A Unified KLD Framework for Duplexity and Deployment Paradigms in Cell-Free mMIMO-ISAC

Abstract: This paper develops a unifying analytical framework for comparing deployment and duplexing paradigms in distributed cell-free massive multiple-input multiple-output (CF-mMIMO) integrated sensing and communication (ISAC) systems. The system comprises distributed access points (APs) serving multiple downlink and uplink users while simultaneously detecting radar targets. Four configurations are analysed - separated and shared AP deployment under half-duplex (HD) and full-duplex (FD) operation, each incorporating realistic impairments: residual self-interference (SI) from transmit-receive leakage, imperfect interference cancellation due to channel estimation errors, and clutter. Kullback-Leibler divergence (KLD) is applied to serve as a unified measure, enabling direct comparison of communication and radar performance on a common scale. A generalised likelihood ratio test (GLRT) framework is developed to produce closed-form expressions linking KLD to detection probability. Monte Carlo simulations are used to verify our expressions, which demonstrate that FD operation achieves substantial gains over HD, provided sufficient SI suppression and IC quality are maintained, while preserving strong radar detection. It is also shown that shared deployment enhances radar performance via a larger effective aperture but exhibits tighter communication-radar coupling than separated deployment. These results establish deployment guidelines and quantitative design thresholds for next-generation CF-mMIMO ISAC systems.