## 6 FWA Transport Network Solutions  
FWA is a universal platform for wired and wireless technologies convergence in the networks. This is owing to the fact that it offers wireless connectivity in the last mile, whereas its transport network between the core network and the BSs can be implemented using wireline and wireless solutions [5]. Meanwhile, substantial enhancements have been recorded in the optical network performance due to many innovative technologies that have been adopted [358]. For instance, the network capacity, reach, and aggregate number of subscribers that can be supported efficiently have been exceptionally improved. In addition, there has been a notable and progressive optical schemes penetration in the access networks. This helps in supporting a number of services like multitenant FTTx and mobile backhaul/fronthaul. The FTTx scheme offers a range of fiber-based broadband network architectures such as FTTN, FTTC, FITP, FITB, and FTTH. The architectures offer viable solutions to the emerging broadband technology last-mile bottleneck, through progressive service delivery in unprecedented proximity to the subscribers [166]. In the light of this, 5G use cases such as eMBB and uRLLC can be supported efficiently by the baseband and the radio processing elements that are closer to the wireless or end user devices (i.e., CPEs). This can be attributed to the principle that proximity not only enables enhanced signal quality but also offers a higher data rate and lower latency [359]. Based on its salient features and service delivery close to the subscribers, the PON scheme can be employed to support the 5G use cases. Note that one of the main challenges is the means of offering various service demands aiming at fulfilling ubiquitous and elastic connections objectives. Consequently, seamless convergence of high-speed fiber and wireless networks is essential for effective 5G and beyond system realization. The system convergence will cost-effectively facilitate broader network penetration to support the envisioned ubiquitous connections [166]. As a result of this, a high-capacity optical link has been deemed as one of the viable solutions for attending to the network demands between the CU and DU [360]. In addition, to address the rising traffic demands, the 5G RAN architectures are envisaged to support heterogeneous networks (HetNets) in which mm-wave small cells will be overlaid on the macrocells [359]. Furthermore, to support the envisioned ultra-dense small-cell BS deployment, cloud RAN (C-RAN) has been adopted as an attractive solution for effective flexibility and scalability in the system performance [2; 3; 47; 361]. The C-RAN is an advanced architecture and in some respect, different from the conventional distributed RAN (DRAN) as described in Section 2.3. The C-RAN architecture shifted the BBU away from its usual cell site location. Therefore, with the arrangement, the BBU pools can be located at the CO, while the remote radio heads (RRHs) are deployed at the cell sites. Consequently, C-RAN presents substantial features like enhanced SE and further flexibility advantages for additional deployment of RRH in relation to the DRAN. Similarly, with the well-centralized BBU pools, C-RAN can support a number of attractive schemes such as better interference coordination/mitigation, improved virtualization, enhanced resource pooling, along with uncomplicated operation and management with scalable and seamless evolution. In addition, it should be noted that the BBU pools and the RRHs are connected through the fronthaul network. As discussed in Section 8.1, the de facto air interface standard generally used for the connection is the digital radio over fiber (D-RoF) based common public radio interface (CPRI) specification. Besides the CPRI, there are other advanced fronthaul interfaces like next-generation fronthaul interface (NGFI), open base station architecture initiative (OBSAI), enhanced CPRI (eCPRI), and open radio interface (ORI) that can be employed as well [362; 363; 364].