## 4 Backgrounds and Methods
### Handover Parameter Optimization Method  
In this section, we describe how the HH and CIO parameters are calculated. The HH, CIO, threshold, offset and TTT parameters are discussed in [17]. Our self-optimization scheme focuses on the HH and CIO parameters. First, we present the calculation for HH, which is the main parameter associated with the triggering of NCHH (Event A3 condition). For example, in a typical hysteresis adjustment scheme, a large HH value is used for the Event A3 HO. Otherwise, if the difference between the signal quality of the neighboring cells and that of the serving cell is high, and MRN is at the cell boundary of the serving cell, the HO of MRN may begin late, in which case, HH should be decreased. However, if a small HH value were being used up to that point, then the HH value should be increased. Our proposed velocity-based HH adjustment value is defined in Equation (7). If the velocity of the vehicle is high, the starting time of the HO procedure may be too late. Therefore, we must reduce the HH of the LTE system. Otherwise, if HH is set too small, the HO procedure decision will be made too early compared with when the default HH of the LTE system is set. \[HH_{adj}=HH_{def}-\log_{HH_{def}}\frac{(V_{max}-V_{c}+1)}{V_{max}} \tag{6}\]  
where \(HH_{def}\) denotes the default hysteresis for all UEs, \(V_{max}\) is the maximum speed of the vehicle and \(V_{c}\) is the current speed of the vehicle, which includes vehicle context information. The maximum speed of a high-speed train is currently 350 km/h in Japan. According to Equation (6), if the velocity of the train increases, the HH will decrease. Otherwise, if the velocity of the train decreases, HH will incur minimal changes. Let us now look at the CIO calculation for the serving DeNB based on HPI. The CIO parameter provides radio resource management for each cell. To monitor the performance of the HO procedures, we compute the HPI as the ratio of the sum of the dropped HOs, HO failures and HOs that are too late to the total number of the HOs. The total number of the HOs is the sum of all of the HOs that are triggered in the target cell. Equation (7) defines the HPI for our proposed CIO adjustment scheme. \[HPI_{late}=\frac{HO_{dropped}+HO_{failure}+HO_{late}}{HO_{total}} \tag{7}\]  
where \(HO_{dropped}\), \(HO_{failure}\) and \(HO_{late}\) are the number of dropped HOs, failed HOs and late HOs, respectively. The unsuccessful HOs are added to these indicators when the procedure has failed, because of the radio link quality. If the CIO of the neighboring cells is high, the HO of MRN may begin early. An HO that is too early must be controlled. Equation (8) shows the calculation carried out for an HPI that is too early. \[HPI_{early}=\frac{HO_{ping-pong}+HO_{early}}{HO_{total}} \tag{8}\]  
where \(HO_{ping-pong}\) and \(HO_{early}\) are the number of ping-pong HOs and early HOs, respectively. The proposed scheme controls the HH and CIO parameters for Event A3 NCHH. The proposed algorithm is outlined in Algorithm 1. ```
Collect of context information  Collect of RLFs, CIO of Neighbor Cells, HOP and HPP  Calculate HH adjustment value \(HPI_{late}\gets Calculate\)\(HPI\)\(Too\)\(Late\)\(Handover\) \(HPI_{early}\gets Calculate\)\(HPI\)\(Too\)\(Early\)\(Handover\) \(HPI_{threshold}\gets threshold\)\(of\)\(HPIs\) \(H_{adj}\gets calculated\)\(HH\)\(adjustment\)\(value\) \(HH\)\(\leftarrow\)\(handover\)\(hysteresis\)\(for\)\(UEs\)\((same\)\(as\)\(NCHH)\) \(\Delta\gets adjustment\)\(value\)\(of\)\(CIO\) if\(HPI_{late}>HPI_{threshold}\)then \(HH\gets HH-HH_{adj}\) for\(Neighbor\)\(Cell\)\(ID\)\(\to\)\(Neighbor\)\(Cell\)\(List\)do if\(count\)\(RLF\)\(of\)\(Neighbor\)\(Cell\)\(ID>0\)then \(CIO\)\(of\)\(Neighbor\)\(Cell\)\(ID\)\(\leftarrow\)\(CIO\)\(of\)\(Neighbor\)\(Cell\)\(ID+\Delta\).