A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <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> BIB001 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <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> BIB002 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> Millimeter-wave (mmW) frequencies between 30 and 300 GHz are a new frontier for cellular communication that offers the promise of orders of magnitude greater bandwidths combined with further gains via beamforming and spatial multiplexing from multielement antenna arrays. This paper surveys measurements and capacity studies to assess this technology with a focus on small cell deployments in urban environments. The conclusions are extremely encouraging; measurements in New York City at 28 and 73 GHz demonstrate that, even in an urban canyon environment, significant non-line-of-sight (NLOS) outdoor, street-level coverage is possible up to approximately 200 m from a potential low-power microcell or picocell base station. In addition, based on statistical channel models from these measurements, it is shown that mmW systems can offer more than an order of magnitude increase in capacity over current state-of-the-art 4G cellular networks at current cell densities. Cellular systems, however, will need to be significantly redesigned to fully achieve these gains. Specifically, the requirement of highly directional and adaptive transmissions, directional isolation between links, and significant possibilities of outage have strong implications on multiple access, channel structure, synchronization, and receiver design. To address these challenges, the paper discusses how various technologies including adaptive beamforming, multihop relaying, heterogeneous network architectures, and carrier aggregation can be leveraged in the mmW context. <s> BIB003 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <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> BIB004 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> We use space shift keying (SSK), which is the simplest form of spatial modulation (SM), to present and analyze the idea of spatial modulation in line-of-sight (LOS) conditions. We show that SSK can operate effectively in LOS conditions provided that the antennas are properly placed at TX and RX such that a high-rank LOS-MIMO channel is constructed. The operating conditions for LOS-SSK with parallel uniform linear arrays are established and two schemes, namely orthogonal SSK and bi-orthogonal SSK, are introduced. The bit error probabilities for both methods are derived and given in closed form. It is shown that LOS-SSK (more generally LOS-SM) is a promising technique and might especially be attractive in millimeter-wave communications which, due to its high frequency and small wavelength, inherently prefers LOS transmission and also enables packing of a large number of antennas in terminals. <s> BIB005 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> In this paper, we propose to use spatial modulation (SM) and the generalized spatial modulation (GSM) MIMO schemes in indoor line-of-sight (LOS) millimeter-wave (mmWave) communication at 60GHz. SM/GSM are known but only at low-GHz frequencies where the channels are typically rich scattered and characterized by Rayleigh or Rician distribution. However, 60-GHz indoor channels are typically not rich but rather sparsely scattered and dominated by the LOS component, thus making them different from low-GHz fading channels and our work novel. We first seek to optimize SM in LOS by finding the channel conditions that minimize its symbol error probability (SEP). Then, we extend our studies to LOS GSM and derive its channel capacity and SEP. Furthermore, we present numerical studies on the behavior and performance of SM/GSM in LOS. LOS spatial multiplexing and beamforming MIMO schemes are used as benchmarks for comparison. At last, we propose novel TX and RX hardware architectures, both of which use only a single RF chain, for implementation of SM/GSM at mmWave frequencies. Both simplicity and good performance are exhibited by LOS SM/GSM thus making them very attractive techniques for short-range indoor mmWave communications at 60GHz. <s> BIB006 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> The distinct ability of spatial modulation (SM) to effectively utilizing spatial degrees of freedom with only a single radio frequency chain renders it promising for millimeter wave (mmWave) communications. In this work we investigate the transmitter design employing the SM concept in mmWave multiple-input multiple-output (MIMO) systems. To introduce beamforming gain while maintaining the advantages of SM, we apply the virtual antenna concept to SM and construct spatial signatures with the aid of analog beamforming. For a given configuration of the space-signal constellation, we formulate an optimization problem for designing the analog beamforming matrix and propose two approaches via exploiting characteristics of mmWave channel and transmitter architecture to acquire effective solutions with low complexity. To further improve the performance, we optimize the configuration of the spacesignal constellation by selecting the optimal sizes of spatial and signal constellations while guaranteeing the transmission rate requirement. The overhead and design complexity of proposed design approaches are analyzed. Besides, we exploit simulations to evaluate the proposed designs, and briefly compare between analog beamforming aided SM-MIMO and conventional