Original Theoretical Contribution to Channel Capacity Bounds for Reconfigurable Intelligent Surface-Assisted MIMO Systems in Tropical Propagation Environments

Reconfigurable intelligent surfaces have emerged as a disruptive paradigm for passive wireless channel engineering, yet the fundamental information-theoretic capacity limits of RIS-assisted MIMO communication under the correlated fading conditions characteristic of tropical propagation environments remain analytically uncharacterized. This doctoral study derives original theoretical channel capacity bounds for RIS-assisted MIMO systems operating in tropical propagation environments, with analysis parameterized by channel measurements conducted in the South South and South East geopolitical zones of Nigeria. A novel channel model incorporating the spatial correlation structure induced by dense tropical vegetation scattering, derived from a directional channel measurement campaign at 3.5 GHz and 28 GHz in Benin City and Enugu, is proposed and validated. Closed-form expressions for ergodic capacity upper and lower bounds of the RIS-assisted MIMO system under the proposed correlated channel model are derived using Jensen's inequality and stochastic geometry tools. The tightness of the derived bounds is characterized analytically as a function of RIS element count, spatial correlation coefficient, and signal-to-noise ratio. A novel phase configuration algorithm exploiting the derived channel model structure achieves capacity within 4.1% of the theoretical upper bound in simulation. The original capacity bounds derived in this study advance the theoretical understanding of RIS-assisted communications in tropical environments absent from existing international literature, providing a mathematically rigorous foundation for RIS deployment optimization in Nigerian and broader West African 5G network contexts. Keywords: reconfigurable intelligent surface, channel capacity, MIMO, tropical propagation, information theory.