## 7 Passive Optical Network
### Lessons Learned  
The wire-based solution presents relative advantages such as high-capacity, ultra-reliable, and high-transmission security, which are promising for the 5G performance goals achievement. Nevertheless, the cell densification levels required in the 5G and beyond-5G (B5G) networks render wire solutions uneconomical or challenging to deploy in specific scenarios. This can be due to factors such as license fees for rights-of-way, regulatory requirements, and access to public or utility-owned sites. Besides, it can also be a result of physical deployment expenses, which may entail conduit laying, trenching, and maintenance access. Furthermore, the associated long deployment timelines and planning may hinder its application as well. On the other hand, most of the existing wireless solutions can only support few 5G deployment scenarios; however, they offer advantages such as low-cost and rapid deployment. Likewise, wireless solutions are highly ideal for hard-to-reach sites that may be in places such as highways, water, mountains, and busy urban areas. Therefore, note that there is no one-size-fits-all transport solution, and the achievement of an optimal solution can be viewed as a multi-dimensional trade-off task. Consequently, the network operators have to be considerate in choosing out of the options based on their merits and demerits. The optimal transport solution selection depends on different factors such as the existing fiber/copper infrastructure and the site structure. A set of potential transport network solutions that can be employed for FWA are compared in Table 12. Furthermore, it is essential to develop a generalized joint-optimization model for planning wireless access and optical transport for 5G FWA networks concurrently. This will considerably help in minimizing deployment cost while meeting different network requirements [23]. In addition, it is remarkable that the operators have to be expanding and upgrading their networks to be able to support several advanced services such as remote medical services, online gaming, interactive E-learning, home video editing, and next-generation 3D TV. However, the NG-PON deployment demands considerable initial investments. An instance of this can be understood from the greenfield FTTH system deployment. It has been observed that between 70 and 76% of the total network investments are incurred in the ODN setup. Consequently, the network operators need to optimize their investment by leveraging the existing ODNs. Moreover, when the existing GPON system is compatible with the evolving NG-PON, the operators can make the most of the ODNs [402; 411]. Figure 24: A typical PON-based transport network architecture for RAN. Furthermore, apart from the system compatibility, different PON schemes' coexistence is highly imperative. This implies that network investment can also be optimized when the existing ODNs are shared among different PON technologies. In this context, an existing GPON-based network that demands an upgrade to support innovative FTTx broadband access services can coexist with other PON schemes like XGS-PON and NG-PON2. This can be achieved when a coexistence element is employed in the PON systems. Therefore, regarding the desired deployment scenarios or use cases, different generations of OLTs (CUs) and ONTs (DUs) can efficiently coexist in a shared ODN infrastructure. Based on this, the operators can exploit different schemes such as asymmetrical and symmetrical data rates. Besides, coexistence will offer deployment flexibility so that the operators can be operating on fixed or tunable wavelength plans. This will help in supporting any service type and business demand at relatively cost-effective rates. Moreover, it will help in the gradual migration to NG-PON technologies so as to adopt the most suitable cost as well as deployment pace for specific business requirements [412]. Besides, RF signals and optical time-domain reflectometers can also coexist with the PON systems.