Prior-Free High-Resolution Sensing for Bi-Static ISAC Systems

Abstract

Uplink sensing in perceptive mobile networks (PMNs), where the user equipment transmits and the base station receives, represents an attractive and important type of integrated sensing and communication (ISAC). However, its bi-static configuration introduces clock asynchrony that leads to time-varying timing offset (TO) and composite phase offset (CPO), which disrupts phase coherence, thereby invalidating conventional sensing algorithms that rely on synchronization assumptions. Moreover, even with phase coherence restored, the delay ambiguity induced by the global TO cannot be fully resolved without prior relative location information, thereby hindering accurate sensing. To address these fundamental challenges, this paper proposes a novel three-step prior-free sensing framework. First, we develop two closed-form subspace-based high-resolution relative TO (RTO) estimators with respect to a reference frame, which enable TO alignment in both line-of-sight (LoS)-dominant and LoS-absent scenarios. Then, we propose a subspace-based relative CPO (RCPO) estimation method, which establishes an analytical relationship between RCPO and the signal subspace and solves the resulting constant-modulus constrained optimization problem efficiently using Riemannian gradient descent (RGD). Finally, leveraging the estimated delay, AoA, and AoD, we develop a geometric-consistency-based association method that resolves the delay ambiguity and establishes parameter associations without requiring prior information. Our simulations demonstrate that the proposed methods robustly deliver superior accuracy even under low signal-to-noise ratio (SNR) conditions.