Non-Uniform Index Modulation for OFDM in VLC Systems

Index modulation (IM) has been proposed and widely demonstrated to improve the spectral efficiency by simultaneously utilizing both the indices of activated subcarriers and their modulated symbols for data transmission in bandwidth-limited visible light communication (VLC) systems. However, current studies assume uniform activation probabilities for all subcarriers without adapting subcarrier index selection to channel conditions, thus limiting system performance maximization. In this paper, we for the first time propose and investigate a novel orthogonal frequency division multiplexing with non-uniform index modulation (OFDM-NUIM) scheme for VLC systems. In OFDM-NUIM, subcarrier interleaving is first introduced to redistribute adjacent subcarriers in the frequency domain into distinct subblocks. This ensures each subblock contains subcarriers with varying signal-to-noise ratios (SNRs). Subsequently, non-uniform index modulation is proposed to dynamically adjust subcarrier activation probabilities based on the SNR distribution within interleaved subblocks. This strategy preferentially activates subcarriers with higher SNRs for data transmission, thereby concentrating the active subcarriers in the low-frequency region. Consequently, it mitigates the high-frequency fading effect inherent in VLC systems and further enhances its anti-interference capability and transmission reliability. Finally, the feasibility and superiority of the proposed OFDM-NUIM scheme are experimentally demonstrated. The results indicate that the proposed OFDM-NUIM scheme can effectively improve the bit error rate (BER) performance of VLC systems.