ces at certain small cells for the benefit of other small cells. In this manner, the inter-cell coordination function which generates the muting patterns considers an appropriate metric for individual users and a proportional fairness scheduling among all users. The muting technique not 5G Network Architecture 55 only takes into account the first dominant interferer but also the second dominant interfer- ence source for optimal interference mitigation. Owing to the network traffic load fluctuation, switching off the base stations/access nodes in the cells with low or no traffic load is an essential method for UDNs to improve energy effi- ciency and to reduce inter-cell interference. In practice, the network load fluctuates over dif- ferent times and locations due to diversity of user behavior and mobility, which is especially true for UDNs that warrants switching off under-utilized base stations. In a sparse network using frequency reuse of one with idle base stations where access node density is less than user density, the average spectral efficiency can still increase linearly with access node den- sity as in the network without idle mode base stations. In a frequency reuse of one UDN with idle mode base stations where access node density is larger than user density, the spectral effi- ciency only increases logarithmically with access node density [68]. Optimal utilization of large amount of radio resources in a UDN can become increasingly com- plex. Improper allocation of abounded radio resources in a UDN can lead to higher inter-cell interference, unbalanced load distributions, and higher power consumption. Furthermore, due to inter-cell interference, local radio resource allocation strategies may have a global impact on a UDN operation. In other words, a localized allocation strategy may not work in a UDN envi- ronment, which necessitates the use of a centralized RRM that has a holistic view of the UDN, allowing tight interworking across the network. Providing sufficient bandwidth over direct- wired backhaul