Chapter 4 Energy-Ef ficient Roaming for Heterogeneous Wireless Networks Dionysis Xenakis, Nikos Passas, Lazaros Merakos and Christos Verikoukis Abstract Heterogeneous networks enable multi-standard terminals to provide increased Quality of Experience with the ability to always stay connected. Never- theless, the price paid for multiplicity of active radio interfaces and increased data rates is the rise in energy consumption of multi-standard terminals. This factor coupled with the slow progress in battery technology creates the so called energy trap, which at times can limit the mobile user to the nearest electricity socket creating a real paradox to the headlines of 4G and future generations in terms of unrestricted mobility. Therefore real solutions need to be in place that can support unrestricted roaming, but in an energy ef ficient manner whilst maintaining Quality of Service (QoS). Today’s mobile networking environment is already considering a HetNet (Heterogeneous networking) environment, with LTE release 12 considering the ABS (almost blank space) approach for coordinated interference management between macro and femto cells (3GPP, Technical report on UMTS heterogeneous networks, 2013, [1]). Therefore, specific user case that can handle mobility management between femto and macro cells, and in a more broader sense vertical handovers will be aligned with current technology trends in 3GPP, and ease the stringent require- ments on interference management as well promote the coexistence and dense deployment of heterogeneous small cells, that can include WLAN hotspots among others. In this chapter, we discuss the key aspects of vertical handover (VHO); focusing on the IEEE 802.11 and 3GPP LTE-Advanced network and propose a novel VHO decision algorithm. Following a similar approach, we discuss the key chal- lenges for (horizontal) handover in macrocell–femtocell network, and propose a novel interference-aware decision algorithm as well. The key features of both algorithms is that they are fully compatible with the IEEE and the 3GPP systems, while they focus on minimizing the energy consumption of mobile terminals; a critical requirement for 4G heterogeneous systems and the foreseen 5G system. D. Xenakis (&)  N. Passas  L. Merakos Department of Informatics and Telecommunications, University of Athens, Athens, Greece e-mail: nio@di.uoa.gr C. Verikoukis Telecommunications Technological Centre of Catalonia, Barcelona, Spain © Springer International Publishing Switzerland 2015 A. Radwan and J. Rodriguez (eds.), Energy Efficient Smart Phones for 5G Networks, Signals and Communication Technology, DOI 10.1007/978-3-319-10314-3_4 83