# Optimization of Pico-eNB Tx Power and the Effects of Picocell Range Expansion in Multiband HetNet
###### Abstract  
The use of heterogeneous networks (HetNets) that combine macrocells and picocells in the same coverage is effective in increasing system capacity and improving user throughput. The use of high carrier frequency bands is also expected to help achieving higher data rates because it promises vast amounts of signal bandwidth. Therefore, multiband HetNets with picocells operating at high carrier frequency bands have attracted significant attention with the aim of increasing system capacity and achieving a high user throughput in fifth-generation mobile systems and beyond. In HetNet deployments, a picocell range expansion (CRE) technique that virtually expands the picocell coverage is well known to allow more user equipment (UE) to access the picocell providing a fixed cell selection offset (CSO) for all UE. Thus far, there has not been sufficient research on optimizing the transmission (Tx) power of pico-evolved node Bs (eNBs) operating at high carrier frequency bands in multiband HetNets. In addition, the effects of CRE in multiband HetNets have not been clarified. In this paper, we first investigated the optimal Tx power of pico-eNB in a multiband HetNet combining macrocells  
which picocells with low-power evolved Node Bs (eNBs) are installed within the same coverage of a macrocell with high-power eNB [8; 9; 10; 11; 12; 13; 14]. Resource allocation and user equipment (UE) association are an essential issue in HetNets. In [15], a resource allocation algorithm based on automatic differentiation is proposed to solve the problems of frequency reusing, resource allocation, and user association jointly. In [16], mobility-based proactive resource scheduling strategy utilizing the predicted information of UE mobility, network residual frequency bandwidth is investigated while maintaining QoS requirements. Traffic offloading in HetNets also has important research issues. In [17], a new traffic offloading scheme that combines big data and machine learning is proposed for 5G as well as device-to-device communications. From the perspective of coexisting network coverage, 5G NR NSA has the same configuration as that of HetNets. The use of high carrier frequency bands is expected to achieve higher data rates because it promises a vast amount of signal bandwidth. Therefore, multiband HetNets with picocells operating at high carrier frequency bands have attracted significant attention with the aim of increasing the system capacity and/or achieving higher data rates in 5G mobile systems and beyond [18; 19; 20; 21; 22; 23; 24; 25; 26]. For example, the use of millimeter wave (mmWave) bands has attracted a lot of attention to increase the system capacity and/or achieve a higher data rate. Furthermore, massive multiple-input and multiple-output (MIMO) and mmWave technologies provide vital means to significantly enhance the spectral efficiency and improve transmission performance for 5G HetNets [21]. In [27], an MIMO antenna for 5G mmWave applications with wideband and high gain is demonstrated. In [28], a 5G MIMO antenna covering 28 GHz bands integrated with 4G is investigated and demonstrated to enhance high gain and isolation. In [29; 30], a multiband HetNet using three-sector picocells with three-dimensional beamforming operating at a 28 GHz band is proposed, and the average and 5-percentile user throughput were clarified using system-level computer simulations. For HetNets, it is important for UE to determine whether communication with macro- or pico-eNBs provides better access, which is referred to as cell selection. Another important issue in HetNet deployment is to ensure that the picocells serve enough UE. The UE is normally connected to a cell with a stronger downlink reference-signal received power (RSRP) or with a higher signal-to-interference plus noise ratio (SINR). Therefore, the connection ratio of macro-eNBs is greater than that of pico-eNBs because the transmission power is larger.