Energy efficient cluster based hybrid MAC protocol with channel dependent optimized intra-cluster scheduling
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Underwater acoustic sensor networks (UW-ASNs) find place in a wide variety of long term applications such as pollution monitoring, ocean sampling, disaster prevention and tactical surveillance. Since underwater system environment is energy constrained, it is imperative to maximize the network lifetime of UW-ASNs to support these long-term applications. The energy concern is aggravated by the fact that transmit power of underwater acoustic (UW-A) modems are extremely high (2W-20W) thus making it necessary to minimize the loss of energy due to retransmission of the packets lost and reducing control overhead. The objective of this research work was to study various existing medium access control (MAC) protocols for UW-ASNs and to design and implement an energy efficient MAC protocol that would maximize the network lifetime and throughput of UW-ASN while maintaining the reliability of the network. This work proposes a clustered based hybrid MAC protocol with channel dependent optimized scheduling in the intra-cluster phase. This scheduling is accomplished in 2 phases to reduce the communication overhead and computational complexity; (i) in the first phase each member node takes a selfish action (number of time slots required during next cycle) obtained by formulating a Markov's decision process (MDP) with an objective to minimize energy consumption and informs this action to the cluster head, (ii) in the second phase after gathering the action of all its member nodes, the cluster head optimizes the scheduling to maximize throughput of the entire cluster and reduce retransmissions, as a result minimizing energy consumption. The optimization at the cluster head also takes two approaches; (a) a greedy approach which only considers maximizing throughput, (b) along with maximizing throughput, the cluster head also considers the urgency of the member nodes to minimize buffer overflow and improve reliability. Simulations demonstrate that the proposed scheduling scheme with urgency factor achieves a 28% improvement in throughput and improves reliability by upto 25% as compared to the scheduling scheme which does not use MDP and optimization.