## 7 Passive Optical Network  
PON is a promising solution for CPRI transport, especially in high-traffic areas such as malls, university campuses, stadiums, and dense urban neighborhoods where densely deployed small-cell are highly essential [387]. Consequently, a PON system can significantly help in reducing the demanded interface number in the network and site space, resulting in an effective approach of saving the required number of optical fibers. Besides, it can considerably help in saving the system power consumption [390]. As discussed in Section 2.1.4, PON schemes have been adopted as promising access network solutions. This is as a result of the offered benefits like high bandwidth, low operation cost, and low maintenance cost [47; 144; 391].  
It is remarkable that, in an effort to meet the growing system demands and enhance the network performance, the PON architectures have been considerably evolving. The inherent benefits and the obtainable technological options offered by the architectures are some of the nice features that have been attracting the network operators in the PON system deployment. For instance, apart from the widely deployed Gigabit PON (GPON) system, several operators have been expanding their deployment to advanced PON architectures such as 10-gigabit symmetric PON (XGS-PON) and the 40-gigabit next-generation PON2 (NG-PON2) [392; 393]. Furthermore, to enhance the capacity and fiber efficiency more, WDM and TDM techniques are generally exploited in the PON system. Therefore, in accordance with access and transmission approaches, the PON system can be generally classified into TDM-PON and/or WDM-PON.  
In addition, the TDM-PON can offer relatively better bandwidth for a number of data applications. Nonetheless, resources accessible to the end-users are comparatively limited. On the other hand, the aforementioned limitation can be attended to with WDM-PON scheme implementation. To achieve this, a specific wavelength is assigned per subscriber. As a consequence, a high-data-rate, distinct, and secured PtP channel can be provided over a high-bandwidth and longer reach network, between each of the densely deployed DU and the CU. Therefore, WDM-PON approaches are appropriate solutions that can be exploited by multiple operators for a virtual partition of ONUs into many distinct PtP network connections, over a shared optical access infrastructure. The salient feature helps  
Figure 22: Transport network technology classification.  
in enhancing the fiber efficiency in comparison with the PtP Ethernet. Likewise, another relative advantage of WDM-PON is that it offers lower latency than the TDM-based schemes. These attributes enable WDM-PON to be a disruptive scheme that is not only suitable for FTTx schemes but also very applicable to the MFH and backhaul architectures. Therefore, apart from the fact that this will assist the operators in developing converged networks, it will also help in improving the existing access networks. In consequence, certain redundant COs in the network can be removed. Therefore, this will help in improving the network performance in an economical manner [47; 144; 391]. In addition, note that the WDM-PON and TDM-PON schemes advantages can be efficiently utilized via a hybrid scheme known as TWDM PON architecture. Further details on different PON architectures and their comparison regarding the standards, wavelength, bit rates, split ratios, ranges, and the employed technologies are discussed in [47; 394; 395]. In general, owing to the requirement for optical splitters for optical signal separation and collection, PON-based solution is susceptible to power loss and latency [387]. This further lowers the cell radius and brings about the difficulty in fault isolation [386; 387]. A number of PON-based schemes and their relative applications are explained further in the following subsections.