MIMO to show advantages of our designs. <s> BIB007 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> A generalized spatial modulation (GSM)-based millimeter-wave communications system is proposed. The GSM transmitter is characterized by a lower number of radio frequency (RF) chains than the number of transmit antennas; hence, it is capable of reducing both the transmitter cost and the energy consumption. The antenna array alignment is optimized so as to maximize the rank of the channel matrix encountered. Furthermore, we employ an array of analog beamformers, which allows us to benefit both from the beamforming gain and from the GSM scheme's high rate. It is demonstrated that the constrained capacity of the GSM transmitter equipped with as few as two RF chains is capable of approaching the performance of the full-RF spatial multiplexing having eight RF chains. <s> BIB008 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> Capacity analysis for millimeter–wave (mmWave) quadrature spatial modulation (QSM) multiple-input multiple-output (MIMO) system is presented in this paper. QSM is a new MIMO technique proposed to enhance the performance of conventional spatial modulation (SM) while retaining almost all its inherent advantages. Furthermore, mmWave utilizes a wide-bandwidth spectrum and is a very promising candidate for future wireless systems. Detailed and novel analysis of the mutual information and the achievable capacity for mmWave–QSM system using a 3-D statistical channel model for outdoor mmWave communications are presented in this paper. Monte Carlo simulation results are provided to corroborate derived formulas. Obtained results reveal that the 3-D mmWave channel model can be closely approximated by a log–normal fading channel. The conditions under which capacity can be achieved are derived and discussed. It is shown that the capacity of QSM system can be achieved, by carefully designing the constellation symbols for each specific channel model. <s> BIB009 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> Generalized spatial modulation (GenSM) aided millimeter wave (mmWave) multiple-input multiple-output (MIMO) has recently received substantial academic attention. However, due to the insufficient exploitation of the transmitter's knowledge of the channel state information (CSI), the achievable rates of state-of-the-art GenSM-aided mmWave MIMO systems are far from being optimal. Against this background, a novel analog precoding scheme is proposed in this paper to improve the spectral efficiency (SE) of conventional GenSM-aided mmWave MIMOs. More specifically, we firstly manage to lower-bound the achievable SE of GenSM-aided mmWave MIMO with a closed-form expression. Secondly, by exploiting this lower bound as a cost function, a low-complexity iterative algorithm is proposed to design the analog precoder for SE maximization. Finally, numerical simulations are conducted to substantiate the superior performance of the proposed design with respect to state-of-the-art GenSM-aided mmWave MIMO schemes. <s> BIB010 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> The concept of generalized spatial modulation (GenSM) aided millimeter wave (mm-wave) multiple-input multiple-output (MIMO) has recently attracted substantial research interest, as it benefits from the large bandwidth of mm-wave MIMO, while maintaining a reduced number of radio frequency (RF) chains. However, due to the lack of precoding design, the preceding GenSM-aided mm-wave MIMO schemes suffered from severe performance loss. Inspired by the reduced-RF-chain structure making implementation cost low while maintaining the benefit of GenSM, in this paper, we incorporate the technique of hybrid precoding into GenSM-aided mm-wave MIMO, where a hybrid digital and analog precoding regime is proposed to enhance the system’s achievable rate. Moreover, a closed-form expression is proposed to provide an accurate approximation to the spectral efficiency (SE) achieved by the proposed scheme. The proposed expression is further simplified in the region of high signal-to-noise ratio (SNR). By exploiting the proposed SE expressions as low-complexity cost functions, two algorithms, i.e. the gradient ascent algorithm and the high-SNR approximation algorithm, are exploited to optimize the hybrid precoders in terms of SE maximization. Finally, numerical simulations are provided to substantiate the superior SE performance achieved by the proposed scheme over other GenSM-aided mm-wave MIMO schemes as well as the state-of-the-art mm-wave MIMO systems. <s> BIB011 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> The orbital angular momentum (OAM) technique provides a new degree of freedom for information transmissions in millimeter wave communications. Considering the spatial distribution characteristics of OAM beams, a new OAM spatial modulation (OAM-SM) millimeter wave communication system is first proposed for future mobile networks. Furthermore, the capacity, average bit error probability, and energy efficiency of OAM-SM millimeter wave communication systems are analytically derived for performance analysis. Compared with the OAM-based multi-input multi-output (MIMO) millimeter wave communication systems, the maximum energy efficiency of OAM-SM millimeter wave communication systems is improved by 227.2%. Moreover, numerical results indicate that the proposed OAM-SM millimeter wave communication systems are more robust to path-loss attenuations than the conventional MIMO millimeter wave communication