230 research outputs found
Power Scaling Laws and Near-Field Behaviors of Massive MIMO and Intelligent Reflecting Surfaces
The use of large arrays might be the solution to the capacity problems in
wireless communications. The signal-to-noise ratio (SNR) grows linearly with
the number of array elements when using Massive MIMO receivers and
half-duplex relays. Moreover, intelligent reflecting surfaces (IRSs) have
recently attracted attention since these can relay signals to achieve an SNR
that grows as , which seems like a major benefit. In this paper, we use a
deterministic propagation model for a planar array of arbitrary size, to
demonstrate that the mentioned SNR behaviors, and associated power scaling
laws, only apply in the far-field. They cannot be used to study the regime
where . We derive an exact channel gain expression that captures
three essential near-field behaviors and use it to revisit the power scaling
laws. We derive new finite asymptotic SNR limits but also conclude that these
are unlikely to be approached in practice. We further prove that an IRS-aided
setup cannot achieve a higher SNR than an equal-sized Massive MIMO setup,
despite its faster SNR growth. We quantify analytically how much larger the IRS
must be to achieve the same SNR. Finally, we show that an optimized IRS does
not behave as an "anomalous" mirror but can vastly outperform that benchmark.Comment: Published in IEEE Open Journal of the Communications Society, 18
pages, 11 figures. Typo in Eq. (64) has been correcte
Holographic Communication using Intelligent Surfaces
Holographic communication is intended as an holistic way to manipulate with
unprecedented flexibility the electromagnetic field generated or sensed by an
antenna. This is of particular interest when using large antennas at high
frequency (e.g., the millimeter wave or terahertz), whose operating condition
may easily fall in the Fresnel propagation region (radiating near-field), where
the classical plane wave propagation assumption is no longer valid. This paper
analyzes the optimal communication involving large intelligent surfaces,
realized for example with metamaterials as possible enabling technology for
holographic communication. It is shown that traditional propagation models must
be revised and that, when exploiting spherical wave propagation in the Fresnel
region with large surfaces, new opportunities are opened, for example, in terms
of the number of orthogonal communication channels.Comment: Submitted to IEEE Comm. Magazin
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
Multiuser MIMO with Large Intelligent Surfaces: Communication Model and Transmit Design
This paper proposes a communication model for multiuser multiple-input
multiple-output (MIMO) systems based on large intelligent surfaces (LIS), where
the LIS is modeled as a collection of tightly packed antenna elements. The LIS
system is first represented in a circuital way, obtaining expressions for the
radiated and received powers, as well as for the coupling between the distinct
elements. Then, this circuital model is used to characterize the channel in a
line-of-sight propagation scenario, rendering the basis for the analysis and
design of MIMO systems. Due to the particular properties of LIS, the model
accounts for superdirectivity and mutual coupling effects along with near field
propagation, necessary in those situations where the array dimension becomes
very large. Finally, with the proposed model, the matched filter transmitter
and the weighted minimum mean square error precoding are derived under both
realistic constraints: limited radiated power and maximum ohmic losses.Comment: 6 pages, 3 figures; This paper is submitted to IEEE International
Conference on Communications (ICC) 202
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