Multi-Mode Index Modulation Aided Multi-Band CAP with Balanced Pairwise Coding for Bandlimited VLC Systems

In this paper, we for the first time propose and demonstrate a multi-mode index modulation aided multi-band carrierless amplitude and phase modulation (mCAP-MM) scheme for bandlimited visible light communication (VLC) systems. More specifically, the information bits of the proposed mCAP-MM scheme are conveyed through both multiple distinguishable modes and their full permutation indices. For mCAP-MM, due to the importance of choosing an appropriate constellation design strategy, we consider partitioning the circular (7,1)-quadrature amplitude modulation (QAM) constellation to obtain multiple distinguishable constellation modes. Moreover, an enhanced log-likelihood ratio (LLR) detector is also developed for mCAP-MM, which can solve the error propagation problem existed in the conventional LLR detector and achieve nearly optimal performance as the maximum likelihood (ML) detector but with a much lower complexity. In addition, aiming at the problem of signal-to-noise ratio (SNR) imbalance among different subbands due to the low-pass effect in practical VLC systems, a novel balanced pairwise coding (B-PWC) technique is further proposed and incorporated into the proposed mCAP-MM scheme. The feasibility and superiority of the proposed mCAP-MM scheme with enhanced LLR detection and B-PWC coding have been successfully verified by both simulations and experiments. Specifically, simulation results verify the superiority of mCAP-MM employing the designed multiple constellation modes and enhanced LLR detection over the additive white Gaussian noise (AWGN) channel, and then the validity of B-PWC in bandlimited mCAP-MM VLC systems is verified via proof-of-concept experiments. Experimental results demonstrate that a bandwidth extension of 19.9% can be achieved by the proposed 8CAP-MM with B-PWC in comparison to that with traditional PWC.