systems, which makes it suitable for long-range transmissions. Therefore, OAM-SM millimeter wave communication systems provide a great growth space for future mobile networks. <s> BIB012 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> In this paper, a novel multiple-input multiple-output (MIMO) transmission scheme, termed as receive antenna selection (RAS)-aided spatial modulation MIMO (SM-MIMO), is proposed for millimeter-wave (mmWave) communications. It employs the spatial modulation (SM) concept and the RAS technique to tackle the costs of the multiple radio frequency (RF) chains at both link ends. Moreover, we develop a pair of RAS algorithms for the proposed mmWave RAS-SM scheme based on the capacity maximization (max-capacity) and the bit-error rate (BER) minimization criteria, which are formulated as two combinatorial optimization problems. The theoretical gradients of the capacity and the BER with respect to RAS variables are derived and the convexities of these problems are discussed. Furthermore, a novel iterative algorithm through jointly designing the log-barrier algorithm (LbA) and the simplified conjugate gradient method is proposed for RAS optimization. Our simulation results show that the proposed RAS-SM schemes are capable of achieving considerable performance gains over conventional norm-based and eigenvalue-based schemes in mmWave MIMO channels, while avoiding an overwhelming complexity imposed by exhaustive search. <s> BIB013 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> In this paper, we propose a novel transmission approach, namely, analog precoding-aided virtual space modulation (APAVSM), and its corresponding multimode hybrid precoder designs in the multiple-input multiple-output system. We consider the system equipped with a single radio frequency chain and a phase shift network. Our proposed designs can attain benefits of both beamforming and spatial modulation, and therefore enable the significant signal-to-noise power ratio enhancement with efficient degrees of freedom utilization and low cost. We elaborate APAVSM and formulate the multimode hybrid precoder design problem. To solve the problem, we impose restrictions on the number of candidate modes for selection, and propose the hybrid precoder design approach in which two non-convex max-min problems are solved sequentially by iterative approaches. To trade off between error rate and complexity, we propose alternative designs by reducing the dimensionality and/or relaxing constraints. Moreover, by the concept of beamspace, heuristic algorithms are proposed with extremely low complexities. We offer complexity analyses for proposed design approaches. Simulations in Ralyigh fading and millimeter-wave (mmWave) channels are used to evaluate the proposed designs. Results show that our designs can outperform the existing SM-MIMO systems, and provide the feasible operation in mmWave channels with severe path loss. Moreover, the superiority of proposed designs to the conventional precoding-aided MIMO systems is numerically demonstrated. <s> BIB014 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> Receive spatial modulation (RSM) is a recently introduced multi-antenna transmission scheme that is capable of achieving high spectral efficiency at low hardware and processing complexity. We study RSM for application in indoor line-of-sight (LOS) millimeter-wave (mmWave) communication. Significantly different from low-gigahertz indoor channels, which are typically modeled by a Rician distribution, indoor mmWave channels are sparsely-scattered and dominated by LOS propagation. Hence, we consider the specific characteristics of the LOS mmWave channel and identify the channel conditions that minimize the symbol error probability (SEP) of RSM. We prove that the SEP of RSM in a LOS channel is minimized by imposing orthogonality conditions to the channel matrix. In addition, we propose a simple hardware architecture for the transmitter (TX), which uses only RF phase shifters for precoding. Simulation results show that RSM can successfully operate in indoor mmWave channels. <s> BIB015 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> In this paper, we study the capacity and symbol error probability (SEP) of generalized spatial modulation (GSM) multiple-input multiple-output (MIMO) using measured channels that are obtained by channel sounding in an indoor office environment at 60GHz. Spatial modulation (SM) and GSM are emerging low-complexity MIMO schemes that have been extensively researched for low-GHz (below 6GHz) communications. Recently, they have been considered and shown to be promising also for (mmWave) communications. In the simplest possible case, they require only one RF chain both at the transmitter (TX) and receiver (RX), and thus, are especially attractive for mmWave communications, in which the number of RF chains needs to be as low as possible. Despite of some early works on the theoretical analysis of SM/GSM for mmWave communications, there have been no investigations using real-world channel data. We focus on the office line-of-sight (LOS) scenario and investigate three problems: 1) the performance of GSM using the extracted LOS component of measured channels; 2) the impact of non-LOS NLOS components on the performance of GSM; and 3) possible simple modulation and reception algorithms for GSM that rely only on the LOS component of the channel. The results being reported in this paper not only validate the main claims of previous studies based on ideal pure