120 research outputs found
Unified and Distributed QoS-Driven Cell Association Algorithms in Heterogeneous Networks
This paper addresses the cell association problem in the downlink of a
multi-tier heterogeneous network (HetNet), where base stations (BSs) have
finite number of resource blocks (RBs) available to distribute among their
associated users. Two problems are defined and treated in this paper: sum
utility of long term rate maximization with long term rate quality of service
(QoS) constraints, and global outage probability minimization with outage QoS
constraints. The first problem is well-suited for low mobility environments,
while the second problem provides a framework to deal with environments with
fast fading. The defined optimization problems in this paper are solved in two
phases: cell association phase followed by the optional RB distribution phase.
We show that the cell association phase of both problems have the same
structure. Based on this similarity, we propose a unified distributed algorithm
with low levels of message passing to for the cell association phase. This
distributed algorithm is derived by relaxing the association constraints and
using Lagrange dual decomposition method. In the RB distribution phase, the
remaining RBs after the cell association phase are distributed among the users.
Simulation results show the superiority of our distributed cell association
scheme compared to schemes that are based on maximum signal to interference
plus noise ratio (SINR)
Secure Transmission in Multi-Cell Massive MIMO Systems
In this paper, we consider physical layer security provisioning in multi-cell
massive multiple-input multiple-output (MIMO) systems. Specifically, we
consider secure downlink transmission in a multi-cell massive MIMO system with
matched-filter precoding and artificial noise (AN) generation at the base
station (BS) in the presence of a passive multi-antenna eavesdropper. We
investigate the resulting achievable ergodic secrecy rate and the secrecy
outage probability for the cases of perfect training and pilot contamination.
Thereby, we consider two different AN shaping matrices, namely, the
conventional AN shaping matrix, where the AN is transmitted in the null space
of the matrix formed by all user channels, and a random AN shaping matrix,
which avoids the complexity associated with finding the null space of a large
matrix. Our analytical and numerical results reveal that in multi-cell massive
MIMO systems employing matched-filter precoding (1) AN generation is required
to achieve a positive ergodic secrecy rate if the user and the eavesdropper
experience the same path-loss, (2) even with AN generation secure transmission
may not be possible if the number of eavesdropper antennas is too large and not
enough power is allocated to channel estimation, (3) for a given fraction of
power allocated to AN and a given number of users, in case of pilot
contamination, the ergodic secrecy rate is not a monotonically increasing
function of the number of BS antennas, and (4) random AN shaping matrices
provide a favourable performance/complexity tradeoff and are an attractive
alternative to conventional AN shaping matrices
Green Cellular Networks: A Survey, Some Research Issues and Challenges
Energy efficiency in cellular networks is a growing concern for cellular
operators to not only maintain profitability, but also to reduce the overall
environment effects. This emerging trend of achieving energy efficiency in
cellular networks is motivating the standardization authorities and network
operators to continuously explore future technologies in order to bring
improvements in the entire network infrastructure. In this article, we present
a brief survey of methods to improve the power efficiency of cellular networks,
explore some research issues and challenges and suggest some techniques to
enable an energy efficient or "green" cellular network. Since base stations
consume a maximum portion of the total energy used in a cellular system, we
will first provide a comprehensive survey on techniques to obtain energy
savings in base stations. Next, we discuss how heterogeneous network deployment
based on micro, pico and femto-cells can be used to achieve this goal. Since
cognitive radio and cooperative relaying are undisputed future technologies in
this regard, we propose a research vision to make these technologies more
energy efficient. Lastly, we explore some broader perspectives in realizing a
"green" cellular network technologyComment: 16 pages, 5 figures, 2 table
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