# A Novel Energy-Efficient Contention-Based MAC Protocol Used for OA-UWSN
## 1 Introduction  
Underwater wireless sensor networks (UWSN) are typically used to monitor underwater environments and collect underwater data, such as marine data collection, pollution monitoring, marine exploration, assisted navigation, and tactical monitoring [1; 2; 3; 4]. The vast areas to be detected in marine environments result in sparse deployment of the network and widespread use of mobile sensors. In order to cover the entire target area, the size of a network is generally large, and the topology is usually based on multi-hop wireless [5]. Three ways are normally employed to transfer data between UWSN nodes, namely, underwater electromagnetic communication, underwater acoustic communication, and underwater opticalcommunication. Electromagnetic waves attenuate very quickly in water, so they are rarely used [6]. Underwater acoustic communication is the most mature technique, because acoustic signal attenuation underwater is small and thus can achieve a relatively long-distance transmission (usually used for UWSN data transmission), but the bandwidth of acoustic communication is narrow and the delay is long, which is not suitable for large-capacity data transmission such as images and video [7]. Underwater optical communication has the advantages of high speed and low power consumption [8], and the attenuation of underwater blue-green light is relatively low, enabling underwater short-distance and large-capacity data transmission [9]. In recent years, combining the advantages of acoustic communication and optical communication, a new underwater data transmission mode has been proposed [10; 11]. The routing algorithm of the design in Reference [12] improved the lifetime of underwater acoustic communication nodes. Compared to acoustic communication, optical communication has inherent advantages in node lifetime. Reference [13] proposed a solution for the combined use of acoustic communication and optical communication that addresses the bandwidth limitation of the acoustic channel while achieving optical alignment through acoustic position, thereby performing optical communication. Reference [14] proposed that autonomous underwater vehicles (AUVs) use long-distance acoustic communications, nodes using high-bandwidth optical communications, and a data transmission mode combining two communication modes. Compared with references [13; 14], we previously proposed and designed a new underwater networking method: A hybrid optical-acoustic underwater wireless sensor network (OA-UWSN) [15]. Using optical communication for high-speed and short-distance data transmission, and using acoustic communication for control commands and node positioning, the OA-UWSN enables solving the problem of high-speed and low-cost wireless transmission of real-time or quasi-real-time video and images in marine information detection, while keeping system power consumption small. In a wireless sensor network, a media access control (MAC) protocol is critical, and is used to prevent collisions when more than two transmissions occur [16]. At present, the network layer routing protocol has only been tested in simulations, such as the multi-level routing protocol for acoustic-optical hybrid underwater wireless sensor networks (MURAO) routing protocol [17]. The existing underwater wireless sensor network MAC protocols are only for a single transmission medium network, and there is no mature MAC protocol applicable to an underwater optical-acoustic hybrid wireless sensor network. At present, MAC protocols adopted by UWSNs based on underwater acoustic communication have been studied for years, generally falling into two types: A MAC protocol based on time slot allocation and a MAC protocol based on contention. References [18; 19] proposed that one node be responsible for scheduling the allocation of time slots across the entire network.