A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> The multiplexing gain of multiple antenna transmission strongly depends on transmit and receive antenna spacing, transmit antenna synchronization, and the algorithm used to eliminate interchannel interference (ICI) at the receiver. In this paper, a new transmission approach, called spatial modulation, that entirely avoids ICI and requires no synchronization between the transmitting antennas while maintaining high spectral efficiency is presented. A block of information bits is mapped into a constellation point in the signal and the spatial domain, i.e. into the location of a particular antenna. The receiver estimates the transmitted signal and the transmit antenna number and uses the two information to de-map the block of information bits. For this purpose, a novel transmit antenna number detection algorithm called iterative-maximum ratio combining (i-MRC) is presented. Spatial modulation is used to transmit different number of information bits and i-MRC is used to estimate both the transmitted signal and the transmit antenna number. The results are compared to ideal V-BLAST (vertical-Bell Lab layered space-time) and to MRC. Spatial modulation outperforms MRC. The (bit-error-ratio) BER performance and the achieved spectral efficiency is comparable to V-BLAST. However, spatial modulation results in a vast reduction in receiver complexity. <s> BIB001 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Spatial modulation (SM) is a recently developed transmission technique that uses multiple antennas. The basic idea is to map a block of information bits to two information carrying units: 1) a symbol that was chosen from a constellation diagram and 2) a unique transmit antenna number that was chosen from a set of transmit antennas. The use of the transmit antenna number as an information-bearing unit increases the overall spectral efficiency by the base-two logarithm of the number of transmit antennas. At the receiver, a maximum receive ratio combining algorithm is used to retrieve the transmitted block of information bits. Here, we apply SM to orthogonal frequency division multiplexing (OFDM) transmission. We develop an analytical approach for symbol error ratio (SER) analysis of the SM algorithm in independent identically distributed (i.i.d.) Rayleigh channels. The analytical and simulation results closely match. The performance and the receiver complexity of the SM-OFDM technique are compared to those of the vertical Bell Labs layered space-time (V-BLAST-OFDM) and Alamouti-OFDM algorithms. V-BLAST uses minimum mean square error (MMSE) detection with ordered successive interference cancellation. The combined effect of spatial correlation, mutual antenna coupling, and Rician fading on both coded and uncoded systems are presented. It is shown that, for the same spectral efficiency, SM results in a reduction of around 90% in receiver complexity as compared to V-BLAST and nearly the same receiver complexity as Alamouti. In addition, we show that SM achieves better performance in all studied channel conditions, as compared with other techniques. It is also shown to efficiently work for any configuration of transmit and receive antennas, even for the case of fewer receive antennas than transmit antennas. <s> BIB002 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> A fundamental component of spatial modulation (SM), termed generalized space shift keying (GSSK), is presented. GSSK modulation inherently exploits fading in wireless communication to provide better performance over conventional amplitude/phase modulation (APM) techniques. In GSSK, only the antenna indices, and not the symbols themselves (as in the case of SM and APM), relay information. We exploit GSSKpsilas degrees of freedom to achieve better performance, which is done by formulating its constellation in an optimal manner. To support our results, we also derive upper bounds on GSSKpsilas bit error probability, where the source of GSSKpsilas strength is made clear. Analytical and simulation results show performance gains (1.5-3 dB) over popular multiple antenna APM systems (including Bell Laboratories layered space time (BLAST) and maximum ratio combining (MRC) schemes), making GSSK an excellent candidate for future wireless applications. <s> BIB003 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> In this paper, we present space shift keying (SSK) as a new modulation scheme, which is based on spatial modulation (SM) concepts. Fading is exploited for multiple-input multiple-output(MIMO) channels to provide better performance over conventional amplitude/phase modulation (APM) techniques. In SSK, it is the antenna index used during transmission that relays information, rather than the transmitted symbols themselves. This absence of symbol information eliminates the transceiver elements necessary for APM transmission and detection (such as coherent detectors). As well, the simplicity involved in modulation reduces the detection complexity compared to that of SM, while achieving almost identical performance gains. Throughout the paper, we illustrate SSK's strength by studying its interaction with the fading channel. We obtain tight upper bounds on bit error probability, and discuss SSK's performance under some non-ideal channel conditions (estimation error and spatial correlation). Analytical and simulation results show performance gains over APM systems (3 dB at a bit error rate of 10-5), making SSK an interesting candidate for future wireless applications. We then extend SSK concepts to incorporate channel coding, where in particular, we consider a bit interleaved coded modulation (BICM) system using iterative decoding for both convolutional and turbo codes. Capacity results are derived, and improvements over APM are illustrated (up to 1 bits/s/Hz), with performance gains of up to 5 dB. <s> BIB004 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Space Shift Keying (SSK) modulation is a new and recently proposed transmission technology for Multiple-Input-Multiple-Output (MIMO) wireless systems, which has been shown to be a promising low-complexity alternative to several state-of-the-art MIMO schemes. So far, only optimal or heuristic transceivers with Full Channel State Information (F-CSI) at the receiver have been investigated, and their performance analyzed over fading channels. In this paper, we develop and study the performance of the optimal Maximum-Likelihood (ML) detector with unknown phase reference at the receiver (i.e., Partial-CSI, P-CSI, knowledge). A very accurate analytical framework for the analysis and optimization of this novel detector over generically correlated and non-identically distributed Nakagami-{m} fading channels is proposed, and its performance compared to the optimal receiver design with F-CSI. Numerical results will point out that: i) the performance of SSK modulation is significantly affected by the characteristics of fading channels, e.g., channel correlation, fading severity, and, particularly, power imbalance among the transmit-receive wireless links, and ii) unlike ordinary modulation schemes, there is a substantial performance loss when the receiver cannot exploit the phase information for optimal receiver design. This latter result highlights the importance of accurate and reliable channel estimation mechanisms