## 5 Simulation Results
### Performance of RL-Based Joint Beamforming Scheme
As the vehicle speed increases, beam alignment errors frequently occur, which results in a decrease in the link stability time, i.e., the sojourn time in the antenna main lobe. Since the proposed scheme dynamically controls the beam alignment interval as well as the antenna beamwidth, the proposed scheme can increase the link stability time. Hence, the proposed joint beamwidth/interval optimization scheme outperforms other schemes in terms of the average link stability ratio. The beam alignment interval optimization scheme with a fixed beamwidth shows the worst average link stability ratio because the fixed narrow beamwidth increases the beam alignment errors with the vehicle speed. In particular, when the vehicle speed is 90 km/h, the proposed scheme increases the average link stability by about 8.6% and 24.8% in comparison with the conventional beamwidth optimization scheme and the random selection, respectively. Figure 8: Average throughput vs. vehicle speed. Figure 7: The cumulative link stability time of the vehicle on the first lane when \(v=60\) km/h. Figure 11 shows the average link stability ratio as the transmit power of the RSU increases. The average link stability ratio depends on both the beam alignment interval and the beamwidth. As the beam alignment interval becomes longer or the beamwidth becomes narrower, the vehicle will deviate from the main lobe of the beam faster and stay longer in the side lobe of the beam. That is, the average link stability ratio depends on the method of determining the beam alignment interval and beamwidth. In the random selection scheme, both the beam alignment interval and the beamwidth are randomly selected; therefore, the average link stability ratio remains constant regardless of the transmit power. Moreover, for all schemes expect the joint beamwidth/interval optimization scheme without future reward, because the beam alignment interval or the beamwidth is optimally selected, the average link stability ratio exhibits approximately a constant value regardless of the transmit power. However, in the joint beamwidth/interval optimization scheme without future reward, the beam alignment interval and beamwidth are not optimized. Since the scheme tries to increase the instantaneous received power at the vehicle, if the transmit power of the RSU is low, a narrow beamwidth is selected; therefore, the sojourn time in the main lobe decreases. On the other hand, as the transmit power of the RSU increases, a wider beamwidth is selected, which increases the sojourn time in the main lobe. However, if the transmit power is too small or too large, the sojourn time in the main lobe becomes independent of the transmit power. Hence, in the joint beamwidth/interval optimization scheme without future reward, the average link stability ratio linearly increases as the transmit power of the RSU increases within a certain range, [24 dBm, 30 dBm]. The proposed joint beamwidth/interval optimization scheme outperforms other schemes. In particular, when the transmit power of the RSU is 20 dBm, the proposed scheme increases the average link stability by about 8.7% and 21.9% in comparison with the conventional beamwidth optimization scheme and the random selection scheme, respectively.