68,011 research outputs found
Reconfigurable Intelligent Surfaces for Wireless Communications: Principles, Challenges, and Opportunities
Recently there has been a flurry of research on the use of reconfigurable
intelligent surfaces (RIS) in wireless networks to create smart radio
environments. In a smart radio environment, surfaces are capable of
manipulating the propagation of incident electromagnetic waves in a
programmable manner to actively alter the channel realization, which turns the
wireless channel into a controllable system block that can be optimized to
improve overall system performance. In this article, we provide a tutorial
overview of reconfigurable intelligent surfaces (RIS) for wireless
communications. We describe the working principles of reconfigurable
intelligent surfaces (RIS) and elaborate on different candidate implementations
using metasurfaces and reflectarrays. We discuss the channel models suitable
for both implementations and examine the feasibility of obtaining accurate
channel estimates. Furthermore, we discuss the aspects that differentiate RIS
optimization from precoding for traditional MIMO arrays highlighting both the
arising challenges and the potential opportunities associated with this
emerging technology. Finally, we present numerical results to illustrate the
power of an RIS in shaping the key properties of a MIMO channel.Comment: to appear in the IEEE Transactions on Cognitive Communications and
Networking (TCCN
A new analysis strategy for detection of faint gamma-ray sources with Imaging Atmospheric Cherenkov Telescopes
A new background rejection strategy for gamma-ray astrophysics with
stereoscopic Imaging Atmospheric Cherenkov Telescopes (IACT), based on Monte
Carlo (MC) simulations and real background data from the H.E.S.S. [High Energy
Stereoscopic System, see [1].] experiment, is described. The analysis is based
on a multivariate combination of both previously-known and newly-derived
discriminant variables using the physical shower properties, as well as its
multiple images, for a total of eight variables. Two of these new variables are
defined thanks to a new energy evaluation procedure, which is also presented
here. The method allows an enhanced sensitivity with the current generation of
ground-based Cherenkov telescopes to be achieved, and at the same time its main
features of rapidity and flexibility allow an easy generalization to any type
of IACT. The robustness against Night Sky Background (NSB) variations of this
approach is tested with MC simulated events. The overall consistency of the
analysis chain has been checked by comparison of the real gamma-ray signal
obtained from H.E.S.S. observations with MC simulations and through
reconstruction of known source spectra. Finally, the performance has been
evaluated by application to faint H.E.S.S. sources. The gain in sensitivity as
compared to the best standard Hillas analysis ranges approximately from 1.2 to
1.8 depending on the source characteristics, which corresponds to an economy in
observation time of a factor 1.4 to 3.2.Comment: 26 pages, 13 figure
- …