Distortion Is Not Noise: On the Limits of the Kappa Model for Monostatic ISAC

TL;DR

This paper investigates the limitations of the Kappa distortion model in monostatic ISAC sensing, deriving new bounds that account for waveform distortions and revealing an irreducible velocity-error floor, with simulations confirming robustness.

Contribution

It introduces PA-aware and PN-aware CRBs for monostatic ISAC, showing the Kappa model's pessimism and quantifying the sensing performance limits.

Findings

Kappa model overestimates sensing degradation in monostatic ISAC.

Derived CRBs reveal an irreducible velocity-error floor.

Simulation results confirm robustness to practical DPD errors.

Abstract

Monostatic ISAC sensing differs from communication because the transmitter can monitor its distorted transmit waveform. Thus, the aggregate $\kappa$ distortion model, which treats impairments as unknown noise, is appropriate for communication but pessimistic for monostatic sensing. We derive PA-aware sensing Cram\'er--Rao bounds (CRBs) and a PN-aware CRB that reveals an irreducible velocity-error floor, and quantify when $\kappa$-based bounds overestimate sensing degradation. Simulations validate the analysis and show robustness to practical DPD template errors (less than 1~dB overhead at a typical $-25$~dB NMSE).