A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> B. Multi-Dimension Entity <s> Motivated by the recent concept of Spatial Modulation (SM), we propose a novel Space-Time Shift Keying (STSK) modulation scheme for Multiple-Input Multiple-Output (MIMO) communication systems, where the concept of SM is extended to include both the space and time dimensions, in order to provide a general shift-keying framework. More specifically, in the proposed STSK scheme one out of Q dispersion matrices is activated during each transmitted block, which enables us to strike a flexible diversity and multiplexing tradeoff. This is achieved by optimizing both the space-time block duration as well as the number of the dispersion matrices in addition to the number of transmit and receive antennas. We will demonstrate that the resultant equivalent system model does not impose any Inter-Channel Interference (ICI), and hence the employment of single-stream Maximum Likelihood (ML) detection becomes realistic at a low-complexity. Furthermore, we propose a Differential STSK (DSTSK) scheme, assisted by the Cayley unitary transform, which does not require any Channel State Information (CSI) at the receiver. Here, the usual error-doubling, caused by the differential decoding, gives rise to 3-dB performance penalty in comparison to Coherent STSK (CSTSK). Additionally, we introduce an enhanced CSTSK scheme, which avoids the requirement of Inter-Antenna Synchronization (IAS) between the RF chains associated with the transmit Antenna Elements (AEs) by imposing a certain constraint on the dispersion matrix design, where each column of the dispersion matrices includes only a single non-zero component. Moreover, according to the turbo-coding principle, the proposed CSTSK and DSTSK schemes are combined with multiple serially concatenated codes and an iterative bit-to-symbol soft-demapper. More specifically, the associated STSK parameters are optimized with the aid of Extrinsic Information Transfer (EXIT) charts, for the sake of achieving a near-capacity performance. <s> BIB001 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> B. Multi-Dimension Entity <s> Inspired by the concept of the Space-Time Shift Keying (STSK) modulation, in this paper we proposed the Space-Frequency Shift Keying (SFSK) modulation as well as the Space-Time-Frequency Shift Keying (STFSK) concept which spreads the transmit signal not only across the space and time domains, but also the frequency domain. The performance of STSK modulation is degraded by about 2 dB, when the channel changes from uncorrelated frequency-flat fading to the frequency-selective environment of the 6-tap COST207 model. By contrast, as a benefit of Frequency Shift keying, the SFSK and STFSK schemes are capable of maintaining their performance also in frequency-selective fading environments. Finally, we demonstrate that the STSK and SFSK schemes constitute special cases of the STFSK modulation. <s> BIB002 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> B. Multi-Dimension Entity <s> Generalized space–time–frequency index modulation (GSTFIM) is proposed for conveying extra information with the aid of the space, time, and frequency indices. GSTFIM subsumes the concept of generalized space–time shift keying (GSTSK), which flexibly activates $Q^{\prime }$ out of $Q$ dispersion matrices, and the orthogonal frequency-division multiplexing (OFDM) with index modulation. The transceiver architecture of the proposed GSTFIM is outlined. GSTFIM is capable of providing higher transmission rate than the classic OFDM-based GSTSK. It also outperforms the classic OFDM-aided GSTSK in terms of the bit-error-probability. <s> BIB003
As discussed in the last subsection, various applications based on the SM concept have been implemented in the space/frequency/time/code/angle domains, which exploit the ON/OFF states or the permutations of transmission entities in one domain to carry index information. To further enhance the system performance and enjoy a more flexible system setting, it is possible and beneficial to integrate resources across multiple dimensions to form a multi-dimension entity for the implementation of SM concept. To unify the SM concept in both space and time domains, both coherent and differential STSK modulation schemes are developed via the activation mechanism on selecting one out of multiple space-time dispersion matrices, which enables a flexible design to compromise between diversity and multiplexing gains BIB001 . By extending the SM concept to include both space and frequency dimensions, the authors of  develop a generalized space and frequency IM (GSFIM) scheme, which shows the potential to achieve better performance than the traditional MIMO-OFDM scheme. Exploiting the ON/OFF keying mechanism across the space, time and frequency domains, space-time-frequency shift keying (STFSK) scheme is also proposed in BIB002 to reap the potential gain from the three domains, which shows an improvement in the system performance and alleviation of inter-symbol interference over frequency-selective fading channels. Based on the dispersion matrix activation over space, time and frequency domains, a generalized space-time-frequency index modulation (GST-FIM) scheme is proposed in BIB003 to provide extra spectral efficiency.