Performance Limits of Hardware-Constrained THz Inter-Satellite MIMO-ISAC Systems

Terahertz inter-satellite links (THz-ISL) offer unprecedented bandwidth for future space networks but face fundamental constraints from onboard power and thermal budgets. This paper establishes theoretical performance limits for MIMO Integrated Sensing and Communication (ISAC) systems under per-element constant-envelope (CE) transmission constraints. We demonstrate that hardware distortions -- specifically power amplifier nonlinearity, ADC quantization, and oscillator phase noise -- impose a capacity ceiling that cannot be overcome by increasing transmit power. A unified link budget framework integrates wideband beam squint, aperture pointing errors, and colored noise sources through a spectral consistency principle that ensures residual phase noise is counted exactly once across communication and sensing analyses. The sensing bounds are derived via the Whittle-Fisher Information Matrix under a Constant Acceleration kinematic model with jerk noise, yielding closed-form scaling laws: residual phase noise variance scales as $α^{-1}$ while dynamic state-estimation error (DSE) variance scales as $α^{-5}$ with pilot overhead $α$. Numerical results show divergent MIMO scaling: sensing precision improves with array size ($\mathrm{RMSE} \propto 1/\sqrt{N_t N_r}$), while the critical SNR exhibits scale invariance regarding array size, implying that the distortion-limited transition point stabilizes regardless of the array scale. The steep $α^{-5}$ DSE scaling creates an operationally infeasible region at $α< α^* \approx 0.16$, where $α^* = (C_{\mathrm{DSE}}/C_{\mathrm{PN}})^{1/4}$ -- a constraint-driven threshold under the adopted baseline for LEO operation. These findings provide design guidelines for hardware-efficient THz-ISL constellations.