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Backscattering by objects is a relevant phenomenon in millimeter-wave (mm-wave) propagation. Accordingly, computationally efficient models characterizing this process are now available in the literature. However, these models are restricted to single-surface contributions. In this article, we propose a multiple-scattering model for calculating backscattered fields due to consecutive interactions with several smooth electrically-large rectangular surfaces. The model is fundamentally based on the cascading of single-scattering events. In addition, the model is later extended to capture single-scattering blockage between backscattering events. The model is implemented using a Fresnel-integral-based method and validated by means of electromagnetic (EM) simulations and measurements. Furthermore, insight on the effects of the model parameters—number of surfaces, frequency, surface size, surface height-to-width ratio, surface displacement, and antenna-to-surface and surface-to-surface distances—is provided. Due to its computational efficiency, the proposed model can be suitable for system- and link-level simulations of wireless systems, particularly when Monte Carlo simulations are applied. [ABSTRACT FROM AUTHOR]

A high-efficiency wideband double-layer transmitarray antenna (TA) using miniaturized array elements is proposed in the study for millimeter-wave (mm-Wave) applications. The transmitarray consists of 484 dual-polarized elements. Each element is composed of only two layers of identical modified Jerusalem crosses printed on a dielectric substrate. Additional stubs are used for size reduction, leading to a compact size with a periodicity of only $0.44 \lambda _{0}$. It contributes to a high aperture efficiency for the proposed TA. TA is designed, fabricated, and measured. It achieves a peak gain of 28.67 dBi at 31 GHz and a high aperture efficiency of 44%–61.3% within the 1 dB gain bandwidth, which is 10.3% (29.5–32.7 GHz) in the measurement. The proposed dual-polarized double-layer TA demonstrates simplified design complexity, reduced thickness and mass, and lower cost compared with the conventional TAs. It also exhibits high radiation gain and high aperture efficiency. [ABSTRACT FROM AUTHOR]

This communication presents a dual-polarized transmitarray antenna with wide bandwidth and independent beam control for each polarization. A new low-insertion-loss transmitarray element allowing wideband dual-polarized operation is proposed by using magnetoelectric (ME) dipole with shared radiating structure and interleaved orthogonal feeding probes; 1 bit phase adjustment can be realized individually for each polarization with additional freedom in manipulating the cross-polarization over a fractional bandwidth of 23%. Three transmitarrays that achieve possibly different collimated beams and/or cross-polarization manipulation are designed and carried out at millimeter-wave frequencies covering from 23.5 to 32.5 GHz. Measurement results of the prototypes agree reasonably well with simulations, which validates the wideband operation and independent beam controls. [ABSTRACT FROM AUTHOR]

A low-profile wide-angle reconfigurable transmitarray antenna (RTA) is proposed in this communication. This RTA consists of a feeding antenna, a phase transforming lens, and a 1 bit phase-reconfigurable transmitting antenna (PRTA). The feeding antenna is composed of a 12 $\times12$ slot array fed by 36-way waveguide power divider, which possesses the characteristics of equal amplitude and in-phase distribution to obtain high efficiency. The desired 1 bit PRTA uses 16 $\times16$ array of C-shaped probe-fed patch elements to achieve flexible wide-angle beam scanning. To eliminate the mirror lobe effects of 1 bit phase quantization, the phase transforming lens with a $16\times16$ array of perforated dielectric element is designed to generate a spherical phase front from virtual focal source. By spatially cascading these three components, a low-profile RTA architecture is constituted, where the two spatial separations between adjacent components are $0.20\lambda _{\mathbf {0}}$ and $0.08\lambda _{\mathbf {0}}$ at 12.0 GHz, respectively. Finally, the proposed antenna prototype with an effective aperture of $7.68\lambda _{\mathbf {0}} \times 7.68\lambda _{\mathbf {0}}$ (192 mm $\times192$ mm) is fabricated and measured. The measured peak gain is 23.5 dBi at 12.0 GHz with aperture efficiency of 30.2% and a 3 dB gain bandwidth of 1.1 GHz. The measured results demonstrate that the proposed design has the 2-D wide-angle beam scanning capability up to 60°. [ABSTRACT FROM AUTHOR]

A design method of programmable transmitarray antenna with independent controls of transmission amplitude and phase is proposed in C-band. The unit cell with cascaded structures mainly consists of four parts, including the receiving antenna, reconfigurable attenuator with p-i-n diodes, reconfigurable phase shifter with varactors, and transmitting antenna. Correspondingly, various manipulations of spatial electromagnetic (EM) fields are achieved by varying the bias voltages of p-i-n diodes and varactors in the transmitarray antenna. The fabricated unit cell is measured in a standard waveguide, and the whole array with $8\times 8$ unit cells is measured with two horns for calibration of the programmable EM features. The experimental results show that the transmission amplitude can range from −16 to −3.6 dB, and the transmission phase achieves 270° coverage independently at the operating frequency of 5.67 GHz under 16 bit programmable control. To further exhibit the capability and functionality of the proposed transmitarray antenna, the waveform engineering of adaptive beamforming and power-allocation beamforming is separately realized in the experiment. The measured results have good agreements with our theoretical calculations, verifying the validity of our design method. [ABSTRACT FROM AUTHOR]

Passive coherent location (PCL) systems using commercial FM radio broadcast stations as illuminators of opportunity (FM-PCL) require an antenna system with a null in its radiation pattern for suppressing the direct path interference (DPI) from the selected FM broadcast station. To maximize the surveillance coverage of the FM-PCL, ideally, the antenna should have a cardioid radiation pattern, which can be obtained by the ideal couplet of isotropic radiators. We carry out a theoretical and experimental analysis to show that such a cardioid pattern cannot be realized by a couplet of cylindrical wire antennas. Therefore, we introduce an alternative array configuration and at the same time seek a simple and low-cost antenna solution for FM-PCL. The proposed antenna is a two-element array of UWB wire bowties, which has an ultrawide bandwidth, a stable main beam, a large beamwidth, and a fixed array setup within the FM-PCL frequency range (88–108 MHz), leading to a simple antenna implementation. At the same time, as it does not require expensive RF and mechanical parts, a low-cost antenna solution for FM-PCL is obtained. Hence, the proposed antenna satisfactorily meets the goal of this work, i.e., simple and low-cost antenna solution for the FM-PCL passive radar. [ABSTRACT FROM AUTHOR]

