Cognitive radio is considered as an efficient way to improve the spectrum efficiency. As one of its key technologies,spectrum handoff can guarantee the transmission continuity of secondary users(SUs). In this paper,we...Cognitive radio is considered as an efficient way to improve the spectrum efficiency. As one of its key technologies,spectrum handoff can guarantee the transmission continuity of secondary users(SUs). In this paper,we address a new and more generalized spectrum handoff problem in cognitive radio networks(CRNs),by considering simultaneously energy efficiency,multiple spectrum handoffs and multiple channels. Furthermore,effects of the primary users'(PUs')arrival and service rate on the target channel sequence selection are also considered. In order to obtain the energy-efficient target channel sequence,we firstly analyze the energy consumption and the number of delivered bits per hertz in the spectrum handoff process,and formulate a ratio-type energy efficiency optimization problem,which can be transformed into a parametric problem by utilizing fractional programming. Then,we propose an algorithm combining dynamic programming with bisection(DPB)algorithm to solve the energy efficiency optimization problem. Our simulation results verify that the designed target channel sequence has better performance than the existing algorithms in terms of energy efficiency.展开更多
In the IEEE g02. 11 protocol, the adoption of the exponential backoff technique leads to throughput performance strongly dependent on the initial contention window size and, most importantly, on the number of contendi...In the IEEE g02. 11 protocol, the adoption of the exponential backoff technique leads to throughput performance strongly dependent on the initial contention window size and, most importantly, on the number of contending stations considered in the network. This paper proposes a simple but accurate method to dynamically estimate the number of contending stations in a wireless local area network （ WLAN ）. Based on estimation, all the mobile stations dynamically adjust the initial contention window in medium access control （ MAC ） layer to avoid collisions. The simulation results show that the proposed algorithm can achieve efficient channel utilization, higher system throughput, and better fairness performance.展开更多
Efficient extraction of crude oil,the major energy resource of current concern and high demand worldwide,is of paramount importance in both energy and environmental fields.However,it remains a great challenge to separ...Efficient extraction of crude oil,the major energy resource of current concern and high demand worldwide,is of paramount importance in both energy and environmental fields.However,it remains a great challenge to separate the crude oil-in-water emulsions with an ultralow oil content of<200 ppm.Here,the three-dimensional and superwetting channels are developed by coating titanium foams with anodic TiO_(2) nanotube arrays.The channels render superhydrophilic and underwater superoleophobic feature,which enables rapid formation of water channels that expel the oil droplets.A high separation efficiency of ∼96.8% and low total organic carbon content of ∼6 ppm are thus achieved for the ultralow-concentration crude oil-in-water emulsions.The pressure and time dependence of the separation process is systematically studied with a critical pressure of 12.25 kPa.Such a high performance is close to the theoretical limit imposed by the ultralow concentration,and shows obvious advances over either organic membranes or inorganic frameworks.展开更多
Due to distinctive lattice and electronic properties,the thiocyanate anion(SCN-)perovskite as an alluring two-dimensional(2D)material system,can be applied in optoelectronic devices.Herein,both photovoltaic and photod...Due to distinctive lattice and electronic properties,the thiocyanate anion(SCN-)perovskite as an alluring two-dimensional(2D)material system,can be applied in optoelectronic devices.Herein,both photovoltaic and photodetection performances of the 2D Cs2Pb(SCN)2I2 have been investigated.Compared with the conventional cationic 2D perovskites,Cs2Pb(SCN)2I2 possesses ultra-small interlayer spacing,additional interlayer nano channels,which is thus beneficial for charge transport ability.The planar heterojunction solar cell based on Cs2Pb(SCN)2I2 as the light absorber,has presented the highest power conversion efficiency among long-chain-cation-based 2D perovskite devices.Besides,the Cs2Pb(SCN)2I2-based photodetector also exhibits much higher photodetection performance(i.e.quantum efficiency,on/off ratio,responsivity,detectivity,response speed,polarization sensitivity and detection stability).It is thus suggested that these outstanding photoelectric characteristics of Cs2Pb(SCN)2I2 could bring huge opportunities for its more abundant optoelectronic applications,such as field-effect transistor and light-emitting diodes.展开更多
基金Heilongjiang Province Natural Science Foundation(Grant No.F2016019)；National Natural Science Foundation of China(Grant No.61571162)；Major National Science and Technology Project(2015ZX03004002004)； China Postdoctoral Science Foundation(Grant No.2014M561347).
