On Estimation of Angles of Arrival in Monostatic ISAC Without Instantaneous Transmit CSI

TL;DR

This paper analyses a monostatic setting where the BS performs multi-target Angle of Arrival (AoA) estimation while simultaneously communicating with one of the targets, and demonstrates that leveraging updated BCRB-based sensing information for the communication receiver enables significantly improved communication rates.

Abstract

This paper explores the fundamental limits of Integrated Sensing and Communication (ISAC) in a more realistic setting compared to previous literature when the Base Staion (BS) has only statistical CSI of the communication user rather than full CSI. We analyze a monostatic setting where the BS performs multi-target Angle of Arrival (AoA) estimation while simultaneously communicating with one of the targets. We assume that the BS has statistical CSI about all AoAs, with less uncertainty in the AoA of the communication receiver. The communication receiver is assumed to have perfect CSI. Utilizing a Bayesian Cramér-Rao Bound (BCRB) framework to characterize the fundamental limits of sensing under minimum mean square error (MMSE) criteria, we derive achievable BCRB-rate trade-off regions. Our approach introduces a number of transmission strategies that share power across sensing and communication beams over a coherence time. Our analysis reveals that beam allocation strategies leveraging the principal eigenvectors of the target-specific sensing matrices minimize individual AoA estimation errors, while strategies balancing sensing and communication directions optimize joint estimation performance at the cost of individual accuracy. We demonstrate that leveraging updated BCRB-based sensing information for the communication receiver, due to its lower channel uncertainty, enables significantly improved communication rates.

Figures (5)

• Figure 1: Representation of the considered setting: one communication target (UE) that is also a sensing target and a separate sensing-only target.
• Figure 2: Achievable BCRB-rate regions under Choices G1–G4 for sensing and communication angles $\bar{\theta}_s = 30^\circ$, $\bar{\theta}_c = 100^\circ$, with initial standard deviation $\sigma_{\theta} = 30^\circ$ for both angle. The black lines indicate the minimum achievable BCRB for each target.
• Figure 3: BCRB-rate regions for deterministic or information-less beam scenario under Choices S1–S6 (Special Case 1).
• Figure 4: Achievable BCRB-rate regions comparison for main strategy where we transmit both deterministic and Gaussian (Main Strategy in \ref{['eq:transmit_main']}), and only Gaussian. The dashed blue and green lines indicate the minimum achievable CRBs for each case, obtained by computing the sum of individual minimum possible CRBs of both targets. The horizontal red dashed line represents the capacity upper bound $C'$ derived in \ref{['eq:c_prime']}.
• Figure 5: Comparison of achievable ISAC regions highlighting the impact of prior standard deviation $\sigma_{\theta}$ and angular separation between sensing and communication targets, with $\bar{\theta}_s$ fixed at $30^\circ$.