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Femtocells are attracting a fast increasing interest nowadays, as a promising solution to improve indoor coverage and system capacity. Due to the short transmit-receive distance, femtocells can greatly lower transmit power, prolong handset battery life, and enhance the user-perceived Quality of Service (QoS). On the other hand, technical challenges still remain, mainly including interference mitigation, security and mobility management, intercepting wide deployment and adoption by both mobile operators and end users. This paper introduces a novel energy-centric handover decision policy and its accompanied algorithm, towards minimizing the power consumption at the mobile terminal side in the integrated LTE macrocell-femtocell network. The proposed policy is shown to extend the widely-adopted strongest cell policy, by suitably adapting the handover hysteresis margin in accordance with standardized LTE measurements on the tagged user's neighbor cells. Performance evaluation results show that significantly lower interference and power consumption can be attained for the cost of a moderately increased number of network-wide handover executions events.

Current mobile devices are equipped with multistandard interfaces to fully utilize both the existing and the imminent Public Land Mobile Network infrastructure. This flexibility comes with significant energy consumption overheads at the mobile terminal side and thus, the adopted interface / network selection scheme, a.k.a. Vertical Handover mechanism, should be based on both Quality of Service oriented and energyefficiency criteria. This paper summarizes the current state of the art on energy-centric vertical handover algorithms, discusses weak aspects of existing approaches and presents a novel contextaware vertical handover framework towards energy-efficiency. Rather than a pure energy-centric vertical handover scheme, this framework is a context-awareness enabler for energy-centric vertical handover decision making.

Femtocells are attracting a fast increasing interest nowadays, as a promising solution to improve indoor coverage, enhance system capacity, and lower transmit power. Technical challenges still remain, however, mainly including interference, security and mobility management, intercepting wide deployment and adoption from mobile operators and end users. This paper describes a novel handover decision policy for the two-tier LTE network, towards reducing power transmissions at the mobile terminal side. The proposed policy is LTE backward-compatible, as it can be employed by suitably adapting the handover hysteresis margin with respect to a prescribed SINR target and standard LTE measurements. Simulation results reveal that compared to the widely-adopted strongest cell policy, the proposed policy can greatly reduce the power consumption at the LTE mobile terminals, and lower the interference network-wide.

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Device-to-Device (D2D) discovery is the inextricable prelude for the direct exchange of local traffic between cellular users in proximity. The D2D discovery process can be based on either autonomous actions taken by D2D-enabled devices, a.k.a. direct D2D discovery, or core network functionalities to estimate proximity, a.k.a. network-assisted D2D discovery. A key advantage of network-assisted D2D discovery is its potential to reduce the energy, signaling, and interference burden required for D2D discovery, by exploiting knowledge of the network layout. We analyze the performance of network-assisted D2D discovery in random spatial networks and derive useful guidelines for its design. Specifically, we derive approximate expressions for the distance distribution between two D2D peers conditioned on the core network's knowledge of the cellular network layout, assuming that the base stations are distributed according to a Poisson point process. The derived expressions are used to assess the interplay between the D2D discovery probability and key system parameters, such as network intensity and transmit power, as well as to identify conditions under which the D2D discovery probability is maximized. Numerical results validate the accuracy of our findings and provide valuable insights on the performance tradeoffs inherent to network-assisted D2D discovery.

The rise of heterogeneity in technology, types of services, and coverage area of radio access in the Fifth Generation (5G) wireless communication, opens new challenges in optimising users' access to the networks. To fully utilise the capacity of such rich field of wireless connectivity, mobile devices can not any more be confined with accessing only their previously agreed home operator's infrastructure. On the other hand, and in order to keep up with the agility required to deliver promises in providing new services and applications, virtualization and softwarization are introduced in the 5G. To this end, we propose an on-the-fly Radio Resource Sharing (RRS) scheme between different mobile infrastructures so as to provide mobile devices with the freedom to access all available radio resources around them. Such on-thefly RRS is empowered by employing the concepts of Softwaredefined Networking (SDN) and virtualization of radio access resources. We argue that the RRS service is a step forwards in achieving the convergence as foreseen by the 5G: convergence of SDN, virtualization, and wireless control, convergence of heterogeneous wireless infrastructure, and above all, convergence of different operators' infrastructure in a transparent manner.

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