Millimeter-wave Systems Research Articles

Low-Resolution Hybrid Beamforming in Millimeter-Wave Multi-User Systems

A new hybrid amalgam of analog and baseband digital beamforming is conceived for millimeter-wave (mmWave) multi-user networks. The base station is equipped with a large-scale antenna-array, which however relies on a limited number of radio frequency chains for mitigating the severe path-loss of mmWave band transmission. Each user is also equipped with a multi-antenna array. The analog beamformer's weight-resolution is set low for the sake of power efficient implementation. The hybrid beamformer design relies on the novel optimization criterion of directly maximizing the geometric means of users' rates, which is shown to result in fair rate-distributions for the users without imposing a minimum user-rate constraint. Furthermore, new computationally efficient algorithms are developed, which are purely based on closed-form low-complexity expressions. Hence these low-complexity solutions are eminently suitable for large-scale mmWave arrays. Numerical examples are provided for demonstrating their efficiency.

Wireless Multi-Casting for Wideband Millimeter-Wave System With 1-bit DAC

Wireless multi-casting is an efficient transmission modality for downlink network control and content sharing. Conventional multi-casting amounts to finding an optimized beamforming vector under a specific performance metric and without the need of a dedicated treatment at the receiver. However, the so-called <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">one-fit-all</i> strategy is not directly applicable to mmWave systems equipped with 1-bit digital-to-analog converters (DAC), prompting us to develop a new multi-casting framework. The overarching design is built upon a vector-based instead of a scalar-based modulation, with which the multi-casting exhibits special features on both the transmitter and receiver. Specifically, we start by revealing the theoretical fundamentals for multi-casting under different 1-bit setups. Then, we design an iterative scheme to generate high-order constellations by establishing the equivalence between the codeword basis and low-order constellations. Finally, effective detection solutions are individually proposed for low-order and high-order constellations per their specific signal structures. Extensive analyses and simulations have been carried out to corroborate the decency of the proposed multi-casting strategy.

Multifrequency Vector Mm-Wave Signal Generation with No Optical Filtering Based on One Dual-Arm MZM with Phase Factor Optimization

In order to reduce the number of devices in and cost of a multifrequency vector millimeter-wave (mm-wave) signal generation system, we propose a novel scheme for multifrequency vector mm-wave signal generation based on one dual-arm Mach-Zehnder modulator (MZM) without optical filtering. This scheme utilizes the carrier suppression modulation of a single modulator, without the need for optical filtering and combines precoding technology to generate high-frequency QPSK and 16 QAM mm-wave signals. The transmitter is applied with precoding technology to make the frequency multiplying vector mm-wave signal display regular vector modulation at the receiver, and the system is optimized with a phase factor to ensure that the phase distribution of QPSK signal at the receiver is symmetrical. We demonstrated the generation of 4-Gbaud 76 GHz QPSK and 16 QAM vector mm-wave signals through simulation. The simulation results show that, at a fiber transmission distance of 12 km, when the launched optical power of the PD is greater than −20.535 dBm, the bit error ratio (BER) of the QPSK signal is lower than the hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10−3. At a fiber transmission distance of 10 km, when the launched optical power of the PD is greater than −18.637 dBm, the BER of 16 QAM signal is also lower than 3.8 × 10−3.

AgRIS: wind-adaptive wideband reconfigurable intelligent surfaces for resilient wireless agricultural networks at millimeter-wave spectrum

