Physically Transparent LBL Modeling of 0.3–1 THz Attenuation: Windowed Foreign-Continuum Scaling and Validation

This study presents a modular Line-By-Line (LBL) model for atmospheric attenuation over the 0.3–1 THz band, separating spectral lines (from HITRAN with exact Voigt evaluation), water-vapor continuum components (MT_CKD 4.3, independently reading self and foreign terms), and dry-air collision-induced absorption (from N₂/O₂). The computational workflow mirrors a reference FTS setup in MATLAB, including high-resolution spectra, 3-GHz transmission binning, and recovery of α. During validation with an independent FTS dataset, the uncorrected LBL model shows a consistent underestimation in specific valley regions, though line peaks remain accurately captured. To mitigate this bias, a frequency dependent foreign-only scaling S(f) is applied—affine in frequency, smoothly windowed between 600–980 GHz, with damping γ = 0.60 and a dominance rule to limit misapplication. Tested across four humidity scenarios, the corrected model significantly reduces valley errors—most notably between 800–900 GHz—yielding MAE reductions of 50–90% in the Z5 window and pulling local biases closer to zero. Band-averaged metrics improve more modestly (around 2–3%) due to unchanged peaks. Under the tested conditions, the dry background remains negligible and does not explain the offset. In link-budget terms, compensating this valley bias improves margin reliability in usable windows by several tens of dB/km. The resulting LBL×S(f) model thus offers a clear, effective framework for planning high-capacity THz links.