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In this article, a linear precoding weight vector is designed to enhance the minimum signal-to-interference plus noise ratio (SINR) of a space-time line code (STLC)-based multicast users. To solve the intractable minimum SINR maximization problem, the optimal weight vector for each STLC user is derived, and then, the individual optimal weight vectors are linearly combined. The designed linear precoding vector improves the minimum and average SINR of the multicast users as verified by numerical results. Numerical results also verify that the STLC-based multicast is a promising scheme, especially, if a transmitter has a large number of transmit antennas and the additive noises are severe at the users. [ABSTRACT FROM AUTHOR]

Since a novel diversity technique called a space–time line code (STLC) was invented, previous studies have focused on the analysis of a rate-1 STLC assuming two receive antennas. For the state-of-the-art STLCs with transmit antenna selection (TAS), in this study, the ergodic capacities of TAS-STLC systems with various code rates are mathematically derived, not only for a TAS-STLC system with two receive antennas but also for those with three or four. To this end, the probability density function of the instantaneous signal-to-noise ratio was characterized. The accuracy of the mathematical analysis was validated by a numerical simulation. This study is significant for investigating the fundamental performance of TAS-STLC systems. [ABSTRACT FROM AUTHOR]

To extend the transmission range governed by limited transmit power, collaborative beamforming (CB) can be employed with a virtual antenna array (VAA) constructed by an unmanned aerial vehicle (UAV) swarm. CB techniques combined with physical layer security (PLS) algorithms are known to better secure transmissions of sensitive information in the presence of eavesdropping attacks, compared to co-located antenna-based schemes. In particular, analog collaborative beamforming (ACB) with random VAA topologies, which is suitable for UAV networks due to its lower signaling/channel estimation overhead compared to the digital CB, can provide a significantly higher secrecy rate by creating noise-like signals at eavesdroppers. However, the increase in energy consumption to improve secrecy rate had been ignored in the existing ACB-based PLS schemes, which is of paramount importance considering the limited onboard energy of UAVs. Thus, motivated by such limitation, in this paper, we examine the security energy efficiency of the ACB-based PLS technique, where a group of UAVs (or VAA elements) randomly change their locations in a distributed manner to randomize the array factor of the VAA, i.e., a stochastic VAA. The proposed stochastic VAA outperforms the existing static VAA in terms of secrecy energy efficiency as verified by analytic and numerical results. Further, the impact of phase errors of the proposed scheme is also presented, which indicates the significance of the accurate phase estimation. [ABSTRACT FROM AUTHOR]

In this article, a generalized space–time line code (GSTLC) is proposed. The proposed GSTLC concurrently transmits multiple data streams obtained by space–time line code encoding and precoding to provide spatial multiplexing and diversity. When full channel state information (CSI) is available to the transmitter and limited CSI is available to the receiver, an optimal precoding matrix is derived for GSTLC with linear combining that is based on the minimum mean square error (MMSE) criterion. Moreover, the detection signal-to-interference-plus-noise ratio (SINR) is derived corresponding to the optimal MMSE precoder. Using the SINR, we propose a receiver combining scheme that selects the best combining matrix, thereby minimizing the bit error rate (BER) from a codebook that includes candidate combining matrices. Through numerical simulations, we demonstrate that the proposed GSTLC with the optimal combiner achieves the theoretical lower bound of BER. In addition, we show that the GSTLC with combiner selection performs better than conventional precoded spatial multiplexing methods in terms of the BER. [ABSTRACT FROM AUTHOR]

In this article, to improve the achievable sum-rate of the multi-user millimeter wave (mmWave) communication in densely blocked areas, we design a multi-route multi-hop (MRMH) cascaded intelligent reflecting surface (IRS)-aided system. In our designed system, we aim to maximize the achievable sum-rate for the multiple users by jointly optimizing the phase shifts (PSs) at the multiple IRSs and the active beamforming vector (ABV) at the base station (BS). To achieve this target, we formulate an optimization problem. However, it is non-convex and difficult to be solved. By considering the sparse scattering and near-orthogonality characteristics of the mmWave channels, we divide the optimization problem into multiple tractable sub-problems and obtain a near-optimal solution. The simulation results prove that the proposed scheme has better performance than the existing schemes. Furthermore, we prove the effectiveness of the multi-hop IRS-aided system by comparing the single-hop IRS-aided system. [ABSTRACT FROM AUTHOR]

