## 1 Introduction
Existing IM schemes can be classified into various signal domain IM schemes, which include spatial-domain IM (SD-IM), channel-domain IM (CD-IM), frequency-domain IM (FD-IM), and time-domain IM (TD-IM) transmission schemes [19]. In [20], a simple OFDM-IM is implemented fusing radar and wireless communication systems. In [21], the basic OFDM-IM system uses modulation based on null subcarriers rather than active subcarriers to transmit the information. This system achieves better bit error rate (BER) performance and improved spectral efficiency (SE) as compared to the conventional OFDM-IM system. The SE of the conventional OFDM-IM scheme can be further improved by dividing the total number of subcarriers into subgroups [22]. In [23], an OFDM-IM system is used for orthogonal multiple access (NOMA) operating in dual mode (DM). The DM-OFDM-IM-NOMA increases the SE using indices selection bits and data transmission through all the OFDM subcarriers employing different constellation sets. With the aim of improving the SE of the system and inspired by the quadrature spatial modulation (QSM) technique, in this paper, we use the sub-carriers of the OFDM system to independently transmit the real and imaginary parts of one QAM symbol over the in-phase and quadrature dimensions, respectively. In particular, QSM is an SM-based technique that has the advantage of doubling the number of bits that can be transmitted in the spatial constellation, which results in improved SE compared to the basic SM scheme [24]. In addition to carrying information bits through conventional amplitude/phase modulation (APM)-based transmission, the proposed scheme allows for the harvesting of additional index bits without consuming extra energy [13]. This makes our proposal capable of employing only part of the resources to achieve the same throughput as its conventional counterpart. Results show that since this technique is based on inserting zeros in the transmission, the systems have a reduced detection complexity while consuming substantially less energy. Mainly, our simulations show that the proposed technique is adequate for high-mobility V2V channels. The contributions of this paper can be summarised as follows:1. A novel IM-MIMO-OFDM system based on QSM is proposed for handling doubly selective MIMO-V2V channels. 2. The MIMO receiver developed here maintains a structure and computational complexity lower than the conventional MIMO-OFDM receiver. 3. The proposed system demonstrates high robustness against uncertainties in channel statistics, achieving a low BER even at high levels of Doppler frequency dispersion. The remainder of this paper is organised as follows. Section 2 presents the system model of the proposed MIMO-OFDM-IM system. Section 3 describes the receiver, channel stimulation, and low-complexity detection for OFDM-IM signals. Section 4 analyses the computational complexity of the proposed system. Section 5 presents the performance analysis of the utilized algorithm and results. Finally, in Section 6, we conclude the work. Notation  
Lowercase (uppercase) bold letters are used for vectors (matrices); \((\cdot)^{T},(\cdot)^{H}\), and \(\lceil\cdot\rceil\) denote the transpose, Hermitian, and rounding up operators, respectively; \(\langle\cdot\rangle_{N}\) denotes circular shift of modulus \(N\); \(E\{\cdot\}\) is the expected value operator. The operator \((\cdot)^{k}\) refers to the \(k\)-th OFDM symbol being considered.