We present a pilot HI survey of 17 Planck Galactic Cold Clumps(PGCCs)with the Five-hundred-meter Aperture Spherical radio Telescope(FAST).HI Narrow Self-Absorption(HINSA)is an effective method to detect cold HI being ...We present a pilot HI survey of 17 Planck Galactic Cold Clumps(PGCCs)with the Five-hundred-meter Aperture Spherical radio Telescope(FAST).HI Narrow Self-Absorption(HINSA)is an effective method to detect cold HI being mixed with molecular hydrogen H2 and improves our understanding of the atomic to molecular transition in the interstellar medium.HINSA was found in 58%PGCCs that we observed.The column density of HINSA was found to have an intermediate correlation with that of 13CO,following log(N(HINSA))=(0.52±0.26)log(N13CO)+(10±4.1).HI abundance relative to total hydrogen[HI]/[H]has an average value of 4.4×10-3,which is about 2.8 times of the average value of previous HINSA surveys toward molecular clouds.For clouds with total column density NH>5×1020 cm-2,an inverse correlation between HINSA abundance and total hydrogen column density is found,confirming the depletion of cold HI gas during molecular gas formation in more massive clouds.Nonthermal line width of 13CO is about 0-0.5 kms-1 larger than that of HINSA.One possible explanation of narrower non-thermal width of HINSA is that HINSA region is smaller than that of 13CO.Based on an analytic model of H2 formation and H2 dissociation by cosmic ray,we found the cloud ages to be within 106.7-107.0 yr for five sources.展开更多
The Five-hundred-meter Aperture Spherical radio Telescope(FAST) has passed national acceptance and finished one pilot cycle of ‘Shared-Risk’ observations. It will start formal operation soon. In this context, this p...The Five-hundred-meter Aperture Spherical radio Telescope(FAST) has passed national acceptance and finished one pilot cycle of ‘Shared-Risk’ observations. It will start formal operation soon. In this context, this paper describes testing results of key fundamental parameters for FAST, aiming to provide basic support for observation and data reduction of FAST for scientific researchers. The 19-beam receiver covering 1.05–1.45 GHz was utilized for most of these observations. The fluctuation in electronic gain of the system is better than 1% over 3.5 hours, enabling enough stability for observations. Pointing accuracy,aperture efficiency and system temperature are three key parameters for FAST. The measured standard deviation of pointing accuracy is 7.9′′, which satisfies the initial design of FAST. When zenith angle is less than 26.4°, the aperture efficiency and system temperature around 1.4 GHz are ~0.63 and less than 24 K for central beam, respectively. The sensitivity and stability of the 19-beam backend are confirmed to satisfy expectation by spectral HI observations toward NGC 672 and polarization observations toward 3 C 286. The performance allows FAST to take sensitive observations for various scientific goals, from studies of pulsars to galaxy evolution.展开更多
This paper investigates the secrecy performance of maximal ratio combining （MRC） and selection combining （SC） with imperfect channel state information （CSI） in the physical layer. In a single-input multiple- out...This paper investigates the secrecy performance of maximal ratio combining （MRC） and selection combining （SC） with imperfect channel state information （CSI） in the physical layer. In a single-input multiple- output （SIMO） wiretap channel, a source transmits confidential messages to the destination equipped with M antennas using the MRC/SC scheme to process the received multiple signals. An eavesdropper equipped with N antennas also adopts the MRC/SC scheme to promote successful eavesdropping. We derive the exact and asymptotic closed-form expressions for the ergodic secrecy capacity （ESC） in two cases： （1） MRC with weighting errors, and （2） SC with outdated CSI. Moreover, two important indicators, namely high signal-to-noise ratio （SNR） slope and high SNR power offset, which govern ESC at the high SNR region, are derived. Finally, simulations are conducted to validate the accuracy of our proposed analytical models. Results indicate that ESC rises with the increase of the number of antennas and the received SNR at the destination, and fades with the increase of those at the eavesdropper. Another finding is that the high SNR slope is constant, while the high SNR power offset is correlated with the number of antennas at both the destination and the eavesdropper.展开更多
In this paper, the transmission of confidential messages through single-input multiple-output （SIMO） independent and identically generMized-K （KG） fading channels is considered, where the eavesdropper overhears th...In this paper, the transmission of confidential messages through single-input multiple-output （SIMO） independent and identically generMized-K （KG） fading channels is considered, where the eavesdropper overhears the transmission from the transmitter to the receiver. Both the receiver and the eavesdropper are equipped with multiple antennas, and both active and passive eavesdroppings are considered where the channel state information of the eavesdropper＇s channel is or is not available at the transmitter. The secrecy performance of SIMO KG systems is investigated. Analytical expressions for secrecy outage probability and average secrecy capacity of SIMO systems are derived via two different methods, in which KG distribution is approximated by the Gamma and mixture Gamma distributions, respectively. Numerical results are presented and verified via the Monte-Carlo simulation.展开更多
