University of the Ryukyus · Integrated Information Infrastructure Center

Research keywordsOrthogonal・frequency-division・multiplexing・Underwater・acoustic・communication・Doppler・effect

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• Development of Novel Flight Modes for Underwater Drones2025 · In this paper, we propose the development of novel flight modes for underwater drones. Currently, most underwater drones are operated manually using a controller. Although each operation method has its advantages and disadvantages depending on the intended application, our target use case is tasks such as seafloor mapping, where operation within a predefined area is required. In such cases, the movement itself is relatively simple, but human operators are still necessary. To address this issue, we developed an autonomous navigation system capable of moving in a predetermined pattern over a fixed distance, thereby reducing the need for human labor and improving operational efficiency. The underwater drone assumed for practical use in this study is the BlueROV2, which allows for system development and integration. The development was carried out using ArduPilot, an open-source software platform, specifically utilizing its underwater drone–dedicated code, ArduSub. The simulation environment was constructed using Mission Planner. The results of Simulations 1 to 4 are presented below. Simulation 1 was designed for applications such as seafloor mapping, and a square flight mode was developed. As a result, the drone was able to move forward the specified distance, rotate its body, and move forward again repeatedly. By repeating this sequence four times, it successfully followed the predetermined square path. Simulation 2 aimed for slightly more complex movements, and a pentagonal flight mode was developed. The drone moved the specified distance, rotated at each vertex, and successfully traced the defined pentagonal path. Simulation 3 tested whether more complex maneuvers could be achieved by developing a diamond-shaped flight mode inspired by playing cards. The drone moved forward while slightly curving, adjusted its orientation at each point, and was able to trace the diamond shape. Simulation 4 was designed to verify smooth movement under continuous curvature, for which a cardioid-shaped flight mode was developed. The drone smoothly followed a curved trajectory, successfully forming the heart-shaped pattern characteristic of a cardioid. The simulations demonstrated that it is possible to develop a system capable of autonomously executing the pre-defined flight patterns. This challenges the conventional notion that underwater drones must be operated manually using a controller. In the future, we plan to conduct experiments to verify whether the system can be implemented and operated on actual hardware. As a next step, we aim to develop coordinated operations using multiple drones for swarm control.
• An Iterative Orthogonal Frequency Division Multiplexing Receiver with Sequential Inter-Carrier Interference Canceling Modified Delay and Doppler Profiler for an Underwater Multipath Channel2024 · Suguru Kuniyoshi, Shiho Oshiro, Rie Saotome, Tomohisa Wada
• Scalability in Autoencoder-based OFDM Communication System2023 · This paper proposed Scalability in Autoencoder-based Orthogonal Frequency Division Multiplexing(OFDM) communication system. In the previous research, only the comparison between IEEE802.11a and Autoencoder by the conventional OFDM communication system was performed, and it was proved that the communication system created by Autoencoder exceeded the performance of the conventional system. Therefore, in this paper, it uses IEEE802.11n and compare whether it can be improved by expanding the bandwidth and using it.IEEE802.11n standard has an FFT length of 128, a subcarrier number of 114 (108 for data), and modulation schemes of Quadrature Phase Shift Keying(QPSK), 16 Quadrature Amplitude Modulation (16QAM). The GI length is 800ns and the symbol length is 4000ns. In the simulation, a computer simulation was performed using a conventional OFDM communication system and a communication system generated by Autoencoder. Assuming that the simulation environment had an Signal-to-Noise Ratio (SNR) of 0 to 30 and an amplitude r of 0.0 to 1.0, the Symbol Error Rate (SER) status for each SNR was output. As a result of computer simulation, QPSK converged at SNR 27 ㏈ at IEEE 802.11a, but was able to reduce SER overwhelmingly to SNR 12 ㏈ at IEEE 802.11n. Also, in 16QAM the convergence at r=0.0 is the same as for SNR22 ㏈, but in IEEE 802.11a it does not converge after r=0.6, but in IEEE 802.11n it does not converge only at r=0.9 and r=1.0 . As a future task, it will use IEEE802.11ac, which enables communication speeds several times faster than IEEE802.11n, examine whether it is possible to further improve accuracy. And it will continue our research to correspond MIMO communication.
• An ICI Canceling Underwater OFDM Communication System with 2-Step Modified Delay and Doppler Profiler2023 · In 2023, the first prototype of an Inter-Carrier-Interference (ICI) Canceling Underwater Orthogonal Frequency Division Multiplexing (OFDM) Communication system was proposed using modified Delay and Doppler Profiler (mDDP). However, the Bit Error Rate (BER) reduction was limited. As a result of continuous research, we found that the two-stage configuration of mDDP can dramatically improve BER reduction performance. This paper presents the results of pool experiments of OFDM communications in forward and reverse two-wave Doppler-shift environments. To mitigate the ICI effect, a modified Delay and Doppler Profiler (mDDP), which estimates not only attenuation, relative delay and Doppler shift but also sampling clock shift of each multi-path component, is utilized. After the 1st stage mDDP, the effect of ICI has been reduced from the Channel Transfer Function (CTF) and the more accurate CTF is processed by the second-stage mDDP, resulting in highly accurate channel parameter estimation and improved ICI cancellation in the final multi-tap equalizer. To verify this effect, two transmit transducers and one receive transducer were used in the pool experiment to create a two-wave inverse Doppler-shifted multipath environment. By increasing the number of equalizer taps, the BER reduction performance of 64QAM modulation was improved by approximately one order of magnitude for 2step mDDP compared to 1step mDDP.

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