sult is essentially showing a better utilization of the small cells in the site-specific cases. The former is again a result of the more accurate envi- ronment and propagation modelling for the site-specific cases, where especially the small cell signal coverage is more widespread in the explicitly represented street canyons. Furthermore, as reported in [9, 10] the use of DC increases both the macro and small cell time-of-stay as compared to the cases without DC, where the UE is subject to more frequent PCell handovers (both intra- and inter-frequency). However, one of the costs of operating with DC is larger RRC signalling overhead from having to manage both PCell and SCell for the users. 8.4 Ultra Dense Network Interference Management 8.4.1 Ultra Dense Network Characteristics Very dense deployment of small cells on a set of dedicated carriers (or on a single carrier) can bring noticeable benefits by providing more transmission resources. The former is also known as ultra dense networks, which can be deployed in either outdoor or indoor traffic hotspots where additional capacity is needed. However, UDN deployments also come with a number of challenges, where co-tier interference is high on the list. Assuming that the cells are deployed with equal maximum transmit power, the interference footprint tends to be rather spread, where many users are subject to a larger number of interfering signals without a clear dominant interference contribution, and only some users are subject to a dominant interfering source (a.k.a. aggressors). Thus, the aggressor-victim relation is much more diffuse for UDN and hence calls for different inter-cell interference coordination (ICIC) solutions as compared to, for example, co-channel macro-pico scenarios, where the macro is a well-defined aggressor for the pico victim users (see detail in Chapter 7). UDN are furthermore characterized by typically only having each cell simultaneously serving a single or few users, while several cells may have no users to serve at certain time instanc