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Intelligent Reflecting Surface (IRS) significantly improves the energy utilization efficiency in 6th generation cellular communication systems. Here, we consider a system with multiple IRS and users, with one user communicating via several IRSs. In such a system, the user to which an IRS is assigned for each unit time must be determined to realize an efficient communication. The previous studies on the optimization of various parameters for IRS based wireless systems did not consider the optimization of such IRS allocation scheduling. Therefore, we propose a new IRS allocation scheduling method which limits the number of users who allocate each IRS to one unit time and sets the reflection coefficients of the IRS specifically to the assigned user resulting in the maximum IRS array gain. Additionally, as the proposed method is a combinatorial optimization problem, we develop a quadratic unconstrained binary optimization formulation to solve this using quantum computing. This will lead to the optimization of the entire system at a high speed and low power consumption in the future. Using computer simulation, we clarified that the proposed method realizes a more efficient communication compared to the method where one IRS is simultaneously used by multiple users.

Phase modulation (PM) techniques based on time modulated arrays (TMAs) are usually used to generate a steerable beam at the carrier frequency. However, the overall efficiency of TMAs constructed by conventional PM techniques vary significantly in the whole scanning space, resulting in abrupt fluctuations in array gain. Thus, this paper presents a geometric method to design the time modulated waveforms. According to the simulation results, TMAs constructed using the proposed approach have suppressed peak sideband levels (PSBLs) and stable overall efficiency.

Millimeter-wave cell-free and user-centric massive multiple input multiple output (MIMO) systems are promising, due to the reliable ultra-high data rate services. Thus, it is a significant issue to design low-complexity hybrid precoders. In this paper, a simplified sum-mean-square-error (SMSE) is used to design hybrid precoders, based on the array gain and spatial macro-diversity. Then, a heuristic hybrid pre-coding scheme is proposed, with two analog design algorithms that are with or without the knowledge of statistical information. The influence of the imperfect state information of beams is theoretically analyzed, from the instantaneous and ergodic aspects, verifying the robustness of the proposed scheme. Simulation results show that our proposed low-complexity hybrid precoder can be easily realized with acceptable bit error rate (BER) and sum-rate performances, especially when the total transmit power is small.

The initial setup at millimeter-wave channels is especially challenging due to the severe propagation loss imposed on the channel gains. To overcome this loss, it is common to use large arrays and concentrate the radiated energy in certain directions through beamforming. During the initial setup process, the base station (BS) has no knowledge of the position and channel state of the user equipment (UE) in its coverage region. Therefore, it is necessary to transmit beams that cover this region, with each beam having sufficient array gain to allow for reliable communication between the BS and the UEs. At the same time, next-generation wireless networks should serve a large number of UEs with minimum latency. In our work, exploring the concept of the worst-case channel, we propose an algorithm that generates the transmitting beamforming codebook. This codebook minimizes the number of beams transmitted, resulting in reduced maximum latency while at the same time guaranteeing a minimal signal-to-noise ratio (SNR) between BS and UEs. Simulation results are conducted to show the efficacy of the proposed beam codebook design, which illustrates latency reduction without sacrificing achievable SNR.

Due to the flexibility of the towed line array, shear currents caused by an internal solitary wave (ISW) distort the array shape. This will lead to a mismatch between the conventional target detection algorithm and the array shape, resulting in a significant decline in the performance of the towed line array gain, azimuth resolution, etc. Based on the improved motion model of towed cable constructed by Ablow and Schechter (Ocean Eng, 10: 443–457, 1983), we propose a motion model of the towed line array under an ISW according to the Korteweg-De Vries (KdV) equation and solve the model by finite difference method combined with the Newton iteration method. In addition, the DFT beamforming theory is used to detect the target signal after the array shape distortion compensation to verify the validity of the model. The data analysis shows that the array shape distortion caused by the ISW is mainly affected by the relative position between the array and the ISW, the amplitude of the ISW, the fluid layer density, the fluid layer depth, the towing velocity, the tangential/normal drag coefficient, the elastic modulus, and the towed cable density. The influence of elastic modulus and the towed cable density on array shape distortion can be ignored. The detection results show that the output signal power is about 6 dB higher than the output noise power, and the array gain and azimuth resolution are improved after array shape distortion compensation.

