# EE-UWSNs: A Joint Energy-Efficient MAC and Routing Protocol for Underwater Sensor Networks
## 1 Introduction  
The water of the oceans, seas, rivers, and lakes cover about seventy percent of the earth's surface [1]. Only less than ten percent of the oceans have been explored, but many areas have not yet been investigated [2]. Recently, ocean exploration is getting more and more common. However, the traditional methods for investigating oceans suffer from many limitations, such as high cost, long delay to receive the outcome, and the difficulty of human existence in such an environment [3]. Therefore, underwater wireless sensor networks are considered an important alternative for studying and discovering the character of the oceans [4]. UWSNs are an important part of the Internet of Things (IoT) technology [5], which is an emerging application of the fifth-generation (5G) technology [6, 7, 8]. A UWSN is a wireless communication network that consists of a number of underwater sensor nodes, a surface station (sink), an on-shore sink, and a satellite (Figure 1). Acoustic communications are used in the underwater environment because they are characterized by their greater efficiency and reliability compared to other communication methods [9]. Nowadays, UWSNs are receiving more attention because of their useful applications [10]. There are challenges encountered by underwater sensor networks, coming from the nature of underwater conditions and the utilization of acoustic links. UWSNs are characterized by high propagation latency, limited available bandwidth, high error rate, and constrained energy, as shown in Figure 2.  
* **Long Propagation Delay:** In the water medium, the velocity of sound waves is around 1500 m/s. It is much slower than radio waves, which travel at the velocity of light (\(3\times 10^{8}\) m/s). Furthermore, several features of the water environment, like depth, temperature, and salinity, have an impact on the speed at which the sound signal spreads. The slow propagation of acoustic waves leads to quite a long propagation latency, even over a limited range [11]. * **Limited Bandwidth:** In comparison with radio networks, the actual bandwidth of the underwater acoustic medium is very low. It is extremely restricted because most acoustic systems work at under 30 kHz. In addition, the available bandwidth of sound media depends on both transmitting range and frequency. Because the bandwidth of sound media depends on the frequency of the sound media, the frequency of sound media depends on the frequency of the sound media. Figure 1: The architecture of an underwater sensor network. Figure 2: The challenges faced by underwater sensor networks. of the channel is limited, the data transmission rate will be low, usually lower than 10 kbps [12; 13]. * **High Error Rate:** The sound link quality is impacted by many things, such as noise, signal attenuation, and multiple paths. The sound channel suffers from several sources of noise (both man-made and ambient). Man-made noise may result from shipping actions and machinery tasks. On the other hand, ambient noise originates from hydrodynamics (e.g., wave movement and storms on the water surface) or biological sources (e.g., seismic risk, the swimming behavior of fishes). If there is no noise, there is still transmission loss caused by the attenuation of the signal. This signal attenuation is a result of the absorption of sound energy and grows with distance and frequency. Multipath propagation mostly originates from reflections from the water surface and the water bottom. In addition, it may be caused by various refracted rays. All these factors give rise to high rates of error in data transmission [14; 15]. * **Constrained Energy:** One of the major challenges when deploying underwater sensor networks is the limitation of energy resources of the sensor nodes. The reason is that they are powered by batteries. The sensor nodes expend their power when they receive, transmit, process, and overhear information. In underwater environments, it is difficult to replace or recharge the batteries of the sensors [16; 17].