## 1 Introduction  
In recent years, train passengers have been transferring increasing amounts of data via their mobile devices. In addition, online applications targeted at mobile users are also proliferating. Consequently, a means of providing broadband mobile communication that satisfies the communication demands of passengers in high-speed trains is essential. To address this need, we propose a mobile relay node (MRN)-based network architecture for communications on high-speed trains. Using this network architecture, Long-Term Evolution (LTE) can achieve high data rates to support multimedia applications, such as Voice over IP (VoIP), video gaming and video streaming, over wireless networks for access anytime and anywhere on mobile broadband networks. The data communication system used by the railway network has been described as facilitating next-generation railway networks based on LTE technology [1]. An LTE network comprises many interconnected Evolved Node Bs (eNBs), which are components of the Evolved Universal Terrestrial Radio Access Network (E-UTRAN). eNBs are connected to the mobility management entity (MME), serving gateway (S-GW) and packet data network gateway (P-GW) in the Evolved Packet Core (EPC) network to support various kinds of services, such as signaling, handover (HO) and security [2]. Long-Term Evolution Advanced (LTE-A) is a developing standard that can satisfy International Mobile Telecommunications (IMT) advanced requirements for 4G mobile networks. Currently, the LTE-A standard working group is considering adoption of mobile relays to support data services in high-speed rail systems [3]. MRN is a feature enhancement for next-generation networks. In a system that utilizes MRN, the user equipment (UE) and the vehicle's passengers do not use the HO; instead, only the MRN executes the HO, using the external antenna of the vehicle [4,5]. However, in LTE-A Release 12, the mobile relay HO reuses the existing UE HO procedure [5]. The HO and MME of the LTE network are unsuitable for high-speed vehicles. Moreover, in vehicle cabins, the effects of high penetration loss, Doppler frequency shift, reduced HO success rate and increased power consumption affect passenger devices [5]. Furthermore, because of rapid movement in overlap areas, increased radio link failures (RLFs) and vehicle service interruptions occur. Many studies on the 3 rd Generation Partnership Project (3 GPP) LTE system have focused on self-organizing networks (SONs) to minimize capital expenditure (CAPEX) and operating expenditure (OPEX) and to maximize operator revenue through automatic network management [6]. This self-configuration and self-optimization comprise dynamic networks. Note that the HO procedure in the network-controlled HO of the 3 GPP LTE system is closely connected to the self-optimization that collects measurement information from the UE and eNB; it then automatically tunes the configuration data to optimize the network. The HO parameter optimization is part of self-optimization. The SON automatically optimizes HO parameters to increase HO performance. Thus, self-optimization schemes can dynamically choose HO parameters in wireless networks [7,8]. To reduce the number of RLFs and connection interruptions that occur during HO procedures in broadband mobile communication systems with MRNs, we dynamically tune the HO hysteresis and cell individual offset (CIO) based on the velocity of the train and the HO performance indicator (HPI) of the cell. In [9], two HO schemes that adjust HO parameters to improve network performance are proposed. The first scheme is a rapid HO algorithm based on UE velocity. In this scheme, the HO hysteresis (HH) and time-to-trigger (TTT) are adjusted to ensure the HO success rate and communication quality at velocities between 0 km/h and 350 km/h. The second scheme focuses on TTT optimization. In [10], an LTE-based cell array solution that supports high throughput and continuous multimedia service is proposed.