than one cell is considered because an SU can easily generate inter-cell interference (ICI) to a PU in an adjacent cell, especially when the cell radius is small. Although a lot of work on CR assumes a single cell model (e.g. [11, 12]) the demand by users for more and more bandwidth means that a multi-cell structure is better for providing the required capacity in a denser population area. Underlay spectrum sharing applies spectrum spreading to the SU's signal in order to spread the transmit power over an ultrawide spectrum SO that the transmit power is very low and the interference from the SUs to the PUs does not exceed a certain threshold [13]. This means that the SUs do not have to find specific holes. Another approach that leads onto spectrum sharing was proposed in Liu et al. [14] that managed spectrum resources to always satisfy PUs, without SUs having to pay attention to the PUs; this was done at the expense of sacrificing the transparency of SUs to the PUs. Such 8 Network Slicing with Spectrum Sharing Single cell overlay Spectrum Cell 1 Spectrum Multiple Inter-cell interference overlay Cell 2 Spectrum Figure 8.3 Overlay spectrum sharing. a network requires no sensing hardware as resource allocation is managed across all users and the division between PUs and SUs can be based on the service provider to which they belong, or to different priorities between users for the same service provider. Effectively, the spectrum is shared between two types of users, in this case with absolute priority being given to one type (the PUs). Such sharing improves the efficiency of the whole network. 8.2.1 Classifications of Spectrum Sharing 8.2.1.1 Orthogonality The different types of user (PU and SU) from CR can be generalized into different net- work operators and Anchora et al. [15] classified spectrum sharing into orthogonal spectrum sharing and non-orthogonal spectrum sharing. If the frequency band is allocated to more than one operator simultaneously, it is classified as non-orthogonal sharing and will intro- duce ad