A hybrid multichannel MAC protocol with low local delay for vehicular ad hoc networks

In vehicular ad hoc networks (VANETs), minimizing the local delay between vehicles and roadside units (RSUs) is crucial for achieving real-time safety communication. In this paper, we propose a hybrid multichannel medium access control (MAC) protocol with low local delay (LLD-HMMAC) that reduces both local delay and transmission overhead by integrating time-division multiple access (TDMA) and carrier-sense multiple access (CSMA) strategies. Firstly, a system model is proposed to estimate the signal strength of the corresponding RSU and neighboring vehicles by dividing vehicle communication range into three concentric circles, i.e. inner circle, outer circle and extending circle. Secondly, we propose a hybrid MAC structure to ensure the prioritized transmission of safety messages and optimize the negotiation processes of non-safety messages. In the CSMA period, we develop a channel dissemination method to mitigate the cumulative collisions caused by the frequent selection of numerous subcarriers and the same transmission batches. Furthermore, we propose a slot identification technique to reduce the probability of channel resource wastage in an RSU entanglement zone. Numerical and simulation results show that compared to existing protocols under different traffic densities, the proposed protocol can achieve substantially lower local delay in both uplink and downlink communications, improve throughput, and decrease the collision rate.