| 1. |
- Liu, Yi, et al.
(author)
-
Laser without population inversion of nitrogen ions pumped by femtosecond pulses
- 2019
-
In: CLEO : QELS_Fundamental Science, CLEO_QELS 2019 - QELS_Fundamental Science, CLEO_QELS 2019. - 9781557528209
-
Conference paper (peer-reviewed)abstract
- We attribute the mechanism of "lasing" action of nitrogen ions pumped by femtosecond IR pulses to a laser without inversion scheme. Numerical simulations reproduce well the temporal dynamics and pressure dependence of the emission.
|
|
| 2. |
- Liu, Yi, et al.
(author)
-
Unexpected Sensitivity of Nitrogen Ions Superradiant Emission on Pump Laser Wavelength and Duration
- 2017
-
In: Physical Review Letters. - 0031-9007. ; 119:20
-
Journal article (peer-reviewed)abstract
- Nitrogen molecules in ambient air exposed to an intense near-infrared femtosecond laser pulse give rise to cavity-free superradiant emission at 391.4 and 427.8 nm. An unexpected pulse duration-dependent cyclic variation of the superradiance intensity is observed when the central wavelength of the femtosecond pump laser pulse is finely tuned between 780 and 820 nm, and no signal occurs at the resonant wavelength of 782.8 nm (2ω782.8 nm=ω391.4 nm). On the basis of a semiclassical recollision model, we show that an interference of dipolar moments of excited ions created by electron recollisions explains this behavior.
|
|
| 3. |
- Ma, Zheng, et al.
(author)
-
Interference Suppression for Railway Wireless Communication Systems : A Reconfigurable Intelligent Surface Approach
- 2021
-
In: IEEE Transactions on Vehicular Technology. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9545 .- 1939-9359. ; 70:11, s. 11593-11603
-
Journal article (peer-reviewed)abstract
- Due to transmission of train control and dispatch commands, the ultra high reliability is required for railway wireless communication systems (RWCS). However, the quality of service (QoS) of RWCS is affected by external interference and jamming seriously in railway transportation. The external interference and jamming generated from intentional and/or unintentional sources can disturb the transmission of RWCS and be extremely dangerous for the trains' safety. In this paper, a reconfigurable intelligent surface (RIS) approach is proposed to suppress the interference and jamming in RWCS. By taking the signal-to-interference-plus-noise (SINR) as the QoS metrics, the interference suppression problem with RIS is transformed into a maximizing receiving SINR problem. Two optimum solutions, which one is based on Charnes-Cooper transformation and the other is based on single linear programming, are provided. In order to deploy the RIS in RWCS practically, two low-complexity sub-optimum solutions, which one is to bypass the channel estimation for interference links and the other is to maximize the receiving desired power, are also discussed. The simulations results show that the RIS can be employed in RWCS with considerable anti-interference gain.
|
|
| 4. |
- Muhammed, Alemu Jorgi, et al.
(author)
-
Energy-Efficient Resource Allocation for NOMA Based Small Cell Networks With Wireless Backhauls
- 2020
-
In: IEEE Transactions on Communications. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 0090-6778 .- 1558-0857. ; 68:6, s. 3766-3781
-
Journal article (peer-reviewed)abstract
- In this paper, we consider a downlink non-orthogonal multiple access (NOMA) enabled heterogeneous small cells network (HSCN), where the macro base station simultaneously communicates with multiple small cell base stations (SBSs) through wireless backhaul. In each small cell, users are grouped by NOMA bases and then served by their respective SBS. The proposed framework considers the realistic imperfect channel state information and quality of service requirements of users. The goal is to investigate an energy-efficient joint power, and bandwidth allocation scheme, which aims to maximize the energy efficiency (EE) of the small cells in downlink NOMA-HSCN constrained by the maximum transmit power and the minimum required data rate simultaneously. The optimization problem is non-convex due to the fractional objective function and non-convex constraint and thus challenging to obtain an exact solution efficiently. To this end, the joint optimization is first decomposed into two subproblems. Then, an iterative algorithm to solve the power optimization subproblem is proposed with guaranteed convergence. Furthermore, we derive a closed-form solution for the bandwidth allocation subproblem. Simulation results reveal that the effectiveness of the proposed schemes in terms of EE compared to the existing NOMA and the orthogonal multiple access schemes.
|
|
| 5. |
- Muhammed, Alemu Jorgi, et al.
