A new type of nonlinear observer for nonlinear systems is presented. Instead of approximating thc cntire nonlinear system with the neural network （NN）, only the un-modeled part left over after the lincarization is a...A new type of nonlinear observer for nonlinear systems is presented. Instead of approximating thc cntire nonlinear system with the neural network （NN）, only the un-modeled part left over after the lincarization is approximated. Compared with the conventional linear observer, the observer provides more accurate estimation of the state. The state estimation error is proved to asymptotically approach zero with the Lyapunov method. The simulation result shows that the proposed observer scheme is effective and has a potential application ability in the fault detection and identification （FDI）, and the state estimation.展开更多
Vehicular beaconing plays an important role to facilitate various applications in the paradigm of Internet of Vehicles(IoV). Due to high dynamic and resource limitation in IoV, how to schedule the vehicular beacon bro...Vehicular beaconing plays an important role to facilitate various applications in the paradigm of Internet of Vehicles(IoV). Due to high dynamic and resource limitation in IoV, how to schedule the vehicular beacon broadcast is challenging, especially in dense scenarios. In this paper, we investigate the beacon broadcast scheduling problem considering the Age of Information(AoI). We first propose an algorithm minimizing the expected sum of AoI considering the limited communication resource and vehicle mobility. Then the performance of the proposed algorithm is analyzed. With the proposed algorithm, the optimal solution can be obtained under certain conditions. Extensive simulations are conducted to verify the efficiency, effectiveness and fairness of the proposed solution.展开更多
This paper proposes a trust ant colony routing algorithm by introducing a node trust evaluation model based on the D-S evidence theory into the ant colony routing protocol to improve the security of wireless sensor ne...This paper proposes a trust ant colony routing algorithm by introducing a node trust evaluation model based on the D-S evidence theory into the ant colony routing protocol to improve the security of wireless sensor networks. To reduce the influence of conflict evidences caused by malicious nods, the consistent intensity is introduced to preprocess conflict evidences before using the D-S combination rule to improve the reliability of the D-S based trust evaluation. The nodes with high trust values will be selected as the routing nodes to insure the routing security, and the trust values are used as heuristic functions of the ant colony routing algorithm. The simulation tests are conducted by using the network simulator NS2 to observe the outcomes of performance metrics of packets loss rate and average end-to-end delay etc. to indirectly evaluate the security issue under the attack of inside malicious nodes. The simulation results show that the proposed trust routing algorithm can efficiently resist malicious attacks in terms of keeping performances of the average end-to-end delay, the throughtput and the routing packet overhead under attacking from malicious nodes.展开更多
In this paper, we describe resourceefficient hardware architectures for softwaredefined radio （SDR） frontends. These architectures are made efficient by using a polyphase channelizer that performs arbitrary sample r...In this paper, we describe resourceefficient hardware architectures for softwaredefined radio （SDR） frontends. These architectures are made efficient by using a polyphase channelizer that performs arbitrary sample rate changes, frequency selection, and bandwidth control. We discuss area, time, and power optimization for field programmable gate array （FPGA） based architectures in an Mpath polyphase filter bank with modified Npath polyphase filter. Such systems allow resampling by arbitrary ratios while simultaneously performing baseband aliasing from center frequencies at Nyquist zones that are not multiples of the output sample rate. A nonmaximally decimated polyphase filter bank, where the number of data loads is not equal to the number of M subfilters, processes M subfilters in a time period that is either less than or greater than the Mdataload ＇ s time period. We present a loadprocess architecture （LPA） and a runtime architecture （RA） （based on serial polyphase structure） which have different scheduling. In LPA, Nsubfilters are loaded, and then M subfilters are processed at a clock rate that is a multiple of the input data rate. This is necessary to meet the output time constraint