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arXiv · 2610.12066

FTN Signaling: Spectral Efficiency from BPSK to 16-QAM

Emre Cerci·Adem Cicek·Melda Yuksel·Gokhan M. Guvensen·Enver Cavus·Yaser Dalveren·Halim Yanikomeroglu
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Abstract

Faster-than-Nyquist (FTN) signaling can improve spectral efficiency by transmitting symbols closer together than the Nyquist limit, at the cost of additional inter-symbol interference (ISI). In this letter, we study the finite-alphabet symbol-wise achievable information rate (AIR) of FTN signaling for BPSK, QPSK, and 16-QAM under a transmit-power constraint and a common minimum mean square error (MMSE) channel-shortening (CS) detection framework, cross-validated for BPSK against an independent reduced-state Ungerboeck Bahl-Cocke-Jelinek-Raviv (BCJR) benchmark. Our results show a clear trend. Lower-order constellations can benefit from more aggressive time acceleration, while higher-order constellations become less tolerant of acceleration and achieve their best spectral efficiency closer to the Nyquist limit. At a reference signal-to-noise ratio (SNR) of 6 dB the optimum acceleration factor shifts from $τ^\star\approx0.65$ for BPSK to 0.75 for QPSK and 0.90 for 16-QAM. We further show that this trend is preserved in coded systems and remains robust to the detector memory. The impact of FTN operation on the instantaneous-to-average power ratio (IAPR) is also examined. Overall, the results highlight how constellation order should be considered when selecting the operating point for FTN signaling.

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Emre Cerci, Adem Cicek, Melda Yuksel, Gokhan M. Guvensen, Enver Cavus, Yaser Dalveren, Halim Yanikomeroglu. 2026-10-08. FTN Signaling: Spectral Efficiency from BPSK to 16-QAM. https://arxiv.org/abs/2610.12066

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