Showing papers by "Thomas L. Marzetta published in 2019"

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

Abstract: 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.

Super-Directive Antenna Arrays: Fundamentals and New Perspectives

Abstract: Wireless power transfer between an array of antennas and a single-antenna terminal is governed by matrix circuit theory. A complex-valued, non-conjugate symmetric impedance matrix constitutes a complete quantitative description of the system. The real-part of the impedance matrix is non-negative definite. The efficiency with which power can be transferred-either down-link (from the antenna array to the single-antenna terminal) or up-link (from the terminal to the array) is equal to the ratio of received power to transmitted power. The maximization of power transfer efficiency with respect to the joint transmit/receive activities yields the fact that the optimized up-link efficiency is equal to the optimized down-link efficiency. The typical operation of antenna arrays seeks to minimize mutual coupling among the constituent antennas by spacing the antennas at least half of a wave-length apart, which yields an array gain proportional to the number of antennas. By utilizing closer spacing, and deliberately creating strong mutual coupling, in principle it is possible to realize considerably higher array gains for the same number of antennas, a phenomenon called super-directivity. Practical super-directivity would benefit not only wireless communications, but also wireless power transfer.

Joint Unicast and Multi-group Multicast Transmission in Massive MIMO Systems.

Abstract: We study the joint unicast and multi-group multicast transmission in massive multiple-input-multiple-output (MIMO) systems We consider a system model that accounts for channel estimation and pilot contamination, and derive achievable spectral efficiencies (SEs) for unicast and multicast user terminals (UTs), under maximum ratio transmission and zero-forcing precoding For unicast transmission, our objective is to maximize the weighted sum SE of the unicast UTs, and for the multicast transmission, our objective is to maximize the minimum SE of the multicast UTs These two objectives are coupled in a conflicting manner, due to their shared power resource Therefore, we formulate a multiobjective optimization problem (MOOP) for the two conflicting objectives We derive the Pareto boundary of the MOOP analytically As each Pareto optimal point describes a particular efficient trade-off between the two objectives of the system, we determine the values of the system parameters (uplink training powers, downlink transmission powers, etc) to achieve any desired Pareto optimal point Moreover, we prove that the Pareto region is convex, hence the system should serve the unicast and multicast UTs at the same time-frequency resource Finally, we validate our results using numerical simulations

Degrees of Freedom of Holographic MIMO Channels

Abstract: We consider spatially-constrained apertures of rectangular symmetry and aim to retrieve the limit to the average number of spatial degrees of freedom (DoF), obtained elsewhere through different analyses and tools. Unlike prior works, we use the Fourier plane-wave series expansion, recently introduced in [1], where a statistical model for the small-scale fading in the far-field is developed on the basis of a continuous-space and physics-based orthonormal expansion over the Cartesian spatial Fourier basis. This expansion yields a set of statistically independent random coefficients whose cardinality directly gives the limit to the average number of DoF. The treatment is limited to an isotropic scattering environment but can be extended to the non-isotropic case through the linear-system theoretic interpretation of plane-wave propagations.

Spatially-Stationary Model for Holographic MIMO Small-Scale Fading

Abstract: Imagine an array with a massive (possibly uncountably infinite) number of antennas in a compact space. We refer to a system of this sort as Holographic MIMO. Given the impressive properties of Massive MIMO, one might expect a holographic array to realize extreme spatial resolution, incredible energy efficiency, and unprecedented spectral efficiency. At present, however, its fundamental limits have not been conclusively established. A major challenge for the analysis and understanding of such a paradigm shift is the lack of mathematically tractable and numerically reproducible channel models that retain some semblance to the physical reality. Detailed physical models are, in general, too complex for tractable analysis. This paper aims to take a closer look at this interdisciplinary challenge. Particularly, we consider the small-scale fading in the far-field, and we model it as a zero-mean, spatially-stationary, and correlated Gaussian scalar random field. Physically-meaningful correlation is obtained by requiring that the random field be consistent with the scalar Helmholtz equation. This formulation leads directly to a rather simple and exact description of the three-dimensional small-scale fading as a Fourier plane-wave spectral representation. Suitably discretized, this leads to a discrete representation for the field as a Fourier plane-wave series expansion, from which a computationally efficient way to generate samples of the small-scale fading over spatially-constrained compact spaces is developed. The connections with the conventional tools of linear systems theory and Fourier transform are thoroughly discussed.

Spatial Characterization of Holographic MIMO Channels.

Abstract: Imagine an array with a massive (possibly uncountably infinite) number of antennas in a compact space. We refer to a system of this sort as Holographic MIMO. Given the impressive properties of Massive MIMO, one might expect a holographic array to realize extreme spatial resolution, incredible energy efficiency, and unprecedented spectral efficiency. At present, however, its fundamental limits have not been conclusively established. A major challenge for the analysis and understanding of such a paradigm shift is the lack of mathematically tractable and numerically reproducible channel models that retain some semblance to the physical reality. Detailed physical models are, in general, too complex for tractable analysis. This paper aims to take a closer look at this interdisciplinary challenge. Particularly, we consider the small-scale fading in the far-field, and we model it as a zero-mean, spatially-stationary, and correlated Gaussian scalar random field. Physically-meaningful correlation is obtained by requiring that the random field be consistent with the scalar Helmholtz equation. This formulation leads directly to a rather simple and exact description of the three-dimensional small-scale fading as a Fourier plane-wave spectral representation. Suitably discretized, this leads to a discrete representation for the field as a Fourier plane-wave series expansion, from which a computationally efficient way to generate samples of the small-scale fading over spatially-constrained compact spaces is developed. The connections with the conventional tools of linear systems theory and Fourier transform are thoroughly discussed.

A Communication Model for Large Intelligent Surfaces

Abstract: The purpose of this paper is to introduce a communication model for Large Intelligent Surfaces (LIS). A LIS is modelled as a collection of tiny closely spaced antenna elements. Due to the proximity of the elements, mutual coupling arises. An optimal transmitter design depends on the mutual coupling matrix. For single user communication, the optimal transmitter uses the inverse of the mutual coupling matrix in a filter matched to the channel vector. We give the expression of the mutual coupling for two types of planar arrays. The conditioning number of the mutual coupling matrix is unbounded as the antenna element density increases, so only the dominant values can be inverted within reasonable computation. The directivity is partial but still significant compared to the conventional gain. When the spacing between elements becomes small (smaller than half a wavelength), the directivity surpasses the conventional directivity equal to the number of antennas, as well as the gain obtained when modelling the surface as continuous. The gain is theoretically unbounded as the element density increases for a constant aperture.