# Personal Picocell Scheme Using Adaptive Control CRE in Heterogeneous Mobile Networks +
## 1 Introduction  
Fifth-generation (5G) mobile systems are being developed, notably in the third-generation partnership project (3 GPP) standards body, and have been partially launched worldwide, and the 5G coverage area is gradually being expanded. The primary objectives of 5G are to increase system capacity and improve data rates, which is called enhanced mobile broadband (eMBB), and achieve ultra-reliable low latency communication (URLLC) while providing massive machine type communications (mMTC) based on the Internet of Things (IoT) [1; 2; 3; 4; 5]. Two types of 5G systems are approved by the 3 GPP standards body; one is 5G New Radio (NR) with non-standalone (NSA) operation which enables 5G NR deployments using existing fourth-generation (4G) mobile systems, such as long-term evolution (LTE). Another one is 5G NR with standalone (SA) operation, which will provide a complete 5G NR with a 5G core network. The initial phase of5G NR NSA focuses on eMBB using both 5G NR and 4G LTE. URLLC and mMTC are expected to be provided by the use of 5G NR SA, although mMTC has already been developed using 3 GPP release 13 low power wide area technology, such as NB-IoT. From the network-density perspective, small cell strategy and heterogeneous network (HetNet) approaches are important for increasing system capacity and/or for improving data rates. The small cell approach is a straightforward method in which each low-power evolved node B (eNB) in the small cell is directly connected to a core mobile network. This approach can increase the system capacity compared with traditional large-scale macrocell networks because system capacity, i.e., radio resources, is proportional to the number of small cells. Considering control channel operation such as handover for user equipment (UE), the concept of a phantom cell has also been proposed in which the control and user-plane are separated within the same coverage [6]. Consequently, the macro-eNB manages all UE within the overlaid cells, although the UE can connect to a neighboring low-power eNB in a small cell. The HetNet approach is also one of the denser mobile networks, which is comprised of a combination of different cell types [7; 8; 9; 10; 11]. For example, picocells with low-power eNBs are installed within the coverage area of a macrocell, and the purpose is to allow UE to access the picocells even though the UE is within the donor macrocell. Consequently, HetNet can increase the system capacity, especially when the traffic in the macro cell enormously increases. One important issue in HetNet deployments is to ensure that the picocells serve enough UE. One way to do that is the cell range expansion (CRE) technique, which can virtually expand the picocell coverage through the use of a positive cell selection offset (CSO) for the downlink reference-signal received power (RSRP) from the picocell [12; 13; 14]. In a normal case without CRE, the cell selection is carried out for UE based on the comparison between the downlink RSRPs from the macrocell and picocell. With CRE, a positive CSO is added to the downlink RSRP from the picocell to expand the coverage served by the picocell. Therefore, CRE can allow more UE to access the picocell. However, the CRE technique has one drawback in that the downlink RSRP of UE that is forcibly served by the picocell located in the CRE zone is lower because the downlink RSRP from the macrocell is in reality higher than that from the picocell. Consequently, the user throughput of UE served by the picocell in the CRE zone may worsen. Based on this background, to overcome this drawback, we have proposed a personal picocell scheme using adaptive control CRE that can provide an optimal different CSO for each piece of UE [15; 16; 17; 18; 19; 20]. For example, in ref. [15], we presented user throughput in co-channel (single-band) HetNets using a 2.0 GHz band by means of a modulation and coding scheme (MCS) up to a 64-quadrature amplitude modulation (QAM). In ref.