# Multiple-Mode Orthogonal Time Frequency Space with Index Modulation  
Huarong Ren  
into a unified time-invariant impulse response in the DD domain for all the symbols under a good approximation. To recover the transmitted symbols at the receiver, the respective multicarrier demodulation followed by an inverse transformation step takes the received signal back to the DD domain. In the preprocessing and postprocessing, inverse symplectic finite Fourier transform (ISFFT) and symplectic finite Fourier transform (SFFT) are used to realize the conversion of signals between the DD domain and the TF domain [5]. Index modulation (IM) is a new information transmission technology; unlike traditional modulation techniques, index modulation can transmit additional information without additional energy consumption by utilizing the switch or call state of the transmission resource. Index modulation expands the switch or calls a state of transmission resources into a new transmission dimension to transmit additional information such as antennas, subcarriers, spreading codes, and time slots [6]. Therefore, index modulation only needs to activate a part of the transmission resources to achieve the same throughput as the traditional modulation scheme. Index modulation not only reduces the energy consumption during information transmission but also improves the energy efficiency of the system. At the same time, because only some transmission resources are activated, the signal shows a certain sparsity after index modulation. This sparsity weakens the interference between transmission signals in a certain extent and improves the anti-interference performance of the system [7]. On the other hand, the index bits of the implicit transmission are transmitted without energy consumption. Index modulation transfers the energy saved from the unactivated transmission resources to the activated transmission resources. Therefore, index modulation can achieve a better bit error rate (BER) performance on the premise of transmitting the same energy. Index modulation in the frequency domain is usually performed with the aid of either the frequency carriers or OFDM subcarriers. OFDM with an index modulation (OFDM-IM) system improves the spectrum efficiency (SE) of the OFDM system and achieves better performance [8; 9]. The OFDM-IM system activates only a subset of all avaliable subcarriers to carry \(M\)-ary data symbols and uses the subcarrier activation patterns as information-bearing units. In [10], an OTFS with an index modulation (OTFS-IM) system was proposed. The OTFS-IM system transmits modulation bits by activating partial grid resources and transmits index information by using the location information of activated grid points. Although the OTFS-IM system achieves better BER performance, only some of the grid resources are activated, resulting in the waste of grid resources and a low SE of the system [11]. An OTFS with improved index modulation (OTFS-IIM) was proposed in [12]. Unlike the OTFS-IM system, each index information is transmitted twice in the OTFS-IIM system, which reduces the error rate of the index bits and further improves the BER performance. However, the problem of grid resource waste has not been solved. To solve this problem, an OTFS with dual-mode index modulation (OTFS-DM-IM) was proposed [13]. In the OTFS-DM-IM system, all grid resources are activated for transmitting modulation bits, achieving a better balance between transmission reliability and SE in the time-varying channel. Moreover, the theoretical BER upper bound was presented to give the benchmark of the OTFS-DM-IM system. In this paper, to improve the SE and energy efficiency (EE) of the OTFS-IM system, a multiple-mode index modulation-aided OTFS system (MM-OTFS-IM) is proposed. In this system, the grid resources of OTFS were divided into several subblocks, and each grid resource in the subblocks carries the modulation information of different constellation modes and uses the combination of different constellation modes to transmit the index information.