# Advances in Improving Energy Efficiency of Fiber-Wireless Access Networks: A Comprehensive Overview
## 4 Energy Efficiency Analyses of Radio-over-Fiber Networks  
The implementation of the FiWi networks using the RoF technology is based on exploiting optical fiber as a means for the direct transport of radio frequency (RF) signals among one or more remote access (antenna) units (RAU) and network operator central office (CO) (Figure 4). The CO is responsible for managing access to both the optical and wireless domains [18]. The optical modulation of the RF signal is realized through electrical-to-optical (E/O) conversion, which is required for transmitting an RF signal over an optical-fiber link. Compared with traditional coaxial-based RAN systems, the RoF network paradigm offers several advantages. Those advantages are related to the high bandwidth of fiber links with high data rates and low signal attenuation. This allows greater transport distances and better resistance to electromagnetic noise and thereby increases the flexibility of the allocation of network devices, reduces BS size due to the BS design with RRUs and BBUs, increases installation flexibility, allowing multiple operators to share the same RoF network and improves the EE through dynamic resource allocation realized using the centralization of baseband functions at CO premises [66]. Depending on how the RF signal is transmitted through an optical-fiber link, there are two basic RoF communication techniques known as digitized RoF (D-RoF) and analog RoF (A-RoF) [67]. In the A-RoF system (Figure 5), the transmission of the RF signal is carried in the analog domain, where analog RF signal is used for light modulation of the optical signal transmitted in the fiber. On the other hand, in the D-RoF system (Figure 6), the D-RoF signal is digitized prior to transmission over an optical communication link. Figure 4: Generic RoF architecture. For example, the MFH link between the RRU and the BBU designed for the LTE advanced (LTE-A) C-RAN architecture based on a common public radio interface (CPRI protocol is a prominent representative of the D-RoF. The main concept of such an MFH solution based on the CPRI interface is to use a digitized version of the baseband signal before being transmitted over the optical-fiber link. Although the CPRI-based fronthauling solution has proven to be effective for previous mobile network generations (i.e., 2G, 3G and 4G), the D-RoF approach has one major issue. Namely, the CPRI has shown to be bandwidth-inefficient, as it uses a constant data rate for transmitting the signals [68]. Hence, this bandwidth inefficiency can become critical in 5G networks using carrier aggregation (CA) and mMIMO techniques wherein high bandwidth and capacity requirements are expected. Moreover, the CPRI does not support flexible rerouting that allows automatic RRU switching to another BBU and the data rate is dependent on a number of antennas which can also be critical in 5G mMIMO implementations [69]. Presumably, the D-RoF-based solutions may have been sufficient for the early stages of the 5G network, but as 5G reaches its full potential, digitized optical MFH transmission based on the conventional CPRI protocol will generally not be sufficient [70]. Analog-optical transmission, commonly referred to as the A-RoF, presents a traditional RoF technique that has promising potential to overcome D-RoF's CPRI limitations (Figure 5). Analog modulation of the optical signal with no prior digitization addresses bandwidth limitations, making it more suitable for high-bandwidth networks. On the other hand, due to the characteristics of the fiber link, transmitted A-RoF signals can experience numerous link impairments such as attenuation, chromatic dispersion and fiber nonlinearities [71]. Depending on the carrier frequency used for modulation of the optical signal, two common A-RoF techniques are usually considered, i.e., the radio frequency-over-fiber (RFoF) and  
Figure 5: General A–RoF optical link architecture. Figure 6: General D–RoF optical link architecture. the intermediate frequency-over-fiber (IFoF).