Direction of arrival (DOA) estimation is an antenna array signal processing technique used in, for instance, radar and sonar systems, source localization, and channel state information retrieval. As new applications and use cases appear with the development of next generation mobile communications systems, DOA estimation performance must be continually increased in order to support the nonstop growing demand for wireless technologies. In previous works, we verified that a deep neural network (DNN) trained offline is a strong candidate tool with the promise of achieving great on-grid DOA estimation performance, even compared to traditional algorithms. In this paper, we propose new techniques for further DOA estimation accuracy enhancement incorporating signal-to-noise ratio (SNR) prediction and an end-to-end DOA estimation system, which consists of three components: source number estimator, DOA angular spectrum grid estimator, and DOA detector. Here, we expand the performance of the DOA detector and angular spectrum estimator, and present a new solution for source number estimation based on DNN with very simple design. The proposed DNN system applied with said enhancement techniques has shown great estimation performance regarding the success rate metric for the case of two radio wave sources although not fully satisfactory results are obtained for the case of three sources.

High gain antennas with narrow-beamforming are required to compensate for the high propagation loss expected in high frequency bands such as the millimeter wave and sub-terahertz wave bands, which are promising for achieving extremely high speeds and capacity. However using narrow-beamforming for initial access (IA) beam search in all directions incurs an excessive overhead. Using wide-beamforming can reduce the overhead for IA but it also shrinks the coverage area due to the lower beamforming gain. Here, it is assumed that there are some situations in which the required coverage distance differs depending on the direction from the antenna. For example, the distance to an floor for a ceiling-mounted antenna varies depending on the direction, and the distance to the obstruction becomes the required coverage distance for an antenna installation design that assumes line-of-sight. In this paper, we propose a novel IA beam search scheme with adaptive beam width control based on the distance to shield obstacles in each direction. Simulations and experiments show that the proposed method reduces the overhead by 20%-50% without shrinking the coverage area in shield environments compared to exhaustive beam search with narrow-beamforming.

In this paper, we propose a novel radial line planar phased array in which helical antenna elements are individually rotated by their respective connected micromotors to realize dynamic beam-scanning. To our knowledge, this is the first radial line planar array (RLPA) that has antenna elements electromechanically rotated by their individual micromotors. To facilitate its fabrication, helix and its probe are directly metallized on a plastic shaft using molded interconnect device technology, and a motor shaft is press-fitted into the plastic shaft. We also present a new design methodology for RLPA, which combines the equivalent circuit theory and electromagnetic simulations of the unit cell element. The proposed procedure is practical to design an RLPA of antenna elements with arbitrary probe shape without large-scale full-wave analysis of the whole structure of the RLPA. We design, fabricate, and evaluate a 7-circle array with 168 helical antenna elements fabricated using molded interconnect device technology. The prototype antenna exhibits dynamic and accurate beam-scanning performance. Furthermore, the prototype antenna exhibits a low reflection coefficient (less than -17dB) and high antenna efficiency (above 77%), which validates the proposed design methodology.

We propose two simple weight calculation methods (primary method and enhanced method), that estimate approximated phase plane from a few antenna phase and calculate weights of all antenna elements, for wireless backhaul systems that utilize millimeter wave band massive antenna arrays. Such systems are expected to be used instead of optical fiber for connecting many small cell base stations (SCBSs) to the core network, and supporting the rapid deployment of SCBSs. However, beamforming with massive antenna arrays requires many analog-digital converters (ADCs) and incurs the issue of implementation complexity. The proposed methods overcome the problem by reducing the number of ADCs. Computer simulations clarify the appropriate layout and the number of ADCs connected to antenna elements; the effectiveness of the proposed methods is confirmed by evaluations with measured channel state information (CSI) in propagation experiments on a wireless backhaul system. Experimental verifications on the case of calculating the weight of 200 elements from the phases of just 9 elements show that the array gain degradation from ideal (the case in which the phases of all elements are used estimation) with both methods is less than 0.4 dB in the direct wave dominant environment. In addition, the enhanced method holds the array gain degradation to under 0.8dB in an environment existing reflected waves. These results show that the proposed methods can attain high accuracy beamforming while reducing ADC number.