The magnetic induction (MI) technique enables wireless communications in extreme environments. However, the existing MI systems rely on large antennas and high power, which is not suitable for Internet of Things (IoT) and other mobile devices. Although the new metamaterial-enhanced MI ($\text{M}^{2}\text{I}$) technique can theoretically enhance the MI signal by 30 dB, none of the existing real-world systems achieve the performance due to: 1) the inherent metamaterial loss is significant; 2) the complicated metamaterial structure is not optimized; and 3) an $\text{M}^{2}\text{I}$ system with fixed parameters cannot keep the optimal performance in dynamic environments. In this article, an active and reconfigurable meta-sphere is designed, optimized, and implemented to approach the theoretical $\text{M}^{2}\text{I}$ enhancement bound. Specifically, the new $\text{M}^{2}\text{I}$ meta-sphere is realized by a spherical array of active and reconfigurable coil units. Given fixed meta-sphere size and power constraints, there exist complicated trade-offs among coil number, coil size, and the power consumption. Based on the theoretical design, the prototype of the active and reconfigurable $\text{M}^{2}\text{I}$ meta-sphere system is implemented and tested. Significant power gain and range extension are proved through comprehensive analytical deduction, COMSOL Multiphysics-based simulations, and real-world experiments. [ABSTRACT FROM AUTHOR]

In this communication, a tunable circularly polarized (CP) antenna with a reconfigurable integrated feed network is proposed for cognitive radio (CR) applications. The integrated feed network is composed of an ultrawideband (UWB) bandpass filter (BPF), a continuously tunable T-shaped resonator, and an L-shaped microstrip line. The feed network employs three p-i-n diodes to switch between different RF paths, resulting in three modes of operation: an UWB mode (3.5–8.5 GHz), a narrowband bandpass (BP) mode and a narrowband bandstop (BS) mode. The continuous tuning range of the BP/BS narrowband mode is realized by two dc-controlled varactor diodes. The stand-alone antenna attains a −10 dB impedance bandwidth of 92.3% (3.5–9.5 GHz), while it attains an axial ratio (AR) bandwidth of 88.3% (3.1–8 GHz) in its wideband mode. The overlapped bandwidth (matching and AR) of the reconfigurable antenna is 78.2% (3.5–8 GHz). The communicating state covers 80% of the sensing spectrum while maintaining CP radiation. The measured peak realized gain values for the wideband mode range from 2 to 3.2 dBic while the maximum gain for the narrowband mode ranged from 1.95 to 3.1 dBic. The measured results show that the proposed filtenna is suitable for UWB CR systems. [ABSTRACT FROM AUTHOR]

This work introduces a multiple-input multiple-output (MIMO) channel model to characterize propagation in a small satellite swarm environment. By using the Rician $K$ -factor, this model appropriately combines fixed line-of-sight (LoS), satellite-reflected, and earth-reflected (ER) channel components with a randomly varying diffuse component that accounts for satellite, earth, and atmospheric scattering. The proposed model is compared to ray tracing and used to elucidate the performance of (4, 4), (8, 8), and (16, 16) point-to-point MIMO systems. Indicatively, assuming 15 dBm of transmit power, a keep-out radius of $R_{KO} \geq 50$ m, and a 24 satellite swarm, (16, 16) MIMO systems achieve approximately 4–5 times the capacity of a (2, 2) polarization diversity-based system, a significant improvement for a swarm that operates as a fractionated system and autonomously makes decisions. It is also shown that a (16, 1) single-input multiple-output system has 130 times the range of a (2, 1) system, increasing proportionally the lifetime of the swarm as satellites drift apart. With an example power budget, we show that a 3U CubeSat with an orbit average power (OAP) of 12.9 W can support 16 MIMO antennas. This MIMO channel model is the first step in developing optimized communication systems for SmallSat swarms. [ABSTRACT FROM AUTHOR]

All aspects of humans connect to devices and applications on wireless networks, such as connecting to computers for online study during the COVID-19 pandemic, connecting to tablets to watch movies, and connecting to mobile phones for financial transactions. These factors have driven the development of higher-performance wireless networks. However, the number of nodes is increasing rapidly. Therefore, the network cannot support all nodes as they use channels simultaneously. Moreover, the bandwidth of wireless networks is limited to a maximum value. So it is a good idea to optimize the wireless network at the lower layer. In the physical layer, the array antennas use to transmit and receive data. The beamforming of the array antenna can form a beam at all angles and connect the nodes from the source. This antenna uses the placement of the antenna elements to create the array pattern. Therefore, this is important for array patterns. In this paper, we analyze the impact of antenna elements on wireless networks based on 802.11 DCF. The indicators are communication links, throughput, and delay. [ABSTRACT FROM AUTHOR]

WiFi energy harvesting is a promising solution for powering microsensors and microsystems through collecting electromagnetic (EM) energies that exist everywhere in modern daily lives. In order to harvest EM energy, we proposed a metamaterial-inspired antenna (MIA) based on the resonant magnetic dipole operating in the WiFi bands. The MIA consists of two metallic split-ring resonators (SRRs), separated by an FR4 dielectric layer, in the broadside coupled configuration. The incident EM waves excite surface currents in the coupled SRRs, and the energy is oscillating between them due to near-field coupling. By varying the vertical distance of the two SRRs, we may achieve impedance matching without complicated matching networks. Collected EM energy can be converted to DC voltages via a rectifier circuit at the output of the coupling coil. Measured results demonstrate that the designed MIA may resonate at 2.4 GHz with a deep-subwavelength form factor (14 mm×14 mm×1.6 mm). The WiFi energy-harvesting capability of the proposed MIA with an embedded one-stage Dickson voltage multiplier has also been evaluated. A rectified DC voltage is approximately 500 mV when the MIA is placed at a distance of 2 cm from the WiFi transmit antenna with a 9 dBm transmitting power. The proposed compact MIA in this paper is of great importance for powering future distributed microsystems. [ABSTRACT FROM AUTHOR]