LOS channels, but also lead to novel findings. One major conclusion is that NLOS components are harmful to the SEP of GSM and should be avoided. As another important outcome, our results strongly motivate the use of precoding in GSM systems to simultaneously improve the channel capacity and reduce the physical size of MIMO arrays (thus eliminating one major issue of LOS GSM). <s> BIB016 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> The application of hybrid precoding in millimeter wave (mmWave) multiple-input multiple-output (MIMO) systems has been proved effective for reducing the number of radio frequency (RF) chains. However, the maximum number of independent data streams is conventionally restricted by the number of RF chains, which leads to limiting the spatial multiplexing gain. To further improve the achievable spectral efficiency (SE), in this paper we propose a novel generalized spatial modulation (GenSM) aided mmWave MIMO system to convey an extra data stream via the index of the active antennas group, while no extra RF chain is required. Moreover, we also propose a hybrid analog and digital precoding scheme for SE maximization. More specifically, a closed-form lower bound is firstly derived to quantify the achievable SE of the proposed system. By utilizing this lower bound as the cost function, a two-step algorithm is proposed to optimize the hybrid precoder. The proposed algorithm not only utilizes the concavity of the cost function over the digital power allocation vector, but also invokes the convex $\ell_\infty$ relaxation to handle the non-convex constraint imposed by analog precoding. Finally, the proposed scheme is shown via simulations to outperform state-of-the-art mmWave MIMO schemes in terms of achievable SE. <s> BIB017 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> A novel low RF-complexity multiple-input multiple-output (MIMO) transmission scheme, termed analog beamforming aided receive spatial modulation (AB-RSM), is proposed for millimeter-wave (mmWave) communications. We investigate the transmitter design employing the RSM concept and analog beamforming technique in mmWave MIMO systems. To introduce beamforming gain while maintaining the advantages of RSM, we employ the phase shifters at the transmitter to make all the transmit antennas aim at the activated receive antenna. Our simulation results show that the proposed AB-RSM scheme is capable of achieving reduced computational complexity as well as hardware cost compared to the conventional RSM, especially for large-scale scenarios. <s> BIB018 </s> A Survey on Spatial Modulation in Emerging Wireless Systems: Research Progresses and Applications <s> A. SM in mmWave Communications <s> Edge caching is emerging as the most promising solution to reduce the content retrieval delay and relieve the huge burden on the backhaul links in the ultra-dense networks by proactive caching popular contents in the small base station (SBS). However, constraint cache resource of individual SBSs significantly throttles the performance of edge caching. In this paper, we propose a device-to-device (D2D) assisted cooperative edge caching (DCEC) policy for millimeter (mmWave) dense networks, which cooperatively utilizes the cache resource of users and SBSs in proximity. In the proposed DCEC policy, a content can be cached in either users’ devices or SBSs according to the content popularity, and a user can retrieve the requested content from neighboring users via D2D links or the neighboring SBSs via cellular links to efficiently exploit the cache diversity. Unlike existing cooperative caching policies in the lower frequency bands that require complex interference management techniques to suppress interference, we take advantage of directional antenna in mmWave systems to ensure high transmission rate whereas mitigating interference footprint. Taking the practical directional antenna model and the network density into consideration, we derive closed-form expressions of the backhaul offloading performance and content retrieval delay based on the stochastic information of network topology. In addition, analytical results indicate that, with the increase of the network density, the content retrieval delay via D2D links increases significantly while that via cellular links increases slightly. Comprehensive simulations validate our theoretical analysis and demonstrate that the proposed policy can achieve higher performance in offloading the backhaul traffic and reducing the content retrieval delay compared with the state-of-the-art most popular caching policy. <s> BIB019
Driven by the high cost and exhaustion of radio resources in low frequency bands (below 6 GHz), the exploitation of mmWave frequency band (around 60 GHz) alleviates the data traffic burden and provides super high data services in a cost-efficient manner BIB003 , BIB019 . However, working in high-frequency bands, mmWave communications suffer from high propagation loss. To overcome this drawback, beamforming technology can be applied with a large number of antennas to compensate for high propagation attenuation. Fortunately, attributed to the significantly reduced wavelength in the mmWave frequency band, mmWave MIMO systems can be equipped with a large number of antennas at the transceiver in a highly compact manner to achieve high beamforming gains BIB002 . For the conventional mmWave MIMO system, the high compaction of antennas imposes the prohibitively high cost at the transceiver due to the use of a large number of RF circuits, which