711 research outputs found
Dealing with Interference in Distributed Large-scale MIMO Systems: A Statistical Approach
This paper considers the problem of interference control through the use of
second-order statistics in massive MIMO multi-cell networks. We consider both
the cases of co-located massive arrays and large-scale distributed antenna
settings. We are interested in characterizing the low-rankness of users'
channel covariance matrices, as such a property can be exploited towards
improved channel estimation (so-called pilot decontamination) as well as
interference rejection via spatial filtering. In previous work, it was shown
that massive MIMO channel covariance matrices exhibit a useful finite rank
property that can be modeled via the angular spread of multipath at a MIMO
uniform linear array. This paper extends this result to more general settings
including certain non-uniform arrays, and more surprisingly, to two dimensional
distributed large scale arrays. In particular our model exhibits the dependence
of the signal subspace's richness on the scattering radius around the user
terminal, through a closed form expression. The applications of the
low-rankness covariance property to channel estimation's denoising and
low-complexity interference filtering are highlighted.Comment: 12 pages, 11 figures, to appear in IEEE Journal of Selected Topics in
Signal Processin
How to Differentiate between Near Field and Far Field: Revisiting the Rayleigh Distance
Future wireless communication systems are likely to adopt extremely large
aperture arrays and millimeter-wave/sub-THz frequency bands to achieve higher
throughput, lower latency, and higher energy efficiency. Conventional wireless
systems predominantly operate in the far field (FF) of the radiation source of
signals. As the array size increases and the carrier wavelength shrinks,
however, the near field (NF) becomes non-negligible. Since the NF and FF differ
in many aspects, it is essential to distinguish their corresponding regions. In
this article, we first provide a comprehensive overview of the existing NF-FF
boundaries, then introduce a novel NF-FF demarcation method based on effective
degrees of freedom (EDoF) of the channel. Since EDoF is intimately related to
spectral efficiency, the EDoF-based border is able to characterize key channel
performance more accurately, as compared with the classic Rayleigh distance.
Furthermore, we analyze the main features of the EDoF-based NF-FF boundary and
provide insights into wireless system design
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
A Tutorial on Extremely Large-Scale MIMO for 6G: Fundamentals, Signal Processing, and Applications
Extremely large-scale multiple-input-multiple-output (XL-MIMO), which offers
vast spatial degrees of freedom, has emerged as a potentially pivotal enabling
technology for the sixth generation (6G) of wireless mobile networks. With its
growing significance, both opportunities and challenges are concurrently
manifesting. This paper presents a comprehensive survey of research on XL-MIMO
wireless systems. In particular, we introduce four XL-MIMO hardware
architectures: uniform linear array (ULA)-based XL-MIMO, uniform planar array
(UPA)-based XL-MIMO utilizing either patch antennas or point antennas, and
continuous aperture (CAP)-based XL-MIMO. We comprehensively analyze and discuss
their characteristics and interrelationships. Following this, we examine exact
and approximate near-field channel models for XL-MIMO. Given the distinct
electromagnetic properties of near-field communications, we present a range of
channel models to demonstrate the benefits of XL-MIMO. We further motivate and
discuss low-complexity signal processing schemes to promote the practical
implementation of XL-MIMO. Furthermore, we explore the interplay between
XL-MIMO and other emergent 6G technologies. Finally, we outline several
compelling research directions for future XL-MIMO wireless communication
systems.Comment: 38 pages, 10 figure
Near-Field Communications: A Comprehensive Survey
Multiple-antenna technologies are evolving towards large-scale aperture
sizes, extremely high frequencies, and innovative antenna types. This evolution
is giving rise to the emergence of near-field communications (NFC) in future
wireless systems. Considerable attention has been directed towards this
cutting-edge technology due to its potential to enhance the capacity of
wireless networks by introducing increased spatial degrees of freedom (DoFs) in
the range domain. Within this context, a comprehensive review of the state of
the art on NFC is presented, with a specific focus on its 1) fundamental
operating principles, 2) channel modeling, 3) performance analysis, 4) signal
processing, and 5) integration with other emerging technologies. Specifically,
1) the basic principles of NFC are characterized from both physics and
communications perspectives, unveiling its unique properties in contrast to
far-field communications. 2) Based on these principles, deterministic and
stochastic near-field channel models are investigated for spatially-discrete
(SPD) and continuous-aperture (CAP) antenna arrays. 3) Rooted in these models,
existing contributions on near-field performance analysis are reviewed in terms
of DoFs/effective DoFs (EDoFs), power scaling law, and transmission rate. 4)
Existing signal processing techniques for NFC are systematically surveyed,
encompassing channel estimation, beamforming design, and low-complexity beam
training. 5) Major issues and research opportunities associated with the
integration of NFC and other emerging technologies are identified to facilitate
NFC applications in next-generation networks. Promising directions are
highlighted throughout the paper to inspire future research endeavors in the
realm of NFC.Comment: 56 pages, 23figures; submit for possible journa
Achieving Large Multiplexing Gain in Distributed Antenna Systems via Cooperation with pCell Technology
In this paper we present pCellTM technology, the first commercial-grade
wireless system that employs cooperation between distributed transceiver
stations to create concurrent data links to multiple users in the same
spectrum. First we analyze the per-user signal-to-interference-plus-noise ratio
(SINR) employing a geometrical spatial channel model to define volumes in space
of coherent signal around user antennas (or personal cells, i.e., pCells). Then
we describe the system architecture consisting of a general-purpose-processor
(GPP) based software-defined radio (SDR) wireless platform implementing a
real-time LTE protocol stack to communicate with off-the-shelf LTE devices.
Finally we present experimental results demonstrating up to 16 concurrent
spatial channels for an aggregate average spectral efficiency of 59.3 bps/Hz in
the downlink and 27.5 bps/Hz in the uplink, providing data rates of 200 Mbps
downlink and 25 Mbps uplink in 5 MHz of TDD spectrum.Comment: IEEE Asilomar Conference on Signals, Systems, and Computers, Nov.
8-11th 2015, Pacific Grove, CA, US
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