## 4 Simulation Results and Discussion
### Effects of the OIS's Speed on Various Aspects of Performance
#### 4.2.2 Handover Frequency  
Figure 19 shows the performance of the OIS's handover frequency in relation to its speed in various channel environments. According to Figure 19a-d, in all three methods, the handover frequency increased in response to increasing velocities. This is because under higher velocities, the OIS quickly leaves the effective coverage area of each selected SBS, and the handover frequency increases as a result. In nonfading environments, the handover frequencies of the toss-and-catch algorithm and Control Group 1 were significantly lower than those of Control Group 2. This is because the toss-and-catch algorithm and Control Group 1 are assisted by the score-based selection mechanism and are able to select more suitable SBSs; meanwhile, when the channel is not affected by fading effects, the SBSs have a large effective signal coverage area, and the OIS has more SBS options.  
Figure 18: Comparing the performance of the OIS’s connection hold rate in relation to its speed using the three methods in (**a**) nonfading environments; (**b**) Nakagami-3 fading environments; (**c**) Nakagami-1 fading environments; and (**d**) Nakagami-1/2 fading environments.  
When Nakagami-3 fading is employed, the channel was affected by slight fading, and the effective signal coverage area was slightly reduced, but the effects on the handover frequencies of all three methods were not substantial. Specifically, the handover frequencies of the toss-and-catch algorithm and Control Group 1 had increased slightly, while those of Control Group 2 had decreased slightly. For instance, in comparison with Figure 19a, the handover frequencies of the toss-and-catch algorithm and Control Group 1 increased slightly by 0.025 Hz, while those of Control Group 2 decreased by 0.050 Hz. From a theoretical perspective, the handover frequency should increase when each SBS effective signal coverage area that the OIS passes through becomes smaller. However, for Control Group 2, the OIS was less able to identify a suitable SBS for a handover and was disconnected, thus reducing the handover frequency. When Nakagami-1 fading is employed, the toss-and-catch algorithm has marginal changes in terms of handover frequency, whereas those of the two control groups have declined significantly (for example, at a train speed of 603 km/hr, and in comparison with Figure 19b, the handover frequencies of Control Groups 1 and 2 had decreased by 0.038 and 0.024 Hz, respectively). The reason for this decline is the same as when Nakagami-3 fading is employed: that is, the greater fading effects had reduced the handover abilities of the control groups and ultimately resulted in disconnection. When Nakagami-1/2 fading is employed, severe channel fading had reduced the handover frequencies of all three methods, which indicates that there were more disconnections. For example, at a train speed of 603 km/hr, the handover frequencies of all three methods were about 0.035 Hz lower compared to the frequencies presented in Figure 19c. This is because when the effects of fading are more significant, the number of deployed SBSs is insufficient for all three methods.