5,679 research outputs found
Fundamental Limits in Correlated Fading MIMO Broadcast Channels: Benefits of Transmit Correlation Diversity
We investigate asymptotic capacity limits of the Gaussian MIMO broadcast
channel (BC) with spatially correlated fading to understand when and how much
transmit correlation helps the capacity. By imposing a structure on channel
covariances (equivalently, transmit correlations at the transmitter side) of
users, also referred to as \emph{transmit correlation diversity}, the impact of
transmit correlation on the power gain of MIMO BCs is characterized in several
regimes of system parameters, with a particular interest in the large-scale
array (or massive MIMO) regime. Taking the cost for downlink training into
account, we provide asymptotic capacity bounds of multiuser MIMO downlink
systems to see how transmit correlation diversity affects the system
multiplexing gain. We make use of the notion of joint spatial division and
multiplexing (JSDM) to derive the capacity bounds. It is advocated in this
paper that transmit correlation diversity may be of use to significantly
increase multiplexing gain as well as power gain in multiuser MIMO systems. In
particular, the new type of diversity in wireless communications is shown to
improve the system multiplexing gain up to by a factor of the number of degrees
of such diversity. Finally, performance limits of conventional large-scale MIMO
systems not exploiting transmit correlation are also characterized.Comment: 29 pages, 8 figure
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
Fundamental Limits of Cooperation
Cooperation is viewed as a key ingredient for interference management in
wireless systems. This paper shows that cooperation has fundamental
limitations. The main result is that even full cooperation between transmitters
cannot in general change an interference-limited network to a noise-limited
network. The key idea is that there exists a spectral efficiency upper bound
that is independent of the transmit power. First, a spectral efficiency upper
bound is established for systems that rely on pilot-assisted channel
estimation; in this framework, cooperation is shown to be possible only within
clusters of limited size, which are subject to out-of-cluster interference
whose power scales with that of the in-cluster signals. Second, an upper bound
is also shown to exist when cooperation is through noncoherent communication;
thus, the spectral efficiency limitation is not a by-product of the reliance on
pilot-assisted channel estimation. Consequently, existing literature that
routinely assumes the high-power spectral efficiency scales with the log of the
transmit power provides only a partial characterization. The complete
characterization proposed in this paper subdivides the high-power regime into a
degrees-of-freedom regime, where the scaling with the log of the transmit power
holds approximately, and a saturation regime, where the spectral efficiency
hits a ceiling that is independent of the power. Using a cellular system as an
example, it is demonstrated that the spectral efficiency saturates at power
levels of operational relevance.Comment: 27 page
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