## 2 Post NG-PON2 Research: Emerging Trends and Open Challenges  
### NG-PONs and Converged Fiber-Wireless (FiWi) Access Networks: State of the Art and Recent Progress  
FiWi access networks, also referred to as hybrid optical-wireless access networks (HOWANs) and wireless-optical broadband access networks (WOBANs), aim at combining the reliability, robustness, and high capacity of optical fiber networks with the flexibility, ubiquity, and cost savings of wireless networks. Over the last few years, significant progress has been made on the design of hybrid optical-wireless access networks [9]. FiWi access networks include conventional radio-over-fiber (RoF) networks, which have been studied for decades as an approach to carry radio frequencies over optical fiber links between a central control station and multiple low-cost remote antenna units, e.g., distributed antenna system (DAS) connected to the BS of a fiber optic microcellular network. RoF networks are well suited to realize networks with centralized control, e.g., traditional cellular networks. The vast majority of RoF studies focused on transmission issues at the physical layer without taking higher-layer protocols into account. However, most of the recent layer-2/layer-3 networking (rather than layer-1 physical transmission) research activities have been focusing on FiWi access networks that are based on integrated low-cost and simple Ethernet passive optical network (EPON), wireless local area network (WLAN), and WiMAX technologies. A comprehensive and up-to-date overview of previously proposed cellular, EPON, WiMAX, and WLAN-mesh based FiWi access network architectures, network planning and reconfiguration, techno-economic analysis of WiMAX _vs._ EPON, wireless and integrated optical-wireless routing algorithms, powerful hierarchical frame aggregation techniques as well as quality-of-service (QoS) continuity techniques across the optical-wireless interface was provided in [10]. The challenges and opportunities of FiWi access networks were described in [11]. Although a few recent FiWi architectural studies exist on the integration of EPON with 4G LTE wireless networks, the vast majority of previous studies considered FiWi access networks consisting of a conventional IEEE 802.3 ah EPON fiber backhaul network and an IEEE 802.11 b/g/n/s WLAN-based wireless mesh front-end network [9; 10; 11]. The design of _energy-efficient_ \"_green_\" _FiWi access networks_ has been receiving considerable attention. An overview of recently proposed energy-efficient architectures as well as energy-efficient medium access control (MAC) and routing protocols for FiWi access networks was provided in [12]. The authors also proposed an optical burst switching (OBS) based dynamic bandwidth allocation (DBA) protocol for the energy-efficient integration of long-reach EPON and LTE-A technologies and investigated the tradeoffs between energy savings and QoS guarantees. The simulation and experimental work in [13] has shed some light on the tradeoff between energy saving and QoS support in FiWi access networks. A unified analytical framework for the throughput-delay performance evaluation of a wide range of FiWi network routing algorithms was developed in [14; 15], taking emerging very high throughput (VHT) IEEE 802.11 ac WLAN technologies and PON fiber faults into account. Beside the energy-efficient design of FiWi access networks, their survivability has been another recent important research area. A variety of _advanced survivability techniques for NG-PONs and FiWi access networks_ were investigated in [16; 17]. A centralized architecture incorporating RoF and cognitive radio technologies into a unified network called cognitive WLAN over fibers by replacing WLAN access points (APs) with remote antenna units was presented in [18], including a testbed architecture built on a PON and software defined radio platform. Network coding based energy management solutions for NG-PONs were presented in [19]. The capacity and delay performance of various types of NG-PON were analyzed in [20].