Capacity limit for faster-than-Nyquist non-orthogonal frequency-division multiplexing signaling

TL;DR

This paper derives the capacity limit of faster-than-Nyquist non-orthogonal FDM signals, demonstrating they can surpass Nyquist capacity, and introduces a fractional cosine transform-based NOFDM implementation with experimental validation.

Contribution

First mathematical expression for FTN NOFDM capacity limit is derived, showing potential for higher capacity than Nyquist signals, with a novel FrCT-NOFDM implementation.

Findings

FTN NOFDM can achieve higher capacity than Nyquist signals.

Experimental demonstration of FrCT-NOFDM with 25% faster transmission rate.

Mathematical proof of FTN NOFDM's capacity advantage.

Abstract

Faster-than-Nyquist (FTN) signal achieves higher spectral efficiency and capacity compared to Nyquist signal due to its smaller pulse interval or narrower subcarrier spacing. Shannon limit typically defines the upper-limit capacity of Nyquist signal. To the best of our knowledge, the mathematical expression for the capacity limit of FTN non-orthogonal frequency-division multiplexing (NOFDM) signal is first demonstrated in this paper. The mathematical expression shows that FTN NOFDM signal has the potential to achieve a higher capacity limit compared to Nyquist signal. In this paper, we demonstrate the principle of FTN NOFDM by taking fractional cosine transform-based NOFDM (FrCT-NOFDM) for instance. FrCT-NOFDM is first proposed and implemented by both simulation and experiment. When the bandwidth compression factor $\alpha$ is set to $0.8$ in FrCT-NOFDM, the subcarrier spacing is equal to $40\%$ of the symbol rate per subcarrier, thus the transmission rate is about $25\%$ faster than Nyquist rate. FTN NOFDM with higher capacity would be promising in the future communication systems, especially in the bandwidth-limited applications.