Joint Power/Rate Congestion Control Optimizing Packet Reception

In Vehicle Safety Communications (VSC) based on IEEE 802.11p, vehicles establish a mutual awareness of their pres-ence by periodically broadcasting status messages, aka bea-cons. If vehicle density is high and beaconing is not regu-lated, the channel can become congested, impairing recep-tion performance and safety benefit. As a countermeasure, a number of congestion control approaches have been sug-gested, adapting transmit (Tx) power, beacon generation rate (Tx rate), or both. However, in general these ap-proaches did not show what the optimal outcome for con-gestion control would be and how and why their solution would lead to the desired result. In this work, we analyze answers to the first question and provide a methodology for the second. We systematically derive a joint power/rate control strategy for VSC which optimizes reception perfor-mance for a targeted sender-receiver distance. We start by laying out why we consider average (or percentile of) packet Inter-Reception Time (IRT) at the targeted awareness dis-tance to be a suitable metric for our purpose. Then, we analyze a wide range of Tx parameters to identify which combinations optimize reception in a homogeneous scenario. We show that for each sender-receiver distance, there is an optimal Tx power which, unlike the corresponding Tx rate, is independent of node density. In addition, we analyze the Pareto optimal Tx parameter combinations for two groups of vehicles with different target distances adapting at the same time. We show that the majority of these combinations use the same Tx power as identified in the homogeneous case. We conclude that a simple and efficient strategy to opti-mize reception performance is to select Tx power w.r.t. the targeted distance and to adapt Tx rate w.r.t. channel load