Adaptive Carry-Store Forward Scheme in Two-Hop Vehicular Delay Tolerant Networks

Stationary roadside units (RSUs) deployed along highways are generally considerably distant with each other, which leads to large areas that are not covered by any RSU. When mobile vehicles enter these areas, their communications are in outage. In order to reduce the outage time, an adaptive carry-store forward (ACSF) scheme is proposed in this letter, where a vehicle is selected to relay the communications for a target vehicle in the uncovered areas. To keep the target and the relaying vehicles to have continuous communications in the uncovered areas, the relaying vehicle should carry and store a sufficient amount of data that are destined to the target vehicle in advance, and the moving speed of the target vehicle can be adaptively changed according to the relaying vehicle. The optimum moving speeds of the target vehicle are solved and the best relaying vehicle is selected in order to minimize the communication outage time of the target vehicle. Numerical results demonstrate that the proposed ACSF scheme can greatly reduce the communication outage time.

I. Introduction

VEHICULAR delay tolerant network (VDTN) is one of the most important applications for the vehicle-to-roadside communications, which have recently gained much research attention [1]–[3]. In a typical VDTN, unconnected stationary roadside units (RSUs) are deployed along highways. Due to deployment cost, the RSUs are usually separated by a considerable distance. Once a vehicle leaves the coverage of one RSU, its communication is in outage until the vehicle moves into the coverage of another RSU. The multi-hop relaying technique can be used to extend the coverage area of an RSU [4], [5]. However, throughput will quickly diminish to zero as the number of hops increases [6]. Besides, multi-hop routing requires much overhead due to frequent handovers, which are caused by dynamic changes to the network topology owing to user movement [5]. In this letter, we introduce a message forwarding mechanism for reducing the outage time for vehicle communications in VDTNs, where some passing by vehicles are used as store-and-forward relays and help other vehicles reduce communication outage time.