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The use of antenna arrays is indispensable in radar and multiple-input multiple-output (MIMO) wireless communications areas, as it permits the treatment of wavefronts in the spatial domain, enabling important applications such as beamforming and target detection. Spatial information is commonly given in terms of spatial correlation functions (SCFs), but its representation in terms of angular domain, such as the power azimuth spectrums (PASs), is sometimes more convenient. Therefore, transforming from SCF to PAS, and vice versa, is often required. This is commonly done by employing so-called beamformers, where the Bartlett beamformer (BB) is the most commonly utilized. However, the use of beamformers alone is not enough for retrieving PAS information from a given SCF. Particularly, this paper shows that a non-linear operation is missing in relating SCFs to PASs as provided by current beamformers. The paper also shows the mathematical expressions that correctly relate SCFs to PASs and wavenumber spectrums (WNSs) for the general case of systems with antenna arrays. The developed theory results in two new beamformers, which, corroborated by simulation results, solve any previous inconsistency in estimating PASs from SCFs. [ABSTRACT FROM AUTHOR]

This correspondence paper studies how correlation of the wiretap channel affects the secrecy rate of the artificial noise (AN) scheme. To this end, we first develop a simple but tight lower bound on the ergodic secrecy rate of the AN scheme on correlated multiple-input single-output wiretap channels. Then, utilizing the derived bound, we investigate the influence of correlation on the secrecy rate, which explicitly manifests how the channel correlation affects the optimal power allocation. Ultimately, this paper answers two fundamental questions: when is the AN signal beneficial? and how much power should be allocated for the AN signal to achieve a higher ergodic secrecy rate? in terms of channel correlation. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surface (RIS) assisted non-orthogonal multiple access (NOMA) transmission can effectively improve the energy/spectrum efficiency in wireless networks. This paper designs a low-complexity scheme to achieve the balanced tradeoff between the sum-rate and power consumption in a RIS-assisted NOMA system, which can be measured by energy efficiency. To solve the formulated problem effectively, the original non-convex problem is first decomposed into two subproblems, i.e., beamforming optimization and phase shift optimization. Alternating optimization is proposed to solve these two subproblems iteratively. In particular, successive convex approximation (SCA) is utilized to convert the non-convex constraints to convex ones. The provided simulation results demonstrate that the proposed scheme can achieve superior performance on energy efficiency compared to the random phase shifts and orthogonal multiple access (OMA) schemes. [ABSTRACT FROM AUTHOR]

A zero forcing (ZF) assisted single layer beamforming is developed for spatial modulation (SM) and space shift keying (SSK) modulation multiple input multiple output (MIMO) systems, termed as SZF-SM/SZF-SSK, to achieve both transmit beamforming and receive diversity gain in this paper. Specifically, the proposed SZF-SM/SZF-SSK maps part of the bits to spatial domain, simultaneously forming a ZF precoding assisted single layer data both in multiple input single output (MISO) and MIMO systems, that differs greatly from conventional ZF beamforming assisted MIMO systems. With MISO configuration, the ZF precoder of SZF-SM/SZF-SSK converges to the maximum ratio transmission (MRT), achieving only the transmit beamforming gain. With MIMO configuration, SZF-SM/SZF-SSK tranceiver could be further modified along with the MIMO channel to achieve single layer transmission and joint detection, attaining both the transmit beamforming and receive diversity gain in SM-MIMO system. The theoretical analysis and the simulation results of the proposed SZF-SM/SZF-SSK with MISO and MIMO configurations are presented in this paper respectively, which show an attractive performance gain over the conventional SM/SSK. Compared with the conventional ZF beamforming assisted MIMO systems and some other CSI assisted SM-MIMO schemes, the proposed scheme exhibits better performance, especially under highly correlated MIMO channels, rendering a new and practical way to achieve low rank MIMO transmission with good performance and high data rate. [ABSTRACT FROM AUTHOR]

