445 research outputs found
Asymptotic Optimality of Massive MIMO Systems Using Densely Spaced Transmit Antennas
This paper investigates the performance of a massive multiple-input
multiple-output (MIMO) system that uses a large transmit antenna array with
antenna elements spaced densely. Under the assumption of idealized uniform
linear antenna arrays without mutual coupling, precoded quadrature phase-shift
keying (QPSK) transmission is proved to achieve the channel capacity of the
massive MIMO system when the transmit antenna separation tends to zero. This
asymptotic optimality is analogous to that of QPSK faster-than-Nyquist
signaling.Comment: submitted to ISIT 2016. arXiv admin note: text overlap with
arXiv:1601.0563
Asymptotic Optimality of Massive MIMO Systems Using Densely Spaced Transmit Antennas
This paper considers a deterministic physical model of massive multiple-input
multiple-output (MIMO) systems with uniform linear antenna arrays. It is known
that the maximum spatial degrees of freedom is achieved by spacing antenna
elements at half the carrier wavelength. The purpose of this paper is to
investigate the impacts of spacing antennas more densely than the critical
separation. The achievable rates of MIMO systems are evaluated in the
large-system limit, where the lengths of transmit and receive antenna arrays
tend to infinity with the antenna separations kept constant. The main results
are twofold: One is that, under a mild assumption of channel instances, spacing
antennas densely cannot improve the capacity of MIMO systems normalized by the
spatial degrees of freedom. The other is that the normalized achievable rate of
quadrature phase-shift keying converges to the normalized capacity achieved by
optimal Gaussian signaling, as the transmit antenna separation tends to zero
after taking the large-system limit. The latter result is based on mathematical
similarity between MIMO transmission and faster-than-Nyquist signaling in
signal space representations.Comment: submitted to IEEE Trans. Inf. Theor
Symbol-level and Multicast Precoding for Multiuser Multiantenna Downlink: A Survey, Classification and Challenges
Precoding has been conventionally considered as an effective means of
mitigating the interference and efficiently exploiting the available in the
multiantenna downlink channel, where multiple users are simultaneously served
with independent information over the same channel resources. The early works
in this area were focused on transmitting an individual information stream to
each user by constructing weighted linear combinations of symbol blocks
(codewords). However, more recent works have moved beyond this traditional view
by: i) transmitting distinct data streams to groups of users and ii) applying
precoding on a symbol-per-symbol basis. In this context, the current survey
presents a unified view and classification of precoding techniques with respect
to two main axes: i) the switching rate of the precoding weights, leading to
the classes of block- and symbol-level precoding, ii) the number of users that
each stream is addressed to, hence unicast-/multicast-/broadcast- precoding.
Furthermore, the classified techniques are compared through representative
numerical results to demonstrate their relative performance and uncover
fundamental insights. Finally, a list of open theoretical problems and
practical challenges are presented to inspire further research in this area.Comment: Submitted to IEEE Communications Surveys & Tutorial
Enabling Large Intelligent Surfaces with Compressive Sensing and Deep Learning
Employing large intelligent surfaces (LISs) is a promising solution for
improving the coverage and rate of future wireless systems. These surfaces
comprise a massive number of nearly-passive elements that interact with the
incident signals, for example by reflecting them, in a smart way that improves
the wireless system performance. Prior work focused on the design of the LIS
reflection matrices assuming full knowledge of the channels. Estimating these
channels at the LIS, however, is a key challenging problem, and is associated
with large training overhead given the massive number of LIS elements. This
paper proposes efficient solutions for these problems by leveraging tools from
compressive sensing and deep learning. First, a novel LIS architecture based on
sparse channel sensors is proposed. In this architecture, all the LIS elements
are passive except for a few elements that are active (connected to the
baseband of the LIS controller). We then develop two solutions that design the
LIS reflection matrices with negligible training overhead. In the first
approach, we leverage compressive sensing tools to construct the channels at
all the LIS elements from the channels seen only at the active elements. These
full channels can then be used to design the LIS reflection matrices with no
training overhead. In the second approach, we develop a deep learning based
solution where the LIS learns how to optimally interact with the incident
signal given the channels at the active elements, which represent the current
state of the environment and transmitter/receiver locations. We show that the
achievable rates of the proposed compressive sensing and deep learning
solutions approach the upper bound, that assumes perfect channel knowledge,
with negligible training overhead and with less than 1% of the elements being
active.Comment: Submitted to IEEE Access. The code will be available soo
Learning on a Grassmann Manifold: CSI Quantization for Massive MIMO Systems
This paper focuses on the design of beamforming codebooks that maximize the
average normalized beamforming gain for any underlying channel distribution.
