Holographic Localization With Synthetic Reconfigurable Intelligent Surfaces

Reconfigurable intelligent surfaces (RISs) are proposed to control complex wireless environments in next-generation networks. In particular, wideband RISs can play a key role in high-accuracy location awareness, which calls for models that consider the frequency-selectivity of metasurfaces. This paper presents a general signal model for wideband systems with RISs and establishes a Fisher information analysis to determine the theoretical limits of wideband localization with RISs. In addition, synthetic RISs are proposed to mitigate the multiplicative fading effect caused by the scattering property of RISs. Special scenarios including complete coupling and complete decoupling are further investigated. Results show that with the proposed models, a wideband RIS with a polynomial phase response per element provides more position information than those with more degrees of freedom (DOFs) in piecewise-constant phase response per element. Furthermore, velocity-induced information allows a dynamic RIS to provide more position information than a static RIS. Additionally, a dynamic RIS can be synthesized through multiple measurements to outperform a large one.