On The Impact Of Fading And Interference On Contention-Based Geographic Routing In Vanets

Geographic routing is a method to distribute information in vehicular ad hoc networks over multiple hops. The basic principle is to forward a message to a neighboring station in the network that is closest to the destination area. In contention-based algorithms this next neighbor selection is done in a distributed way, where potential next hop candidates delay their forwarding attempt in a way that the candidate with the largest progress to the destination forwards the message first; other candidates cancel their scheduled retransmissions then. This elegant principle unfortunately depends on a consistent timing and the absence of transmission failures, which cannot be expected in vehicular networks based on IEEE 802.11.This paper illustrates how the re-ordering of channel access attempts through the 802.11 MAC layer and the transmission errors caused by frame collisions impact delivery rate and communication overhead. A counter-intuitive finding is that the delivery rate of contention-based forwarding increases under fading conditions (i.e. higher frame errors), but at the cost of packet duplication and thus higher overhead. Without fading, duplication is low, but more end-to-end delivery failures occur.In this paper, we explain the reasons for this behaviour analytically and illustrate it in simulations. Furthermore, we propose a randomized contention-based routing algorithm that balances the trade-off between high end-to-end delivery and low overhead better than the deterministic algorithm and shows a significant performance increase independent of whether fading or frame collisions are present.