In this article, we investigate a secure uplink multiple-input multiple-output communication system, in which the multiple users transmit information to a legitimate access point (AP) when a passive eavesdropper exists. A secure multiple access method is proposed for single-antenna users by employing space–time line codes (STLCs), which enables noncoherent detection at the legitimate AP without channel state information (CSI) and achieves full spatial diversity. While extending the secure STLC scheme from the single-antenna to two-antenna users by combining the STLC with artificial noise (AN) injection, we design a low-complexity AN signals with the optimal power control scheme. The proposed AN is ensured to be eliminated after STLC decoding at the legitimate AP, while sustained in eavesdropper's received signals as interference. Theoretical sum secrecy rates are derived by considering time-varying fading channels with the uncertainty of CSI at the users, and the numerical results verify that the proposed STLC schemes outperform the existing secure transmission schemes in terms of the sum secrecy rate. [ABSTRACT FROM AUTHOR]

In this paper, we consider a reconfigurable intelligent surface (RIS)-assisted two-way relay network, in which two users exchange information through the base station (BS) with the help of an RIS. By jointly designing the phase shifts at the RIS and beamforming matrix at the BS, our objective is to maximize the minimum signal-to-noise ratio (SNR) of the two users, under the transmit power constraint at the BS. We first consider the single-antenna BS case, and propose two algorithms to design the RIS phase shifts and the BS power amplification parameter, namely the SNR-upper-bound-maximization (SUM) method, and genetic-SNR-maximization (GSM) method. When there are multiple antennas at the BS, the optimization problem can be approximately addressed by successively solving two decoupled subproblems, one to optimize the RIS phase shifts, the other to optimize the BS beamforming matrix. The first subproblem can be solved by using SUM or GSM method, while the second subproblem can be solved by using optimized beamforming or maximum-ratio-beamforming method. The proposed algorithms have been verified through numerical results with computational complexity analysis. [ABSTRACT FROM AUTHOR]

In this paper, the dimension of artificial noise (AN) and corresponding transmit power in multiple-input multiple-output single-eavesdropper channels are investigated. It is widely known that AN is transmitted in the null space of the channel matrix of a legitimate link between a source and a destination. Here, the secrecy outage probabilities are derived for various dimensions of AN. Recognizing the fact that the secrecy outage performance depends on the dimension of the AN, an optimization problem is formulated to maximize the secrecy rate under a secrecy outage constraint. The optimal power splitting factor between the information and AN signals is then derived. Furthermore, the asymptotic behaviors of the optimal power splitting factor and the resulting maximum secrecy rate are examined. Through asymptotic analyses and numerical verification, it is formally shown that the full-dimension AN maximizes the secrecy rate under a secrecy outage constraint. Because the complexity of AN design increases as the dimension of the AN increases, we can achieve a tradeoff between secrecy performance and AN design complexity by determining the dimension of the AN. This study can provide a guideline of AN design specifically pertaining to the dimension of the AN. [ABSTRACT FROM AUTHOR]

In this study, we consider a space–time line code (STLC) system with $N$ transmit antennas and two receive antennas. Although the STLC system is scalable to an arbitrary number of transmit antennas, a large number of them will increase the hardware cost and complicate the implementation of the system. To resolve this practical limitation, we propose a transmit antenna selection (TAS) scheme for the $N$ -by-2 STLC system. If $S\leq N$ antennas are used for the transmission, the STLC transmitter selects the $S$ antennas such that the instantaneous signal-to-noise ratio (SNR) is maximized. For a comprehensive performance analysis, the instantaneous SNR of the proposed TAS is characterized in terms of the statistical mean and moment-generating function (MGF). To investigate the performance of the proposed TAS approach, the approximated expression of error rate is derived based on the MGF method. Moreover, using asymptotic analysis, we show that the proposed TAS-STLC system achieves full spatial diversity of $2N$ , while the coding gain is affected by $N$ , $S$ , and the modulation order. As a special case, we also provide a mathematical analysis of the error rate of the conventional STLC system when $N=S$. [ABSTRACT FROM AUTHOR]