In this paper, we investigate joint retrospective interference alignment (RIA) and caching schemes for multiple-input single-output (MISO) broadcast channels (BC) with delayed channel state information at the transmitter (CSIT). By exploiting the cached content, we propose one sophisticated cache-aided RIA scheme to cancel the interference. Importantly, our scheme takes the number of symbols in a packet into account, as different number of symbols in a packet result in different system configuration, which may have significant impact on the design of the transmission scheme and achievable degrees of freedom (DoF). Furthermore, we present two theorems to quantify the achievable sum DoF of the proposed cached-aided RIA scheme for different system configuration. Moreover, the system configuration of the existing cache-aided RIA scheme with mixed CSIT can be treated as a special case in our paper. And also our scheme can simultaneously support more users than the existing cache-aided RIA scheme with mixed CSIT to invoke zero forcing (ZF) precoding, which limits the number of users simultaneously supported. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the construction of a transmit signal for downlink multiuser multiple-input single-output (MU-MISO) systems under the cost-effective 1-bit digital-to-analog converters (DACs). Due to the coarse nonliner property, conventional precoding methods, developed under infinite-resolution DACs, could not yield an attractive performance having a severe error-floor. Finding an optimal transmit signal, moreover, is computationally implausible due to its combinatorial nature. This challenging problem has been partially solved in our earlier work focusing on phase-shift-keying constellations. Our major contribution is to extend it into more practical quadrature-amplitude-modulation constellations in a non-trivial way. We first derive the so-called feasibility condition to guarantee that every user's noiseless received signal belongs to a desired decision region, and then transform it as linear constraints. Taking the robustness to an additive noise into account, the proposed construction method is formulated as a mixed integer linear programming problem. We construct an efficient algorithm to solve it (equivalently, to generate an 1-bit transmit signal). Via simulation results, we demonstrate the superiority of the proposed method with respect to a computational complexity and detection performance. [ABSTRACT FROM AUTHOR]

Cache-enabled small base stations (SBS) are capable of relieving the heavy burden of the backhaul link and reducing the transmission latency. The hit probability depends on the coverage probability and caching placement probabilities. However, the interference in the small-cell networks may significantly degrade the coverage probability. In this paper, for MIMO small-cell networks consisting of SBS and users, where both of them are equipped with multiple antennas, a joint interference alignment (IA) and probabilistic caching (JIA-ProbC) scheme is proposed. Using tools from stochastic geometry, the $K$ -th order Voronoi cells are constructed to form clusters, where $K$ SBSs cooperatively serve users within each of the $K$ -th order Voronoi cells. Then, the IA scheme for MIMO interference channel (IC) is employed to cancel the intra-cluster interference within each $K$ -th order Voronoi cell. By exploiting the advantage of multiples antennas at users, the IA scheme can simultaneously support more users interference-free than both the zero forcing (ZF) based interference cancellation scheme for MISO systems and SISO systems without interference management, as more interference can be canceled. Furthermore, the coverage probability is analytically approximated by the a closed-form expression. Moreover, the optimal caching placement probability is analytically derived. Numerical Simulation results show that the proposed JIA-ProbC can significantly outperform the existing joint ZF and probabilistic caching (JZF-ProbC) scheme for MISO systems and SISO probabilistic caching (SISO-ProbC) scheme as well as the joint IA and most popular caching (JIA-MPC) caching scheme. [ABSTRACT FROM AUTHOR]

Non-orthogonal multiple access (NOMA) is a prospective technology for radio resource constrained future mobile networks. However, NOMA users far from a base station (BS) tend to be more susceptible to eavesdropping because they are allocated more transmit power. In this paper, we aim to jointly optimize the precoding vectors at the BS to ensure the legitimate security in a downlink multiple-input single-output (MISO) NOMA network. When the eavesdropping channel state information (CSI) is available at the BS, we can maximize the sum secrecy rate by joint precoding optimization. Owing to its non-convexity, the problem is converted into a convex one, which is solved by a second-order-cone-programming-based iterative algorithm. When the CSI of the eavesdropping channel is not available, we first consider the case that the secure user is not the farthest from the BS, and the transmit power of the farther users is maximized via joint precoding optimization to guarantee its security. Then, we consider the case when the farthest user from the BS requires secure transmission, and the modified successive interference cancellation order and joint precoding optimization can be adopted to ensure its security. A similar method can be exploited to solve the two non-convex problems when the CSI is unknown. Simulation results demonstrate that the proposed schemes can improve the security performance for MISO NOMA systems effectively, with and without eavesdropping CSI. [ABSTRACT FROM AUTHOR]

A tradeoff between sum rate and fairness for multiple-input single-output (MISO) broadcast communication employing dirty paper coding or zero-forcing dirty paper coding at physical layer is investigated in this paper. The tradeoff is based on a new design objective termed “tri-stage” approach as well as a new $\ell _1$ -based fairness measure that is much more robust than the well-known Jain's index for comparing fairness levels achieved by various design objectives at a much finer resolution in high SNR regime. The newly proposed tri-stage design also introduces a new concept of statistical power allocation that randomly allocates power to users based on an optimal probability distribution derived from the tradeoff between sum rate and fairness. Simulation results show that the proposed approach can simultaneously achieve a larger sum rate and better fairness than the reputable proportional fairness criterion. A performance upper bound is also given in the paper to show the excellent performance of the proposed approach at moderate and high SNR regimes as well as some potential for further improvement in low SNR regime. [ABSTRACT FROM AUTHOR]

