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 simultaneous broadcast transmissions impact delivery rate and communication overhead. A counter-intuitive finding is that the delivery rate of contention-based forwarding increases when fading is significant (i.e. higher frame errors), but at the cost of higher overhead due to packet duplication. Without fading, duplication is low, but more 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.