1,967 research outputs found
On the Optimal Feedback Rate in Interference-Limited Multi-Antenna Cellular Systems
We consider a downlink cellular network where multi-antenna base stations
(BSs) transmit data to single-antenna users by using one of two linear
precoding methods with limited feedback: (i) maximum ratio transmission (MRT)
for serving a single user or (ii) zero forcing (ZF) for serving multiple users.
The BS and user locations are drawn from a Poisson point process, allowing
expressions for the signal- to-interference coverage probability and the
ergodic spectral efficiency to be derived as a function of system parameters
such as the number of BS antennas and feedback bits, and the pathloss exponent.
We find a tight lower bound on the optimum number of feedback bits to maximize
the net spectral efficiency, which captures the overall system gain by
considering both of downlink and uplink spectral efficiency using limited
feedback. Our main finding is that, when using MRT, the optimum number of
feedback bits scales linearly with the number of antennas, and logarithmically
with the channel coherence time. When using ZF, the feedback scales in the same
ways as MRT, but also linearly with the pathloss exponent. The derived results
provide system-level insights into the preferred channel codebook size by
averaging the effects of short-term fading and long-term pathloss.Comment: to appear in IEEE Transactions on Wireless Communication
Large Antenna Analysis of Multi-Cell Full-Duplex Networks
We study a multi-cell multi-user MIMO full-duplex network, where each base
station (BS) has multiple antennas with full-duplex capability supporting
single-antenna users with either full-duplex or half-duplex radios. We
characterize the up- and downlink ergodic achievable rates for the case of
linear precoders and receivers. The rate analysis includes practical
constraints such as imperfect self- interference cancellation, channel
estimation error, training overhead and pilot contamination. We show that the
2X gain of full-duplex over half-duplex system remains in the asymptotic regime
where the number of BS antennas grows infinitely large. We numerically evaluate
the finite SNR and antenna performance, which reveals that full-duplex networks
can use significantly fewer antennas to achieve spectral efficiency gain over
the half-duplex counterparts. In addition, the overall full-duplex gains can be
achieved under realistic 3GPP multi-cell network settings despite the increased
interference introduced in the full-duplex networks.Comment: Submitted to IEEE Transactions on Wireless Communication
Fog Massive MIMO: A User-Centric Seamless Hot-Spot Architecture
The decoupling of data and control planes, as proposed for 5G networks, will
enable the efficient implementation of multitier networks where user equipment
(UE) nodes obtain coverage and connectivity through the top-tier macro-cells,
and, at the same time, achieve high-throughput low-latency communication
through lower tiers in the hierarchy. This paper considers a new architecture
for such lower tiers, dubbed fog massive MIMO, where the UEs are able to
establish high-throughput low-latency data links in a seamless and
opportunistic manner, as they travel through a dense fog of high-capacity
wireless infrastructure nodes, referred to as remote radio heads (RRHs).
Traditional handover mechanisms in dense multicell networks inherently give
rise to frequent handovers and pilot sequence re-assignments, incurring, as a
result, excessive protocol overhead and significant latency. In the proposed
fog massive MIMO architecture, UEs seamlessly and implicitly associate
themselves to the most convenient RRHs in a completely autonomous manner. Each
UE makes use of a unique uplink pilot sequence, and pilot contamination is
mitigated by a novel coded "on-the-fly" pilot contamination control mechanism.
