# FD-LTDA-MAC: Full-Duplex Unsynchronised Scheduling in Linear Underwater Acoustic Chain Networks
## 1 Introduction  
A recent research into underwater acoustic communication, sensor and acoustic modem technologies has paved the way for various undersea and ocean operations [1, 2, 3, 4, 5]. Emerging application areas include oceanographic exploration for marine life, archaeological studies, and marine search and rescue missions. Other important applications, such as improved offshore petroleum exploration, monitoring, and control of underwater pipelines, border and military operations, fish farming, freshwater reservoir management, and tsunami and sea quake early warning systems [1, 2, 3, 4, 5] are also being explored. Monitoring subsea oil and gas infrastructure is a key application area of underwater acoustic sensor technology. This is because there are a number of underwater pipeline networks spanning long distances such as the Langleed pipeline in the North Sea measuring about 1200 km [6]. Most of these pipelines carry petroleum products. Timely detection of leakages and corrosion along these pipelines is critical in order to avoid financial loss and, more importantly, prevent water body pollution caused by oil spillage. Thus, for effective live monitoring of these pipelines, a multi-hop linear network topology is employed whereby packets are relayed from a source nodes via the neighbouring nodes to one or more sink nodes. A typical multi-hop topology linear underwater pipeline sensor network based monitoring system is shown in Figure 1.  
Acoustic waves are preferable for underwater communication because they propagate much further than electromagnetic and optical waves [7]. Acoustic systems can also operate with lower transmission power compared to electromagnetic and optical systems [1]. However, establishment of communication among nodes underwater is a challenging task because of the complicated underwater channel characteristics, slow propagation of sound waves, and limited usable frequency bandwidth [3, 8, 9, 10]. These challenges (notably longpropagation delays and low available bandwidth) have made designing medium access control (MAC) protocols for underwater networks difficult [1; 7]. This has also made the traditional MAC approaches unsuitable or only able to provide poor network performance. Various MAC protocols that operate in a half-duplex fashion have been developed in order to improve network performance in UANs. Orthogonal access schemes, such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), and Space Division Multiple Access (SDMA), involve the division of resources (time, frequency, code, and space) into sub-resources to enable collision-free channel access for the network nodes [12]. Alternative approaches to sharing a single channel among a group of users are either scheduling or contention based. Contention based schemes utilise carrier sensing, handshaking, or random access techniques [12] to access a shared channel. However, problems of quality of service (QoS) and energy efficiency still persist, mainly due to long propagation delays and limited available bandwidth in the underwater channel [13]. These network performance problems become more evident in multi-hop UANs. Time-based synchronisation schemes may be an option for short term applications, however, maintaining synchronisation is challenging in underwater networks and may incur significant overheads and, thus, makes synchronisation-based access techniques less viable. In the same vein, long propagation delays also create some uncertainty around channel idle/busy status prediction, which reduces the effectiveness of carrier sense protocols in UANs and this is amplified in multi-hop UANs. Additionally, handshaking techniques as employed in Request-To-Send/Clear-To-Send (RTS/CTS) based protocols [14; 15; 16; 17] are also highly impacted by long propagation delay, since they can create significant idle time on the channel challenging their suitability for multi-hop UANs.