Complementary failure modes of SRAM-PUF and RF fingerprinting under compound thermal–distance stress in LoRaWAN sensor nodes: A device-level empirical study

Physical-layer authentication for LoRaWAN sensor nodes increasingly relies on SRAM Physical Unclonable Functions (PUFs) or Radio Frequency (RF) fingerprinting, yet prior work has not reported the joint failure trajectories of both modalities under identical, directly measured stress on the same physical device. This paper reports a device-level finding (referred to here as D4) obtained with a calibrated DS18B20 temperature probe ((Formula presented) °C) bonded adjacent to the SRAM, so that every authentication outcome is paired with a measured rather than estimated temperature.Using a stressor-decoupling protocol on Device C (STM32 B-L072Z-LRWAN1 + Microchip 23K640 SRAM), we show that no single stressor drives the PUF past its 15% authentication threshold: at a fixed 50cm range the genuine fractional Hamming distance (FHD) rises smoothly and monotonically from 3.0% to only 10.9% across 27.4–56.1°C (0.374%/°C), with a False Reject Rate of 0% throughout. Distance alone (＋2.2pp) and elevated temperature alone (＋7.0pp) each leave the PUF below threshold. PUF authentication fails (Genuine Accept Rate (Formula presented), FHD 17.7%) only under the compound condition (50°C ＋ 100cm), where the observed degradation exceeds the sum of the individual stressors, indicating a super-additive thermal–distance interaction. Throughout the same sweep the Carrier Frequency Offset (CFO) fingerprint drifts only −14.9Hz/°C (Spearman (Formula presented)), matching an AT-cut crystal model and remaining within-device stable (158Hz) against thousands-of-Hz inter-device separation (ANOVA (Formula presented)), so RF discriminability is preserved precisely where the PUF fails.The compound failure reproduces across both MCU platforms (Feather M0 and STM32), while a device sharing the SRAM2 chip with the failing STM32 node but hosted on a Feather remains robust (FHD 7.6%, GAR 100%), indicating platform-dependent rather than chip-intrinsic behaviour. These findings motivate, as a design implication, adaptive hybrid authentication in which RF-only fallback is invoked when PUF reliability degrades. The observations are drawn from a small device set and require validation on a larger population.