# A Novel Base-Station Selection Strategy for Cellular Vehicle-to-Everything (C-V2X) Communications
## 7 Simulation Results
Figure 11 shows the expected RSS we obtained at middle speed with the choice of the femtocell base station. As shown in Figure 12, there was a small efficiency improvement when compared with a traditional base station selection policy at middle speed. Then, we assumed that mobility speed reached 30 m/s (high speed), and \(\theta_{0}\) was also randomly set. The three different colors represent the different locations where the macrocell and femtocell base stations were located. Figure 13 shows the expected RSS we obtained in high speed with the choice of a macrocell base station. Figure 14 shows the expected RSS we obtained in high speed with the choice of a femtocell base station. From Figure 15, we can see that, compared with the traditional base station selection policy, selection with base station choice was efficient. Therefore, in high speed, the priority of the vehicle selecting the femtocell base station was high. Hence, from Figures 7-15, we can conclude that, when a vehicle made a base station selection, the traditional base station selection policy had difficulty in making a selection at low and middle speeds, and this can be solved by our proposed base station selection policy, which is based on the Markov decision policy. Figure 9: When r = 2 m/s, expected received signal strength (RSS) with base station choice. Figure 11: When r = 4.5 m/s, expected received signal strength of femtocell base station \(RSS_{F}\). Figure 10: When r = 4.5 m/s, expected received signal strength of macrocell base station \(RSS_{M}\). Figure 12: When r = 4.5 m/s, expected RSS with base station choice. Figure 13: When r = 30 m/s, expected received signal strength of macrocell base station \(RSS_{M}\). To compare our proposed base station policy with the traditional base station policy, we randomly distributed 150 vehicles at coverage of femtocell base station. The initial-motion angles of all vehicles  
Figure 14: When r = 30 m/s, expected received signal strength of femtocell base station \(RSS_{F}\). Figure 15: When r = 30 m/s, expected RSS with base station choice. were the same. Since motion speed was different, the expected RSS was different in the next second. We assumed different initial vehicle motion angles when the running trend of all vehicles was the femtocell base station. According to the traditional base station selection policy, the predicted base stations of all vehicles were selected as femtocell base stations. However, since the femtocell base station had an upper limit of 100, there would be 50 random vehicles selecting the macrocell base station. According to our proposed Markov decision policy, we proposed the base station selection sequence and obtained the best choice based on the value function. In Figure 16, the result of the blue line is the average RSS obtained from the vehicle under the traditional base station selection strategy. In the next-second selection, the RSS did not change much, regardless of movement angle. However, with the Markov decision policy, we selected the higher-priority vehicle to prefer the femtocell base station, and the average RSS of the vehicle was improved.