mto-cells) and macro-extension access points (relays and RRHs). A full-functional base station is capable of performing all functions of a macro-cell with a lower power in a smaller coverage area and encompasses the full RAN protocol stack. On the other hand, a macro-extension access node is an extension of a macro-cell to effec- tively increase the signal coverage, and it performs all or some of the lower protocol layer functions. Moreover, the small cells feature different capabilities, transmission powers, cover- age, and deployment scenarios. The UDN deployment scenarios introduce a different cover- age environment where any given user would be in close proximity to many cells. Small-cell architectures using low-power nodes were considered promising to mitigate the substantial increase in network traffic, especially for hotspot deployments in indoor and out- door scenarios. A low-power node generally means a node whose transmit power is lower than the corresponding macro-node and base station classes, for example, pico-cell and femto-cell access nodes. Small-cell enhancements for LTE focused on additional functional- ities for improved performance in hotspot areas for indoor and outdoor using low-power nodes. Network architectures comprising small cells (of various types including non-3GPP access nodes) and macro-cells can be considered as practical realization of the heteroge- neous networks. Increasing the density of the 3GPP native or non-indigenous small cells overlaid by macro-cells constitutes what is considered as UDNs. Small cells can be deployed sparsely or densely with or without overlaid macro-cell cover- age as well as in outdoor or indoor environments using ideal or non-ideal backhaul. Small cells deployment scenarios include small-cell access nodes overlaid with one or more macro-cell layer(s)30 in order to increase the capacity of an already deployed cellular Note: 3GPP uses the term layer in this context to refer to different radio frequencies or different component carriers. 48 Chapter 1 Tabl