ARQ_HMAC: An Automatic Repeat reQuest Strategy for Hybrid MAC Protocol in Underwater Acoustic Sensor Networks

In underwater sensor networks, the use of acoustic waves with large propagation delays and narrow channel bandwidth poses significant challenges for Medium Access Control (MAC) protocols in reducing the number of retransmissions caused by packet collisions at the receiver node, thereby improving channel throughput. When packet collisions occur, the transmitting node enters a waiting state and will retransmit the packet at an appropriate time. However, if a node retransmits the packet at inappropriate times, it can lead to further collisions in subsequent transmission attempts. To reduce the number of retransmissions caused by collisions and improve channel throughput, in this study, we propose a Hybrid MAC protocol that combines asynchronous contention-free MAC with Carrier Sense Multiple Access (CSMA) and synchronous Time Division Multiple Access (TDMA) MAC protocols, along with an ARQ strategy applied to underwater communication channels, called the ARQ_HMAC (Automatic Repeat reQuest Strategy for Hybrid MAC) strategy. In the ARQ_HMAC protocol, when a node has a packet to transmit, it senses the channel state and operates according to the Contention-Free MAC protocol. If a packet collision occurs, the node switches to a waiting state and retransmits the packet using the TDMA protocol. The waiting time and the TDMA schedule are calculated based on the ARQ_HMAC strategy to avoid collisions in subsequent retransmission attempts. The simulation results will demonstrate that the ARQ_HMAC protocol helps reduce the number of packet retransmissions caused by collisions, thereby increasing and stabilizing the channel throughput as the arrival rate of new packets increases.