Reliable and robust underwater acoustic and optical wireless communication solutions

In recent years, underwater communications have demonstrated immense potential across various applications, including natural resource detection, scientific ocean exploration, oceanic environmental monitoring, and communications between submarines/autonomous robots. For many of these applications, a reliable underwater wireless communication system is in urgent demand (for various ranges: short, medium, and long). Diverging from the well-established wired communications on land, underwater wireless communications offer unparalleled advantages for the unhindered future development of underwater systems, including communication range and locations, which fosters dynamic and flexible exploration and monitoring capabilities. Underwater Acoustic Communication (UAC) and Underwater Optical Wireless Communication (UOWC) are two mainstream underwater wireless communications. UAC experiences less attenuation and covers up to several kilometers, making it the preferred choice for medium/long-range underwater communications. However, UAC's propagation speed is as slow as 1500m/s, leading to high latency and multipath delay. The bandwidth is as narrow as approximately 100 kHz, and the data rate is limited. Additionally, the dynamic and unpredictable water waves further introduce severe Doppler effects. In comparison, UOWC achieves a wide bandwidth of hundreds of MHz and a high data rate of Gbps, covering distances up to about 100m. Nonetheless, UOWC contends with attenuation caused by absorption, scattering, and turbulence in water. Moreover, solar background noise also interferes with its performance, especially in shallow water and during daytime operations. Hence, this research aims to design novel communication solutions to support robust, reliable, and high-data-rate underwater acoustic or optical wireless communications. In particular, these objectives will be achieved by: Proposing a novel nonlinear and noncoherent modulation scheme for UAC, called Circular Time Shift Modulation (CTSM), to defend against the multipath effects and additive noise in the underwater acoustic environment. Improving the system reliability and transmission rate for UAC by introducing a novel transmission scheme called Spatial Modulation-based Orthogonal Signal Division Multiplexing (SM-OSDM). Proposing a novel transmission method, called Circular Spatio-Temporal Modulation (C-STM), which utilizes spatial diversity, Zero-Correlation-Zone (ZCZ) sequences, and Inverse Discrete Fourier Transform (IDFT) matrix to improve the performance of UACs in doubly spread channels.Designing a new multiple access technique for underwater acoustic networks, called Orthogonal Signal Division Multiple Access (OSDMA) based on the orthogonality of IDFT matrix. Developing Carrier Frequency Offset Compensated Orthogonal Signal Division Multiplexing (CFO-C-OSDM) to mitigate the effects of time-varying multipath delays and varying Doppler frequency offsets across different communication paths. Developing a new transceiver structure for UOWC with hybrid Orthogonal Frequency Division Multiplexing-Pulse Position Modulation (OFDM-PPM) and time-frequency spreading to improve the system robustness and coverage distance.