llocating spectrum in conformity with the internationally agreed spectrum allocations of the International Telecommunication Union (ITU). With the increasing use of mobile devices there is greater demand on radio resources, and spectrum sharing is seen as an effective way to utilize the limited frequency resources available. This is different from the traditional allocation of frequencies where each oper- ator can occupy exclusively the allocated block of frequency bands. This sole ownership guarantees simple intra-operator spectrum management and coordination, but the lack of flexibility causes a waste of spectrum. This is illustrated in Figure 8.1. In this scenario there are two network operators (NO A A and NO) that are allocated a certain amount of spectrum: in Figure 8.1a both network operators have sufficient resources to serve all their users, but in (b) NO has used all its allocated resources and SO cannot add any new users, while NO A still has unused spectrum. Spectrum sharing provides a possible solution to efficiently uti- lize the whole spectrum. Blocks of spectrum are no longer exclusive to one operator only but are made available to all operators as illustrated in Figure 8.1c where it can be seen, in this simple example, that there would only be unserved users if the total demand exceeds the total allocation. This example implicitly assumes that there is a central allocation and transmission mech- anism to avoid co-channel interference and the easiest way to achieve that is to have one radio access network (RAN) that all operators share. However, the situation and choices 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. 8 Network Slicing with Spectrum Sharing Unused Unused Unused Block 1 Used by Allocated to NOA Used by Used by Block 2 Used by Used by Allocated to NO Used by (a) No sharing - (b) No sharing - (c) Shar