Near Field Sensing using OFDM and Reconfigurable Intelligent Surfaces

Sammanfattning

This thesis investigates near-field sensing using orthogonal frequency division multiplexing (OFDM) waveforms and a reconfigurable intelligent surface (RIS) in a bistatic integrated sensing and communication (ISAC) scenario. The work focuses on RIS-assisted inverse scattering under limited bandwidth conditions, where conventional delay based radar methods provide insufficient range resolution. Instead, the sensing approach exploits the spatial structure of near field wavefronts. A mutual coupling aware RIS model is developed to account for electromagnetic interactions between RIS elements, addressing limitations of conventional independent element models commonly used in the signal processing literature. Based on this model, sensing formulations and RIS constellation design methods are developed for OFDM systems, including the effects of synchronization impairments such as carrier and sampling frequency offsets. Simulation results demonstrate how mutual coupling influences RIS assisted sensing behavior and illustrate the potential of near-field OFDM sensing using practical low-resolution RIS hardware.