# Asymmetric Propagation Delay-Aware TDMA MAC Protocol for Mobile Underwater Acoustic Sensor Networks
## 1 Introduction  
Recent advances in mobile underwater acoustic sensor networks (MUASNs) have led to various applications, including ocean sampling, environmental monitoring, undersea exploration, disaster prevention, communication/navigation aid, distributed tactical surveillance, and mine reconnaissance [1; 2; 3; 4; 5; 6]. Nevertheless, there are many challenges in designing MUASNs due to limited bandwidth, time-varying multi-path effects, and long propagation delay [1; 2; 3; 4; 5; 6]. Although these limitations are primarily mitigated by physical layer techniques, it is important to handle the long propagation delay at medium access control (MAC) protocol for high network efficiency. MAC protocols for MUASNs can be classified into two categories: contention-based MAC protocols and contention-free MAC protocols. Since underwater sensor nodes (UNs) compete with each other to occupy the common acoustic communication channel formed by the propagation of the slow sound waves, the contention-based MAC protocols [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21] suffer from the collision risk. The collision probability of the contention-based MAC protocols is increased as the traffic load increases. In high traffic scenarios, packet collisions of MUASNs not only severely degrade network throughput, but also cause intolerably long latency due to uncertain number of retransmissions. On the other hand, contention-free MAC protocols [22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37] are significantly superior to contention-based ones in high traffic scenarios because the medium is orthogonalized in a certain domain. The classical TDMA uses guard time to deal with the propagation delay, which causes unaffordable inefficiency in MUASN [38]. The guard time of the classical TDMA in MUASN might be larger even than the packet duration due to the slow propagation speed of sound, approximately 1500 m/s [39] at sea-water. However, this large propagation delay is being re-interpreted rather positively, and TDMA approaches in MUASN are considered to be one of the most efficient MAC protocols. In particular, spatial reuse MACs [28; 29; 30; 31; 32; 33; 34; 35; 36; 37] take advantage of a large propagation delay for concurrent transmissions without collisions at receivers by considering possible interferences from neighbor nodes. Spatial reuse MACs that operate in a distributed manner [29; 30; 31; 32; 33; 34] make concurrent multiple connections as frequently as possible, and show significantly improved throughput. However, it is not easy to operate these distributed methods in a centralized manner without sacrificing performance. For example, although Chitre's transmission scheduling algorithm in MUASNs possibly achieves higher throughput than other ones in terrestrial networks, optimal schedules can be applied to a topology that has a specific geometry, which could be unacceptable in MUASNs. On the other hand, spatial reuse MACs that have a centralized topology inherit the advantage of contention-free MACs. However, there is still room to for improvement. In the ST-MAC [35], it is assumed that time-delay between any two nodes is shared through a sink node (SN) in a stationary topology, and the transmission schedule is established in a heuristic manner. The GSR-TDMA [36] works in multi-hop stationary topology, and it is not straightforward to optimize in a centralized one-hop topology. Although the BTB-TDMA [37] works in a dynamic topology, the idle interval at a SN is not optimum due to the conservative response time with respect to a beacon packet. Moreover, in previous literature, time synchronization is required, and asymmetric propagation delay is not considered. In spatial reuse MAC protocols, collision is resolved assuming symmetric propagation delay. However, the propagation delay in MUASNs becomes asymmetric in a dynamic topology.