Cognitive Spectrum Access for Underwater Acoustic Communications

While very successful in traditional radio communications, the usage of TDMA and CSMA schemes for underwater acoustic communications is severely limited in efficiency and scalability, primarily due to the very large propagation delays. FDMA seems a viable alternative in that the propagation delay does not impact significantly its efficiency. However, in underwater communications, the capacity achievable on a particular channel depends strongly both on its frequency and on the communication distance, unlike in traditional radio transmissions where FDMA channels usually have comparable performance. Therefore, fixed channel allocation schemes traditionally used for radio FDMA do not perform well in underwater communications. In this paper, we investigate the application of the principles of cognitive radio and dynamic spectrum access to underwater communications. In particular, we propose a channel allocation scheme which exploits user location knowledge in order to maximize the minimum channel capacity among those achieved by the users. This provides maximum fairness and makes a more efficient use of the available spectrum resources. Performance evaluation carried out by means of simulation shows that our approach can achieve a great improvement in fairness among users, with respect to fixed allocation schemes, while at the same time scaling much better and thus allowing effective communications over larger distances.