## 1 Introduction
In the separated architecture, a macro cell covers a large area and manages the control signal, and a small cell covers a small area and manages data, as shown in Figure 1. In this architecture, a small cell BS can be turned off, if there is little UE with data service in the coverage area. The power of a BS can be saved, while connectivity with network can be provided using a macro cell. Also, UE can be connected with a macro cell always and therefore, handover can be managed seamlessly and power of a BS can be saved, since signaling overhead due to handover can be reduced. Of works regarding separate network architectures, the work in [30; 31] analyzed the efficiency of accommodating UEs for varying the number of small cells and the distance between a UE and a BS. The work in [32] defines the threshold ratio of a small cell BS in sleep state. Then, if the ratio of small cell BSs in sleep state is less than the threshold ratio, small cell BSs with low traffic load are turned off, and traffic load covered by small cell BSs are handed over to neighboring small cell BSs. Otherwise, BSs with high traffic load are turned off, and the traffic load managed by small cell BSs are distributed to neighboring small cell BSs. Using these algorithms, power consumption of BSs can be minimized. In [33], the authors propose dynamic traffic BS switching on/off scheme, in separated network architecture with coverage BS and traffic BSs, where random sleeping is considered. In [34], two sleeping schemes, i.e., random and repulsive schemes, are proposed in a separated network, where vertical offloading between a macro cell and a small cell is assumed. In random scheme, a small  
Figure 1: Separation of data plane and control plane for 5G networks. cell BS is turned off with probability p. In repulsive scheme, small cell BSs which are located within a distance R from a macro cell BS are turned off. Then, they derive maximum p and R, while satisfying the outage constraints. In a call admission control (CAC)-based handover scheme [35], the authors classified the types of handovers in heterogeneous networks. Also, a call is admitted based on the signal-to-noise-plus-interference ratio (SINR), signal level, speed of UE, so unnecessary handover signaling can be reduced. As an extension of the work in [35], the authors in [36] proposed an optimized CAC-based handover based on context information such as reference signal received power (RSRP), speed of UE, traffic load of target cell, and handover admission control. In the optimized CAC-based handover [36], traffic load is directed to some BSs with high traffic load. Then, BSs with low traffic load are turned off so significant BS power consumption can be saved. Figure 2 shows that a UE hands over to a BS with better RSRP in a traditional handover approach, but in CAC-based handover, a UE hands over based on context information, and thus, a high speed UE hands over to a macro cell BS and a low speed UE hands over to a small cell BS. In [37], a prediction-based association control scheme is proposed in order to reduce unnecessary handover in a femtocell environment, without degrading throughput. In [38], reduced early handover is proposed in order to save the energy of BSs, while maintaining acceptable radio link failure, where early handover is carried out based on prediction using RSRP from BSs. In [39], the authors used the concept of information theory for the efficient mobility and resource management in future cellular networks, which are two main problems in cellular networks. Since overhead in the mobility and resource management are basically generated from the random behavior of UEs' movement and resulting uncertainty of UEs' position, the authors developed an information-theoretic approach for utilizing this uncertainty using the entropy or information content of UEs' movement. The authors revealed that the proposed information-theoretic mobility management scheme can reduce overhead by approximately 80 percent.