Wireless networks in agricultural environments are unique in many ways. Recent measurements reveal that the dynamics of crop growth impact wireless propagation channels with a long-term seasonal pattern. Additionally, short-term environmental factors, such as strong wind, result in variations in channel statistics. Next-generation agricultural fields, populated by autonomous tractors, drones, and high-throughput sensing systems, require high-throughput connectivity infrastructure, resulting in the future deployment of high-frequency networks, where they have not been deployed before. More specifically, when millimeter-wave (mmWave) communication systems, a viable candidate for 5G and 6G high-throughput solutions, are deployed for higher throughput, these issues become more prominent due to the relatively small wavelength at this frequency band. To improve coverage in the mmWave spectrum in agricultural settings, reconfigurable intelligent surfaces (RISs) are a promising solution with low energy consumption and high cost efficiency when compared to half-duplex active relays with multiple antennas. To ensure link resiliency under dynamic channel behavior, an adaptive RIS for broadband wireless agricultural networks (AgRIS) at mmWave band is designed in this work. AgRIS relies on output from a time-series model that forecasts the short-term wind speed based on measured wind data, which is readily available in most farms. The temporal correlation between link reliability and wind speed is demonstrated through extensive field experiments. Our simulation results demonstrate that AgRIS with a small footprint of 11 × 11 elements can help mitigate the adversarial effects of wind-induced signal level drop by up to 8 dB and provides high energy efficiency of 1 Gbits/joule.

Integrated beam training and environmental sensing in the mmWave prototype system

Integrated sensing and communication (ISAC) promises to be one of the major technologies in the future mobile communications. However, the lack of co-designed hardware systems and algorithms that combine both functions poses a challenge for further study of ISAC. In this case, firstly, we apply the inertial measurement unit (IMU) to access auxiliary motion information of user equipment (UE). Second, we design a 64-antenna millimeter-wave communication prototype system and estimate localization of UE and environmental features after beam measurements. Finally, the belief propagation simultaneous localization and mapping (BP SLAM) algorithm is utilized to combine IMU estimation with beam measurements to improve localization accuracy of UE and environmental features. The experiment results show that BP SLAM fused with IMU measurements can locate UE within 1 m error and virtual anchor within 2 m error in average, which increases the UE localization and environment sensing accuracy by ten times approximately.

Fast Multibeam Training for mmWave MIMO Systems With Subconnected Hybrid Beamforming Architecture

In this article, we present novel multibeam training strategies for millimeter-wave communication systems with the subconnected hybrid architecture. To begin with, we propose an iterative selecting search (ISS) algorithm that can reduce the training complexity of multibeam training by a large margin, and its central concept is exploiting the prior information of scanned beam combinations to iteratively optimize the beam of each subarray in order to determine the possible optimal beam combination. Then, we apply the concept of the ISS algorithm to optimize the original beam set in line with channel sparsity, with the goal of effectively characterizing the large original beam set with a small beam subset, resulting in a much smaller search space for the optimized beam subset than for the original beam set. Furthermore, we adopt two linear search algorithms on our optimized beam subset to develop efficient multibeam training methods. As shown in the simulation results, the ISS scheme shines in terms of low training complexity but performs poorly in spectral efficiency. In comparison to individual linear search methods, beam-subset-optimization-based linear search schemes not only have a greater spectral efficiency, but also have a very low training complexity.

Two-Stage Channel Estimation Using Convolutional Neural Networks for IRS-Assisted mmWave Systems

Intelligent reflecting surface (IRS) is expected to be an essential component of next-generation wireless communication networks due to its potential to provide similar or higher array gain with lower hardware cost and energy consumption compared with massive multiple-input--multiple-output (MIMO) technology. However, channel estimation in IRS-assisted communication systems is more challenging than that in conventional systems. In this article, we propose a two-stage channel estimation approach using deep learning in millimeter-wave (mmWave) communication systems with a hybrid passive/active IRS structure. In the first stage, the sparsity of the mmWave massive MIMO channel in the angular domain is exploited to estimate the amplitude of the sparse channel through a convolutional neural network. In this way, the indices of the nonzero entries of the sparse channel can be simultaneously obtained. In the second stage, the channel is reconstructed by solving a least squares problem with the acquired indices. The simulation results show that the proposed channel estimation scheme could achieve better performance with manageable complexity over the existing solutions.

