ed to one operator SO suitable constraints would need to be imposed. Explicit Spectrum Sharing Here the aim is to maximize the number of qualified users; the RBs are allocated to users SO that those users requiring the smallest number of RBs receive the allocation first, irre- spective of which operator they belong to. Users are ranked in ascending order of distance from the BS, distance being a first approx- imation to the number of RBs required. As each user is considered, the RBs are ordered in terms of the channel conditions, with those providing the best capacity (taking into account the co-channel interference from neighboring cells) being ranked first. Going down the ordered list of users, enough RBs are allocated from the top of the list of RBs to provide the necessary capacity. However, allocating RBs in particular sectors will affect the co-channel interference, which is why the allocation has to be implemented iteratively taking into account the interference map generated by the allocation. A running total of the numbers of RBs allocated to users of each operator is required SO that once the total that an operator may use (from the sharing ratio) is reached no further users from that operator are served. Implicit Sharing Mechanism The overall serving order is adjusted to take into account the share of the spectrum to be taken by each operator. A simple illustration of this principle is given in Figure 8.5 where NOA is allocated twice the spectrum of NO SO two NO A users are served for every user from NO- Within the list for each operator, the serving order is still in ascending order of distance from the BS as illustrated in Figure 8.5. It is important to note that, with the example of Figure 8.5, this reordering means that the channel conditions may be worse in some of the NOA users that are served before NO 8.2 Historical Approaches to Spectrum Sharing Increasing distance From NOA Queue of users From NO Overall serving order 2A:1B 2A:1B Left over Figure 8.5 Creating non-equal allocation order. Source