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SUjet de thèse financé

27 Juillet 2021

Catégorie : Doctorant

Fully Funded PhD Offer :

Title : Optimization of the PHY and MAC Layers of LiFi Links in a Cell-Free Network

Research Unit: Laboratoire d’Ingénierie des Systèmes de Versailles (LISV) / University of Paris-Saclay (UVSQ).

Work location: Vélizy-Villacoublay, Paris Area (78140 France).

PhD enrollment: University of Paris-Saclay.

Application process: Please send your CV and motivation letter to the contacts listed below. Contacts: - Luc Chassagne, Professor and LISV Director, Thesis director – - Bastien Béchadergue, Associate Professor, Thesis co-supervisor – bastien.bé


Application process: Please send your CV and motivation letter to the contacts listed below. Contacts: - Luc Chassagne, Professor and LISV Director, Thesis director – - Bastien Béchadergue, Associate Professor, Thesis co-supervisor – bastien.bé

1. Context of the PhD Offer

This PhD offer is proposed by the Laboratoire d’Ingénierie des Systèmes de Versailles (LISV), attached to the University of Paris-Saclay (UVSQ). The LISV develops multidisciplinary theoretical and experimental research activities on topics ranging from robotics to mobility, including optical wireless communications (OWC). On this last theme, the laboratory has been developing cutting-edge expertise for nearly 15 years, particularly for vehicular communication applications [Béchadergue17] or for indoor network access, a use case also known as LiFi. For example, the company OLEDCOMM, which is now one of the world leaders in LiFi, was born out of LISV and still has close ties with the laboratory. From a more scientific point of view, LISV has recently carried out work on high-speed LiFi links (around 300 Mbps) with a standard white light source [Merah19]. This work demonstrated the transmission capabilities of a LiFi cell with a reception area of about 4 m² to a maximum of 20 users. It also demonstrated that thanks to modulations and access protocols less usual than those developed for RF waves (m-CAP modulation and sub-carrier multiplexing access, for example), the properties of the light carrier could allow a more agile allocation that corresponds as closely as possible to user demand, and thus allow a first level of optimisation of the quality of service (QoS). However, the LISV would now like to take this optimisation to a higher level by exploring a new LiFi network topology. To this end, the laboratory is receiving funding from the French National Research Agency through the "Steerable Light Antenna for Enhanced QoS Cell-Free LiFi Networks" (SAFELiFi) project, which includes two fully funded PhD thesis.

2. The SAFELiFi Project: Problems Addressed, Solution Proposed and Objectives

Most of the current LiFi products and academic demonstrators operate on their own and are not interoperable with radiofrequency (RF) technologies such as WiFi, 4G/5G etc. More importantly, they often rely on the concept of cells, generated by multiple access points (AP) installed at regular intervals on the ceiling to ensure complete coverage of the room and thus continuous connectivity for the user. Such a topology, represented on the left side of Figure 1, then requires conventional interference management and handover mechanisms to ensure that all user equipment (UE) is continuously connected to the network. Although functional, this cell network topology has several limitations. In particular, it is preferable to have cells large enough to avoid too frequent horizontal handovers, which would reduce the time dedicated to traffic transmission in favour of signalling transmissions. At the same time, it is difficult to provide coverage of more than a few square meters with a conventional light source. The signal-to-noise ratio (SNR), and therefore the QoS, degrades rapidly as the receiver moves away from the transmitter's transmission axis, and is therefore generally relatively low at the edge of the cell. In addition, each change of cell results in a short loss of connection, which contributes to further degradation of QoS.

To overcome these problems and ensure an optimal QoS at all times, we propose to develop the cell-free topology shown on the right hand side of Figure 1. In this topology, each AP would be composed of a steerable transmit/receive light antenna capable of tracking one or more UEs simultaneously (e.g. AP2) and providing them with a narrow and focused light beam. Such a light beam would thus ensure a high SNR link that would be hard to eavesdrop for an attacker who would not be in the line of sight and would thus be more secure than in the usual cell-based topology. A UE could in turn be served by several APs simultaneously (e.g. AP1 and AP2 for UE1), thus making its connection more robust to obstacles. Each AP could also provide several parallel links through wavelength division multiplexing (WDM) to support multiple-input multiple output (MIMO) modes of operation and thus provide an even greater QoS. The resulting LiFi network could operate alongside various RF networks (WLAN, 5G femtocells etc.), which would be coordinated by a network controller. Therefore, the proposed network topology would be in line with the overall 6G requirements, as it would be very flexible to adapt to the dynamic user environment and yet provide high QoS and secure service while ensuring sensing at the same time.

The cell-free topology for indoor networking just presented needs several technological locks to be lifted in order to be demonstrated. The main objectives of the SAFELiFi project are thus: - Objective 1 – Ensure UE tracking by the APs, and conversely: o Objective 1.1: Ensure accurate location of UEs/APs, so that it is possible to determine where the APs/UEs light beams should be directed. o Objective 1.2: Develop steerable light antennas able to direct their different optical beams in controllable directions. - Objective 2 – Optimize QoS at UE level: o Objective 2.1: Optimize the throughput, latency and security of each AP/UE link in real time to ensure optimal QoS for users. o Objective 2.2: Optimize the coordination of LiFi APs with each other (horizontal coordination) and with RF APs (vertical coordination) to optimize traffic management. - Objective 3 – Demonstrators development: Project SAFELiFi aims to fulfil the previous objectives with an additional constraint of controlled complexity in order to achieve demonstrators allowing technology transfers to market products within a few years.

3. Thesis Offer and Profile Required

The PhD offer presented here is one of the two fully funded offers included in the SAFELiFi project. It consists in achieving the Objective 2 presented in the previous section, i.e. to design and experimentally validate PHY and MAC layers including multiple access and handover (horizontal and vertical) schemes that optimize the user QoS. To do this, it will be necessary to develop the following core bricks: - Firstly, to develop an optimized PHY layer, and especially its modulation scheme, using for example the basis provided by standardized PHYs such as IEEE 802.11bb or ITU-T G.9991 [ITU19]. The proposed modulation scheme should also support MIMO modes of operation, for example WDM [Bian19] or link aggregation from different APs etc., which will also require the development of load balancing schemes. - Secondly, to develop an optimized MAC layer and especially its multiple access [Shahjalal19], [Hasan20] and handover schemes. Techniques such as rate splitting multiple access (RSMA) could be explored [Naser20], whereas horizontal as well as vertical handover with LiFi are wide subjects that are just starting to be explored by the research community.

Thus, a candidate interested in this subject should hold a Master of Science in one of the following fields, ideally with demonstrated skills in all of them: - Telecommunications and networks, especially with knowledge in modulation, multiple access and handover schemes, - Programming, for the implementation of the proposed scheme in hardware platforms such as universal software radio peripheral (USRP). Simulation skills with MATLAB and ns3 (or equivalent) will also be a plus. In addition, strong communication and interpersonal skills, including collaborative skills, initiative, autonomy, result oriented and capacity to work in an interdisciplinary environment will also be appreciated. Good level of English is a must, knowledge of French is an asset but not a requirement.