  The vehicular ad-hoc network (VANET) based on dedicated short-range
communication (DSRC) is a distributed communication system, in which all the
nodes share the wireless channel with carrier sense multiple access/collision
avoid (CSMA/CA) protocol. However, the competition and backoff mechanisms of
CSMA/CA often bring additional delays and data packet collisions, which may
hardly meet the QoS requirements in terms of delay and packets delivery ratio
(PDR). Moreover, because of the distribution nature of security information in
broadcast mode, the sender cannot know whether the receivers have received the
information successfully. Similarly, this problem also exists in no-acknowledge
(non-ACK) transmissions of VANET. Therefore, the probability of packet
collisions should be considered in broadcast or non-ACK working modes. This
paper presents a connection-level scheduling algorithm overlaid on CSMA/CA to
schedule the start sending time of each transmission. By converting the object
of reducing collision probability to minimizing the overlap of transmission
durations of connections, the probability of backoff-activation can be greatly
decreased. Then the delay and the probability of packet collisions can also be
decreased. Numerical simulations have been conducted in our unified platform
containing SUMO, Veins and Omnet++. The result shows that the proposed
algorithm can effectively improve the PDR and reduce the packets collision in
VANET.