This correspondence paper considers a multiple-input single-output non-orthogonal multiple access (MISO-NOMA) system that spatially multiplexes NOMA signals formed by superimposing two user symbols in the power domain. We aim to design practical beamforming (BF), power allocation (PA), and user clustering methods for the system by minimizing the transmit power under the rate constraints. The two-step BF and conditionally optimal PA are devised in closed-form expressions that facilitate user clustering based on message passing and effective channel correlation. The results show that the MISO-NOMA system with the proposed BF, PA, and user clustering methods outperforms the benchmark systems of comparable complexity. [ABSTRACT FROM AUTHOR]

In this paper, we investigate a novel channel estimation (CE) method for multiple-input and single-output (MISO) systems in visible light communication (VLC). Direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) is commonly used in VLC where half of the available subcarriers are spent to guarantee a real-valued output after the inverse fast Fourier transform operation. Besides, dedicated subcarriers are typically used for CE, thus, many resources are wasted and the spectral efficiency is degraded. We propose a superimposed training approach for CE in MISO DCO-OFDM VLC scenarios. Analytical expressions of mean squared error (MSE) and spectral efficiency are derived when the least squares estimator is considered. This analysis is valid for outdoor and indoor scenarios. For the CE error, simulation results of MSE show a perfect match with analytical expressions. Moreover, results prove that this technique guarantees a larger spectral efficiency than previous schemes where dedicated pilots were used. Finally, the optimal data power allocation factor is also analytically derived. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surface (RIS) is a passive antenna array composed of a large number of reflecting elements. In a RIS-assisted communication system, it is a challenging task to acquire accurate channel state information (CSI). In this paper, we propose a semi-blind channel estimation method for a RIS-assisted massive multiple-input single-output (MISO) system. Assuming a Gaussian priori on the data symbols, an iterative expectation maximization (EM)-based algorithm is developed to obtain the maximum likelihood (ML) estimate of the cascaded RIS channel. Different from existing pilot-based methods, the proposed semi-blind channel estimation method exploits the data symbols for channel estimation enhancement and only a fraction of full-pilot signaling overhead is required. Simulation results verify that the proposed method achieves significant improvement in the accuracy of the channel estimation while it noticeably reduces the training pilot overhead. [ABSTRACT FROM AUTHOR]

This paper studies a secure multiuser multiple-input single-output (MISO) communication system aided by an intelligent reflecting surface (IRS), where multiple colluding eavesdroppers (EVEs) coexist. We aim to maximize the sum secrecy rate (SSR) via jointly optimizing the beamforming vectors, the artificial noise (AN) and the phase shifts at the IRS subject to the maximum transmit power constraint and unit modulus constraints. To address the non-convex optimization problem, we first propose an alternating optimization (AO) algorithm based on semidefinite relaxation (SDR) and obtain a high-quality sub-optimal solution. In order to reduce the high computational complexity, a low-complexity alternating optimization (LC-AO) algorithm is developed, in which the beamforming vectors, AN and the IRS phase shifts are optimized alternately by the generalized power iteration (GPI) and the Riemannian manifold conjugate gradient (RMCG) algorithm, respectively. Simulation results show the advantages of deploying the IRS in improving the system secrecy performance. [ABSTRACT FROM AUTHOR]

In this paper, we propose a physical layer secure transmission technology based on multi-components for multiple-input single-output (MISO) systems. The scheme mainly realizes physical layer security (PLS) in General Multi-Fractional Fourier Transform (GMFRFT) signal domain, which is different from traditional schemes that rely on the spatial domain. Different GMFRFT components are transmitted by multiple antennas at the transmitter, then the legitimate receiver and the eavesdropper receive the superposition of multiple non-orthogonal components. The resulting mutual interference will reduce the signal-to-interference and noise (SINR) at the eavesdropper, but without affecting the legitimate receiver. Because the legitimate receiver receives the non-contaminated GMFRFT signal due to designing multi-components at the transmitter, while the eavesdropper receives the signal whose constraint relations in the GMFRFT signal domain are destroyed, which will cause the energy loss of the information bearing signal. Furthermore, in order to achieve the maximum secrecy capacity, the selection method of GMFRFT transform order is given to adjust the power allocation among GMFRFT components. Compared with the scheme based on artificial noise (AN), the advantages of our scheme are: 1) our scheme can further reduce the capacity of wiretap channel while not requiring the transmitter to use partial power to transmit meaningless artificial noise signals; 2) our scheme outperforms AN-based schemes when the available spatial degrees of freedom are limited. Simulation results are provided to demonstrate the secrecy performance of the proposed scheme. [ABSTRACT FROM AUTHOR]

