# Advances in Improving Energy Efficiency of Fiber-Wireless Access Networks: A Comprehensive Overview
###### Abstract
This design decouples BS hardware into a remote radio unit (RRU) dedicated to the transmission and reception of wireless signals and a Baseband Unit (BBU) dedicated to baseband digital signal processing (Figure 1). Such D-RAN architecture, also known as Fiber to the Antenna (FTTA) architecture, creates two physically separated BS components that are connected with optical fiber at the location of the BS site, or up to a few hundred meters away. The introduction of such modular BS design with fiber-optic cabling between BBU and RRU provides numerous advantages which primarily include higher bandwidth, reduced transmission losses and lower sensitivity to electromagnetic interference and noise. Figure 1: Comparison of traditional RAN and D-RAN architecture. However, one additional positive aspect of such architecture is reducing the increased energy consumption of network equipment, since RRU can exploit the concept of natural air cooling, which reduces the energy needed for cooling overall BSs composed of BBU and RRU in the same cabinet (Figure 1). Additionally, the introduction of optical communication between RRU and BBU in D-RAN becomes the basis for the development of the newest generation of RAN architectures, which are based on centralized wireless access networks known as Cloud-RAN (C-RAN). In the C-RAN, decoupled BS RRU and BBU components are placed at different locations which can be up to tens of kilometers away (Figure 1). Locations are connected with fiber-optic technology which establishes a new RAN entity defined as a mobile fronthaul network (MFN). Although the realization of MFN based on the C-RAN architecture was initially introduced for fourth-generation (4G) mobile networks, the superior bandwidth capabilities and network scalability that C-RAN provides set the basis for the full exploitation of the C-RAN concept in the realization of the MFN of 5G networks and future sixth-generation (6G) mobile networks. Although most of the modern BSs currently use D-RAN architecture, the C-RAN can be viewed as the architectural evolution of the D-RAN and the C-RAN architecture and will be essential for the full practical deployment of the 5G and the future 6G RANs. Besides implementation in mobile cellular networks, the integration of fiber optics and wireless technology in access networks through the combination of fiber and wireless technologies, also known as the fiber-wireless (FiWi) concept, presents an alternative cost-effective solution to the realization of access networks. FiWi access networks integrate the high capacity of optical fiber networks with the coverage and flexibility of wireless networks. They create a robust infrastructure for the development and deployment of current and future applications and services and they are considered to be the most promising option for the realization of next-generation access networks [5]. By using optical networks as the backhaul and wireless networks as the front-end, FiWi networks aim to provide straightforward access for users [6]. These networks are expected to meet the demands of future access networks, such as providing high bandwidth, reliability, low cost and flexibility [7]. Additionally, it has been reported that access networks account for approximately 70% of energy usage in the ICT industry, and as a result of an increasing number of communication devices and bandwidth rates, this percentage is likely to increase in the future [6]. Therefore, to decrease overall network energy consumption, it is crucial to develop energy-efficient architectures of access networks, and architecture based on the FiWi concept is seen as a promising contributor to this goal. In a FiWi network, both radio-over-fiber (RoF) and radio-and-fiber (R&F) technologies are utilized. Radio-over-fiber (RoF) constitutes a popular communication system design that addresses the increasing bandwidth demand and enables optical and wireless integration in modern C-RAN systems.