645 research outputs found
Energy-Efficient Power Control in Cell-Free and User-Centric Massive MIMO at Millimeter Wave
In a cell-free massive MIMO architecture a very large number of distributed
access points simultaneously and jointly serves a much smaller number of mobile
stations; a variant of the cell-free technique is the user-centric approach,
wherein each access point just serves a reduced set of mobile stations. This
paper introduces and analyzes the cell-free and user-centric architectures at
millimeter wave frequencies, considering a training-based channel estimation
phase, and the downlink and uplink data transmission phases. First of all, a
multiuser clustered millimeter wave channel model is introduced in order to
account for the correlation among the channels of nearby users; second, an
uplink multiuser channel estimation scheme is described along with
low-complexity hybrid analog/digital beamforming architectures. Third, the
non-convex problem of power allocation for downlink global energy efficiency
maximization is addressed. Interestingly, in the proposed schemes no channel
estimation is needed at the mobile stations, and the beamforming schemes used
at the mobile stations are channel-independent and have a very simple
structure. Numerical results show the benefits granted by the power control
procedure, that the considered architectures are effective, and permit
assessing the loss incurred by the use of the hybrid beamformers and by the
channel estimation errors.Comment: To appear on the IEEE Transactions on Green Communications and
Networking; originally submitted on April 24, 2018 and finally accepted for
publication on March 24, 201
Energy-Efficient Downlink Power Control in mmWave Cell-Free and User-Centric Massive MIMO
This paper considers cell-free and user-centric approaches for coverage
improvement in wireless cellular systems operating at millimeter wave
frequencies, and proposes downlink power control algorithms aimed at maximizing
the global energy efficiency. To tackle the non-convexity of the problems, an
interaction between sequential and alternating optimization is considered. The
use of hybrid analog/digital beamformers is also taken into account. The
numerical results show the benefits obtained from the power control algorithm,
as well as that the user-centric approach generally outperforms the cell-free
one.Comment: 4 pages; to be presented at the IEEE 5G Worls Forum Conference, Santa
Clara, July 2018. arXiv admin note: text overlap with arXiv:1710.0781
A Holistic Investigation on Terahertz Propagation and Channel Modeling Toward Vertical Heterogeneous Networks
User-centric and low latency communications can be enabled not only by small
cells but also through ubiquitous connectivity. Recently, the vertical
heterogeneous network (V-HetNet) architecture is proposed to backhaul/fronthaul
a large number of small cells. Like an orchestra, the V-HetNet is a polyphony
of different communication ensembles, including geostationary orbit (GEO), and
low-earth orbit (LEO) satellites (e.g., CubeSats), and networked flying
platforms (NFPs) along with terrestrial communication links. In this study, we
propose the Terahertz (THz) communications to enable the elements of V-HetNets
to function in harmony. As THz links offer a large bandwidth, leading to
ultra-high data rates, it is suitable for backhauling and fronthauling small
cells. Furthermore, THz communications can support numerous applications from
inter-satellite links to in-vivo nanonetworks. However, to savor this harmony,
we need accurate channel models. In this paper, the insights obtained through
our measurement campaigns are highlighted, to reveal the true potential of THz
communications in V-HetNets.Comment: It has been accepted for the publication in IEEE Communications
Magazin
Millimeter Wave Cellular Networks: A MAC Layer Perspective
The millimeter wave (mmWave) frequency band is seen as a key enabler of
multi-gigabit wireless access in future cellular networks. In order to overcome
the propagation challenges, mmWave systems use a large number of antenna
elements both at the base station and at the user equipment, which lead to high
directivity gains, fully-directional communications, and possible noise-limited
operations. The fundamental differences between mmWave networks and traditional
ones challenge the classical design constraints, objectives, and available
degrees of freedom. This paper addresses the implications that highly
directional communication has on the design of an efficient medium access
control (MAC) layer. The paper discusses key MAC layer issues, such as
synchronization, random access, handover, channelization, interference
management, scheduling, and association. The paper provides an integrated view
on MAC layer issues for cellular networks, identifies new challenges and
tradeoffs, and provides novel insights and solution approaches.Comment: 21 pages, 9 figures, 2 tables, to appear in IEEE Transactions on
Communication
Subspace Tracking and Least Squares Approaches to Channel Estimation in Millimeter Wave Multiuser MIMO
The problem of MIMO channel estimation at millimeter wave frequencies, both
in a single-user and in a multi-user setting, is tackled in this paper. Using a
subspace approach, we develop a protocol enabling the estimation of the right
(resp. left) singular vectors at the transmitter (resp. receiver) side; then,
we adapt the projection approximation subspace tracking with deflation and the
orthogonal Oja algorithms to our framework and obtain two channel estimation
algorithms. We also present an alternative algorithm based on the least squares
approach. The hybrid analog/digital nature of the beamformer is also explicitly
taken into account at the algorithm design stage. In order to limit the system
complexity, a fixed analog beamformer is used at both sides of the
communication links. The obtained numerical results, showing the accuracy in
the estimation of the channel matrix dominant singular vectors, the system
achievable spectral efficiency, and the system bit-error-rate, prove that the
proposed algorithms are effective, and that they compare favorably, in terms of
the performance-complexity trade-off, with respect to several competing
alternatives.Comment: To appear on the IEEE Transactions on Communication
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