# Cross-Medium Photoacoustic Communications: Challenges, and State of the Art
## 1 Introduction  
Recent years have witnessed growing interest in the use of networks in underwater applications, such as environmental state monitoring, search and rescue, marine biology, leak detection in oil fields, tracking seismic activities, seabed profiling, distributed surveillance, and navigation [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. While lots of research has focused on establishing robust communication links to enable reliable exchange of information among the underwater nodes, little attention has been given to interfacing such an underwater network to terrestrial-based centers. Fundamentally, it is often required for the data collected by underwater nodes to be relayed to a remote off-water command post. However, interconnecting underwater nodes with terrestrial assets have to be provisioned. Basically, the terrestrial systems utilize Electromagnetic waves (EM) for communication while EM suffers from high attenuation loss in the underwater environment. Moreover, acoustic signals which are deemed the best means for underwater communication [21, 22, 23] cannot cross the air-water interface due to the high impedance mismatch between the two mediums. To overcome such an interconnection issue, published underwater networking solutions incorporate floating nodes, e.g., buoys, to act as bridges between the acoustic and EM networks and as reference anchors for underwater localization. However, in many application scenarios deploying surface nodes could be unjustified due to the incurred cost relative to the task duration, or undesirable due to mission constraints. For example, in search and rescue missions, e.g., finding the black box of a crashed plane, autonomousunderwater vehicles (AUVs) survey the area for a limited duration. In such a case, requiring the deployment of surface nodes is logistically challenging and very slow. The search for the Malaysia Airlines Flight 370 in 2014 demonstrated how tardiness could lead to failure; basically, the water current and the uncertainty about the flight path complicated the process where boats are involved in the search, mainly to deploy probes and interact with AUVs. Other examples include, but are not limited to, scientific data collection and covert military underwater surveillance where the presence of the underwater network has to be concealed. In addition, the design of underwater acoustic networks (UANs) has its own challenges [24; 25]. First, nodes tend to drift, causing some communication links to be lost and the network to get partitioned. Moreover, the inhomogeneity of the underwater environment causes acoustic waves to travel with varying speeds that depend on the temperature, density, and salinity of the water, which complicates both link and network formation [25; 26; 27; 28]. Therefore, UANs ought to adapt to changing seawater conditions and the inherent node mobility in order to reestablish broken communication links. The use of floating nodes has been conventionally used to facilitate management of the UAN topologies, where the floating nodes can serve as anchor nodes with known GPS coordinates to guide the underwater nodes. In other words, the floating nodes not only interface the UAN to airborne and terrestrial command centers and base-stations, but also generate acoustic beacons to help underwater nodes determine their relative position using multi-literation [29; 30; 31; 32; 33]. To address the aforementioned networking challenges, there is a need for developing network architecture and protocols that take into consideration the underwater physical layer dynamics and the constraints on interfacing the network with off-water nodes. Given the transmissivity of light from air to water is very high, visible light, e.g., LED, and laser could be a viable candidate for cross medium communication [34; 35]. However, neither LED nor laser light is good for long range communication due to the high light absorption and beam scattering, especially for a wavy water surface, which is the most common scenario.