Spectral-Efficient Gapped Index Modulation for FTN-<i>m</i>CAP in Bandlimited VLC

A low-complexity gapped index modulation (GIM) scheme was proposed to mitigate inter-band interference (IBI) in faster-than-Nyquist multi-band carrierless amplitude and phase modulation (FTN-<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i>CAP), but incurs spectral efficiency (SE) loss due to silent subbands. To address this, three spectral-efficient GIM schemes are proposed. First, the quadrature GIM aided FTN-<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i>CAP (FTN-<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i>CAP-QGIM) is proposed, and an enhanced LLR detector is designed accordingly. Second, a variable GIM aided FTN-<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i>CAP (FTN-<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i>CAP-VGIM) scheme is proposed to support variable subband activation. Third, a variable quadrature GIM aided FTN-<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i>CAP (FTN-<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i>CAP-VQGIM) is proposed to further enhance the SE of FTN-<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i>CAP-VGIM. Extensive simulations and hardware experiments are conducted to evaluate the performance of three spectral-efficient GIM schemes. The experimental results demonstrate that FTN-8CAP-VQGIM supports a compression factor of 0.53 at a target bit error rate (BER) of 3:8 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup>, and achieves a 113% enhancement in SE compared with 8CAP.