During an experiment conducted in the deep South China Sea, a bottom-mounted horizontal linear array (HLA) collected the radiated noise from a moving cooperative ship at a distance less than 13 km. Bearing-time records of the HLA through plane-wave beamforming showed significant bearing estimation errors and two or three split bearing tracks of the ship that leads to misjudgment of target numbers. To reveal the physics underlying the experimental phenomena, the directional response of the HLA to an acoustic source at close range was developed by normal mode theory. Numerical simulations were conducted to analyze the acoustic intensity distribution characteristics and the arrival structures of the acoustic field. Comparative results of beamforming obtained with normal mode and ray theory show that the ray model provides an easy way to predict the split bearings for HLA beamforming. When sources are near the endfire of the array, significant bearing estimation errors and the beam splitting effect for a large aperture array were the main issues. For sources near the broadside of the array, the beam broadening effect was the most significant concern. The effect of the array length on the beamforming anomalies and the array gain loss was also investigated. With the present theories, the HLA beamforming anomalies during the experiment were well explained.

Range-coherent matched field processing (MFP) coherently combines snapshots to localize a moving, narrowband source. This approach differs from existing MFP approaches that treat each snapshot as having a random phase due to both unknown motion through the medium and imprecise knowledge of the source frequency. Range-coherent MFP requires determination of the source phase acquired between snapshots. With that information, MFP can be applied to the cross-spectrum of snapshots acquired at different times, since relative phase between snapshots is determined by the medium properties, source location, and source velocity. Viewed another way, range-coherent MFP is simply MFP applied to a passive synthetic aperture formed from a moving source. The synthetic aperture geometry depends on source velocity, which is included in the MFP search space. Range-coherent MFP produces robust velocity estimates at low signal-to-noise ratio (SNR), which permits the use of a longer fast Fourier transform in pre-processing. The synthetic aperture array gain plus the increased input SNR afforded by the enhanced pre-processing significantly lowers the required signal level for successful localization. In data from the SWellEx-96 experiment, range-coherent MFP successfully localizes a source that is too quiet for conventional methods to localize.

Direction of arrival (DOA) estimation has paramount importance in array signal processing tasks. A practical issue regarding DOA estimation is the uncertainty in gain/phase of some of the sensor elements. In this paper, we focus on the 2-dimensional DOA (2D-DOA) estimation in the presence of gain/phase uncertainty. We propose a grid-based modeling for the 2D-DOA estimation of the uniform rectangular array considering the array gain/phase uncertainty. In order to solve the modeled problem, we suggest an algorithm to exploit the joint sparse properties of the problem. Furthermore, we have used a zooming-based technique to estimate the 2D-DOAs with much lower computational complexity. The efficiency of the proposed method has been validated through various simulation scenarios which confirm the superiority of the suggested algorithm over its counterparts.

In this paper, we consider a mmWave massive multiple-input multiple-output (MIMO) communication system with one static base station (BS) serving a fast-moving user, both equipped with a very large array. The transmitted signal arrives at the user through multiple paths, each with a different angle-of-arrival (AoA) and hence Doppler frequency offset (DFO), thus resulting in a fast time-varying multipath fading MIMO channel. In order to mitigate the Doppler-induced channel aging for reduced pilot overhead, we propose a new angular-domain selective channel tracking and Doppler compensation scheme at the user side. Specifically, we formulate the joint estimation of partial angular–domain channel and DFO parameters as a dynamic compressive sensing (CS) problem. Then we propose a Doppler-aware-dynamic variational Bayesian inference (DD-VBI) algorithm to solve this problem efficiently. Finally, we propose a practical DFO compensation scheme which selects the dominant paths of the fast time-varying channel for DFO compensation and thereby converts it into a slow time-varying effective channel. Compared with the existing methods, the proposed scheme can enjoy the huge array gain provided by the massive MIMO and also balance the tradeoff between the CSI signaling overhead and spatial multiplexing gain. Simulation results verify the advantages of the proposed scheme over various baseline schemes.