5G and future generation networks require physical layer security (PLS) to reinforce the security at the higher communication layers. A novel technique of securely transmitting data in a wireless network is presented, by ensuring its security and reliability on the physical layer. Information is encoded in two domains: the spatial domain, in terms of the sequence of transmitting antennas and in the signal domain, in terms of the symbol that the antennas transmit. Hence the name antenna sequence spatial modulation (ASSM). The case of three antennas is presented, where each antenna transmits the same symbol of the two possible symbols, of the bipolar phase shift keying (BPSK) modulation, in one of the three time slots. Both the transmitter and the receiver have a mapper table relating the antenna transmitting sequence and the symbol transmitted to a code word. Using the information theoretic approach, the performance analysis of the system is presented through the bit error rate (BER) and the secrecy capacity metrics on Rayleigh fading channel. The comparison of simulation and analytical results are presented. It is inferred that the ASSM has a higher secrecy rate than the conventional spatial modulation (SM), albeit at a lower spectral efficiency. [ABSTRACT FROM AUTHOR]

This paper discusses layered baud‐space modulation system under arbitrary spatial correlation both at the transmitter and the receiver. Layered baud‐space modulation provides better reliability in spatial modulation systems as well as flexibility in data rates compared to multiple‐active space modulation. Several transmissions occur asynchronously within the time slot. As a result, fractional sampling is employed at the receiver to detect the signals. For this scheme, it is more practical to analyse spatial correlation both at the transmitter and the receiver, since temporal correlation is a likely occurrence. However, asynchronous transmission will lead to higher probability of packet drops when synchronization errors are present. Lower bounds of bit error rates are derived from theoretical criteria based on mutual information and sample covariance of the joint spatial and modulation space. Analytical results for bit error probability are tightly verified from simulations. The channel is assumed to be of frequency‐flat Rayleigh fading form. The tests are carried out under different antenna spacing, amplitude and phase modulation schemes and various number of transmit and receive antenna configurations. [ABSTRACT FROM AUTHOR]

Artificial Neural Network (ANN) approaches are applied to detect and determine the object class using a special set of the UltraWideBand (UWB) pulse Ground Penetrating Radar (GPR) sounding results. It used the results of GPR sounding with the antenna system, consisting of one radiator and four receiving antennas located around the transmitting antenna. The presence of four receiving antennas and, accordingly, the signals received from four spatially separated positions of the antennas provide a collection of signals received after reflection from an object at different angles and, due to this, to determine the location of the object in a coordinate system, connected to the antenna. We considered the sums and differences of signals received by two of the four antennas in six possible combinations: (1 and 2, 1 and 3, 2 and 3, 1 and 4, etc.). These combinations were then stacked sequentially one by one into one long signal. Synthetic signals constructed in such a way contain many more notable differences and specific information about the class to which the object belongs as well as the location of the searched object compared to the signals obtained by an antenna system with just one radiating and one receiving antenna. It therefore increases the accuracy in determining the object's coordinates and its classification. The pulse radiation, propagation, and scattering are numerically simulated by the finite difference time domain (FDTD) method. Results from the experiment on mine detection are used to examine ANN too. The set of signals from different objects having different distances from the GPR was used as a training and testing dataset for ANN. The training aims to recognize and classify the detected object as a landmine or other object and to determine its location. The influence of Gaussian noise added to the signals on noise immunity of ANN was investigated. The recognition results obtained by using an ANN ensemble are presented. The ensemble consists of fully connected and recurrent neural networks, gated recurrent units, and a long-short term memory network. The results of the recognition by all ANNs are processed by a meta network to provide a better quality of underground object classification. [ABSTRACT FROM AUTHOR]

As an efficient and interference-resistant coding technique, multiple description coding (MDC) is proposed with the aim to solve the transmission unreliability problem caused by packet errors, loss or blocking delays. The integration of MDC with non-orthogonal multiple access (NOMA) scheme (MDC-NOMA) can effectively boost the robustness and throughput of the underlying system. Additionally, space-time block coding (STBC) can further enhance the system reliability with increased diversity gain, as well as reduced decoding complexity. In this paper, we propose a novel framework referred to as MDC-NOMA-STBC, in which we apply Alamouti STBC integrated with MDC and NOMA to implement more reliable transmission. Specifically, NOMA signals are constructed by superimposing the descriptions from different users, then transmitted by the base station (BS) using Alamouti STBC. In order to substantiate the performance of the proposed framework, first, we derive closed-form expressions of both the outage probability and ergodic rate for each user in a two-antenna BS scenario. Second, for multi-antenna BS, we investigate different antenna selection strategies to further enhance the outage performance under the considered framework, whose analytical expression is provided wherever possible. Third, Monte Carlo simulation results are presented to validate our theoretical framework and to corroborate the superiority of the proposed MDC-NOMA-STBC over state-of-the-art MDC-NOMA scheme. Moreover, under realistic contexts with image transmission, it is confirmed that MDC-NOMA-STBC outperforms its counterpart MDC-NOMA in terms of the peak signal-to-noise ratio and the bit error rate. [ABSTRACT FROM AUTHOR]

