971 research outputs found
Deploying Dense Networks for Maximal Energy Efficiency: Small Cells Meet Massive MIMO
How would a cellular network designed for maximal energy efficiency look
like? To answer this fundamental question, tools from stochastic geometry are
used in this paper to model future cellular networks and obtain a new lower
bound on the average uplink spectral efficiency. This enables us to formulate a
tractable uplink energy efficiency (EE) maximization problem and solve it
analytically with respect to the density of base stations (BSs), the transmit
power levels, the number of BS antennas and users per cell, and the pilot reuse
factor. The closed-form expressions obtained from this general EE maximization
framework provide valuable insights on the interplay between the optimization
variables, hardware characteristics, and propagation environment. Small cells
are proved to give high EE, but the EE improvement saturates quickly with the
BS density. Interestingly, the maximal EE is achieved by also equipping the BSs
with multiple antennas and operate in a "massive MIMO" fashion, where the array
gain from coherent detection mitigates interference and the multiplexing of
many users reduces the energy cost per user.Comment: To appear in IEEE Journal on Selected Areas in Communications, 15
pages, 7 figures, 1 tabl
Downlink Spectral Efficiency of Cell-Free Massive MIMO with Full-Pilot Zero-Forcing
Cell-free Massive multiple-input multiple-output (MIMO) ensures ubiquitous
communication at high spectral efficiency (SE) thanks to increased
macro-diversity as compared cellular communications. However, system
scalability and performance are limited by fronthauling traffic and
interference. Unlike conventional precoding schemes that only suppress
intra-cell interference, full-pilot zero-forcing (fpZF), introduced in [1],
actively suppresses also inter-cell interference, without sharing channel state
information (CSI) among the access points (APs). In this study, we derive a new
closed-form expression for the downlink (DL) SE of a cell-free Massive MIMO
system with multi-antenna APs and fpZF precoding, under imperfect CSI and pilot
contamination. The analysis also includes max-min fairness DL power
optimization. Numerical results show that fpZF significantly outperforms
maximum ratio transmission scheme, without increasing the fronthauling
overhead, as long as the system is sufficiently distributed.Comment: Paper published in 2018 IEEE Global Conference on Signal and
Information Processing (GlobalSIP). {\copyright} 2019 IEEE. Personal use of
this material is permitted. Permission from IEEE must be obtained for all
other use
Cell-Free Massive MIMO versus Small Cells
A Cell-Free Massive MIMO (multiple-input multiple-output) system comprises a
very large number of distributed access points (APs)which simultaneously serve
a much smaller number of users over the same time/frequency resources based on
directly measured channel characteristics. The APs and users have only one
antenna each. The APs acquire channel state information through time-division
duplex operation and the reception of uplink pilot signals transmitted by the
users. The APs perform multiplexing/de-multiplexing through conjugate
beamforming on the downlink and matched filtering on the uplink. Closed-form
expressions for individual user uplink and downlink throughputs lead to max-min
power control algorithms. Max-min power control ensures uniformly good service
throughout the area of coverage. A pilot assignment algorithm helps to mitigate
the effects of pilot contamination, but power control is far more important in
that regard.
Cell-Free Massive MIMO has considerably improved performance with respect to
a conventional small-cell scheme, whereby each user is served by a dedicated
AP, in terms of both 95%-likely per-user throughput and immunity to shadow
fading spatial correlation. Under uncorrelated shadow fading conditions, the
cell-free scheme provides nearly 5-fold improvement in 95%-likely per-user
throughput over the small-cell scheme, and 10-fold improvement when shadow
fading is correlated.Comment: EEE Transactions on Wireless Communications, accepted for publicatio
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