Mohamed Amine Arfaoui

This paper investigates the channel aging problem of mobile light-fidelity (LiFi) systems. In the LiFi physical layer, the majority of the optimization problems for mobile users are non-convex and require the use of dual decomposition or heuristics techniques. Such techniques are based on iterative algorithms, and often cause a high processing delay at the physical layer. Hence, the obtained solutions are rendered sub-optimal since the LiFi channels are evolving. In this paper, a proactive-optimization (PO) approach that can alleviate the LiFi channel aging problem is proposed. The core idea is to design a long-short-term-memory (LSTM) network that is capable of predicting posterior positions and orientations of mobile users, which can be then used to predict their channel coefficients. Consequently, the obtained channel coefficients can be exploited to derive near-optimal transmission-schemes prior to the intended service-time, which enables real-time service. Through various simulations, the performance of the designed LSTM model is evaluated in terms of prediction error and inference complexity, as well as its application in a practical LiFi optimization problem.

We consider a two-user uplink cooperative rate-splitting multiple access (C-RSMA) and seek to maximize the minimum signal-to-interference-plus-noise ratio (SINR) by jointly optimizing the beamforming at the base station (BS) and device transmit power. Due to the high coupling among the variables, the problem is non-convex and difficult to solve directly. To handle this challenge, we utilize successive convex approximation (SCA) and difference of convex (DC) based approach to approximate the non-convex problem to a convex one. The simulation results demonstrate that our proposed scheme can outperform cooperative non-orthogonal multiple access (C-NOMA) and other baseline schemes.

In this paper, we consider a wireless network consisting of a base station that is serving multiple real-time traffic streams forwarding information updates to their destinations in order to sustain the freshness of information for time-critical applications. Since the wireless channels may be unreliable due to the impurities of the propagation environments, such as deep fading, blockages, etc., we integrate a reconfigurable intelligent surface to the wireless system in order to mitigate the propagation-induced impairments, enhance the quality of the wireless links, and ensure that the required freshness of information is achieved for these real time applications. For this network set-up, we investigate the joint optimization of the traffic streams scheduling and the reconfigurable intelligent surface phase-shift matrix with the goal of minimizing the long-term average Age of Information. The formulated optimization problem is a mixed integer non-convex optimization problem, which is difficult to solve. To circumvent the high-coupled optimization variables, and with the aid of bi-level optimization, we decompose the original problem into an outer traffic stream scheduling problem and an inner reconfigurable intelligent surface phase-shift matrix problem. For the outer problem, owing to its complexity and stochastic nature of packet arrivals, we resort to deep reinforcement learning solution where the traffic stream scheduling is modeled as a Markov Decision Process, and Proximal Policy Optimization is invoked to solve it. Whereas, the inner problem that determines the reconfigurable intelligent surface configuration is solved through semi-definite relaxation. Finally, we show through extensive simulations that our approach evaluates the combined impact of scheduling policy and reconfigurable intelligent surface configuration on the long term average Age of Information, where we demonstrate its superiority against other baseline schemes.

This paper investigates the potential of integrating reconfigurable intelligent surface (RIS) and cooperative non-orthogonal multiple access (C-NOMA) in preserving the freshness of information in real-time Internet of Things (IoT) applications. The system model comprises one base stations (BS), one RIS, and two IoT devices (IoTDs), in an uplink setting, where the IoTD with poor channel quality is assisted by the RIS and by the IoTD with the strong quality through a full duplex (FD) device-to-device (D2D) communication. In this setup, an optimization problem has been formulated to minimize the average sum Age of Information (AoI) by optimizing the transmit power of the IoTDs and the RIS phase shift matrix, which is non-convex and is hard to solve directly. In order to resolve this issue, the formulated optimization problem is divided into a power control sub-problem and a RIS configuration sub-problem. Capitalizing on that, a closed-form solution has been derived for the power control sub-problem and the RIS configuration sub-problem is solved by resorting to difference-of-convex (DC) along with successive convex approximation (SCA). The simulation results demonstrate that the proposed RIS-empowered uplink C-NOMA scheme achieves higher AoI-reduction compared to all considered baseline schemes.

This paper investigates the benefits of integrating reconfigurable intelligent surface (RIS) on minimizing the average sum age of information in uplink NOMA-based IoT networks. A problem is formulated to optimize the RIS configuration, the transmit power of IoT devices (IoTDs) and their clustering policy. The formulated problem is a mixed-integer non-convex one, and in order to solve it, we obtain first the RIS configuration by resorting to difference-of-convex and successive convex approximation. Afterwards, the joint power allocation and clustering problem is solved using the concept of bi-level optimization and is decomposed into an outer IoTDs clustering problem and an inner power allocation problem. Optimal closed-form expressions are derived for the inner problem and one-to-one matching is employed to solve the outer one. Numerical results demonstrate the performance superiority of our scheme.

Reconfigurable intelligent surface (RIS) emerged as a promising technology for programming the wireless propagation environment, ushering wireless networks into a new era of high connectivity, and better spectrum and energy efficiency. Research has gained momentum over the past few years to study the gains of RIS and the optimal configuration of its elements. This paper is concerned with the optimal configuration of RIS elements; we consider a single base station (BS) serving multiple single-antenna users and formulate the problem of minimizing the total transmit power while jointly optimizing the allocated power to users and the RIS phase shifts. In line with the existing literature, the problem is decomposed into power control and RIS phase shift sub-problems. The power control sub-problem is the conventional power problem which is solved optimally through closed-form expression. Then, we tackle the problem of RIS passive beamforming optimization and propose two solutions leveraging a linear transformation and element-wise Karush-Kuhn-Tucker (KKT), respectively. The main idea of the linear transformation approach is to reduce the number of optimization variables to the number of users associated with the RIS from the number of its elements which is, in general, very large. On the other hand, the main idea of the element-wise KKT-based approach is to obtain a closed-form expression for the RIS phase shifts for a multi-user scenario, similar to the single-user case. Simulation results show that the proposed schemes have performance in total transmitted power close to the numerical solution and have much lower computational complexity than the baseline solutions. Moreover, simulation results show that the proposed solutions have a competitive performance in terms of optimality and complexity for large-scale RIS. Hence, our approach represents a general framework for configuring RIS elements in real scenarios.

