Multimodal underwater networks: Recent advances and a look ahead

The recent uptake of non-acoustic underwater transmission systems suggests that in the near future it will be common for underwater devices to incorporate different physical communication technologies. Such devices are typically described as multimodal. They seek flexibility by compensating for the shortcomings of a given technology through the advantages of another. For example, a system encompassing acoustic and optical communication systems can provide long-range, lowbit rate communications, while enabling faster data transfer at very short range. As the development of non-acoustic underwater communications is taking momentum, so is the research on how to optimally exploit the multimodal communications capabilities in different scenarios. This paper presents a survey of past and recent work on this topic, covering the development both of the communication technologies and of the networking schemes and protocols for multimodal networks. As an example of the opportunities offered by multimodal communications, we discuss two different case studies. We conclude with an outlook on likely future developments for multimodal communications.