A Telemetry-Native Boundary Drift Index for Detecting Receiver Tracking Collapse Under Finite Adaptation Bandwidth

Receiver performance analyses commonly assume stationary additive white Gaussian noise (AWGN) and sufficient estimator adaptation bandwidth to track time-varying channel parameters. In deployed systems, adaptation bandwidth is finite, and parameter drift can induce decoder mismatch leading to abrupt block error rate (BLER) escalation.This preprint introduces the Boundary Drift Index (BDI), a telemetry-native scalar derived from pilot-based residual variance tracking. BDI quantifies the ratio between observed boundary drift rate and estimator adaptation bandwidth. Under controlled log-variance drift, simulation demonstrates a sharp operational transition between nominal and collapse BLER regimes when BDI exceeds an empirical threshold BDI_c. The transition is stable across signal-to-noise ratios of 0–12 dB and confirmed across independent random seeds.The contribution is operational rather than theorem-level. BDI does not alter classical capacity or mismatched decoding bounds; it provides a receiver-side diagnostic for identifying when stationary-AWGN characterisations cease to be operationally reliable under finite tracking dynamics.Two explicit falsifiers are pre-registered and evaluated:F1 — No threshold (failure of sharp BLER transition aligned to BDI)F2 — No diagnostic surplus (threshold behaviour reducible to instantaneous SNR alone)Across 360 parameter combinations, 99.0% of collapsed runs satisfy the sharpness criterion Delta_BDI ≤ 10. Threshold location is stable across SNR but conditional on estimator parameters (alpha, W_d), which must be reported alongside any BDI measurement.All simulation code, raw scalar results, and a reproducibility guide are included in the Zenodo record.Filed within the author’s TRIDENT analytic corpus as INFO-BDI-001.v1 under USDOP v1.2 falsifiability standards.boundary drift indexadaptive receiverschannel estimationnonstationary variancetracking collapsefinite adaptation bandwidthtelemetry diagnosticsmismatched decodingpilot-aided estimationoperational threshold detection Filed within the author’s TRIDENT analytic corpus as INFO-BDI-001.v1 under USDOP v1.2 falsifiability standards.