# Cross-Layer Protocol Based on Directional Reception in Underwater Acoustic Wireless Sensor Networks
## 4 Proposed CLIC Protocol
### Collision-Tolerant MAC Protocol with CT Priority  
A single vector hydrophone with directional reception characteristics can concentrate the reception gain on one direction, which can reduce energy loss and increase spatial reuse. Therefore, for directional reception mode, if the MAC protocol of the omnidirectional reception mode is also used, it will cause channel waste. Thus, a contention-based collision-tolerant MAC protocol (DMAC) is presented based on directional reception. The DMAC combines time slot and channel contention mechanisms to provide a MAC protocol with efficient channel utilization, low interference, and low latency. The DMAC defines three types of packets: CT, data packet (DATA), and acknowledge (ACK). To keep the neighbor table up to date, all three packets contain node information within them. Figure 3: Relay selection scheme. To avoid packet collision, each packet must be transmitted at the beginning of the time slot. The length of the time slot is  
\[slot=\tau+T_{trans}+T_{guard}, \tag{15}\]  
\[\tau=\frac{range}{v}, \tag{16}\]  
where \(\tau\) is the maximum propagation delay, and \(T_{trans}\) is the packet transmission delay. Different packets have different transmission delays. \(T_{guard}\) is the guard time slot, which is 0.0001 s.  
The design for the directional receive handshake mechanism in this paper is presented as  
1. When a node aims to send a packet, it waits until the slot start and competes for a channel by sending a CT packet that includes the next-hop address determined by the relay selection algorithm, its own remaining energy, buffer queue length, number of potential interferences, and the priority of the contention channel. After receiving the CT, the node first updates the neighbor information table and then determines the next slot action based on the CT that is given by \[P=\xi\cdot(1-\frac{|Inter_{j}^{t}|}{|Nei_{j}|})+\delta\cdot(1-\frac{Buff_{now}} {Buff_{\max}}),\] (17) where \(\xi\) and \(\delta\) are the weights of interference and congestion, respectively. If the node has a low level of congestion and the packet transmission has a low potential interference, the CT has a high priority and is more likely to compete for channel success. The priority is designed to make the network less congested, balance the traffic, and reduce packet interference. 2. After receiving a CT packet intended for another node (xCT), it does not need to enter into silent state immediately but makes a judgment based on the interference situation. * A node that has sent a CT first compares the priority of two CT packets. If its own CT packet priority is lower than that of the xCT, it will determine whether its next data transmission will interfere with the data transmission of the node with the higher priority. If the interference power is greater than the acceptable average interference power threshold (\(MeanP\)), it is silent long enough for the node with high priority to transmit the DATA and ACK. * If the interference power is less than the \(MeanP\), then it determines whether the transmission with higher priority will interfere with its own data transmission. If they do not interfere with each other, the node does not silence, keeping its current state and sending DATA for the next slot. The \(MeanP\) is defined as \[MeanP=\frac{S_{th}-\text{SINR}}{|Inter_{j}^{t}|},\] (18) where \(S_{th}\) is the received power, and SINR is the received power threshold. * Nodes that do not send CT packets silence two time slots (DATA and ACK) to allow another terminal to transmit DATA and receive the corresponding ACK. If a DATA packet sent to itself is received during silence, it determines whether sending an ACK in the next slot will affect the receiving ACK of the node with high priority. If there is no effect (interference power is less than MeanP), an ACK is sent in the next slot after receiving the DATA. Otherwise, it will remain silent.