A photonics-based leakage cancellation and echo signal de-chirping approach for frequency-modulated continuous-wave radar systems is proposed based on a dual-drive Mach–Zehnder modulator (DD-MZM). The de-chirp reference signal and the leakage cancellation reference signal are combined and applied to the upper arm of the DD-MZM, while the received signal, including the leakage signal and echo signals, is applied to the lower arm of the DD-MZM. When the amplitudes and delays of the leakage cancellation reference signal and the leakage signal are precisely matched, the leakage signal is canceled in the optical domain. The de-chirped signal is obtained after the leakage-free optical signal is detected in a photodetector. An experiment is performed. The cancellation depth of the leakage signal after de-chirping is around 23 dB when the center frequency and bandwidth of the linearly frequency-modulated signal are 11.5 and 2 GHz, respectively. When the leakage cancellation is not employed, the leakage will seriously affect the imaging results and distance measurement accuracy. When the leakage cancellation is enabled, the imaging results of multiple targets can be clearly distinguished, the distance measurement error of a moving target is significantly reduced to less than 10 cm, and a dynamic range increase of 21 dB is achieved. [ABSTRACT FROM AUTHOR]

This paper studies intelligent reflecting surface (IRS) aided wireless power transfer (WPT) to batteryless Internet of Everything (IoE) devices. A practical energy receiver (ER) with multiple antennas is investigated. Multiple RF energy flows gleaned by all the receive antennas are input multiple energy harvesters, which are further rectified to direct-current (DC) energy. The resultant multiple DC energy flows are then combined in the DC domain for energy storage. Three classic waveforms, namely deterministic waveform, M-QAM waveform, and Gaussian waveform, are considered for WPT. We maximize the output DC power by jointly designing the active transmit beamformer of the transmitter and the passive reflecting beamformer of the IRS with the above-mentioned waveforms, respectively, subject to the transmit power constraint at the transmitter and to the limited resolution constraints on the phase-shifters of the IRS. A low complexity alternating optimization (AO) algorithm is proposed, which converges to a Karush-Kuhn-Tucker (KKT) point and thus results in a locally optimal solution. The numerical results demonstrate that the Gaussian waveform has the best energy performance with a low input RF power to the energy harvesters. By contrast, the deterministic waveform becomes superior with a high input RF power to the energy harvesters. [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]

Recent studies have shown that low-resolution analog-to-digital-converters and digital-to-analog-converters (ADCs and DACs) can make fully-digital beamforming more power efficient than its analog or hybrid beamforming counterpart over wide-band millimeter-wave (mmWave) channels. Inspired by this, we propose a computationally efficient fully-digital beamformer relying on low-resolution ADCs/DACs for multi-user mmWave communication networks. Both a generalized (unstructured) beamformer (GB) and a structured zero-forcing beamformer (ZFB) are proposed. For maintaining fairness among all users in the network, specifically tailored objective functions are considered under sum-power constraints, namely that of maximizing the geometric mean (GM) of users’ rate and their max-min rate. These computationally challenging beamforming design problems are tackled by developing computationally efficient steep ascent algorithms, which have the radical benefit of relying on a closed-form solution at each iteration. Moreover, to facilitate the employment of low-cost amplifiers at each antenna, the GB design problem subject to the equal-gain transmission constraint is considered, which assigns equal transmit power to each transmit antenna. The proposed algorithms promise a user-rate distribution having a reduced deviation among the user-rates, i.e., improved rate-fairness. Our extensive simulation results show an approximately upto 45% reduction for the GM-rate of a 2-bit ADC (4-bin quantization) compared to the $\infty$ -resolution ADC. [ABSTRACT FROM AUTHOR]

Most of the existing works in millimeter wave (mmWave) and non-orthogonal multiple access (NOMA) communications (mmWave NOMA) employs phased array (PA) antennas which yield only angle beamspace without range dependent, range-interference suppression and “dot-shaped” array resolution. In this paper, we propose a collocated transmitter (Tx) - receiver (Rx) mmWave NOMA system where frequency diverse array (FDA) antennas are utilized. FDA employs small frequency increments across the element indices which yields angle-range dependent beamspace. Herein, an equivalent radiate beamspace is designed to serve the users. Further, to ensure user equity, we formulate objective function to max-min achievable sum rate subject to power allocation and BS equivalent FDA transmit beamspace along the users. Since the problem is not convex, a boundary-compressed Bat meta-heuristic optimization algorithm is devised to provide a sub-optimal solution by optimizing the frequency increments to synthesis the BS beamspace along the users. The achievable secrecy rate, cost efficiency (CE), economic efficiency (ECE), and secrecy energy efficiency (SEE) are derived for the users. Both theoretical analysis and simulation results indicate that the proposed scheme would constitute an excellent nominee for 5G mmWave NOMA communication applications. [ABSTRACT FROM AUTHOR]

In this paper, we propose a reference-free artificial noise (AN) waveform design and cancellation scheme for single-input-single-output (SISO) wireless secure transmission. First, a system structure with the aid of AN is presented, where we provide a specific method extracting the sign information of each symbol from the desired signal (DS) to form AN at the transmitter. Then, we notice that the legitimate receiver enable to reconstruct the AN from the received signal by almost the same method as the transmitter without AN reference, which reduces the system complexity in practice. Subsequently, the AN cancellation performance and the secrecy capacity at the receiver are analyzed, and their upper bounds are also derived. Finally, simulation results confirm that the proposed reference-free AN design and cancellation scheme can effectively suppress the AN at legitimate receiver, while degrades the signal to noise ratio (SNR) at eavesdropper seriously, which provides impressive secure performance in terms of secrecy capacity. [ABSTRACT FROM AUTHOR]

A wideband transmitarray antenna based on the magnetoelectric (ME) dipole element and 2-bit phase compensation scheme is presented for vehicular communications in this paper. A wideband 90° phase shifter is proposed and combined with the rotating method to achieve four discrete phase states. Thanks to the polarization-twisting technique and the simple structure of the phase shifter, the proposed element can be easily designed. For the transmitarray design, a multi-frequency phase compensation strategy is applied to guarantee stable performance over the operating band. To validate the design, a 12 × 12 transmitarray antenna simultaneously covering vehicle-to-satellite band (20 GHz), and 5G millimeter-wave (mmWave) bands (24 GHz/28 GHz) is designed, fabricated and measured. The experimental results show that the prototype can achieve high aperture efficiencies (>40%) within a wide bandwidth (34%). The measured peak aperture efficiency is up to 53% at 23 GHz. These results demonstrate that the proposed 2-bit transmitarray antenna enjoys the advantages of wide operating bandwidth, high aperture efficiency and comparatively low design complexity. With these characteristics, the proposed transmitarray antenna can be a promising candidate for vehicular communications that require reliable satellite links and 5G millimeter-wave connections simultaneously. [ABSTRACT FROM AUTHOR]