(author)
-
Resource Allocation for Energy-Efficient NOMA System in Coordinated Multi-Point Networks
- 2021
-
In: IEEE Transactions on Vehicular Technology. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9545 .- 1939-9359. ; 70:2, s. 1577-1591
-
Journal article (peer-reviewed)abstract
- This paper studies user scheduling and power allocation problem to maximize the energy efficiency (EE) for non-orthogonal multiple access (NOMA) in downlink Coordinated Multi-Point networks. In the proposed framework, a more practical scenario the imperfect channel state information, imperfect successive interference cancellation and data outage are investigated. To address the considered problem, the optimization problem is formulated constrained by the total power and the outage probability requirements. However, the EE objective function is with a non-convex structure. Accordingly, we first convert the optimization problem to make the objective function concave and analytically tractable. Furthermore, we split the joint optimization problem to find a suboptimal solutions to the original problem. As a result, we first propose a suboptimal user-scheduling algorithm to improve the system's EE. Due to the non-convex function of the transmit power, we invoke a sequential successive convex approach to address the non-convex problem by its lower bound concave function. Besides, the fractional objective function is converted to its equivalent subtractive form. Finally, we derive a power control scheme to address the proposed framework. Simulation results endorse the effectiveness of the proposed algorithm and their performance gains in terms of EE compared to both NOMA and OFDMA variants.
|
|
| 6. |
- Zhang, Xiang, et al.
(author)
-
Backward lasing of singly ionized nitrogen ions pumped by femtosecond laser pulses
- 2020
-
In: Applied Physics B: Lasers and Optics. - : Springer Science and Business Media LLC. - 0946-2171. ; 126:3
-
Journal article (peer-reviewed)abstract
- We report on the observation of backward lasing at 391.4 nm of nitrogen ions pumped by linearly polarized intense femtosecond pulses at 800 nm. The strongly enhanced spectral intensity at 391.4 nm, as well as the amplification of an externally injected backward seeding pulse, confirm that the backward 391.4 nm signal is due to optical amplification in the nitrogen gas plasma. Compared to the forward emission at 391.4 nm, the optimal backward emission is achieved at a lower gas pressure around 10 mbar, which is due to asymmetry of the backward and forward directions rooted in the traveling excitation geometry. Comparison of the signals in pure nitrogen and air revealed a strong quenching effect of the oxygen molecules, preventing backward lasing action in ambient air.
|
|
| 7. |
- Zhang, Zhengquan, et al.
(author)
-
6G WIRELESS NETWORKS Vision, Requirements, Architecture, and Key Technologies
- 2019
-
In: IEEE Vehicular Technology Magazine. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 1556-6072 .- 1556-6080. ; 14:3, s. 28-41
-
Journal article (peer-reviewed)abstract
- A key enabler for the intelligent information society of 2030, 6G networks are expected to provide performance superior to 5G and satisfy emerging services and applications. In this article, we present our vision of what 6G will be and describe usage scenarios and requirements for multi-terabyte per second (Tb/s) and intelligent 6G networks. We present a large-dimensional and autonomous network architecture that integrates space, air, ground, and underwater networks to provide ubiquitous and unlimited wireless connectivity. We also discuss artificial intelligence (AI) and machine learning [1], [2] for autonomous networks and innovative air-interface design. Finally, we identify several promising technologies for the 6G ecosystem, including terahertz (THz) communications, very-large-scale antenna arrays [i.e., supermassive (SM) multiple-input, multiple-output (MIMO)], large intelligent surfaces (LISs) and holographic beamforming (HBF), orbital angular momentum (OAM) multiplexing, laser and visible-light communications (VLC), blockchain-based spectrum sharing, quantum communications and computing, molecular communications, and the Internet of Nano-Things.
|
|
| 8. |
- Zhang, Zhengquan, et al.
