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Terahertz (THz) communication signals are susceptible to severe degradationbecause of the molecular interaction with the atmosphere in the form ofsubsequent absorption and re-radiation. Recently, reconfigurable intelligentsurface (RIS) has emerged as a potential technology to assist in THzcommunications by boosting signal power or providing virtual line-of-sightpaths. However, the re-radiated energy has either been modeled as anon-line-of-sight scattering component or as additive Gaussian noise in theliterature. Since the precise characterization is still a work in progress,this paper presents the first comparative investigation of the performance ofan RIS-aided THz system under these two extreme re-radiation models. Inparticular, we first develop a novel parametric channel model that encompassesboth models of the re-radiation through a simple parameter change, and thenutilize that to design a robust block-coordinate descent (BCD) algorithmicframework which maximizes a lower bound on channel capacity consideringimperfect channel state information. In this framework, the original problem issplit into two sub-problems: a) receive beamformer optimization, and b) RISphase-shift optimization. We also analytically demonstrate the limitedinterference suppression capability of a passive RIS. Our numerical resultsalso demonstrate that slightly better throughput is achieved when there-radiation manifests as scattering.

Robust Optimization of RIS in Terahertz under Extreme Molecular Re-radiation Manifestations