In this paper, we investigate the power allocation and decoding order at the base station (BS) of two-user uplink (UL) cooperative non-orthogonal multiple access (C-NOMA)-based cellular networks. We formulate a joint optimization problem aiming at maximizing the minimum user achievable rate. Given that the problem at hand is non-convex, as an alternative, we derive low-complexity and near optimal analytical expressions for the power allocation coefficients. We demonstrate through numerical results that the proposed scheme provides superior performance in comparison with the traditional UL NOMA. We also demonstrate that, in UL C-NOMA, decoding the far NOMA user first at the BS provides the best performance.

This paper investigates the benefits of integrating multiple reconfigurable intelligent surfaces (RISs) in enhancing the timeliness performance of uplink Internet-of-Things (IoT) network, where IoT devices (IoTDs) upload their time-stamped status update information to a base station (BS) using non-orthogonal multiple access (NOMA). Accounting to the potential unreliable wireless channels due to the impurities of the propagation environments, such as deep fading, blockages, etc., multiple RISs are deployed in the considered IoT network to mitigate the propagation-induced impairments, to enhance the quality of the wireless links, and to ensure that the required freshness of information is achieved. In this setup, an optimization problem has been formulated to minimize the average sum Age of Information (AoI) by optimizing the transmit power of the IoTDs, the IoTDs clustering policy, and the RISs configurations. The formulated problem ends up to be a mixed-integer non-convex problem. In order to tackle this challenge, the RISs configurations are first obtained by adopting a semi-definite relaxation (SDR) approach. Then, the joint power allocation and user-clustering problem is solved using the concept of bi-level optimization, where the original problem is decomposed into an outer IoTDs clustering problem and an inner power allocation problem. Optimal closed-form expressions are derived for the inner problem and the Hungarian method is invoked to solve the outer problem. Numerical results demonstrate that our proposed approach achieves lowest AoI compared to the other baseline approaches.

Ultra-Reliable and Low Latency Communications (URLLC) is one of the essential services in 5G networks and beyond. The coexistence of URLLC alongside other services, namely, enhanced Mobile BroadBand (eMBB) and massive Machine-Type Communications (mMTC), calls for developing spectrally efficient multiplexing techniques. In this work, we study the problem of scheduling URLLC traffic in a downlink system in the presence of eMBB traffic. Based on the proposed superposition/puncturing scheme, a resource allocation problem is formulated with the objective to minimize the rate loss of the eMBB service and URLLC packet segmentation loss while satisfying the eMBB and URLLC quality of service (QoS) constraints. The resulting problem is formulated as a mixed-integer non-linear program (MINLP) which is generally very hard to solve in polynomial time. Hence, we reformulate the problem as a one-to-one pairing problem and we derive its feasibility region as well as the optimal solutions for the power and spectral resource allocation. Subsequently, we propose a low complexity algorithm to support the many-to-many pairing. Simulation results show that the proposed algorithm achieves higher URLLC packet admission rate and lower rate loss for eMBB. For instance, the URLLC packet admission rate, unlike baseline methods, is shown to be preserved under the proposed method even at higher URLLC load. It is shown that at least 30% more URLLC users can be served without degrading their QoS, while keeping the impact on eMBB rate minimal. Detailed numerical evaluation is presented to quantify the benefits of the proposed method.

In this paper, we investigate the dynamic power allocation for a visible light communication (VLC) cellular system consisting of two coordinating attocells, each equipped with one access-point (AP). The coordinated multipoint (CoMP) between the two cells is introduced to assist users experiencing high inter-cell-interference (ICI). Specifically, the coordinated zero-forcing (ZF) precoding is used to cancel the ICI at the users located near the centers of the cells, whereas the joint transmission (JT) is employed to eliminate the ICI at the users located at the edge of both cells and to improve their receptions as well. Furthermore, two multiple access techniques are invoked within each cell, namely, non-orthogonal-multiple-access (NOMA) and cooperative non-orthogonal-multiple-access (C-NOMA). Hence, two multiple access techniques are proposed for the considered multi-user multi-cell system, namely, the CoMP-assisted NOMA scheme and the CoMP-assisted C-NOMA scheme. For each scheme, two power allocation frameworks are formulated each as an optimization problem, where the objective of the former is maximizing the network sum data rate while guaranteeing a certain quality-of-service (QoS) for each user, whereas the goal of the latter is to maximize the minimum data rate among all coexisting users. The formulated optimization problems are not convex, and hence, difficult to be solved directly unless using heuristic methods, which comes at the expense of high computational complexity. To overcome this issue, optimal and low complexity power allocation schemes are derived. In the simulation results, the performance of the proposed CoMP-assisted NOMA and CoMP-assisted C-NOMA schemes are compared with those of the CoMP-assisted orthogonal-multiple-access (OMA) scheme, the C-NOMA scheme and the NOMA scheme, where the superiority of the proposed schemes are demonstrated. Finally, the performance of the proposed schemes and the considered baselines is evaluated while varying various system parameters.