# Energy Harvesting Hybrid Acoustic-Optical Underwater Wireless Sensor Networks Localization
## 1 Introduction  
The development of underwater wireless communication capable sensor nodes has provided new opportunities for ocean exploration. The spatial-temporal data for oceanic exploration is of utmost importance for the applications of marine scientific research, ocean energy development and utilization of ecological underwater environment. Currently, the technologies used for underwater wireless communication consists of acoustic communication, electromagnetic waves communication, and optical communication. Acoustic communication systems have been one of the most used underwater wireless communication technology due to its ability to provide connectivity over very long distances. However, acoustic waves still have many drawbacks including scattering, high delay due to low propagation speeds, high attenuation, low bandwidth. Acoustic waves also have a bad impact on the health of underwater fishes and mammals. According to a recent report by natural resources defensecouncil (NRDC), military sonar and other acoustic communication networks are rising the ocean noise, which has a serious impact on the health of underwater mammals [1; 2]. Electromagnetic waves suffer from serious attenuation in water, the attenuation in the ocean is about 169 dB/m for the 2.4 GHz band, and the attenuation in freshwater is much higher, 189 dB/m [3]. Moreover, electromagnetic waves based underwater communication requires huge antennas and is limited to the shallow areas of the sea. On the other hand, operating at ultra-low frequencies yields reduced attenuation levels, in return for high hardware costs and low data rates. Underwater propagation of light waves also exhibits distinctive characteristics in different wavelengths as shown in [4]. In 1963, the authors [5] found that attenuation within the range of 450-550 nm wavelengths (blue and green lights) is much smaller compared to other wavelengths. In 1966, [PERSON] et al. [6] experimentally confirmed this behavior of optical waves, which provided the foundation of underwater optical communication. Table 1 shows the comparison between three different kinds of underwater wireless communication carrier waves. Optical waves have the advantage of higher data rate and low power consumption but low transmission ranges. In this paper, a novel hybrid acoustic-optical underwater wireless sensor network (AO-UWSN) is considered that benefits from the advantages of both underwater acoustic communication and optical communication. While the optical wireless communication is employed to transmit at higher data for shorter range, the acoustic communication is used for the command and control within the long ranges, which does not require high data rates. Notable research has been previously conducted on the energy consumption and lifetime maximization aspects of RF wireless sensor networks. In [7], the authors proposed an energy efficient routing protocol to transfer information in WSNs. Stable election protocol (SEP) and prolong stable election protocol (P-SEP) are proposed to balance the energy consumption in WSNs [8] and to increase the network stability [9]. Energy efficiency of acoustic UWSNs has been recently investigated in [10] for amplify-and-forward scheme along with a minimum energy routing protocol. An enhanced energy efficient protocol is addressed in [11], which also takes the depth of sensor nodes into account. A comprehensive survey is also presented on research challenges, localization schemes, and deployment of UWSNs in [12]. However, to the best of our knowledge, no literature exists that considers the energy efficiency, network lifetime, energy harvesting and localization of hybrid AO-UWSNs. Today's UWSNs consist of battery-powered sensor nodes with a limited energy budget. Albeit the notable research efforts on designing different protocols for underwater communication networks, no significant research has been carried out on the energy harvesting methods for UWOSNs.