This paper considers a vehicular system that enables high data rate wireless service for moving vehicle.
In vehicular communications, higher data transmission rate is required in order to meet the needs of vehicles and a directional transmission beam must be used for the reliable communications. Beam alignment is essential when the directional beam is used, but there is a lot of overhead because the vehicle that moves at high speed require more frequent beam realignment. The beam switching technique using initial information of the vehicle can reduce the overhead because the transmitter predicts the position of vehicle and switches the transmission beam sequentially. The scenario, where the road side unit (RSU) deployed along the roadside is trying to deliver the data to the moving vehicle, is considered. There may be a change in speed of the vehicle as it passes through the coverage of the road side unit. As the RSU knows only the initial speed and position of the vehicle, the actual position of the vehicle is different from the vehicle's position predicted by the RSU. Considering the speed variation of the vehicle, an optimal beam design that maximizes the average data rate is analytically proposed. Moreover, the analytic results are very close to the simulation results and the optimal beam design obtained analytically is almost same as the optimal beam design obtained from the exhaustive search.