Section: New Results

Wireless Networks

Participants : Osama Arouk, Btissam Er-Rahmadi, Adlen Ksentini, Meriem Bouzouita, Pantelis Frangoudis, Yassine Hadjadj-Aoul, Gerardo Rubino.

We are continuing our activities around wireless and mobile networks, by focusing more on leveraging the current mobile and wireless architecture toward building the 5G systems.

LTE improvements. One of the 5G objectives is to support a high number of devices. This not only concerns User Equipment (UE) devices, but also other devices such as sensors and actuators (known also as Internet of Things (IoT)). Sensor and actuator devices communicate generally with a remote server in an automatic way, without any human intervention. This type of communication is known as Machine to Machine (M2M) communication, or Machine Type Communication (MTC). The corresponding traffic is known by its intensity and impact on increasing congestion in both main parts of 4G networks, the Radio Access Network (RAN) and the Core Network. To improve the current LTE system to support MTC, we did several contributions. We proposed in [51] an important enhancement to the Group Paging (GP) mechanism, which is responsible for relaying requests to sensors, in order to gather data. After modeling analytically the GP procedure, we proposed a mechanism that, instead of paging all MTC devices in the same period, calculates the appropriate number of MTCs that reduces the collision probability as well as increases the success probability. In [52] , we modeled the Radio Access Channel (RACH) procedure when the MTC devices are activated in a highly synchronized manner during a certain period (synchronized traffic), which is represented by a Beta distribution. The proposed model estimates for each period the exact number of MTC devices that may win the contention.

To control the Random Access Network (RAN) overload and alleviate the access network congestion, 3GPP developed the Access Class Barring (ACB) procedure that depends on an access probability called the ACB factor, without proposing a procedure for calculating such probability. In [72] , we have proposed a fluid-based random access model for M2M communications, which was used to determine dynamically the value of the ACB factor that avoids system overload and the radio resources' underutilization at the same time. We proposed in [60] a novel implementation of the ACB mechanism in the context of multiple M2M traffic classes. Based on a scheduling algorithm, we have applied a PID controller to adjust dynamically multiple ACB factors related to each class category, guaranteeing a number of devices around an optimal value that maximizes the Random Access (RA) success probability. In [61] , we first present a simple fluid model of MTC devices' random access. This model is then used to derive a novel adaptive regulator of the ACB factor, somehow in contrast with previous existing contributions which generally rely on heuristics. The main advantages of the proposed approach are twofold. First, the proposal is fully compliant with the standard while it reduces significantly the computation and the signaling overheads. Second, it provides an efficient mean to regulate adaptively the ACB factor as it guarantees having an optimal number of MTC devices accessing concurrently to the RAN. The obtained results based on simulations show clearly the robustness of the proposed approach, and its superiority compared to existing proposals.

Another important objective of 5G mobile networks is to accommodate a diverse and ever-increasing number of user equipments (UEs). Coping with the massive signaling overhead expected from UEs is an important hurdle to tackle so as to achieve this objective. In [38] , we devised an efficient tracking area list management framework that aims for finding optimal distributions of tracking areas (TAs) in the form of TA lists (TALs) and assigning them to UEs. The objective is to minimize two conflicting metrics: paging overhead and tracking area update (TAU) overhead. We used bargaining games to find the Pareto optimal solution that satisfies both objectives.

WiFi networks improvements. It is well established that WiFi is complementing LTE connections to ensure, wirelessly, high data rate. One idea to improve WiFi towards high data rates is to multiple users' transmissions on both directions, i.e. on the Down Link (DL) and the Up Link (UL). In [50] we devised a novel solution to enhance the TXOP Sharing mechanism, introduced in the 802.11ac amendment, to achieve efficient Down-Link Multi-User Multiple-Input Multiple-Output (DL-MU-MIMO) transmission. First, we give new definitions about both events of successful and failed DL-MU-MIMO transmission. Then, we devise a revised Backoff procedure for the primary Access Category (AC). In [40] we proposed a novel 802.11ax MAC protocol aiming at reducing the elapsed time in managing the establishment of an UL-MU communication, thus enhancing considerably the system's performance.

On the other hand, the volume of mobile multimedia traffic is fast-growing, challenging the radio and backhaul network infrastructure and calling for alternative content dissemination schemes. To improve user experience and reduce infrastructure load, we exploit implicit social relationships among users and take into account content popularity, proposing push-based prefetching mechanisms which take advantage of the caching and mobile ad hoc networking capabilities of user devices. We use, in [65] , bloom filters as summaries of user caches, and design mechanisms to estimate the social distance between users and the popularity of content items, which drive our algorithms. Our simulation-based evaluation shows that our scheme brings caching performance improvements in an order of 10% in terms of absolute cache hit ratio in most of the cases studied, and from 3% to 82% in terms of normalized cache hit ratio gain.

Network selection. With the explosion of mobile data traffic, the Fixed and Mobile Converged (FMC) network are being heavily required. Mobile devices have the capability of connecting to different access networks in the FMC architecture simultaneously. Access network selection becomes an issue when mobile devices are under coverage of different access networks, since a bad selection may lead to network congestion and degrade the QoE of users. In order to address this problem, we model and analyze, in [62] and [63] , the interface selection procedure using control theory in the FMC architecture. Based on our model, we designed a controller which can send to mobile devices a network selection command calculated instantly for the access network selection. In [29] , we investigated network decentralization in conjunction with the selective IP traffic offload approaches to handle the increased data traffic. We first devised different approaches based on a per-destination-domain-name basis, which offer operators a fine-grained control to determine whether a new IP connection should be offloaded or accommodated via the core network.

Energy efficiency. Due to the ever-growing gap between battery lifetime and hardware/software complexity in addition to application's computing power needs, the energy saving issue becomes crucial. In this context, we proposed, in [13] , an end-to-end study of video decoding on different architectures. The study was achieved thanks to a two steps methodology: (1) a comprehensive characterization and evaluation of the performance and the energy consumption of video decoding, (2) an accurate high level energy model based on the characterization step. In [24] , we proposed to apply data fragmentation, in slotted CSMA/CA, in a way to allow improving the bandwidth occupation while reducing the latency. We proposed to introduce a network allocation vector (NAV) in the fragmentation mechanism to reduce energy consumption in IEEE 802.15.4. A Markov chain-based analytical model of the fragmentation mechanism was given as well as an analytical model of the energy consumption using a NAV. The analytical results show that the fragmentation technique improves at the same time the throughput, the access delay and the bandwidth occupation. They also show that the NAV mechanism reduces energy consumption when applying the fragmentation technique in slotted CSMA-CA for IEEE 802.15.4.