Enhancing Shadow Fading Mitigation with RIS Using Two-Dimensional Orthogonal Pattern Division Multiple Access.

This paper addresses multipath interference introduced by reconfigurable intelligent surface (RIS) in mitigating shadow fading in wireless communication systems. To tackle this, we propose the use of two-dimensional orthogonal pattern division multiple access (OPDMA) with optimal frequency-hopping patterns. OPDMA effectively reduces interference from multipath propagation, Doppler effect, and inter-user interactions, eliminating the need for complex equalization and channel estimation. This simplifies receiver design and enhances system performance. The paper introduces optimal frequency-hopping patterns with ideal autocorrelation and cross-correlation properties, constructed using a two-dimensional cyclic shift based on algebraic Costas arrays. Simulation results demonstrate that the RIS-assisted OPDMA system significantly reduces the bit error rate (BER), improves system throughput, and outperforms existing one-dimensional methods, improving communication quality and system efficiency.