Enabling Spherical Beam Coverage for Millimeter-Wave Stationary and Mobile Applications: A stackable patch antenna

The limited beam-scanning angles of planar phased arrays are one of the most critical challenges for millimeter-wave (mm-wave) wireless communication systems. This article proposes a new class of antenna topology, denoted as a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">stackable patch antenna</i> ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SPA</i> ), that achieves near-spherical beam-steering coverage for mm-wave applications. The proposed SPA consists of two identical rectangular patch antenna elements. By separately controlling the magnitude and phase of the excitation ports, the proposed antenna can radiate in multiple directions that are normal or parallel to the patch elements. In addition, the proposed structure has the advantage of enabling end-fire radiation when the structure is encompassed by the full metallic frame of a device. This is accomplished without any deformation or modification of the metal frame.

Passive millimeter-wave imaging system with variable focal length optical configuration

Passive millimeter-wave imaging system with variable focal length optical configuration

A Double-Layer Dual-Polarized Huygens Metasurface and Its Meta-Lens Antenna Applications.

In this paper, a dual-polarized Huygens unit is proposed, which has a double-layer metallic pattern etched on both sides of one dielectric substrate. Induced magnetism enables the structure to support Huygens' resonance, thus obtaining nearly complete available transmission phase coverage. By optimizing the structural parameters, a better transmission performance can be achieved. When the Huygens metasurface was used for the design of a meta-lens, good radiation performance was exhibited, with a maximum gain of 31.15 dBi at 28 GHz, an aperture efficiency of 42.7% and a 3 dB gain bandwidth of 26.4 GHz to 30 GHz (12.86%). Due to its excellent radiation performance and very simple fabrication, this Huygens meta-lens has important applications in millimeter-wave communication systems.

A Two-Stage Fast Beam Search for Multi-User Millimeter-Wave Massive MIMO Systems

Millimeter-wave systems deploy spatial beam search for initial access. A narrow beam search results in significant discovery delays whereas wide beams cause high user misdetection. In this work, a novel two-stage fast beam search algorithm is proposed that utilizes a beamforming architecture with an appropriate antenna spacing for subarray design. The first stage employs analog beamforming to identify the coarse beam space. The second stage based on hybrid beamforming makes use of pattern multiplication for beam space refinement. Simulation results demonstrate that the proposed scheme achieves improved performance in comparison to existing techniques in terms of significantly reduced beam search latency with a marginal increase in user misdetection.

Comparison of Terahertz Transceiver Array Topologies in Multiobject Imaging

We investigate the effect of side lobe level (SLL) and multipath propagation in millimeter-wave(mmWave) and terahertz (THz) imaging systems for various transceiver array topologies. This work considers practical limitations in high-resolution active imaging systems and focuses on electrically large, planar antenna array topologies. As such, we investigate the imaging performance of sparse transmitting/receiving (Tx/Rx) array topologies that enable high angular/spatial resolution imaging with minimum hardware complexity. We show that 1) properly tapering the sparse array excitation can mitigate image clutter caused by high side lobes and 2) sparse arrays are more vulnerable to multipath reflections and clutter in dense scenes with highly reflective surfaces. We present both full-wave numerical analysis and laboratory experiments (250-300 GHz) to verify and compare the imaging array performance for several exemplary scenes.

Beam-Tracking Technique for IRS-Assisted mmWave Cellular Systems With Uniform Planar Array

In this paper, a beam-tracking technique is proposed for millimeter-wave (mmWave) cellular systems with an intelligent reflective surface (IRS). A preamble design technique for IRS-assisted cellular systems is proposed using the complex conjugate property of the Zadoff-Chu sequence and reflecting nature of IRS. Moreover, a beam-tracking technique for mmWave cellular systems with a uniform planar array (UPA) is also proposed to track the variation in the angle of departure (AoD) using subpanel structures of the UPA. Through simulation, it is demonstrated that the beam tracking technique with the proposed preamble can successfully track 2-D AoD trajectories of the base station and IRS.

