Joint Optimization Scheme Based on Beam Selection and Interference Cancellation in Lens Millimeter Wave NOMA Systems

Non-orthogonal multiple access(NOMA)has been integrated in millimeter-wave(mmWave)Massive MIMO systems to further enhance the spectrum efficiency, but NOMA has not been fully considered in lens mmWave systems. The fusion of these two technologies requires the joint design of beam selection and interference cancellation. In addition, when users follow the spatial cluster distribution, although the user clustering schemes based on K-means algorithm have been applied, the influence of the virtual and actual cluster center users on achievable sum rate has not been differentiated and analyzed in detail. To solve the above problems, a joint optimization scheme is proposed to maximize the achievable sum rate. The optimization problem is NP-hard, which is solved by using the divide-and-conquer approach. Concretely,based on the signal power loss analysis of directional deviation, a beam selection algorithm is designed for inter-cluster interference cancellation based on the Kmeans algorithm. Further for intra-cluster interference cancellation, a power allocation algorithm based on the bisection method is designed to guarantee the fairness of users in each cluster. The simulation results show that the proposed scheme offers a significant performance improvement in terms of both achievable sum rate and energy efficiency, and it is suitable for the large-scale user scenario due to its low complexity.

Fingerprint-Based Millimeter-Wave Beam Selection for Interference Mitigation in Beamspace Multi-User MIMO Communications

Millimeter-wave communications are suitable for application to massive multiple-input multiple-output systems in order to satisfy the ever-growing data traffic demands of the next-generation wireless communication.However,their practical deployment is hindered by the high cost of complex hardware,such as radio frequency(RF)chains.To this end,operation in the beamspace domain,through beam selection,is a viable solution.Generally,the conventional beam selection schemes focus on the feedback and exhaustive search techniques.In addition,since the same beam in the beamspace may be assigned to a different user,conventional beam selection schemes suffer serious multi-user interference.In addition,some RF chains may be wasted,since they do not contribute to the sum-rate performance.Thus,a fingerprint-based beam selection scheme is proposed to solve these problems.The proposed scheme conducts offline group-based fingerprint database construction and online beam selection to mitigate multi-user interference.In the offline phase,the contributing users with the same best beam are grouped.After grouping,a fingerprint database is created for each group.In the online phase,beam selection is performed for purposes of interference mitigation using the information contained in the group-based fingerprint database.The simulation results confirm that the proposed beam selection scheme can achieve a signal-to-interference-plus-noise ratio and sum-rate performance which is close to those of a fully digital system,and having much higher energy efficiency.

Coverage and rate analysis of user beam selection in mm-wave heterogeneous networks

Millimeter wave(mm-wave)communication is widely considered to be a promising technique for 5G(Fifth Generation)cellular systems.Owing to the high path loss of mm-wave channels,5G networks could employ a heterogeneous structure that consists of an MBS(Macro Base Station)and numerous SBSs(Small Base Stations).In this paper,we analyze the coverage and rate performance of an mm-wave heterogeneous network.Using user beam selection,we derive analytical expressions for the coverage probabilities of the SBSs and MBS.Furthermore,the average achievable rate for a typical user is also investigated.It is shown that the analytical results closely follow those of the simulations with marginal differences.

Corrosion and wear properties of micro-arc oxidation treated Ti6Al4V alloy prepared by selective electron beam melting

In order to analyze the effect of voltage during micro-arc oxidation(MAO)on corrosion and wear properties of Ti6Al4V(TC4),the MAO technology was employed to treat TC4 samples fabricated by selective electron beam melting(SEBM)at the voltages of 400,420 and 450 V.The results show that the metastable anatase phase gradually transforms to rutile phase with oxidation time and temperature increasing.The surface morphology of coating contains numerous micropores with uniform size distribution.Cracks and pores over 10μm are found on MAO-TC4 sample with applied voltage of 450 V.The thickness of MAO coating is positively correlated with the voltage.The corrosion resistance and wear resistance are related to phase composition,micropore size distribution on the surface and film thickness.When the voltage is 420 V,the coating shows the smallest corrosion current density(0.960×10^-7 A/cm^2)and the largest resistance(7.17×10^5Ω·cm^2).Under the same load condition,the coating exhibits larger friction coefficient and wear loss than the TC4 substrate.With the increase of voltage,the wear mechanism of the coating changes from abrasive wear to adhesive wear,and the adhesive wear is intensified at applied voltage of 450 V,with a maximum friction coefficient of 0.821.

