 up with bandwidth and data rate increases: Bandwidth surge and applications with high data rate have been a major chal- lenge to today's mobile network. This is the number one priority that 5G network should address. Providing seamless connectivity across technologies: Multiple radio technologies will coexist, and the future network should support seamless migration and roaming across technologies. 5.1.1 Architecture To meet these requirements, the 5G employs a two-tier hierarchical structure with both the macrocell base station (MBS) and the small cell base station (SBS). Figure 5.1 shows the high-level picture of 5G. The smallest unit of coverage is constructed by various SBSs, examples of which include femtocell and picocell. The MBS oversees a set of SBSs that are located close by. For example, a building can be connected 5.1 5G Overview (((C))) (((C))) Figure 5.1 5G architecture. by an SBS, and the SBS connects to an MBS in the region. Another example is having an SBS in the train SO that all the UEs are connected to the internal SBS and will not need to deal with the frequent han- dovers due to fast moving of the train. This two-tier hierarchy is the fundamental design of 5G network. It is deployed as a process of network densification. On the one hand, the number of antennas for each US and MBS increases. On the other hand, the density of base stations also increases. The main advantages of having SBS include the following: (1) high data rate and efficient spectrum usage; (2) reducing energy consumption; (3) SBS is a cost-effective solution; (4) the SBS is often plug and play and thus easy to manage; (5) reducing congestion on the MBS since SBS can offload the UEs; and (6) improving handoff performance because the SBS can handle handoff on behalf of its connected UEs. The SBS can be connected to core network from the backhaul tech- nologies. Typical backhaul technologies include wired optical fiber, wireless point to multipoint, and wireless point to point using bidi- rectional links. The SBS and MBS comm