  Most V2X applications/services are supported by the continuous exchange of
broadcast messages. One of the main challenges is to increase the reliability
of broadcast transmissions that lack of mechanisms to assure the correct
delivery of the messages. To address this issue, one option is the use of
acknowledgments. However, this option has scalability issues when applied to
broadcast transmissions because multiple vehicles can transmit acknowledgments
simultaneously. To control scalability while addressing reliability of
broadcast messages, this paper proposes and evaluates a context-based broadcast
acknowledgement mechanism where the transmitting vehicles selectively request
the acknowledgment of specific/critical broadcast messages, and performs
retransmissions if they are not correctly received. In addition, the V2X
applications/services identify the situations/conditions that trigger the
execution of the broadcast acknowledgment mechanism, and the receiver(s) that
should acknowledge the broadcast messages. The paper evaluates the performance
of the context-based broadcast acknowledgment mechanism for a Collective
Perception Service. The obtained results show the proposed mechanism can
contribute to improve the awareness of crossing pedestrians at intersections by
increasing the reliability in the exchange of CPM messages between vehicles
approaching the intersection. This solution is being discussed under IEEE
802.11bd, and thus can be relevant for the standardization process.