This paper investigates the capacity results of dimmable visible light communications (VLCs) channel under practical range-limited constraints. Upper and lower bounds for both single-input single-output (SISO) and multiple-input single-output (MISO) scenarios which are valid in all signal-to-noise (SNR) region are derived. We reveal that the SISO capacity is a symmetrical function of the dimming target, and is strictly increasing at its left half. Tight MISO upper bounds are obtained by solving non-convex problems, and alternating optimization method is employed with constraint transformations. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the performance of downlink multiple-input-single-output (MISO) non-orthogonal multiple access (NOMA) aided vehicular communication systems. To prepare the background for MISO NOMA-based vehicular systems, we first derive the exact outage probability (OP), average bit error rate (ABER), and ergodic capacity (EC) expressions for the single-input-single-output (SISO) NOMA-based vehicular systems over independent but not necessarily identically distributed (i.ni.d.) double Nakagami-m fading channels. Furthermore, we propose two transmit antenna selection schemes at multiple-antenna source vehicle for both far user and near user. Based on these antenna selection strategies, we derive the exact expressions for the OP, ABER, and EC, of the MISO NOMA-based vehicular systems over i.ni.d. double Nakagami-m fading channels. The derived ABER expressions for both SISO and MISO NOMA-enabled vehicular systems can be applicable for various modulation schemes. Moreover, to reveal better insights into the diversity orders of SISO and MISO NOMA-enabled vehicular systems, we present the asymptotic OP analysis in the high signal-to-noise ratio (SNR) regime. The numerical and simulation results corroborate our theoretical and analytical findings, and demonstrate the effects of transmit power, power allocation factor, fading severity parameters, and number of antennas, on the system's performance. We further compare the performance of both systems, and show that the multiple antennas can improve the performance of vehicular systems, which can be further improved significantly with the simultaneous improvement in fading severity parameters. [ABSTRACT FROM AUTHOR]

This paper presents a single-inductor multiple-input single-output (SI-MISO) boost converter with a deadbeat-based control method. The proposed deadbeat-based control has the capability to simultaneously conduct the power regulation from multiple inputs and the output voltage regulation for the SI-MISO boost converter. The advantages of the proposed method are less component count, lower system cost, compatibility for arbitrary number of inputs, fast response, and simplicity for implementation. Hence, the SI-MISO boost converter is a promising interface for the regulation of multiple energy sources as a standalone power system, which can be utilized in electric vehicle applications. To verify the performance of the proposed deadbeat-based control method, a laboratorial prototype of the SI-MISO boost converter is built. The steady-state and dynamic operations are investigated based on the prototype. The experimental results demonstrate that input power and output voltage of the SI-MISO boost converter can be effectively regulated by the proposed control method. The comprehensive comparison with the existing methods is conducted to further illustrate the advantages of the proposed deadbeat-based method. [ABSTRACT FROM AUTHOR]

Intelligent reflecting surface (IRS) has recently emerged as a promising technology for 6 G wireless systems, due to its capability to reconfigure the wireless propagation environment. In this paper, we investigate a Rate-Splitting Multiple Access (RSMA) for IRS-assisted downlink system, where the base station (BS) communicates with single-antenna users with the help of an IRS. RSMA relies on rate-splitting (RS) at the BS and successive interference cancellation (SIC) at the users and provides a generalized multiple access framework. We derive a new architecture called IRS-RS that leverages the interplay between RS and IRS. For performance analysis, we utilize an on-off control technique to control the passive beamforming vector of the IRS-RS and derive the closed-form expressions for outage probability of cell-edge users and near users. Moreover, we also analyze the outage behavior of cell-edge users for a sufficiently large number of reflecting elements. Additionally, we also analyze the outage performance of cooperative RS based decode-and-forward (DF)-assisted framework called DF-RS. Through simulation results, it is shown that the proposed framework outperforms the corresponding DF-RS, RS without IRS and IRS-assisted conventional non-orthogonal multiple access (NOMA) schemes. Furthermore, the impact of various system's parameters such as the number of IRS reflecting elements and the number of users on the system performance is revealed. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the physical layer security (PLS) for a multiple-input single-output (MISO) non-orthogonal multiple access (NOMA) visible light communication (VLC) system, whose information is intercepted by collusive eavesdroppers. For the case of known eavesdroppers’ locations, a precoding scheme is proposed to maximize the secrecy capacity of the system by suppressing the rate of collusive eavesdroppers. Then we extend the scheme to the case of unknown eavesdroppers’ locations. Specifically, the artificial noise is introduced in the precoding scheme to further suppress the average rate of eavesdroppers. To solve the non-convex optimization problem, we propose an iterative algorithm based on concave-convex procedure. Simulation results illustrate the outperformance of the proposed schemes in guaranteeing the PLS for the MISO NOMA VLC system under the interception of collusive eavesdroppers. [ABSTRACT FROM AUTHOR]

