ing sufficient NO has no spare resource resource Figure 8.1 Advantages of spectrum sharing. Block Block Block Block Block 1 + 2 1 + 2 NOA & NOB ((()))) Figure 8.2 Different approaches to sharing. (a) No sharing. (b) Common pool of spectrum and transmission. (c) Common pool of spectrum but separate transmission. are more complicated than that, as shown simply in Figure 8.2. Figure 8.2a shows the non-shared approach and (b) and (c) show two different approaches to sharing a com- mon pool of spectrum: (b) has one pool of spectrum and both operators share the same transmission equipment SO the same spectrum will only be used by a single transmission whereas in (c) the same pool of spectrum is made available to the base stations of both operators but the transmission equipment is not shared SO spectrum could be used by sev- eral transmissions. Logically these are the same, but a more complex control mechanism is required for (c) as the co-channel interference map is effectively that of the two operators overlaid. It is clear from Figure 8.2c that operators being allocated the same block of spectrum in the same areas will unavoidably suffer co-channel interference without a high level of coop- eration between them - the problem about such a high level of cooperation is that it is likely to lead to a high signaling load. However, there is a trend for infrastructure sharing and this 8.2 Historical Approaches to Spectrum Sharing cooperation and deeper integration will have a lower signaling cost, thus making spectrum sharing more practical. For example, the China Tower Company took over the tower assets of the three major network operators in China to reduce redundant construction and the towers will be shared by the three operators [2]. Sharing is now being encouraged by trade bodies and regulators, for example [3, 4]. 8.2 Historical Approaches to Spectrum Sharing Once spectrum has been allocated there may be a need to share that in different ways. For instance, the well-known cellular concept [5, 6] shares spectrum geog