Efficient Data Traffic Forwarding for Infrastructure-to-Infrastructure Communications in VANETs

In this paper, we consider roadside infrastructure-to-roadside infrastructure communications in a vehicular ad hoc network. A remote roadside unit (RSU), which does not have connection to the backbone network, needs to send its data traffic to a central RSU (which has backbone connection) by using help from vehicles passing by. Cost is assigned to information transmission energy consumption, as well as possible violation of a soft delay bound. For each passing vehicle, the remote RSU needs to decide whether or not to ask for help from the vehicle, with a target at a minimal rate of cost. We derive an optimal decision strategy of the remote RSU, which is shown to have a conditional pure-threshold structure, i.e., when a vehicle arrives at the remote RSU, if the queuing delay of the data traffic at the remote RSU is above a threshold, it is optimal for the remote RSU to ask for help from the vehicle, with a condition that the vehicle's speed satisfies a requirement. We also provide a method that can theoretically derive the threshold. The conditional pure-threshold structure makes our derived strategy very easy to implement with very low computation complexity.

I. Introduction

A vehicular ad hoc network (VANET) can support the communications among roadside units (RSUs, the infrastructure) and vehicles. By providing safety messages, road conditions, and commercial services, VANET is essential to make a safe, intelligent, and convenient transportation system [1]–[6]. In the literature, VANET has been well investigated, mainly in two research directions: vehicle-to-infrastructure (V2I) communications and vehicle-to-vehicle (V2V) communications. It is in general assumed that all the RSUs are connected to a backbone network via wired links. However, in some cases, some RSUs may not be connected to the backbone network. For example, in remote areas, it is costly to connect all RSUs to the backbone network. Those remote RSUs (i.e, RSUs without backbone connection) need to send their data traffic to central RSUs (i.e., RSUs with backbone connection), and then the central RSUs forward the data traffic to the backbone network. This communication is referred to as roadside infrastructure-to-roadside infrastructure (I2I) communication. A cost effective method to achieve I2I communications is to use passing-by vehicles, which can carry messages from the remote RSUs and forward them to central RSUs on their path.