Non-orthogonal multiple access (NOMA) has a great potential to offer a higher spectral efficiency of multi-user wireless networks than orthogonal multiples access (OMA). Previous work has established the condition, referred to quasi-degradation (QD) probability, under which NOMA has no performance loss compared to the capacity-achieving dirty paper coding for the two-user case. Existing results assume Rayleigh fading channels without line-of-sight (LOS). In many practical scenarios, the channel LOS component is critical to the link quality where the channel gain follows a Rician distribution instead of a Rayleigh distribution. In this work, we analyze the QD probability over multi-input and single-output (MISO) channels subject to Rician fading. The QD probability heavily depends on the angle between two user channels, which involves a matrix quadratic form in random vectors and a stochastic matrix. With the deterministic LOS component, the distribution of the matrix quadratic form is non-central that dramatically complicates the derivation of the QD probability. To remedy this difficulty, a series of approximations is proposed that yields a closed-form expression for the QD probability over MISO Rician channels. Numerical results are presented to assess the analysis accuracy and get insights into the optimality of NOMA over Rician fading channels. [ABSTRACT FROM AUTHOR]

This paper investigates the power allocation problem for secure secondary transmission in a multiple-input single-output cognitive wiretap system with artificial noise (AN). The secondary transmitter exchanges confidential information with the secondary destination in the presence of an unknown eavesdropper. Since the eavesdropper's channel state information (CSI) is unavailable, we propose an optimal adaptive power allocation scheme, and derive a closed-form expression of the optimal power allocation factor that minimizes the secrecy outage probability (SOP). A suboptimal fixed power allocation scheme is also proposed to reduce the system complexity. Moreover, exact closed-form expressions of SOP under both schemes are derived. Numerical results are provided to corroborate the accuracy of our analytical results and the superiority of the proposed schemes to the benchmarks. [ABSTRACT FROM AUTHOR]

In this paper, a very accurate approximation method for the statistics of the sum of Málaga-M random variates with pointing error (MRVs) is proposed. In particular, the probability density function of MRV is approximated by a Fox's H-function through the moment-based approach. Then, the respective moment-generating function of the sum of N MRVs is provided, based on which the average symbol error rate is evaluated for an N-branch maximal-ratio combining (MRC) receiver. The retrieved results show that the proposed approximate results match accurately with the exact simulated ones. Additionally, the results show that the achievable diversity order increases as a function of the number of MRC diversity branches. [ABSTRACT FROM AUTHOR]

Combining intelligent reflecting surface (IRS) and non-orthogonal multiple access (NOMA) is an effective solution to enhance communication coverage and energy efficiency. In this paper, we focus on an IRS-assisted NOMA network and propose an energy-efficient algorithm to yield a good tradeoff between the sum-rate maximization and total power consumption minimization. We aim to maximize the system energy efficiency by jointly optimizing the transmit beamforming at the BS and the reflecting beamforming at the IRS. Specifically, the transmit beamforming and the phases of the low-cost passive elements on the IRS are alternatively optimized until the convergence. Simulation results demonstrate that the proposed algorithm in IRS-NOMA can yield superior performance compared with the conventional OMA-IRS and NOMA with a random phase IRS. [ABSTRACT FROM AUTHOR]

Non-orthogonal multiple access (NOMA) has been deemed a promising technology to achieve high spectral efficiency. This paper investigates transmit power optimization for a multi-user multiple-input single-output (MISO) NOMA downlink system endowed with energy harvesting and physical layer security. More specifically, the system is designed to satisfy the data-link security requirements of its legitimate users while charging energy harvesting devices. To achieve that goal, we investigate a joint design on beamforming vectors and artificial noise (AN) covariance matrix. Then, an iterative difference-of-two-convex-function (D.C.) algorithm is proposed by applying the successive convex approximation (SCA) approach. To further reduce the computational complexity, a sub-optimal alternating D.C. (ADC) algorithm is devised by decoupling beamforming and mapping the AN to the null space of legitimate users. In the ADC algorithm, optimal null space and beamforming vectors are alternately solved by convex programming. Furthermore, the convergence of the proposed D.C. and ADC algorithms is derived. Extensive simulation results show that the two proposed AN-aided algorithms significantly reduce the transmit power cost over OMA and NOMA scheme without AN, especially when there are more eavesdroppers. [ABSTRACT FROM AUTHOR]

