llocat- ing a certain proportion of RBs but in both these papers only a single cell is considered. Zhao et al. [37] discusses a multi-cell problem but models a uniform arrangement SO that all cells have the same sharing arrangement. The partial resource reservation is an "in-between solution" and does require complex management. However, it can also be seen as a "mixed solution" where part of the available spectrum is guaranteed to each slice with the remainder being shared. Full resource sharing: In this model [31], all of the spectrum is available to users from any slice (Figure 8.8c), but management of the sharing will allow users "belonging" to any slice to get the QoS they require. Previous work on spectrum sharing (for instance, [18]) can be extended to network slicing, not only considering a multi-cell environment but also removing any restriction that slicing in each cell would be the same. Limits can be placed on the total amount of bandwidth available to each slice if needed. Having determined a structure for slices that may, or may not, include sharing, it is nec- essary to provision resources on a per-slice basis [33]. 8.4.1 Example Radio Resource Allocation for Sharing Through Network Slicing There is a very large amount of literature on spectrum sharing, much of the earlier material considering CR networks but latterly including network slicing in the RAN. A good survey paper is [38] with Fig. 3 in that paper illustrating a framework for spectrum sharing. The authors note that most of the spectrum sharing mechanisms are at layer 2, although a few are at layer 1. Constraints are identified as (i) the requirements of applications (e.g. the type of QoS needed), (ii) business models and cooperation agreements, and (iii) the regulatory framework. Although that paper does not consider network slicing specifically, a lot of the material is relevant. In Voicu et al. [38] it is also noted that the spectrum sharing in layer 2 can be done in (i) time, (ii) frequency, (iii) code, or (iv) space. From a conceptua