# Faster-than-Nyquist Non-Orthogonal Frequency-Division Multiplexing for Visible Light Communications

## TL;DR

This paper introduces a faster-than-Nyquist non-orthogonal frequency-division multiplexing scheme using FrCT for visible light communications, offering higher spectral efficiency, lower bandwidth usage, and improved BER performance.

## Contribution

The paper proposes a real-valued FrCT-NOFDM scheme for VLC that reduces bandwidth and enhances spectral efficiency compared to traditional OFDM, with better BER and security performance.

## Key findings

- 20% bandwidth saving at bandwidth compression factor 0.8
- Better BER performance than OFDM at similar transmission rates
- Enhanced security features of FrCT-NOFDM

## Abstract

In this paper, we propose a faster-than-Nyquist (FTN) non-orthogonal frequency-division multiplexing (NOFDM) scheme for visible light communications (VLC) where the multiplexing/demultiplexing employs the inverse fractional cosine transform (IFrCT)/FrCT. Different to the common fractional Fourier transform-based NOFDM (FrFT-NOFDM) signal, FrCT-based NOFDM (FrCT-NOFDM) signal is real-valued which can be directly applied to the VLC systems without the expensive upconversion. Thus, FrCT-NOFDM is more suitable for the cost-sensitive VLC systems. Meanwhile, under the same transmission rate, FrCT-NOFDM signal occupies smaller bandwidth compared to OFDM signal. When the bandwidth compression factor $\alpha$ is set to $0.8$, $20\%$ bandwidth saving can be obtained. Therefore, FrCT-NOFDM has higher spectral efficiency and suffers less high-frequency distortion compared to OFDM, which benefits the bandwidth-limited VLC systems. As the simulation results show, bit error rate (BER) performance of FrCT-NOFDM with $\alpha$ of $0.9$ or $0.8$ is better than that of OFDM. Moreover, FrCT-NOFDM has a superior security performance. In conclusion, FrCT-NOFDM shows great potential for application in the future VLC systems.