# Cross-Layer Protocol Based on Directional Reception in Underwater Acoustic Wireless Sensor Networks
## 1 Introduction  
The long propagation delay and low transmission rate of underwater acoustic communications and the dynamic marine communication environment pose serious challenges in designing underwater acoustic wireless sensor networks (UASNs) protocols [1; 2; 3; 4]. For an omnidirectional underwater acoustic communication network scenario, nodes use omnidirectional transmission to send and receive packets. Due to the nature of broadcasting, the network performance is seriously affected by the large propagation delay and high packet collision rate [5; 6; 7]. Although the handshake mechanism of the competitive MAC protocol widely used in UASNs can reduce the occurrence of collision, there are still problems of wasting idle channels and low spatial reuse [8]. The MAC protocol in [7] uses a handshake to compete for channels, reducing network collisions, but it still suffers from low spatial reuse. Development of underwater acoustic directional communication technology has been paid more attention with the increasing interest in applying directional reception technology to UASNs to improve network performance. Different from omnidirectional transceiver networks, the sound pressure and vibration velocity of the acoustic vector sensors form unilateral directivity through linear weighting combination to achieve directional reception of signals in a certain direction, thereby improving the spatial reuse rate of the network. Although the application of vector hydrophone in wireless networks has been widely studied, the research in UASNs is relatively limited, which is mainly limited by the mediaaccess control protocol and channel difference. Some studies of UASNs routing and MAC schemes using directional transmission are reported in [9; 10]. [PERSON] et al. proposed a directional MAC framework based on the vector hydrophone and multimodal transducer, along with a benchmark MAC protocol, which can achieve spatial reuse and energy conservation [9]. Simulation results show that the MAC protocol outperforms existing representative protocols in terms of throughput, end-to-end delay, and energy efficiency. [PERSON] et al. introduced the validity of conventional routing schemes in underwater ad hoc networks with directional antennas and presented a special design of multipath routing algorithm for directional transmission. The experimental results show a significant performance improvement in throughput and latency [10]. Although there are many routing or MAC protocols based on directional transmission, most do not take into account the collision-prone and congestion-prone nature of packets in hydroacoustic networks. The node deployment of the UASNs is sparse, so it is easy to cause network congestion under heavy and burst data input loads such as earthquake monitoring applications. Network congestion always leads to packet loss, packet interference, and queuing delays, which degrades network performance [11]. As we know, more interference increases packet collisions and reduces the packet delivery rate (PDR). Although directional reception techniques obtain higher network capacity, the long propagation delays of acoustic links still lead to packet collisions. Therefore, interference avoidance and congestion control are critical for the UASNs to achieve high throughput and long network lifetime. This paper describes a cross-layer protocol with low interference and low congestion (CLIC) based on directional reception. An integrated routing-MAC is employed in the CLIC to use directional beams to create the least-interfering, highest-capacity data transmission links, weighing key factors affecting network performance to obtain routes with low collisions and low congestion. Simulation results show that the CLIC has a better PDR and higher energy efficiency compared to the machine-learning based adaptive routing protocol (QELAR) [12] and vector-based forwarding (VBF) [13] protocols. The main contributions of this paper are presented as follows:  
1.