# Orthogonal Subblock Division Multiple Access for OFDM-IM-Based Multi-User VLC Systems
## 1 Introduction  
In recent years, visible light communication (VLC) using white light-emitting diodes (LEDs) for illumination and communication has been triggering great attention, which has been widely recognized as one of the key enabling technologies of 6G and Internet of Things (IoT) systems [1, 2]. VLC has the inherent advantages of abundant unlicensed spectrum resources, anti-electromagnetic interference and simple deployment in comparison to traditional radio frequency (RF) communication [3]. For a practical multi-user VLC (MU-VLC) system based on commercial white LEDs, its performance mainly face two key challenges which are the limited modulation bandwidth and severe nonlinearity of LED [4]. For the bandwidth limitation issue of LEDs, various spectral-efficient modulation techniques and multiple access techniques such as orthogonal frequency division multiplexing (OFDM) and carrierless amplitude and phase (CAP) modulation employing high-order constellations, multiple-input multiple-output (MIMO) and non-orthogonal multiple access (NOMA) can be adopted to achieve high-speed MU-VLC systems [5, 6, 7, 8]. In practical orthogonal frequency division multiple access (OFDMA)-based bandlimited MU-VLC systems, the allocated subcarriers of different users might experience different low-pass frequency responses, leading to the degraded overall bit error rate (BER) performance. To address this issue, analog or digital equalization techniques can be applied to flat the system spectrum [9]. Nevertheless, excessive amplification of the power of high-frequency subcarriers might not be feasible due to the limited dynamic range of LEDs [10]. It has beendemonstrated in [11] that interleaved subcarrier multiplexing can be an effective way to ensure that the subcarriers of different users can have comparable signal-to-noise ratio (SNR) performance. For the LED nonlinearity issue, the transmitted signals with reduced peak-to-average power ratios (PAPRs) are less vulnerable to the nonlinearity, and discrete Fourier transform (DFT) spreading can be an efficient approach to significantly reduce the PAPR of OFDM signals [12; 13]. Moreover, DFT-spread OFDM (DFT-S-OFDM), which is also known as single-carrier frequency-division multiplexing (SC-FDM), has been widely applied in various scenarios such as local area IMT-A [14] and intensity-modulation/direct-detection (IM/DD) optical interconnects [15]. Recently, OFDM with index modulation (OFDM-IM) has being emerging as a promising technique for 5G and VLC systems, which can transmit additional index bits through subcarrier index selection [16; 17]. More specifically, OFDM-IM can be implemented in the single-mode (SM) manner or the dual-mode (DM) manner. For OFDM-SM-IM, only a subset of subcarriers within each subblock are selected and activated to transmit constellation symbols, while the remaining subcarriers are simply left unmodulated. For OFDM-DM-IM, all the subcarriers within each subblock are used to transmit constellation symbols, where the selected subcarriers transmit constellation symbols corresponding to mode 1 while the remaining subcarriers transmit constellation symbols corresponding to mode 2 [18; 19]. It has already been shown that OFDM-SM-IM is more power efficient than classical OFDM, which achieves better BER performance than OFDM for relatively low spectral efficiencies. Nevertheless, it is challenging for OFDM-SM-IM to achieve high spectral efficiency due to the existence of unmodulated subcarriers. In contrast, OFDM-DM-IM can achieve high spectral efficiencies, which is able to outperform both classical OFDM and OFDM-SM-IM by carefully designing the dual-mode constellations. Nevertheless, both OFDM-SM-IM and OFDM-DM-IM suffer from high PAPR as classical OFDM, which might not be suitable for practical bandlimited MU-VLC systems with LED nonlinearity.