(author)
-
Full-Duplex Device-to-Device-Aided Cooperative Nonorthogonal Multiple Access
- 2017
-
In: IEEE Transactions on Vehicular Technology. - : Institute of Electrical and Electronics Engineers (IEEE). - 0018-9545 .- 1939-9359. ; 66:5, s. 4467-4471
-
Journal article (peer-reviewed)abstract
- This paper presents a full-duplex device-to-device (D2D)-aided cooperative nonorthogonal multiple access (NOMA) scheme to improve the outage performance of the NOMA-weak user in a NOMA user pair, where the NOMA-weak user is helped by the NOMA-strong user with the capability of full-duplex D2D communications. The expressions for the outage probability are derived to characterize the performance of the proposed scheme. The results show that the proposed cooperative NOMA scheme can achieve superior outage performance compared to the conventional NOMA and orthogonal multiple access (OMA). In order to further improve the outage performance, an adaptive multiple access (AMA) scheme is also studied, which dynamically switches between the proposed cooperative NOMA, conventional NOMA, and OMA schemes, according to the level of residual self-interference and the quality of links. The results show that the AMAscheme outperforms the above multiple access schemes in terms of outage performance.
|
|
| 9. |
- Zhang, Zhengquan, et al.
(author)
-
Full-Duplex Two-Way and One-Way Relaying : Average Rate, Outage Probability, and Tradeoffs
- 2016
-
In: IEEE Transactions on Wireless Communications. - 1536-1276 .- 1558-2248. ; 15:6, s. 3920-3933
-
Journal article (peer-reviewed)abstract
- In this paper, we systematically study the average rate and outage probability tradeoffs of full-duplex two-way and one-way relaying under residual self-interference. Among various relaying protocols, two common of them are considered: amplify-and-forward (AF) and decode-and-forward (DF). Furthermore, we consider the application of physical-layer network coding (PNC) and analog network coding (ANC) to full-duplex two-way relaying. Novel closed-form expressions for the average rate and outage probability, are presented. The results show that full-duplex two-way relaying can achieve higher rate than one-way relaying in the medium to high signal-to-noise ratio (SNR) region, at the cost of a certain loss in the outage performance. Moreover, DF protocol can achieve better outage performance than the AF one, but it suffers from a certain loss in the rate in the high SNR region. It is also shown that PNC can further improve the rate and outage performance. In addition, the results clearly reveal the effects of time multiplexing, forward protocol, and network coding on relaying systems, which would shed light on designing practical full-duplex relaying schemes.
|
|
| 10. |
- Zhang, Zhengquan, et al.
(author)
-
Heterogeneous Ultradense Networks with NOMA System Architecture, Coordination Framework, and Performance Evaluation
- 2018
-
In: IEEE Vehicular Technology Magazine. - : Institute of Electrical and Electronics Engineers (IEEE). - 1556-6072 .- 1556-6080. ; 13:2, s. 110-120
-
Journal article (peer-reviewed)abstract
- Heterogeneous ultradense networks (H-UDNs) are one key enabler for fifth-generation (5G) wireless networks and beyond to satisfy the explosive growth of mobile data traffic, which exploits spatial reuse of scarce spectrum by deploying massive base stations (BSS) to boost network capacity and enhance network coverage. In this article, we present the system architecture for 5G H-UDNs, consisting of virtualized integrated ground-Air-space radio access networks (RANs) and core networks and study network coordination for virtualized H-UDN to efficiently manage computing resources and intercell interference. We look at a cloud-fog-computing coordination framework for efficient computing resource management by achieving reasonable computing task distribution and transfer; computing load balance for computing tasks among virtual computing resources to improve network performance and computing resource efficiency; and a macro-small cell coordination framework for virtualized H-UDN with nonorthogonal multiple access (NOMA) to efficiently manage intercell interference and improve network performance. The joint macro-small enhanced intercell interference coordination (eICIC) and small-small coordinated multipoint joint transmission (CoMP-JT) scheme can efficiently eliminate macro-small intercell interference and utilize small-small intercell interference.
|
|
