Salah Eddine Elayoubi

Performance Evaluation of Cellular Networks Offering TV Services

In this paper, the capacity of orthogonal frequency-division multiple-access (OFDMA) networks is evaluated when serving mobile TV traffic. To introduce the service within the network, we develop analytical cross-layer models to assess the performance of the system when carrying TV calls over unicast connections. The presented cross-layer analysis is general and can be adapted to all types of multimedia services. We then focus on a TV-specific capacity analysis, exploiting the possibility of broadcasting TV channels over point-to-multipoint (PtM) connections. This includes investigating an enhanced broadcast solution where the modulation is dynamically adapted to the radio conditions of connected TV users. The more complete broadcast solution over single-frequency networks (SFNs) is then evaluated, based on a novel physical-layer model integrating the combined signal gain from neighboring cells. We finally show how to maximize the system capacity through a secondary usage of the SFN resources by elastic traffic. The presented models are evaluated using numerical applications and further validated through network simulations.

Sleep mode implementation issues in green base stations

In this paper, we focus on practical issues when implementing sleep mode in base stations. Our aim being to reduce energy consumption without affecting the Quality of Service (QoS) perceived by users, we expose some problems that my occur when applying sleep mode and propose practical solutions to cope with the resulting QoS degradation. We namely consider activation time issue that may result in blocking new or handover calls, and the ping-pong effect resulting in unnecessary ON/OFF oscillations. We show, using a realistic large-scale simulator, how to choose sleep mode implementation that achieves the best tradeoff between QoS preservation and energy consumption reduction.

A Hybrid Decision Approach for the Association Problem in Heterogeneous Networks

The area of networking games has had a growing impact on wireless networks. This reflects the recognition in the important scaling advantages that the service providers can benefit from by increasing the autonomy of mobiles in decision making. This may however result in inefficiencies that are inherent to equilibria in non-cooperative games. Due to the concern for efficiency, centralized protocols keep being considered and compared to decentralized ones. From the point of view of the network architecture, this implies the co-existence of network-centric and terminal centric radio resource management schemes. Instead of taking part within the debate among the supporters of each solution, we propose in this paper hybrid schemes where the wireless users are assisted in their decisions by the network that broadcasts aggregated load information. We derive the utilities related to the Quality of Service (QoS) perceived by the users and develop a Bayesian framework to obtain the equilibria. Numerical results illustrate the advantages of using our hybrid game framework in an association problem in a network composed of HSDPA and 3G LTE systems.

Impact of flow-level dynamics on QoE of video streaming in wireless networks

The Quality of Experience (QoE) of streaming service is often degraded by frequent playback interruptions. To mitigate the interruptions, the media player prefetches streaming contents before starting playback, at a cost of delay. We study the QoE of streaming from the perspective of flow dynamics. First, a framework is developed for QoE when streaming users join the network randomly and leave after downloading completion. We compute the distribution of prefetching delay using partial differential equations (PDEs), and the probability generating function of playout buffer starvations using ordinary differential equations (ODEs). Second, we extend our framework to characterize the throughput variation caused by opportunistic scheduling at the base station in the presence of fast fading. Our study reveals that the flow dynamics is the fundamental reason of playback starvation. The QoE of streaming service is dominated by the average throughput of opportunistic scheduling, while the variance of throughput has very limited impact on starvation behavior.

Mobility-Aware Admission Control Schemes in the Downlink of Third-Generation Wireless Systems

In this paper, novel connection admission control (CAC) algorithms that take into account the effect of mobility of users both inside and outside the cell in the downlink of third-generation mobile systems are developed. First, the system capacity, including the other-cell interference, subject to feedback between cells is studied. Then, effective bandwidth expressions for calls are obtained as a function of both their location in the cell as well as their class of traffic (i.e., voice versus data). Next, this formulation is used to derive two mobility-aware admission control algorithms, i.e., a priority CAC, where calls are accepted not only upon resource availability, but also through acceptance ratios that reflect their levels of priority, and a squeezing CAC, where elastic calls may be squeezed to a minimum agreed value, giving way to admit more calls in the system and to secure further ongoing mobile users. Using Markovian analysis, several performance measures are obtained, namely the blocking probability, the dropping probability, both intracell and intercell, as well as the overall cell throughput. The authors eventually investigate the performance of our CAC and show how to extend the Erlang capacity bounds, i.e., the set of arrival rates such that the corresponding blocking/dropping probabilities are kept below predetermined thresholds

