# Energy Harvesting Hybrid Acoustic-Optical Underwater Wireless Sensor Networks Localization
## 1 Introduction
However, energy harvesting can play a significant role in performance enhancement and improve the lifetime of UWOSNs. Since underwater sensor nodes cannot survive on the battery power for a long time, energy harvesting is a promising solution to provide energy from the ambient sources in the aquatic environment. Noting that energy harvesting techniques used for terrestrial communications are not applicable for underwater sensor nodes in the aquatic environment, it is necessary to consider alternative energy harvesting methods, e.g., acoustic piezo-electric harvesters [13] and microbial fuel cells [14]. The underwater aquatic monitoring demands accurate localization techniques as the collected data is only useful if the location of the nodes is known to a certain accuracy. Nonetheless, localization of sensors within UWSNs is also a crucial and challenging task especially for the surveillance applications, which can scale for a large number of sensor nodes. A number of acoustic underwater sensor networks' \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **EM Waves** & **Acoustic Waves** & **Optical Waves** \\ \hline Communication Distance & 100 m & Upto 20 Km & 10–100 m \\ Transmit Power & Few mW to Hundred of Watts & 10–100 W & Few Watts \\ Cost & High & High & Low \\ Data Rate & Up to 100 Mbps & In Kbps & Up to Gbps \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Underwater wireless communication channels comparison. localization techniques based on time difference of arrival (TDoA) has already been proposed, which consider different parameters such as signal propagation model, network topology, environmental factors, localization accuracy, number of anchor nodes, the geometry of anchor nodes and the relative location of the sensor node to the anchors [15; 16; 17; 18; 19; 20; 21]. However, the TDoA measurements for distance estimation in underwater acoustic communication channels are highly affected by multipath. Received signal strength (RSS) based distance estimation did not get much attention for UWSNs' localization since the multipath propagation of acoustic communication makes it hard to achieve accurate distance estimation [22]. However, the underwater acoustic channels show good transmission features at certain water depths and RSS based distance estimation can be considered [23]. In this paper, we propose RSS based AO-UWSNs' localization with the capability of energy harvesting. The contributions of this paper are summarized as follows:  
1. A novel hybrid acoustic-optical underwater wireless sensor network localization technique is proposed in order to benefit from the advantages of both acoustic and optical communication. 2. A weighted multiple observations paradigm is proposed for hybrid estimated distances to suppress the noisy observations and give more importance to accurate observations. Considering the hybrid acoustic and optical RSS model, the closed form solution for Cramer-Rao lower bound (CRLB) is also derived to improve the localization accuracy of the proposed technique. 3. We consider energy harvesting for the battery limited sensor nodes and show the impact of harvested energy on the network lifetime maximization and energy efficiency.