11,562 research outputs found
Massive MIMO with Non-Ideal Arbitrary Arrays: Hardware Scaling Laws and Circuit-Aware Design
Massive multiple-input multiple-output (MIMO) systems are cellular networks
where the base stations (BSs) are equipped with unconventionally many antennas,
deployed on co-located or distributed arrays. Huge spatial degrees-of-freedom
are achieved by coherent processing over these massive arrays, which provide
strong signal gains, resilience to imperfect channel knowledge, and low
interference. This comes at the price of more infrastructure; the hardware cost
and circuit power consumption scale linearly/affinely with the number of BS
antennas . Hence, the key to cost-efficient deployment of large arrays is
low-cost antenna branches with low circuit power, in contrast to today's
conventional expensive and power-hungry BS antenna branches. Such low-cost
transceivers are prone to hardware imperfections, but it has been conjectured
that the huge degrees-of-freedom would bring robustness to such imperfections.
We prove this claim for a generalized uplink system with multiplicative
phase-drifts, additive distortion noise, and noise amplification. Specifically,
we derive closed-form expressions for the user rates and a scaling law that
shows how fast the hardware imperfections can increase with while
maintaining high rates. The connection between this scaling law and the power
consumption of different transceiver circuits is rigorously exemplified. This
reveals that one can make the circuit power increase as , instead of
linearly, by careful circuit-aware system design.Comment: Accepted for publication in IEEE Transactions on Wireless
Communications, 16 pages, 8 figures. The results can be reproduced using the
following Matlab code: https://github.com/emilbjornson/hardware-scaling-law
Resolved energy budget of superstructures in Rayleigh-B\'{e}nard convection
Turbulent superstructures, i.e. large-scale flow structures in turbulent
flows, play a crucial role in many geo- and astrophysical settings. In
turbulent Rayleigh-B\'{e}nard convection, for example, horizontally extended
coherent large-scale convection rolls emerge. Currently, a detailed
understanding of the interplay of small-scale turbulent fluctuations and
large-scale coherent structures is missing. Here, we investigate the resolved
kinetic energy and temperature variance budgets by applying a filtering
approach to direct numerical simulations of Rayleigh-B\'{e}nard convection at
high aspect ratio. In particular, we focus on the energy transfer rate between
large-scale flow structures and small-scale fluctuations. We show that the
small scales primarily act as a dissipation for the superstructures. However,
we find that the height-dependent energy transfer rate has a complex structure
with distinct bulk and boundary layer features. Additionally, we observe that
the heat transfer between scales mainly occurs close to the thermal boundary
layer. Our results clarify the interplay of superstructures and turbulent
fluctuations and may help to guide the development of an effective description
of large-scale flow features in terms of reduced-order models
Wave modelling - the state of the art
This paper is the product of the wave modelling community and it tries to make a picture of the present situation in this branch of science, exploring the previous and the most recent results and looking ahead towards the solution of the problems we presently face. Both theory and applications are considered.
The many faces of the subject imply separate discussions. This is reflected into the single sections, seven of them, each dealing with a specific topic, the whole providing a broad and solid overview of the present state of the art. After an introduction framing the problem and the approach we followed, we deal in sequence with the following subjects: (Section) 2, generation by wind; 3, nonlinear interactions in deep water; 4, white-capping dissipation; 5, nonlinear interactions in shallow water; 6, dissipation at the sea bottom; 7, wave propagation; 8, numerics. The two final sections, 9 and 10, summarize the present situation from a general point of view and try to look at the future developments
A dissipative random velocity field for fully developed fluid turbulence
We investigate the statistical properties, based on numerical simulations and
analytical calculations, of a recently proposed stochastic model for the
velocity field of an incompressible, homogeneous, isotropic and fully developed
turbulent flow. A key step in the construction of this model is the
introduction of some aspects of the vorticity stretching mechanism that governs
the dynamics of fluid particles along their trajectory. An additional further
phenomenological step aimed at including the long range correlated nature of
turbulence makes this model depending on a single free parameter that
can be estimated from experimental measurements. We confirm the realism of the
model regarding the geometry of the velocity gradient tensor, the power-law
behaviour of the moments of velocity increments (i.e. the structure functions),
including the intermittent corrections, and the existence of energy transfers
across scales. We quantify the dependence of these basic properties of
turbulent flows on the free parameter and derive analytically the
spectrum of exponents of the structure functions in a simplified non
dissipative case. A perturbative expansion in power of shows that
energy transfers, at leading order, indeed take place, justifying the
dissipative nature of this random field.Comment: 38 pages, 5 figure
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