Building an Underwater Wireless Sensor Network based on Optical Communication: Research Challenges and Current Results. Davide Anguita, Davide Brizzolara, Giancarlo Parodi DIBE - Department of Biophysical and Electronic Engineering University of Genoa Via Opera Pia 11A, 16045 Genoa (Italy) {Davide.Anguita, Davide.Brizzolara, Giancarlo.Parodi}@unige.it Abstract In this paper current research for an Underwater Wire- less Sensor Network based on optical communication be- tween nodes is described. Radio Frequencies are strongly attenuated in water and optical communication can be con- sidered as a feasible solution to explore as an alternative to acoustic communication in case of short-range distance. At the moment, our work focuses on an optical Physical Layer developed considering the characteristics of IEEE 802.11 Infrared Physical Layer and the compatibility with the cur- rent IEEE 802.15.4 protocol for terrestrial Wireless Sensor Networks. This approach, that allows the interface between the optical Physical Layer and the current terrestrial tech- nology for Wireless Sensor Networks, is a first step that will be followed by works on upper layers. In this paper, the main research challenges and current results are illustrated. Keywords: Underwater optical communication, under- water technology, wireless LAN. 1. Introduction One of the challenging aspects in building an efficient Underwater Wireless Sensor Network (UWSN) is under- water wireless communication. High Frequency (HF) ra- dio waves are strongly attenuated in water: the available radio modules such as Bluetooth or Wireless LAN (IEEE 802.11), which operate in the gigahertz range, around 2.4 GHz, cannot be used underwater where, currently, acoustic communication is mainly employed [2]. In this paper the research challenges in building an UWSN based on optical communication are explored and the development of an optical communication system based on LEDs is illustrated: it can be effective because of its low power consumption, low operating voltage, long lifetime, low cost and it can establish high rate communication, as shown in [10], between fixed nodes and an autonomous un- derwater vehicle (up to 320 kb/s). The proposed solution is to replace the traditional infrared communication (IR), such as the one described in IEEE 802.11 standard, with visible light generated by LEDs considering the minimum absorp- tion wavelength window of the water [8]. The described approach starts from the Physical (PHY) Layer, taking into account both the circuits for transmission and reception and the implementation of the PHY Layer used to provide services requested by the Medium Access Control (MAC) Layer. The next step will focus on the up- per layers and on the hardware structure used to support the effective implementation of a prototype. This paper is organized as follows: a brief overview of the main aspects of underwater communication, a brief il- lustration of the possible applications of an optical UWSN, a presentation of the proposed optical network focusing on the different layers and a report of the current results. 2. Wireless Underwater Communication Due to the impossibility of using Radio Frequencies (RF), traditionally wireless underwater communication em- ploys acoustic waves because sound propagates well in wa- ter and its range can be very long (∼km). However, it has several disadvantages such as narrow bandwidth and la- tency in communication due to the slow speed of acoustic wave in water. For instance, at ranges of less than 100 m the data transmission rates of these systems in shallow littoral waters are ∼10 kb/s [5]. An alternative feasible solution is optical communica- tion especially in blue/green light wavelengths, even if lim- ited to short distances (up to 100 m) [7]. Compared to acoustic communication it offers a practical choice for high- bandwidth communication and it propagates faster in the water (2.255 x 10 8 ) [10].