This paper proposes a dual linear-polarized open-ended waveguide subarray designed for use in phased array antennas. The proposed subarray is a one-dimensional linear array that consists of open-ended waveguide antenna elements and suspended stripline feed networks to realize vertical and horizontal polarizations. The antenna includes a novel suspended stripline-to-waveguide transition that combines double- and quad-ridge waveguides to minimize the size of the transition and enhance the port isolation. Metal posts are installed on the waveguide apertures to eliminate scan-blindness. Prototype subarrays are fabricated and tested in an array of 16 subarrays. The experimental tests and numerical simulations indicate that the prototype subarray offers a low reflection coefficient of less than -11.4dB, low cross-polarization of less than -26dB, and antenna efficiency above 69% in the frequency bandwidth of 14%.

The sidelobe level at tilts around 30-40 degrees in both the E and H planes due to a tapered excitation of units of 2×2 radiation slots is suppressed by introducing slit layers over a corporate-feed waveguide slot array antenna. The slit layers act as averaging the excitation of the adjacent radiating slots for sidelobe suppression in both planes. A 16×16-element array in the 70GHz band is fabricated. At the design frequency, the sidelobe levels at tilts around 30-40 degrees are suppressed from -25.4dB to -31.3dB in the E-plane and from -27.1dB to -38.9dB in the H-plane simultaneously as confirmed by measurements. They are suppressed over the desired range of 71.0-76.0GHz frequencies, compared to the conventional antenna.

An analytic expression of the Capon spectrum is derived for two uncorrelated incident signals. On the basis of this theoretical formulation, we discuss the effect of a factor arising from the inner product of mode vectors with respect to the incident angles, which compromises the resolution. We show numerical examples to demonstrate the effect that the inner product of mode vectors has on the shape of the Capon spectrum.

This study discusses an area-averaged incident power density to estimate surface temperature elevation from patch antenna arrays with 4 and 9 elements at the frequencies above 10 GHz. We computationally demonstrate that a smaller averaging area (1 cm2) of power density should be considered at the frequency of 30 GHz or higher compared with that at lower frequencies (4 cm2).

The combination of large-scale antenna arrays and simultaneous wireless information and power transfer (SWIPT), which can provide enormous increase of throughput and energy efficiency is a promising key in next generation wireless system (5G). This paper investigates efficient transceiver design to minimize transmit power, subject to users' required data rates and energy harvesting, in large-scale SWIPT system where the base station utilizes a very large number of antennas for transmitting both data and energy to multiple users equipped with time-switching (TS) or power-splitting (PS) receive structures. We first propose the well-known semidefinite relaxation (SDR) and Gaussian randomization techniques to solve the minimum transmit power problems. However, for these large-scale SWIPT problems, the proposed scheme, which is based on conventional SDR method, is not suitable due to its excessive computation costs, and a consensus alternating direction method of multipliers (ADMM) cannot be directly applied to the case that TS or PS ratios are involved in the optimization problem. Therefore, in the second solution, our first step is to optimize the variables of TS or PS ratios, and to achieve simplified problems. After then, we propose fast algorithms for solving these problems, where the outer loop of sequential parametric convex approximation (SPCA) is combined with the inner loop of ADMM. Numerical simulations show the fast convergence and superiority of the proposed solutions.

A link adaptation scheme is devised for vector-perturbation (VP) zero-forcing beamforming (ZFBF) MIMO precoding and a link-adaptable VP-ZFBF precoder is applied to multi-point three-dimensional (3D) beamformers to be used in mmWave-band wireless access systems. Channel coding schemes used in current systems, e.g., turbo codes, possess systematic code structures. The VP gain can thus be predicted by searching for perturbation vectors for the symbol vectors mapped from information bits. On the basis of this principle, we constructed an efficient iterative modulation-and-coding-set (MCS) selection procedure for VP-ZFBF precoding. Simulation results demonstrate that our proposed scheme suitably passed on the VP gain to the selection of an appropriate higher-rate MCS index and thus achieved high throughputs by incorporating with multi-point 3D-beamformers.