While the existing techniques use statistical channel models, we utilize a
model-free data-driven approach with foundations in machine learning to
generate beamforming codebooks that adapt to the surrounding propagation
conditions. The key technical contribution lies in reducing the codebook design
problem to an unsupervised clustering problem on a Grassmann manifold where the
cluster centroids form the finite-sized beamforming codebook for the channel
state information (CSI), which can be efficiently solved using K-means
clustering. This approach is extended to develop a remarkably efficient
procedure for designing product codebooks for full-dimension (FD)
multiple-input multiple-output (MIMO) systems with uniform planar array (UPA)
antennas. Simulation results demonstrate the capability of the proposed design
criterion in learning the codebooks, reducing the codebook size and producing
noticeably higher beamforming gains compared to the existing state-of-the-art
CSI quantization techniques
RF Lens-Embedded Massive MIMO Systems: Fabrication Issues and Codebook Design
In this paper, we investigate a radio frequency (RF) lens-embedded massive
multiple-input multiple-output (MIMO) system and evaluate the system
performance of limited feedback by utilizing a technique for generating a
suitable codebook for the system. We fabricate an RF lens that operates on a 77
GHz (mmWave) band. Experimental results show a proper value of amplitude gain
and an appropriate focusing property. In addition, using a simple numerical
technique--beam propagation method (BPM)--we estimate the power profile of the
RF lens and verify its accordance with experimental results. We also design a
codebook--multi-variance codebook quantization (MVCQ)--for limited feedback by
considering the characteristics of the RF lens antenna for massive MIMO
systems. Numerical results confirm that the proposed system shows significant
performance enhancement over a conventional massive MIMO system without an RF
lens
Limited Feedback Designs for Machine-type Communications Exploiting User Cooperation
Multiuser multiple-input multiple-output (MIMO) systems are a prime candidate
for use in massive connection density in machine-type communication (MTC)
networks. One of the key challenges of MTC networks is to obtain accurate
channel state information (CSI) at the access point (AP) so that the spectral
efficiency can be improved by enabling enhanced MIMO techniques. However,
current communication mechanisms relying upon frequency division duplexing
(FDD) might not fully support an enormous number of devices due to the
rate-constrained limited feedback and the time-consuming scheduling
architectures. In this paper, we propose a user cooperation-based limited
feedback strategy to support high connection density in massive MTC networks.
In the proposed algorithm, two close-in users share the quantized version of
channel information in order to improve channel feedback accuracy. The
cooperation process is performed without any transmitter interventions (i.e.,
in a grant-free manner) to satisfy the low-latency requirement that is vital
for MTC services. Moreover, based on the sum-rate throughput analysis, we
develop an adaptive cooperation algorithm with a view to
activating/deactivating the user cooperation mode according to channel and
network conditions.Comment: 15 Pages, 9 figure
Amplitude Retrieval for Channel Estimation of MIMO Systems with One-Bit ADCs
This letter revisits the channel estimation problem for MIMO systems with
one-bit analog-to-digital converters (ADCs) through a novel
algorithm--Amplitude Retrieval (AR). Unlike the state-of-the-art methods such
as those based on one-bit compressive sensing, AR takes a different approach.
It accounts for the lost amplitudes of the one-bit quantized measurements, and
performs channel estimation and amplitude completion jointly. This way, the
direction information of the propagation paths can be estimated via accurate
direction finding algorithms in array processing, e.g., maximum likelihood. The
upsot is that AR is able to handle off-grid angles and provide more accurate
channel estimates. Simulation results are included to showcase the advantages
of AR
A Survey on MIMO Transmission with Discrete Input Signals: Technical Challenges, Advances, and Future Trends
Multiple antennas have been exploited for spatial multiplexing and diversity
transmission in a wide range of communication applications. However, most of
the advances in the design of high speed wireless multiple-input multiple
output (MIMO) systems are based on information-theoretic principles that
demonstrate how to efficiently transmit signals conforming to Gaussian
distribution. Although the Gaussian signal is capacity-achieving, signals
conforming to discrete constellations are transmitted in practical
communication systems. As a result, this paper is motivated to provide a
comprehensive overview on MIMO transmission design with discrete input signals.
We first summarize the existing fundamental results for MIMO systems with
discrete input signals. Then, focusing on the basic point-to-point MIMO
systems, we examine transmission schemes based on three most important criteria
for communication systems: the mutual information driven designs, the mean
square error driven designs, and the diversity driven designs. Particularly, a
unified framework which designs low complexity transmission schemes applicable
to massive MIMO systems in upcoming 5G wireless networks is provided in the
first time. Moreover, adaptive transmission designs which switch among these
criteria based on the channel conditions to formulate the best transmission
strategy are discussed. Then, we provide a survey of the transmission designs
with discrete input signals for multiuser MIMO scenarios, including MIMO uplink
transmission, MIMO downlink transmission, MIMO interference channel, and MIMO
wiretap channel. Additionally, we discuss the transmission designs with
discrete input signals for other systems using MIMO technology. Finally,
technical challenges which remain unresolved at the time of writing are
summarized and the future trends of transmission designs with discrete input
signals are addressed.Comment: 110 pages, 512 references, submit to Proceedings of the IEE
Spatial Self-Interference Isolation for In-Band Full-Duplex Wireless: A Degrees-of-Freedom Analysis
The challenge to in-band full-duplex wireless communication is managing
self-interference. Many designs have employed spatial isolation mechanisms,
such as shielding or multi-antenna beamforming, to isolate the
self-interference wave from the receiver. Such spatial isolation methods are
effective, but by confining the transmit and receive signals to a subset of the
available space, the full spatial resources of the channel be under-utilized,
expending a cost that may nullify the net benefit of operating in full-duplex
mode. In this paper we leverage an antenna-theory-based channel model to
analyze the spatial degrees of freedom available to a full-duplex capable base
station, and observe that whether or not spatial isolation out-performs
time-division (i.e. half-duplex) depends heavily on the geometric distribution
of scatterers. Unless the angular spread of the objects that scatter to the
intended users is overlapped by the spread of objects that backscatter to the
base station, then spatial isolation outperforms time division, otherwise time
division may be optimal
- …