License Assisted Access-WiFi Coexistence with TXOP Backoff for LTE in Unlicensed Band

License assisted access(LAA) was proposed as a solution to the challenge of satisfying high throughput requirement in cellular network with limited licensed band. LAA uses long term evolution(LTE) carrier aggregation technique to combine licensed and unlicensed bands, specifically 5 GHz Wi Fi band, in order to achieve high data rates. However, LAA has several critical concerns for services provision on 5 GHz unlicensed band. In this article, an overview of the techniques concerning the coexistence of LTE and Wi Fi on the same unlicensed band is presented. We first present several general approaches including power control of LTE evolved Node B(e NB), carrier selection, LTE duty cycling, self clear to send(CTS) and request to send/CTS(RTS/CTS), and listen before talk(LBT). Then, we focus on LBT techniques for LAA-Wi Fi coexistence and discuss how LBT based coexistence solutions can dynamically respond to the variations in network load and ensure fair coexistence in a distributed manner. Based on the LBT mechanism, we also present a fairness-aware LAA-WiF i coexisting scheme with TXOP(transmitted in a single transmission opportunity) backoff for LAA e NB. Finally, the performances of various LBT solutions for LAA-Wi Fi coexistence are evaluated using Markov chain analytical model.

Boosting the efficiency of GeSe solar cells by low-temperature treatment of p-n junction

Germanium monoselenide(GeSe)is an emerging promising photovoltaic absorber material due to its attractive optoelectronic properties as well as non-toxic and earth-abundant constitutes.However,all previously reported GeSe solar cells rely on a superstrate configuration coupled with a CdS buffer layer,and suffer from unsatisfactory performance.Here we demonstrate that this low efficiency arises from the inevitable high-temperature treatment of p-n junction in superstrate configuration.This results in the diffusion of Cd atoms from CdS layer into GeSe film that introduces detrimental deep trap states inside the bandgap of GeSe(~0.34 eV below conduction band minimum).We adopt therefore a substrate configuration that enables the deposition of CdS atop pre-deposited polycrystalline GeSe film at room temperature,avoiding the Cd diffusion.By optimizing the annealing temperature of complete devices via a highthroughput screening method,the resulting substrate solar cells annealed at 150℃achieve an efficiency of 3.1%,two times that of the best previously reported superstrate GeSe results.

Melt-and air-processed selenium thin-film solar cells

Elemental selenium(Se), as the world’s first but long-neglected photovoltaic material, has regained great interest recently in tandem solar cells as top cells due to its wide bandgap(~1.8 eV), simple, non-toxic and earth-abundant composition, and intrinsic environmental stability. In particular, Se possesses the lowest melting point of 217 °C among the photovoltaic absorbers reported so far, endowing Se with a unique advantage of film fabrication by blade coating the Se melt on substrate. However, the poor wettability of Se melt on widely-used photovoltaic functional layers such as TiO_(2) limits its melt processing. Here we introduce a wettability-modification strategy that decreases the contact angle of Se melt on substrate and improves the wettability by appropriately enhancing the heating temperature of molten Se while avoiding Se volatilization. We further reveal the mechanism of the inherent air stability of Se that originates from the high activation energy of oxygen chemisorption on Se(3.21 eV). This enables the realization of compact Se films through melt-based blade coating in ambient air. The resulting Se solar cells exhibit an efficiency of 3.5%. Unencapsulated devices show no efficiency loss after 1,000 h of storage under ambient conditions.

Enhancing the performance of future wireless networks with software-defined networking

To provide ubiquitous Internet access under the explosive increase of applications and data traffic,the current network architecture has become highly heterogeneous and complex,making network management a challenging task.To this end,software-defined networking(SDN) has been proposed as a promising solution.In the SDN architecture,the control plane and the data plane are decoupled,and the network infrastructures are abstracted and managed by a centralized controller.With SDN,efficient and flexible network control can be achieved,which potentially enhances network performance.To harvest the benefits of SDN in wireless networks,the software-defined wireless network(SDWN) architecture has been recently considered.In this paper,we first analyze the applications of SDN to different types of wireless networks.We then discuss several important technical aspects of performance enhancement in SDN-based wireless networks.Finally,we present possible future research directions of SDWN.

Wide-bandgap perovskites for indoor photovoltaics

With the rapid development of the Internet of Things(IoT),selfsustainable power sources are highly demanded for the wireless devices(e.g.,sensors,actuators,and communication devices)that are connected and automated by IoT technology.