Ultrawideband RF-IQ Modulator Using Segmented Nonlinearly Scaled RF-DACs and Nonoverlapping LO Signals

A wideband Cartesian in-phase and quadrature (I/Q) modulator based on dual 10 b RF-digital-to-analog converters (DACs) is presented. Nonoverlapping local oscillator (LO) signals and a segmented nonlinear scaling with scaled unit cells contribute to a high linearity and allow for a low-complexity digital predistortion (DPD). Unit-cell flip-flops (FFs) and a balanced clock distribution enable a high sample rate with little skew. Out-of-band emissions are reduced through drive-slope control of the data-switch inputs. The modulator, implemented in the 22-nm fully-depleted silicon-on-insulator (FDSOI) CMOS, operates between 20 and 26 GHz with a peak sample rate of 11 GS/s. It has been used to demonstrate the transmission of a 64-quadrature amplitude modulation (QAM) single-carrier (SC) signal at 13.2 Gb/s, a 256-QAM SC signal at 7.33 Gb/s, and an orthogonal frequency division multiplexing (OFDM) signal comprising four aggregated 400-MHz 64-QAM channels with an error vector magnitude (EVM) of 6.43%. These results demonstrate the potential of the proposed modulator for the realization of ultrawideband transmitters in high-performance millimeter-wave (mmW) systems.

High-Speed and Low-Latency Transmission by Millimeter-Wave Digital Wireless System for Si-IGBT/SiC-MOSFET Driver Control

High-Speed and Low-Latency Transmission by Millimeter-Wave Digital Wireless System for Si-IGBT/SiC-MOSFET Driver Control

Joint Localization and Communication Study for Intelligent Reflecting Surface Aided Wireless Communication System

The intelligent reflecting surface (IRS) is promising in assisting user localization and wireless communication in the future wireless networks. In this paper, a novel IRS-aided joint localization and communication (L&C) scheme is designed in a millimeter-wave transmission system. For the proposed scheme, the user position/orientation estimation error bound (POEB) and the effective achievable data rate (EADR) are derived in closed-form as L&C performance metrics, which reveal the inherent trade-off between L&C capabilities. To achieve the joint optimal point of the POEB and EADR in consideration of the localization errors, a worst-case robust beamforming and time allocation optimization problem is formulated. To solve the original non-convex problem, a novel joint optimization approach is developed. Specifically, from an equivalent minimax problem, the local optimal solutions of the transceiver beamformers, the IRS phase-shift matrix, and the time allocation ratio between user localization stage (ULS) and effective data transmission stage (EDTS), are obtained in closed-form with respect to the localization errors. Then, the worst-case localization error is iteratively found by a dedicated majorize-minimization (MM) based algorithm. Subsequently, potential extensions to general wireless channels and discrete phase-shift models are discussed in detail. Finally, simulations are carried out to show the optimization results and the L&C performance trade-off. In comparison with the conventional non-robust method, the proposed approach is validated to be robust against the user localization uncertainty.

Three-Dimensional Active Incoherent Millimeter-Wave Imaging

A new 3-D active incoherent millimeter-wave (3-D AIM) imaging method is introduced in this article. By utilizing a sparse interferometric receive array and pulse modulation in the transmitted noise signals we demonstrate far-field incoherent imaging in three dimensions. Interferometric image formation is based on the spatio-temporal incoherence of the signals emitted by or scattered from the imaging scene. To support this, we use active noise transmission from multiple independent transmitters which provide incoherent illumination in space (azimuth and elevation) and time. Two-dimensional (azimuth-elevation) image formation in this manner only requires statistical knowledge of the waveforms, greatly simplifying the imaging system. In this work, we modulate the transmitted waveforms to impart range-dependent information that can be used to also obtain range, supporting imaging in three dimensions. We modulate a Gaussian pulse onto the noise waveforms and apply incoherent pulse integration to provide sufficient spatio-temporal decorrelation of the signals. We introduce a new 3-D Fourier image formation algorithm, provide imaging simulations, and include experimental image reconstructions from a 38 GHz millimeter-wave imaging system. Video-rate 3-D imagery of a moving target is presented to demonstrate the feasibility of the approach for real-time 3-D imaging applications.

