Phase-Dislocation-Mediated High-Dimensional Fractional Acoustic-Vortex Communication

Traditional multiplexing communication techniques include time division multiplexing (time domain), frequency division multiplexing (frequency domain), wavelength division multiplexing (wavelength domain), etc. Its essence is to transmit information in different dimensions to increase the channel capacity. The orbital angular momentum (OAM) carried by vortex beams is different from the traditional physical multiplexing methods, providing a brand-new dimension for data communication. Integer OAMs can be identified using the spectrum decomposition method based on the orthogonal property. However, the channel capacity is still limited by the available OAM modes, which are determined by the source number of the transmitter array. And the long-distance acoustic communication is also restricted by the obvious spiral divergence of vortex beams.

Fractional vortex beams have infinite fractional OAM (FOAM) modes in theory, and they can be used to increase the channel capacity by multiplexing FOAMs within a limited range. However, due to limitations of non-orthogonality, susceptibility to interference, and divergence of FOAMs, there is currently no precedent for its application in acoustic communication. Although the image-based machine learning can recognize optical OAMs, the real-time distribution of acoustic fields cannot be achieved through photography, making it difficult to apply in acoustic communication. Given the unique advantages of acoustic waves in underwater communication, the development of a stable high-speed FOAM communication technology with low-divergence and high-capacity shows its great significance.

Now, researchers from Nanjing Normal University, along with Peking University and Nanjing University, have made breakthrough progress in the FOAM communication. They proposed a phase-dislocation-medicated high-dimensional fractional acoustic-vortex communication method based on FOAM complexing, circular sparse sampling, and machine learning for the first time internationally.

Focused on the core issues of channel capacity, anti-interference, system feasibility, and real-time performance in acoustic communication, positive and negative FOAMs were employed to excite the traditional ring-array of transducers to construct multiplexed acoustic-vortex fields, with phase dislocations independent of transmission distance and sampling radius. The stable azimuth angle of the phase dislocation only determined by the pair of FOAMs provides an important physical foundation for OAM recognition and data decoding. Meanwhile, the circular sparse sampling also provides possibilities for the practical application of multiplexed fractional acoustic-vortex beams in real-time communication.

By considered various non-ideal conditions of turbulence, array translation, rotation, and deflection, researchers constructed the 5.17-bit, 8-bit, and 10-bit channels with the FOAMs from ±0.6 to ±2.4, and the accurate transmission and decoding of characters “NJNU” were accomplished successfully through the single-ring or double-ring sampling. Furthermore, the FOAM recognition resolution of up to 0.025 was also realized by utilizing the 128-point single-ring sampling, demonstrating the application potential of infinitely increasing the channel capacity. Compared with the other OAM-based acoustic communication technologies, they achieved the 3-fold utilization efficiency of OAM, 4-fold information transmission level, and 5-fold OAM resolution, and provided a groundbreaking new technology for the significant expansion of channel capacity within a limited range of OAM in acoustic communication.

This study has successfully solved the scientific challenges of OAM multiplexing, transmission, and decoding. Further optimization of synchronous timing of the transmission and reception system and introduction of more advanced deep learning networks with more experimental datasets will lead to the improved performance of accuracy, stability and reliability in real-time acoustic communication. This study is expected to be applied in the field of underwater acoustic communication with significantly enhanced channel capacity, thereby promote the development and application of the next-generation of acoustic communication.

Tag: Information Science

Sources: https://spj.science.org/doi/10.34133/research.0280