The vast proliferation of the mobile devices along with the increasing data demands pave the way for the fifth generation (5G) wireless communication systems, which are envisaged to support significantly higher connection speeds, such as 10 Gb/s peak data rates. Heterogeneous networks (HetNets), cloud computing, pervasive networking, Internet of Things (IoT), Machine-to-Machine (M2M) communications, user cooperation and cognitive radio are only some indicative topics that are expected to attract notable research attention in the context of 5G technologies. However, all these concepts should be studied under a common framework defined by the European Union and the Information and Communications Technology (ICT) industry: energy awareness.

In particular, the development of novel technologies implies higher energy consumption both at the operator’s and the end user’s side. On the other hand, mobile operators seek the least possible energy consumption to reduce their costs and provide “green” services, while the end users set as high priority the maximization of the battery autonomy. To meet these demands, the energy efficiency should be explicitly considered in the design of new 5G systems, something that will potentially bring changes to the default protocol stack.

This Special Issue of Springer Telecommunications Systems includes energy-oriented multidisciplinary aspects of future networks, ranging from physical layer techniques to communication protocols and network architectures. In response to our call, thirty interesting high-quality research papers have been submitted and, after a rigorous and thorough review process, the top eleven articles have been selected for publication. A summary of the accepted papers is discussed next.

In the first paper, “Exploiting Diversity of Usage to Enhance User Equipment Energy Efficiency in LTE Networks” by Ta and Baras, the authors propose a cooperative framework to improve the lifetime of the mobile terminals in device-to-device (D2D) communications.

In the second paper, “Backhaul-aware Energy Efficient Heterogeneous Networks with Dual Connectivity” by Prasad and Maeder, the authors foster the energy efficient network operation, taking into account the backhaul connections in dense heterogeneous networks.

In the third paper, “A Lifetime Extended Multi-levels Heterogeneous Routing Protocol for Wireless Sensor Networks” by Tyagi et al., the authors focus on the wireless sensor networks that consist of nodes with different energy levels and propose a new routing scheme to increase the network lifetime.

In the fourth paper, “An Energy-Aware Scheme for Efficient Spectrum Utilization in a 5G Mobile Cognitive Radio Network Architecture” by Mavromoustakis et al., the authors propose an energy efficient scheme for the cognitive exploitation of the TV White Space in next generation networks.

In the fifth paper, “Towards Energy-Efficient Cooperative Spectrum Sensing for Cognitive Radio Networks – An Overview” by Althunibat et al., the authors provide a survey of the state of the art research in cooperative spectrum sensing for cognitive radio networks and they propose a framework to improve the energy efficiency of such systems.

In the sixth paper, “An Energy Efficient DOA Estimation Algorithm for Uncorrelated and Coherent Signals in Virtual MIMO Systems” by Wan et al., the authors propose an energy efficient scheme for the direction-of-arrival (DOA) estimation of different user equipment terminals by the base station in a parallel way.

In the seventh paper, “Dynamic Two-Stage Beam Training for Energy-Efficient Millimeter-Wave 5G Cellular Systems” by Kim and Hong, the authors introduce an energy efficient protocol for link configuration in millimeter-wave communication, which reduces the number of training signal transmissions in the network.

In the eighth paper, “Energy Efficient Load Balancing for Future Self-Organized Shared Networks” by Monteiro et al., the authors present a novel load balancing scheme that enhances the energy efficiency in self-organized networks shared among multiple operators.

In the ninth paper, “Optimized Handover Strategy with Hybrid Satisfaction Mechanisms” by Zorba and Hassanein, the authors propose a novel vertical handover mechanism that jointly considers the provided data rate and the energy consumption in heterogeneous networks.

In the tenth paper, “A Quantitative Analysis of the Throughput Gains and the Energy Efficiency of Multi-Radio Transmission Diversity in Dense Access Networks” by Koudouridis et al., the authors investigate the benefits of multi-radio transmission diversity in dense networks under different scheduling and resource allocation strategies.

Finally, in the last paper, “Relay Selection for Secure 5G Green Communications” by Nomikos et al., the authors deal with application-oriented secure relay selection policies and they propose two schemes for delay-critical applications and energy-efficient communications, respectively.

We would like to thank all the authors for submitting their work in this special issue and the volunteer reviewers for their effort and time during the whole review process. Finally, we would like to express our gratitude to the Editors-in-Chief, Prof. Gavish and Prof. Khan, for their guidance and Mr. Levine for his administrative support.