hinders the practical implementation. To explore the practical implementation of RF-chainlimited mmWave systems, the (G)SM technique is a promising low-cost and high-efficiency design option by activating a subset of transmit antennas connected to a small number of RF chains. As a result, (G)SM becomes an attractive technology in mmWave communications for reaping benefits of both beamforming gain and low cost with reduced RF chains. Fig. 8 shows the transmitter diagram of the (G)SM aided mmWave communication systems. A total number of N T transmit antennas are partitioned into M P analog precoders, each of which comprises N K antenna elements for analog precoding, i.e., N T = M P · N K . For the implementation of (G)SM aided mmWave scheme, the number of RF chains N RF is less than the number of analog precoders M P . Unlike the conventional mmWave MIMO scheme that conveys information in the constellation domain, the incoming data of (G)SM aided mmWave scheme are separated into two parts in the constellation domain and space domain, respectively. Specifically, the space domain data are fed to the index selector to determine the indices of N RF out of M P analog precoders for activation. Since the remaining M P − N RF analog precoders are set to be idle, the outputs of N RF RF chains are connected with N RF activated analog precoders via a fast switcher and the total combination number of N RF analog precoders is MP NRF . On the other hand, the constellation data are fed to the symbol modulator to generate N RF constellation symbols, and then independently processed with N RF RF chains. For each analog precoder, N K antenna elements are deployed as phase shifters for the analog procession. A diagonal matrix of size N T is applied as an analog precoder, whose n-th diagonal entry is given by where φ n ∈ [−π, π] is the rotation angle of the n-th transmit antenna. Similar to BIB004 , we denote the (G)SM block as x [x 1 x 2 . . . x MP ] T , and the received signal model can be expressed as where 1 NK denotes the all-one vector of size N K , ⊗ denotes the Kronecker product, A is the mmWave MIMO channel matrix of size N R × N T , and w is an AWGN vector at the receiver. Note that when N RF = 1, the system in BIB001 specializes to a SM aided mmWave system with only a single RF chain, which enjoys an extremely low cost of implementation and detection in mmWave communications. (G)SM aided mmWave scheme not only saves the transmitter cost by reducing the number of RF chains, but also achieves the precoding gains via the analog precoder, which meets the requirement of mmWave communications. In BIB008 , the implementation of analog beamformers in (G)SM-based mmWave systems is investigated, where the array of analog beamformers is optimally designed to attain the maximum rank of the equivalent channel matrix. Moreover, it is shown that with a reduced RF-chain structure, the (G)SMbased mmWave scheme of BIB008 approaches the same constrained capacity as the full-RF spatial-multiplexing counterpart. The application of SSK and (G)SM in indoor mmWave communications at 60 GHz is studied in BIB005 - BIB016 , where the channel is characterized as line-of-sight (LOS) components due to the high reflection attenuation. Based on the maximum criterion of the minimum distance of received symbols, SSK BIB005 and (G)SM BIB006 , BIB016 in LOS channels are designed to achieve optimal performance by orthogonalizing the channel matrix with proper antenna placement. By approximating the 3-D statistical channel as a logâŁ"normal fading channel, BIB009 analyzes the capacity of QSM for outdoor mmWave communications. Assuming the CSI at the transmitter, BIB010 proposes an analog precoding aided (G)SM scheme for mmWave communications to improve the spectral efficiency. Specifically, after deriving a tight lower bound on the achievable spectral efficiency, the analog precoding aided (G)SM scheme in BIB010 is designed in an iterative way to meet the maximal spectral efficiency. Furthermore, hybrid precoding aided (G)SM schemes using hybrid digital and analog precoding are proposed for mmWave communications, which achieve an optimal spectral efficiency based on some approximations BIB011 , BIB017 . To save the transmitter cost with a reduced number of antennas, (G)SM with a variable number of activated antennas is designed in  for indoor LoS mmWave communications to achieve higher transmission efficiency. Moreover, the capacity and energy efficiency are analyzed, and the optimization problems in terms of power allocation and antenna separation are investigated. To save energy consumption and allow robustness against path-loss attenuations, an orbital angular momentum aided SM scheme is proposed for mmWave communication systems in BIB012 , where the capacity, error performance, and energy efficiency are also discussed. In BIB013 , a SM aided receive antenna selection technology is developed for saving the RF cost at the transceiver in mmWave communications, where the combinatorial optimization problems of capacity and error performance are studied. To attain beamforming gain and reduce the cost of RF chain, an analog precoding-aided virtual SM BIB007 and its variant using multi-mode hybrid precorder BIB014 are proposed, which enhance the received SNR and spatial degrees of freedom with the low cost of single RF chain. Receive SM is also studied in indoor and outdoor mmWave communications BIB015 - BIB018 , where the performance is optimized with minimum symbol error probability in LOS channels BIB015 .