Required throughput for a fronthaul in a cloud-radio access network tremendously increases and becomes a bottleneck in a communication network with wide bandwidth and a large number of antennas. To reduce the fronthaul capacity requirement, more digital function blocks are shifted from a central unit to a distributed unit. In this study, we consider separated delivery of data and pilot signals with different compression rates under time-varying channels to reduce fronthaul burden. By analyzing channel prediction error based on a Kalman filter, an achievable rate for uplink data is derived. The fronthaul rates for data and pilots are derived using rate-distortion theory. Both uplink rate maximization and fronthaul rate minimization are simultaneously designed by considering the data and pilot signal distortions as well as the frame structure. First, the optimization of signal distortion with a fixed pilot interval is investigated. Next, a sub-optimal algorithm to find the pilot interval with the fixed signal distortion is developed. Finally, an iterative algorithm to obtain the sub-optimal solution, which is the joint solution of signal distortion and pilot interval, is proposed. The numerical results demonstrate that the proposed algorithm can provide convergent solutions. [ABSTRACT FROM AUTHOR]

Cognitive radio (CR) is an effective solution to improve the spectral efficiency (SE) of wireless communications by allowing the secondary users (SUs) to share spectrum with primary users (PUs). Meanwhile, intelligent reflecting surface (IRS), also known as reconfigurable intelligent surface (RIS), has been recently proposed as a promising approach to enhance energy efficiency (EE) of wireless communication systems through intelligently reconfiguring the channel environment. To improve both SE and EE, in this paper, we introduce multiple IRSs to a downlink multiple-input single-output (MISO) CR system, in which a single SU coexists with a primary network with multiple PU receivers (PU-RXs). Our design objective is to maximize the achievable rate of SU subject to a total transmit power constraint on the SU transmitter (SU-TX) and interference temperature constraints on the PU-RXs, by jointly optimizing the beamforming at SU-TX and the reflecting coefficients at each IRS. Both perfect and imperfect channel state information (CSI) cases are considered in the optimization. Numerical results demonstrate that IRS can significantly improve the achievable rate of SU under both perfect and imperfect CSI cases. [ABSTRACT FROM AUTHOR]

We consider a homogeneous cellular network where a multi-antenna base station (BS) in each cell transmits messages to its intended user over a common frequency band. To improve the system capacity of this multi-cell multi-input single-output (MISO) interference channel, one of the state-of-the-art algorithms, namely, downlink-beamforming coordination, allows all BSs to cooperate with one another to mitigate the effect of inter-cell interference. However, most existing algorithms are suboptimal and impractical in a dynamic wireless environment, due to the high computational complexity and the overhead involved in collecting global channel state information (CSI). In this study, we exploit deep reinforcement learning (DRL) and propose a distributed dynamic downlink-beamforming coordination (DDBC) method with partial observability of the CSI. Each BS is able to train its own deep Q-network and employs appropriate beamformer depending on its environment, which is observed through a designed limited-information exchange protocol. The simulation results show that the proposed DRL-based DDBC method, with a considerably lower system overhead, achieves a system capacity that is very close to that of the fractional programming algorithm with global and instantaneous CSI measurements. In addition, this work demonstrates the potential of utilizing DRL to solve DDBC problems in a more practical manner. [ABSTRACT FROM AUTHOR]