In this paper, we study the secrecy performance of a $K$ -user multi-input single-output (MISO) broadcast channel with random beamforming (RB), where the transmitter is equipped with $N$ antennas, and the eavesdropper has $N_e$ antennas. We first propose a signal-splitting random beamforming (SSRB) scheme for a single-user MISO wiretap channel, and then improve the SSRB scheme with power-minimizing (PM), which is called a PM-SSRB scheme. To improve the spectral efficiency of a multiuser network, we combine PM with non-orthogonal multiple access (NOMA), and propose a hybrid NOMA (H-NOMA) scheme and a signal-splitting NOMA (SS-NOMA) scheme for the $K$ -user MISO wiretap channel, respectively. The sum ergodic secrecy rate of the $K$ -user scenario is analyzed comprehensively, and a closed-form expression for the lower bound on the ergodic secrecy rate is also derived for the single-user case. Simulation results show that, compared with the traditional hybrid artificial fast-fading scheme (AFF) and artificial noise (AN) scheme, the proposed SSRB scheme and PM-SSRB scheme perform much better in terms of the ergodic secrecy rate in all power regimes. More importantly, when the eavesdropper has more antennas than the transmitter, our schemes always outperform the AFF and AN schemes. The PM-SSRB scheme is also shown to be superior to the secret-key AFF scheme. For the multiuser case, the SS-NOMA scheme can achieve higher ergodic secrecy rate than the H-NOMA scheme. [ABSTRACT FROM AUTHOR]

We propose a semi-closed-form solution to the problem of computing the capacity and optimal signaling for a Gaussian MIMO channel under a joint sum power constraint (SPC) and per-antenna power constraint (PAPC). Existing efficient solutions to this fundamental problem are only applicable to some special cases: multiple-input single-output (MISO) systems, or full column rank MIMO channels with sufficiently high transmit power, or full-rank optimal signaling. For the general case, we present an efficient numerical method to solve the considered problem which does not make any assumptions on the rank of the channel matrix or the maximum transmit power. To achieve this, the considered problem is transformed into an equivalent minimax problem. We then exploit the special structure of the minimax problem to derive a closed-form solution based on a concave-convex procedure (CCP)-like algorithm. Extensive simulation results show that our proposed algorithm outperforms the existing solutions in terms of complexity and generality. [ABSTRACT FROM AUTHOR]

In this paper, a multiple-input multiple-output wiretap channel model is studied, where the information packet sent from the transmitter to the desired receiver is wiretapped by a potential eavesdropper. When ideal channel state information (CSI) is available, we design the secure beamforming using a proposed iterative closed-form updating scheme based on majorization–minimization approach. Numerical results demonstrate that the proposed scheme acquires the same secrecy rate as the existing algorithms whereas it has much lower complexity. When ideal CSI is not available and channel errors are modeled by deterministically bounded model, we investigate robust secure beamforming (RSB) optimization problem whose goal is worst-case secrecy rate maximization. In the formulated optimization problem, the objective function includes the expression of a logarithm of a determinant ($\log \det$) minus $\log \det$. Instead of approximating the $\log \det$ expression as a trace, we propose to linearize two $\log \det$ terms. After linearization, epigraph reformulation is used to deal with deterministically bounded channel errors and semi-infinite constraints are recast as linear matrix inequalities. Finally, an alternating optimization algorithm is utilized to find the RSB design. After obtaining the RSB design, we derive the worst-case secrecy rate. Numerical results illustrate that the proposed RSB design is superior to the nonrobust one. [ABSTRACT FROM AUTHOR]

In this paper, we develop various beamforming techniques for downlink transmission for multiple-input single-output nonorthogonal multiple access (NOMA) systems. First, a beamforming approach with perfect channel state information is investigated to provide the required quality of service for all users. Taylor series approximation and semidefinite relaxation (SDR) techniques are employed to reformulate the original nonconvex power minimization problem to a tractable one. Furthermore, a fairness-based beamforming approach is proposed through a max–min formulation to maintain fairness between users. Next, we consider a robust scheme by incorporating channel uncertainties, where the transmit power is minimized while satisfying the outage probability requirement at each user. Through exploiting the SDR approach, the original nonconvex problem is reformulated in a linear matrix inequality form to obtain the optimal solution. Numerical results demonstrate that the robust scheme can achieve better performance compared to the nonrobust scheme in terms of the rate satisfaction ratio. Furthermore, simulation results confirm that NOMA consumes a little over half transmit power needed by orthogonal multiple access for the same data rate requirements. Hence, NOMA has the potential to significantly improve the system performance in terms of transmit power consumption in future 5G networks and beyond. [ABSTRACT FROM AUTHOR]