A yttrium-containing high-temperature titanium alloy additively manufactured by selective electron beam melting

A yttrium-containing high-temperature titanium alloy(Ti-6Al-2.7Sn-4Zr-0.4Mo-0.45Si-0.1Y, mass fraction, %) has been additively manufactured using selective electron beam melting(SEBM). The resulting microstructure and textures were studied using scanning electron microscopy(SEM), transmission electron microscopy(TEM), X-ray diffraction(XRD) and electron backscattered diffraction(EBSD) and compared with the conventionally manufactured form. A notable distinct difference of microstructures is that additive manufacturing by SEBM enables homogeneous precipitation of fine Y2O3 dispersoids in the size range of 50-250 nm throughout the as-fabricated alloy, despite the presence of just trace levels of oxygen(7×10-4, mass fraction) and yttrium(10-3, mass fraction) in the alloy. In contrast, the conventionally manufactured alloy shows inhomogeneously distributed coarse Y2O3 precipitates, including cracked or debonded Y2O3 particles.

Performance of Ti6Al4V fabricated by electron beam and laser hybrid preheating and selective melting strategy

Electron beam selective melting(EBM)and selective laser melting(SLM)are regarded as significant manufacturing processes for near-net-shaped Ti6Al4V components.Generally,in the conventional EBM process,preheating is necessitated to avoid"smoke"caused by the charging of electrons.In the conventional SLM process,laser as an energy source without the risk of"smoke"can be employed to melt metal powder at low temperatures.However,because of the low absorption rate of laser,the powder bed temperature cannot reach a high level.It is difficult to obtain as-built TiAl4V with favorable comprehensive properties via conventional EBM or SLM.Hence,two types of electron beam and laser hybrid preheating(EB-LHP)combined with selective melting strategies are proposed.Using laser to preheat powder allows EBM to be performed at a low powder bed temperature(EBM-LT),whereas using an electron beam to preheat powder allows SLM to be performed at a high powder bed temperature(SLM-HT).Ti6Al4V samples are fabricated using two different manufacturing strategies(i.e.,EBM-LT and SLM-HT)and two conventional processes,i.e.,EBM at a high powder bed temperature(EBM-HT)and SLM at a low powder bed temperature(SLM-LT).The temperature-dependent surface quality,microstructure,density,and mechanical properties of the as-built Ti6Al4V samples are characterized and compared.Results show that EBM-LT Ti6Al4V exhibits a higher ultimate tensile strength(981±43 MPa)and a lower elongation(12.2%±2.3%)than EBM-HT Ti6Al4V owing to the presence ofα′martensite.The SLM-HT Ti6Al4V possesses the highest ultimate tensile strength(1,059±62 MPa)and an elongation(14.8%±4.0%)comparable to that of the EBM-HT Ti6Al4V(16.6%±1.2%).

Microstructural evolution and dynamic recrystallization mechanisms of additively manufactured TiAl alloy with heterogeneous microstructure during hot compression

Microstructural evolution and dynamic recrystallization(DRX)mechanisms of a Ti-48Al-2Cr-2Nb(at.%)alloy prepared by selective electron beam melting(SEBM)during hot deformation at 1150℃and 0.1 s^(-1)were investigated by hot compression tests,optical microscope(OM),scanning electron microscope(SEM),electron back-scattered diffraction(EBSD)and transmission electron microscope(TEM).The results show that the initial microstructure of the as-SEBMed alloy exhibits layers of coarseγgrains and fineγ+α_(2)+(α_(2)/γ)lamellar mixture grains alternately along the building direction.During the early stage of hot deformation,deformation twins tend to form within the coarse grains,facilitating subsequent deformation,and a small number of DRX grains appear in the fine-grained regions.With the increase of strain,extensive DRX grains are formed through different DRX mechanisms in both coarse and fine-grained regions,involving discontinuous dynamic recrystallization mechanism(DDRX)in the fine-grained regions and a coexistence of DDRX and continuous dynamic recrystallization(CDRX)in the coarsegrained regions.

