# Optimal Deployment of Vector Sensor Nodes in Underwater Acoustic Sensor Networks
## 4 Summary and Conclusions  
Most previous studies on underwater acoustic communication used a multichannel underwater communication system with an array of spatially separated hydrophone receivers to increase the communication performance. Our previous paper [13] proposed a methodology of optimal deployment of underwater sensor nodes, which employed hydrophone arrays. Our algorithm used the communication PS, which can reflect the effects of ocean environmental variations on underwater communication. However, a large-sized receiver system compromises the spatial efficiency of a UWASN system.  
This paper considers a single-vector sensor that measures particle velocities as an underwater sensor node in a UWASN system, instead of a hydrophone array measuring acoustic pressure, which is a scalar quantity. Since a vector sensor measures the three directional components of acoustic vector quantities, it can be used as a SIMO system. Therefore, diversity gain can be achieved during the decoding process of underwater acoustic communication, even with a single-vector sensor. Our method to determine the optimal deployment of sensor nodes (except for the difference of signal demodulation process owing to sensor type) was the same as in our previous work [13]. As a result, in our simulation, the performance of a single-vector sensor was better than that of the three-channel vertical-pressure sensor array. Specifically, the communication coverage rate obtained using one hundred single-vector sensors (which was 95.3%) was a \(\sim\)10% improvement over that obtained using one hundred three-channel vertical-pressure sensor arrays (which was 85.2%). The simulation was further carried out and the results indicated that the communication coverage rate for one hundred three-channel hydrophone arrays could be achieved using 80 single-vector sensor nodes, with a coverage rate of 84.7%. In addition, when the simulation was repeated using one hundred single pressure sensor nodes, the coverage rate was estimated to be only 45.6%.  
In this paper, we proposed an algorithm for determining the optimal locations of underwater sensor nodes under the assumption that each sensor node was composed of a single-vector sensor. If a vector sensor array is used as a sensor node, the spatial efficiency might be further improved, thus achieving better communication performance and larger communication coverage. However, both the advantages and disadvantages should be considered with respect to spatial efficiency and cost efficiency.  
[PERSON] designed and led the research, and [PERSON] applied the algorithm to the UWASN and performed the result analysis. Both authors wrote the paper. The authors declare no conflicts of interest.  
Figure 6: Results of the optimal deployment simulation using two different sensor node types: (**a**) the three-channel vertical-pressure sensor array, and (**b**) the single-vector sensor. The solid lines between the sensor nodes indicate their connectivity, which was estimated by comparing the communication range of each sensor node with the range from each sensor node to its neighboring nodes. In both cases, every sensor node was 100% connected.  
Funding: This work was supported by Agency for Defense Development, Korea, under grant UD170006 DD, the National Research Foundation of Korea (NRF-2016R1D1A1B03930983), a \"Development of the wide-area underwater mobile communication systems\" grant (PMS4110) from National R&D Project, funded by Ministry of Oceans and Fisheries (MOF) and a \"Development of maritime defense and security technology\" grant (PE99741), promoted by the Korea Institute of Ocean Science and Technology (KIOST).  
The authors declare no conflict of interest.