What Does an RF Fingerprint Identify? A Cross-Corpus Evaluation of Hardware-Family-Associated and Unit-Level Signal

Hyojung Han

RF fingerprinting benchmarks report how well a classifier separates transmitters, not whether that separability reflects the individual unit or its hardware family. We compare a benchmark's real hardware grouping with size-matched random groupings of the same devices, using calibrated, unclipped variational lower bounds under label-blind, device-level cross-fitting on devices never seen in training. On WiSig (OFDM Wi-Fi), the generation grouping exceeded all 999 matched random groupings (permutation p = 0.001, the resolution floor), although no grouping attained a positive certified bound, and the direction survives removing channel equalization. A preregistered replication on a LoRa corpus (chirp spread spectrum, three hardware models), with a front-end frozen on synthetic data, again exceeded all 999 (p = 0.001), and two sensitivity analyses judged by a reversal rule fixed in advance did not reverse the direction. This is a directional replication only: the excess magnitude reflects a miscalibrated null, the chain-rule decomposition leaves a residual of +0.296 bits, and capture order is undocumented. In a same-model corpus of 122 ESP32 radios, a temperature-calibrated held-out…