In this study, we present a unifying framework for future reconfigurable intelligent surface (RIS)-assisted space shift keying (SSK) systems; we additionally propose two novel transmission schemes. The key strategies surrounding this concept, namely power-sensing RIS-SSK and partitioned RIS-SSK, grant knowledge of the activated transmitter (Tx) antenna index at the RIS, which, when using SSK, allows us to adjust the reflection phases. The performance of the proposed scheme is investigated in terms of the theoretical average bit error rate (ABER), and the effect of non-ideal transceivers. The performance is then compared to that of a reference RIS-SSK scheme, and a complexity analysis is provided. The obtained results, verified by extensive computer simulations, demonstrate that the partitioned and power-sensing RIS-SSK schemes achieve a higher ABER than the reference one. Moreover, hardware impairments clearly have a critically degrading impact on the system performance for all schemes, and should be carefully taken into account in future communication systems. [ABSTRACT FROM AUTHOR]

Underlay spectrum sharing addresses spectrum scarcity but imposes constraints on the interference the secondary system causes to the primary receiver. For a secondary transmitter that is subject to the stochastic and general interference-outage constraint and the peak transmit power constraint, we present a novel joint antenna selection and power adaptation rule. It addresses the twin goals of improving the spectral efficiency and reducing the power consumed of an underlay secondary system with low hardware complexity and cost. We prove that the rule maximizes the energy-efficiency (EE) of the secondary system. Its form differs from all other rules considered in the literature. We then present an insightful geometrical characterization of the optimal power of the selected antenna in terms of the channel gains within the secondary system and between the secondary and primary systems. Our approach leads to several special cases, which are themselves novel, and novel analytical insights about the performance and structure of the optimal rule. We also present an iterative subgradient-based algorithm and a simpler non-iterative bound-based algorithm to compute the rule’s parameters. The rule achieves a markedly higher EE compared to conventional approaches, and serves as a new fundamental benchmark for antenna selection in underlay spectrum sharing. [ABSTRACT FROM AUTHOR]

In this paper, we consider a downlink non-orthogonal multiple access (NOMA) using multiple transmit antennas, where the transmit antenna selection scheme is used to reduce the system complexity. Thus, in this paper, we propose radio resource allocation and receiver selection schemes in order to improve the outage probability performance of the downlink NOMA system using the transmit antenna selection scheme. In particular, the receiver selection scheme is a method of grouping the receivers to improve the outage probability performance, and the radio resource allocation scheme is a method of allocating radio resources to each group in order to enhance the outage probability performance. In addition, through the computer simulation under the assumption of independent Rayleigh fading channels, we show that the proposed radio resource allocation and receiver selection schemes can improve the outage probability performance at the expense of the sum rate performance. [ABSTRACT FROM AUTHOR]

We propose an Omega-K algorithm that uses plane wave approximation for image formation in forward-looking imaging radar (FIRA) with the multi-input/double-output configuration. We assume that each of the transmitting antennas is located at the center of the receiving antenna array by applying a virtual antenna array. Then, we solve numerical equations in an approximation of the plane wave with the direction normal to the antenna array. Finally, we can obtain an image by proceeding with the following steps in order: the matched filtering, Stolt interpolation, two-dimensional inverse fast Fourier transform, phase compensation, image registration, and image merging. The performance of the proposed algorithm is verified through a simulation and a real experiment with neighboring targets. The results show that the proposed Omega-K algorithm with plane wave approximation can be successfully applied to FIRA systems with bistatic synthetic aperture radar configuration. [ABSTRACT FROM AUTHOR]

In this paper, we propose an interface layer using the coplanar waveguide (CPW) between an array antenna and transmitting receiver modules (TRMs) for X-band military ship radar systems. To improve transmission characteristics, the interface layer with the CPW has three transition parts: Transition part A is between the sub miniature push-on (SMP) connector and the interface layer. Transition part B is the CPW in which the thickness gradually increases. Transition part C is for connecting the interface layer and the antenna. The measured reflection and transmission coefficients of the fabricated interface layer are −22.4 dB and −0.82 dB, respectively. To verify the proposed interface layer, we then apply the layer to a 2 × 2 X-band array antenna and measure the array antenna characteristics, such as reflection coefficients, array antenna gain, and half-power beam widths (HPBWs). The measured reflection coefficient is under −10 dB from 8.6 GHz to 10.2 GHz, and the bore-sight array gain of the 2 × 2 array antenna is 10.5 dBi at 9.5 GHz. In addition, the measured HPBWs under the same conditions are 47.8° and 39.1° in the zx- and zy-planes, respectively. The results demonstrate that the proposed interface layer using the CPW is suitable for X-band radar systems. [ABSTRACT FROM AUTHOR]

With the advent of the space industry, hybrid satellite-terrestrial cooperative systems (HSTCSs) have drawn significant attention due to their prominent advantages. Besides, a novel modulation technique called spatial modulation (SM), which uses transmit antenna indices as additional information conveying source, is another technology envisioned to be used for next-generation communication systems. In this article, we propose to use SM in an HSTCS scheme consisting of a satellite, multiple terrestrial relays with multiple-input and multiple-output configuration, and a destination node equipped with multiple antennas on the ground. We derive a union bound for the symbol error probability in a fairly exact form and diversity order to shed light on the significant system parameters on the performance. Our theoretical results are validated through Monte Carlo simulations, and it is shown that using the SM technique provides a better error performance in the considered HSTCS system when compared to vertical Bell Laboratories layered space-time and space-time Alamouti codes. [ABSTRACT FROM AUTHOR]

