A Compact Ultra-Wideband Circularly Polarized Antenna Array for Vehicular Communications

In this paper,a new compact ultrawideband(UWB)circularly polarized(CP)antenna array for vehicular communications is proposed.The antenna array consists of a 2×2 sequentially rotated T-shaped cross dipole,four parasitic elements,and a feeding network.By loading the T-shaped cross dipoles with parasitic rectangular elements with cut corners,the bandwidth can be expanded.On this basis,the radiation pattern can be improved by the topology with sequential rotation of four T-shaped cross-dipole antennas,and the axial ratio(AR)bandwidth of the antenna also can be further enhanced.In addition,due to the special topology that the vertical arms of all Tshaped cross dipoles are all oriented toward the center of the antenna array,the gain of proposed antenna is improved while the size of the antenna is almost the same as the traditional cross dipole.Simulated and measured results show that the proposed antenna has good CP characteristics,an impedance bandwidth for S11<-10 d B of about 106.1%(3.26:1,1.57-5.12 GHz)and the 3-d B AR bandwidth of about 104.1%(3.17:1,1.57-4.98 GHz),a wide 3-d B gain bandwidth of 73.3%as well as the peak gain of 8.6 d Bic at 3.5 GHz.The overall size of antenna is 0.56λ×0.56λ×0.12λ(λrefers to the wavelength of the lowest operating frequency in free space).The good performance of this compact UWB CP antenna array is promising for applications in vehicular communications.

Joint User Association and Fair Scheduling for Dual Connectivity Heterogeneous Networks

Heterogeneous small cell network is one of the most effective solutions to overcome spectrum scarcity for the next generation of mobile networks.Dual connectivity(DC)can improve the throughput for each individual user by allowing concurrent access to two heterogeneous radio networks.In this paper,we propose a joint user association and fair scheduling algorithm(JUAFS)to deal with the resource allocation and load balancing issues for DC heterogeneous small cell networks.Considering different coverage sizes,numbers of users,and quality of experience characteristics of heterogeneous cells,we present a proportional fair scheduling for user association among cells and utilize interference graph to minimize the transmission conflict probability.Simulation results show the performance improvement of the proposed algorithm in spectrum efficiency and fairness comparing to the existing schemes.

Green Distributed Antenna Systems for Smart Communities: A Comprehensive Survey

Smart communities are an emerging communication means in which humans and smart devices will interact with each other and deliver ubiquitous services by exploiting social intelligence.Distributed antenna system(DAS),one of the key technologies to realize smart decisions in smart communities,can settle network smart coverage problem and improve system energy/spectrum efficiency significantly.Considering that energy consumption is an important element for community communications,in this paper,we survey the existing green DAS research for smart communities.In particular,our investigation covers antenna distribution,system capacity,spectrum efficiency,energy efficiency,and green access issues.Moreover,we analyze the existing application opportunities and challenges.This survey contributes to better understanding of the challenges and approaches for green DAS in existing smart community networks and further shed novel light on some future research directions.

Unlicensed Band Sharing Based on Bargaining Game for Heterogeneous Networks

Currently,limited licensed frequency bands cannot meet the increasing demands for various wireless communication applications any more.It is necessary to extend wireless communication networks to unlicensed spectrum.In this paper,we propose a new bargaining framework for unlicensed band access to achieve high spectrum efficiency,where one radio access technology(RAT)(such as macro cellular network)“competes”the unlicensed bands with multiple other RATs(such as small cellular networks or Wi-Fi)virtually.Considering that macro cell can share unlicensed frequencies with multiple small cells which are in the same coverage area for more freedom,we use bargaining game theory to fairly and effectively share the unlicensed spectrum between macro and multiple heterogeneous small cell networks,where bargaining loss and time dissipation loss for virtual“price”of unlicensed bands are mainly considered.In the oneto-many bargaining process,we also develop a multiple RAT alliance game strategy to reduce transmission loss in a joint manner.Simulation results show that the proposed unlicensed band sharing algorithm significantly improves the spectrum efficiency performance compared with the other practical schemes for heterogeneous networks.