This paper deals with a beamforming and cell ID detection technique for a mobile station (MS) with multiple antenna arrays in millimeter wave (mm-wave) cellular communication systems. Multiple antenna arrays, required to cover the entire space around the MS, can be used to estimate the direction of arrivals (DoAs) and cell IDs, form beams in the direction of DoAs, select a serving cell in a cooperative manner, and improve BER performance by signal combining. However, a signal may enter the overlapped region formed by two adjacent arrays in the MS, resulting in a double-counting problem during the cell searching period. In this paper, a beamforming and cell detection technique without double-counting is proposed to handle this problem, and they are evaluated by simulation in a simple scenario of an mm-wave cellular system with spatial channel model (SCM).

This paper presents the design and radiation properties of a linearly polarized radial line microstrip antenna array (RL-MSAA) with U-slot circular microstrip antennas. A circular microstrip antenna (C-MSA) with U-shaped slot is used as a radiation element of the RL-MSAA. Radiation phase of the U-slot C-MSA is controlled by tuning the radius of the C-MSA and dimensions of the U-slot on the C-MSA; therefore, the desired phase distribution of the RL-MSAA can be realized. In this paper, a linearly polarized RL-MSAA with three concentric rows of C-MSAs at a spacing of 0.65 wavelengths is designed for 12GHz operation. In order to realize uniform phase distribution, the U-slot C-MSAs are arranged for inner two rows and normal C-MSAs are arranged for the termination row. Validity of the linearly polarized RL-MSAA with the U-slot C-MSAs for radiation phase control is demonstrated by simulation and measurement.

This paper presents a method of synthesizing covariance matrix elements of array input signal for high resolution 2-D Direction-Of-Arrival (DOA) estimation via antenna (sensor) switching. Antenna array generally has the same number of array elements and receiver modules which often leads large receiver hardware cost. Two of the authors have already studied a way of antenna switching to reduce receiver cost, but it can be applied only for periodic incident signals like sinusoid. In this paper, we propose two simple methods of DOA estimation from sparse data by synthesizing covariance matrix elements of array input signal via antenna switching, which can also be applied to DOA estimation of antiperiodic incident signals. Performance of the proposed approach is evaluated in detail through some computer simulation.

Near range microwave imaging systems have broad application prospects in the field of concealed weapon detection, biomedical imaging, nondestructive testing, etc. In this paper, the technique of optimized sparse antenna array is applied to near range microwave imaging, which can greatly reduce the complexity of imaging systems. In detail, the paper establishes three-dimensional sparse array imaging geometry and corresponding echo model, where the imaging geometry is formed by arranging optimized sparse antenna array in elevation, scanning in azimuth and transmitting broadband signals in range direction; and by analyzing the characteristics of near range imaging, that is, the maximum interval of transmitting and receiving elements is limited by the range from imaging system to targets, we propose the idea of piecewise sparse line array; secondly, by analyzing the convolution principle, we develop a method of arranging piecewise sparse array which can generate the same distribution of equivalent phase centers as filled antenna array; then, the paper deduces corresponding imaging algorithm; finally, the imaging geometry and corresponding algorithm proposed in this paper are investigated and verified via numerical simulations and near range imaging experiments.

This paper presents design and radiation properties of a radial line microstrip antenna array (RL-MSAA) for linear polarization. A stacked circular microstrip antenna (C-MSA) is used as a radiation element for the RL-MSAA. Radiation phase of the stacked C-MSA is controlled by tuning radii of the lower and upper patches, therefore, the desired phase distribution of the RL-MSAA can be designed. In this paper, a linearly polarized RL-MSAA with three concentric rows of the stacked C-MSAs at a spacing of 0.65 wavelengths for uniform aperture distribution is designed and tested in 12GHz. The experimental results reveal that validity of the linearly polarized RL-MSAA with the stacked C-MSAs for radiation phase control is demonstrated.