for the efficient operation of SSK modulation over fading channels. Analytical frameworks and theoretical findings will also be substantiated via Monte Carlo simulations. <s> BIB005 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> In this paper, a generalised technique for spatial modulation (SM) is presented. Generalised spatial modulation (GSM) overcomes in a novel fashion the constraint in SM that the number of transmit antennas has to be a power of two. In GSM, a block of information bits is mapped to a constellation symbol and a spatial symbol. The spatial symbol is a combination of transmit antennas activated at each instance. The actual combination of active transmit antennas depends on the random incoming data stream. This is unlike SM where only a single transmit antenna is activated at each instance. GSM increases the overall spectral efficiency by base-two logarithm of the number of antenna combinations. This reduces the number of transmit antennas needed for the same spectral efficiency. The performance of GSM is analysed in this paper, and an upper bound on the bit-error-ratio (BER) performance is derived. In addition, an algorithm to optimise the antenna combination selection is proposed. Finally, the performance of GSM is validated through Monte Carlo simulations. The results are compared with traditional SM. It is shown that for the same spectral efficiency GSM performs nearly the same as SM, but with a significant reduction in the number of transmit antennas. <s> BIB006 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Trellis coded modulation (TCM) is a well known scheme that reduces power requirements without any bandwidth expansion. In TCM, only certain sequences of successive constellation points are allowed (mapping by set partitioning). The novel idea in this paper is to apply the TCM concept to the antenna constellation points of spatial modulation (SM). The aim is to enhance SM performance in correlated channel conditions. SM considers the multiple transmit antennas as additional constellation points and maps a first part of a block of information bits to the transmit antenna indices. Therefore, spatial multiplexing gains are retained and spectral efficiency is boosted. The second part of the block of information bits is mapped to a complex symbol using conventional digital modulation schemes. At any particular time instant, only one antenna is active. The receiver estimates the transmitted symbol and the active antenna index and uses the two estimates to retrieve the original block of data bits. In this paper, TCM partitions the entire set of transmit antennas into sub-sets such that the spacing between antennas within a particular sub-set is maximized. The scheme is called trellis coded spatial modulation (TCSM). Tight analytical performance bounds over correlated fading channels are proposed in this paper. In addition, the performance and complexity of TCSM is compared to the performance of SM, coded V-BLAST (vertical Bell Labs layered space-time) applying near optimum sphere decoder algorithm, and Alamouti scheme combined with TCM. Also, the performance of all schemes with turbo coded modulation is presented. It is shown that under the same spectral efficiency, TCSM exhibits significant performance enhancements in the presence of realistic channel conditions such as Rician fading and spatial correlation (SC). In addition, the complexity of the proposed scheme is shown to be 80% less than the V-BLAST complexity. <s> BIB007 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> This paper proposes and studies a transmitter preprocessing aided spatial modulation (PSM) scheme, which conveys information jointly by a conventional amplitude-phase modulation (APM) and a preprocessing aided space shift keying (pre-SSK) modulation. In contrast to the existing SSK modulation, which carries information using the indexes of transmitter antennas and assumes channel state information at receiver (CSIR), the pre-SSK modulation extracts the transmitted information using the indexes of receive antennas and assumes channel state information at transmitter (CSIT). This paper addresses the issues of preprocessing optimization and detection of PSM signals. Furthermore, the bit-error-rate (BER) performance of the PSM is investigated, when assuming that the channel from any transmit antenna to any receive antenna experiences independent Rayleigh fading. Our studies show that, when appropriately designed, the pre-SSK and APM invoked can enhance each other, resulting in that the PSM is capable of attaining a better BER performance than the corresponding pure pre-SSK and pure APM. <s> BIB008 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> In this letter, an adaptive spatial modulation (ASM) transmission scheme is proposed to achieve better system performance under a fixed data rate. The proposed scheme is based on a developed modulation order selection criterion (MOSC), so as to minimize the conditioned pairwise error probability (PEP) for each channel realization. Furthermore, the special information-conveying mode of spatial modulation (SM) is utilized to reduce the complexity of the proposed algorithm. Simulation results show that the proposed adaptive SM scheme provides considerable system performance improvement compared to original SM system. <s> BIB009 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> In this paper, we study the performance of Space Shift Keying (SSK) modulation for a generic Multiple-Input-Multiple-Output (MIMO) wireless system over correlated Rician fading channels. In particular, our contribution is twofold, i) First, we propose a very general framework for computing the Average Bit Error Probability (ABEP) of SSK-MIMO systems over a generic Rician fading channel with arbitrary correlation and channel parameters. The framework relies upon the Moschopoulos method. We show that it is exact for MIMO systems with two transmit-antenna and arbitrary receive-antenna, while an asymptotically-tight upper-bound is proposed to handle the system setup with an arbitrary number of transmit-antenna. ii) Second, moving from the consideration that conventional SSK-MIMO schemes can offer only receive-diversity gains, we propose a novel SSK-MIMO scheme that can exploit the transmit-antenna to increase the diversity order. The new method has its basic foundation on the transmission of signals with good time-correlation properties, and is called Time-Orthogonal-Signal-Design (TOSD-) assisted SSK modulation (TOSD-SSK). It is shown that the proposed method can increase twofold the diversity order for arbitrary transmit- and receive-antenna. In particular, for MIMO systems with two transmit-antenna and Nr receive-antenna full-diversity equal to 2Nr can be achieved. Analytical frameworks and theoretical findings are substantiated via Monte Carlo simulations for various system setups. <s> BIB010 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Spatial modulation (SM), in which multiple antennas are used to convey information besides the conventional M-ary signal constellations, is a new multiple-input multiple-output (MIMO) transmission technique, which has recently been proposed as an alternative to V-BLAST (vertical Bell Labs layered space-time). In this paper, a novel MIMO transmission scheme, called spatial modulation with trellis coding (SM-TC), is proposed. Similar to the conventional trellis coded modulation (TCM), in this scheme, a trellis encoder and an SM mapper are jointly designed to take advantage of the benefits of both. A soft decision Viterbi decoder, which is fed with the soft information supplied by the optimal SM decoder, is used at the receiver. A pairwise error probability (PEP) upper bound is derived for the SM-TC scheme in uncorrelated quasi-static Rayleigh fading channels. From the PEP upper bound, code design criteria are given and then used to obtain new 4-, 8- and 16-state SM-TC schemes using quadrature phase-shift keying (QPSK) and 8-ary phase-shift keying (8-PSK) modulations for 2,3 and 4 bits/s/Hz spectral efficiencies. It is shown via computer simulations and also supported by a theoretical error performance analysis that the proposed SM-TC schemes achieve significantly better error performance than the classical space-time trellis codes and coded V-BLAST systems at the same spectral efficiency, yet with reduced decoding complexity. <s> BIB011 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> A novel multiple-input multiple-output (MIMO) transmission scheme, called space-time block coded spatial modulation (STBC-SM), is proposed. It combines spatial modulation (SM) and space-time block coding (STBC) to take advantage of the benefits of both while avoiding their drawbacks. In the STBC-SM scheme, the transmitted information symbols are expanded not only to the space and time domains but also to the spatial (antenna) domain which corresponds to the on/off status of the transmit antennas available at the space domain, and therefore both core STBC and antenna indices carry information. A general technique is presented for the design of the STBC-SM scheme for any number of transmit antennas. Besides the high spectral efficiency advantage provided by the antenna domain, the proposed scheme is also optimized by deriving its diversity and coding gains to exploit the diversity advantage of STBC. A low-complexity maximum likelihood (ML) decoder is given for the new scheme which profits from the orthogonality of the core STBC. The performance advantages of the STBC-SM over simple SM and over V-BLAST are shown by simulation results for various spectral efficiencies and are supported by the derivation of a closed form expression for the union bound on the bit error probability. <s> BIB012 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> In this paper, we evaluate the energy efficiency of a multi-antenna Base Station (BS) employing Spatial Modulation (SM). Taking advantage of the single Radio Frequency (RF) chain configuration of SM, we show that SM offers a significant total power reduction compared to other multi-RF chain Multiple-Input Multiple-Output (MIMO) architectures. For the same RF transmit power, we demonstrate that the total power saving of SM scales linearly with the number of RF chains required by other MIMO schemes. Furthermore, we clarify that for the same ergodic capacity, SM has a significant advantage in energy efficiency compared to the studied multi-RF chain MIMO configurations. In addition, for a number of transmit antennas larger than two, we establish that SM results in higher ergodic capacity than Space-Time Block-Coding (STBC), combined with significant power saving. For a BS with 8 transmit antenna, the achieved power saving of SM can reach up to almost 90%. Finally, based on simulation results, we demonstrate that the energy efficiency (bits/J) of the studied MIMO schemes exhibits a maximum as function of ergodic capacity, and it is further shown that this maximum is several times higher in SM compared to the studied state-of-the-art MIMO schemes. <s> BIB013 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Space shift keying (SSK) modulation and its extension, the generalized SSK (GSSK), present an attractive framework for the emerging large-scale MIMO systems in reducing hardware costs. In SSK, the maximum likelihood (ML) detector incurs considerable computational complexities. We propose a compressed sensing based detector, NCS, by formulating the SSK-type detection criterion as a convex optimization problem. The proposed NCS requires only O(ntNrNt) complexity, outperforming the O(NrNtnt) complexity in the ML detector, at the cost of slight fidelity degradation. Simulations are conducted to substantiate the analytical derivation and the detection accuracy. <s> BIB014 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Spatial modulation (SM) and space shift keying (SSK) use only one out of several transmit antennas at a time to transmit data via an antenna index. In such a system, the information is encoded by exploiting channel randomness i.e. the fact that channels between different transmit and receive antennas are random. This difference is used to distinguish among the transmit antennas. While SSK uses only antenna index to transmit data, SM also uses ordinary signal modulation. In wireless secrecy systems, one of the key performance measures is secrecy capacity. It specifies the rate at which the transmitter can communicates on the main link to the desired receiver while this information cannot be decoded by the eavesdropper. We investigate SM and SSK in the context of wireless secrecy capacity when the underlying modulation and the difference between the legitimate and eavesdropper signal to noise ratios (SNRs) are varied. <s> BIB015 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> In this paper, the spatial modulation (SM) is considered as a novel physical layer technique to improve the physical layer security of wireless networks. Exploiting the index of antenna as an additional dimension to convey part of source information bits, SM helps to achieve multiplexing gain and realize higher data rates. Moreover, by properly designing the transmission signal, SM can also improve physical layer security. The secrecy mutual information of SM with finite alphabet input is derived in this work, and a preceding scheme is proposed to improve the physical layer security performance of SM. Analysis and numerical results verify the validity and effectiveness of the proposed scheme, and it also shows that the maximum achievable secrecy mutual information is the logarithm of the number of transmit antennas. <s> BIB016 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> In this paper, we aim to study the Energy Efficiency (EE) of Spatial Modulation (SM) at different Base Stations (BSs) taking into account the total power consumption. Compared to conventional Multiple-Input Multiple-Output (MIMO) schemes, SM benefits from a single Radio Frequency (RF) chain which results in decreased power supply (W), higher EE (Mbits/J), and reduced complexity. Using the fundamental limits of Shannon capacity, we show that SM achieves a range of average data rates with only a fraction, which can be as low as 24% for four transmit antennas, of the total power supply of conventional MIMO. In addition, we demonstrate that the EE of the studied schemes is maximized for a certain average data rate and that SM achieves the highest EE among them. Finally, we note that a BS employing SM can be up to 67% more energy efficient compared to a BS under a conventional MIMO transmission scheme, for four transmit antennas. <s> BIB017 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> In