A third motivation for carrier aggregation is support of heterogeneous networks. A
heterogeneous network deployment typically consists of a layer of high-power macrocellsand a layer of low-power small cells (e.g. picocells, Closed Subscriber Group (CSG)
femtocells or relay nodes – see Chapters 24 and 30) with at least one carrier being used
by both layers. In such a deployment, transmissions from one cell can interfere strongly
with the control channels of another, thus impeding scheduling and signalling. Rather than
simply using separate carriers for the two layers, which would result in inefficient spectrum
usage, carrier aggregation enables multiple carriers to be used for a given layer, while
interference can be avoided by means of cross-carrier scheduling. Cross-carrier scheduling
allows the Physical Downlink Control Channel (PDCCH) on the CC of one serving cell to
schedule transmission resources on a CC of another serving cell, as explained in detail in
Section 28.3.1.