Multiple-input multiple-output (MIMO) beamforming array gain can make multi-antenna transmitters (TXs) more power efficient. However, these MIMO TXs require more complex digital predistortion (DPD). In this work, analytical expressions are derived for the power consumption of both MIMO power amplifier (PA) arrays and their respective DPD, taking into account the effects of precoding on PA output power and peak-toaverage power ratio (PAPR). It is shown that for complex DPD algorithms such as cross-over DPD (CO-DPD) in combination with wide bandwidths, the overall DPD power consumption can exceed the overall PA power consumption already for TXs for scenarios with more than two antennas.

Hybrid analog/digital precoding in millimeter-wave (mmWave) multi-input multi-ouput (MIMO) systems is capable of achieving the near-optimal full-digital performance at reduced hardware cost and power consumption compared to its full-RF digital counterpart. However, having numerous phase shifters is still costly, especially when the phase shifters are of high resolution. In this paper, we propose a novel twin-resolution phase-shifter network for mmWave MIMO systems, which reduces the power consumption of an entirely high-resolution network, whilst mitigating the severe array gain reduction of an entirely low-resolution network. The connections between the twin phase shifters having different resolutions and the antennas are either fixed or dynamically configured. In the latter, we jointly design the phase-shifter network and the hybrid precoding matrix, where the phase of each entry in the analog precoding matrix can be dynamically designed according to the required resolution. This method is slightly modified for the fixed network’s hybrid precoding matrix. Furthermore, we extend the proposed method to multi-user MIMO systems and provide its performance analysis. Our simulation results show that the proposed dynamic hybrid precoding method strikes an attractive performance vs. power consumption trade-off.

The perfectly partly calibrated antenna array is a frequently assumption in most of the existing array gain/phase calibration methods. In practice, however, the partly calibrated array is usually not available. In this letter, a tail optimization method for direction finding with unknown gains and phases in the presence of spatially non-uniform noise is proposed. Specifically, the unknown gain/phase entry is firstly merged into the signal power by using the sparse representation. Subsequently, a tail optimization method that can significantly suppress the occurrence of pseudo-peaks is designed to determine the signal DOAs without a priori information of unknown sensor gain and phase errors. In addition, the spatially non-uniform noise can be removed by a linear transformation to improve the robustness against the noise. Numerical simulations examples are presented to demonstrate the effectiveness and superior performance of the proposed approach over the other existing counterparts.

A dielectric rod antenna array fed by slot antenna radiators without the use of metallic waveguide is presented here. Communication systems, especially those related to 5G, are moving up into mm-wave bands where metal losses can become significant and many traditional fabrication and manufacturing techniques become more difficult. Dielectric rod antennas (DRA) are entirely made of dielectric and may be injection-molded or 3D-printed as solid rods, layered rods, or tubes. While the normal approach to feeding DRAs involves some version of metallic waveguide, we show here that the DRA may be integrated with a planar radiator feed and used effectively in an array configuration. The array demonstrated is a $2 \times 2$ array designed for operation at 15 GHz. Each DRA is $6\lambda $ long. The array is fabricated on a 100 mm $\times$ 100 mm substrate with 52 mm separation between elements. The array radiation efficiency is 80% for an array gain of 19 dBi.

The number of elements reduction in different antenna array configurations such as linear, circular, rectangular, planar, and non-planar is of main concern for several research groups. This minimizes the complexity of the feeding network in static antenna arrays, whereas, in adaptive antenna arrays, it reduces the number of active components. The main issue in the array synthesis is to preserve the radiation pattern with minimum distortion compared to the original pattern. These issues are addressed by many researchers in linear, planar, and concentric circular arrays, but no attempt is made in the synthesis of uniform circular antenna arrays (UCAAs) that have elements distributed on a single circle. This is because the synthesis is almost performed in one plane where the radiation patterns in other planes are not predicted. In this paper, most of the aforementioned challenges are treated; the number of elements is reduced, the radiation pattern has minimum distortion, and the radiation pattern is symmetric in all array orthogonal planes. This is performed by dividing the original UCAA’s 3D radiation pattern into a suitable number of 2D plane patterns, each of which is separately synthesized using the specified number of elements. The 2D pattern synthesis process for a particular plane is confined to estimating a new set of non-uniform excitation coefficients while keeping the original UCAA’s array radius. To construct the final set of synthesized excitation coefficients for the 3D array pattern synthesis, we aggregate all of the estimated sets of excitation coefficients and take the average. To verify the efficacy of the proposed technique, the original and synthesized arrays are realized using the CST Microwave Studio using $\lambda \mathord {\left /{ {\vphantom {\lambda 2}} }\right. } 2$ dipole elements. The results indicated that substantially matched patterns were obtained. Furthermore, the coupling between the synthesized array elements is decreased, which enhances the radiation efficiency and realized array gain.

