Introduction and prior art
Frequency allocation for heterogeneous networks
In heterogeneous networks, frequency resources can be allocated to different tiers in a co-channel (shared-spectrum) or dedicated channel (split-spectrum) manner, or through a hybrid technique, which is a combination of the two approaches. In the co-channel approach shown in Figure 15.1(a), while the spectrum resources are fully reused in different tiers, cross-tier interference may cause crucial setbacks to the system. For example, macrocell users in the vicinity of closed subscriber group (CSG) femtocells are not allowed to connect to the femtocells, even if their link quality with these femtocells is good. Therefore, such macrocell users receive strong downlink interference from CSG femtocells and may fall into outage.
The split spectrum approach shown in Figure 15.1(b), on the other hand, partitions the allocated spectrum between multiple tiers. Each tier can use its own segment of resource and therefore there is no cross-tier interference . However, the amount of bandwidth available to each tier is reduced. Hybrid methods as shown in Figure 15.1(c) use a mixture of co-channel and dedicated channel methods, and aim to reuse the spectrum resources whenever feasible. For example, in , the macrocell users are dedicated a component carrier (CC), referred as the “escape carrier,” which is not used by the femtocell network. Any macrocell mobile station (MMS) that is close by to a femtocell is scheduled within this escape carrier, if the interference observed from the femtocell network is above a threshold.