Sporadic Ultra-Time-Critical Crowd Messaging in V2X

Life-critical warning message, abbreviated as warning message, is a special event-driven message that carries emergency information in Vehicle-to-Everything (V2X). Three important characteristics that distinguish warning messages from ordinary vehicular messages are sporadicity, crowding, and ultra-time-criticality. Specifically, warning messages come only once in a while in a sporadic manner; however, when they come, they tend to come as a crowd and they need to be delivered in short order. This paper puts forth a medium-access control (MAC) protocol for warning messages. The overall MAC protocol operates by means of interrupt-and-access. To circumvent potential inefficiency arising from message sporadicity, we adopt an override network architecture whereby warning messages are delivered on the spectrum of the ordinary vehicular messages. A vehicle with a warning message first sends an interrupt signal to pre-empt the transmission of ordinary messages, so that the warning message can use the wireless spectrum originally allocated to ordinary messages. In this way, no exclusive spectrum resources need to be pre-allocated to the sporadic warning messages. Following the interrupt, for transmissions of ultra-time-critical crowd messages, we employ advanced channel access techniques to ensure reliable message delivery within an ultra-short time in the order of 10 ms.

I. Introduction

With the explosive growth of vehicles on road, safety has become a major concern for future intelligent transportation systems (ITS) [1]. Statistical data show that the number of crashes in the United States is nearly 6 million each year [2]. Vehicle-to-Everything, abbreviated as V2X, is a promising means to cut the road toll [3]. Through V2X, all the entities on the road (e.g., vehicles, road side units and pedestrians) are connected, hence, they can exchange safety messages and cooperate to prevent road accidents or cut down fatality and injury rates when they do occur.