Self-energyrecycling (sER), which allows transmit energy reutilization, has emerged as a viable option for improving the energy efficiency (EE) in low-power Internet of Things networks. In this article, we investigate its benefits also in terms of reliability improvements and compare the performance of full-duplex (FD) and half-duplex (HD) schemes when using multiantenna techniques in a communication system. We analyze the tradeoffs when considering not only the energy spent on transmission but also the circuitry power consumption, thus making the analysis of much more practical interest. In addition to the well-known spectral efficiency improvements, results show that FD also outperforms HD in terms of reliability. We show that sER introduces not only benefits in EE matters but also some modifications on how to achieve maximum reliability fairness between uplink and downlink transmissions, which is the main goal in this article. In order to achieve this objective, we propose the use of a dynamic FD scheme where the small base station determines the optimal allocation of antennas for transmission and reception. We show the significant improvement gains of this strategy for the system outage probability when compared to the simple HD and FD schemes. [ABSTRACT FROM AUTHOR]

This article studies the secondary users’ sum-throughput in an in-band full-duplex (I-FD) cognitive wireless powered backscatter communication network. The secondary network consists of a hybrid access point (H-AP) and a finite number of geographically distributed secondary backscatter sensors (SBSs), each capable of using either the conventional or backscatter communication. The secondary network shares the primary network’s spectrum using an underlay spectrum sharing model. In this model, the H-AP and SBSs operate in I-FD mode to achieve improved spectral and time efficiency. Moreover, when an SBS has little available energy, it switches to the low-energy consumption backscatter method instead of the conventional transmission method. The goal is to maximize the sum-throughput of the SBSs. It is shown that such a problem is a convex optimization problem. Closed-form expressions for the optimal allocated time and energy to SBSs are derived and solved via a low complexity and efficient algorithm called joint optimal time and energy allocation (JOTEA). Numerical results illustrate that the JOTEA algorithm achieves a higher sum-throughput than the benchmark equal time allocation method. Furthermore, if the self-interference cancellation circuit considerably cancels self-interference in the H-AP, the I-FD mode achieves a higher sum-throughput performance than that of the half-duplex mode. Moreover, using backscatter communication results in a further increase in the sum-throughput. [ABSTRACT FROM AUTHOR]

Massive multi-input multi-output (MIMO) has aroused extensive interest in communication industry, as it can effectively increase communication system capacity and reduce transmit power. For massive MIMO-orthogonal frequency division multiplexing (OFDM) systems, traditional pilot assisted channel estimation methods need to obtain the channel response of data subcarriers by interpolation. However, when the channel response of continuous data subcarriers changes dramatically, the accuracy of channel estimation will be decreased. In this article, therefore, a novel channel prediction framework that integrates the imperfect channel estimation of the massive MIMO-OFDM into the deep neural network scheme is proposed, thus replacing the interpolation method and achieving higher accuracy of channel estimation. From algorithm analysis and simulation results, the performance of proposed deep neural network channel prediction method based on the imperfect channel estimation outperforms the conventional least square method based on interpolation. Furthermore, compared with minimum mean square error, there are no need for channel autocorrelation matrix, prior statistics of noise variance, and the complex matrix inversion operations. In summary, deep neural network is an efficient method for the imperfect channel estimation in massive MIMO-OFDM systems. [ABSTRACT FROM AUTHOR]

In this article, outage probability (OP) of a joint transmit-and-receive antenna selection (JTRAS) scheme is analyzed in multiple-input–multiple-output nonorthogonal multiple-access-based downlink energy harvesting (EH) relaying networks. In this dual-hop and amplify-and-forward relaying-based network, since the first and second hops are types of single-user and multiuser systems, respectively, the optimal JTRAS and suboptimal majority-based JTRAS schemes are employed in the first and second hops. The theoretical OP analysis is carried out over Nakagami-m fading channels in the cases of perfect and imperfect successive interference cancellation. An asymptotic OP expression is also obtained at a high signal-to-noise ratio regime. Finally, Monte Carlo simulations are performed to substantiate the accuracy of the theoretical analysis. It is shown that the optimal power splitting ratios at the EH relay are different for users and the users with good channel conditions have minimum optimal ratios. [ABSTRACT FROM AUTHOR]

The nonlinear distortions attributed by radio frequency power amplifiers (PAs) are inevitable in millimeter-wave (mmWave) systems due to the high frequency and large bandwidth. This nonlinearity induces multiplicative distortion and intercarrier interference in mmWave multiple-input-multiple-output (MIMO) orthogonal frequency-division multiplexing systems, which in collusion with a frequency-selective channel and a carrier frequency offset (CFO) brings great challenges to signal detection. Furthermore, the nonlinearity destroys orthogonality of the training pilots and degrades the estimation accuracy of the channel gains and the CFO. Also, the target posterior distribution for data detection becomes analytically intractable due to the nonlinearity. In this article, we present an importance-sampling-based framework for estimating the CFO in the presence of PA impairment, which utilizes few pilots and initializes the channel estimator. We propose a particle-filter-based iterative algorithm for data detection, where the intractable posterior distribution is approximated by weighted random measures. Furthermore, a sequential-maximum-likelihood-based semiblind channel estimator in the presence of nonlinearity is proposed. Performances of the estimator and the detector are evaluated by means of normalized mean square error, mean square error, and bit error rate. To validate the efficacy of the channel and the CFO estimator, the Cramer–Rao bound (CRB) and the weighted Bayesian CRB are derived, respectively. [ABSTRACT FROM AUTHOR]

An algorithm for measuring the Muller matrix of a radar target using non-polarized probing signals is presented. Estimates are obtained for the measurement error of polarization characteristics depending on the orientation angles and ellipticity of irradiating radio signals. An algorithm for eliminating the influence of the transmitting and receiving radar antennas on the result of measuring the polarization characteristics of radar targets using a radar reflector with switchable polarization properties is described. The results of numerical calculation of the Muller matrix of dihedral corner and non-reciprocal reflectors before and after compensation for the influence of transmitting and receiving antennas on the measured polarization characteristics are presented. [ABSTRACT FROM AUTHOR]

