DoF-achieving distance-aligned structure for layered underwater acoustic 2x3x3 X networks

In this paper, we investigate the degrees of freedom (DoF) of a layered 2 × 3 × 3 X channel for underwater acoustic networks (UANs), where each of the two source nodes sends an independent message to each of the three destination nodes via the relay layer with three nodes. The key problem is to maximize the achievable DoF with proper interference management. Using the long propagation delay property of UANs, we propose a feasible scheme based on distance-aligned structure (DAS), which achieves the DoF upper bound of 3/2. The core idea is to obtain six decodable message symbols over four time slots by proper transceiver design with an aligned interference neutralization approach.