Most of the researches pay attention to minimize the end-to-end delay rather than reduce the usage of radio. However, some information is not urgent and radio is a valuable and limited resource. Recently, delay-bounded routing protocol has become a popular issue, whose goal is to deliver messages to the destination within user-defined delay and minimize the usage of radio. To decrease the burden on the environment and save energy, the hybrid-solar vehicle is one of the most prominent solutions. The existing delay-bounded routing protocols do not consider the charge and the energy expenditure factors and only choose the path with the least forwarding times. To improve previous works, we propose a mechanism based on a cost function, which includes forwarding times and power gain, to choose a minimum cost path. Besides, we propose a novel mechanism to deliver message to the destination by the hybrid of data muling (carried by the vehicle) and forwarding (transmitted through radio). In the existing protocol, when the remaining time is not enough the vehicle should forward the message by radio and when the remaining time is enough the vehicle should carry the message by itself. However, forwarding message by radio consumes more energy and hence the energy factor should be added into the consideration. The proposed protocol contains two schemes: the greedy and centralized schemes. In the centralized scheme, the information such as the solar illumination, length of each road segment, and the average velocity of the vehicles on each road segment are collected and then the dynamic programming is used to find the least cost routing path. In the greedy scheme, it only has the knowledge of the average velocity, the solar illumination and the length of all the possible next road segments to choose the next road segment with the least cost. Simulation results illustrate that the proposed routing protocol can save more energy than existing protocols.
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