Fractional Fourier Transform-Based OFDM in LoS-Dominant Multi-Cluster NTN Channels: A Structural Analysis

Abstract

Fractional Fourier transform (FrFT)-based OFDM has been reported to improve banded equalization when channel energy is approximately aligned with a single FrFT-diagonal ridge. This letter examines that premise for a LoS-dominant multi-cluster non-terrestrial network (NTN) channel under a TR 38.811-inspired Ka-band parameter regime. A concise structural argument indicates that the optimal FrFT order tends to collapse to the DFT point when the line-of-sight (LoS) concentration loss caused by rotation exceeds the clustered-part concentration gain available in the FrFT domain. Synthetic-operator sanity tests verify that the discrete FrFT implementation and the orderselection rule recover non-DFT optima when such optima exist. Monte Carlo BER results indicate no measurable FrFT gain over FFT-OFDM across κ ∈ {1000, 1300, 1600} Hz/sample and two geometry assumptions, while a slight penalty appears at narrow equalization bandwidths.