Capacity-Optimized Access Scheduling Control for Heterogeneous MANETs with MIMO Links

Multiple-input and multiple-output (MIMO) technique can significantly improve transmission reliability and bandwidth efficiency in wireless networks. However, many upper layer aspects of MIMO links, especially in mobile networks with heterogeneous antennas and strong interference environments, need further investigation. In this paper, we study its impact on medium access and network capacity. Since MIMO links can enhance physical layer capacity and co-channel interference suppression that affects network access scheduling directly, we develop a capacity-optimized access scheduling control (COASC) scheme for mobile ad hoc networks (MANETs) with MIMO links. We formulate the distributed scheduling taking the key of optimization into design to improve the network capacity and transmission reliability. Simulation results are presented to show the effectiveness of the proposed scheme.

Strengthening mechanism of different morphologies of nanosized MSH on tribological performance of phosphate/MoS2 bonded solid lubricating coatings

Magnesium silicate hydroxides(MSHs)with granular,schistose,and tubular morphologies were separately incorporated to enhance the tribological properties of phosphate/MoS2 composite coatings.The nano-schistose MSH demonstrated superior tribological performance due to its effective interactions with the worn surface and frictional synergies with solid lubricants.Incorporation of nano-schistose MSH decreased the friction coefficient of composite coatings by about 34.7%and increased the anti-wear performance of composite coatings by about thirteen times.Nano-schistose MSH facilitated the formation of a friction-induced multi-layer heterogenous slipping structure with layered solid lubricants at the friction interface.Moreover,tribo-chemical reactions between nano-schistose MSH and worn surface promoted the in-situ formation of a cermet supporting film,and this also induced the gradual in-situ formation of a lubrication film on the top of worn surface.Consequently,the contact state between tribo-pairs was timely regulated and the invalidation of the nanocomposite slipping structure was effectively restrained during the friction process.As a result,the service life of the phosphate composite coatings was significantly extended and further abrasion on the worn surface was notably reduced.

Preparation of Cu-Ni bimetallic nanoparticles surface-capped with dodecanethiol and their tribological properties as lubricant additive

Cu-Ni bimetallic nanoparticles surface-capped with dodecanethiol were prepared via the controlled reduction of nickel nitrate and copper nitrate in the presence of dodecanethiol as ligand and hydrazine hydrate as reductant. The morphology and structure of the Cu-Ni nanoparticles were analyzed by trans- mission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectrometry, and thermal gravimetric analysis. Furthermore, the tribological properties of the Cu-Ni nanoparticles as a lubricant additive in liquid paraffin were evaluated with a four-ball machine, and the morphology and elemental composition of the worn surfaces were analyzed by three-dimensional non-contact surface profilometry and X-ray photoelectron spectroscopy, respectively, The results indi- cated that Cu-Ni nanoparticles prepared at a sufficient dodecanethiol concentration showed almost no signs of aggregation, and exhibited good dispersibility in various apolar solvents. The addition of the Cu-Ni nanoparticles improved the load-carrying capacity as well as friction-reducing and anti-wear abilities of liquid paraffin, attributed to the roiling effect of agglomerated Cu-Ni nanocores released by the Cu-Ni nanoparticles during sliding, and the formation of a boundary lubricating film containing tribochemical reaction products such as iron oxides and iron sulfate.

多羰基共价有机骨架在二次电池中的应用

共价有机骨架(covalent organic frameworks,COFs)是一类由构建单元通过共价键连接形成的新兴晶体多孔材料。凭借超高的孔隙率、规则的一维通道、稳定的骨架结构和出色的结构可设计性等特点,COFs被认为在二次电池中极具应用前景。本文综述了含有多羰基构建单元的COFs(multi-carbonyl COFs,Mc-COFs)材料在不同金属离子二次电池中的研究进展,对Mc-COFs作为电极材料和固态电解质材料面临的挑战进行了概括,并且详细介绍了电池性能的提升策略,最后对Mc-COFs在二次电池领域的发展方向进行展望。