We consider a cooperative system consisting of a source node, a destination node, N （N 〉 1） wirelesspowered relays, and an eavesdropper. Each relay is assumed to be with a nonlinear energy harvester, in which there...We consider a cooperative system consisting of a source node, a destination node, N （N 〉 1） wirelesspowered relays, and an eavesdropper. Each relay is assumed to be with a nonlinear energy harvester, in which there exists a saturation threshold, limiting the level of the harvested power. For decode-and-forward and power splitting protocols, the Kth best relay is selected to assist the source-relay-destination transmission. An analytical expression for the secrecy outage probability is derived, and also verified by simulation.展开更多
We investigate the secrecy outage performance of maximal ratio combining(MRC) in cognitive radio networks over Rayleigh fading channels. In a single-input multiple-output wiretap system, we consider a secondary user(S...We investigate the secrecy outage performance of maximal ratio combining(MRC) in cognitive radio networks over Rayleigh fading channels. In a single-input multiple-output wiretap system, we consider a secondary user(SU-TX) that transmits confidential messages to another secondary user(SU-RX) equipped with M(M ≥ 1)antennas where the MRC technique is adopted to improve its received signal-to-noise ratio. Meanwhile, an eavesdropper equipped with N(N ≥ 1) antennas adopts the MRC scheme to overhear the information between SU-TX and SU-RX. SU-TX adopts the underlay strategy to guarantee the service quality of the primary user without spectrum sensing. We derive the closed-form expressions for an exact and asymptotic secrecy outage probability.展开更多
基金supported by the National Key R&D Program of China(2017YFA0402600 and 2016YFA0400702)the National Natural Science Foundation of China(Grant Nos.11988101,11803051,11725313 and 11721303)+3 种基金CAS International Partnership Program(114A11KYSB20160008)CAS“Light of West China”ProgramCAS Interdisciplinary Innovation Team and Young Researcher Grant of National Astronomical Observatoriessupported in part by the Youth Innovation Promotion Association of CAS(No.2018075)
文摘We present a pilot HI survey of 17 Planck Galactic Cold Clumps(PGCCs)with the Five-hundred-meter Aperture Spherical radio Telescope(FAST).HI Narrow Self-Absorption(HINSA)is an effective method to detect cold HI being mixed with molecular hydrogen H2 and improves our understanding of the atomic to molecular transition in the interstellar medium.HINSA was found in 58%PGCCs that we observed.The column density of HINSA was found to have an intermediate correlation with that of 13CO,following log(N(HINSA))=(0.52±0.26)log(N13CO)+(10±4.1).HI abundance relative to total hydrogen[HI]/[H]has an average value of 4.4×10-3,which is about 2.8 times of the average value of previous HINSA surveys toward molecular clouds.For clouds with total column density NH>5×1020 cm-2,an inverse correlation between HINSA abundance and total hydrogen column density is found,confirming the depletion of cold HI gas during molecular gas formation in more massive clouds.Nonthermal line width of 13CO is about 0-0.5 kms-1 larger than that of HINSA.One possible explanation of narrower non-thermal width of HINSA is that HINSA region is smaller than that of 13CO.Based on an analytic model of H2 formation and H2 dissociation by cosmic ray,we found the cloud ages to be within 106.7-107.0 yr for five sources.
基金supported by the National Key R & D Program of China (No. 2017YFA0402701)the National Natural Science Foundation of China (NSFC, Nos. 11673039, 11803051 and 11833009)+1 种基金supported by the CAS “Light of West China” programsupported by the Youth Innovation Promotion Association CAS
文摘The Five-hundred-meter Aperture Spherical radio Telescope(FAST) has passed national acceptance and finished one pilot cycle of ‘Shared-Risk’ observations. It will start formal operation soon. In this context, this paper describes testing results of key fundamental parameters for FAST, aiming to provide basic support for observation and data reduction of FAST for scientific researchers. The 19-beam receiver covering 1.05–1.45 GHz was utilized for most of these observations. The fluctuation in electronic gain of the system is better than 1% over 3.5 hours, enabling enough stability for observations. Pointing accuracy,aperture efficiency and system temperature are three key parameters for FAST. The measured standard deviation of pointing accuracy is 7.9′′, which satisfies the initial design of FAST. When zenith angle is less than 26.4°, the aperture efficiency and system temperature around 1.4 GHz are ~0.63 and less than 24 K for central beam, respectively. The sensitivity and stability of the 19-beam backend are confirmed to satisfy expectation by spectral HI observations toward NGC 672 and polarization observations toward 3 C 286. The performance allows FAST to take sensitive observations for various scientific goals, from studies of pulsars to galaxy evolution.
