equency bands are the complementary bands to deploy 5G, whereas low-frequency bands (i.e., sub-6 GHz) are still the primary bands of the 5G spectrum. High frequency also enables unified access and backhaul since the same radio resources are shared. 5G is expected to use a unified air interface and hierarchical schedul- ing for both radio access and backhaul which enables flexible backhauling and low-cost ultra-dense network (UDN) architectures. 1 Future radio access may also employ bands with different levels of access regulation including exclusively licensed, non-exclusively licensed UDN is the most promising solution to address high traffic rates in wireless networks. It increases network capacity, improves link efficiency, and spectrum utilization while reducing power consumption. In fact, densi- fication of infrastructure has already been used in current cellular networks, where the minimum distance between base stations has been drastically reduced. Nonetheless, UDN faces many technical challenges, including difficulties in mobility management and backhauling (including self-backhaul). Advanced interfer- ence and mobility management schemes need to be developed at the physical and network layers to support UDN. UDNs further require comprehensive characterization in terms of cost, power consumption, and spec- trum utilization efficiency [4]. xxxii Introduction and Background (shared), and unlicensed bands. The 5G system supports both licensed and unlicensed spec- trum with a flexible and unified air interface framework [35]. The ever increasing user traffic in mobile networks necessitates increasing the amount of spectrum that may be utilized by the 5G systems. High-frequency bands in the centimeter wave and millimeter wave range are being used due to their potential for supporting wider channel bandwidths and eventually the capability to deliver ultra-high data rates. Some new spectrum below 6 GHz was allocated for mobile communication at the World Radio Conference (WRC) 2015, and the bands above 6 GHz are 