Resource management for network-assisted D2D communication

Sammanfattning: During the last decade, the widespread use of smart devices and mobile applications has led to a massive growth of the mobile traffic demand. Efficiency and scalability are therefore key criteria for the development of future cellular systems, in which device-to-device (D2D) communication is recognized as one of the promising technologies. D2D communication allows mobile users in physical proximity to communicate directly, bypassing the base station as in conventional cellular networks.In this thesis, we investigate some of the possible benefits and challenges brought by the introduction of D2D communication in cellular systems. In particular, we focus on resource management techniques for network-assisted D2D communication using cellular spectrum. Our main contributions lie in the context of mode selection, power control and (frequency/time) resource allocation mechanisms, recognized as key techniques to realize the promises of this technology.First, we investigate how the integration of D2D communication in cellular systems operating under dynamic Time Division Duplex (TDD) can enhance their energy efficiency. We perform joint optimization of mode selection, uplink/downlink transmission period, and power allocation to minimize the transmission energy consumption. The resource management problems for different scenarios are formulated as mixed-integer nonlinear programming problems. In several cases, we exploit the problems’ structure to design efficient algorithms that achieve optimal solutions in polynomial time. In the remaining cases, we propose a heuristic algorithm that computes near-optimal solutions while respecting practical constraints in terms of execution times and signalling overhead. Our simulations demonstrate that D2D communications in dynamic TDD systems can yield significant energy savings and improved spectral efficiency compared to traditional cellular communication.Second, we study the performance of various power control strategies applicable to D2D communications in 3GPP LTE networks. We compare them with an utility maximization approach that trades off spectrum efficiency and total transmit power consumption. Our numerical results suggest that the LTE power control scheme is well prepared for network-assisted D2D communications, especially from the cellular user perspective. However, for D2D users, the utility based scheme can provide gains in terms of SINR and power consumption.Finally, we investigate the subcarrier allocation problem for uplink transmissions ina D2D-enabled network. We focus on maximizing the aggregate transmission rate of the system. In addition to the traditional inter-cell interference, we also account for the intra-cell interference caused by D2D pairs reusing cellular resources. This problem is computationally hard due to its nonconvex and combinatorial nature. However, we show that it can be described as a potential game; hence, we can find a Nash equilibrium using iterative algorithms based on best/better response dynamics.