this paper, we contribute to the theoretical understanding, analysis, and design of spatial modulation multiple-input-multiple-output (SM-MIMO) systems for transmit diversity without channel state information at the transmitter. The contribution is threefold: 1) The achievable transmit diversity of SM-MIMO is analytically studied by analyzing the impact of various design parameters, notably spatial constellation diagram and shaping filters at the transmitter; 2) the design of SM-MIMO providing transmit diversity and maximum-likelihood (ML) optimum single-stream decoding is investigated; and 3) via Monte Carlo simulations, a comprehensive performance assessment of SM-MIMO against state-of-the-art MIMO (e.g., spatial multiplexing, orthogonal space-time block codes, Golden code, and double space-time transmit diversity) is conducted. It is shown that, for many system setups, a properly designed SM-MIMO outperforms, with lower decoding complexity, state-of-the-art MIMO. In particular, SM-MIMO is particularly useful in the downlink, where many antenna elements (with only few of them active) are available at the transmitter, and few antenna elements are available at the receiver. <s> BIB018 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Recently, there has been considerable interest in new tiered network cellular architectures, which would likely use many more cell sites than found today. Two major challenges will be i) providing backhaul to all of these cells and ii) finding efficient techniques to leverage higher frequency bands for mobile access and backhaul. This paper proposes the use of outdoor millimeter wave communications for backhaul networking between cells and mobile access within a cell. To overcome the outdoor impairments found in millimeter wave propagation, this paper studies beamforming using large arrays. However, such systems will require narrow beams, increasing sensitivity to movement caused by pole sway and other environmental concerns. To overcome this, we propose an efficient beam alignment technique using adaptive subspace sampling and hierarchical beam codebooks. A wind sway analysis is presented to establish a notion of beam coherence time. This highlights a previously unexplored tradeoff between array size and wind-induced movement. Generally, it is not possible to use larger arrays without risking a corresponding performance loss from wind-induced beam misalignment. The performance of the proposed alignment technique is analyzed and compared with other search and alignment methods. The results show significant performance improvement with reduced search time. <s> BIB019 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> In this paper, we compare the performance of multiple-input-multiple-output (MIMO) techniques applied to indoor optical wireless communications (OWC) assuming line-ofsight (LOS) channel conditions. Specifically, several 4 × 4 setups with different transmitter spacings and different positions of the receiver array are considered. The following MIMO algorithms are considered: Repetition Coding (RC), Spatial Multiplexing (SMP) and Spatial Modulation (SM). Particularly, we develop a framework to analytically approximate the bit error ratios (BERs) of these schemes and verify the theoretical bounds by simulations. The results show that due to diversity gains, RC is robust to various transmitter-receiver alignments. However, as RC does not provide spatial multiplexing gains, it requires large signal constellation sizes to enable high spectral efficiencies. In contrast, SMP enables high data rates by exploiting multiplexing gains. In order to provide these gains, SMP needs sufficiently low channel correlation. SM is a combined MIMO and digital modulation technique. We show that SM is more robust to high channel correlation compared to SMP, while enabling larger spectral efficiency compared to RC. Moreover, we investigate the effect of induced power imbalance between the multiple transmitters. It is found that power imbalance can substantially improve the performance of both SMP and SM as it reduces channel correlation. In this context, we also show that blocking some of the links is an acceptable method to reduce channel correlation. Even though the blocking diminishes the received energy, it outweighs this degradation by providing improved channel conditions for SMP and SM. For example, blocking 4 of the 16 links of the 4 × 4 setup improves the BER performance of SMP by more than 20 dB, while the effective signal to noise ratio (SNR) is reduced by about 2 dB due to the blocking. Therefore, MIMO techniques can provide gains even under LOS conditions which provide only little channel differences. <s> BIB020 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Massive multiple-input multiple-output (MIMO) wireless communications refers to the idea equipping cellular base stations (BSs) with a very large number of antennas, and has been shown to potentially allow for orders of magnitude improvement in spectral and energy efficiency using relatively simple (linear) processing. In this paper, we present a comprehensive overview of state-of-the-art research on the topic, which has recently attracted considerable attention. We begin with an information theoretic analysis to illustrate the conjectured advantages of massive MIMO, and then we address implementation issues related to channel estimation, detection and precoding schemes. We particularly focus on the potential impact of pilot contamination caused by the use of non-orthogonal pilot sequences by users in adjacent cells. We also analyze the energy efficiency achieved by massive MIMO systems, and demonstrate how the degrees of freedom provided by massive MIMO systems enable efficient single-carrier transmission. Finally, the challenges and opportunities associated with implementing massive MIMO in future wireless communications systems are discussed. <s> BIB021 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> What will 5G be? What it will not be is an incremental advance on 4G. The previous four generations of cellular technology have each been a major paradigm shift that has broken backward compatibility. Indeed, 5G will need to be a paradigm shift that includes very high carrier frequencies with massive bandwidths, extreme base station and device densities, and unprecedented numbers of antennas. However, unlike the previous four generations, it will also be highly integrative: tying any new 5G air interface and spectrum together with LTE and WiFi to provide universal high-rate coverage and a seamless user experience. To support this, the core network will also have to reach unprecedented levels of flexibility and intelligence, spectrum regulation will need to be rethought and improved, and energy and cost efficiencies will become even more critical considerations. This paper discusses all of these topics, identifying key challenges for future research and preliminary 5G standardization activities, while providing a comprehensive overview of the current literature, and in particular of the papers appearing in this special issue. <s> BIB022 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> A key challenge of future mobile communication research is to strike an attractive compromise between wire- less network's area spectral efficiency and energy