Sporadic Ultra-Time-Critical Messaging in V2X

Abstract

Life-critical warning message, abbreviated as warning message, is a special event-driven message that carries emergency warning information in Vehicle-to-Everything (V2X). Two important characteristics that distinguish warning messages from ordinary vehicular messages are sporadicity and ultra-time-criticality. This paper puts forth a medium-access control (MAC) protocol for warning messages. To circumvent potential inefficiency arisen from sporadicity, we propose an override network architecture whereby warning messages are delivered on the network of the ordinary vehicular messages. Specifically, 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. To meet the ultra-time- criticality requirement, we use advanced MAC techniques (e.g., coded ALOHA) to ensure highly reliable delivery of warning messages within an ultra-short time in the order of 10 ms. The overall MAC protocol operates by means of interrupt-and-access. We investigate the use of spread spectrum sequences as interrupt signals. Simulation results show that the missed detection rate (MDR) of the interrupt signals can be very small given sufficient sequence length, e.g., when SIR is -32 dB, a 0.43 ms sequence (64512 symbols, 150 MHz) can guarantee an MDR of 0.0001. For channel access, simulation results indicate that coded ALOHA can potentially satisfy the ultra- time-criticality requirements of warning messages. In the stringent scenario where 30 emergency nodes broadcast warning messages simultaneously, the message loss rate can be kept lower than 0.0001 with delay less than 10 ms.