacity of 900 Mbit/s under normal operation, due to concurrency effects 18 connected
users will each experience a rate of 6 Mbit/s. This is an overall throughput far less than the capacity of the Radio Frequency (RF) channels available. This effect worsens with more connected
devices. As a solution, manufacturers are including more RF channels on their new APs, which will eventually become saturated, only pushing the problem further down the road. A longer term solution is required, so as an alternative to RF a VLC channel could be used to augment RF systems
and remove RF congestion. The suggested augmentation is using a VLC channel to serve a data link, allowing for ensured quality of service.

The Silicon Photomultiplier (SiPM) has been identified as the world's most sensitive optical reciever, and has been demonstrated to be an ideal candidate for VLC systems. This reciever can be
paired with low-power indicator LEDs to create inexpensive, practical links. The combination of indicator LEDs and SiPMs is something which hasn't been achieved before, and has the potential to
perform strongly. Further characterisation of SiPM recievers must be undertaken, as well as understanding the impacts of ambient light, output pulse width, equalisation methods and more on VLC links.

A suggestion for practical VLC systems is to use optical fibre to transport the optical channels throughout the serviced area from a central hub, rather than use distributed transmitters. POF has been identified as a suitible fibre, which offers mechanical benefits along with being inexpensive. Each POF fibre will carry a separate VLC channel dividing space so multiple users can be serived simultaneously without interference, other benefits which arise include increases in security, and decreases in average transmission rate. This optical fibre could inexpensively be placed in an office's suspended ceiling, and protruded through at a desired transmit location in a minimally invasive fashion. The separate optical channels would need to be distributed throughout the space covered by the WiFi, filling the space with (not necessarily separate) regions of VLC coverage. Although POF usually works in the red, it can work in the blue region and therefore emit light that can be detected by SiPMs. The 405nm wavelength is interesting as it is far less prevalent in the target home and office environments, and SiPMs are more sensitive to it. Exploration of the impacts of using POF to transport optical wireless links is also research which must be undertaken.

For down-links, I aim to exploit the high sensitivity, and large area of Silicon Photomultipliers, to detect a link from POF at a range of > 3m. It would be feasible to use another POF as an up-link from a client back to the AP. Coupling the up-link light emitted from a client back into a POF would
be a challenge here, as POF typically has a diameter of 1mm. The low power of the emitted light in
conjunction with the small area of the POF may require a solution involving a fluorophore coating on the end of a POF, or fluorophore-doped fibres connected to a POF to collect and couple more light. Since up-links are a lower rate by nature, it is crucial to explore what allows the up-link to be robust and achieve a high rate in order to guarantee quality of service.