Massive MIMO is a reality—What is next?: Five promising research directions for antenna arrays

Massive MIMO (multiple-input multiple-output) is no longer a "wild" or "promising" concept for future cellular networks - in 2018 it became a reality. Base stations (BSs) with 64 fully digital transceiver chains were commercially deployed in several countries, the key ingredients of Massive MIMO have made it into the 5G standard, the signal processing methods required to achieve unprecedented spectral efficiency have been developed, and the limitation due to pilot contamination has been resolved. Even the development of fully digital Massive MIMO arrays for mmWave frequencies - once viewed prohibitively complicated and costly - is well underway. In a few years, Massive MIMO with fully digital transceivers will be a mainstream feature at both sub-6 GHz and mmWave frequencies. In this paper, we explain how the first chapter of the Massive MIMO research saga has come to an end, while the story has just begun. The coming wide-scale deployment of BSs with massive antenna arrays opens the door to a brand new world where spatial processing capabilities are omnipresent. In addition to mobile broadband services, the antennas can be used for other communication applications, such as low-power machine-type or ultra-reliable communications, as well as non-communication applications such as radar, sensing and positioning. We outline five new Massive MIMO related research directions: Extremely large aperture arrays, Holographic Massive MIMO, Six-dimensional positioning, Large-scale MIMO radar, and Intelligent Massive MIMO.

Massive MIMO is a Reality -- What is Next? Five Promising Research Directions for Antenna Arrays

A fully textile microstrip topology with ultra wideband (UWB) characteristics useful in wireless body area networks (WBAN) is proposed. The antenna is operable within the full UWB band and incorporates a full textile shielding ground plane. The full ground plane is shown to be crucial in maintaining the performance when worn on-body. It successfully reduces any on-body performance degradation, resulting in a very robust structure. A detailed numerical and experimental evaluation of the antenna is performed in free space and on body.

UWB All-Textile Antenna With Full Ground Plane for Off-Body WBAN Communications

This letter presents an Exponential Slot Edge Antipodal Vivaldi Antenna (ESE-AVA), with improved radiative features as compared to the conventional Antipodal Vivaldi Antenna (AVA) design. It extends the low-end bandwidth limitation, mitigates the side and back lobe levels, corrects squint effect, and increases its main lobe gain. In order to confirm those features, a comparative study among the ESE-AVA, the low directivity conventional AVA and two popular modifications, regular slot edge (RSE) and the tapered slot edge (TSE) AVA is performed. A comparison between AVA and the proposed ESE-AVA at 6 GHz shows an improved gain of 8.3 dB, $-$ 15.5 dB of Side Lobe Level (SLL), and 0 degrees of main lobe squint (MLS), in contrast with 5 dB of gain, $ - 5~\hbox{dB}$ of SLL, and 5 degrees of MLS in the conventional AVA. By comparing the ESE-AVA with RSE-AVA and TSE-AVA, it was observed that its notches in exponential shape, similar to open the main radiator, besides mitigating the SLL also directs the E-fields distributions towards the main lobe. It reflects into a main lobe gain improvement.

A Palm Tree Antipodal Vivaldi Antenna With Exponential Slot Edge for Improved Radiation Pattern

This communication presents a new approach to implement planar antipodal Vivaldi antenna design. A nature fern inspired fractal leaf structure is implemented here. Impedance bandwidth (−10 dB) of the proposed antenna is around 19.7 GHz starting from 1.3 to 20 GHz. The lower operating frequency of this antenna is reduced by 19% with the second iteration as compared to the first iteration of fractal leaf structure. The prototype antenna is fabricated and tested in frequency as well as in time domains to obtain various transfer characteristics along with common antenna parameters. Experimental results show that good wideband feature, stable radiation pattern, and promising group delay of less than 1 ns signatures are obtained, which agree well with the simulated data. The miniaturized proposed antenna structure becomes an attractive choice in microwave imaging applications because of its ultrawide fractional bandwidth at 175%, high directive gain of 10 dBi, and finally appreciably large fidelity factor above (>90%).

A Fern Fractal Leaf Inspired Wideband Antipodal Vivaldi Antenna for Microwave Imaging System

A novel way capable of improving low-frequency performance of Vivaldi antennas is presented in this paper. Traditional Vivaldi antennas are modified via introducing the loading structure, i.e., circular-shape-load or slot-load, to match the termination. This modified antenna has been demonstrated to have the impedance bandwidth of over 25:1. It also exhibits symmetric radiation patterns in both the E- and H-plane in addition to the gain varying from 3 to 12 dBi in the measurement bandwidth of 4-50 GHz.

Modified Compact Antipodal Vivaldi Antenna for 4–50-GHz UWB Application

A novel optically driven ultrawideband phased array is presented. In the array, two coherent optical signals mixing at photodiodes produce RF signals that are tunable over an ultra-wide bandwidth. An optical interleaving feed network is used to drive the array, which is capable of mitigating phase drift due to acoustic vibration and maintaining phase fidelity. In addition, excess noises in the system are analyzed and found to be responsible for spurious beams. The excess noise can be suppressed by optimizing polarization states of optical signals in polarization-maintaining (PM) fibers. Accordingly, the sensitivity can be significantly increased. In the end, the concept is proven by demonstrating experimental results from a 1 × 2 array consisting of two wideband Vivaldi antennas.

Optically Driven Ultrawideband Phased Array With an Optical Interleaving Feed Network

In this paper we present a wideband optically addressed transmitting phased array antenna, consisting of an ultra-wideband tunable RF synthesizer, an optical phase-feed network, and a high-speed photodetector-integrated conformal array antenna. RF source generation and the optical feed network are capable of covering a frequency range of up to 50 GHz. A conformal 4 × 4 Ka-band patch array antenna is designed, fabricated and characterized. The array is designed to sweep over ±40
 $^{o}$
 from the broadside. An optically based detector is implemented to measure the far-field of the transmitting phased array. Embedded electronic control is developed for the RF synthesizer, optical feed network control, and photonic receiver. This system is consolidated into a portable cart for field testing. Extensive studies of the subsystems have been performed. Electrically controlled optical beam steering is demonstrated and characterized, and far-fields are measured, and compared with simulation results, showing excellent agreement.

Conformal Wideband Optically Addressed Transmitting Phased Array With Photonic Receiver

An optical feed network for a phased-array antenna may comprise a phase-based feed network that may include electro-optical phase shifting.

Optical feed network for phased array antennas

An optically addressed phase feed network for an ultra-wideband RF phased array is proposed. The architecture leverages recent advances in the development of optical RF source generation and coherent optical system in the fiber optics. By using polarization maintaining (PM) fibers two coherent optical tones can be encoded in two orthogonal states and transmitted to a high-speed photomixer at the antenna head for RF signal transmission. The copropagation in the same fiber enables two optical signals experience same environmental impact, thereby significantly minimizing phase fluctuation. To phase the antenna array, electro-optic z-cut LiNbO3 phase modulators are used, in which phase modulation is dominated at one of the polarizations, consequently leading to electrically addressed phase shift at the mixer. In addition, dynamic polarization control is also investigated to compensate the polarization fluctuation due to bending, thermal and mechanical stresses. A 1×2 array is implemented to demonstrate the feasibility and robustness of the proposed system.

Jian Bai

Papers

Modified Compact Antipodal Vivaldi Antenna for 4–50-GHz UWB Application

Optically Driven Ultrawideband Phased Array With an Optical Interleaving Feed Network

Conformal Wideband Optically Addressed Transmitting Phased Array With Photonic Receiver

Optical feed network for phased array antennas

Optical phase feed network and ultra-wideband phased array