In this paper, sparse activated subcarriers OFDM index modulation (SAS-OFDM-IM) and interpolated sparse activated subcarriers OFDM index modulation (ISAS-OFDM-IM) are proposed to improve the spectrum efficiency of OFDM modulation technique in visible light communication (VLC) systems. To be suitable for intensity modulation (IMod) by Light Emitting Diode (LED) used in VLC systems, the Unipolar Orthogonal Frequency Division Multiplexing (U-OFDM) is used. U-OFDM is more power efficient in an Additive White Gaussian Noise (AWGN) channel, which is prevalent in an optical wireless system. At the receiver, threshold detector has been used for detecting the activated subcarriers by taking the highest K out of N, where N is the total number of OFDM subcarriers and K is the total number of activated subcarriers. Results indicate that, using the proposed schemes can improve both the spectral efficiency …