The request for higher quality Internet services and the increase of data traffic has recently led the communication world to face up the problem of bandwidth scarcity due to the RF spectrum saturation. In this regard, optical wireless communication and one of its paradigm, namely Li-Fi (Light Fidelity), has emerged as innovative technology to support wireless connectivity in both indoor scenarios and other harsh environments such as the underwater case. Furthermore, Li-Fi represents a green solution to be employed in the so-called smart environments relying on the 5G technology and beyond. For these reasons, in 2009 the IEEE 802.15.7 Visible Light Communication Task was formed, with the first standard for VLC PHY and MAC released in 2011 and further developments currently ongoing.
The great perspective of Li-Fi is significantly due to the spread of light emitting diodes (LEDs) as incoherent solid state lighting sources. From a communication point of view, by conveniently modulating the optical signal of a LED, high data rates can be achieved.
The ease of integration of optoelectronic hardware has pushed the researchers to the investigation of massive multiple-input multiple-output (M-MIMO) architectures, originally developed for 5G cellular networks, with the goal to achieve higher communication performance for a large number of simultaneous users.
This project focuses on Li-Fi technology for the implementation of optical Massive MIMO systems for:
- Indoor optical communication
- Underwater optical communication
By referring to such challenging scenarios, the main issues to be addressed will concern the physical layer, including the investigation of efficient modulation and equalization schemes, and data link layer, especially regarding the problem of multiple access.
The goal of the project is to provide advances with respect to the state of art by means of both theoretical and practical investigation of the problems disclosed above.
Li-Fi for indoor communications
The initial step of the proposed study concerns the investigation of the MIMO propagation channel characteristics, with specific focus on the effects of channels spatial correlation on cross-talk interference. As channels spatial correlation represents a significant impairment to the achievement of good communication performance, having the channel state information as reliable as possible is in general recognized as a necessary condition to resolve the crosstalk with equalization. However, to the best of our knowledge, the current literature does not provide any specific discussion about the convenience of using accurate CSI to perform equalization in MIMO VLC systems suffering from crosstalk due to high channels spatial correlation. In this direction, the study is meant to proceed with the investigatation of equalization and precoding techniques that, even in absence of perfect channel knowledge, allow the mitigation of cross-talk even in highly-correlated MIMO channel scenarios.
Li-Fi for underwater communications
From an overview of what the scientific literature proposes as solutions for underwater optical communications, it is possible to infer that there are no specific ideas developed for the underwater context, but only techniques borrowed from the RF communication field.
So, underwater systems performance appears to be far from being optimized.
Dealing specifically with transmission issues, such as for example modulation, the schemes and constellations are not tailored to exploit the properties of the transmission medium. For this reason, the goal to be achieved will be to identify, on the basis simulation and experiments about the channel behavior, the transmission architectures optimized for the scenario under investigation. So, wavelength, spatial and temporal correlation will be some of the aspects to consider in order to propose a transmission scheme that takes into account the peculiarities, strengths and weaknesses of the underwater propagation.
Particular attention will be paid to channels spatial correlation since representing the most challenging impairment to deal with when realizing massive MIMO systems.
Finally, it is worth emphasizing that the proposed project will consider not only a theoretical study of massive MIMO Li-Fi systems, but also experimental validation. This latter aspect is particularly important since it is recognized by the research community as fundamental to realize high quality studies. This becomes even more true in the context of underwater communications, where performing experiments and measurement campaigns is typically very difficult.