QoE analysis of media streaming in wireless data networks

The purpose of this paper is to model quality of experience (QoE) of media streaming service in a shared fast-fading channel. In this context, the arrival and the service processes of the playout buffer do not have the same job size. We present an analytical framework based on Takacs Ballot theorem to compute the probability of buffer starvation and the distribution of playback intervals. We model the arrival processes of Proportional Fair and Round Robin schedulers, and feed them into this framework to study the impact of prefetching on the starvation behavior. Our simulations match the developed model very well if the base station knows the playback rate and the channel gain. Furthermore, we make an important observation that QoE metrics predicted by users are very sensitive to the measurement error of arrival process.

An Energy-Efficient Framework for the Analysis of MIMO Slow Fading Channels

In this paper, a new energy-efficiency performance metric is proposed for multiple-input multiple-output (MIMO) point-to-point systems. In contrast with related works on energy-efficiency, this metric translates the effects of using finite blocks for transmitting, using channel estimates at the transmitter and receiver, and considering the total power consumed by the transmitter instead of the radiated power only. The main objective pursued is to choose the best precoding matrix used at the transmitter in the following two scenarios : 1) the one where imperfect channel state information (CSI) is available at the transmitter and receiver and 2) the one where no CSI is available at the transmitter. In both scenarios, the problem of optimally tuning the total used power is shown to be nontrivial. In scenario 2), the optimal fraction of training time can be characterized by a simple equation. These results and others provided in the paper, along with the provided numerical analysis, show that the present work can therefore be used as a good basis for studying power control and resource allocation in energy-efficient multiuser networks.

A Nash-Stackelberg Fuzzy Q-Learning Decision Approach in Heterogeneous Cognitive Networks

Motivated by the fact that when selfish users choose their policies independently without any coordination mechanism, Nash equilibria could result in a network collapse, we develop in this paper a hierarchical distributed learning framework for decision-making in heterogeneous cognitive networks. We introduce the Nash-Stackelberg fuzzy Q-learning, with the network as leader that aims at maximizing its utility (revenue) and the mobiles as followers that have their individual objectives (maximizing their QoS). We validate our results through extensive simulations of the algorithm in a practical setting of a geographical area covered by a global HSDPA and 3G LTE system that serves both streaming and elastic traffic.

An Interaction-Based Mobility Model for Dynamic Hot Spot Analysis

In this paper, we analyze phenomena related to user clumps and hot spots occuring in mobile networks at the occasion of large urban mass gatherings. Our analysis is based on observations made on mobility traces of GSM users in several large cities. Classical mobility models, such as the random waypoint, do not allow one to represent the observed dynamics of clumps in a proper manner. This motivates the introduction and the mathematical analysis of a new interaction-based mobility model, which is the main contribution of the present paper. This model is shown to allow one to describe the dynamics of clumps and in particular to predict key phenomena such as the building of hot spots and the scattering between hot spots, which play a key role in the engineering of wireless networks. We show how to obtain the main parameters of this model from simple communication activity measurements and we illustrate this calibration process on real cases.

A Cross-Layer Approach for Distributed Energy-Efficient Power Control in Interference Networks

In contrast with existing works that rely on the same type of energy-efficiency (EE) measure to design distributed power control policies, this paper takes into account the presence of a finite packet buffer at the transmitter side and the impact of transport protocols. This approach is relevant when the transmitters have a nonzero energy cost, even when the radiated power is zero. A generalized EE performance metric integrating these features is constructed under two different scenarios in terms of transport layer protocols characterized by a constant or an adaptive packet arrival rate. The derived performance metric is shown to have several attractive properties in both scenarios, which ensures convergence of the used distributed power control algorithm to a unique point. This point is the Nash equilibrium (NE) of a game for which the equilibrium analysis is conducted. Although the equilibrium analysis methodology is not new in itself, conducting it requires several nontrivial proofs, including the proof of quasi-concavity of the payoff functions. A thorough numerical analysis is provided to illustrate the effects of the proposed approach and provides several valuable insights in terms of designing interference management policies.