Research Article ARS: An Adaptive Retransmission Scheme for Contention-Based MAC Protocols in Underwater Acoustic Sensor Networks Thi-Tham Nguyen and Seokhoon Yoon Department of Electrical and Computer Engineering, University of Ulsan, Ulsan 680-749, Republic of Korea Correspondence should be addressed to Seokhoon Yoon; seokhoonyoon@ulsan.ac.kr Received 11 August 2014; Accepted 13 January 2015 Academic Editor: Nianbo Liu Copyright © 2015 T.-T. Nguyen and S. Yoon. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Due to the limited capacity and high propagation delay of underwater communication channels, contention-based media access control (MAC) protocols sufer from a low packet delivery ratio (PDR) and a high end-to-end (E2E) delay in underwater acoustic sensor networks due to the reliance on packet retransmission for reliable data delivery. In order to address the problem of low performance, we propose a novel adaptive retransmission scheme, named ARS, which dynamically selects an optimal value of the maximum number of retransmissions, such that the successful delivery probability of a packet is maximized for a given network load. ARS can be used for various contention-based protocols and hybrid MAC protocols that have contention periods. In this paper, ARS is applied to well-known contention-based protocols, Aloha and CSMA. Simulation results show that ARS can achieve signiicant performance improvement in terms of PDR and E2E delay over original MAC protocols. 1. Introduction Underwater acoustic sensor networks (UASNs) have received growing interest due to their potential application to oceano- graphic data collection, environment monitoring, undersea exploration, disaster prevention, assisted navigation, and tactical surveillance [1, 2]. Unfortunately, establishing an efective UASN brings about new challenges due to unique characteristics of the underwater acoustic communication channel. First, the underwater acoustic communication channel has a high propagation delay due to the low speed of acoustic signals, which is approximately 1500 m/s, ive orders of magnitude slower than radio waves. Second, the available bandwidth for an acoustic channel is limited, which leads to a low data rate, typically only tens of kilobits per second [1, 3, 4]. hird, the high bit error rate is another challenge on an underwater acoustic communication channel [1]. Media access control (MAC) protocols for UASNs have been extensively studied to mitigate the limitations of under- water communication channels. Among a lot of MAC pro- tocols that have been studied for UASNs, contention-based MAC protocols, most of which are based on Aloha [5–8] and CSMA [9–14], have particularly received a great deal of attention due to their low complexity and high applicability in UASNs [5–17]. It has also been shown that a simple contention-based MAC protocol can achieve acceptable throughput and low latency with a low network load without requiring time synchronization [14, 16]. Contention-based MAC protocols for a UASN can be further classiied into handshake-based and random access- based protocols. here have been a lot of studies on handshake-based protocols [10, 15, 16, 18, 19] that attempted to address the long propagation delay in UASNs. However, the exchange of control packets causes a long packet delay, and control packets also have a long preamble, which leads to degraded network performance [20]. As a result, those protocols are not appropriate for applications that require a low delay. here have also been a considerable number of studies on random access-based MAC protocols in UASNs [5, 6, 8, 9, 12]. A drawback to random access-based MAC protocols comes from their reliance on packet retransmission. More speciically, they depend on retransmission for reliable data delivery, which is suitable for terrestrial wireless networks. However, in a UASN, packet retransmission can quickly Hindawi Publishing Corporation International Journal of Distributed Sensor Networks Volume 2015, Article ID 826263, 15 pages http://dx.doi.org/10.1155/2015/826263