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Non-orthogonal multiple access (NOMA) has recently attracted significant attention as a promising multiple access scheme for the 5th generation (5G) wireless communication due to its superior spectral efficiency, which has also been studied and shown to achieve a superior performance in visible light communication (VLC) networks. However, the error propagation (EP) problem due to successive interference cancellation (SIC) decoding has not yet been resolved, which degrades the system BER performance and causes user unfairness. In this work, symmetric superposition coding (SSC) and symmetric SIC (SSIC) decoding are proposed for a downlink NOMA-based VLC network, in which the distribution of the demodulation regions of the user allocated with more power will be symmetrical in terms of the decision threshold of the user allocated with less power. Furthermore, the proposed method is experimentally tested and the results show that more than 90% demodulation errors caused by EP are eliminated compared with traditional NOMA VLC.

Recently, a hybrid multiple access (MA) scheme, non-orthogonal MA (NOMA) combined with orthogonal frequency-division MA (OFDMA), has attracted significant attention in the fifth-generation (5G) wireless communication due to its superior spectrum efficiency. However, the advantage of the hybrid MA scheme cannot be fully realized in visible light communication (VLC) networks, since current optical orthogonal frequency-division multiplexing (OFDM) technologies are unable to provide high spectrum efficiency and power efficiency at the same time. In this paper, a hierarchical pre-distorted layered asymmetrically clipped optical OFDM (HPD-LACO-OFDM) scheme is proposed for NOMA, which offers superior spectrum efficiency as well as high optical power efficiency. In HPD-LACO-OFDM, multiple layers of asymmetrically clipped optical OFDM (ACO-OFDM) signals are generated to fill the odd subcarriers successively, and the inter-layer interference is eliminated with successive signal pre-distortion. A comparison of the bit-error-rate (BER) performance between the traditional dc-biased optical OFDM (DCO-OFDM), the layered ACO-OFDM (LACO-OFDM), and the proposed HPD-LACO-OFDM scheme is experimentally performed. The results show that with the same signal power, the HPD-LACO-OFDM-based NOMA-VLC network shows the best BER performance, whereas the demand for direct current voltage is reduced by half of the signal voltage compared to the traditional DCO-OFDM-based NOMA-VLC network. INDEX TERMS Visible light communication (VLC), non-orthogonal multiple access (NOMA), orthogonal frequency division multiplexing (OFDM).