The synthesis of pencil beam and arbitrarily shaped beam patterns of linear antenna arrays (LAA) using reduced number of antenna elements attracts the attention of researchers in recent years. In this paper, a hybrid beamforming technique based on the combination of the genetic algorithm (GA) optimization technique and the $l_{1}$ minimization method denoted as (GA/ $l_{1}$ ) is introduced for LAAs synthesis. The proposed GA/ $l_{1}$ beamforming technique optimizes both the elements excitations and interelement spacing to synthesize the desired LAA pattern with a minimum number of antenna elements. The GA/ $l_{1}$ technique provides an excellent approximation to the desired radiation pattern with high accuracy and low complexity (less number of iterations and computational time) compared to the other synthesis approaches introduced in the literature. In addition, as an application of this work, the proposed GA/ $l_{1}$ technique is used to build up a proposed hybrid precoding and beamforming (HP-BF) structure for Massive Multi-input Multi-output (M-MIMO) systems. In this structure, the transmit antenna array is synthesized for maximum gain realization using the existing number of antenna elements. In the HP-BF structure, the proposed GA/ $l_{1}$ technique is used to make full use of the existing transmit array elements to synthesize the radiation pattern of much larger size and higher gain arrays without the need for additional elements. Thereby, significant savings in the number of antenna elements and their corresponding radio frequency (RF) chains are achieved, which reduces the system complexity. In addition, the array gain maximization will maximize the received signal to noise ratio (SNR) giving rise to higher system performance in terms of spectral efficiency (SE) and power utilization.

In this paper, new virtual antenna array (VAA) based synthesis techniques are introduced for side lobe level (SLL) reduction, beam thinning, and number of elements minimization for elliptical cylindrical antenna arrays (ECAA) of radar systems. Thereby, significant improvements in the array gain and directivity are achieved, which enhance the detection range and angular resolution of the radar. Furthermore, the overall implementation cost of the system is highly reduced by saving the number of elements and the corresponding RF chains and simplifying the feeding network. Firstly, the proposed technique decomposes the single transmit/receive ECAA into a separate transmit linear antenna array (LAA) and receive elliptical antenna array (EAA). Secondly, the number of antenna elements, element spacing, and excitations of the created LAA and EAA are optimized using particle swarm optimization (PSO) to produce efficient beamformed patterns. Finally, the Kronker product of the optimized LAA and EAA patterns is performed to form the optimized virtual ECAA (V-ECAA) pattern. We also introduced both the uniform feeding based V-ECAA technique and the non-uniform feeding based V-ECAA synthesis technique for more flexibility and better productivity in antenna arrays design. The simulation results revealed that the uniform feeding based V-ECAA provides an identical pattern to that of the traditional uniform feeding ECAA while saves the number of elements by 66.6%. While in the case of non-uniform feeding based V-ECAA, it provides much lower SLL and narrower HPBW than those of the ECAA while saving the number of elements by 63.8%. Furthermore, the HB is applied to provide additional beam thinning and SLL reduction of the proposed non-uniform V-ECAA that is denoted as (HBV-ECAA). The possibility of practical validations of the synthesized V-ECAAs is verified using the computer simulation technology (CST) microwave studio package, which gives users an integrated design environment and achieves realizable and robust designs.