Exploring the Validity of Plane and Spherical Millimeter-Wave Incidences for Multiple-Diffraction Calculations in Wireless Communication Systems

The focus of this work is to determine at which threshold can the results for both plane and spherical wave incidence assumptions either converge or deviate when performing multiple diffraction attenuation calculations. The analysis has been carried out—for various millimeter-wave frequencies, inter-obstacle spacings, and angles of incidence—by employing a pair of two-dimensional (2D) hybrid formulations based on both the uniform theory of diffraction and physical optics (UTD-PO). This way, we seek to demonstrate under which circumstances each wave incidence assumption can be valid in environments that entail millimeter-wave bands. Based on this, we may ensure the minimum necessary distance from the transmitter to the first diffracting obstacle for the convergence of the spherical wave incidence solution onto that of the plane wave with a relative error below 0.1%. Our results demonstrate that for less than four diffracting elements, the minimum necessary distance engages in quasi-linear behavior under variations in both the angle of incidence and obstacle spacing. Notably, the considered frequencies (60–100 GHz) have almost no bearing on the results. Our findings will facilitate the simplified, more accurate and realistic planning of millimeter-wave radio communication systems, with multiple diffractions across various obstacles.

Design of a Compact Analog Complex Correlator for Millimeter-Wave Radiation Temperature Measurement System.

Human body temperature is a fundamental physiological sign that reflects the state of physical health. It is important to achieve high-accuracy detection for non-contact human body temperature measurement. In this article, a Ka band (32 to 36 GHz) analog complex correlator using the integrated six-port chip is proposed, and a millimeter-wave thermometer system based on the designed correlator is completed for human body temperature measurement. The designed correlator utilizes the six-port technique to achieve large bandwidth and high sensitivity, and miniaturization of the correlator is achieved through an integrated six-port chip. By performing the single-frequency test and the broadband noise measurement on the correlator, we can determine that the dynamic range of input power of the correlator is -70 dBm to -35 dBm, and the correlation efficiency and equivalent bandwidth are 92.5% and 3.42 GHz, respectively. Moreover, the output of the correlator varies linearly with the input noise power, which reveals that the designed correlator is suitable for the field of human body temperature measurement. Then, a handheld thermometer system, with a size of 140 mm × 47 mm × 20 mm, is proposed using the designed correlator, and the measurement results show that the temperature sensitivity of the thermometer is less than 0.2 K.

Ka Band Low Channel Mutual Coupling Integrated Packaged Phased Array Receiver Front-End for Passive Millimeter-Wave Imaging

This paper presents a Ka band eight-channel integrated packaged phased array receiver front-end for a passive millimeter-wave imaging system. Since multiple receiving channels are integrated in a given package, the mutual coupling issue affecting the channel will deteriorate imaging quality. Therefore, in this study, the influence of channel mutual coupling on the system array pattern and amplitude phase error is analyzed, and the design requirements are proposed according to the results. During the design implementation, the coupling paths are discussed, and passive circuits in the path are modeled and designed to reduce the level of channel mutual coupling and spatial radiation. Finally, an accurate coupling measurement method for a multi-channel integrated phased array receiver is proposed. The receiver front-end achieves a 28~31 dB single channel gain, a 3.6 dB noise figure, less than −47 dB of channel mutual coupling. Furthermore, the array pattern of the two-dimensional 1024 channel system composed of the front end of the receiver is consistent with the simulation, and the receiver’s performance is verified by a human-body-imaging experiment. The proposed coupling analysis, design, and measurement methods are also applicable to other multi-channel integrated packaged devices.