## 3 Overview of HO Management in 5G UDSC Network
### 5G Small Cells Architecture
With cell densification, connection distances are small, ensuring high network resources per UE. Large distribution of small cells BSs in UD networks make connection distance small compared to macrocell network, leading to offer better signal quality at every user equipment. UEs thus can support high mobile services, online gaming and streamline videos [46]. Besides, cells in UD networks have limited number of UEs, enabling them to experience more network resources. States of RRC On the air interface, 5G new radio technology is using a network layer protocol named radio resource control (RRC) protocol. This protocol is used between UE and base station that specified by 3 GPP in TS 38.331. PDCP-Protocol is used for RRC message transportation. In general, the main functions of the RRC protocol follows as: 1. Broadcasting system information; 2. Paging notification and release; 3. Connection establish and releasing; 4. Reconfiguration and release; 5. Outer loop power control; 6. RRC connection mobility procedures. According to the network status, RRC protocol configures user and control planes using signalling functions and grant radio resource management strategies for implementation. The operation of the RRC protocol is controlled by a state machine which characterize specific states of a UE and different amounts of radio resources correspondent with different state. Idle, connected, and inactive are the three states of RRC protocol. Among these three, idle and connected are introduced by 4G LTE, whereas inactive state is initiated in 5G cellular network. 1. Idle State; In the RRC idle mode, the content of user equipment access stratum is not available in UE and network. It means that its main function is to save power and energy, for instance, when there is no need to transfer or receive data, the UE changes its mode to RRC idle by turning off both Tx and Rx. Similarly, when UE is following RRC idle mode, it regularly checks the call channel, handles incoming cases, and chooses the cell for camping by following mobility measurements [47]. Camping of user equipment performs the following purposes: * Getting system information for the camping in the cell; * Establishing RRC connection setup over the camped cell; * Getting call messages to shut the mobile calls within the camped cell; * Getting public warning system alerts. 2. Connected State; In the RRC connected mode, the context of UE AS is present in both network and user equipment. When UE is present in RRC connected mode, it not only transmits or receives user plane data but also control plane signaling. It means that in this case, UE requires to monitor link quality of the former and target cell to get the radio link information. Subsequently, the consumption of battery is higher than idle mode [48]. However, DRX (discontinuous reception) is deployed in RRC connected mode to save power. 3. Inactive State; Although when UE is present in RRC connected mode, networks do not suffer communication delays, yet high power consumption is a major challenge to address. And regular transition of UE from idle to connected or connected to idle creates undesirable signaling load, which amplifies latency. RRC inactive state or mode therefore is introduced by 5G NR to reduce power consumption, network signaling load and latency. In RRC inactive mode, the content of user equipment access stratum is stored in both core network and UE, whereas the connection between radio access network and core network is remained active to avoid power consumption along with the control plane delay. In this mode, the quantity of CN signals needed to paging a UE is estimated to be reduced, which is notable to improve latency performance. In the prior study, it is estimated that RC inactive states saves over 200% latency reduction in contrast to RRC idle states and 40% UE power consumption comparing to RRC connected mode [49]. 4. Registration and Paging of UE.