This letter presents an effective direction of arrival (DOA) estimator that is based on the orthogonal projection (OP) technique. When an OP matrix is attained, the proposed estimator, which dispenses with spatial smoothing (SS) preprocessing, can form the maximizing orthogonality for a single snapshot. Since this technique does not need to perform eigen-decomposition while maintaining better DOA estimates, it also has real-time DOA estimation capability. Numerical results are presented to illustrate the efficiency of this method.

This paper presents a simple 3-D array configuration for high-resolution 2-D Direction-Of-Arrival (DOA) estimation. Planar array structures like Uniform Rectangular Array (URA) or Uniform Circular Array (UCA) often well estimate azimuth angle but cannot well estimate elevation angle because of short antenna aperture in elevation direction. One may put more number of array elements to improve elevation angle estimation accuracy, however it will require very large hardware and software cost. This paper presents a simple 3-D array structure for high-resolution 2-D DOA estimation only by modifying the height of some array elements in a planar array. Based on the analysis of Cramer-Rao Lower Bound (CRLB) formulation and its dependency on the height of array elements, we develop a simple 3-D array structure which improves elevation angle estimation accuracy while preserving azimuth angle estimation accuracy.

This letter realizes direction of arrival (DOA) estimation by exploiting the noise subspace based estimator. Since single subspace feature extraction fails to achieve satisfactory results under unknown noise fields, we propose a two-step subspace feature extraction technique that is effective even in these fields. When a new noise subspace is attained, the proposed estimator without prewhitening can form the maximizing orthogonality especially for unknown noise fields. Simulation results confirm the effectiveness of the proposed technique.

In this paper, a simple type of printed dipole is proposed for Multi-Input Multi-Output (MIMO) applications in cognitive radio. The antenna is composed of a transmission line and a dipole. Some examinations of key factors and optimized parameters of the antenna are presented. The measured results illustrate that the proposed antenna offers a bandwidth of over 50% for Voltage Standing Wave Ratio (VSWR) less than 2, extending from 2.4 GHz to 4.0 GHz. The antenna peak gain in E-plane and radiation patterns at different frequencies are also explored. In addition, based on the proposed antenna, we introduce two simple broadband arrays for MIMO applications in cognitive radio. One has two ports and the other has four ports. Measurement results indicate that the arrays also work in a broad bandwidth. Mutual couplings between ports in each array are kept under -10 dB at the low frequencies and under -20 dB at the high frequencies of bandwidth of the arrays. Furthermore, we utilized the antenna arrays for some MIMO experiments to estimate the channel capacity in a wide frequency range.

This paper presents a 65-nm CMOS 8-antenna array transmitter operating in 117–130-GHz range for short range and portable millimeter-wave (mm-wave) active imaging applications. Each antenna element is a new on-chip antenna located on the top metal. By using on-chip transformer, pulse output of each resistor-less mm-wave pulse generators (PG) are sent to each integrated antenna. To adjust pulse delays for the purpose of pulse beam-forming, a 7-bit digitally programmable delay circuit (DPDC) is added to each of PGs. Moreover, in order to dynamically adjust pulse delays among eight SW's outputs, we implemented on-chip jitter and relative skew measuring circuit with 20-bit digital output to achieve cumulative distribution (CDF) and probability density (PDF) functions from which DPDC's input codes are decided to align eight antenna's output pulses. Two measured radiation peaks after relative skew alignment are obtained at (θ; φ) angles of (-56; 0) and (+57; 0). Measurement results shows that beam-forming angles of the fully integrated antenna array can be adjusted by digital input codes and by the on-chip skew adjustment circuit for active imaging applications.