As a nonorthogonal mode, faster-than-Nyquist (FTN) signaling transmission can effectively improve the spectrum efficiency of a system. However, FTN signaling inevitably causes inter-symbol interference (ISI). The ISI hinders traditional synchronization methods from operating accurately, and this makes the carrier synchronization of FTN systems more complex. To overcome this issue, we propose an iterative carrier frequency offset (CFO) estimation scheme for an FTN signaling system. The proposed CFO estimation scheme consists of two steps: i) a coarse estimation step based on discrete Fourier transform and ii) a fine estimation step based on a golden-section search algorithm. The simulation results show that the mean square error of the proposed scheme is lower than that of the conventional CFO estimation schemes and approaches the Cramer-Rao Bound, which is an optimal estimation performance bound. [ABSTRACT FROM AUTHOR]

This paper investigates the rotary-wing unmanned aerial vehicle (UAV)-enabled full-duplex wireless-powered Internet-of-Things (IoT) networks, in which a rotary-wing UAV equipped with a full-duplex hybrid access point (HAP) serves multiple sparsely-distributed energy-constrained IoT sensors. The UAV broadcasts energy when flying and hovering, and collects information only when hovering. It is assumed that the transmission range of the UAV is limited and the sensors are sparsely distributed in the IoT network. Under these practical assumptions, we formulate three optimization problems: a sum-throughput maximization (STM) problem, a total-time minimization (TTM) problem, and a total-energy minimization (TEM) problem. For the TEM problem, we further take into consideration that the power needed for hovering, flying, and transmitting are different. For the STM, TTM and TEM problems, optimal solutions are obtained. Finally, numerical results show that the performance achieved by the proposed optimal time allocation schemes outperform existing time allocation schemes. It is also observed that i) the time allocation between hovering and flying time has different trends for different goals; ii) there is an optimal UAV transmit power range that minimizes the energy consumed by the UAV during the entire cycle. [ABSTRACT FROM AUTHOR]

This work proposes a random access system with uplink non-orthogonal multiple access (NOMA), where users transmit their packets by using the space-time line code (STLC) to fully exploit the spatial diversity without channel state information at the receiver. More specifically, users with a single antenna estimate channel gain from the base station (BS) with two antennas, encode the information using the STLC, and then transmit it using transmit power control so that the received power of their packets at the BS can be one of the predetermined values. The BS decodes the received packets via STLC decoding and the successive interference cancellation technique. Owing to spatial diversity, we show that the proposed scheme improves energy efficiency in comparison to the conventional system with a single antenna BS. [ABSTRACT FROM AUTHOR]

In this paper, an $M\times 4$ double space-time line code (D-STLC) is designed for a system with $M$ -transmit and four-receive antennas whose channel state information is available at the transmitter. By delivering two independent STLC data streams simultaneously, both spatial diversity and multiplexing gains are obtained, like a $4\times M$ double space-time transmit diversity (DSTTD) system that transmits two space-time block coded data streams. It is analytically shown that the decoding signal-to-interference-plus-noise ratios of D-STLC and DSTTD are identical to each other, and furthermore, it is numerically verified that the proposed D-STLC significantly outperforms a conventional $M\times 4$ STLC scheme and other existing spatial-diversity and spatial-multiplexing schemes, such as precoded space-time block code and precoded spatial multiplexing, in terms of achievable rate and bit-error-rate performance as the signal-to-noise ratio and/or transmission data rate increase. [ABSTRACT FROM AUTHOR]

This article proposes beamforming strategies for multiuser communication networks by jointly considering the data rate and battery lifetime of each user. We consider a practical power consumption model, where the decoding power of each user device is a nonlinear function of the data rate. We assume each user device is powered by a battery with limited capacity. The power consumption of the user device determines the expected battery lifetime which conforms to Peukert's law. Two non-convex optimization problems are considered based on two criteria: maximizing the minimum of user throughputs (MaxMin) and maximizing the weighted sum of user throughputs (MaxSum), during the battery lifetime. For the MaxMin problem, we solve it by using a bisection search algorithm. For the MaxSum problem, we propose an algorithm based on the successive convex approximation, which can find a stable point that approximates the optimum solution. Numerical simulations verify that the expected data throughput of the multiuser system can be greatly improved by properly choosing the beamforming strategies using the proposed algorithms. [ABSTRACT FROM AUTHOR]