of the down-sampled data. In RA, Msubfilters processes are efficiently scheduled within Ndataload time while simultaneously loading N subfilters. This requires reduced clock rates compared with LPA, and potentially less power is consumed. A polyphase filter bank that uses different resampling factors for maximally decimated, underdecimated, overdecimated, and combined upand downsampled scenarios is used as a case study, and an analysis of area, time, and power for their FPGA architectures is given. For resourceoptimized SDR frontends, RA is superior for reducing operating clock rates and dynamic power consumption. RA is also superior for reducing area resources, except when indices are prestored in LUTs.展开更多
This paper presents efficient processing engines for software-defined radio （SDR） front-ends. These engines, based on a polyphase channelizer, perform arbitrary sample-rate changes, frequency selection, and bandwidt...This paper presents efficient processing engines for software-defined radio （SDR） front-ends. These engines, based on a polyphase channelizer, perform arbitrary sample-rate changes, frequency selection, and bandwidth control. This paper presents an M-path polyphase filter bank based on a modified N-path polyphase filter. Such a system allows resampling by arbitrary ratios while performing baseband aliasing from center frequencies at Nyquist zones that are not multiples of the output sample rate. This resampling technique is based on sliding cyclic data load interacting with cyclic-shifted coefficients. A non-maximally-decimated polyphase filterbank （where the number of data loads is not equal to the number of M subfilters） processes M subfilters in a time period that is less than or greater than the M data loads. A polyphase filter bank with five different resampling modes is used as a case study for embedded resamp/ing in SDR front-ends. These modes are （i） maximally decimated, （ii） Under-decimated, （iii） over-decimated, and combined up- and down-sampling with （iv） single stride length, and （v） multiple stride lengths. These modes can be used to obtain any required rational sampling rate change in an SDR front-end based on a polyphase channelizer. They can also be used for translation to and from arbitrary center frequencies that are unrelated to the output sample rates.展开更多
This paper reports a simple and controllable post-synthesis method for engineering the number of graphene layers based on oxygen plasma etching. Singular sheet etching(SSE) of graphene was achieved with the optimum pr...This paper reports a simple and controllable post-synthesis method for engineering the number of graphene layers based on oxygen plasma etching. Singular sheet etching(SSE) of graphene was achieved with the optimum process duration of 38 seconds. As a demonstration of this SSE process, monolayer graphene films were produced from bilayer graphenes. Experimental investigations verified that the oxygen plasma etching removes a single layer graphene sheet in an anisotropic fashion rather than anisotropic mode. In addition,etching via the oxygen plasma at the ground electrodes introduced fewer defects to the bottom graphene layer compared with the conventional oxygen reactive ion etching using the powered electrodes. Such defects can further be reduced with an effective annealing treatment in an argon environment at 900-1000?C. These results demonstrate that our developed SSE method has enabled a microelectronics manufacturing compatible way for single sheet precision subtraction of graphene layers and a potential technique for producing large size graphenes with high yield from multilayer graphite materials.展开更多
While having enabled human beings to realize the dream of communicating with whomever whenever and wherever,wireless technologies and applications are being developed at a very rapid speed and in a massively large sca...While having enabled human beings to realize the dream of communicating with whomever whenever and wherever,wireless technologies and applications are being developed at a very rapid speed and in a massively large scale as we have entered the 5G era and several grand 6G plans have also been clearly visible.However,most electronic products,including mobile phones,still have not gotten rid of the shackles of power wires.展开更多