We design the optimal transmission strategy, which maximizes the average achievable data rate of the multiple-input single-output system that adopts short-packet communications. In this system, the $N_{A}$ -antenna access point (AP) transmits to the single-antenna user with finite blocklength $T$ after estimating the AP-user channel via downlink training and uplink feedback. For this system, we determine the optimal allocation of the finite resource (e.g., the total transmit power and a finite number of symbol periods) for downlink training, uplink feedback, and data transmission to maximize the average data rate. Specifically, we derive an approximate closed-form lower bound on the average data rate, an explicit result for the optimal number of symbol periods for downlink training, an easy-to-implement method to find the optimal number of symbol periods for uplink feedback, and a simple expression for the optimal power allocation between data transmission and downlink training. By using numerical results, we demonstrate the effectiveness of our analytical solutions and examine the impact of system parameters, e.g., $N_{A}$ and $T$ , on the optimal strategy. [ABSTRACT FROM AUTHOR]

Analog precoder was introduced recently to millimeter wave (mmWave) systems with large-scale antenna arrays as a promising hardware-efficient transceiver architecture. In practice, low-resolution phase shifters (PSs) are generally adopted to achieve low hardware cost and low power consumption, which, however, makes fine beam tuning challenging. Recent achievements in antenna selection technology motivate us to exploit the augmented antenna diversity of a large-scale antenna array to compensate the accuracy loss caused by this low-resolution PS implementation. In this correspondence, we consider a narrow-band multiple-input single-output downlink mmWave system. Our goal is to jointly select a set of transmit antennas and design the analog precoder with constant amplitude and discrete phase constraints to maximize the spectral efficiency. In particular, we propose a novel joint antenna selection and analog beamformer design algorithm, which can provide satisfactory performance with low complexity. Simulation results demonstrate the advances of the proposed algorithm compared with the low-resolution analog precoding schemes without antenna selection. [ABSTRACT FROM AUTHOR]

Intelligent reflecting surfaces (IRSs) enhance the robustness of wireless transmissions against shielding. However, addressing channel-estimation overheads caused by the large number of IRS elements remains a major challenge. In particular, these overheads increase proportionately with an increase in the number of IRS elements, thereby making it difficult to handle multiple IRSs. Therefore, we investigate a passive beamforming method with partial channel state information (CSI) in distributed IRS communication systems called selective reflection control (SRC). Specifically, the proposed method determines the associations between the IRS and user equipment (UE) to obtain the CSI of the associated IRS-UE links. Simulations were conducted, and the obtained results confirm that the proposed method based on SRC outperforms the benchmark method in terms of the user sum rate. Furthermore, the proposed method realizes robust transmissions against shielding owing to appropriate IRS–UE associations. Therefore, the proposed SRC achieves better communication performance with less channel estimation overheads, which is expected to accelerate large-scale IRS deployment. [ABSTRACT FROM AUTHOR]

A simultaneous wireless information and power transfer (SWIPT)-assisted cognitive overlay-based multiple input multiple output (MIMO) downlink (DL) cooperative relaying system has been considered wherein a secondary network comprising of an energy harvesting (EH) base station (BS) and possibly multiple fully wireless powered secondary users (SUs) share the spectrum of a primary network for information transmission and energy harvesting. In exchange, the secondary BS acts as a relay for the primary network and aids in transmitting information from the primary BS to the primary users (PUs) along with its own information using non-orthogonal transmission and the harvested energy. Closed-form expressions for the end-to-end outage probability (OP) at all the PUs and SUs have been obtained considering Rayleigh fading channel links and also sensitivity and saturation impairments at the energy harvesting nodes. Moreover, all the energy harvesting nodes have been assumed to be equipped with a finite battery for storing the harvested energy and rely solely on this stored energy for running the transceiver circuitry. Closed-form expressions have also been obtained to characterize the degradation due to the energy harvesting impairments and finite battery on the end-to-end OP of all the users. Furthermore, closed-form expressions for the asymptotic outage floor and diversity order have been obtained to analyze the performance of the system at high transmit power. Finally, simulation results have been presented to validate the analytical results derived and provide further insights into the performance of the system. [ABSTRACT FROM AUTHOR]

This work investigates the effect of double intelligent reflecting surface (IRS) in improving the spectrum efficient of multi-user multiple-input multiple-output (MIMO) network operating in the millimeter wave (mmWave) band. Specifically, we aim to solve a weighted sum rate maximization problem by jointly optimizing the digital precoding at the transmitter and the analog phase shifters at the IRS, subject to the minimum achievable rate constraint. To facilitate the design of an efficient solution, we first reformulate the original problem into a tractable one by exploiting the majorization-minimization (MM) method. Then, a block coordinate descent (BCD) method is proposed to obtain a suboptimal solution, where the precoding matrices and the phase shifters are alternately optimized. Specifically, the digital precoding matrix design problem is solved by the quadratically constrained quadratic programming (QCQP), while the analog phase shift optimization is solved by the Riemannian manifold optimization (RMO). The convergence and computational complexity are analyzed. Finally, simulation results are provided to verify the performance of the proposed design, as well as the effectiveness of double-IRS in improving the spectral efficiency. [ABSTRACT FROM AUTHOR]

