174 research outputs found
Cutting Wi-Fi Scan Tax for Smart Devices
Today most popular mobile apps and location-based services require near always-on Wi-Fi connectivity (e.g., Skype, Viber, Wi-Fi Finder). The Wi-Fi power drain resulting from frequent Wi-Fi active scans is undermining the battery performance of smart devices and causing users to remove apps or disable important services. We collectively call this the scan tax problem. The main reason for this problem is that the main processor has to be active during Wi-Fi active scans and hence consumes a significant and disproportionate amount of energy during scan periods. We propose a simple and effective architectural change, where the main processor periodically computes an SSID list and scan parameters (i.e. scan interval, timeout) taking into account user mobility and behavior (e.g. walking); allowing scan to be offloaded to the Wi-Fi radio. We design WiScan, a complete system to realize scan offloading, and implement our system on the Nexus 5. Both our prototype experiments and trace-driven emulations demonstrate that WiScan achieves 90%+ of the maximal connectivity (connectivity that the existing Wi-Fi scan mechanism could achieve with 5 seconds scan interval), while saving 50-62% energy for seeking connectivity (the ratio between the Wi-Fi connected duration and total time duration) compared to existing active scan implementations. We argue that our proposed shift not only significantly reduces the scan tax paid by users, but also ultimately leads to ultra-low power, always-on Wi-Fi connectivity enabling a new class of context-aware apps to emerge
Doping and energy evolution of spin dynamics in the electron-doped cuprate superconductor PrLaCeCuO
The doping and energy evolution of the magnetic excitations of the
electron-doped cuprate superconductor PrLaCeCuO
in the superconducting state is studied based on the kinetic energy driven
superconducting mechanism. It is shown that there is a broad commensurate
scattering peak at low energy, then the resonance energy is located among this
low energy commensurate scattering range. This low energy commensurate
scattering disperses outward into a continuous ring-like incommensurate
scattering at high energy. The theory also predicts a dome shaped doping
dependent resonance energy.Comment: 8 pages, 4 figures, added discussions, replotted figures, and updated
references, accepted for publication in Phys. Rev.
Electronic band gaps and transport properties in periodically alternating mono- and bi-layer graphene superlattices
We investigate the electronic band structure and transport properties of
periodically alternating mono- and bi-layer graphene superlattices (MBLG SLs).
In such MBLG SLs, there exists a zero-averaged wave vector
(zero-) gap that is insensitive to the lattice constant. This
zero- gap can be controlled by changing both the ratio of the
potential widths and the interlayer coupling coefficient of the bilayer
graphene. We also show that there exist extra Dirac points; the conditions for
these extra Dirac points are presented analytically. Lastly, we demonstrate
that the electronic transport properties and the energy gap of the first two
bands in MBLG SLs are tunable through adjustment of the interlayer coupling and
the width ratio of the periodic mono- and bi-layer graphene.Comment: More discussion is added and the English is polished. Accepted for
publication in EP
Electronic band gaps and transport in aperiodic graphene superlattices of Thue-Morse sequence
We have studied the electronic properties in aperiodic graphene superlattices
of Thue-Morse sequence. Although the structure is aperiodic, an unusual Dirac
point (DP) does exist and its location is exactly at the position of the
zero-averaged wave number (zero-. Furthermore, the zero- gap
associated with the DP is robust against the lattice constants and the incident
angles, and multi-DPs can appear under the suitable conditions. A resultant
controllability of electron transport in Thue-Morse sequence is predicted,
which may facilitate the development of many graphene-based electronics.Comment: Accepted for publication in Applied Physics Letters; 4 pagese, 5
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