In this paper, a low-cost dualtone radio interferometric positioning system using a single mobile anchor is proposed to locate multiple targets at the same time. We name it mDRIPS. In mDRIPS, each target continuously ...In this paper, a low-cost dualtone radio interferometric positioning system using a single mobile anchor is proposed to locate multiple targets at the same time. We name it mDRIPS. In mDRIPS, each target continuously transmits a dual-tone signal with its dedicated frequencies, and the mobile anchor receives the signals at different positions along its trajectory, Neither time synchronization between the mobile anchor and the targets nor time synchronization among the targets is required. We take the instability of targets clocks into consideration and develop an ESPRIT-type algorithm to estimate frequencies of each dual-tone signal. Furthermore, the time of arrival （TOA） of each target signal is extracted from the phase estimates of the received dual-tone signals. After measuring several TOAs at different locations along the anchor＇s trajectory, each target can be located. Since the frequency difference of the two tones of each dualtone signal is designed to be smaller than the channel coherence bandwidth, the same fading effect on these two tones can be eliminated. Moreover, the integer ambiguity problem due to phase wrapping is investigated, and a localization algorithm to deal with a simplified ambiguity problem is proposed. Numerical results demonstrate the efficiency of the proposed mDRIPS.展开更多
High electronic density is achieved by polarization doping without an impurity dopant in graded AIGaN films. Low specific contact resistance is studied on the polarization-doped A1GaN/GaN heterojunctions by using the ...High electronic density is achieved by polarization doping without an impurity dopant in graded AIGaN films. Low specific contact resistance is studied on the polarization-doped A1GaN/GaN heterojunctions by using the transmission line method （TLM）. The sheet density of polarization-doped A1GaN/GaN heterojunction is 6 × 10 14 cm-2 at room temperature. The linearly graded material structure is demonstrated by X-ray diffraction. The cartier concentration and mobility are characterized by a temperature-dependent Hall measurement. Multiple-layer metal （Ti/A1/Ti/Au） is deposited and annealed at 650 ℃ to realize the Ohmic contacts on the graded A1GaN/GaN heterojunctions.展开更多
Electrically pumped high power terahertz （THz） emitters that operated above room temperature in a pulse mode were fabricated from nitrogen-doped n-type 6H-SiC. The emission spectra had peaks centered on 5 THz and 12...Electrically pumped high power terahertz （THz） emitters that operated above room temperature in a pulse mode were fabricated from nitrogen-doped n-type 6H-SiC. The emission spectra had peaks centered on 5 THz and 12 THz （20 meV and 50 meV） that were attributed to radiative transitions of excitons bound to nitrogen donor impurities. Due to the relatively deep binding energies of the nitrogen donors, above 100 meV, and the high thermal conductivity of the SiC substrates, the THz output power and operating temperature were significantly higher than previous dopant based emitters. With peak applied currents of a few amperes, and a top surface area of 1 mm2, the device emitted up to 0.5 mW at liquid nitrogen temperature （77 K）, and tens of microwatts up to 333 K. This result is the highest temperature of THz emission reported from impurity-based emitters.展开更多
We choose 8-hydroxyquinoline derivative-metal complexes（Beq,Mgq,and Znq） as the acceptors（A） and 4,4＇,4”-tri-（2-methylphenyl phenylamino） triphenylaine（m-MTDATA） as the donor（D） respectively to study the e...We choose 8-hydroxyquinoline derivative-metal complexes（Beq,Mgq,and Znq） as the acceptors（A） and 4,4＇,4”-tri-（2-methylphenyl phenylamino） triphenylaine（m-MTDATA） as the donor（D） respectively to study the existing energy transfer process in the organic ultraviolet（UV） photodetector（PD）,which has an important influence on the sensitivity of PDs.The energy transfer process from D to A without exciplex formation is discussed,differing from the working mechanism of previous PDs with Gaq[Zisheng Su,Wenlian Li,Bei Chu,Tianle Li,Jianzhuo Zhu,Guang Zhang,Fei Yan,Xiao Li,Yiren Chen and Chun-Sing Lee 2008 Appl.Phys.Lett.93 103309）]and REq[J.B.Wang,W.L.Li,B.Chu,L.L.Chen,G.Zhang,Z.S.Su,Y.R.Chen,D.F.Yang,J.Z.Zhu,S.H.Wu,F.Yan,H.H.Liu,C.S.Lee 2010 Org.Electron.111301]used as an A material.Under 365-nm UV irradiation with an intensity of 1.2 mW/cm^2,the m-MTDATA：Beq blend device with a weight ratio of 1：1 shows a response of 192 mAAV with a detectivity of 6.5 × 10^（11） Jones,which exceeds those of PDs based on Mgq（146 mA/W） and Znq（182 mA/W） due to better energy level alignment between m-MTDATA/Beq and lower radiative decay.More photophysics processes of the PDs involved are discussed in detail.展开更多