In this paper, we analyze the coverage probability and average rate of millimeter-wave (mmWave) communication systems with directional cell discovery based on beam scanning. We first model the system and derive the coverage probability and average rate using stochastic geometry. We then show that the appropriate beamwidth can balance cell search delay and beamforming gains, improving average rate. Finally, we design the beamwidth that achieves a near-optimal average rate, which is approximately $\text{1.5}\%$ less than the maximum average rate, and verify it by simulation. Furthermore, some useful remarks, which serve guidelines for mmWave beamforming system design, are provided. [ABSTRACT FROM AUTHOR]

In this paper, a general two-way relay (TWR) transmission method is proposed under per-antenna power constraints by combining space-time line codes (STLCs) with transmit power allocation for two source nodes. We introduce a general STLC-based encoding scheme for a decode-and-forward TWR and derive the detection signal-to-interference-plus-noise ratio (SINR) values at two source nodes. An optimal encoder structure with power allocation is proposed in terms of maximizing the minimum SINRs, and it is verified that the optimal encoder is identical to the superposition of two conventional STLCs. An iterative method is proposed to find the optimal power allocation for detection at two source nodes. Moreover, a low-complexity suboptimal encoder is proposed for practical implementation. Numerical simulations present that the proposed STLC-based transmission method outperforms a conventional eigen-beamforming scheme with nulling and an STLC-based scheme with equal power allocation, in terms of the average bit error rate of source nodes, regardless of the distribution of source nodes, the number of TWR antennas, and the transmit power. [ABSTRACT FROM AUTHOR]

In this correspondence paper, a novel machine learning (clustering) based blind decoding method is proposed for the space–time line code (STLC) systems without the information of modulation size and channels. The number of clusters, which is equivalent to the modulation size, is estimated from the received signals by using a $k$ -mean cluster validation metrics, such as silhouette score and Davies–Bouldin index, and the cluster indices are directly mapped to the transmitted binary information bits. To improve the clustering performance, received signal normalization and the initial centroids of the clusters are designed by exploiting the combined STLC signal structure. From the numerical results, it is verified that the proposed blind decoding method can achieve near-coherent decoding performance with either small-size modulation, low noise at the receiver, or a large number of transmit antennas. [ABSTRACT FROM AUTHOR]

In this letter, the altitude of an unmanned aerial vehicle base station (UAV-BS) is optimized to maximize an average worst case secrecy rate (AWSR) without eavesdropper location information. The air-to-ground channels are modeled based on the derived line-of-sight probability under a general urban model with Poisson point process and Rayleigh distribution for the locations and heights of the buildings, respectively. To solve the AWSR maximization problem, which is intractable, we reformulate it by the first-order Taylor series approximation and apply the Davies–Swann–Campey algorithm to effectively find the solution of the modified AWSR maximization problem. Numerical results verify that the altitude of UAV-BS affects the secrecy rate significantly and that the proposed UAV-BS can improve the secrecy rate compared with a static terrestrial BS. [ABSTRACT FROM AUTHOR]

A zero-forcing (ZF)-based transmission method is proposed for a wireless powered communication network (WPCN) with multiuser multi-input multi-output (MU-MIMO) channels. We formulate the max–min user rate optimization problem based on ZF-based uplink transmission, under the transmit power constraints determined by the downlink power transfer, and derive a Newton-like iterative algorithm to find the optimal solution of the proposed problem. Using the proposed ZF-based method, we design a downlink energy beamformer, and optimize the uplink power allocation vector, precoder, and combiner in terms of the max–min user rate for a MU-MIMO WPCN system. Numerical simulations show that the proposed method provides at least 3 dB signal-to-noise ratio gain compared to the conventional transmission schemes for WPCN. Moreover, through complexity analysis and simulations, it is shown that the proposed method can reduce at least 92% of the computation load in terms of runtime with moderate achievable rate loss, compared to the optimal method obtained by the interior-point method. [ABSTRACT FROM AUTHOR]

