Deployment of Acoustic Reconfigurable Intelligent Surface in Underwater Acoustic Communications Under Deep-Sea Long-Range Scenarios

As the only technology that can support kilometer-level underwater communications, the UnderWater Acoustic Communications (UWAC) suffers from low data rate which is unable to support various emerging marine applications. The recently proposed acoustic reconfigurable intelligent surface (aRIS) has been considered to have the ability to improve the data rate and the coverage of UWAC systems, since it can control the direction of acoustic signal propagation and realize beamforming. The deployment of aRIS is critical to realize its data rate and coverage enhancement capabilities. However, how to optimize the aRIS deployment in deep-sea long-range scenarios has not been studied. To fill in this gap, a transmission loss (TL) model for aRIS-aided UWAC systems is established in this paper, where the beamforming gain and signal amplification gain of aRIS are characterized. Then, based on the TL model, a deep sound channel axis focusing (DSCAF) deployment architecture is proposed and analyzed for aRIS-aided UWAC systems in deep-sea long-range scenarios. In the proposed architecture, one aRIS each is deployed near the transmitter and receiver at the depth of the deep sound channel axis. By utilizing the beamforming capability of aRISs, the proposed DSCAF deployment architecture can guide acoustic signals into and out of the deep sound channel. This will ensure the transmission is fully concentrated in the deep sound channel, thus reducing the TL and enhancing the coverage in a distant receiving area. The analysis proves that the proposed scheme enjoys asymptotic optimality of TL as the number of aRIS elements increases compared to deploying aRISs at other depths. Simulation results have illustrated that the proposed architecture can reduce the TL and improve the spectrum efficiency in a long-distance receiving area.