Multidomain Index Modulation for Vehicular and Railway Communications: A Survey of Novel Techniques

Vehicular communications have recently attracted considerable attention due to a range of new applications and valueadded features. Compared to conventional communication systems, vehicular communications suffer from graver challenges due to the high-Doppler spread caused by the mobility of vehicles. In this context, index modulation (IM) constitutes a promising design alterative for vehicular communications due to its appealing benefits. A key feature of IM is that it conveys the information bits using the specific indices of the transmit resources, such as antenna, subcarrier, precoder, and channel-path indices. The resulting transmission signals tend to be sparse in one or several of the resource domains, which increases their robustness and reduces the detection complexity. In this article, we introduce the basic IM concept and highlight the applications of IM in the context of road vehicle-to-infrastructure (V2I), vehicle-to-vehicle (V2V), and conventional railway and high-speed train communication systems. Finally, some design challenges of IM-based vehicular communications are listed for future studies.