# Concurrent Transmission Based on Distributed Scheduling for Underwater Acoustic Networks
## 2 Related Work  
Many MAC protocols have been proposed for underwater acoustic networks [1; 2; 3]. In [PERSON] et al. [2], these protocols were divided into three categories: contention-free, contention-based, and hybrid protocols. Contention-free MAC protocols divide the spectral resources into discrete channels and separately assign them to nodes to avoid contentions. FDMA assigns different frequency bands to individual users. However, the limited bandwidth of underwater acoustic channels greatly restricts the scale of network. To overcome this problem, orthogonal frequency division multiple access (OFDMA) techniques [11; 12] for underwater acoustic networks were proposed to improve the use of the limited bandwidth. OFDMA allows multiple user symbols to transmit simultaneously using different subcarriers with mutually orthogonal overlapping frequency bands so that more users can be supported. CDMA [13; 14] is a physical and MAC layer technique that allows multiple users to communicate simultaneously in the same frequency band by spreading the user messages with pseudo-noise (PN) codes. The CDMA-based MAC protocols in underwater acoustic networks mainly focus on spreading code assignment, power control and energy efficiency. TDMA allows users to occupy the whole channel in non-overlapping time intervals. It is simple in implementation, so it is widely used in practical underwater acoustic networks. However, due to the large propagation delay in acoustic channels, the conventional TDMA protocol has lower channel use in underwater acoustic networks. Therefore, some modified TDMA protocols were proposed to improve the efficiency of the conventional TDMA through temporal and/or spatial reuse [15; 16; 17; 18; 19]. In the contention-based protocols, all nodes compete to assess a shared channel, which means that a node has the chance to exploit the full bandwidth of the channel when it wins in the contention, but will lose the chance for communication in the current transmission cycle when it fails in the contention. The contention-based protocols can be further divided into random access and handshaking protocols [2]. Random access protocols allow a node to transmit its data when needed. The ALOHA based protocols are typical random access protocols. In the basic version of ALOHA, a node sends data whenever it needs without any control. It is simple and easy to implement but many collisions occur when the traffic load is heavy. Some variances based on ALOHA have been proposed to improve its performance in underwater environments, such as Slotted ALOHA, propagation delay tolerant ALOHA, ALOHA with carrier sense, and ALOHA with advance notification [1; 2; 3]. Another type of widely used random access protocol is Carrier Sense Multiple Access (CSMA), in which all nodes have to sense the channel before sending data to reduce collisions. Improvements can be achieved by optimizing sensing duration, back-off window, scheduling, enabling interleaving, and energy-efficiency [1; 2; 3]. Without careful transmission controls, collisions become severe in the ALOHA- and CSMA-based protocols as the traffic loads increase. To reduce collision, handshaking protocols use channel reservation to control the transmission of each node, in which a node has to capture the channel by handshaking before sending data. Floor Acquisition Multiple Access (FAMA) and Multiple Access Collision Avoidance (MACA) are two of the earliest handshaking protocols that use RTS and CTS for handshaking. New protocols were proposed based on FAMA and MACA by leveraging long propagation delays, improving handshaking efficiency, and improving handshaking success [2]. Among these improvements, using the long propagation delay to accommodate concurrent transmissions by scheduling or reservation is an effective method to improve overall performance.