# Performance Evaluation of Routing Protocols for Underwater Wireless Sensor Networks
## 1 Introduction  
UWSNs are currently the most considerable research field. A 3/4 portion of the world is enclosed by water. It contains multiple applications, such as oil, gas, mines, and other minerals [1]. Therefore, the UWSNs are taking the consideration of scientists to explore these things with modern UWSN technology. UWSNs differ from Terrestrial Wireless Sensor Networks (TWSNs) and IoT-based WSNs in many aspects, such as node mobility and communication techniques. TWSNs use radio signals to transmit data from source to destination [2]. In contrast, UWSNs mostly use acoustic communications to transfer data from the sensor node to the base station or sink node. Figure 1 shows the basic structure of the UWSNs. UWSNs and Under-water Acoustic Networks (UWANs) have several unique properties, such as localization, flexibility, and scalability [3, 4, 5]. UWSNs mainly contain multiple sensor nodes, autonomous vehicles, sink nodes, and a satellite base station [6]. All of these communications are done through routing protocols. Still, compared to TWSNs, UWSN routing protocols have many limitations, such as low bandwidth, transmission power, network topology, energy consumption, and high propagation delay [7, 8]. It is important to understand the minimum energy consumption of sensor devices, packet delivery ratio, battery, and transmission loss [9, 10]. One of the fundamental issues in UWSNs is the design of routing protocols. Most researchers are working nowadays to overcome these challenges. They are developingenergy-efficient routing protocols to increase the network lifetime in UWSN environments [11]. The main issue in these routing protocols is focused on the physical layer instead of the network layer, as routing techniques are the most important era. There are three types of UWSN routing protocols: Localization based, localization-free, and cooperative routing protocols. Those protocols in which the sensor nodes already know the sink node's location and itself are called Localization based. Moreover, due to the mobility nature of UWSNs, localization-based routing protocols are not better. Localization-free routing strategies mean GPS signals, which are also not better because, due to the nature of UWSNs, electromagnetic signals do not propagate in these environments. Another disadvantage of these routing protocols is the waste of node resources to obtain information about the sensor node location [12]. Cooperative routing is the technique of using the other sensor node resource to transfer from source to destination. The cooperative method normally transfers data between the sink and sensor node through multiple hops. However, cooperative routing (C.R.) takes the underwater channel advantage for transmitting packets through relay nodes in every hop. Initial C.R. ideas were proposed by [PERSON] [13], who characterized C.R. as a routing method at the physical layer using cooperative transmission. To improve energy efficiency, link reliability, network performance, and throughput, nodes with a single antenna and a limited amount of available energy can use the resources of other nodes to transmit data through the cooperative method. Data are transferred through multiple relay nodes instead of direct communication. The main cooperative routing methods for UWSNs considered in this paper are RACE-SM, Energy-efficient regional-based cooperative routing protocol for underwater sensor networks with sink mobility (EERBCR), an energy-efficient data gathering scheme in underwater wireless sensor networks (EEDG), Improved Cooperative Routing Scheme for UWSNs Using Energy Harvesting (EH-UWSN), Multilayer Sinks and Cooperation-Based Data Routing Techniques for Underwater Acoustic Wireless Sensor Networks (MuLSi-Co),  
Figure 1: The basic structure of UWSNs. and Geographic and Cooperative Opportunistic Routing Protocol for Underwater Sensor Networks (GCORP). RACE-SM improved the end-to-end delay, PDR, and energy consumption of the UWSNs by using the sink mobility technique instead of the single relay node.