Microstructure and mechanical properties of WMoTaNbTi refractory high-entropy alloys fabricated by selective electron beam melting

WMoTaNbTi RHEAs formed by SEBM with negative defocus distance were investigated.Four scanning speeds were applied,an electron beam with scanning speed at 2.5 m/s completely fused the premixed WMoTaNb alloyed powder and pure Ti powder.Significant vaporization of Nb and Ti elements happened during the formation of WMoTaNbTi RHEAs,however,the single BCC phase remains stable.Weakened solid-solute strengthening caused by elemental vaporization,dropping percentage of Nb and Ti solutes in the matrix as well as improved ductilizing effects with decreasing scanning speeds leads to falling microhardness and better local ductility.Microhardness of scanning speed at 4.0 m/s,3.5 m/s,3.0 m/s and 2.5 m/s is 578±17 HV,576±12 HV,573±10 HV and 511±2 HV,respectively.The as-deposited WMoTaNbTi RHEA formed at a scanning speed of 2.5 m/s displays ultimate strength of 1312 MPa.

Hierarchical microstructure of a titanium alloy fabricated by electron beam selective melting

Here,a near alpha-type Ti6.5 Al2 Zr1 Mo1 V alloy has been fabricated by electron beam selective melting(EBSM).Near-equiaxed grains existed in the first few layers,whereas elongated columnar priorβgrains almost parallel to the building direction formed in the subsequent built layers.Interspacing ofβphase gradually decreased as the build height increased.Martensiteα'with twins and dislocations emerged and microhardness value reached the maximum in the top region,whereas onlyα/βphase appeared in other regions in the EBSMed sample.Multiple phase transformations can be observed with the change of peak temperatures during each thermal cycle.With a sufficient dwell time,martensiteα'in the middle and bottom regions in-situ decomposed intoα+βand coarsened by the heat conduction from the subsequent layers.Fineβprecipitates nucleated heterogeneously insideα'plates and at plate-plate interfaces during the subsequent EBSM process.Considering the phase transformation during the heating process and the cooling process,the existence time of different phases was combined with cycle heating and cooling to clarify the dynamic evolution of micro structure under complex thermal history of EBSM,favoring the fabrication of high-performance titanium alloy components.

Microstructure and high-temperature tensile property of TiAl alloy produced by selective electron beam melting

The microstructure and high-temperature tensile property of a Ti-47Al-2Cr-2 Nb alloy fabricated via selective electron beam melting(SEBM) with hatch spacings of 85,100,and 115 μm were systematically investigated.When the hatch spacing increased from 85 to 115 μm,the microstructure comprising the horizontal cross section changed from coarse lamellar(y/B2) colonies to an inhomogeneous structure and the grain morphology transformed from elongated grains to inhomogeneous and equiaxed grains along the building direction of the vertical cross section.The boundary population of the SEBMproduced TiAl alloy samples was dominated by high-angle grain boundaries(≥ 15°),and the volume fraction of these boundaries decreased with hatch spacing increasing.Additionally,the as-built TiAl alloy sample produced under a spacing of 100 μm exhibited the highest room-and elevated-temperature tensile strengths,with the ultimate tensile strength at room temperature(642 MPa) increasing to 674 MPa at 700 ℃.Furthermore,the mechanism of anomalous strengthening at 700 ℃ was discussed in detail.

Hot rolling and annealing effects on the microstructure and mechanical properties of ODS austenitic steel fabricated by electron beam selective melting

The grain morphology, nano-oxide particles and mechanical properties of oxide dispersion strengthened （ODS）-316L austenitic steel synthesized by electron beam selective melting （EBSM） technique with different post-working processes, were explored in this study. The ODS-316L austenitic steel with superfine nano-sized oxide particles of 30-40 nm exhibits good tensile strength （412 MPa） and large total elongation （about 51%） due to the pinning effect of uniform distributed oxide particles on dislocations. After hot rolling, the specimen exhibits a higher tensile strength of 482 MPa, but the elongation decreases to 31.8% owing to the introduction of high-density dislocations. The subsequent heat treatment eliminates the grain defects induced by hot rolling and increases the randomly orientated grains, which further improves the strength and ductility of EBSM ODS-316L steel.

