GeoMAC: Geo-backoff based co-operative MAC for V2V networks

This paper presents a preliminary design of Geo-MAC, a MAC protocol that exploits spatial diversity in highly mobile wireless networks. It aims to achieve low latency and high reliability, goals that are intrinsic to the success of many envisioned vehicular safety applications. Conventional MAC layers address reliability through ARQ mechanisms that re-transmit messages from the source, if earlier transmissions were not acknowledged. These schemes essentially exploit temporal diversity since retransmissions are only likely to succeed if the channel has improved. GeoMAC exploits spatial diversity, by allowing other nearby nodes to opportunistically forward and retransmit messages. Through a geo-backoff mechanism it uses geographic distance to the destination as a heuristic to select the forwarder most likely to succeed. This mechanism does not require nodes to monitor channel state or position of their neighbors, except for approximate node density, thus enabling their use in highly mobile networks with low coordination overhead. The performance of GeoMAC is evaluated via trace-driven ns2 simulation using packet error measurements from a freeway environment. GeoMac leads to lower delay jitter combined with up to 50% packet delivery rate gains, compared to the AODV and GPSR routing protocols, which also take advantage of nearby nodes for packet forwarding. Spatial diversity is also shown to better utilize available channel opportunities than ARQ mechanisms.