Interference Mitigation for Faster-Than-Nyquist $m$CAP in Bandlimited VLC: A Low-Complexity Gapped Index Modulation Approach

In this paper, a novel gapped index modulation (GIM) scheme is proposed and investigated to mitigate the inter-band interference (IBI) with low complexity in faster-than-Nyquist multi-band carrierless amplitude and phase modulation (FTN-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula>CAP) based bandlimited visible light communication (VLC) systems. More specifically, the introduction of FTN sampling reduces the required bandwidth to achieve the same data rate via spectrum compression in the frequency domain at the transmitter, which results in increased IBI. To efficiently mitigate the IBI, by innovatively combining low-complexity GIM with FTN-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula>CAP, the proposed GIM aided FTN-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula>CAP (FTN-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula>CAP-GIM) scheme achieves a target data rate with a narrower bandwidth. Compared with FTN-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula>CAP, a larger compression factor can be supported at a target bit error rate (BER) when applying the proposed FTN-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula>CAP-GIM scheme in bandlimited VLC systems. Numerical simulations and hardware experiments have been conducted to verify the superior BER performance of the proposed FTN-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula>CAP-GIM in comparison to the conventional FTN-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$m$</tex-math></inline-formula>CAP scheme. The experimental results show that the feasible compression factor is increased from 0.22 to 0.38 when FTN-4CAP is replaced with the proposed FTN-4CAP-GIM at a target BER of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$3.8 \times 10^{-3}$</tex-math></inline-formula> with a roll-off factor of 0.5, while FTN-6CAP-GIM with a roll-off factor of 0.5 achieves a maximum compression factor of 0.44 at BER = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$3.8 \times 10^{-3}$</tex-math></inline-formula>. Moreover, FTN-6CAP-GIM with a roll-off factor of 0.5 also achieves a substantial spectral efficiency (SE) improvement of 78.2% in comparison to conventional 6CAP.