# Concurrent Transmission Based on Distributed Scheduling for Underwater Acoustic Networks
## 1 Introduction  
As the need for monitoring the marine environment for scientific exploration, commercial exploitation and coastline protection continues to grow, underwater acoustic communication and networking are receiving considerable attention [1; 2; 3]. Underwater acoustic channels, which are characterized by limited bandwidth, high and variable propagation delay, time-varying multi-path and fading, resulting in high bit error rates and temporary losses of connectivity, creates challenges for underwater acoustic networking [3]. The medium access control (MAC) protocol, which controls the nodes in a network to share a common broadcast channel, is one of the key techniques for underwater acoustic networking. Many MAC protocols for underwater acoustic networks have been proposed [1; 2; 3]. They can be broadly classified into three categories: contention-free, contention-based and hybrid protocols [2]. In the contention-free protocols, a node does not need to compete with other nodes when accessing the medium. Collisions are avoided by assigning different frequency bands, time slots, or spreading codes to different users. Typical contention-free schemes include frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In the contention-based protocols, the nodes compete for the opportunity to access the medium. Because contention causes collisions which significantly reduce the efficiency of the communication, minimizingthe collisions is the key task for contention-based protocols. Besides, the required overheads for the contentions should also be considered and minimized. Random access and handshaking are two major schemes used in contention-based protocols. Usually, contention-free protocols are suitable for continuous transmission, whereas contention-based protocols are more efficient for burst data transmission. To take the advantage of both techniques, the hybrid MAC protocols integrate different medium access techniques so that better performance can be achieved in underwater environments. Handshaking is a technique widely used to avoid collision in contention-based terrestrial networks and was standardized in IEEE 802.11i. However, the conventional handshaking mechanism, which only allows one successful handshake and one pair of nodes to communicate per transmission cycle, becomes less effective in underwater environments due to the long propagation delay of underwater acoustic channels. Exchanging request-to-send (RTS) and clear-to-send (CTS) packets in the underwater environment requires much more time than in terrestrial radio channels, so the transmission efficiency decreases significantly. Many modified handshake-based protocols have been proposed to address this problem. One of the most effective methods is to exploit the opportunity of the temporal and/or spatial reuse of the acoustic channel that is created by the long propagation latency to allow multiple concurrent packets to propagate in the same contention domain [4]. In the previous research, several handshake protocols with temporal and/or spatial reuse techniques have been developed in sender- or receiver-initiated modes for centralized or distributed underwater acoustic networks [4; 5; 6; 7; 8; 9; 10]. However, little attention has been paid to using this technique to implement multiple handshakes and multiple concurrent transmissions in one transmission cycle for distributed underwater acoustic networks. Besides, the sending sequence of the source node affects the transmission efficiency due to the different propagation delays between the source and destination nodes, which has not been mentioned in previous studies. In this paper, we propose a new distributed scheduling method for underwater acoustic networks that supports multiple handshakes and concurrent transmissions in one transmission cycle for one-hop clusters using the temporal reuse technique.