8,041 research outputs found
Harmonized Cellular and Distributed Massive MIMO: Load Balancing and Scheduling
Multi-tier networks with large-array base stations (BSs) that are able to
operate in the "massive MIMO" regime are envisioned to play a key role in
meeting the exploding wireless traffic demands. Operated over small cells with
reciprocity-based training, massive MIMO promises large spectral efficiencies
per unit area with low overheads. Also, near-optimal user-BS association and
resource allocation are possible in cellular massive MIMO HetNets using simple
admission control mechanisms and rudimentary BS schedulers, since scheduled
user rates can be predicted a priori with massive MIMO.
Reciprocity-based training naturally enables coordinated multi-point
transmission (CoMP), as each uplink pilot inherently trains antenna arrays at
all nearby BSs. In this paper we consider a distributed-MIMO form of CoMP,
which improves cell-edge performance without requiring channel state
information exchanges among cooperating BSs. We present methods for harmonized
operation of distributed and cellular massive MIMO in the downlink that
optimize resource allocation at a coarser time scale across the network. We
also present scheduling policies at the resource block level which target
approaching the optimal allocations. Simulations reveal that the proposed
methods can significantly outperform the network-optimized cellular-only
massive MIMO operation (i.e., operation without CoMP), especially at the cell
edge
On cost-effective reuse of components in the design of complex reconfigurable systems
Design strategies that benefit from the reuse of system components can reduce costs while maintaining or increasing dependability—we use the term dependability to tie together reliability and availability. D3H2 (aDaptive Dependable Design for systems with Homogeneous and Heterogeneous redundancies) is a methodology that supports the design of complex systems with a focus on reconfiguration and component reuse. D3H2 systematizes the identification of heterogeneous redundancies and optimizes the design of fault detection and reconfiguration mechanisms, by enabling the analysis of design alternatives with respect to dependability and cost. In this paper, we extend D3H2 for application to repairable systems. The method is extended with analysis capabilities allowing dependability assessment of complex reconfigurable systems. Analysed scenarios include time-dependencies between failure events and the corresponding reconfiguration actions. We demonstrate how D3H2 can support decisions about fault detection and reconfiguration that seek to improve dependability while reducing costs via application to a realistic railway case study
Molecular gas in extreme star-forming environments: the starbursts Arp220 and NGC6240 as case studies
We report single-dish multi-transition measurements of the 12^CO, HCN, and
HCO^+ molecular line emission as well as HNC J=1-0 and HNCO in the two
ultraluminous infra-red galaxies Arp220 and NGC6240. Using this new molecular
line inventory, in conjunction with existing data in the literature, we
compiled the most extensive molecular line data sets to date for such galaxies.
The many rotational transitions, with their different excitation requirements,
allow the study of the molecular gas over a wide range of different densities
and temperatures with significant redundancy, and thus allow good constraints
on the properties of the dense gas in these two systems. The mass (~(1-2) x
10^10 Msun) of dense gas (>10^5-6 cm^-3) found accounts for the bulk of their
molecular gas mass, and is consistent with most of their IR luminosities
powered by intense star bursts while self-regulated by O,B star cluster
radiative pressure onto the star-forming dense molecular gas. The highly
excited HCN transitions trace a gas phase ~(10-100)x denser than that of the
sub-thermally excited HCO^+ lines (for both galaxies). These two phases are
consistent with an underlying density-size power law found for Galactic GMCs
(but with a steeper exponent), with HCN lines tracing denser and more compact
regions than HCO^+. Whether this is true in IR-luminous, star forming galaxies
in general remains to be seen, and underlines the need for observations of
molecular transitions with high critical densities for a sample of bright
(U)LIRGs in the local Universe -- a task for which the HI-FI instrument on
board Herschel is ideally suited to do.Comment: 38 pages (preprint ApJ style), 3 figures, accepted for Ap
Supporting group maintenance through prognostics-enhanced dynamic dependability prediction
Condition-based maintenance strategies adapt maintenance planning through the integration of online condition monitoring of assets. The accuracy and cost-effectiveness of these strategies can be improved by integrating prognostics predictions and grouping maintenance actions respectively. In complex industrial systems, however, effective condition-based maintenance is intricate. Such systems are comprised of repairable assets which can fail in different ways, with various effects, and typically governed by dynamics which include time-dependent and conditional events. In this context, system reliability prediction is complex and effective maintenance planning is virtually impossible prior to system deployment and hard even in the case of condition-based maintenance. Addressing these issues, this paper presents an online system maintenance method that takes into account the system dynamics. The method employs an online predictive diagnosis algorithm to distinguish between critical and non-critical assets. A prognostics-updated method for predicting the system health is then employed to yield well-informed, more accurate, condition-based suggestions for the maintenance of critical assets and for the group-based reactive repair of non-critical assets. The cost-effectiveness of the approach is discussed in a case study from the power industry
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