基金Project supported by the National Natural Science Foundation of China （No. 61401372） and the Fundamental Research Funds for the Central Universities, China （Nos. XDJK2015B023 and XDJK2016A011）
文摘This paper investigates the secrecy performance of maximal ratio combining （MRC） and selection combining （SC） with imperfect channel state information （CSI） in the physical layer. In a single-input multiple- output （SIMO） wiretap channel, a source transmits confidential messages to the destination equipped with M antennas using the MRC/SC scheme to process the received multiple signals. An eavesdropper equipped with N antennas also adopts the MRC/SC scheme to promote successful eavesdropping. We derive the exact and asymptotic closed-form expressions for the ergodic secrecy capacity （ESC） in two cases： （1） MRC with weighting errors, and （2） SC with outdated CSI. Moreover, two important indicators, namely high signal-to-noise ratio （SNR） slope and high SNR power offset, which govern ESC at the high SNR region, are derived. Finally, simulations are conducted to validate the accuracy of our proposed analytical models. Results indicate that ESC rises with the increase of the number of antennas and the received SNR at the destination, and fades with the increase of those at the eavesdropper. Another finding is that the high SNR slope is constant, while the high SNR power offset is correlated with the number of antennas at both the destination and the eavesdropper.
基金Project supported in part by the National Natural Science Foundation of China （Nos. 61471076 and 61401372）, the Program for Changjiang Scholars and Innovative Research Team in University, China （No. IRT1299）, the Natural Science Foundation Project of CQ CSTC （No. cstc2013jcyjA40040）, the Project of Fundamental and Frontier Research Plan of Chongqing, China （No. cstc2015jcyjBX0085）, the Special Fund of Chongqing Key Laboratory （CSTC）, the Scientific and Technological Research Program of Chongqing Municipal Education Commission, China （No. KJ1600413）, the Research Fund for the Doctoral Program of Higher Education of China （No. 20130182120017）, and the Fundamental Research Funds for the Central Universities, China （No. XDJK2015B023）. Parts of this publication, specifically Sections 1, 3, and 4, were made possible by PDRA （Post- Doctoral Research Award） from the Qatar National Research Fund （QNRF） （a member of Qatar Foundation （QF））, Qatar （No. PDRA1-1227-13029）
文摘In this paper, the transmission of confidential messages through single-input multiple-output （SIMO） independent and identically generMized-K （KG） fading channels is considered, where the eavesdropper overhears the transmission from the transmitter to the receiver. Both the receiver and the eavesdropper are equipped with multiple antennas, and both active and passive eavesdroppings are considered where the channel state information of the eavesdropper＇s channel is or is not available at the transmitter. The secrecy performance of SIMO KG systems is investigated. Analytical expressions for secrecy outage probability and average secrecy capacity of SIMO systems are derived via two different methods, in which KG distribution is approximated by the Gamma and mixture Gamma distributions, respectively. Numerical results are presented and verified via the Monte-Carlo simulation.
基金Project supported by tile National High-Tech R＆D Program （863） of China （No. 2015AA016304）, the National Natural Sci- ence Foundation of China （Nos. 61401372 and 61531016）, the Research Fund for the Doctoral Program of Higher Education of China （No. 20130182120017）, the Natural Science Foundation of CQ CSTC （Nos. cstc2013jeyjA40040 and cstc2016jcyjA2002）, the Postdoctoral Science Foundation of China （No. 2016M590875）, and the Fundamental Research Funds for the Central Uni- versities, China （Nos. XDJK2014A017, XDJK2015B023, and XDJK2016A011）
文摘We consider a cooperative system consisting of a source node, a destination node, N （N 〉 1） wirelesspowered relays, and an eavesdropper. Each relay is assumed to be with a nonlinear energy harvester, in which there exists a saturation threshold, limiting the level of the harvested power. For decode-and-forward and power splitting protocols, the Kth best relay is selected to assist the source-relay-destination transmission. An analytical expression for the secrecy outage probability is derived, and also verified by simulation.
基金Project supported in part by the National Natural Science Foundation of China(Nos.61401372 and 61531016)the Research Fund for the Doctoral Program of Higher Education of China(No.20130182120017)+1 种基金the Natural Science Foundation of CQ CSTC(No.cstc2013jcyj A40040)the Fundamental Research Funds for the Central Universities,China(No.XDJK2015B023)
文摘We investigate the secrecy outage performance of maximal ratio combining(MRC) in cognitive radio networks over Rayleigh fading channels. In a single-input multiple-output wiretap system, we consider a secondary user(SU-TX) that transmits confidential messages to another secondary user(SU-RX) equipped with M(M ≥ 1)antennas where the MRC technique is adopted to improve its received signal-to-noise ratio. Meanwhile, an eavesdropper equipped with N(N ≥ 1) antennas adopts the MRC scheme to overhear the information between SU-TX and SU-RX. SU-TX adopts the underlay strategy to guarantee the service quality of the primary user without spectrum sensing. We derive the closed-form expressions for an exact and asymptotic secrecy outage probability.
