# Optimization of Pico-eNB Tx Power and the Effects of Picocell Range Expansion in Multiband HetNet
###### Abstract
One way to increase the connection ratio of pico-eNBs is the cell range expansion (CRE) technique, which can virtually expand the picocell coverage using a positive cell selection offset (CSO) for the downlink RSRP from the picocell [31; 32]. In a normal case without CRE, cell selection is carried out for UE based on a 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. In this case, the CRE can allow more UE to access the picocell. However, the CRE technique has one drawback in that the downlink RSRP of the UE, which 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 the UE served by the picocell in the CRE zone may deteriorate. A higher-order modulation scheme is a promising technique to increase the data rate within a limited bandwidth. As specified by 3 GPP Release 8, 4G mobile systems first applied quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM), and 64-QAM for symbol modulation of orthogonal frequency division multiplexing (OFDM). Then, 256-QAM was added to the modulation and coding scheme (MCS) of 3 GPP Release 12. In 3 GPP Release 15 for 5G, 1024-QAM was added to the MCS. Nevertheless, the application areas and/or conditions for these post-64-QAM schemes are currently very limited [33; 34; 35; 36; 37]. Furthermore, OFDM-based 4096-QAM has been investigated using link-level simulations for 5G and beyond [38; 39]. In [12], adaptive control CRE technique was proposed in singleband HetNet using 2 GHz band; however, the pico-eNB transmission (Tx) power was fixed. In [26], the throughput of the UE served by the picocell in the CRE zone may deteriorate. In [26], the throughput of the UE served by the picocell in the CRE zone may deteriorate. A key advantage of the proposed scheme is that the UE served by the  
put of singleband HetNet using 4 GHz without CRE technique was investigated on the condition of a given pico-eNB Tx power. In [29], sectorized picocell architecture for multiband HetNet using 2 and 28 GHz bands was proposed; however, the pico-eNB Tx power was fixed and CRE technique was not considered. In [40], fundamental research on optimizing the pico-eNB Tx power was carried out in multiband HetNet using 2 and 4.5 GHz bands; however, CRE technique was not considered. Thus, there is no research on evaluating the performance of multiband HetNet from the viewpoint of both pico-eNB Tx power optimization and CRE activation. That is, how to optimize the pico-eNB Tx power has not been investigated when the CRE is activated in multiband HetNet. On the basis of this background, this paper investigates the relationship between optimal pico-eNB Tx power and a CSO used in CRE technique from the user throughput perspective. We first investigate the optimal Tx power of a pico-eNB in a multiband HetNet combined with macrocells operating at a 2 GHz band and picocells with a wide signal bandwidth operating at a 4.5 GHz band with a new multipath fading channel model. Specifically, we clarify the average and 5-percentile user throughput as a function of pico-eNB Tx power using system-level computer simulations. Then, we investigate the effects of CRE providing a positive CSO for UE given two pico-eNB Tx powers close to the optimal Tx power. Here, we show the average and 5-percentile user throughput as a function of CSO for the two pico-eNB Tx powers. Furthermore, we analyze the use rates of modulation methods including 256- and 1024-QAM in the MCS as a function of the pico-eNB Tx power. This paper is organized as follows: In Section 2, we introduce a multiband HetNet combined with a macrocell operating at 2.0 GHz and picocells with a wide bandwidth operating at 4.5 GHz. In Section 3, we describe the HetNet model and MCS index including 1024-QAM used in system-level computer simulations.