The Effect of TDMA Frame Size and the Hidden Node Problem on the Performance of a Neighbor-Aware VANET MAC

Despite various proposed Medium Access Control (MAC) approaches, efficient medium access in VANET remains a significant challenge for the broadcast of safety messages. A key contributing factor, especially with the high node mobility apparent in VANET, is the hidden terminal problem. Since the hidden terminal problem is fundamentally a problem of lack of awareness, increased neighbor awareness at the MAC layer has been suggested as a possible solution. An increased awareness of neighboring nodes' slot allocation requires an increased number of TDMA slots. In this paper we evaluate the impact of TDMA frame size in a neighbor-aware MAC. An analytical model is developed to support the simulation results, based on a geographical probability of RF interference and the slot allocation scheme used in the TDMA MAC. The results show that the packet delivery increases with increased TDMA frame size to support neighbor awareness, as the probability for packet collisions decrease. Packet delivery decreases with increases in neighbor count, since more nodes cause packet collisions. The receiver throughput initially increases as TDMA frame size is increased, since more transmission opportunities occur, and packet delivery increases. However, when the TDMA increases beyond an optimal, throughput is adversely affected by underutilization. Despite the decrease in packet delivery ratio, receiver throughput is higher for more nodes.