Device-Free Localization in ISAC Networks: Performance Limits and Fisher Information-Based Cooperative Localization

Cooperative device-free localization, which involves node cooperation for location-awareness services, represents pivotal opportunities for forthcoming integrated sensing and communication (ISAC) networks. In this paper, a theoretical framework is established to quantify the performance limits of device-free localization accuracy in ISAC networks. It is achieved through Fisher information analysis, accounting for signal randomness, transceiver deployment, multi-target, multipath, etc. To explore node cooperation, the theoretically optimal linear fusion, inspired by the best linear unbiased estimator, is derived. It linearly combines independent target location estimates from multi-static transceivers for localization accuracy enhancement. The optimal weights are determined by the corresponding effective Fisher information, which later constitutes the definition of the Fisher Information Field (FIF), as a spatial field to quantify useful information that can be gained through location estimation. An FIF-based practical location estimator using multi-source estimates is further developed. Finally, a case study on an orthogonal frequency division multiplexing ISAC network is conducted. The target is independently localized from multi-static transceivers using joint range-angle maximum likelihood estimation; closed-form evaluations of the Fisher information are derived for performance limits study and FIF-based cooperative localization. Numerical results offer insights into the Fisher information analysis and demonstrate low complexity and high accuracy of the FIF-based estimator compared with benchmarks.