## 3 Overview of HO Management in 5G UDSC Network
### 5G Small Cells Architecture  
3 GPP (3 rd Generation Partnership Project) has demonstrated the 5G structure to improve the cellular communication. Different service based communication models were deployed between control plane functions as proposed by 3 GPP. Primary approaches are included but not limited to overcome dependencies among core network (CN) & access network (AN), boosting the access service, and obtaining a well defined separation between control plane (CP) & user plane (UP) to amplify the flexibility in deployment and scaling phases. 5G structure is also supporting concurrent access, which is highly paramount for low latency use cases [41]. The architecture of UDSC networks was proposed in [42] and compared with traditional network architecture. In traditional cellular network architecture, a tree network infrastructure is used in which BS managers are used to handle all macrocell BS in the core network, whereas all the back haul traffic is moved towards the core network from the gateway. A hybrid architecture is required to support the deployment of microcell in traditional cellular network. In hybrid architecture, microcell BS managers are used to handle microcell BS and the back haul traffic is moved to core network through fiber routes. In contrast to macrocell BS, microcell BS can deliver high speed wireless broadcast at home and hotspots use cases. Nonetheless, both have the potential to independently spread user data and management data to corresponding users [43]. In Figure 2, we show the 5G NR small cells architecture and its working model. Mobile users can also HO in macrocell or microcell depending upon the requirements. Microcell network architecture therefore is used as a helping component for high speed wireless transmissions and communication purposes. Likewise, a similar approach is used to deploy small cells in 5G cellular networks, but it is quite challenging to forward back haul traffic from every small BS through fiber links or by using broadband internet, specifically in urban environments where cost and geographic installments are a major concern. Small cells BS cannot transmit back haul traffic to given gateway due to limitation of wireless transmission distance. To combat this challenge, a distributed UD 5G cellular network is proposed in which the functions of microcell BS and macrocell BS are distributed [44]. This means that the configuration of microcell BS is performed to handle management data and to monitor HO issues in small cells. While on the other side, transmission of user data is controlled by small cell BS. Furthermore, two different distribution architectures of UD cellular networks as follow:  
1. Using single gateway in UDSC network; A single gateway is deployed, configured, and installed at the macrocell BS having the capacity to embed large number of MIMO mmWave antennas. The purpose of integrating these antennas is to accept wireless back haul traffic coming from small cells located in macrocell. After collecting all the back haul traffic, it is moved to the macrocell BS through multi-hope mmWave links, which is then further forwarded to core network by using fiber links. Figure 2: 5G NR Small Cells Deployment. 2. Using multiple gateways in UDSC network; In this scenario, multiple gateways are used and deployed at massive microcell BSs by examining the conditions of back haul traffic and geographic scenarios. The reason to use multiple gateways instead of single gateway is that they are more flexible to handle back haul traffic and forward it to the core network. In this case, all the back haul traffic from small cells is dispersed over the multiple gateways in the macrocell, and then combined at gateway to move towards the core network by fibre-to-the-cabinet (FTTC) links [45]. Both the traffic capacity and network efficiency of 5G is 100x times than 4G-LTE, allowing it to deliver 20 Gbps peak data rates and 100+ Mbps average data rates. Small cell technology enables 5G networks to provide these benefits.