efficiency. This necessitates a clean-slate approach to wireless system design, embracing the rich body of existing knowledge, espe- cially on multiple-input-multiple-ouput (MIMO) technologies. This motivates the proposal of an emerging wireless commu- nications concept conceived for single-radio-frequency (RF) large-scale MIMO communications, which is termed as SM. The concept of SM has established itself as a beneficial transmission paradigm, subsuming numerous members of the MIMO system family. The research of SM has reached sufficient maturity to motivate its comparison to state-of-the-art MIMO communica- tions, as well as to inspire its application to other emerging wireless systems such as relay-aided, cooperative, small-cell, optical wireless, and power-efficient communications. Further- more, it has received sufficient research attention to be im- plemented in testbeds, and it holds the promise of stimulating further vigorous interdisciplinary research in the years to come. This tutorial paper is intended to offer a comprehensive state-of-the-art survey on SM-MIMO research, to provide a critical appraisal of its potential advantages, and to promote the discussion of its beneficial application areas and their research challenges leading to the analysis of the technological issues associated with the implementation of SM-MIMO. The paper is concluded with the description of the world's first experimental activities in this vibrant research field. <s> BIB023 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> This paper proposes a new multiple-input–multiple-output (MIMO) technique called quadrature spatial modulation (QSM). QSM enhances the overall throughput of conventional SM systems by using an extra modulation spatial dimension. The current SM technique uses only the real part of the SM constellation, and the proposed method in this paper extends this to in-phase and quadrature dimensions. It is shown that significant performance enhancements can be achieved at the expense of synchronizing the transmit antennas. Additionally, a closed-form expression for the pairwise error probability (PEP) of generic QSM system is derived and used to calculate a tight upper bound of the average bit error probability (ABEP) over Rayleigh fading channels. Moreover, a simple and general asymptotic expression is derived and analyzed. Obtained Monte Carlo simulation results corroborate the accuracy of the conducted analysis and show the significant enhancements of the proposed QSM scheme. <s> BIB024 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Spatial modulation (SM) is a newly emerging multiple-input–multiple-output technique that activates only a single antenna for transmission at any time instant and uses the index of the active antenna as an additional information-carrying mechanism. However, by its nature, SM decoding is coherent in that channel state information (CSI) is required at the receiver. In fact, coherent SM decoding can be very complex due to the heavily entangled channel estimation and symbol detection. In this correspondence, a differential SM scheme that completely bypasses any CSI at the transmitter or receiver, while preserving the single active transmit antenna property, is developed. The proposed scheme can be applied to any constant energy constellation such as phase-shift keying (PSK) and to systems with arbitrary numbers of transmit and receive antennas. Simulation results are presented under various system configurations. With the same spectral efficiency, the proposed scheme is capable of paying no more than 3 dB of signal-to-noise ratio penalty compared with coherent SM and outperforming the single-antenna differential PSK and differential space–time coding schemes. <s> BIB025 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Differential spatial modulation (DSM) is a newly-emerging differential scheme tailored to the spatial modulation technique, which selects only one among a group of antennas for transmission at any time instant. DSM, however, gives rise to prohibitive search complexity when the number of transmit antennas is large. In this letter, a low-complexity suboptimal detector is proposed for DSM. It is designed based on the maximum-likelihood criterion but takes more candidates for the antenna activation orders into account. The detection is performed in two steps: the first step is to confine the number of candidates for the modulated symbols to a small portion by exploiting the symmetry of the signal constellation; the second step is to select the most likely modulated symbols from the output of the first step according to the determined antenna activation order via a Viterbi-like algorithm. Analyses and simulations show that the proposed detector achieves near-optimal performance yet largely reduces the search complexity. <s> BIB026 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> The recently proposed multiple input multiple output (MIMO) transmission scheme termed as generalized pre-coding aided spatial modulation (GPSM) is analyzed, where the key idea is that a particular subset of receive antennas is activated and the specific activation pattern itself conveys useful implicit information. We provide the upper bound of both the symbol error ratio (SER) and bit error ratio (BER) expression of the GPSM scheme of a low-complexity decoupled detector. Furthermore, the corresponding discrete-input continuous-output memoryless channel (DCMC) capacity as well as the achievable rate is quantified. Our analytical SER and BER upper bound expressions are confirmed to be tight by our numerical results. We also show that our GPSM scheme constitutes a flexible MIMO arrangement and there is always a beneficial configuration for our GPSM scheme that offers the same bandwidth efficiency as that of its conventional MIMO counterpart at a lower signal to noise ratio (SNR) per bit. <s> BIB027 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Adaptive power allocation (PA) algorithms based on optimization of the minimum distance $d_{\min} $ between signal points at the receiver side are investigated in spatial modulation (SM) systems. First, a closed-form solution of the optimal maximum- $d_{\min} $ -aided PA algorithm is derived in the case of two transmit antennas (TAs) . Moreover, for a higher number of TAs, we propose a numerical approach, which appropriately splits the power between the specific TA pair associated with $d_{\min} $ to increase this distance. Furthermore, our PA-aided SM systems may be readily combined with adaptive modulation (AM) to further improve the system's performance. Our numerical results show that the proposed algorithms provide beneficial system performance improvements compared with both the equal-gain PA-based SM and the identical-throughput PA-aided spatial multiplexing systems. <s> BIB028 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> The main limitations of employing large-scale antenna (LSA) architectures for broadband frequency-selective channels include, but are not limited to their complexity, power consumption, and the high cost of multiple radio frequency (RF) chains. Promising solutions can be found in the recently proposed family of single-carrier (SC) spatial modulation (SM) transmission techniques. Since the SM scheme’s transmit antenna (TA) activation process is carried out in the context