1,153 research outputs found
Interference Alignment with Partially Coordinated Transmit Precoding
In this paper, we introduce an efficient interference alignment (IA)
algorithm exploiting partially coordinated transmit precoding to improve the
number of concurrent interference-free transmissions, i.e., the multiplexing
gain, in multicell downlink. The proposed coordination model is such that each
base-station simultaneously transmits to two users and each user is served by
two base-stations. First, we show in a K-user system operating at the
information theoretic upper bound of degrees of freedom (DOF), the generic IA
is proper when , whereas the proposed partially coordinated IA is
proper when . Then, we derive a non-iterative, i.e., one shot, IA
algorithm for the proposed scheme when . We show that for a given
latency, the backhaul data rate requirement of the proposed method grows
linearly with K. Monte-Carlo simulation results show that the proposed one-shot
algorithm offers higher system throughput than the iterative IA at practical
SNR levels.Comment: 19 pages, 8 figure
Degrees of Freedom of MIMO Cellular Networks: Decomposition and Linear Beamforming Design
This paper investigates the symmetric degrees of freedom (DoF) of MIMO
cellular networks with G cells and K users per cell, having N antennas at each
base station and M antennas at each user. In particular, we investigate
achievability techniques based on either decomposition with asymptotic
interference alignment (IA) or linear beamforming schemes, and show that there
are distinct regimes of (G,K,M,N) where one outperforms the other. We first
note that both one-sided and two-sided decomposition with asymptotic IA achieve
the same degrees of freedom. We then establish specific antenna configurations
under which the DoF achieved using decomposition based schemes is optimal by
deriving a set of outer bounds on the symmetric DoF. For linear beamforming
schemes, we first focus on small networks and propose a structured approach to
linear beamforming based on a notion called packing ratios. Packing ratio
describes the interference footprint or shadow cast by a set of transmit
beamformers and enables us to identify the underlying structures for aligning
interference. Such a structured beamforming design can be shown to achieve the
optimal spatially normalized DoF (sDoF) of two-cell two-user/cell network and
the two-cell three-user/cell network. For larger networks, we develop an
unstructured approach to linear interference alignment, where transmit
beamformers are designed to satisfy conditions for IA without explicitly
identifying the underlying structures for IA. The main numerical insight of
this paper is that such an approach appears to be capable of achieving the
optimal sDoF for MIMO cellular networks in regimes where linear beamforming
dominates asymptotic decomposition, and a significant portion of sDoF
elsewhere. Remarkably, polynomial identity test appears to play a key role in
identifying the boundary of the achievable sDoF region in the former case.Comment: 25 pages, 14 figures, submitted to IEEE Transactions on Information
Theor
Signal Processing and Optimal Resource Allocation for the Interference Channel
In this article, we examine several design and complexity aspects of the
optimal physical layer resource allocation problem for a generic interference
channel (IC). The latter is a natural model for multi-user communication
networks. In particular, we characterize the computational complexity, the
convexity as well as the duality of the optimal resource allocation problem.
Moreover, we summarize various existing algorithms for resource allocation and
discuss their complexity and performance tradeoff. We also mention various open
research problems throughout the article.Comment: To appear in E-Reference Signal Processing, R. Chellapa and S.
Theodoridis, Eds., Elsevier, 201
Interference Exploitation Precoding for Multi-Level Modulations: Closed-Form Solutions
In this paper, we study closed-form interference-exploitation precoding for
multi-level modulations in the downlink of multi-user multiple-input
single-output (MU-MISO) systems. We consider two distinct cases: first, for the
case where the number of served users is not larger than the number of transmit
antennas at the base station (BS), we mathematically derive the optimal
precoding structure based on the Karush-Kuhn-Tucker (KKT) conditions. By
formulating the dual problem, the precoding problem for multi-level modulations
is transformed into a pre-scaling operation using quadratic programming (QP)
optimization. We further consider the case where the number of served users is
larger than the number of transmit antennas at the BS. By employing the pseudo
inverse, we show that the optimal solution of the pre-scaling vector is
equivalent to a linear combination of the right singular vectors corresponding
to zero singular values, and derive the equivalent QP formulation. We also
present the condition under which multiplexing more streams than the number of
transmit antennas is achievable. For both considered scenarios, we propose a
modified iterative algorithm to obtain the optimal precoding matrix, as well as
a sub-optimal closed-form precoder. Numerical results validate our derivations
on the optimal precoding structures for multi-level modulations, and
demonstrate the superiority of interference-exploitation precoding for both
scenarios
Interference Alignment for Multicell Multiuser MIMO Uplink Channels
This paper proposes a linear interference alignment (IA) scheme which can be
used for uplink channels in a general multicell multiuser MIMO cellular
network. The proposed scheme aims to align interference caused by signals from
a set of transmitters into a subspace which is established by the signals from
only a subset of those transmitters, thereby effectively reducing the number of
interfering transmitters. The total degrees of freedom (DoF) achievable by the
proposed scheme is given in closed-form expression, and a numerical analysis
shows that the proposed scheme can achieve the optimal DoF in certain scenarios
and provides a higher total DoF than other related schemes in most cases.Comment: Submitted to IEEE Transactions on Signal Processing, Jan., 201
