Transition-metal
dichalcogenide monolayers and heterostructures
are highly tunable material systems that provide excellent models
for physical phenomena at the two-dimensional (2D) limit. While most
studies to date have focused on electrons and electron–hole
pairs, phonons also play essential roles. Here, we apply ultrafast
electron diffraction and diffuse scattering to directly quantify,
with time and momentum resolution, electron–phonon coupling
(EPC) in monolayer molybdenum disulfide and phonon transport from
the monolayer to a silicon nitride substrate. Optically generated
hot carriers result in a profoundly anisotropic distribution of phonons
in the monolayer within ∼5 ps. A quantitative comparison with ab initio ultrafast dynamics simulations reveals the essential
role of dielectric screening in weakening EPC. Thermal transport from
the monolayer to the substrate occurs with the phonon system far from
equilibrium. While screening in 2D is known to strongly affect equilibrium
properties, our findings extend this understanding to the dynamic
regime.
The mushroom growth of cellular users requires novel advancements in the existing cellular infrastructure. One way to handle such a tremendous increase is to densely deploy terrestrial small-cell base stations (TSBSs) with careful management of smart backhaul/fronthaul networks. Nevertheless, terrestrial backhaul hubs significantly suffer from the dense fading environment and are difficult to install in a typical urban environment. Therefore, this paper considers the idea of replacing terrestrial backhaul network with an aerial network consisting of unmanned aerial vehicles (UAVs) to provide the fronthaul connectivity between the TSBSs and the ground core-network (GCN). To this end, we focus on the joint positioning of UAVs and the association of TSBSs such that the sum-rate of the overall system is maximized. In particular, the association problem of TSBSs with UAVs is formulated under communication-related constraints, i.e., bandwidth, number of connections to a UAV, power limit, interference threshold, UAV heights, and backhaul data rate. To meet this joint objective, we take advantage of the genetic algorithm (GA) due to the offline nature of our optimization problem. The performance of the proposed approach is evaluated using the unsupervised learning-based k-means clustering algorithm. We observe that the proposed approach is highly effective to satisfy the requirements of smart fronthaul networks.
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