We analyze the spectral efficiency and the outage probability of the proposed
architecture via stochastic geometry, using some recent results on unique
coverage in Boolean models, and provide a detailed comparison with respect to
an idealized baseline massive MIMO cellular system, that neglects protocol
overhead and latency due to explicit user-cell association. Our analysis,
supported by extensive system simulation, reveals that there exists a "sweet
spot" of the per-pilot user load (number of users per pilot), such that the
proposed system achieves spectral efficiency close to that of an ideal cellular
system with the minimum distance user-base station association and no
pilot/handover overhead.Comment: 32 pages, 7 figures and 1 Tabl
Performance of Network-Assisted Full-Duplex for Cell-Free Massive MIMO
Network assisted full-duplex (NAFD) is a spatial-division duplex technique
for future wireless networks with cell-free massive multiple-input
multiple-output (CF massive MIMO) network, where a large number of remote
antenna units (RAUs), either using half-duplex or full-duplex, jointly support
truly flexible duplex including time-division duplex, frequency-division duplex
and full duplex on demand of uplink and downlink traffic by using network MIMO
methods. With NAFD, bi-directional data rates of the wireless network could be
increased and end-to-end delay could be reduced. In this paper, the spectral
efficiency of NAFD communications in CF massive MIMO network with imperfect
channel state information (CSI) is investigated under spatial correlated
channels. Based on large dimensional random matrix theory, the deterministic
equivalents for the uplink sum-rate with minimum-mean-square-error (MMSE)
receiver as well as the downlink sum-rate with zero-forcing (ZF) and
regularized zero-forcing (RZF) beamforming are derived. Numerical results show
that under various environmental settings, the deterministic equivalents are
accurate in both a large-scale system and system with a finite number of
antennas. It is also shown that with the downlink-to-uplink interference
cancellation, the uplink spectral efficiency of CF massive MIMO with NAFD could
be improved. The spectral efficiencies of NAFD with different duplex
configurations such as in-band full-duplex, and half-duplex are compared. With
the same total numbers of transmit and receive antennas, NAFD with half-duplex
RAUs offers a higher spectral efficiency. To alleviate the uplink-to-downlink
interference, a novel genetic algorithm based user scheduling strategy (GAS) is
proposed. Simulation results show that the achievable downlink sum-rate by
using the GAS is greatly improved compared to that by using the random user
scheduling
Fundamental Green Tradeoffs: Progresses, Challenges, and Impacts on 5G Networks
With years of tremendous traffic and energy consumption growth, green radio
has been valued not only for theoretical research interests but also for the
operational expenditure reduction and the sustainable development of wireless
communications. Fundamental green tradeoffs, served as an important framework
for analysis, include four basic relationships: spectrum efficiency (SE) versus
energy efficiency (EE), deployment efficiency (DE) versus energy efficiency
(EE), delay (DL) versus power (PW), and bandwidth (BW) versus power (PW). In
this paper, we first provide a comprehensive overview on the extensive on-going
research efforts and categorize them based on the fundamental green tradeoffs.
We will then focus on research progresses of 4G and 5G communications, such as
orthogonal frequency division multiplexing (OFDM) and non-orthogonal
aggregation (NOA), multiple input multiple output (MIMO), and heterogeneous
networks (HetNets). We will also discuss potential challenges and impacts of
fundamental green tradeoffs, to shed some light on the energy efficient
research and design for future wireless networks.Comment: revised from IEEE Communications Surveys & Tutorial
Adaptive Pilot Clustering in Heterogeneous Massive MIMO Networks
We consider the uplink of a cellular massive MIMO network. Acquiring channel
state information at the base stations (BSs) requires uplink pilot signaling.
Since the number of orthogonal pilot sequences is limited by the channel
coherence, pilot reuse across cells is necessary to achieve high spectral
efficiency. However, finding efficient pilot reuse patterns is non-trivial
especially in practical asymmetric BS deployments. We approach this problem
using coalitional game theory. Each BS has a few unique pilots and can form
coalitions with other BSs to gain access to more pilots. The BSs in a coalition
thus benefit from serving more users in their cells, at the expense of higher
pilot contamination and interference. Given that a cell's average spectral
efficiency depends on the overall pilot reuse pattern, the suitable coalitional
game model is in partition form. We develop a low-complexity distributed
coalition formation based on individual stability. By incorporating a base
station intercommunication budget constraint, we are able to control the
overhead in message exchange between the base stations and ensure the
algorithm's convergence to a solution of the game called individually stable
coalition structure. Simulation results reveal fast algorithmic convergence and
substantial performance gains over the baseline schemes with no pilot reuse,
full pilot reuse, or random pilot reuse pattern.Comment: IEEE Transactions on Wireless Communications, 13 pages, 13 figures, 2
table
A Stochastic Analysis of Network MIMO Systems
This paper quantifies the benefits and limitations of cooperative
communications by providing a statistical analysis of the downlink in network
multiple-input multiple-output (MIMO) systems. We consider an idealized model
where the multiple-antenna base-stations (BSs) are distributed according to a
homogeneous Poisson point process and cooperate by forming disjoint clusters.
We assume that perfect channel state information (CSI) is available at the
cooperating BSs without any overhead. Multiple single-antenna users are served
using zero-forcing beamforming with equal power allocation across the beams.
