# EE-UWSNs: A Joint Energy-Efficient MAC and Routing Protocol for Underwater Sensor Networks
## 2 Related Works
### Energy-Aware Routing Protocols for UWSNs  
The Focused Beam Routing (FBR) protocol was proposed by [PERSON] et al. in [33]. The FBR protocol is considered a position-based routing protocol, where each node must know its location and the position of its final destination, but the intermediate nodes' positions are not needed. The protocol determines the transmission area using the cone angle concept. Therefore, the reduction of unnecessary transmission results in minimizing power utilization. In addition, the FBR protocol uses a finite set of power levels in transmission. However, the FBR protocol may have some difficulties caused by water movements. The FBR protocol can be coupled with any MAC protocol. In general, the protocol can achieve high performance and high power efficiency with less delay. Simulation results indicate that the performance of the FBR protocol, which is measured in energy per bit consumption and average packet end-to-end delay, is near to the ideal case [33; 34]. In [35], [PERSON], [PERSON] and [PERSON], [PERSON] introduced a routing algorithm called Distributed Underwater Clustering Scheme (DUCS). It aims to reduce the exchanging of proactive routing messages and to decrease data loss. The simulation result demonstrates that the DUCS scheme enhances the packet delivery ratio and the achievable throughput and reduces the network overhead. The Reliable Energy-efficient Routing Protocol based on Physical distance and Residual energy (R-ERP2R) was proposed by [PERSON] et al. in [36]. The main idea of this protocol is to balance energy consumption between nodes to prolong the UWSN's lifetime. This is achieved by taking the physical distance and residual energy into account during the packet forwarding process. The simulation results show that the R-ERP2R protocol works well in UWSNs and improves the network lifetime, end-to-end delay, and delivery ratio. However, the analytical study in [37] found that when the number of sensors increases, the R-ERP2R would not achieve much improvement in terms of energy consumption. [PERSON] et al. proposed a routing protocol called Opportunistic Void Avoidance Routing (OVAR) in [38]. The main goal of the OVAR protocol is to minimize the number of dropped packets by bypassing void areas. It has three phases: constructing an adjacency graph; adjusting the number of forwarding nodes, and calculating packet holding time. Simulation results show that the proposed OVAR protocol outperforms other protocols in terms of energy-saving, end-to-end delay, packet delivery ratio, and hop counts. In [39], [PERSON] et al. proposed a routing protocol called Q-learning-based Delay-Aware Routing (QDAR). The QDAR protocol has five main phases: (1) data ready, (2) routing decision, (3) interest, (4) packet forwarding, and (5) acknowledgment. In the data ready phase, information is collected and a DATA_READY packet is broadcast if a source node has data to transmit to the sink. In the routing decision phase, the sink performs a routing decision based on the received data. A path based on the QDAR algorithm is constructed in the interest phase and an INTEREST packet is sent to the source node. In the acknowledgment phase, the sink node transmits the ACK packet when it successfully receives the data. Simulation results verify that the proposed protocol can decrease the end-to-end delay by up to 25% compared with other methods. [PERSON] et al. [40] introduced a network coding routing protocol (NCRP) for UWSNs. It efficiently forwards data packets to sinks using network coding and cross-layer design. The NCRP fully utilizes multicast transmission and decodes encoded packets received from various potential nodes across the whole network. The NCRP performs two processes: initial routing construction and route maintenance. The transmission power is optimized to lengthen the network's lifetime. A real-time routing maintenance protocol is created to update the route when inefficient relay nodes are detected.