DFT-Based Spectral Precoding for Scalable Unique-Word OFDM

Abstract

Unique-word orthogonal frequency division multiplexing (UW-OFDM) addresses the spectral inefficiency of cyclic prefix (CP) OFDM by embedding the guard interval within the symbol period. However, its practical adoption is hindered by a prohibitive computational bottleneck. Conventional methods for embedding unique word within the DFT window rely on complex matrix inversions at the transmitter (Tx). While alternative zero-padded (ZP) schemes avoid this complexity, they place guard interval outside DFT window, breaking natural circularity and necessitating noise-enhancing Overlap-Add receiver (Rx). This letter proposes a scalable, closed-form isometric Spectral Precoding architecture that resolves this trade-off. By exploiting frequency-domain interpolation, we implicitly construct the required near-zero-energy tail within the DFT window using only deterministic linear transformations. We prove this construction reduces Tx complexity to log-linear order while preserving standard single-tap equalization without Rx noise enhancement. Validated with 5G-NR numerologies, the scheme matches