## 2 Post NG-PON2 Research: Emerging Trends and Open Challenges
### Emerging Trends and Open Challenges
Complementing fast evolving HetNets with already widely deployed WiFi access points in addition to femtocells, which together are projected to carry over 60% of the global data traffic by 2015, represents a key aspect of the strategy of today's operators to offload mobile data traffic from their cellular networks [36]. It was recently shown by means of real-world iPhone measurements that significant offloading as well as battery power saving gains can be obtained if data transfers are delayedwith some deadline until users enter a WiFi zone, though the achievable performance gains highly depend on given user mobility and spatial/temporal WiFi coverage patterns [37]. Beside traffic-aware scheduling, _backhaul sharing_ is gaining significant importance to enhance the performance of cellular networks overlaid with WiFi offloading hotspots [35]. Another important trend is the convergence of mobile cellular networks and WSNs in order to support _machine-to-machine (M2M) communications_. It was shown by means of simulation that the throughput-delay performance and lifetime of WSNs can be remarkably increased in such a converged network, but a number of essential issues still need to be resolved, most notably jointly optimized channel access techniques, two-level resource allocation schemes, and robust re-selection algorithms [38]. Especially for event-driven M2M communications, where a large number of devices become activated within a short period of time, traffic-aware random MAC protocols were shown to provide superior delay performance and robustness [39]. According to a recent OECD report on the future digital economy, one of the most promising applications of M2M communications is smart metering, which represents one of the first steps toward realizing the vision of the _smart power grid_[40]. The European Union (EU) has mandated the use of smart meters by 2020 in Directive 2006/32/EC, creating a market for around 180 million meters at a rate of one per household. Beside wireless technologies, the superior security and immunity features of PON based communications infrastructures, which are already installed in many countries, will be leveraged for the realization of large-scale sensor-actuator networks in support of future smart grid applications [41]. Smart grids will be the backbone of the future electricity network of the EU integrating the high penetration of renewable energy sources and enabling the flexible participation of customers, as witnessed by the EcoGrid EU project, which establishes the first prototype to provide a market-based platform and information and communication technology infrastructure of the future European intelligent power grid [42]. Today, Internet technology and renewable energies are beginning to merge in Europe, North America, and other regions worldwide in order to create an interactive, integrated, and seamless _Energy Internet_ infrastructure for the so-called _Third Industrial Revolution (TIR)_ economy, which goes well beyond current austerity measures and has been officially endorsed by the European Commission as economic growth roadmap toward a competitive low carbon society by 2050 [43], as discussed in more detail next.