Sovereign Cognitive Digital Twins: Fusing 6G ISAC, AI-RAN, and Zero-Trust Edge Grids for National Resilience in the Global South
Small-island developing states face accelerating sea-level rise, intensifying cyclones, and storm surge, while suffering the sparse ground instrumentation that makes timely hazard perception difficult. This paper argues for a shift from passive cellular connectivity to the Network as a Sensor, realized through a Sovereign Cognitive Digital Twin (S-CDT): a federated national digital twin whose perceptual substrate is the 6G radio interface itself. Instead of disjoint sensing systems, a nation can reuse the Integrated Sensing and Communication (ISAC) waveforms of its own network as a distributed radar mesh. We specify a six-layer S-CDT stack in which ISAC collapses the boundary between the dynamic-data and communication layers; we map the physical layer to the ETSI GR ISC 001 and 3GPP Release 19 sensing frameworks; and we formulate a belief-state control loop, an Extended Kalman Filter feeding a Proximal Policy Optimization agent, designed to absorb O-RAN telemetry delay. That loop is specified but not evaluated here. Beyond the reference architecture, we implement a reproducible, CPU-only geodata-to-ray-tracing pipeline over a 2 km study area at the Barbados Heritage District, Newton Plantation: 576 LiDAR-height buildings, a 70x70 terrain grid, and the government tower register become a Sionna RT scene. On identical geometry, median best-server path gain falls from -106 dB at 1.8 GHz to -122 dB at 10 GHz; concrete-only 28/60 GHz runs yield -130/-136 dB, with coverage contracting to line-of-sight lobes. These uncalibrated, 1x1 V-polarized simulations are framed as a towards-6G site model of the background channel, not an operational ISAC deployment. We treat data sovereignty and physical-layer zero-trust security as first-order design constraints. Barbados (166 km2, ~280k population) is the reference deployment, with the Philippines as an archipelagic generalization.