ditional interference into the system that must be mitigated by suitable control. Orthogonal spectrum sharing ensures that, in any given area, no operator can be allocated a frequency that is already in use by another. In a multi-cell environment, orthogonality, neither the no sharing case (Figure 8.4a) nor orthogonality (Figure 8.4b) will cause interference, but non-orthogonality allows a more dynamic allocation that is different between cells (so better for an uneven load distribution) but could cause interference between network operators (Figure 8.4c). It is worth noting that in any multi-cell configuration, whether it is orthogonal or non-orthogonal, there exists the possibility of ICI and it is a function of the radio resource management to actually allocate resource blocks (RBs) in such a way as to mitigate the effects of this interference. 8.2 Historical Approaches to Spectrum Sharing Figure 8.4 Orthogonal and non-orthogonal sharing. (a) No spectrum sharing. (b) Inter-cell orthogonal spectrum sharing. Cell 1 (c) Inter-cell non-orthogonal spectrum sharing. Cell 2 Cell 1 Cell 2 Reserved Reserved for NOA for NOB Cell 1 Cell 2 Reserved Reserved for NOA for NOB 8.2.1.2 Sharing Rights the literature, the type of spectrum sharing between two operators is generally "equal-rights sharing," although CR is a good example where the users of the spectrum do not have equal rights. Equal rights sharing is where each operator has the same right to frequencies on the shared band. In Jorswieck et al. [16] the full cooperation case is considered with the aim being to maximize the sum of the throughput of two operators. In Singh et al. [17] coordina- tion protocols are proposed to allow negotiations between operators. Equal-rights sharing has been the center of consideration in the literature as this is the simple case and it avoids any priority mechanism between operators. However, such a constraint restricts general- ity and it is necessary to consider the case where operators could have access to different proportions of 