of a SC-SM architecture, the benefits of a low-complexity and low-cost single-RF transmitter are maintained, while a high MIMO multiplexing gain can be attained. Moreover, owing to its inherent SC structure, the transmit signals of SC-SM have attractive peak power characteristics and a high robustness to RF hardware impairments, such as the RF carrier frequency offset (CFO) and phase noise. In this paper, we present a comprehensive overview of the latest research achievements of SC-SM, which has recently attracted considerable attention. We outline the associated transceiver design, the benefits and potential tradeoffs, the LSA aided multiuser (MU) transmission developments, the relevant open research issues as well as the potential solutions of this appealing transmission technique. <s> BIB029 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> In this paper, an emerging wireless communication concept, which is termed as spatial modulation (SM) for large-scale multiple-input–multiple-output (MIMO), is considered. The results show that from the information-theoretic perspective, SM achieves capacity comparable to the open-loop MIMO capacity, although a subset of transmit antennas is activated in every channel use because both the channel coefficients and the input symbols carry information in SM. As a result, SM compensates the loss of information capacity due to a subset of antennas being activated by modulating information in the antenna index; therefore, the total information rate remains high. In particular, an upper bound for the capacity of SM is derived in closed form, and it is shown analytically that this upper bound is almost certainly achievable in the massive MIMO regime. Moreover, it is shown that the upper bound is achievable with no channel state information at the transmitter (CSIT) but with channel distribution information (CDI) at the transmitter (CDIT). The optimum transmission strategy should adapt the channel input distribution to fading using CDI such as the $K$ factor in Rician fading or the shape parameter $m$ in Nakagami- $m$ fading. <s> BIB030 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> For high-speed railway (HSR), an essential characteristic of the propagation channels is the specific spatial–temporal correlation caused by a dominant strong line-of-sight (LOS) component. Multiple-antenna gain is therefore far from being achieved due to this strong channel correlation. However, it is interesting to note that if the resulted intercarrier interference (ICI) is elaborately controlled, high mobility may decrease the channel correlation. Spatial modulation (SM) is one of the promising multiple-antenna technologies for wireless communication systems, which needs only the activation of one transmit antenna to convey information bits implicitly through the index of the active transmit antenna, in addition to conveying information bits through modulation symbols during transmission. In SM, ICI is avoided, which therefore helps in the implementation of the idea of decreasing channel correlation by increasing the velocity in an HSR scenario. In this paper, the performance of massive SM multiple-input multiple-output (MIMO) over a spatial–temporal correlated Rician fading channel is investigated under an HSR scenario. The impacts of velocity, Rician factor K, and signal-to-noise ratio (SNR) are discussed. In addition, the correlation factor and the correlation comparison function are defined as metrics of channel correlation. We theoretically find out that higher velocity makes the impact of temporal correlation more dominant, which weakens the influence of spatial correlation. In other words, spatial correlation can be weakened by high mobility. Our theoretical analysis and simulations further demonstrate that, with an appropriate scheme, massive SM MIMO outperforms Vertical Bell Laboratories Layered Space–Time (VBLAST), except at a lower SNR region with high velocity (usually more than 360 km/h). Consequently, massive SM MIMO is a promising solution within an appropriate range of velocity for HSR wireless communication systems. <s> BIB031 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> In this paper, the bit error rate (BER) performance of a new multiple-input-multiple-output technique, named spatial modulation (SM), is studied under a novel non-stationary wideband high-speed train (HST) channel model in different scenarios. Time-varying parameters obtained from measurement results are used to configure the channel model to make all results more realistic. A novel statistic property called the stationary interval in terms of the space-time correlation function is proposed to describe the channel model’s time-varying behavior. The accurate theoretical BER expression of SM systems is derived under the time-varying wideband HST channel model with the non-ideal channel estimation assumption. The simulation results demonstrate that the BER performance of SM systems shows a time-varying behavior due to the non-stationary property of the employed HST channel model. The system performance can maintain a relative stationary status within the specified stationary interval. It can also be observed that the BER performance of SM systems under the HST channel model is mainly affected by the correlation between sub-channels, inter-symbol-interference, Doppler shift, and channel estimation errors. <s> BIB032 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Generalized spatial modulation (GSM) is a spectral and energy efficient multiple-input–multiple-output transmission technique. The low-complexity detection algorithm design with near maximum likelihood (ML) performance at the receiver is very challenging, and is the focus of this letter. In specific, we exploit the fixed sparsity constraint in the transmitted GSM signals, and take advantage of Bayesian compressive sensing (BCS) in sparse signal recovery. A new detection algorithm, referred to as enhanced Bayesian compressive sensing (EBCS), is proposed. It features more than 75% complexity reduction at high signal-to-noise ratios compared with the ordered-blocked minimum-mean-squared-error algorithm. Furthermore, it is shown by simulation that its error performance is comparable to the ML algorithm, and the performance gap is negligible in many cases. <s> BIB033 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Generalized spatial modulation (GSM) is a relatively new multi-input multi-output transmission technique that enables a flexible trade-off between the achievable transmission rate and the cost of radio frequency chains. However, due to the constraint of transmit antenna combination and the variation of interchannel interference, the efficient low-complexity demodulation of GSM signals is challenging, especially when soft demodulation is needed. In this paper, we propose two low-complexity algorithms based on the deterministic sequential Monte Carlo (SMC) technique for the demodulation of GSM. The type-I SMC demodulator, which uses the conventional successive interference cancellation as the kernel and draws antenna-wise samples from the extended constellation, is proposed for the overdetermined GSM system. The type-II SMC demodulator, which consists of two stages and uses the orthogonal matching pursuit as the kernel in the first stage, is proposed for the underdetermined GSM