# Advances in Improving Energy Efficiency of Fiber-Wireless Access Networks: A Comprehensive Overview
## 2 Fiber-Wireless Broadband Access Network
### Architecture of FiWi Networks  
Figure 2: Illustration of R&F integration between optical and wireless networks. The most common architecture of FiWi access networks comprises optical and wireless network domains. Ethernet passive optical networks (EPONs) or gigabit passive optical networks (GPONs) are commonly used in the optical segment of the network. In the wireless domain, a wireless point-to-point or mesh network (WMN) is operated according to the IEEE 802.11, WiMAX and LPWAN standards [11] and specific routing protocols [13], and cellular mobile network (4G, 5G) low-power wireless access network (LPWAN) IoT standards (LoRa, Sigfox, NB-IoT, etc.) in the wireless segment of the network [14]. The generic architecture of the FiWi access network is shown in Figure 2[15]. The optical transport network between the optical line terminal (OLT) unit and the optical network unit (ONU) can reach up to 100 km. In the downstream direction, an OLT connects the FiWi access network to the core network, while in the upstream direction it is responsible for scheduling resources toward the ONUs located near mobile users. The allocation of resources by OLTs is usually dynamic. It involves resource planning in terms of allocating time slots for data transmission to a single ONU device using time-division multiplexing (TDM) or assigning a single wavelength using the wavelength-division multiplexing (WDM) mechanisms. Such dynamic bandwidth allocation (DBA) is a key component of the TDM-PON mechanism that prevents collisions and improves bandwidth utilization through a polling scheme based on determining the ONU bandwidth requirements [16]. Connecting the OLT device with several ONU devices is realized through a shared optical fiber, at the ends of which the passive optical splitters are installed (Figure 2). Such architecture enables point-to-multipoint topology between single ONU and multiple OLT devices. It is also possible to combine these two solutions using time and wavelength division multiplexing (TWDM), which is characteristic of Next-Generation Passive Optical Network 2 (NG-PON2) and 100G-Ethernet Passive Optical Network (100G-EPON). The TWDM-PON uses the same physical network architecture as TDMA-PON (Figure 2). However, the OLT in TWDM-PON requires multiple transceivers that operate at different wavelengths and the ONUs are equipped with either a tunable transceiver (NG-PON2) or multiple transceivers (100G-EPON) to match specific OLT wavelengths in upstream transmission. The OLT's transceiver sends packets to all ONUs on the same wavelength in the downstream direction, and each ONU determines whether to receive or ignore the packet that is based on its type and destination address. In order to allow the ONUs to access the shared upstream channel, the OLT employs dynamic wavelength and bandwidth allocation (DWBA) scheduling. Depending on the size of its queue, this system allocates a specific time slot and wavelength for transmission to each ONU. In classical PON networks, ONU devices terminate at user locations, where it is possible to offer the service to end users exclusively through optical media. However, in FiWi networks, ONU devices are deployed in the characteristic locations (Figure 2). These characteristic locations are planned according to the needs of the spatial coverage of individual BSs or mesh portal points (MPPs) which are integrated with ONU devices. Integrated ONU-BS or ONU-MPP devices represent the interface between the optical and wireless segments of the FiWi access network (Figure 2). The transmission of optical signals in FiWi access networks can be implemented by RoF and R&F technology. Generally, both RoF and R&F technologies can be deployed in FiWi access networks; however, R&F technology is a very good solution for high-range FiWi networks based on the WLAN standard [14]. In comparison with RoF, the R&F approach also has certain shortcomings in terms of coordination between optical and wireless domains. These shortcomings are related to ensuring QoS and service viability [17].