# Cooperative Position Aware Mobility Pattern of AUVs for Avoiding Void Zones in Underwater WSNs
## 2 Related Work
### Mobility Focused Protocols  
The authors in [12] use AUVs and gateway nodes to maximize the data delivery ratio and to reduce the energy consumption. In the AUV-aided underwater routing protocol (AURP), distant data transmissions are minimized by using AUVs as relay nodes. Normal nodes send their sensed data to the gateway nodes, then AUVs collect those data and forward them to the sink. The three-dimensional sink mobility (3D-SM) scheme is proposed in [13], in which the authors divide the three-dimensional network volume into four rectangular cuboids (RC). In one cuboid, a mobile sink (MS) is deployed to gather data packets from sensor nodes, and in the remaining three cuboids, courier nodes (CNs) are deployed to gather data from nodes at relatively shorter distances. The energy consumption of normal nodes is minimized due to MS and CNs due to shorter distances between the source node and the destination (MS or CN). A network of randomly-deployed identical sensor nodes is considered in [14]. In this scheme, the sensed data are gathered from pathway nodes, and the AUV visits pathway nodes to save energy \begin{table}
\begin{tabular}{p{71.1 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Technique** & **Features** & **Achievements** & **Limitations** \\ \hline QUO VADIS [1] & Optimization technique based on position estimation and uncertainty of gliders & Low end to end delay & Transmission power is increased \\ \hline Close-range tracking of AUVs [8] & Use of light beacon messages and camera to estimate the position of AUV & Exact and approximate position of the AUV is computed & Range is very short \\ \hline SINS [9] & Positioning algorithm based on the strap down inertial navigation system (SINS) and the long baseline (LBL) positioning system & Exact position of the AUV is estimated & High energy consumption is estimated \\ \hline AUV state estimation [10] & Kalman filter is used to estimate the state of the AUV & Navigation and state estimation of the AUV is done & High energy consumption estimation \\ \hline Trajectory-aware routing [11] & Model for the position uncertainty of the underwater glider & Minimization in uncertainty or error & High packet drop ratio \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Performance tradeoffs in existing position estimation-based routing protocols. by avoiding distant transmissions. An efficient data-gathering (AEDG) routing protocol is proposed in [15]. Sensor nodes in the network are associated with gateway nodes, and gateway nodes gather the data from sensor nodes. An AUV visits gateway nodes to collect the data and ensures the reliability. To avoid gateway nodes being overloaded, the authors limit the number of sensor nodes associated with the gateway nodes. In [16], a routing protocol is proposed in which three regions or potential fields are defined, and the network area is divided into three different layers based on: the depth, residual energy and density of the sensor nodes. The authors in [17] propose a link state-based adaptive feedback routing (LAFR) protocol for underwater acoustic sensor networks (UASNs). By using the link state detection mechanism, the link state is determined, then LAFR uses the link state information in the adaptive feedback method to fully utilize the asymmetric links underwater for conserving energy and to maintain routing tables. The channel-aware routing protocol (CARP) [18] uses link quality information for data forwarding. For the selection of relay nodes, the authors propose that the nodes are selected as relay nodes, if they exhibit the latest history of successful data transmissions. By using hop count, CARP avoids loops to successfully route around void and shadow zones. In [19], a void-aware pressure routing (VAPR) protocol is proposed that uses enhanced beacons and opportunistic directional data forwarding in the network to propagate control information to sensor nodes. Sensor's depth, hop count to the sonobuoy and the data forwarding direction of a sensor node are included in the control information.