rom the network architecture aspect, slices are logically virtualized and iso- lated over shared physical networks. Hence, both physical and virtual resource constraints need to be considered to form a function chain for a specific service. Second, from the user association aspect, UE should be associated with an NS via a specific physical access point (AP), such as base station (BS). Not all APs are able to provide the specific slice/service due to either a missing functionality or limited resources. Hence, joint optimization of NS and BS selection should be addressed. Third, from the service aspect, NS-based networks provide guaranteed QoS for all serving UEs [7] instead of the traditional best effort model [8]. Owing to the aforementioned differences, applying traditional resource management Radio Access Network Slicing and Virtualization for 5G Vertical Industries, First Edition. Edited by Lei Zhang, Arman Farhang, Gang Feng, and Oluwakayode Onireti. © 2021 John Wiley & Sons Ltd. Published 2021 by John Wiley & Sons Ltd. 9 Access Control and Handoff Policy Design for RAN Slicing mechanisms to RAN slicing may lead to low resource utilization, poor QoS provisioning, and frequent NS reconfigurations. Therefore, designing new resource management mecha- nisms dedicated for RAN slicing to optimize network performance and resource utilization becomes an essential yet challenging issue. In this chapter, we aim for a unified framework for access control in RAN slicing. Owing to resource limitation, not all users' QoS requirements can be satisfied simultaneously. Hence, we design two user access control policies to select admissible users for optimiz- ing the QoS and the number of admissible users respectively. Numerical results show that in typical scenarios, our proposed access control policies can significantly outperform tra- ditional policies in terms of the number of admissible users. 9.1.1 System Model for RAN Slicing We consider an NS-based mobile network shown in Figure 9.1, which consists of core and radio acc