system. A key component in both algorithms is an efficient online scheme to eliminate the illegal samples during the sampling process. Both proposed algorithms achieve near-optimal performances with complexity linear in terms of the antenna size. Moreover, owing to their soft-input soft-output nature, they can be employed in a turbo receiver for a coded GSM system. <s> BIB034 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <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> BIB035 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Due to the interchannel interference and the varying active antennas in generalized spatial modulation (GSM), it is challenging to detect the transmitted data effectively while imposing low computational burden to the receiver. In this paper, we propose a novel algorithm based on the K-best sphere decoding for the soft detection of the GSM signal, which performs the breadth-first tree search in two stages sequentially. By exploiting the null space of the GSM channel, the inactive antenna searching is carried out antennawise in the first stage, in which a recursive algorithm for updating the QL decomposition (a decomposition of a matrix into a product of an orthogonal matrix and a lower triangular matrix) is proposed to calculate the branch metrics and an effective examination scheme is developed to prune those illegal or repetitive child nodes. Based on the available QL decomposition structure that has been updated recursively during the first stage, the second stage of the proposed detector is then readily concatenated to search for the modulated symbols carried on the active antennas in a one-by-one manner. Moreover, owing to the soft-input soft-output nature, the proposed detector can be applied in a turbo decoder for a coded GSM system. Both complexity analysis and simulation results corroborate the superiority of the proposed soft detector for the GSM system. <s> BIB036 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Spatial modulation (SM) has emerged as a low-complexity and energy-efficient multiple-input multiple-output transmission technique, where the information bits are not only transmitted by amplitude phase modulation but also conveyed by the index of activated transmit antenna (TA). By deploying SM in downlink multi-user (DL-MU) scenarios, conventional orthogonal multiple access-based SM (OMA-SM) allocates exclusive time-frequency resources to users, but suffers from low spectral efficiency. TA grouping-based SM (TAG-SM) divides TAs into sub-groups to serve different users independently, but suffers from severe inter-user interference. By introducing non-OMA (NOMA) into SM for DL-MU transmission, NOMA-based SM (NOMA-SM) is proposed to mitigate inter-user interference, while maintaining high spectral efficiency. Specifically, by applying successive interference cancellation at user side, the inter-user interference could be effectively eliminated with the sacrifice of increased computational complexity. Afterward, based on a symbol error rate analysis, a low-complexity power allocation scheme is provided to achieve high spectral efficiency through power domain multiplexing. When considering near-far effect from user distribution, user pairing issue is also discussed. Numerical simulation compares NOMA-SM with OMA-SM and TAG-SM, and verifies the effectiveness of the proposed low-complexity power allocation and user pairing methodologies. <s> BIB037 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> Multi-antenna non-orthogonal multiple access (NOMA) is a promising technique to significantly improve the spectral efficiency and support massive access, which has received considerable interests from academic and industry. This article first briefly introduces the basic idea of conventional multi-antenna NOMA technique, and then discusses the key limitations, namely, the high complexity of successive interference cancellation(SIC) and the lack of fairness between the user with a strong channel gain and the user with a weak channel gain. To address these problems, this article proposes a novel spatial modulation (SM) assisted multi-antenna NOMA technique, which avoids the use of SIC and is able to completely cancel intra-cluster interference. Furthermore, simulation results are provided to validate the effectiveness of the proposed novel technique compared to the conventional multi-antenna NOMA. Finally, this article points out the key challenges and sheds light on the future research directions of the SM assisted multi-antenna NOMA technique. <s> BIB038 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> I. INTRODUCTION <s> The combination of non-orthogonal multiple access (NOMA) and multiple-input multiple-output (MIMO) signaling is capable of providing significantly improved spectral efficiency and effective multiple access. However, conventional MIMO-NOMA entails significantly increased power consumption and implementation complexity. Inspired by the essence of spatial modulation (SM), in this paper, a novel three-node cooperative relaying system (CRS) using SM-aided NOMA, termed CRS-SM-NOMA, is proposed, in which the messages for two users are conveyed by two different information-bearing units of SM. As the reference scheme, conventional CRS-NOMA is further re-investigated with multiple receive antennas. Besides the achievable rate, which is adopted by most of the existing NOMA works as the performance metric, the bit error rate (BER) of CRS-SM-NOMA and CRS-NOMA is analyzed assuming maximum-likelihood detection. Approximate bit error probability expressions are derived in closed form for both users. Simulation results verify the analysis and show that the proposed CRS-SM-NOMA outperforms conventional CRS-NOMA and SM-OMA in terms of the BER as well as ergodic sum rate. <s> BIB039
D RIVEN by the skyrocketing growth of mobile devices and the wide applications of the Internet of things (IoT), future wireless communication systems have triggered the explosive demand and urgent need for ultra-high capacity, ultra-low latency, and massive connectivity over the scarce wireless resources BIB021 -  . Specifically, an ever-increasing volume of mobile data traffic is expected to appear in the coming years, which may overwhelm the limited spectrum resources significantly and increase the power consumption dramatically BIB022 -  . Due to the unprecedented surge of mobile data traffic, researchers have been motivated to develop new transmission technologies for maximizing the achievable throughput and minimizing the deployment cost. Among various technologies, spatial modulation (SM) has been envisioned as one prospective digital modulation technology to achieve high spectral efficiency and energy efficiency yet enjoy a simple design principle. Note that the first works on SM date back to the beginning of the 21-st century, when the preliminary SM was first explored in BIB001 , BIB002 . Unfortunately, they did not receive much attention in the early days. After 2008, research work on SM has begun to grow explosively and the systematic introduction of the SM concept has been reported in some tutorial/overview literature BIB023 - BIB029 , which attract a lot of attention from researchers. By activating one transmit antenna to convey index information, SM reaps the spatial gain with a single radio frequency (RF) chain and