A Bistatic ISAC Framework for LEO Satellite Systems: A Rate-Splitting Approach

TL;DR

This work proposes a RSMA-based bistatic LEO-ISAC system with a radar receiver separated from the satellite to mitigate echo-path loss while delivering global connectivity and sensing. It develops a CRB-constrained precoder design solved via SDR, SROCR, and SCA to maximize the minimum user rate using geometry- and statistics-based channel knowledge in the absence of iCSI. The framework also includes a bistatic-specific radar parameter estimation pipeline combining MUSIC with a joint delay–Doppler–AOD matcher, and demonstrates via simulations that bistatic geometry and common-stream management yield superior interference control and radar performance. The results indicate strong potential for practical LEO satellite networks capable of simultaneous communication and radar sensing under limited power and CSI resources.

Abstract

Aiming to achieve ubiquitous global connectivity and target detection on the same platform with improved spectral/energy efficiency and reduced onboard hardware cost, low Earth orbit (LEO) satellite systems capable of simultaneously performing communications and radar have attracted significant attention. Designing such a joint system should address not only the challenges of integrating two functions but also the unique propagation characteristics of the satellites. To overcome severe echo signal path loss due to the high altitude of the satellite, we put forth a bistatic integrated sensing and communication (ISAC) framework with a radar receiver separated from the satellite. For robust and effective interference management, we employ rate-splitting multiple access (RSMA), which splits and encodes users messages into private and common streams. We optimize the dual-functional precoders to maximize the minimum rate among all users while satisfying the Cramer-Rao bound (CRB) constraints. Given the challenge of acquiring instantaneous channel state information (iCSI) for LEO satellites, we exploit the geometrical and statistical characteristics of the satellite channel. To develop an efficient optimization algorithm, semidefinite relaxation (SDR), sequential rank-1 constraint relaxation (SROCR), and successive convex approximation (SCA) are utilized. Numerical results show that the proposed framework efficiently performs both communication and radar, demonstrating superior interference control capabilities. Furthermore, it is validated that the common stream plays three vital roles: i) beamforming towards the radar target, ii) interference management between communications and radar, and iii) interference management among communication users.

Figures (8)

• Figure 1: Overall system architecture of the proposed bistatic LEO-ISAC framework.
• Figure 2: The proposed bistatic LEO-ISAC system model.
• Figure 3: Geometrical relationship among the radar receiver, LEO satellite, and the radar target in the proposed framework.
• Figure 4: Echo path loss with increasing target altitude in the bistatic and monostatic structures at the different target's $x$-$y$ coordinates.
• Figure 5: Minimum rate performance under the various transmission modes with the CRB thresholds $\gamma_\theta^{\sf th}=\gamma_\phi^{\sf th}=8\times10^{-7}~{\rm rad}$.

Theorems & Definitions (4)

• Remark 1
• Remark 2
• Remark 3
• Remark 4