Scan Strategy in Electron Beam Selective Melting

In electron beam selective melting process, powder pushed-away phenomena and uneven temperature field are two main obstacles, which are greatly associated with the electron beam scan mode. In this paper, various scan strategies, including iterative scan mode, reverse scan mode, interlaced reverse scan mode, randomized block scan mode, and constant length scan mode, are investigated. The analyses for each scan strategy are presented based on the influence to the temperature field over the formation zone and the powder pushed-away phenomena. The most promising strategy, interlaced reverse scan mode, is approved by the ANSYS simulation and a two-dimensional scan experiment. The result shows interlaced reverse scan mode can improve the uniformity of the temperature field and reduce the powder pushed-away phenomena.

增材制造制备钨及钨合金材料研究进展

钨及钨合金具有熔点高、硬度大、高温强度高等优异性能,广泛应用于航空航天、国防军工、核工业、医疗放射屏蔽材料、电子等领域。增材制造(additive manufacturing, AM)是制备复杂结构钨及钨合金成形件的有效方法。激光选区熔化(selective laser melting, SLM)和电子束选区熔化(selective electron beam melting, SEBM)是制备钨及钨合金常用的两种AM技术,主要围绕原料粉末、成形工艺、合金化和后处理等因素对钨及钨合金成形件的致密度、微观组织、力学性能和缺陷等角度开展研究。由于纯钨的表面张力大、粘度大、润湿性差,纯钨成形件中易出现球化现象、裂纹和气孔3种缺陷,阐述了缺陷形成机制及解决措施。从材料体系、制备工艺、材料性能和数值模拟方面对未来SLM和SEBM技术制备钨和钨合金成形件的发展方向进行了展望,以期建立工艺参数-微观组织-缺陷特征-力学性能的内在联系,快速实现成形工艺的调控和优化。

Additive Manufacturing of Nickel-Based Superalloy Single Crystals with IN-738 Alloy

The production of non-weldable nickel-based superalloys,especially single-crystal superalloys,is important for additive manufacturing.Single-crystal specimens of non-weldable nickel-based superalloys were produced by electron beam selective melting using an IN-738 alloy.In this study,single-crystal nickel-based superalloy specimens of diff erent sizes were prepared by a multiple preheating process and tight control of the melting parameters without the need for a grain selector or single-crystal seed for the first time.Electron backscattered diff raction measurements were performed to confi rm the presence of a single crystal.The transition boundaries between the polycrystalline and single-crystal regions at the edges and bottom of the samples were characterized to analyze the formation of single crystals.

A comparative study of surface characterization and corrosion behavior of micro-arc oxidation treated Ti-6Al-4V alloy prepared by SEBM and SLM

Additively manufactured Ti-6Al-4V(TC4)parts have been successfully employed as artificial implants in dental and orthopedic surgery due to their excellent mechanical properties.However,the suboptimal corrosion resistance limits their applications.The surface characterization and corrosion behavior of micro-arc oxidation(MAO)treated TC4 alloy prepared by selective electron beam melting(SEBM)and selective laser melting(SLM)technologies were compared.The corrosion resistance mechanism of SLM-MAO and SEBM-MAO was clarified through the analysis of the microstructure evolution,surface morphology,and electrochemical experiments.The results show that the anatase-type TiO_(2) is partially transformed into the slankite phase after MAO treatment.The surface roughness of SEBM sample was reduced by MAO coating,while the surface roughness of SLM sample increased after MAO,which is related to the difference between the microstructure of the deposited samples caused by different additive manufacturing technologies.When MAO time was 15 min,SLM-MAO and SEBM-MAO coating displayed the best and the worst corrosion resistance,respectively.