Content distribution in large-scale vehicular ad hoc networks is difficult due to the scalability issue. A message may need to be carried by several vehicles till it reaches the destination. To select an appropriate n...Content distribution in large-scale vehicular ad hoc networks is difficult due to the scalability issue. A message may need to be carried by several vehicles till it reaches the destination. To select an appropriate next-hop carrier, the current carrier should ex- change control messages with a large number of vehicles encountered, and thus the pure ad hoc solution is not scalable. In this paper, we introduce a hybrid-network solution. We first divide the area into regions, and select a hot spot in each region to install a road-side unit （RSU）. RSUs can coordinate message exchanges between vehicles, and storage devices are used to temporarily hold a message waiting for the next-hop carrier. The RSUs and the vehicles traveling between them construct an overlay store-car- ry-and-forward content distribution network. Two types of vehicles exist, one with fixed mobility patterns such as buses, and the other with random patterns such as taxis. Considering one or both types of vehicles, utility-based optimization problems can be formulated to find the optimal routing solutions. Using the bus and taxi traces of Shanghai city, we demonstrate the effectiveness of the hybrid framework in terms of delivery delay, delivery ratio and overhead ratio.展开更多
Ultrashort multi-ke V x-ray pulses are generated by electron plasma produced by the irradiation of femtosecond pulses on metals. These sub-picosecond x-ray pulses have extended the field of x-ray spectroscopy into the...Ultrashort multi-ke V x-ray pulses are generated by electron plasma produced by the irradiation of femtosecond pulses on metals. These sub-picosecond x-ray pulses have extended the field of x-ray spectroscopy into the femtosecond time domain. However, pulse-to-pulse instability and long data acquisition time restrict the application of ultrashort x-ray systems operating at low repetition rates. Here we report on the performance of a femtosecond laser plasma-induced hard x-ray source that operates at 1-k Hz repetition rate, and provides a flux of 2.0 × 10^10 photons/s of Cu Kαradiation. Using this system for time-resolved x-ray diffraction experiments, we record in real time, the transient processes and structural changes induced by the interaction of 400-nm femtosecond pulse with the surface of a 200-nm thick Au（111） single crystal.展开更多
We propose a novel strategy numed basis-splitting scheme to split the intercepted quanta into several portions based on different bases, for eavesdropping in the process of quantum cryptography. Compared with intercep...We propose a novel strategy numed basis-splitting scheme to split the intercepted quanta into several portions based on different bases, for eavesdropping in the process of quantum cryptography. Compared with intercept- resend strategy, our simulation results of the basis-splitting scheme under the non-ideal condition show a greater performance, especially with the increase of the length of shifted bits. Consequently our scheme can aid eaves- dropper to gather much more useful information.展开更多
文摘A new type of nonlinear observer for nonlinear systems is presented. Instead of approximating thc cntire nonlinear system with the neural network （NN）, only the un-modeled part left over after the lincarization is approximated. Compared with the conventional linear observer, the observer provides more accurate estimation of the state. The state estimation error is proved to asymptotically approach zero with the Lyapunov method. The simulation result shows that the proposed observer scheme is effective and has a potential application ability in the fault detection and identification （FDI）, and the state estimation.
基金supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC)China Scholarship Council (CSC)
文摘Vehicular beaconing plays an important role to facilitate various applications in the paradigm of Internet of Vehicles(IoV). Due to high dynamic and resource limitation in IoV, how to schedule the vehicular beacon broadcast is challenging, especially in dense scenarios. In this paper, we investigate the beacon broadcast scheduling problem considering the Age of Information(AoI). We first propose an algorithm minimizing the expected sum of AoI considering the limited communication resource and vehicle mobility. Then the performance of the proposed algorithm is analyzed. With the proposed algorithm, the optimal solution can be obtained under certain conditions. Extensive simulations are conducted to verify the efficiency, effectiveness and fairness of the proposed solution.
