such as 60 and 71 GHz [102, 103]. In comparison with LTE-LAA and muLTEfire, NR-U supports several deployment scenarios including carrier aggregation, dual connectivity, and standalone [101]. Moreover, NR-U reduces latency and enables URLLC applications by deploying several numerologies with mini-slots. Another advantage of NR-U is regarding the exposed node problem by permitting directional carrier sensing which increases the spectrum efficiency. Despite the various benefits of standalone NR-U, it still faces several challenging issues. These challenges were highlighted in [101] and [104]. 1.4 5G Enabling Technologies The SDN, NFV, network slicing, AI/ML, and MEC have been considered as key enabling technologies to automate, virtualize, softwarize, and optimize the 5G networks. This section investigates these enabling technologies and addresses the concept, standards, protocols, architectural frameworks, challenges, and recent research works. 1.4.1 Software-Defined Networking (SDN) In the traditional cellular networks, data and control planes are tightly coupled on the same device that increases the complexity of managing and controlling the end-user devices. In contrast, the SDN technology enables the 5GC to configure and manage the control plane separately from the data plane [45]. According to open networking 1 A Comprehensive Study on 5G foundation (ONF), 7 the main advantage of SDN is enabling a centralized and a fully controlled network by decoupling the data plane from the control plane of several nodes. Consequently, the end-user devices act as forwarding-packet devices where the control logic is moved from the devices onto a centralized layer [3]. The centralized control layer is defined as SDN controller that acts as the intelligence of the network. Consequently, the data plane delivers the packets by checking the flow tables that are controlled by the SDN controller [105]. The control plane, in turn, sends commands to manage the forwarding rules of the data plane [3, 106]. In [105], an example for the 