High-Speed Underwater Acoustic Orbital Angular Momentum Communications

Acoustic vortex waves carrying orbital angular momentum (OAM) have been demonstrated to transmit multiple independent data streams simultaneously. By virtue of the inherent orthogonality of OAM modes, ring-shaped intensity pattern, and vortex phase distribution, acoustic vortex waves can enhance both underwater communication and positioning. Such acoustic vortex-based communications have great potential to improve the data rate of underwater acoustic communications (UWAC) and achieve the underwater integration of sensing and communications. Here, we demonstrate a high-speed OAM-mode division multiplexing (OAM-MDM) system in UWAC. By employing a pair of uniform circular arrays (UCA), the coaxially transmitted underwater acoustic OAM modes are utilized to introduce extra degrees of freedom (DoFs). The data rate is increased multifold, and the theoretical limit of DoFs is attained by exploiting the generalized OAM modes. The theoretical power penalty and channel capacity of an ideal UCA-generated OAM mode are derived. In experiments, a high spectral efficiency of ${\sim }4$ bit/s/Hz is achieved by multiplexing four acoustic OAM modes. A decision-feedback equalizer is employed to inhibit the crosstalk between OAM modes and decrease the bit error rate of OAM-MDM. Moreover, a partial receiving aperture scheme is demonstrated to miniaturize the size of conventional OAM communications. This study provides a theoretical and experimental basis for underwater acoustic OAM communications.