The search for physical-layer technologies that can play a key role in beyond-5G systems has started. One option is reconfigurable intelligent surfaces (RIS), which can collect wireless signals from a transmitter and passively beamform them towards the receiver. The technology has exciting prospects and is quickly gaining traction in the communication community, but in the current hype we have witnessed how several myths and overstatements are spreading in the literature. In this article, we take a neutral look at the RIS technology. We first review the fundamentals and then explain specific features that can be easily misinterpreted. In particular, we debunk three myths: 1) Current network technology can only control the transmitter and receiver, not the environment in between; 2) A better asymptotic array gain is achieved than with conventional beamforming; 3) The pathloss is the same as with anomalous mirrors. To inspire further research, we conclude by identifying two critical questions that must be answered for RIS to become a successful technology: 1) What is a convincing use case for RIS?; 2) How can we estimate channels and control an RIS in real time?, Comment: To appear in IEEE Communications Magazine, 7 pages, 6 figures

In this paper, we investigate the secrecy outage performance in simultaneous wireless information and power transfer (SWIPT) systems taking artificial noise assistance into account. Multiple antennas in the source and a single antenna in both the legitimate receiver and the eavesdropper are assumed. Specifically, the transmitted signal at the source is composed of two parts, where the first part is the information symbols and the other is the noise for the eavesdropper. To avoid making noise in the legitimate receiver, these two parts in the transmitted signals are modulated into two orthogonal dimensions according to the instantaneous channel state between the source and the legitimate receiver. We derive an approximate closed-form expression for the secrecy outage probability (SOP) by adopting the Gauss-Laguerre quadrature (GLQ) method, where the gap between the exact SOP and our approximate SOP converges with increase of the summation terms in the GLQ. To obtain the secrecy diversity order and secrecy array gain for the considered SWIPT system, the asymptotic result of the SOP is also derived. This is tight in the high signal-to-noise ratio region. A novel and robust SOP approximation is also analyzed given a small variance of the signal-to-interference-plus-noise ratio at the eavesdropper. Some selected Monte-Carlo numerical results are presented to validate the correctness of the derived closed-form expressions.

This paper proposes a new data-dependent beamforming strategy to adaptively maximize the output power, while nulling preset directions-of-arrival. This new beamformer needs no iterative computation and needs no prior information of the signals-of-interest’s incident directions nor temporal waveforms. Surprisingly, this proposed beamformer can “blindly” null any dominant interferences with no prior information of their incident directions nor of their waveforms, even as this beamformer simultaneously preserves the signals-of-interest as mentioned above. Monte Carlo simulations verify this new method’s superior array gain relative to the “linearly constrained minimum-variance” beamformer’s.

Energy-efficient wireless system design has gained considerable attraction owing to the widespread proliferation of wireless devices. The next-generation wireless systems should also be spectrally efficient to sustain the tremendous demand for wireless services. Full-duplex (FD) systems have the potential to double the spectral efficiency (SE) of current half-duplex communication systems. The major challenge in FD systems is the suppression of strong self-interference (SI) imposed by the transmit signal on its receiving antennas. However, SI can also be utilized for recycling of energy by exploiting the radio frequency energy harvesting (RFEH) techniques, which is commonly referred to as self-energy recycling (S-ER). S-ER improves the overall energy efficiency (EE) of the system via recycling a portion of transmitted power. In this article, we evaluate the EE for a downlink FD nonlinear multiuser multi-input–multi-output (MU-MIMO) orthogonal frequency division multiplexing (OFDM) system in the presence of S-ER. The proposed analytical formulation consists of closed-form derivation for EE and SE of the aforementioned system. Furthermore, we have also obtained the outage performance and based on this performance proposed a methodology to allocate the antennas for S-ER while satisfying the quality of service. It has been observed that reserving few antennas for S-ER improves the EE albeit with a slight degradation in SE due to reduced array gain. Consequently, with S-ER, the tradeoff between the SE and EE has been studied for a nonlinear MU-MIMO-OFDM system. Analytical derivation for EE, SE, and outage are followed by simulations for corroborating the proposed analysis.

In this paper, we propose the hybrid-precoding design that alleviates the performance loss caused by beam-misalignment in the mmWave multi-user communication systems. To this end, we firstly design the beam-misalignment aware fully-digital precoders for two distinct scenarios. First, for a base-station (BS) with full estimated channel-state-information (CSI), the minimum-mean-squared-error metric incorporating the ‘error-statistics’ of the beam-misalignment error is used to analytically derive a closed-form expression for the fully-digital precoder, which maximizes the array gain while suppressing the inter-user interference for each user-equipment (UE). Second, for a BS which can only acquire partial estimated CSI, a min-max non-convex optimization is considered to obtain the fully-digital precoder, which minimizes the maximum loss in the array gains of the expected beam-misalignment ‘error-range’ over the UEs while cancelling the inter-user interference. Subsequently, we propose the hybrid-precoding design that approximates the fully-digital designs based on the gradient-projection method, which is mathematically proven to converge to an approximate local solution with further reduced complexity compared to the state-of-the-art algorithms. Finally, the proposed hybrid-precoding design is further extended to the wideband mmWave communication systems. Numerical results show that the proposed hybrid-precoding design can effectively alleviate the performance degradation incurred by the beam-misalignment.