The Feasibility of Interference Alignment for Reverse TDD Systems in MIMO Cellular Networks
The feasibility conditions of interference alignment (IA) are analyzed for
reverse TDD systems, i.e., one cell operates as downlink (DL) but the other
cell operates as uplink (UL). Under general multiple-input and multiple-output
(MIMO) antenna configurations, a necessary condition and a sufficient condition
for one-shot linear IA are established, i.e., linear IA without symbol or time
extension. In several example networks, optimal sum degrees of freedom (DoF) is
characterized by the derived necessary condition and sufficient condition. For
symmetric DoF within each cell, a sufficient condition is established in a more
compact expression, which yields the necessary and sufficient condition for a
class of symmetric DoF. An iterative construction of transmit and received
beamforming vectors is further proposed, which provides a specific beamforming
design satisfying one-shot IA. Simulation results demonstrate that the proposed
IA not only achieve lager DoF but also significantly improve the sum rate in
the practical signal-to-noise ratio (SNR) regime.Comment: 12 pages; 5 figure
Sum-rate Maximization in Sub-28 GHz Millimeter-Wave MIMO Interfering Networks
MIMO systems in the lower part of the millimeter-wave spectrum band (i.e.,
below 28 GHz) do not exhibit enough directivity and selectively, as their
counterparts in higher bands of the spectrum (i.e., above 60 GHz), and thus
still suffer from the detrimental effect of interference, on the system
sum-rate. As such systems exhibit large numbers of antennas and short coherence
times for the channel, traditional methods of distributed coordination are
ill-suited, and the resulting communication overhead would offset the gains of
coordination. In this work, we propose algorithms for tackling the sum-rate
maximization problem, that are designed to address the above limitations. We
derive a lower bound on the sum-rate, a so-called DLT bound (i.e., a difference
of log and trace), shed light on its tightness, and highlight its decoupled
nature at both the transmitters and receivers. Moreover, we derive the solution
to each of the subproblems, that we dub non-homogeneous waterfilling (a
variation on the MIMO waterfilling solution), and underline an inherent
desirable feature: its ability to turn-off streams exhibiting low-SINR, and
contribute to greatly speeding up the convergence of the proposed algorithm. We
then show the convergence of the resulting algorithm, max-DLT, to a stationary
point of the DLT bound. Finally, we rely on extensive simulations of various
network configurations, to establish the fast-converging nature of our proposed
schemes, and thus their suitability for addressing the short coherence
interval, as well as the increased system dimensions, arising when managing
interference in lower bands of the millimeter wave spectrum. Moreover, our
results also suggest that interference management still brings about
significant performance gains, especially in dense deployments.Comment: 16 page
Linear Transceiver Design for Interference Alignment: Complexity and Computation
Consider a MIMO interference channel whereby each transmitter and receiver
are equipped with multiple antennas. The basic problem is to design optimal
linear transceivers (or beamformers) that can maximize system throughput. The
recent work [1] suggests that optimal beamformers should maximize the total
degrees of freedom and achieve interference alignment in high SNR. In this
paper we first consider the interference alignment problem in spatial domain
and prove that the problem of maximizing the total degrees of freedom for a
given MIMO interference channel is NP-hard. Furthermore, we show that even
checking the achievability of a given tuple of degrees of freedom for all
receivers is NP-hard when each receiver is equipped with at least three
antennas. Interestingly, the same problem becomes polynomial time solvable when
each transmit/receive node is equipped with no more than two antennas. Finally,
we propose a distributed algorithm for transmit covariance matrix design, while
assuming each receiver uses a linear MMSE beamformer. The simulation results
show that the proposed algorithm outperforms the existing interference
alignment algorithms in terms of system throughput
MIMO Precoding in Underlay Cognitive Radio Systems with Completely Unknown Primary CSI
This paper studies a novel underlay MIMO cognitive radio (CR) system, where
the instantaneous or statistical channel state information (CSI) of the
interfering channels to the primary receivers (PRs) is completely unknown to
the CR. For the single underlay receiver scenario, we assume a minimum
information rate must be guaranteed on the CR main channel whose CSI is known
at the CR transmitter. We first show that low-rank CR interference is
preferable for improving the throughput of the PRs compared with spreading less
power over more transmit dimensions. Based on this observation, we then propose
a rank minimization CR transmission strategy assuming a minimum information
rate must be guaranteed on the CR main channel. We propose a simple solution
referred to as frugal waterfilling (FWF) that uses the least amount of power
required to achieve the rate constraint with a minimum-rank transmit covariance
matrix. We also present two heuristic approaches that have been used in prior
work to transform rank minimization problems into convex optimization problems.
The proposed schemes are then generalized to an underlay MIMO CR downlink
network with multiple receivers. Finally, a theoretical analysis of the
interference temperature and leakage rate outage probabilities at the PR is
presented for Rayleigh fading channels.We demonstrate that the direct FWF
solution leads to higher PR throughput even though it has higher interference
"temperature (IT) compared with the heuristic methods and classic waterfilling,
which calls into question the use of IT as a metric for CR interference.Comment: 11 page
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
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