基金supported by the National Natural Science Foundation of China(NSFC)under Grant No.61373126the Natural Science Foundation of Jiangsu Province of China under Grant No.BK20131107the Fundamental Research Funds for the Central Universities under Grant No.JUSRP51510
文摘This paper proposes a trust ant colony routing algorithm by introducing a node trust evaluation model based on the D-S evidence theory into the ant colony routing protocol to improve the security of wireless sensor networks. To reduce the influence of conflict evidences caused by malicious nods, the consistent intensity is introduced to preprocess conflict evidences before using the D-S combination rule to improve the reliability of the D-S based trust evaluation. The nodes with high trust values will be selected as the routing nodes to insure the routing security, and the trust values are used as heuristic functions of the ant colony routing algorithm. The simulation tests are conducted by using the network simulator NS2 to observe the outcomes of performance metrics of packets loss rate and average end-to-end delay etc. to indirectly evaluate the security issue under the attack of inside malicious nodes. The simulation results show that the proposed trust routing algorithm can efficiently resist malicious attacks in terms of keeping performances of the average end-to-end delay, the throughtput and the routing packet overhead under attacking from malicious nodes.
文摘In this paper, we describe resourceefficient hardware architectures for softwaredefined radio （SDR） frontends. These architectures are made efficient by using a polyphase channelizer that performs arbitrary sample rate changes, frequency selection, and bandwidth control. We discuss area, time, and power optimization for field programmable gate array （FPGA） based architectures in an Mpath polyphase filter bank with modified Npath polyphase filter. Such systems allow resampling by arbitrary ratios while simultaneously performing baseband aliasing from center frequencies at Nyquist zones that are not multiples of the output sample rate. A nonmaximally decimated polyphase filter bank, where the number of data loads is not equal to the number of M subfilters, processes M subfilters in a time period that is either less than or greater than the Mdataload ＇ s time period. We present a loadprocess architecture （LPA） and a runtime architecture （RA） （based on serial polyphase structure） which have different scheduling. In LPA, Nsubfilters are loaded, and then M subfilters are processed at a clock rate that is a multiple of the input data rate. This is necessary to meet the output time constraint of the down-sampled data. In RA, Msubfilters processes are efficiently scheduled within Ndataload time while simultaneously loading N subfilters. This requires reduced clock rates compared with LPA, and potentially less power is consumed. A polyphase filter bank that uses different resampling factors for maximally decimated, underdecimated, overdecimated, and combined upand downsampled scenarios is used as a case study, and an analysis of area, time, and power for their FPGA architectures is given. For resourceoptimized SDR frontends, RA is superior for reducing operating clock rates and dynamic power consumption. RA is also superior for reducing area resources, except when indices are prestored in LUTs.
文摘This paper presents efficient processing engines for software-defined radio （SDR） front-ends. These engines, based on a polyphase channelizer, perform arbitrary sample-rate changes, frequency selection, and bandwidth control. This paper presents an M-path polyphase filter bank based on a modified N-path polyphase filter. Such a system allows resampling by arbitrary ratios while performing baseband aliasing from center frequencies at Nyquist zones that are not multiples of the output sample rate. This resampling technique is based on sliding cyclic data load interacting with cyclic-shifted coefficients. A non-maximally-decimated polyphase filterbank （where the number of data loads is not equal to the number of M subfilters） processes M subfilters in a time period that is less than or greater than the M data loads. A polyphase filter bank with five different resampling modes is used as a case study for embedded resamp/ing in SDR front-ends. These modes are （i） maximally decimated, （ii） Under-decimated, （iii） over-decimated, and combined up- and down-sampling with （iv） single stride length, and （v） multiple stride lengths. These modes can be used to obtain any required rational sampling rate change in an SDR front-end based on a polyphase channelizer. They can also be used for translation to and from arbitrary center frequencies that are unrelated to the output sample rates.
