Researchers at Nanjing University have made significant steps toward realizing real-time free-space acoustic communication with high spatial information density, in a passive postprocessing-free and sensor-scanning-free paradigm by multipath twisting of acoustic wave.
The new work is reported in the journal Nature Communications, in a paper entitled “Metamaterial based real-time communication with high information density by multipath twisting of acoustic wave” [Nat. Commun.13, 5171 (2022). DOI: https://doi.org/10.1038/s41467-022-32778-z], by the co-first authors PhD student Kai Wu and associate researcher Jingjing Liu and the co-corresponding authors associate researcher Jingjing Liu, Professor Bin Liang and Professor Jianchun Cheng.
In recent years, many researches have proved that the orbital angular momentum (OAM) carried by acoustic vortex beam can serve as a new degree of freedom independent of phase and amplitude for encoding information and consequently increase the communication capacity. Based on the orthogonality of OAM, different modes of OAM beams carrying independent information can be multiplexed into a single path without inter-modal crosstalk and can be compatible with the conventional multiplexing techniques. Despite some recent efforts to apply OAM multiplexing in acoustic communication, they usually need postprocessing of signals received with complicated sensor arrays or multi-layered metamaterials with de-multiplexing capability limited by insertion loss and diffraction effect. More importantly, the existing OAM multiplexing strategies rely on multiple coaxially-overlapped twisted beams used as orthogonal communication channels, while leaving the potential of non-coaxial beams to further boost acoustic communication efficiency unexplored. To date, the realization of real-time acoustic OAM-based communication system with high information density in free space still remains challenging.
The researchers proposed a meta material-based mechanism that uses multipath twisting of acoustic wave to realize real-time high-capacity communication in free space, as shown in Fig. 1a. In the existing acoustic OAM-based communication system, the channel number has to be increased by including more high-order OAM modes, leading to more severe diffraction and spatial aliasing effect, which limits the maximum number of available communication channels. By combining coaxial mode multiplexing and non-coaxial spatial multiplexing, the total number of communication channels reaches NM (N is the number of transmission paths, and M is the number of OAM modes multiplexed in a single path), which is a remarkable boost in comparison to the existing acoustic OAM-based communication mechanisms. Besides, the introduction of non-coaxial acoustic vortex beam as a new encoding degree of freedom also helps to reduce the dependence on high-order OAM beam and significantly increase the effective communication distance and signal-to-noise rate (SNR).At the receiving terminal, the researchers proposed a new demultiplexing mechanism and designed a monolayer meta surface as a passive and efficient demultiplexer that can simultaneously remove the OAM of all the multiplexed vortex beams and converge the energy of different OAM orders into predesigned detection locations, as shown in Fig. 1b. The proposed demultiplexing system possess high spatial selectivity, which can solve the problem of channel crosstalk caused by the diffraction effect in multi-link systems, ensuring the high robustness and low spatial crosstalk of the communication system. And the most important feature of the proposed system is the record-breaking high spatial information density which is ensured by more available channels and the compactness of both the transmitting and receiving terminals. Based on this mechanism, researchers fabricated the demultiplexer via 3D printing and established a two-path two-mode communication system (N = 2, M = 2), and experimentally demonstrated the effectiveness of the proposed mechanism via real-time high-quality transmission of a complicated image of Nanjing University logo, as shown in Fig. 2. Besides, researchers also quantitatively evaluated the low crosstalk and low bit error rate (BER) of the communication system. The experimental results in Fig. 3 show that the communication system can still maintain extremely-low BER under long transmission distance and low signal-to-noise ratio, which suggests that the designed system has high robustness and anti-noise-interference ability.
Figure 1. a The designed communication system using multipath twisting of acoustic wave and metamaterial. b The schematic diagram of demultiplexing the synthesized vortex beams in free space. c The acoustic response of metamaterial unit cell.
Figure 2.aThe original input image which is divided into four separate parts for a parallel transmission along four independent channels. b The enlarged view of pixel block, and the two colors of pixel are encoded by binary information “0” and “1.” c The target and output data streams of four lines in b. d The transmission results of Nanjing University logo.
Figure 3.aMeasured BERs of school badge image versus transmission distance.b The measured BERs curves of four channels versus SNR.
This work presents a new mechanism of acoustic communication with high spatial information density using multipath twisting acoustic wave and designed and fabricated a compact and passive demultiplexer with high spatial selectivity. The excellent performance of this communication system in terms of channel capacity, spatial information density and transmission distance is systematically demonstrated. The effective communication distance and channel capacity can be further improved by reasonably designing the device scale and using high-order shift keying to encode data. This new OAM-based communication mechanism is excepted to have practical application value in underwater communication, ocean exploration and other fields.
This work was supported by the National Key R&D Program of China, the National Natural Science Foundation of China, Nanjing University Deng Feng Scholars Program and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Full-text links:https://www.nature.com/articles/s41467-022-32778-z