A new precoder design method is proposed for multi-input multioutput (MIMO) systems with per-antenna power constraints (PAPCs). In the proposed scheme, the optimal zero-forcing (ZF) based precoder maximizing the sum rate is evaluated for MIMO systems under the conventional sum power constraint (SPC), after which a new ZF-based precoder conforming to the PAPCs is designed by finding the linear precoder closest to the optimal ZF-based SPC precoder. Using the proposed design procedure, we derive an iterative algorithm based on the modified Newton's method to construct a PAPC precoder for a multiuser MIMO system. The complexity order of the proposed method is compared with those of the existing precoding schemes, and it is shown through numerical simulations that the proposed PAPC precoder achieves near-optimal performance while reducing the power consumption at macrocell base stations. [ABSTRACT FROM AUTHOR]

A transmit antenna shuffling method combined with an orthogonal space-time block code (OSTBC) is proposed for a system using $M$ transmit antennas with asymmetric power amplifiers (PAs) having uneven gains. Considering the uneven gains of PAs, we design an optimal antenna shuffling method with $\lceil {log}_2(M!) \rceil$ -bit feedback to maximize the detection signal-to-noise ratio (SNR). To reduce the feedback, a suboptimal method with $\lceil {log}_2M\rceil$ -bit feedback is proposed. A theoretical analysis and numerical results verify that antenna shuffling can significantly improve the detection SNR of the OSTBC system with uneven PA gains and that the SNR loss of the suboptimal method is marginal. [ABSTRACT FROM AUTHOR]

In this study, we investigate distributed antenna systems (DASs) with partial channel state information (CSI). In the DAS considered here, one control unit coordinates multiple baseband units (BBUs), and each BBU with multiple distributed antennas (DAs) knows only the direct channels from its DAs to its associated users and supports them simultaneously. For fairness among users, a distributed precoder structure is designed to maximize the upper bound of the minimum user rate, under the assumption that the direct channel dominates the intra-BBU interfering links. We then propose a distributed general precoder that allows multiple BBUs to serve multiple users simultaneously. Numerical results verify that, in terms of the minimum and average achievable rates, the proposed scheme significantly outperforms an existing scheme in which each BBU supports a single user. The results also demonstrate that the proposed scheme is comparable to a full-CSI based scheme. [ABSTRACT FROM PUBLISHER]

A practically efficient resource allocation scheme is designed for multiple device-to-device cluster (D2DC) multicast communications underlay cellular networks. The transmit power and the channel are allocated to D2DC to maximize the sum effective throughput provided that the cellular communications maintain a certain level of quality of service. We devise a channel allocation scheme using partial information of device locations. For this, we derive the outage probability and an effective throughput of D2DC communications in approximate forms. Numerical results reveal that, when the number of D2DC receivers is large, the proposed scheme achieves the near optimal sum effective throughput, which can be obtained by using full information of device locations. [ABSTRACT FROM PUBLISHER]

In this paper, an amplify-and-forward two-way relay (TWR) precoding is designed with no source node (SN) processing in order to maximize sum rate and concurrently reduce the network latency and signaling overhead. For the arbitrary pre-/postprocessing at the SNs that have $N$ antennas, it is rigorously shown that an $N$ -by-$N$ optimal TWR precoder matrix is symmetric when the noise power of the TWR node is relatively small compared with that of the SNs and/or each of the channel matrices is spatially orthogonal. Using the symmetric structure of the TWR precoding matrix, a constrained nonlinear-multivariable optimization problem is formulated that can be solved with polynomial time complexity. Furthermore, a closed-form optimal TWR precoding matrix is designed for a TWR system with $N=2$. When compared with an optimal joint SN and RN precoding method, it is verified that the proposed precoding method provides a good tradeoff between the system complexity and sum rate performance. [ABSTRACT FROM PUBLISHER]