## 2 Post NG-PON2 Research: Emerging Trends and Open Challenges
### Emerging Trends and Open Challenges  
The explosion of mobile data traffic has mandated the need for a new cellular architecture. _Femtocells_, which need to be more autonomous and self-adaptive than traditional _small cells_, are now widely deployed as small, inexpensive, low-power BSs. Beside economic and regulatory issues, the new key challenges arising in femtocell deployments are (i) interference coordination; (ii) cell association and biasing; (iii) mobility and soft handover; and (iv) self-organizing networks (SONs) [28]. Key to the cost-effective deployment and operation of small-cell networks will be the sharing of already existing high-capacity _FTTx backhaul infrastructures_[29]. The importance of high-capacity and low-latency fiber backhaul infrastructures in HetNets is also emphasized by the introduction of CoMP, one of the important performance-enhancing features of 3 GPP LTE-A Release-11, which will have a major impact on the backhaul and will demand a careful design of backhaul links between BSs in order to provide high data rates and very low latency in the range of 1 ms or lower for the efficient coordination of transmission and reception among multiple BSs [30]. When migrating from macro-only to HetNet environments, mobility management becomes more challenging due to increased interference and decreased (femto)cell radius. According to [31], there is a clear trend toward including additional mobility enhancements in future LTE releases to ensure a smooth migration from traditional network controlled to _user equipment (UE) assisted mobility_ as a remedy for reducing signaling overhead via fiber interconnections between macrocells and small cells. Among other mobility-based UE distribution techniques, _traffic steering_ represents a promising approach for both load balancing and minimization of unnecessary handovers in HetNets [32]. Moreover, the introduction of SON technologies holds great promise to not only optimize network capacity, coverage, and service quality, but also to substantially reduce (i) the complexity of interference coordination and, arguably more importantly; (ii) both operational and capital expenditures (OPEX/CAPEX) of LTE-A HetNets by minimizing human intervention [33]. Among the various SON functionalities, _self-healing_ has so far received the least attention as opposed to widely studied self-configuration and self-optimization, making the development of real-time fault-compensation algorithms a key research topic in LTE-A HetNets [34]. Recall from above that the vast majority of previous FiWi network studies considered architectures based on low-cost simple EPON fiber backhaul and capacity-centric WLAN solutions. With the advent of high-speed IEEE 802.3 av 10G-EPON and commercially available multichannel WDM PONs in conjunction with the emergence of Gigabit-class IEEE 802.11 ac VHT WLAN technologies providing raw data rates of up to 6,900 Mb/s, future FiWi access networks will give rise to fiber backhaul and wireless front-end infrastructures of unprecedented capacity. In the past, WiFi and cellular radio access networks (RANs) have been in constant competition, until recently when tighter integration of both capacity-centric WLAN and coverage-centric mobile networks has emerged as a necessary paradigm [35]. The WLAN mesh front-end of FiWi access networks may be used to provide _WiFi offloading_ of rapidly increasing mobile data traffic from cellular networks, thereby avoiding the need for capacity upgrades while at the same time helping reduce the complexity of future mobile networks. An interesting precursor of this trend is Deutsche Telekom's recently announced deal with crowdsourced WiFi hotspot provider FON, whose subscribers have signed up to mutually share their home broadband services in order to provide free \"WiFi To Go\" service on millions of hotspots worldwide.