文摘This paper reports a simple and controllable post-synthesis method for engineering the number of graphene layers based on oxygen plasma etching. Singular sheet etching(SSE) of graphene was achieved with the optimum process duration of 38 seconds. As a demonstration of this SSE process, monolayer graphene films were produced from bilayer graphenes. Experimental investigations verified that the oxygen plasma etching removes a single layer graphene sheet in an anisotropic fashion rather than anisotropic mode. In addition,etching via the oxygen plasma at the ground electrodes introduced fewer defects to the bottom graphene layer compared with the conventional oxygen reactive ion etching using the powered electrodes. Such defects can further be reduced with an effective annealing treatment in an argon environment at 900-1000?C. These results demonstrate that our developed SSE method has enabled a microelectronics manufacturing compatible way for single sheet precision subtraction of graphene layers and a potential technique for producing large size graphenes with high yield from multilayer graphite materials.
文摘While having enabled human beings to realize the dream of communicating with whomever whenever and wherever,wireless technologies and applications are being developed at a very rapid speed and in a massively large scale as we have entered the 5G era and several grand 6G plans have also been clearly visible.However,most electronic products,including mobile phones,still have not gotten rid of the shackles of power wires.
基金partially supported by the National Nature Science Foundation of China under Grants 61301223,61174127,61221003, and 61273181the Nature Science Foundation of Shanghai under Grant 13ZR1421800the New Faculty Foundation of Ministry of Education of China under Grant 13Z102090106
文摘In this paper, a low-cost dualtone radio interferometric positioning system using a single mobile anchor is proposed to locate multiple targets at the same time. We name it mDRIPS. In mDRIPS, each target continuously transmits a dual-tone signal with its dedicated frequencies, and the mobile anchor receives the signals at different positions along its trajectory, Neither time synchronization between the mobile anchor and the targets nor time synchronization among the targets is required. We take the instability of targets clocks into consideration and develop an ESPRIT-type algorithm to estimate frequencies of each dual-tone signal. Furthermore, the time of arrival （TOA） of each target signal is extracted from the phase estimates of the received dual-tone signals. After measuring several TOAs at different locations along the anchor＇s trajectory, each target can be located. Since the frequency difference of the two tones of each dualtone signal is designed to be smaller than the channel coherence bandwidth, the same fading effect on these two tones can be eliminated. Moreover, the integer ambiguity problem due to phase wrapping is investigated, and a localization algorithm to deal with a simplified ambiguity problem is proposed. Numerical results demonstrate the efficiency of the proposed mDRIPS.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.61204098 and 61371046)
文摘High electronic density is achieved by polarization doping without an impurity dopant in graded AIGaN films. Low specific contact resistance is studied on the polarization-doped A1GaN/GaN heterojunctions by using the transmission line method （TLM）. The sheet density of polarization-doped A1GaN/GaN heterojunction is 6 × 10 14 cm-2 at room temperature. The linearly graded material structure is demonstrated by X-ray diffraction. The cartier concentration and mobility are characterized by a temperature-dependent Hall measurement. Multiple-layer metal （Ti/A1/Ti/Au） is deposited and annealed at 650 ℃ to realize the Ohmic contacts on the graded A1GaN/GaN heterojunctions.
基金supported by the NSF Award No.DMR-0601920ONR Contract No.N0001-4-00-1-0834
文摘Electrically pumped high power terahertz （THz） emitters that operated above room temperature in a pulse mode were fabricated from nitrogen-doped n-type 6H-SiC. The emission spectra had peaks centered on 5 THz and 12 THz （20 meV and 50 meV） that were attributed to radiative transitions of excitons bound to nitrogen donor impurities. Due to the relatively deep binding energies of the nitrogen donors, above 100 meV, and the high thermal conductivity of the SiC substrates, the THz output power and operating temperature were significantly higher than previous dopant based emitters. With peak applied currents of a few amperes, and a top surface area of 1 mm2, the device emitted up to 0.5 mW at liquid nitrogen temperature （77 K）, and tens of microwatts up to 333 K. This result is the highest temperature of THz emission reported from impurity-based emitters.