This article presents techniques into prediction and control of the radiated harmonic levels in time-modulated antenna arrays (TMAs). The effect of ramping on the controlling switching waveform, due to slew in the RF switches, is analyzed first. This is followed by a novel technique for controlling and reducing the fundamental carrier component (due to the Fourier series 0 Hz term) that is always produced by TMAs. A new bespoke, steerable, TMA RF hardware test platform is presented, using three gain states to pragmatically implement the proposed carrier reduction technique. The platform implementation utilizes binary logic control interfaces to the TMA, rather than analog control interfaces. Laboratory measurements using the test platform demonstrate a 16.5 dB reduction (limited by PCB emissions) in the level of the fundamental carrier component relative to the steered first harmonic, from a possible reduction of 21.3 dB. The array gain for the first harmonic beam is 2 dBi.

Deconvolution applied to conventional beamforming (CBF) has been demonstrated for a uniform line array and circular array of hydrophones resulting in much improved performance than CBF. However, the existing Richardson–Lucy (R–L) method is limited to arrays with a shift-invariant beam pattern. An extended R–L deconvolution algorithm is proposed in this paper so that the deconvolved CBF can be applied to arrays with a shift-variant beam pattern, such as a vector sensor line array. The new extended R–L algorithm uses the predefined beam pattern from all steering angles to replace the shift of the point spread function. It is shown with both simulated and sea data that deconvolved beam power yields a narrower beamwidth, higher peak-to-sidelobe ratio, higher left/right suppression ratio, and higher directivity index or array gain than CBF. The extended R–L algorithm is shown to yield superior performance, particularly at low signal-to-noise ratios, compared with other deconvolution algorithms, and much less computation time for the same number of iterations, which are of practical importance.

Modal beamformer for circular arrays, which is based on a circular harmonics decomposition of the sound field, is flexible for use because it is decoupled from the sensor positions. A time-domain implementation of broadband modal beamformer for circular arrays is presented in this paper. This beamforming structure consists of a real-valued circular harmonics transform and steering unit, followed by a pattern generation unit implemented by a set of real-valued finite impulse response (FIR) filters. The beam pattern is controlled through the FIR filters’ tap weights, whose design is decoupled from beam pattern steering and can be formulated as a constrained optimization problem that can be solved efficiently. Multiple array performance measures such as array gain, sensitivity, mainlobe, and sidelobes can be taken into consideration in beamformer design, which is very important in practical applications. Simulation and experimental results are presented to illustrate the effectiveness of the proposed time-domain broadband modal beamforming approach. Performance comparisons between the proposed and the classical methods in weight coefficients, beam patterns, and quality of audio output are also provided. The results show that the proposed approach possesses high robustness and can achieve good frequency invariance.

In this paper, we provide a performance analysis of communication systems over Rayleigh-product channels with two popular diversity combining techniques, namely maximal ratio combining (MRC) and selection combining (SC). We first derive new closed-form expressions for the exact cumulative distribution function (CDF) and probability density function (PDF) of the post-processing signal-to-noise ratio (SNR) for these two schemes. Secondly, we present the first-order asymptotic expansions for these CDF and PDF functions. Performance of MRC and SC techniques, in terms of outage probability, average symbol error rate (SER) and ergodic capacity, is derived using the exact expressions of CDF and PDF. Furthermore, we present new expressions for key metrics characterizing the system performance at the high and low SNR regimes. Thanks to the asymptotic CDF and PDF expressions, we compute the average SER in the high SNR regime and derive the diversity order and array gain parameters. In addition, we provide simple expressions for the ergodic capacity in the asymptotic low and high SNR regimes. Monte-Carlo simulations are conducted and their results agree well with the analytical results.