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.Comment: 20 pages, 9 figures, submitted to Digital Signal Processin

Polarization diversity and adaptive beamsteering for 5G reflectarrays: a review

The growing demands of advanced future communication technologies require investigating the possible enhancement in the current features of a reflectarray antenna. Its design and experimental features need a thorough investigation before a plausible transition towards millimeter wave frequencies. This paper provides a detailed review covering various fundamental and advanced design tactics for polarization diversity and beamsteering in the reflectarray antenna. The diversity in the polarization has been discussed for linear and circular polarized designs in reflectarrays. The importance of electronically tunable materials and different lumped components for adaptive beamsteering in reflectarrays has also been highlighted. Each design has been critically analyzed and possibilities of its compatibility with future 5G systems have been provided

A millimeter wave reflectarray antenna with tilted side patch elements for fifth generation communication systems

A flat surface reflectarray antenna is becoming an impending competitor for fifth generation (5G) communications among the generally known conventional antenna systems. Its narrow bandwidth and high loss performance lead to restrict its gain and effciency at millimeter wave frequencies. Additionally, high design sensitivity is also an issue at millimeter waves that can trigger the problem of imperfect fabrications. Therefore, a simple design of reflectarray patch element is required with wide reflection phase range to achieve wideband and high gain performance. Effciency of reflectarray antenna is also needed to be formulated properly to acquire polarization diversity. In this work, a new reflectarray patch element with a tilted side is recommended for a wideband dual resonance operation within 24 GHz to 28 GHz frequency range. Dual resonance of the tilted side patch element offers a reflection phase range of more than 600' and a reflection loss of 1.6 dB with a novel design. Simulated results of the patch element have been verified by the scattering parameter measurements using a waveguide simulator. Additionally, a mathematical relationship has been formulated to predict the effciency of the reflectarray antenna based on its aperture shape and feed distance. It has been found that, a circular aperture reflectarray attains 21.46% higher effciency than its equivalent square aperture reflectarray of the same feed distance. Consequently, a circular aperture reflectarray consisting of 332 variable size tilted side patch elements has been designed and tested at 26 GHz with various possible configurations. The high cross polarization issue due to the asymmetric design of the tilted side patch element has been tackled by mirroring the orientations of the elements on the surface of reflectarray. Moreover, circular ring slots with variable radius have been embedded in reflectarray ground plane for gain improvement. Experimental results show that, the slotted ground reflectarray antenna offers a 3.5 dB higher gain with 22.9% higher effciency and 3% wider bandwidth than a full grounded reflectarray antenna. A maximum of 26.1 dB gain with 41.3% effciency and 11.5% (3 GHz) bandwidth has been acquired with the slotted ground reflectarray antenna. The tilted side patch reflectarray has offered dual linear polarization when its elements are mirrored to each other and dual circular polarization when its elements are not mirrored to each other. Its main beam has been numerically steered up to +20' by a progressive phase shift of 80'. The acquired parameters of the tilted side patch reflectarray antenna fit within the requirements of the 5G communication systems