MAC Scheduling for High Throughput Underwater Acoustic Networks Yang Guan Chien-Chung Shen Department of Computer and Information Sciences University of Delaware, Newark, DE, USA {yguan,cshen}@cis.udel.edu Justin Yackoski Intelligent Automation, Inc. Rockville, MD, USA Email: jyackoski@i-a-i.com Abstract—Underwater acoustic networks (UWANs) have emerged as the primary tool to monitor and act upon the well-being of marine environment. However, the significantly slower propagation speed of acoustic signals, in contrast to RF signals, introduces the spatio-temporal uncertainty, which makes existing medium access control (MAC) solutions for terrestrial RF wireless networks not suitable for UWANs. In this paper, we investigate transmission scheduling for time- based MAC protocols and design scheduling algorithms that take advantage of the long propagation delay of acoustic signals to facilitate concurrent transmission and reception of acoustic communications. Specifically, we specify the constraints that MAC protocols need to satisfy to avoid collisions, and model these constraints into a conflict graph. We show that the scheduling problem is equivalent to the Traveling Salesman Problem (TSP) over the conflict graph, and demonstrate that simple heuristics can provide significant network throughput improvement. I. I NTRODUCTION More than 70% of the Earth surface is covered by water, and underwater acoustic networks (UWANs) have emerged as the primary tool to monitor and act upon the well-being of marine environment for operations such as environmental monitoring, resources exploration, and early disaster warning [1], [2]. Since radio frequency (RF) electromagnetic signals do not propagate well in the underwater environment, UWANs use acoustic wave as the communication carrier [3], which differs fundamentally from RF signals in several ways. Foremost, the propagation speed of acoustic signals (1500 m/s) is roughly five orders of magnitude slower than that of RF signals. In addition, an effective transmission range of acoustic signals can reach several kilometers, which is much longer than that of typical Wi-Fi networks. However, in the state-of-the-art, the bandwidth of acoustic communications is relatively low, only accommodating transmission rate of less than 50 kbps. These characteristics challenge the networking protocols of UWANs. The major function of a Medium Access Control (MAC) protocol over a shared channel is to coordinate the transmis- sions from different nodes to reduce the chance of collisions. In RF wireless networks, due to the negligible propagation delay, concurrent transmissions by two (or more) nearby nodes on the same channel nearly always collide. Therefore, classical MAC protocols mitigate collisions by enforcing no more than one transmitter within a receiver’s carrier sense range to transmit at any time. B Receive #1 A->B Receive #3 C->B C Send #3 C->B A Send #1 A->B Time 1 2 0 3 4 Collision B Receive #1 A->B Receive #3 C->B C Send #3 C->B A Send #1 A->B Time 1 2 0 3 4 (a) (b) (c) B Send #2 B->C Send #4 B->A Receive #1 A->B Receive #3 C->B Send #9 B->C Receive #7 C->B Receive #8 A->B Receive #10 C->B Interference from #5,#6 C Send #3 C->B Send #5 C->A Send #7 C->B Receive #2 B->C Send #10 C->B Receive #6 A->C Receive #9 B->C Interference from #1,#4 Interference from #8 A Send #1 A->B Send #6 A->C Receive #4 B->A Send #8 A->B Receive #5 C->A Interference from #2 Interference from #3 Interference from #7,#9 Interference from #10 Time 1 2 0 3 4 5 6 7 8 (d) Fig. 1. (a) Example topology with propagation delays; (b) a schedule that causes collision due to Spatio-Temporal Uncertainty; (c) and (d) are both collision-free schedules [5]. In contrast, the propagation delay of acoustic signals in UWANs becomes much more significant, which can be equal to or even exceed the transmission time of data frames. When coordinating the transmissions of MAC-layer frames, the distance between different transmitters and the receiver (or propagation delay) should be taken into account so that signals arriving from different transmitters will not overlap at the receiver around the same time. Such issue is termed the Spatio-Temporal Uncertainty [4]. Fig. 1(a) depicts a 3- node UWAN topology, where the integer next to each link denotes its propagation delay in unit of time. Figs. 1(b) and 1(c) depict two different schedules of two transmissions A→B and C→B, each with a transmission time of one time unit. The schedule depicted in Fig. 1(b), although valid for terrestrial RF networks as nodes A and C do not transmit simultaneously (i.e., exclusive access), results in collision at the receiver node B in the context of UWANs as the two acoustic signals arrive at the receiver node B at the same time. In contrast, in Fig. 1(c) where nodes A and C transmit simultaneously, no collision occurs as the propagation delay interleaves two arriving signals at receiver node B. Due to the non-negligible propagation delay incurred in UWANs, exclusive access is actually not necessary for col- lisions avoidance. Instead, the successful reception of a trans- mission by the intended receiver must only be separated in time from the reception of any interfering signals by that