Ultra-Dense Network (UDN) has emerged as a key enabler in enhancing the capacity of mobile networks in order to deliver super-speed connectivity and high data rates, provide seamless coverage and support diverse use cases whilst satisfying a wide range of other performance requirements, such as improved reliability, latency, energy and spectral efficiencies. However, the reduced cell size in UDNs poses serious challenges in the areas of inter-cell interference (ICI) coordination and mobility management (due to increased frequency of handovers and signaling overheads). In this study, we simulate scenarios using Network Simulator version 3 (NS-3) to study the impact of cell size on user throughput at the point of handover using pedestrian mobility (3 kmph) as case study. The simulation results show improved spectral (and energy) efficiency with small cells over macrocells but significantly shorter handover times, which translate to more frequent handovers. And since the Long Term Evolution (LTE) and next-generation cellular networks are required to support mobility without serious impact on connectivity and performance, we align with the decoupling of the user and control planes where the macro-layer manages control signals (e.g., handover signaling) while the small cell provides the users with high data rates. By allocating the small cells more bandwidth, preferably in the millimeter wave (mmWave) bands with abundant spectrum, this decoupled framework will guarantee better spectrum management to support the fifth-generation (5G) broadband services and applications.