文摘Cognitive radio is considered as an efficient way to improve the spectrum efficiency. As one of its key technologies,spectrum handoff can guarantee the transmission continuity of secondary users(SUs). In this paper,we address a new and more generalized spectrum handoff problem in cognitive radio networks(CRNs),by considering simultaneously energy efficiency,multiple spectrum handoffs and multiple channels. Furthermore,effects of the primary users'(PUs')arrival and service rate on the target channel sequence selection are also considered. In order to obtain the energy-efficient target channel sequence,we firstly analyze the energy consumption and the number of delivered bits per hertz in the spectrum handoff process,and formulate a ratio-type energy efficiency optimization problem,which can be transformed into a parametric problem by utilizing fractional programming. Then,we propose an algorithm combining dynamic programming with bisection(DPB)algorithm to solve the energy efficiency optimization problem. Our simulation results verify that the designed target channel sequence has better performance than the existing algorithms in terms of energy efficiency.
基金Supported by National Natural Science Foundation of China ( No. 60472078) , and Cisco University Research Program Fund at Community Foundation Silicon Valley( No. 20029303 ).
文摘In the IEEE g02. 11 protocol, the adoption of the exponential backoff technique leads to throughput performance strongly dependent on the initial contention window size and, most importantly, on the number of contending stations considered in the network. This paper proposes a simple but accurate method to dynamically estimate the number of contending stations in a wireless local area network （ WLAN ）. Based on estimation, all the mobile stations dynamically adjust the initial contention window in medium access control （ MAC ） layer to avoid collisions. The simulation results show that the proposed algorithm can achieve efficient channel utilization, higher system throughput, and better fairness performance.
基金supported by the National Natural Science Foundation of China(51871037)Chongqing Youth Talents Program(CQYC201905023)the National Key Research and Development Program of China(2020YFF0421893).
文摘Efficient extraction of crude oil,the major energy resource of current concern and high demand worldwide,is of paramount importance in both energy and environmental fields.However,it remains a great challenge to separate the crude oil-in-water emulsions with an ultralow oil content of<200 ppm.Here,the three-dimensional and superwetting channels are developed by coating titanium foams with anodic TiO_(2) nanotube arrays.The channels render superhydrophilic and underwater superoleophobic feature,which enables rapid formation of water channels that expel the oil droplets.A high separation efficiency of ∼96.8% and low total organic carbon content of ∼6 ppm are thus achieved for the ultralow-concentration crude oil-in-water emulsions.The pressure and time dependence of the separation process is systematically studied with a critical pressure of 12.25 kPa.Such a high performance is close to the theoretical limit imposed by the ultralow concentration,and shows obvious advances over either organic membranes or inorganic frameworks.
基金supported by the National Key R&D Program of China(2018YFB1500101)the National Natural Science Foundation of China(11874402,51421002,51627803,91733301 and 51761145042)the International Partnership Program of Chinese Academy of Sciences(112111KYSB20170089)。
文摘Due to distinctive lattice and electronic properties,the thiocyanate anion(SCN-)perovskite as an alluring two-dimensional(2D)material system,can be applied in optoelectronic devices.Herein,both photovoltaic and photodetection performances of the 2D Cs2Pb(SCN)2I2 have been investigated.Compared with the conventional cationic 2D perovskites,Cs2Pb(SCN)2I2 possesses ultra-small interlayer spacing,additional interlayer nano channels,which is thus beneficial for charge transport ability.The planar heterojunction solar cell based on Cs2Pb(SCN)2I2 as the light absorber,has presented the highest power conversion efficiency among long-chain-cation-based 2D perovskite devices.Besides,the Cs2Pb(SCN)2I2-based photodetector also exhibits much higher photodetection performance(i.e.quantum efficiency,on/off ratio,responsivity,detectivity,response speed,polarization sensitivity and detection stability).It is thus suggested that these outstanding photoelectric characteristics of Cs2Pb(SCN)2I2 could bring huge opportunities for its more abundant optoelectronic applications,such as field-effect transistor and light-emitting diodes.
