Francesco Pantisano

Interference Alignment for Cooperative Femtocell Networks: A Game-Theoretic Approach

The use of small cells serviced by low-power base stations such as femtocells is envisioned to improve the spectrum efficiency and the coverage of next-generation mobile wireless networks. However, one of the major challenges in femtocell deployments is managing interference. In this paper, we propose a novel cooperative solution that enables femtocells to improve their achievable data rates, by suppressing intratier interference using the concept of interference alignment (IA). We model this cooperative behavior among the femtocells as a coalitional game in partition form and we propose a distributed algorithm for the coalition formation. The proposed algorithm allows the femtocell base stations to independently decide on whether to cooperate or not, while maximizing a utility function capturing both the gains and costs from cooperation. Using the proposed algorithm, the femtocells can self-organize into a stable network partition composed of disjoint femtocell coalitions and which constitutes the recursive core of the game. Inside every coalition, cooperative femtocells use advanced IA techniques to improve their downlink transmission rate. Simulation results show that the proposed coalition formation algorithm yields significant gains, in terms of average payoff per femtocell, reaching up to 30 percent relative to the noncooperative case for a network of N=300 femtocells.

Coalition formation games for femtocell interference management: A recursive core approach

Overlaying low-power, low-cost, femtocells, over existing wireless networks has recently emerged as a means to significantly improve the coverage and performance of next-generation wireless networks. While most existing literature focuses on spectrum sharing and interference management among non-cooperative femtocells, in this paper, we propose a novel cooperative model that enables the femtocells to improve their performance by sharing spectral resources, minimizing the number of collisions, and maximizing the spatial reuse. We model the femtocell spectrum sharing problem as a coalitional game in partition form and we propose a distributed algorithm for coalition formation. Using the proposed algorithm, the femtocells can take autonomous decisions to cooperate and self-organize into a network partition composed of disjoint femtocell coalitions and that constitutes a stable partition which lies in the recursive core of the considered game. Whenever a coalition forms, the femtocells inside this coalition can cooperatively pool the occupied spectral resources. Additionally, the members of any given coalition jointly schedule their transmissions in order to avoid collisions, in a distributed way. Simulation results show that the proposed coalition formation algorithm yields a performance advantage, in terms of the average payoff (rate) per femtocell reaching up to 380% relative to the non-cooperative case.

Cooperative Interference Alignment in Femtocell Networks

Underlay femtocells have recently emerged as a key technology that can significantly improve the coverage and performance of next- generation wireless networks. In this paper, we propose a novel approach for interference management that enables a number of femtocells to cooperate and improve their downlink rate, by sharing spectral resources and suppressing intra-tier interference using interference alignment. We formulate a coalitional game in partition form among the femtocells and propose a distributed algorithm for coalition formation. Using our approach, the femtocell access points can make individual decisions on whether to cooperate or not, while maximizing a utility function that captures the cooperative gains and the costs in terms of transmit power for information exchange. We show that, using the proposed coalition formation algorithm, the femtocells can self-organize into a network partition composed of disjoint femtocell coalitions, which constitutes the recursive core of the game. Simulation results show significant gains in terms of average payoff per femtocell, reaching up to 30% relative to the non-cooperative scheme.

Interference avoidance via resource scheduling in TDD underlay femtocells

In this paper, we proposes a resource reuse scheme for femtocell networks. In this context, macrocells operate in a Frequency Division Duplexing mode while all femtocells employ the macrocellular uplink spectrum in a Time Division Duplexing mode. We propose two solutions. In the first one, a centralized scheduling is performed via list-coloring based on the overall conflict graph. The second solution is distributed and autonomously performed by each femto access point (FAP), based on local conflict graph. The two solutions are compared to a coordinated LTE-A compatible and a random allocation schemes in terms of spectral efficiency and activity factor in the superframe. Finally, given a target Quality of Service requirement, the proposed distributed scheme is evaluated. Results show that self-organizing femtocell operating in an uncoordinated way achieves a good tradeoff of performance when a limited feedback from femto users is exchanged.

On the impact of heterogeneous backhauls on coordinated multipoint transmission in femtocell networks

The choice of a suitable backhaul constitutes one of the main performance bottlenecks in the emerging femtocell networks. In this paper, we study the impact of adopting a heterogenous backhaul (i.e., wired or over-the-air) with realistic quality-of-service requirements on coherent coordinated multipoint (CoMP) transmission in the downlink of femtocell networks. We formulate a cooperative game with continuum among the femtocell access points (FAPs) for performing CoMP in order to maximize the downlink rate while accounting for the constraints on the heterogeneous backhaul. In this respect, we propose a distributed algorithm that enables the FAPs to jointly decide on their cooperative partners as well as the choice of a backhaul strategy. In this respect, the proposed algorithm jointly addresses the problem of coalition formation as well as the optimization of the tradeoff between OTA and wired backhaul transmission modes, each of which is limited by a different factor such as delay or spectrum resources availability. We show that the proposed algorithm converges to a stable partition which constitutes the continuum core of the studied cooperative game. Simulation results show that our proposed scheme yields interesting gains in terms of the average downlink rate per FAP, reaching up to 26% relative to the classical of non-cooperative transmissions.

