Bigger is better — Combining contention window adaptation with geo-based backoff generation in DSRC networks

The vision of safer transportation is strongly driven by the introduction of Vehicular Safety Communications (VSC) to enable new cooperative safety applications. In highly dense traffic scenarios, however, the current Dedicated Short Range Communications (DSRC) technology is expected to face serious performance problems due to simultaneous transmissions making packets to collide with each other. In this paper, we first analyze the sources of packet collisions. The analysis reveals a significant amount of simultaneous transmissions as vehicles have chosen the same backoff counter, especially in the close vicinity, which is the most critical area with respect to safety. Based on these observations, we then introduce a new concept for DSRC backoff generation called geo-backoff. It implements two countermeasures: First, we increase the Contention Window (CW) to reduce the probability of simultaneous transmissions in general. Second, we exploit geographical information for generating the current backoff counter to further reduce the probability of packet collisions at short (critical) ranges. We analyze our concept from a traditional TX-RX perspective (latency) as well as an RX-centric perspective (update delay). The simulation results indeed have shown that geo-backoff is able to improve the communication performance, but the improvement is mainly dominated by just increasing the CW.