A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> VI. APPLICATIONS OF SM IN NEW DOMAINS <s> The ambitious goals set for 5G wireless networks, which are expected to be introduced around 2020, require dramatic changes in the design of different layers for next generation communications systems. Massive MIMO systems, filter bank multi-carrier modulation, relaying technologies, and millimeter-wave communications have been considered as some of the strong candidates for the physical layer design of 5G networks. In this article, we shed light on the potential and implementation of IM techniques for MIMO and multi-carrier communications systems, which are expected to be two of the key technologies for 5G systems. Specifically, we focus on two promising applications of IM: spatial modulation and orthogonal frequency-division multiplexing with IM, and discuss the recent advances and future research directions in IM technologies toward spectrum- and energy-efficient 5G wireless networks. <s> BIB001 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> VI. APPLICATIONS OF SM IN NEW DOMAINS <s> What is index modulation (IM)? This is an interesting question that we have started to hear more and more frequently over the past few years. The aim of this paper is to answer this question in a comprehensive manner by covering not only the basic principles and emerging variants of IM, but also reviewing the most recent as well as promising advances in this field toward the application scenarios foreseen in next-generation wireless networks. More specifically, we investigate three forms of IM: spatial modulation, channel modulation and orthogonal frequency division multiplexing (OFDM) with IM, which consider the transmit antennas of a multiple-input multiple-output system, the radio frequency mirrors (parasitic elements) mounted at a transmit antenna and the subcarriers of an OFDM system for IM techniques, respectively. We present the up-to-date advances in these three promising frontiers and discuss possible future research directions for IM-based schemes toward low-complexity, spectrum- and energy-efficient next-generation wireless networks. <s> BIB002 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> VI. APPLICATIONS OF SM IN NEW DOMAINS <s> 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. <s> BIB003
As a novel digital modulation technology for achieving both spectral efficiency and energy efficiency, the basic concept of SM is to convey additional information through the ON/OFF states of transmit antennas. Consequently, the concept of SM can be generalized and used in various applications, in which the transmission media for embedding additional information can be either physical (e.g., antenna) or virtual (e.g., space-time matrix). Instead of using the full-activation state, the distinguishing feature of SM is to embed the implicit information via the activation states of transmission media in addition to explicit information (e.g, constellation symbols), which is carried by the partially activated transmission media BIB002 , BIB001 - BIB003 . The information loss due to the deactivation of some transmission media can be compensated for by the implicit information based on the ON/OFF states, saving the cost of transmission media (the number of actual transmission media is significantly reduced) and striking an attractive compromise between energy efficiency and spectral efficiency. Relying on the activation states of transmit antennas, SM provides a new perspective on the digital modulation, which is different from the conventional digital modulation using the amplitude, frequency, and phase on the sinusoidal carrier. Inspired by the concept of SM, researchers have proposed various forms and applications for SM, which alter the ON/OFF states of different transmission entities to embed additional information. In Table I , we summarize extensive representative forms of SM, which are classified according to their transmission entities that perform the ON/OFF keying mechanism. As Table I , the concept of SM can be performed in either a single-dimension entity or a multi-dimension entity. So far, various single-dimension entities have been explored to increase the spectral efficiency and energy efficiency of communication systems, which mainly includes spatial entities (e.g., antenna, RF mirror, and LED), frequency entities (e.g., subcarrier), time entities (e.g., time slot), code entities (e.g., spreading code and modulation type), angle entities (e.g., AoA and polarization state), etc. To further enhance the system performance and enjoy a more flexible system setting, several multi-dimension entities are also developed, which include more than one dimension for performing the ON/OFF keying mechanism.