Christopher Merlhe will defend his doctoral thesis, titled Resource Allocation in a Cell-less Context for 5G Wireless Networks, on Monday, September 16, 2024, at 10:00 AM at IMT Atlantique on the Rennes campus (Petit amphi).

The defense of PhD thesis of Tania Alhajj will be held on May 17th, 2023 at 10:00 in the Rennes campus of IMT Atlantique (Petit amphi). It will be accessible with a conf call. The thesis is entitled “Impact of centralized-radio access network architecture on 5G performances”. It was partially funded by Région Bretagne.

Fifth Generation (5G) mobile networks are paving the way for a new Radio Access Network (RAN) architecture. This is the Centralized-RAN (C-RAN) which groups some of the Base Station (BS) functions in a Central Unit (CU) connected to Radio Units (RU) distributed in different sites where the other BS functions are implemented. This thesis studies how to take advantage of C-RAN to combine high reliability and low latency on the one hand and to minimize the power consumption of the BS on the other hand. Both single and multi-RU transmissions, as well as a mixture of both, are evaluated. We compare a single-RU approach where the Hybrid Automatic Repeat reQuest (HARQ) mechanism, which combines all the ARQ re-transmissions, is located in the RU to a multi-RU transmission with an HARQ centralized in the CU. We show that multi-RU transmissions provide high reliability and low latency due to spatial diversity, even though centralization increases the round trip time. We then consider radio resource allocation. We evaluate the energy consumed by the BSs using a consumption model that integrates both the transmission power and the energy consumed for processing. At low load, multi-RU transmissions in a C-RAN, where all RUs serve one user, save BS energy consumption, without coverage degradation, compared to serving a user with a single RU. We pose and solve an optimization problem to minimize the energy consumption and increase the capacity to moderate load by reusing radio resources between RUs.

The defense of PhD thesis of Juan Carlos Vargas Rubio will be held on March 23th, 2023 at 10:00 in the Rennes campus of IMT Atlantique (Petit amphi). It will be accessible with a conf call. The thesis is entitled “Unicast versus Broadcast in Cellular Networksn 5G Radio Access Networks”. It was made in the framework of a CIFRE contract within Enensys.

Data traffic on mobile networks increases every year, especially video content. However, spectrum is scarce and expensive and operators need to optimize its use. In scenarios where the same content is transmitted at the same time to many devices in the same geographical area, the preferred solution to reduce bandwidth consumption is broadcast transmission.
Unicast transmission benefits from link adaptation techniques. However, the same content is transmitted as many times as the number of users demanding the same service. Conversely, a single broadcast transmission can cover a large number of users. Nevertheless, the bitrate in broadcast is fixed considering the users with the worst channel quality. Multicast-Broadcast Single-Frequency-Network (MBSFN) is a broadcast technique in which a group of synchronized cells transmit the same waveform. On the other hand, with Single-Cell Point-To-Multipoint (SC-PTM) each cell performs broadcast transmission independently. The problem is to determine when is it better to use unicast, MBSFN or SC-PTM.
In our work, we compare the performance of unicast, MBSFN and SC-PTM through system level simulations and analytical models. We consider base stations located according to Poisson distributions, the use of beamforming in unicast and different broadcast configurations. Furthermore, we propose an analytical method to calculate the number of users demanding the same content from which MBSFN or SC-PTM become more efficient than unicast. We prove that a switching mechanism based on this user threshold reduces bandwidth utilization and energy consumption. This method is based on stochastic geometry results for wireless networks.

Bruno Stevant defended his thesis on May 23, 2022 at 10:00 am in room Petri at IRISA. The title of the thesis is “Dynamic orchestration of microservices according to usage conditions.” The supervision of this thesis was provided by Jean-Louis Pazat, professor at INSA Rennes, and Alberto Blanc.

Summary

A participatory infrastructure aims to provide a hosting solution for services for a virtual community of users. Such a community has needs that are not fully met by data center hosting solutions. We therefore propose a new approach where community members host these services on their home devices. Such a participatory infrastructure raises issues related to the heterogeneity of the participating devices and networks, as well as to the variations in their computing and communication capabilities.

Through the study of existing solutions, the software architecture based on microservices has aroused our interest. It allows a more flexible placement of applications on the participating devices. The first part of our work consisted in finding a placement of the microservices on these devices that optimizes the response time of the applications. After defining a model of this response time, we used the PSO heuristic to find a solution close to the optimal one.

We tested this placement in real use conditions to evaluate the influence of network QoS variations on the application response time. This study showed that, under certain conditions, an adaptation of the placement could improve the measured performance. We have therefore added to our participatory infrastructure a mechanism capable of deciding whether an adaptation is necessary based on the measured response times, and to compute a placement adapted to the new conditions.

Amanvon Ferdinand Atta defended his thesis on December9, 2021 at 8:30 am (GMT) in B3 room at Cocody Danga Campus of Institut National Polytechnique Félix Houphouët Boigny in Côte d’Ivoire. The title of the thesis is “Reconfiguration of Multicast Routing in Sparse Wavelength Converter Networks”. The direction of this thesis has been assured by Prof. Bernard Cousin and Prof. Souleymane Oumtanaga. The jury members were Michel Babri, Miklos Molnar, Eyouléki T. G. Palanga, Mouhamadou Dosso, Bernard Cousin, Souleymane Oumtanaga.

Abstract

Optical networks using Wavelength Division Multiplexing (WDM) and wavelength conversion technologies are an efficient solution for very high-speed core networks. They make it possible to cope with the very high bandwidth and short delay requirements of applications such as telemedicine, distance learning, distributed computing, and online games. However, to reduce the costs of network dimensioning, it is quite practical that only some nodes of the network are equipped with wavelength conversion capability. Such a network is called a sparse wavelength converter network.

In the WDM networks, operators establish optical connections for their customers who need to use unicast or multicast applications. It is common for a customer, given the critical nature of its business, to require from the operator little or no interruption in the delivery of the optical flow it generates. In addition, following the occurrence of events such as an increase in connection requests, the network operator could be required to reconfigure the routing in order to improve network performance. However, the requirement for continuity of flow is not always easy to meet during the reconfiguration process if it is not done carefully. Reconfiguration becomes even more challenging for multicast routing in a WDM optical network with the property of partial wavelength conversion. Existing works address the reconfiguration of unicast or multicast connection routing in networks having the property of full wavelength conversion.

In this thesis, we deal on the one hand with the problem of reconfiguring the routing of a single multicast connection in a sparse wavelength converter network and on the other hand with the problem of reconfiguring the routing of a set of several multicast connections. In the first case, one aims to migrate the flow from an initial multicast routing to a final multicast routing without flow interruption. In the second case, we aim to migrate the optical flow from the initial multicast routing to the final multicast routing with a low number of flow interruptions. Note that the initial multicast routing and the final multicast routing are each supposed to be a set of light-trees.

The method denoted Para_SbSRA (Parallel Subtree by Subtree Reconfiguration Algorithm) has been proposed to solve the problem of routing reconfiguration relating to a single multicast connection and the methods LSRA (Light-tree Set Reconfiguration Algorithm) and LSRA_2 have been proposed to solve the problem of routing reconfiguration relating to a set of multicast connections. The effectiveness of these methods has been confirmed through simulations.

Keywords : Reconfiguration, flow interruption, multicast routing, WDM optical network, light-tree, wavelength conversion.