Abstract
The Society of Automotive Engineers (SAE) J2945/1 standard prescribes a wireless channel congestion control algorithm for periodic Basic Safety Messages (BSMs) in vehicular safety communication. The algorithm is designed to work with the Dedicated Short Range Communication (DSRC) technology as the radio interface, and utilizes transmit (Tx) power control and rate control elements for the periodic BSM transmissions, which are supposed to work in a complementary manner. An equivalent standard is being made for the cellular vehicle-to-everything (C-V2X) communication environment, and it is expected that the J2945/1 congestion control algorithm will be reused. Unfortunately, we find that when transplanted to the C-V2X communication environment, the J2945/1 algorithm exhibits a poor balance between the two control elements under congestion. Specifically, the Tx rate control is pushed to its limit much earlier than the Tx power control. We believe that this is due to the new relation formed between the respective inputs to the control elements, channel busy ratio (CBR) and the vehicle density (VD), induced by the different radio technology used in the C-V2X communication. The consequence of the unbalanced control dynamics is the degraded update delay of neighboring vehicles' kinematics due to the over-exerted rate control, which potentially undermines the driving safety. In this paper, we propose to adjust the rate control function to react to congestion in a more relaxed manner so that the power control contributes more actively to the overall congestion control. With the re-balancing between the control elements, we show that the update delay improves, where larger improvements are obtained in closer, more safety-critical distances from the BSM transmitter. We believe that in future efforts to define the standard congestion control algorithm for the C-V2X environment, the balanced control aspect as explored in this paper should be considered in the design phase.
Highlights
In vehicle-to-vehicle (V2V) communication, the periodic exchange of safety messages facilitates the tracking and short-term prediction of neighboring vehicles’ kinematics, which is indispensable for reducing the collision risk between vehicles
With no standardized congestion control algorithm defined for the cellular V2X environment yet, the effort by Society of Automotive Engineers (SAE) to transcribe the J2945/1 algorithm to the new environment is notable
This paper explores the expected consequences of applying the J2945/1 algorithm to the cellular V2X environment, and finds that the imbalance between the two control elements in the algorithm is aggravated, which causes undesirable consequences
Summary
In vehicle-to-vehicle (V2V) communication, the periodic exchange of safety messages facilitates the tracking and short-term prediction of neighboring vehicles’ kinematics, which is indispensable for reducing the collision risk between vehicles. We first find through our exploration that the congestion level estimations by the power and rate control elements of the transplanted J2945/1 algorithm become ill-aligned in the new C-V2X communication environment. The Tx power lower than in the standard J2945/1 algorithm leads to higher packet error rates at farther distances from the transmitter We discuss how this trade-off is justified in congested roads, from the perspective of the vehicle traffic engineering. It briefly discusses how it can be transplanted to the C-V2X framework, and shows how it performs without the aforementioned modification.
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