In this paper, a new spectrally efficient modulation scheme called quadrature spatial modulation sub-carrier intensity modulation (QSM-SIM) is proposed for multiple-input multiple-output (MIMO) visible light communication (VLC) systems. In QSM-SIM, in addition to the digital modulation symbols, two LED indices are used simultaneously for transmission of data bits with the help of a sinusoidal signal pair. Then, spatial modulation quad-LED complex modulation (SM-QCM), which is based on quad-LED complex modulation (QCM), is enhanced such that the number of LEDs is greater than four. Bit error rate (BER) performances of the considered modulation schemes are analyzed for different MIMO plans. The upper bound on BER performances of the QSM-SIM and SM-QCM schemes are derived analytically in order to validate the simulation results. In this work, pulse amplitude modulation (PAM), QCM, and SIM are used as benchmarks. In addition, for the purpose of reducing channel correlation, the power imbalance (PI) method is applied to the VLC-MIMO system. Best power imbalance factors are obtained for the considered modulation schemes and benchmarks under fixed signal to noise ratio (SNR) values. The results show that the proposed schemes outperform the benchmarks with PI communication scenarios. (C) 2019 Elsevier B.V. All rights reserved.