# Cross-Medium Photoacoustic Communications: Challenges, and State of the Art
## 6 Conclusions and Future Work  
In this article, we have provided an overview of the state-of-the-art techniques for enabling air-to-underwater communication using the photoacoustic mechanism. We have described the generation, propagation, and characteristics of the acoustic signal and how to control the properties of such a signal through the laser beam. We have also outlined the research challenges in establishing photoacoustic communication links and reported progress made to date. The use of photoacoustic as a cross-medium communication has high potential to morph the classical way of approaching a solution in underwater applications. Since developments are still in an early stage, there are many open research issues that warrant further investigation. These issues include the following:  
* Improving the bitrate is indeed warranted for photoacoustic links. To date the achievable bitrate is in the kHz range, which is extremely low for real-time audio and video communications. The use of traditional modulation techniques does not fully exploit the optoacoustic capabilities and often leads to slower data transmission rates. There is a clear need for innovative modulation and channel coding techniques that can leverage the different characteristics of the optoacoustic channel to increase the bitrate. * Optimizing the laser source energy is an obvious means for strengthening the generated acoustic signal and improving the signal-to-noise ratio. The generation of a vapor cloud associated with each optoacoustic effect hinders the generation of new acoustic signals at the same location until that cloud decimates. Since high energy laser pulses tend to make larger vapor clouds that eventually require more time to decimate into the underwater environment, research is needed to find the optimum balance between the laser energy and the bitrate. * A complete protocol stack is yet to be developed by taking advantage of the photoacoustic effect, not only the physical layer but also at the link and transport layers. * Focusing laser light inside the water medium is extremely difficult in practice. In lab setups, one can do so by using focusing lenses. However, such an approach is not practical in an open ocean environment. Auto-focusing laser light could be a viable option that is worth investigation. * Improving the bit error rate warrants more research efforts. Since the optoacoustic effect produces acoustic signals that propagate in an underwater environment, the channel parameters need to be estimated to ensure successful delivery of the transmitted data. Channel estimators for free space communication have been addressed in many recent publications [105; 106]; yet little progress is made for underwater environments. Ideas portrayed in free space can be mapped to an underwater medium where the underwater dynamics are considered such as the medium inhomogeneity [107]. In the context of photoacoustic communication, underwater channel estimators can be leveraged for improving reception where the directionality of the acoustic beam can be controlled by the laser focusing angle. Furthermore, the creation of multiple bubbles in the nonlinear photoacoustic case can be exploited to mitigate the multipath effect where reflected waves either from the surface or bottom are used to establish communication and reduce the multipath fading [108]. Machine learning techniques would be a viable option for conducting such optimization, as demonstrated by recent work [106]. * Field testing of photoacoustic communication is necessary for gauging viability and performance. PA is an emerging area of research; most of the developments have been validated in lab settings using water tanks. The exception is work done at the US naval research laboratory where some sea-based experiments were conducted. As noted in Section 5.2, these experiments mainly focused on physics to explore the properties of the photoacoustic effect. Hence, more effort should be made to open water settings. Conceptualization, M.S.I. and M.M. ; methodology, M.S.I. and M.M.