This study considers the simultaneous optimization problem of the transmit beamforming at the access point and the reflecting beamforming at the intelligent reflecting surface (IRS) in an IRS-assisted multiuser downlink system. This joint optimization problem is nonconvex and challenging due to the intricate coupling of transmit and reflecting beamforming variables and the highly nonconvex constant modulus constraints. All the known solutions apply the alternating optimization framework to decouple the joint optimization problem into two problems. Then, these problems are optimized individually with different optimization methods in an alternating manner. However, such an alternative procedure may cause performance degradation. On the basis of the cross-entropy (CE) framework, which was developed initially for machine learning applications, we propose a machine learning-inspired algorithmic framework for simultaneously optimizing transmit and reflecting beamforming in an IRS-assisted wireless system. Extensive simulation results reveal that the proposed CE-based algorithms significantly perform better than the state-of-the-art test algorithms for various system configurations. [ABSTRACT FROM AUTHOR]

In practical multi-antenna systems, the design of precoding and power control is not straightforward even for multi-input single-output (MISO) systems because of the distortion caused by nonlinear power amplifiers. Through analysis of the signal-to-interference-plus-noise ratio (SINR), we found that the equal gain transmission scheme can achieve the maximum SINR of a nonlinear MISO system only at the point of local optimal transmit power. Then we derived an approximated optimal transmit power for the nonlinear MISO system. To maximize the SINR in all transmit power region, we propose a precoding and power control algorithm for a MISO system with nonlinear power amplifiers under power constraint. By simulation, we show that the proposed scheme can achieve the local optimum SINR, and outperform other baseline precoding techniques. [ABSTRACT FROM AUTHOR]

We develop a novel technique to accelerate minorization-maximization (MM) procedure for the non-orthogonal multiple access (NOMA) weighted sum rate maximization problem. Specifically, we exploit the Lipschitz continuity of the gradient of the objective function to adaptively update the MM algorithm. With fewer additional analysis variables and low complexity second-order cone program (SOCP) to solve in each iteration of the MM algorithm, the proposed approach converges quickly at a small computational cost. By numerical simulation results, our algorithm is shown to greatly outperform known solutions in terms of achieved sum rates and computational complexity. [ABSTRACT FROM AUTHOR]

Although there have been extensive studies on transmit beamforming in multi-input single-output (MISO) multicell networks, achieving optimal sum-rate with limited channel state information (CSI) is still a challenge even with a single user per cell. A novel cooperative downlink multicell MISO beamforming scheme is proposed with highly limited information exchange among the base stations (BSs) to maximize the sum-rate. In the proposed scheme, each BS can design its beamforming vector with only local CSI based on limited information exchange on CSI. Unlike previous studies, the proposed beamforming design is non-iterative and does not require any vector or matrix feedback but requires only quantized scalar information. The proposed scheme closely achieves the optimal sum-rate bound in almost all signal-to-noise ratio regime based on non-iterative optimization with lower amount of information exchange than existing schemes, which is justified by numerical simulations. [ABSTRACT FROM AUTHOR]

A multi-user multiple-input single-output simultaneous wireless information and power transfer full-duplex system with imperfect channel state information is considered. Under the assumption that the unknown channel estimation error is within a certain range, the self-interference cannot be suppressed completely at the base station and at the multi-user nodes. The proposed design problem is formulated as maximizing the average harvested energy of multi-user nodes and maintaining the signal-to-interference-plus-noise ratio requirement, which is proved to be non-convex. Utilizing semidefinite relaxation and S-procedure methods, a convex optimization problem of optimal beamforming vector for the multi-antenna transmitter is obtained. Moreover, semidefinite relaxation is proved to be tight by showing the existence of rank-one optimal solutions. Simulation results are presented to verify the robustness and effectiveness of the proposed algorithm compared with the non-robust design. [ABSTRACT FROM AUTHOR]

Constant envelope (CE) precoding is a transmission scheme facilitating the use of low-cost radio-frequency power amplifiers in the transmitter. Herein, we consider CE precoder designs for multiple-input–single-output multicast systems using phase-shift-keying modulation. A new CE precoder is proposed that first smooths the objective function in the max–min received signal-to-noise-ratio (RSNR) design problem, which is then solved through the projected gradient framework. Simulation results revealed that the proposed CE precoders can achieve similar or superior minimum RSNR with much lower computational complexity compared with existing semidefinite-relaxation-based CE precoders. [ABSTRACT FROM AUTHOR]