Feedback-Based Hidden-Terminal Mitigation for Distributed Scheduling in Cellular V2X

Hidden terminal situations, in which two transmitters may interfere at a single receiver, but the transmitters cannot notice each other's interference, can have severe effects. Traditional access technologies use RTS/CTS to remedy this. This is infeasible in vehicular scenarios, because the broadcast communication would lead to many RTS/CTS dialogues and every potential receiver might not be known to the transmitter. Hence, there are no mitigations in IEEE 802.11p, ITS-G5 or LTE-V2X mode 4.In this paper, we describe a method to leverage the periodicity of the semi-persistent scheduling in LTE-V2X mode 4 to proactively consider and reactively detect hidden terminal problems. Our approach uses the padding bits of periodic messages, e.g., CAMs, to indicate which resource has been successfully decoded by the terminal. This information can be used by terminals to detect whether its own transmissions have not been decodable by others, and react to this situation by selecting new resources. Additionally, we present an extended candidate resource selection procedure that considers these acknowledgements to extend the station's sensing range. Due to semi-persistent scheduling, already acknowledged resources are likely to result in hidden terminal situations in future. Our approach significantly outperforms standard LTE-V2X mode 4 in our evaluation.