Faster-Than-Nyquist N-Dimensional CAP Modulation Scheme Based on Slicing Sphere Decoder for Visible Light Communication

Visible light communication (VLC) based on light-emitting diodes (LEDs) is foreseen to be a key technology in indoor scenarios for next-generation (6 G) networks due to its low cost, high data rate, and immunity to electromagnetic interference. However, the limited modulation bandwidth of LED poses great challenges to increasing data rate, and thus higher spectral efficiency (SE) is pivotal in LED-based VLC systems. Carrierless amplitude and phase (CAP) is a high SE modulation scheme, which is widely used in VLC systems. To facilitate multi-user communication, CAP has been extended to multi-band CAP or N-dimensional (N-D) CAP. However, the minimum bandwidth required for orthogonality hinders further improvement of SE. To address this issue, we introduce Faster-than-Nyquist (FTN) technique to the N-D CAP for the first time, such that the signal bandwidth can be further compressed and the pulse shaping filters (PSFs) are non-orthogonal. Compared with multi-band CAP, such a design does not require guard bands and ensures similar performance among multiple users. We develop a FTN N-D CAP PSFs approach by modifying minimax optimization program. Moreover, a low-complexity FTN N-D CAP modulation scheme based on slicing sphere decoder (SD) is proposed to mitigate intrinsic inter-symbol interference (ISI) and inter-channel interference (ICI) resulted from the non-orthogonality. FTN 4-D CAP and FTN 3-D CAP are experimentally demonstrated in a white-LED-based VLC system. The demo confirms that the maximum compression ratio reaches up to 70% with the BER threshold of 3.8E-3, which indicates that the SE is improved by 42.9% compared to orthogonal CAP.