## 4 Simulation Results and Discussion
### Effects of the OIS's Speed on Various Aspects of Performance
#### 4.2.3 Average RSS  
Figure 20 shows the performance of the OIS's average RSS in relation to its speed in various channel environments. In nonfading environments, the toss-and-catch algorithm  
Figure 19: Comparing the performance of the OIS’s handover frequency 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.  
has a better average RSS than the two control groups at various velocities. On average (in terms of the average RSSs of various velocities), the performance of the toss-and-catch algorithm is about 15 dB higher than that of Control Group 2. This is due to the fact that the super mode was applied to the MSBSs, in addition to the assistance of the overload support mechanism of the ASBSs. Therefore, the SBSs selected by the moving train had a larger average effective signal coverage area and thus a higher average RSS.  
As shown in Figure 20b-d, in all fading environments, the toss-and-catch algorithm performed better than the two control groups. As the fading became more severe, while the performance of all three methods declined, the performance of the toss-and-catch algorithm became significantly better than those of the control groups. This is because the effective signal coverage area of the SBS selected by the OIS becomes smaller in response to the increasing severity of fading, and the OIS has a higher chance of selecting an SBS that offers less RSS. Moreover, since the super mode was not established in the SBSs of both control groups, both groups were less able to cover the track paths, which resulted in a significantly lower average RSS performance compared to the toss-and-catch algorithm. Specifically, when Nakagami-3 fading is employed, the performance of the toss-and-catch algorithm decreased by about 5 dB in comparison with nonfading environments, while the performance of Control Groups 1 and 2 decreased by about 13 dB and about 10 dB, respectively. The reason for this is as follows: after the OIS using the toss-and-catch algorithm is connected to a suitable SBS through the score-based selection mechanism and overload support mechanism, it is able to select an SBS that offers more RSS (it has a higher chance of receiving a stronger signal), and as a result, the decline in the average RSS is relatively low. When Nakagami-1 fading is employed, the performance of the toss-and-catch algorithm was about 18 dB lower than that in Nakagami-3 fading channel environments, while those of the control groups decreased by about 24 dB. Therefore, it is evident that even though the OIS's decline in average RSS caused by channel fading  
Figure 20: Comparing the performance of the OIS’s average received signal strength (RSS) 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.  
would further decrease in all three methods, the level of decline of the toss-and-catch algorithm is relatively lower. Furthermore, when Nakagami-1/2 fading is employed, the difference between the performance of the toss-and-catch algorithm and the control groups is significantly larger. For example, at a train speed of 301.5 km/hr, Figure 20c,d show that the performance of Control Group 1 is, respectively, about 26 dB and about 29 dB lower than that of the toss-and-catch algorithm.