Multiple-input-multiple-output (MIMO) can provide superior performances such as system capacity, linkage, etc. But also it will bring high RF costs and system complexity, especially in large scale MIMO systems. Antenna selection (AS) is proved to be a trade-off between good performances and complexity. Specifically, from the perspective of both transmit and receive antennas, the joint transmit and receive antenna selection (JTRAS) is employed in MIMO systems. Up to now, some algorithms of JTRAS have been studied in MIMO systems. However, most of them are mainly focused on just one aspect about accuracy or complexity. Especially, compared to numerical analysis, the implementation of swarm intelligence algorithm in JTRAS needs to be studied extensively. In the paper, three intelligent algorithms, i.e. genetic algorithm, cat swarm algorithm and particle swarm algorithm are studied and compared in terms of accuracy, cost, and complexity. In addition, fractional coding is proposed in the algorithms instead of binary integer coding. The simulation results demonstrate that all three algorithms can efficiently accomplish the antenna selection. PSO has the best accuracy and stability, but the complexity of PSO is also highest. If we take overall performances in consideration, CSO is the best choice especially in practical implementation. Moreover, fractional coding will provide better performance than binary integer coding. [ABSTRACT FROM AUTHOR]

This letter considers the joint design of transmit waveform and receive beamforming for low probability of intercept (LPI) of multiple-input multiple-output with frequency diverse array radar in cluttered environments. The goal is to maximize the output signal-to-interference-plus-noise ratio while controlling the transmit power at the target region which enhances LPI capability, subject to peak-to-average power ratio and similarity constraints. To tackle the formulated non-convex optimization problem, an alternating optimization algorithm based on the alternating direction method of multipliers is proposed. Simulation results are provided to validate the effectiveness of the developed method in terms of the objective function values as well as the transmit and receive beampatterns. [ABSTRACT FROM AUTHOR]

This letter proposes a novel transport-layer encapsulation scheme, which integrates the permutation-based transmission and the packet grouping to improve spectral efficiency. For a group of packets, each data unit (DU) is divided into three portions: permutation-conveyed DU (PCDU), grouping-conveyed DU (GCDU), and conventionally conveyed DU (CCDU). The PCDU is conveyed through the selection among various lengths for GCDU plus CCDU. The GCDU is used for forming sub-groups so that, in each sub-group, the GCDU is encapsulated into the first packet only while removed from the other packets. Unlike CCDUs, the delivery of PCDUs and removed GCDUs does not consume any communication resource. To promote the improvement of spectral efficiency, an algorithm based on the binary search tree is proposed for the GCDU design in each sub-group, and a lower bound on the number of bits in the removed GCDUs is achieved in closed-form. Illustrative numerical results substantiate the spectral efficiency gain obtained by our permutation-based scheme in comparison with the conventional transport-layer encapsulation. [ABSTRACT FROM AUTHOR]

Importance of the signal reliability has started to increase in today’s communication technologies. One of the most important methods of increasing signal reliability for communication systems is the selection of antennas at the receiver, transmitter, or both. On the other hand, having low complexity, low processing times and high performance, deep learning (DL) based technologies have started to find a place in various fields such as communication, warehouse management, health services, image processing, and so on. In this letter, we first introduce the joint transmit and receive antenna selection (JTRAS) for spatial modulation (SM) systems, which, as far as we know, has not been studied in detail before, and second, we perform the DL algorithms for JTRAS instead of its exhaustive search. [ABSTRACT FROM AUTHOR]

In this letter, a single-layer compact copolarized shorted TM1/2,0 mode microstrip patch antenna is presented for in-band full-duplex application. Two shorted TM1/2,0 mode patch antennas, used for transmission and reception purposes, make the structure compact. The wideband behavior of the patch antenna is obtained by fusing the two resonances, one because of the shorted TM1/2,0 mode radiating patch and the other because of the λ/4 line resonator. To enhance the isolation between the transmitter and receiver antenna, a novel defected ground structure is used. Slots are introduced into the ground plane to create a weak field region for the basic antenna element. The other antenna element (receiver) is located in the weak field region to enhance isolation. Further enhancement in isolation is achieved by introducing a T-shaped metallic strip into the slots. The structure is proposed with the credits of compactness, enhanced bandwidth, colinear polarization, very less fabrication complexity, low profile, and high isolation. The proposed structure is fabricated and measured. The structure experimentally exhibits a bandwidth (≤ −10 dB) of 2.42–2.52 GHz with the isolation of 25 dB, peak isolation of 36 dB, and an average realized gain of 4 dBi in the entire band. [ABSTRACT FROM AUTHOR]

The multiple-input–multiple-output (MIMO) phase of the received signal is the most important parameter in the line-of-sight MIMO (LOS MIMO) system. The MIMO phase turbulence often occurs in practical applications, and the LOS MIMO system cannot operate normally in severe cases. However, there is no practical MIMO phase prediction method presently. A tropospheric radio-duct during phase turbulence was observed using a mesoscale numerical weather prediction (NWP) model. In this study, a novel MIMO phase prediction method was proposed. In the method, the tropospheric refractivity profile was predicted using a mesoscale NWP model, and then MIMO phase was predicted by the parabolic equation model. The comparison of the predicted MIMO phases with the measured results shows good agreement. MIMO phase turbulence prediction method can provide support for LOS MIMO system performance prediction. [ABSTRACT FROM AUTHOR]