enjoys the following additional benefits BIB002 , BIB023 : • higher energy efficiency BIB017 , BIB013 ; • lower detection complexity with a smaller number of receive antennas and lower complexity of RF circuits; • free of inter-channel interference; • no need for inter-antenna synchronization; • compatibility with massive multiple-input multiple-output (MIMO) BIB030 . In SM, extra information bits are typically conveyed through the index of one active transmit antenna, in addition to the information bits conveyed by the conventional constellation 0733-8716 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. symbol. Based on the antenna-switching mechanism, the active antenna index changes according to the spatial information bits. In particular, only one RF chain is required at the SM transmitter to activate one out of multiple transmit antennas for a constellation symbol transmission, significantly saving the energy consumption in downlink communications, and dramatically reducing the hardware cost at the user terminal in uplink communications. Moreover, due to freedom of the inter-channel interference, SM can be a better candidate technology than Vertical Bell Laboratories Layered Space-Time (VBLAST) for high-mobility wireless communication systems, where the channel correlation is weakened while the inter-channel interference effect is aggravated BIB031 . Space shift keying (SSK) can be viewed as a simplified variant of SM BIB004 - BIB005 , which embeds the overall information into the index of one active antenna only without involving any constellation symbol. Other variants of SM technology typically consist of generalized (G)SM BIB006 - BIB034 , quadrature SM (QSM) BIB024 , differential (D)SM BIB025 - BIB026 , receive (R)SM BIB008 - BIB027 , and generalized (G)SSK BIB003 . Generally speaking, SM refers to a new modulation family of communication systems that conveys additional information typically through the activation states of transmit antennas. By choosing different activation patterns at the transmitter, various SM members provide a flexible design to meet different specific requirements and trade-offs among the spectral efficiency, energy efficiency, deployment cost, and system performance. For information recovery, the receivers of SM have to execute two main tasks BIB035 : • detecting the indices/states of active antennas; • demodulating the constellation symbols embedded on the active antennas/states (if applicable). However, it is not trivial to effectively detect both spatial and constellation information effectively while maintaining low complexity for SM members under different channel conditions BIB034 , BIB036 . On the other hand, link-adaptive SM, which relies on the feedback from the receiver to alter its transmission pattern (e.g., modulation order, transmit power and antenna selection), was extensively investigated to achieve better system performance and channel utilization BIB009 - BIB028 . Please note that the detection performance of plain SM highly relies on the distinctness of the channel signatures/fingerprints associated with different transmit antennas. As a result, plain SM enjoys the technical challenge to operate in rich scattering propagation and stationary BIB032 environments. With further leverage of advantage preprocessing techniques, such as orthogonal pulse shaping BIB010 and trellis coded modulation (TCM) BIB007 , BIB011 , the problem of lack of channel distinctness can be well addressed in SM systems. Furthermore, as plain SM utilizes the space domain to convey index information via one active antenna, no transmit-diversity gain is provided by plain SM to combat channel fading effects. To overcome the lack of transmit-diversity inherent in plain SM, transmit-diversity enhanced SM using space-time block coding (STBC) is also a promising direction to improve the error performance BIB012 - BIB018 . Due to its many promising advantages, SM serves as an attractive energy-efficient modulation technology with flexibility of working together with others in emerging communication systems. Thanks to the sparsity inherent in SM signals, compressed-sensing (CS) theory is a powerful tool for low-complexity signal reconstruction even when the number of available measurements is much smaller than the signal dimension (i.e., the number of receive antennas is much less than that of transmit antennas in underdetermined systems), particularly in the case of large-scale MIMO BIB014 - BIB033 . On the other hand, due to the significantly reduced wavelength in the millimeter-wave (mmWave) frequency band, mmWave systems can be equipped with a large number of antennas at the transceiver in a highly compact manner for the implementation of large-scale MIMO BIB019 . As a result, the SM family emerges as a promising low-cost and high-efficiency candidate for large-scale MIMO with a large number of transmit antennas while a much smaller number of RF chains are required for antenna activation. More recently, noticing the great potentials of non-orthogonal multiple access (NOMA) in supporting massive connectivity and low transmission latency, NOMA aided SM emerges as an attractive and novel technology for multi-user communications, which achieves high spectral efficiency and energy efficiency while maintaining low-complexity transceiver design BIB037 - BIB039 . As a result, NOMA aided SM technology strikes an appealing trade-off among spectral efficiency, energy efficiency, deployment cost, and interference mitigation BIB038 . Simultaneous wireless information and power transfer (SWIPT) is another emerging important technology, which aims at delivering wireless information and energy concurrently. SM finds its special fit to SWIPT-enabled wireless systems since it has the potential to leverage the inactive antennas for energy harvesting without incurring any loss of the spectral efficiency. Moreover, due to the broadcast nature of wireless communications, the security of SM transmission is an essential problem in practice BIB015 - BIB016 . Interestingly, both physical layer security (PLS) and SM highly rely on the randomness and discrimination properties of the wireless interface, which can be exploited to achieve confidential information exchange among legitimate nodes while impairing the potential eavesdropper at the same time. Specifically, SM employs a fast antenna-switching mechanism to achieve the random selection of transmit antennas according to the spatial information bits and the legitimate channel state information (CSI), which can incur fast time-varying environments to confuse the eavesdropper and degrade its decoding performance. In other words, SM shows a great potential not only in its increased energy efficiency, but also in the feasibility of secure transmission. Furthermore, since visible light communications (VLC) enjoys high security and supports the fast switching in light-emitting diodes (LEDs), SM technology can also be readily integrated into VLC, which provides the flexibility to control the luminance of the LED array and the communication throughput BIB020 . Although originated in the space domain, the concept of SM is not exclusive to the space domain, but can be generalized and applied to other signal domains such as frequency/time/code/angle domain or even across multiple domains. As a consequence, considerable interest and