3,391 research outputs found
Two-Layered Superposition of Broadcast/Multicast and Unicast Signals in Multiuser OFDMA Systems
We study optimal delivery strategies of one common and independent
messages from a source to multiple users in wireless environments. In
particular, two-layered superposition of broadcast/multicast and unicast
signals is considered in a downlink multiuser OFDMA system. In the literature
and industry, the two-layer superposition is often considered as a pragmatic
approach to make a compromise between the simple but suboptimal orthogonal
multiplexing (OM) and the optimal but complex fully-layered non-orthogonal
multiplexing. In this work, we show that only two-layers are necessary to
achieve the maximum sum-rate when the common message has higher priority than
the individual unicast messages, and OM cannot be sum-rate optimal in
general. We develop an algorithm that finds the optimal power allocation over
the two-layers and across the OFDMA radio resources in static channels and a
class of fading channels. Two main use-cases are considered: i) Multicast and
unicast multiplexing when users with uplink capabilities request both
common and independent messages, and ii) broadcast and unicast multiplexing
when the common message targets receive-only devices and users with uplink
capabilities additionally request independent messages. Finally, we develop a
transceiver design for broadcast/multicast and unicast superposition
transmission based on LTE-A-Pro physical layer and show with numerical
evaluations in mobile environments with multipath propagation that the capacity
improvements can be translated into significant practical performance gains
compared to the orthogonal schemes in the 3GPP specifications. We also analyze
the impact of real channel estimation and show that significant gains in terms
of spectral efficiency or coverage area are still available even with
estimation errors and imperfect interference cancellation for the two-layered
superposition system
Distortion Minimization in Gaussian Layered Broadcast Coding with Successive Refinement
A transmitter without channel state information (CSI) wishes to send a
delay-limited Gaussian source over a slowly fading channel. The source is coded
in superimposed layers, with each layer successively refining the description
in the previous one. The receiver decodes the layers that are supported by the
channel realization and reconstructs the source up to a distortion. The
expected distortion is minimized by optimally allocating the transmit power
among the source layers. For two source layers, the allocation is optimal when
power is first assigned to the higher layer up to a power ceiling that depends
only on the channel fading distribution; all remaining power, if any, is
allocated to the lower layer. For convex distortion cost functions with convex
constraints, the minimization is formulated as a convex optimization problem.
In the limit of a continuum of infinite layers, the minimum expected distortion
is given by the solution to a set of linear differential equations in terms of
the density of the fading distribution. As the bandwidth ratio b (channel uses
per source symbol) tends to zero, the power distribution that minimizes
expected distortion converges to the one that maximizes expected capacity.
While expected distortion can be improved by acquiring CSI at the transmitter
(CSIT) or by increasing diversity from the realization of independent fading
paths, at high SNR the performance benefit from diversity exceeds that from
CSIT, especially when b is large.Comment: Accepted for publication in IEEE Transactions on Information Theor
Principles of Physical Layer Security in Multiuser Wireless Networks: A Survey
This paper provides a comprehensive review of the domain of physical layer
security in multiuser wireless networks. The essential premise of
physical-layer security is to enable the exchange of confidential messages over
a wireless medium in the presence of unauthorized eavesdroppers without relying
on higher-layer encryption. This can be achieved primarily in two ways: without
the need for a secret key by intelligently designing transmit coding
strategies, or by exploiting the wireless communication medium to develop
secret keys over public channels. The survey begins with an overview of the
foundations dating back to the pioneering work of Shannon and Wyner on
information-theoretic security. We then describe the evolution of secure
transmission strategies from point-to-point channels to multiple-antenna
systems, followed by generalizations to multiuser broadcast, multiple-access,
interference, and relay networks. Secret-key generation and establishment
protocols based on physical layer mechanisms are subsequently covered.
Approaches for secrecy based on channel coding design are then examined, along
with a description of inter-disciplinary approaches based on game theory and
stochastic geometry. The associated problem of physical-layer message
authentication is also introduced briefly. The survey concludes with
observations on potential research directions in this area.Comment: 23 pages, 10 figures, 303 refs. arXiv admin note: text overlap with
arXiv:1303.1609 by other authors. IEEE Communications Surveys and Tutorials,
201
A Survey of Physical Layer Security Techniques for 5G Wireless Networks and Challenges Ahead
Physical layer security which safeguards data confidentiality based on the
information-theoretic approaches has received significant research interest
recently. The key idea behind physical layer security is to utilize the
intrinsic randomness of the transmission channel to guarantee the security in
physical layer. The evolution towards 5G wireless communications poses new
challenges for physical layer security research. This paper provides a latest
survey of the physical layer security research on various promising 5G
technologies, including physical layer security coding, massive multiple-input
multiple-output, millimeter wave communications, heterogeneous networks,
non-orthogonal multiple access, full duplex technology, etc. Technical
challenges which remain unresolved at the time of writing are summarized and
the future trends of physical layer security in 5G and beyond are discussed.Comment: To appear in IEEE Journal on Selected Areas in Communication
Amplify-and-Forward in Wireless Relay Networks
A general class of wireless relay networks with a single source-destination
pair is considered. Intermediate nodes in the network employ an
amplify-and-forward scheme to relay their input signals. In this case the
overall input-output channel from the source via the relays to the destination
effectively behaves as an intersymbol interference channel with colored noise.
Unlike previous work we formulate the problem of the maximum achievable rate in
this setting as an optimization problem with no assumption on the network size,
topology, and received signal-to-noise ratio. Previous work considered only
scenarios wherein relays use all their power to amplify their received signals.
We demonstrate that this may not always maximize the maximal achievable rate in
amplify-and-forward relay networks. The proposed formulation allows us to not
only recover known results on the performance of the amplify-and-forward
schemes for some simple relay networks but also characterize the performance of
more complex amplify-and-forward relay networks which cannot be addressed in a
straightforward manner using existing approaches.
Using cut-set arguments, we derive simple upper bounds on the capacity of
general wireless relay networks. Through various examples, we show that a large
class of amplify-and-forward relay networks can achieve rates within a constant
factor of these upper bounds asymptotically in network parameters.Comment: Minor revision: fixed a typo in eqn. reference, changed the
formatting. 30 pages, 8 figure
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