A dual-band beam-scanning antenna by rotation of two transmintarrays (TAs) operating at 20 and 30 GHz has been presented in this letter. TA's unit cell has three metal layers and demonstrates almost completely independent performance at two frequency bands. Two designed TAs have unit cells of different sizes to achieve dual-band with different linear phases. By rotating the two TAs synchronously around the axis orthogonal to the planes of the TAs, it enables two-dimensional (2-D) beam scanning with maximum scanning angles of ±40°. The achieved simulated peak gains are 19.9 dBi, 20.5 dBi and 22.5 dBi, 23.2 dBi at phi = 0°, phi = 90° at 20 and 30 GHz, respectively. The maximum beam scanning losses are 1.7 dB, 2.6 dB and 1.5 dB, 2.7 dB. Moreover, the two beams at the two frequencies can basically point to the same direction at the same time. To validate the design, a prototype is fabricated and tested. The measured results are in good agreement with the simulated results. [ABSTRACT FROM AUTHOR]

The industrial Internet of Things (IIoTs) is a key component of the fourth industrial revolution (Industry 4.0) which is faced with privacy issues as the scale and sensitivity of user and system data constantly increases. Eavesdropping attack is one of such privacy issue of the IIoT system especially when the number of transmitting antennas is increased. Thus, the focus of this article is on establishing efficient privacy in an IIoT-multiple-input–multiple-output–multiple-antenna eavesdropping communications scenario. To achieve this, a closed-form derivation for asymptotic regularized prompt privacy rate is first formulated for IIoT network system. Then, the study further examines the design of optimal jamming parameters by proposing a model referred as optimal counter-eavesdropping channel approximation technique for tackling eavesdropping attack in IIoT. The simulated performance of the proposed model clearly shows that provided that the channel coherence time is less than two times the number of transmitting nodes, a high privacy precision is achieved even without deploying any artificial noise. [ABSTRACT FROM AUTHOR]

Large antenna arrays and high-frequency bands are two key features of future wireless communication systems. The combination of large-scale antennas with high transmission frequencies often results in the communicating devices operating in the near-field (Fresnel) region. In this paper, we study the potential of beam focusing, feasible in near-field operation, in facilitating high-rate multi-user downlink multiple-input multiple-output (MIMO) systems. As the ability to achieve beam focusing is dictated by the transmit antenna, we study near-field signalling considering different antenna structures, including fully-digital architectures, hybrid phase shifter-based precoders, and the emerging dynamic metasurface antenna (DMA) architecture for massive MIMO arrays. We first provide a mathematical model to characterize near-field wireless channels as well as the transmission pattern for the considered antenna architectures. Then, we formulate the beam focusing problem for the goal of maximizing the achievable sum-rate in multi-user networks. We propose efficient solutions based on the sum-rate maximization task for fully-digital, (phase shifters based-) hybrid and DMA architectures. Simulation results show the feasibility of the proposed beam focusing scheme for both single- and multi-user scenarios. In particular, the designed focused beams provide a new degree of freedom to mitigate interference in both angle and distance domains, which is not achievable using conventional far-field beam steering, allowing reliable communications for uses even residing at the same angular direction. [ABSTRACT FROM AUTHOR]

As a widely used localization and sensing technique, radars will play an important role in future wireless networks. However, the wireless channels between the radar and the targets are passively adopted by traditional radars, which limits the performance of target detection. To address this issue, we propose to use the reconfigurable intelligent surface (RIS) to improve the detection accuracy of radar systems due to its capability to customize channel conditions by adjusting its phase shifts, which is referred to as MetaRadar. In such a system, it is challenging to jointly optimize both radar waveforms and RIS phase shifts in order to improve the multi-target detection performance. To tackle this challenge, we design a waveform and phase shift optimization (WPSO) algorithm to effectively solve the multi-target detection problem, and also analyze the performance of the proposed MetaRadar scheme theoretically. Simulation results show that the detection performance of the MetaRadar scheme is significantly better than that of the traditional radar schemes. [ABSTRACT FROM AUTHOR]

End-to-end autoencoder (AE) learning has the potential of exceeding the performance of human-engineered transceivers and encoding schemes, without a priori knowledge of communication-theoretic principles. In this work, we aim to understand to what extent and for which scenarios this claim holds true when comparing with fair benchmarks. Our particular focus is on memoryless multiple-input multiple-output (MIMO) and multi-user (MU) systems. Four case studies are considered: two point-to-point (closed-loop and open-loop MIMO) and two MU scenarios (MIMO broadcast and interference channels). For the point-to-point scenarios, we explain some of the performance gains observed in prior work through the selection of improved baseline schemes that include geometric shaping as well as bit and power allocation. For the MIMO broadcast channel, we demonstrate the feasibility of a novel AE method with centralized learning and decentralized execution. Interestingly, the learned scheme performs close to nonlinear vector-perturbation precoding and significantly outperforms conventional zero-forcing. Lastly, we highlight potential pitfalls when interpreting learned communication schemes. In particular, we show that the AE for the considered interference channel learns to avoid interference, albeit in a rotated reference frame. After de-rotating the learned signal constellation of each user, the resulting scheme corresponds to conventional time sharing with geometric shaping. [ABSTRACT FROM AUTHOR]

The use of low-resolution digital-to-analog and analog-to-digital converters (DACs and ADCs) significantly benefits energy efficiency (EE) at the cost of high quantization noise for massive multiple-input multiple-output (MIMO) systems. This paper considers a precoding optimization problem for maximizing EE in quantized downlink massive MIMO systems. To this end, we jointly optimize an active antenna set, precoding vectors, and allocated power; yet acquiring such joint optimal solution is challenging. To resolve this challenge, we decompose the problem into precoding direction and power optimization problems. For precoding direction, we characterize the first-order optimality condition, which entails the effects of quantization distortion and antenna selection. We cast the derived condition as a functional eigenvalue problem, wherein finding the principal eigenvector attains the best local optimal point. To this end, we propose generalized power iteration based algorithm. To optimize precoding power for given precoding direction, we adopt a gradient descent algorithm for the EE maximization. Alternating these two methods, our algorithm identifies a joint solution of the active antenna set, the precoding direction, and allocated power. In simulations, the proposed methods provide considerable performance gains. Our results suggest that a few-bit DACs are sufficient for achieving high EE in massive MIMO systems. [ABSTRACT FROM AUTHOR]