Adaptive traffic load-balancing for green cellular networks

The sleeping strategy has become popular to reduce power consumption of base stations (BSs) by shutting down underutilized BSs in the management of green cellular networks. In this paper, we propose a novel solution for an energy efficient use of cellular networks, based on traffic load balancing. By modeling the power consumption for BSs connected to uniformly distributed users, the relationship between the optimal number of active (or shut down) BSs and the traffic load is then derived through the power ratio, which is the ratio between dynamic and fixed power part of BS power consumption. Both analytical and simulation results demonstrate that, in order to achieve significant energy savings, less BSs should be turned on at low traffic load while more BSs turned on at high traffic load.

A self-organizing solution for interference avoidance in TDD underlay femtocells

In this paper, we draw our attention to the problem of interference avoidance in a macrocell-femtocell network. Further, macro user equipments operate in a Frequency Division Duplexing mode while all femtocells employ the macrocellular uplink spectrum in a Time Division Duplexing mode. We propose a self-adapting algorithm to tackle highly interfering MUEs through handover procedure. Results reveal that the proposed solution provides lower outage probability than closed and open access policies, without significant loss in terms of signal-to-noise-and-interference ratio.

On the dynamic formation of cooperative multipoint transmissions in small cell networks

The ever-growing data rate requirements of next-generation mobile networks mandate a highly efficient exploitation of the spectral and energy resources. In this respect, a key technology which can allow high quality-of-service provisioning with reasonable energy expenditures is given by the deployment of Coordinated Multi-point (CoMP) transmissions for small cells. While the benefits of CoMP have been explored in the literature, little has been done to study how a network can adaptively decide on whether or not to adopt CoMP. To this end, in this paper, we propose a cooperative framework for small cell networks, in which groups of small cells dynamically decide when to perform CoMP transmission while optimizing the tradeoff between rate improvement and energy consumption. We formulate the problem as a dynamic coalitional game with the small base stations as players, and we provide a distributed algorithm that allows the small cells to self-organize into CoMP-based coalitions. The resulting network partition represents the ϵ-core of the game, which is a key solution concept for dynamic coalition games. Simulation results show that the proposed dynamic coalition formation algorithm leads to a significant gain of up to 41% sum rate to transmit power ratio in the small cell tier.

Interference Management in Femtocell Networks Using Distributed Opportunistic Cooperation

Femtocells are envisioned to be deployed in indoor environments in order to improve both radio coverage and spectrum efficiency. This paper focuses on the self-organization of indoor femtocells, which includes mechanisms of cooperation. In this context, we propose a solution to automatically form cooperating groups among severely interfered femtocells in order to avoid interference. A hybrid access policy is proposed and compared to the closed and open policies. Results show that, as stated in the Braess Paradox, pervasive cooperation may be detrimental, when available resources are highly contended. Conversely, in particular cases, a marginal selfish behavior of femtocells can be preferable.

Improving Macrocell-Small Cell Coexistence Through Adaptive Interference Draining

The deployment of underlay small base stations (SBSs) is expected to significantly boost the spectrum efficiency and the coverage of next-generation cellular networks. However, the coexistence of SBSs underlaid to a macro-cellular network faces important challenges, notably in terms of spectrum sharing and interference management. In this paper, we propose a novel game-theoretic model that enables the SBSs to optimize their transmission rates by making decisions on the resource occupation jointly in the frequency and spatial domains. This procedure, known as interference draining, is performed among cooperative SBSs and allows to drastically reduce the interference experienced by both macro- and small cell users. At the macrocell side, we consider a modified water-filling policy for the power allocation that allows each macrocell user (MUE) to focus the transmissions on the degrees of freedom over which the MUE experiences the best channel and interference conditions. This approach not only represents an effective way to decrease the received interference at the MUEs but also grants the SBS tier additional transmission opportunities and allows for a more agile interference management. Simulation results show that the proposed approach yields significant gains at both macrocell and small cell tiers, in terms of average achievable rate per user, reaching up to 37%, relative to the non-cooperative case, for a network with 150 MUEs and 200 SBSs.