1,959 research outputs found
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
Power Allocation for Conventional and Buffer-Aided Link Adaptive Relaying Systems with Energy Harvesting Nodes
Energy harvesting (EH) nodes can play an important role in cooperative
communication systems which do not have a continuous power supply. In this
paper, we consider the optimization of conventional and buffer-aided link
adaptive EH relaying systems, where an EH source communicates with the
destination via an EH decode-and-forward relay. In conventional relaying,
source and relay transmit signals in consecutive time slots whereas in
buffer-aided link adaptive relaying, the state of the source-relay and
relay-destination channels determines whether the source or the relay is
selected for transmission. Our objective is to maximize the system throughput
over a finite number of transmission time slots for both relaying protocols. In
case of conventional relaying, we propose an offline and several online joint
source and relay transmit power allocation schemes. For offline power
allocation, we formulate an optimization problem which can be solved optimally.
For the online case, we propose a dynamic programming (DP) approach to compute
the optimal online transmit power. To alleviate the complexity inherent to DP,
we also propose several suboptimal online power allocation schemes. For
buffer-aided link adaptive relaying, we show that the joint offline
optimization of the source and relay transmit powers along with the link
selection results in a mixed integer non-linear program which we solve
optimally using the spatial branch-and-bound method. We also propose an
efficient online power allocation scheme and a naive online power allocation
scheme for buffer-aided link adaptive relaying. Our results show that link
adaptive relaying provides performance improvement over conventional relaying
at the expense of a higher computational complexity.Comment: Submitted to IEEE Transactions on Wireless Communication
Non-Orthogonal Multiple Access for FSO Backhauling
We consider a free space optical (FSO) backhauling system which consists of
two base stations (BSs) and one central unit (CU). We propose to employ
non-orthogonal multiple access (NOMA) for FSO backhauling where both BSs
transmit at the same time and in the same frequency band to the same
photodetector at the CU. We develop a dynamic NOMA scheme which determines the
optimal decoding order as a function of the channel state information at the CU
and the quality of service requirements of the BSs, such that the outage
probabilities of both BSs are jointly minimized. Moreover, we analyze the
performance of the proposed NOMA scheme in terms of the outage probability over
Gamma-Gamma FSO turbulence channels. We further derive closed-form expressions
for the outage probability for the high signal-to-noise ratio regime. Our
simulation results confirm the analytical derivations and reveal that the
proposed dynamic NOMA scheme significantly outperforms orthogonal transmission
and existing NOMA schemes.Comment: This paper has been submitted to IEEE WCNC 201
NOMA Assisted Wireless Caching: Strategies and Performance Analysis
Conventional wireless caching assumes that content can be pushed to local
caching infrastructure during off-peak hours in an error-free manner; however,
this assumption is not applicable if local caches need to be frequently updated
via wireless transmission. This paper investigates a new approach to wireless
caching for the case when cache content has to be updated during on-peak hours.
Two non-orthogonal multiple access (NOMA) assisted caching strategies are
developed, namely the push-then-deliver strategy and the push-and-deliver
strategy. In the push-then-deliver strategy, the NOMA principle is applied to
push more content files to the content servers during a short time interval
reserved for content pushing in on-peak hours and to provide more connectivity
for content delivery, compared to the conventional orthogonal multiple access
(OMA) strategy. The push-and-deliver strategy is motivated by the fact that
some users' requests cannot be accommodated locally and the base station has to
serve them directly. These events during the content delivery phase are
exploited as opportunities for content pushing, which further facilitates the
frequent update of the files cached at the content servers. It is also shown
that this strategy can be straightforwardly extended to device-to-device
caching, and various analytical results are developed to illustrate the
superiority of the proposed caching strategies compared to OMA based schemes
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