## 6 FWA Transport Network Solutions
### Wireless Solution  
As discussed in Section 2.2, wireless transport schemes have been known to be highly feasible solutions with remarkable advancements in the communication system evolutions. This can be attributed in part to the schemes' inherent benefits like roaming support, scalability, operational simplicity, cost-effectiveness, and ease of deployment. In addition, wireless transport schemes are very suitable solutions for complementing fiber-based transport schemes. Nonetheless, their susceptibility to transmission channel conditions limits their employment to short-range applications. Moreover, most of the existing solutions can just support the lower bit-rate options of the CPRI. This results in bandwidth limitation for the wireless transport schemes. In addition, attractive wireless technology such as mm-wave has been proposed to address the limitation [370; 371; 372; 373; 374]. As stated earlier in Section 3, an enormous amount of unexploited, as well as underutilized, frequencies are available at high bands. Consequently, there is a significant research interest in the mm-wave partly owing to a number of high-dimensional and compact antenna arrays being designed for commercial use in the band. In addition, as a consequence of the 60 GHz standards such as 802.15.3c, 802.11 ad, and Wireless-HD being issued, significant consideration has been given to communications at the band. Nevertheless, mm-wave applications are limited to a relatively shorter communication range due to the inherent high propagation losses [375; 376; 377; 378]. In [301], some enabling approaches that can make mm-wave relevant for the 5G networks and viable means of addressing its challenges are presented. Furthermore, there has been a considerable scarcity of spectrum in the RF band because of several advanced wireless technologies that have been progressively deployed in the communication networks [368; 375]. Therefore, to meet the wireless network growing demands, a number of chip set suppliers, as well as mobile wireless operators, have been given considerable interest in the unlicensed spectrum. The main attention is to the 2.4 GHz and 5 GHz bands that have been employed for Wi-Fi applications. This has also been considered by mobile operators who are employing LTE in the unlicensed (5G LTE-U) spectrum for deploying additional small cells [376; 47]. Through this, the unlicensed spectrum resources can be well allocated to the LTE system, to have further bandwidth for supporting the Wi-Fi subscribers [376; 377; 378]. In addition, there is a possibility of reusing the same spectrum in the transport networks. This can be effectively achieved through frequency-division multiplexing and time-division multiplexing (TDM) schemes. Furthermore, it can be realized through opportunistic fronthauling, in which unlicensed spectrum can be sensed. For example, the DU can sense available unlicensed spectrum (unexploited unlicensed spectrum) and subsequently utilize it for fronthauling. Moreover, when the signal of an active user is relatively below the predefined threshold, the spectrum can also be used by the DU. Besides, the associated transport network constraints can be alleviated by Wi-Fi through offloading [376]. Despite the fact that Wi-Fi network can offer comparatively high data rates, it presents limited coverage and mobility. The weaknesses can be addressed by Wi-Fi mesh network implementations [379; 47]. As aforementioned in Section 2.2.4, FSO communication offers an alternative/ complementary solution to the optical fiber systems. It can be utilized for RF signal transmission between the apertures at the CU and DU through the free space. Therefore, being an optical wireless solution, optical fiber media deployment is not obligatory. As a result, there is no need for trench digging to implement it, resulting in cost and time-saving. Additionally, similar to RoF technology that is a feasible, well-developed, and widely employed solution, the FSO system can effectively support the transmission of multiple RF signals.