# Cross-Layer Protocol Based on Directional Reception in Underwater Acoustic Wireless Sensor Networks
## 2 Related Work  
Many different routing schemes have been presented and used for the UASNs due to the unique characteristics of underwater acoustic channels, including long propagation delays, low data rate, and high error probability.  
Vector-based forwarding (VBF) is a geographical location routing method [13] that limits the forwarding range of data packets to the virtual pipeline determined by the source node and the destination node. An effective anycast routing algorithm called HydroCast [14] uses a new opportunistic routing mechanism to select a subset of forwarders that maximizes greedy processes but limits interference on the same channel and employs an efficient method for recovering underwater dead zones. HydroCast has achieved superior performance to both depth-based routing (DBR) [15] and an efficient 3D cube-based protocol [16].  
An RL-based routing protocol for UASNs named QELAR has been proposed [12]. In QELAR, the reward function is composed of the residual energy of the nodes and the energy distribution among a group of nodes, and the network life is extended by balancing the residual energy. Simulation results show that the average service life of QELAR is 20% longer than that of VBF [13]. A congestion avoidance routing protocol for UASNs based on RL has been developed [17] in which the reward function is defined by the current buffer state, residual energy, and location information of the neighbors. To accelerate the convergence of the algorithm, a dynamic virtual routing pipeline with a variable radius is introduced, which is related to the average residual energy of the sender node's neighbors. A localization-free interference and energy holes minimization (LF-IEHM) routing protocol for UASNs has been proposed [18]. The proposed algorithm overcomes interference during data packet forwarding by defining a unique packet holding time for every sensor node. The energy holes formation is mitigated by a variable transmission range of the sensor nodes.  
Cross-layer design, where more than one layer in the protocol stack interacts with each other, has been drawing the attention of researchers over the past few years as a promising performance management technology in UASNs with complex channels.  
[PERSON] et al. [19] followed a cross-layering design and provided a class of scheduling, power control, and routing policies that result in low-power, interference-aware effective solutions for underwater sensor networks. Considering the complete interference model, they assumed the accurate description of an underwater acoustic channel and modeled the acoustic signal attenuation and propagation through Bellhop ray tracing software.  
Inspired by the unpredictability of both route interruptions and packet collisions in UASNs, a collision and interruption tolerant protocol (CITP) was proposed to jointly address the two challenges in a smart way [20]. By exploiting the opportunistic transmission strategy, CITP forms the optimized route on the fly, which bypasses nodes that undergo collisions or interrupted links and void regions. An energy-efficient, channel-aware, and depth-based scalable and multipath-agent-based routing protocol (ABA) is proposed in [21]. ABA considers multiple parameters, such as node energy, propagation delay, link quality, hop count, and queue size to select the relay node. ABA adopts a cross-layer approach to estimate the quality of link that assures higher reliability than the probabilistic measure. The propagation time is also taken into account with varying depth, temperature, and salinity. The propagation time and quality of link are vital factors that ensure the speed of data transmission and reliability.