Scalable Cell-Free ISAC for Drone Surveillance with Hybrid Signaling and Dynamic TDD

Future 6G wireless networks are expected to support both reliable communication and native sensing, making Integrated Sensing and Communication (ISAC) over Cell-Free Massive MIMO a promising architecture for low-altitude drone surveillance. This task is a concrete and demanding mission for future systems: the network must monitor a three-dimensional airspace, detect small and mobile aerial targets, and maintain situational awareness while continuing to serve communication users. This application motivates moving beyond generic sensing formulations, because reliable surveillance requires voxel-wise search, multi-static visibility, bounded fronthaul, and communication-constrained operation. However, combining distributed multi-static sensing with user-centric Cell-Free operation raises major challenges in scalability, duplexing, fronthaul, and Cross-Link Interference. These challenges become even more critical when sensing must be integrated into communication-native protocols without relying on unscalable centralized processing or excessive signaling overhead. This thesis proposes a scalable Cell-Free ISAC framework based on Dynamic Time Division Duplex (DTDD), in which distributed Access Points are dynamically assigned to downlink illumination or uplink communication-and-sensing roles, and sensing is performed under explicit communication constraints. The proposed framework combines a hybrid downlink signaling strategy, a bounded-complexity scheduling and control architecture, and a scalable uplink reception and distributed coherent detection pipeline. In particular, the system is designed so that communication feasibility is preserved at all times, while the remaining spatial and power resources are exploited to support reliable voxel-based drone surveillance over the monitored airspace. Overall, the thesis shows that multi-static drone surveillance can be embedded into Cell-Free wireless networks in a way that is both operationally explicit and structurally scalable, while preserving the main benefits of coordinated processing, communication-native probing, and distributed sensing diversity. The results support the view that Cell-Free ISAC can move beyond conceptual formulations and be developed as a practical protocol-level architecture for next-generation wireless surveillance systems.