For such a system, we obtain tractable, but accurate, approximations of the
signal power and inter-cluster interference power distributions and derive a
computationally efficient expression for the achievable per-BS ergodic sum rate
using tools from stochastic geometry. This expression allows us to obtain the
optimal loading factor, i.e., the ratio between the number of scheduled users
and the number of BS antennas, that maximizes the per-BS ergodic sum rate.
Further, it allows us to quantify the performance improvement of network MIMO
systems as a function of the cooperating cluster size. We show that to perform
zero-forcing across the distributed set of BSs within the cluster, the network
MIMO system introduces a penalty in received signal power. Along with the
inevitable out-of-cluster interference, we show that the per-BS ergodic sum
rate of a network MIMO system does not approach that of an isolated cell even
at unrealistically large cluster sizes. Nevertheless, network MIMO does provide
significant rate improvement as compared to uncoordinated single-cell
processing even at relatively modest cluster sizes.Comment: Accepted for publication at IEEE Transactions on Signal Processin
Joint User Selection, Power Allocation, and Precoding Design with Imperfect CSIT for Multi-Cell MU-MIMO Downlink Systems
In this paper, a new optimization framework is presented for the joint design
of user selection, power allocation, and precoding in multi-cell multi-user
multiple-input multiple-output (MU-MIMO) systems when imperfect channel state
information at transmitter (CSIT) is available. By representing the joint
optimization variables in a higher-dimensional space, the weighted sum-spectral
efficiency maximization is formulated as the maximization of the product of
Rayleigh quotients. Although this is still a non-convex problem, a
computationally efficient algorithm, referred to as generalized power iteration
precoding (GPIP), is proposed. The algorithm converges to a stationary point
(local maximum) of the objective function and therefore it guarantees the
first-order optimality of the solution. By adjusting the weights in the
weighted sum-spectral efficiency, the GPIP yields a joint solution for user
selection, power allocation, and downlink precoding. The GPIP is also extended
to a multi-cell scenario, where cooperative base stations perform joint user
selection and design their precoding vectors by sharing global yet imperfect
CSIT within the cooperative BSs. System-level simulations show the gains of the
proposed approach with respect to conventional user selection and linear
downlink precoding.Comment: 35 pages, 6 figure
Full-Duplex Non-Orthogonal Multiple Access for Modern Wireless Networks
Non-orthogonal multiple access (NOMA) is an interesting concept to provide
higher capacity for future wireless communications. In this article, we
consider the feasibility and benefits of combining full-duplex operation with
NOMA for modern communication systems. Specifically, we provide a comprehensive
overview on application of full-duplex NOMA in cellular networks, cooperative
and cognitive radio networks, and characterize gains possible due to
full-duplex operation. Accordingly, we discuss challenges, particularly the
self-interference and inter-user interference and provide potential solutions
to interference mitigation and quality-of-service provision based on
beamforming, power control, and link scheduling. We further discuss future
research challenges and interesting directions to pursue to bring full-duplex
NOMA into maturity and use in practice.Comment: Revised, IEEE Wireless Communication Magazin
Feedback Design for Multi-Antenna K-tier Heterogeneous Downlink Cellular Networks
We characterize the ergodic spectral efficiency of a non-cooperative and a
cooperative type of K-tier heterogeneous networks with limited feedback. In the
non-cooperative case, a multi-antenna base station (BS) serves a single-antenna
user using maximum-ratio transmission based on limited feedback. In the
cooperative case, a BS coordination set is formed by using dynamic clustering
across the tiers, wherein the intra-cluster interference is mitigated by using
multi-cell zero-forcing also based on limited feedback. Modeling the network
based on stochastic geometry, we derive analytical expressions for the ergodic
spectral efficiency as a function of the system parameters. Leveraging the
obtained expressions, we formulate feedback partition problems and obtain
solutions to improve the ergodic spectral efficiency. Simulations show the
spectral efficiency improvement by using the obtained feedback partitions. Our
major findings are as follows: 1) In the non-cooperative case, the feedback is
only useful in a particular tier if the mean interference is small enough. 2)
In the cooperative case, allocating more feedback to stronger intra-cluster BSs
is efficient. 3) In both cases, the obtained solutions do not change depending
on instantaneous signal-to-interference ratio
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