## 1 Introduction  
The term LiFi was first coined by Prof. [PERSON] in one of his TED talks, in 2011. The technique has since, attracted a lot of research interest and is maturing very fast. The idea behind LiFi is to take advantage of the vast deployment of LED lights for illumination, and to use them to carry data, simultaneously. LiFi proposes fast modulation of LEDs to ensure that the flicker is undetectable to the naked eye, providing both illumination and communication at the same time. The advantages of LiFi range from higher security and larger bandwidth to better power efficiency [1]. Despite the benefits, LiFi comes with its own set of challenges such as flicker mitigation, dimming control, ambient light interference, and the low switching speed of typical LEDs used for illumination [2]. Some of these challenges have prompted research to switch from single-carrier to multicriter modulation. Orthogonal Frequency Division Multiplexing (OFDM) is a well-known digital communication technique and has been around for many years. It has found great use in visible light communication (VLC) [3]. OFDM provides a superior bandwidth performance, along with good flicker mitigation properties. Since a low-cost implementation is desired, intensity modulation and direct detection (IM/DD) are our techniques of choice, meaning the modulated signals should be non-negative and real-valued [4]. Several OFDM variants have been proposed to satisfy these constraints [5; 6; 7; 8; 9] such as pulse amplitude-modulated digital multi-tone (PAM-DMT), asymmetrically clipped optical OFDM (ACO-OFDM), DC-biased optical OFDM (DCO-OFDM), Unipolar OFDM and Flip OFDM. To ensure the signals are real-valued, Hermitian symmetry is used on the subcarriers. A DC bias may be imposed on the signals to make them non-negative. Alternate arrangements in the time and frequency domain, are used on signals to ensure non-negativity without the need for DC bias in [5; 6; 7; 8; 9]. These arrangements, however, result in a loss of spectral efficiency. Hybrid approaches [10; 11; 12; 13], such as asymmetrically clipped DC-biased optical OFDM (ADO-OFDM), hybrid ACO-OFDM, layered ACO-OFDM (LACO-OFDM) and enhanced U-OFDM have been proposed to overcome this loss of spectral efficiency. A detailed discussion of index modulation techniques is presented in [14], where time domain, space domain and frequency domain index modulation were discussed in detail. Transmitter and receiver models and a detailed error rate analysis for each approach is presented in detail [14]. The authors in [15] discuss the implementation of Index Modulation (IM) in OFDM systems and present their views on the future research direction in this field. The spectral and power efficiencies of DCO-OFDM have been investigated in [16]. The authors have proposed a multilevel mercury-water filling power allocation scheme to improve the spectral efficiency, while a Dinkelback-type power allocation scheme has been proposed to improve the energy efficiency [16]. SIM-OFDM has been proposed in [17], where each subcarrier is turned ON or OFF, carrying one bit of information, while the individual subcarriers are modulated by conventional modulation techniques. This provides better energy efficiency by turning some of the subcarriers OFF, while extra bits are also carried by the indices of the activated (ON) subcarriers. Enhanced SIM-OFDM proposed in [18] pairs two subcarriers together, and only one subcarrier is ON. This further improves power efficiency, but the number of bits transmitted through the indices are reduced by half, providing a poorer spectral efficiency. OFDM with index modulation (OFDM-IM) is proposed in [19], where subcarriers are divided into subgroups and then index modulation is performed within each subgroup. The technique has a much-improved power efficiency; however, the number of inactive subcarriers means wastage of bandwidth. It has also been shown that OFDM-IM outperforms the conventional OFDM with low spectral efficiency only.