基金supported by the National Natural Science Foundation of China(Grant Nos.61371046,61405026,61474016,and 61421002)China Postdoctoral Science Foundation(Grant No.2014M552330)
文摘We choose 8-hydroxyquinoline derivative-metal complexes（Beq,Mgq,and Znq） as the acceptors（A） and 4,4＇,4”-tri-（2-methylphenyl phenylamino） triphenylaine（m-MTDATA） as the donor（D） respectively to study the existing energy transfer process in the organic ultraviolet（UV） photodetector（PD）,which has an important influence on the sensitivity of PDs.The energy transfer process from D to A without exciplex formation is discussed,differing from the working mechanism of previous PDs with Gaq[Zisheng Su,Wenlian Li,Bei Chu,Tianle Li,Jianzhuo Zhu,Guang Zhang,Fei Yan,Xiao Li,Yiren Chen and Chun-Sing Lee 2008 Appl.Phys.Lett.93 103309）]and REq[J.B.Wang,W.L.Li,B.Chu,L.L.Chen,G.Zhang,Z.S.Su,Y.R.Chen,D.F.Yang,J.Z.Zhu,S.H.Wu,F.Yan,H.H.Liu,C.S.Lee 2010 Org.Electron.111301]used as an A material.Under 365-nm UV irradiation with an intensity of 1.2 mW/cm^2,the m-MTDATA：Beq blend device with a weight ratio of 1：1 shows a response of 192 mAAV with a detectivity of 6.5 × 10^（11） Jones,which exceeds those of PDs based on Mgq（146 mA/W） and Znq（182 mA/W） due to better energy level alignment between m-MTDATA/Beq and lower radiative decay.More photophysics processes of the PDs involved are discussed in detail.
文摘Content distribution in large-scale vehicular ad hoc networks is difficult due to the scalability issue. A message may need to be carried by several vehicles till it reaches the destination. To select an appropriate next-hop carrier, the current carrier should ex- change control messages with a large number of vehicles encountered, and thus the pure ad hoc solution is not scalable. In this paper, we introduce a hybrid-network solution. We first divide the area into regions, and select a hot spot in each region to install a road-side unit （RSU）. RSUs can coordinate message exchanges between vehicles, and storage devices are used to temporarily hold a message waiting for the next-hop carrier. The RSUs and the vehicles traveling between them construct an overlay store-car- ry-and-forward content distribution network. Two types of vehicles exist, one with fixed mobility patterns such as buses, and the other with random patterns such as taxis. Considering one or both types of vehicles, utility-based optimization problems can be formulated to find the optimal routing solutions. Using the bus and taxi traces of Shanghai city, we demonstrate the effectiveness of the hybrid framework in terms of delivery delay, delivery ratio and overhead ratio.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.61222509 and 11421064)the W.M.Keck Foundation
文摘Ultrashort multi-ke V x-ray pulses are generated by electron plasma produced by the irradiation of femtosecond pulses on metals. These sub-picosecond x-ray pulses have extended the field of x-ray spectroscopy into the femtosecond time domain. However, pulse-to-pulse instability and long data acquisition time restrict the application of ultrashort x-ray systems operating at low repetition rates. Here we report on the performance of a femtosecond laser plasma-induced hard x-ray source that operates at 1-k Hz repetition rate, and provides a flux of 2.0 × 10^10 photons/s of Cu Kαradiation. Using this system for time-resolved x-ray diffraction experiments, we record in real time, the transient processes and structural changes induced by the interaction of 400-nm femtosecond pulse with the surface of a 200-nm thick Au（111） single crystal.
基金Supported by the National Natural Science Foundation of China under Grant Nos 61301171 and 61372076the Fundamental Research Funds for the Central Universities of China under Grant No K5051301018the National 111 Project of Higher Education of China under Grant No B8038
文摘We propose a novel strategy numed basis-splitting scheme to split the intercepted quanta into several portions based on different bases, for eavesdropping in the process of quantum cryptography. Compared with intercept- resend strategy, our simulation results of the basis-splitting scheme under the non-ideal condition show a greater performance, especially with the increase of the length of shifted bits. Consequently our scheme can aid eaves- dropper to gather much more useful information.
