Performance Analysis of OTFS-RSMA for LEO-ISAC: Ergodic SINR and Sensing Bounds

Abstract:
This paper develops a comprehensive analytical framework for a downlink (DL) Low Earth Orbit (LEO) Integrated Sensing and Communication (ISAC) system using Orthogonal Time Frequency Space (OTFS) and Rate-Splitting Multiple Access (RSMA) to address the challenges posed by high Doppler shifts. For communication, we derive a refined Signal-to-Interference-plus-Noise Ratio (SINR) expression for a Bussgang Linear Minimum Mean Square Error (BLMMSE) receiver that rigorously accounts for practical impairments such as Imperfect Channel State Information (iCSI). For sensing, we establish the fundamental Cramer-Rao Bound (CRB) and reveal that symmetric waveforms induce parameter orthogonality, a condition manifested by a diagonal Fisher Information Matrix (FIM) with significant information-geometric implications. The accuracy of our analytical models is validated via Monte Carlo simulations showing excellent agreement with theory. The findings demonstrate that the proposed framework is a robust solution for LEO- ISAC and that the discovered orthogonality offers a clear design principle for reliable sensing