Classification and comparison of massive MIMO propagation channel models

Considering great benefits brought by massive multiple-input multiple-output (MIMO) technologies in Internet of things (IoT), it is of vital importance to analyze new massive MIMO channel characteristics and develop corresponding channel models. In the literature, various massive MIMO channel models have been proposed and classified with different but confusing methods, i.e., physical vs. analytical method and deterministic vs. stochastic method. To have a better understanding and usage of massive MIMO channel models, this work summarizes different classification methods and presents an up-to-date unified classification framework, i.e., artificial intelligence (AI)-based predictive channel models and classical non-predictive channel models, which further clarify and combine the deterministic vs. stochastic and physical vs. analytical methods. Furthermore, massive MIMO channel measurement campaigns are reviewed to summarize new massive MIMO channel characteristics. Recent advances in massive MIMO channel modeling are surveyed. In addition, typical non-predictive massive MIMO channel models are elaborated and compared, i.e., deterministic models and stochastic models, which include correlation-based stochastic model (CBSM), geometry-based stochastic model (GBSM), and beam domain channel model (BDCM). Finally, future challenges in massive MIMO channel modeling are given

The progress of TMT Laser Guide Star Facility

The Laser Guide Star Facility (LGSF) is responsible for generating the artificial laser guide stars required by the TMT Laser Guide Star (LGS) AO systems. The LGSF uses multiple sodium lasers to generate and project several LGS asterisms from a laser launch telescope located behind the TMT secondary mirror. The LGSF includes 3 main subsystems: (1) the laser system, (2) the beam transfer optics (BTO) system, (3) the associated laser safety system. At present, the LGSF is in the preliminary design phase. During this phase, the laser launch telescope trade study, Beam transfer optical path trade study are compared carefully, and some critical components prototypes have been carried out to verify the requirements, such as the polarization status control and test, the Fast Steer Mirror (FSM) prototype test

The progress of TMT Laser Guide Star Facility

The Laser Guide Star Facility (LGSF) is responsible for generating the artificial laser guide stars required by the TMT Laser Guide Star (LGS) AO systems. The LGSF uses multiple sodium lasers to generate and project several LGS asterisms from a laser launch telescope located behind the TMT secondary mirror. The LGSF includes 3 main subsystems: (1) the laser system, (2) the beam transfer optics (BTO) system, (3) the associated laser safety system. At present, the LGSF is in the preliminary design phase. During this phase, the laser launch telescope trade study, Beam transfer optical path trade study are compared carefully, and some critical components prototypes have been carried out to verify the requirements, such as the polarization status control and test, the Fast Steer Mirror (FSM) prototype test

Luminescence spectra of doped and undoped lead tungstate crystals

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Effect of Reduction Annealing on the Coloration Mechanism of Yellow Sapphire with High Iron Content

The color of yellow sapphire from Africa characterized by high iron content and low levels of other transition metal elements was changed from yellow to grayish-blue after high-temperature reduction annealing. Before reduction annealing, the optical absorption spectra showed that the outer d–d electron transitions of Fe3+ were the main coloring cause of yellow sapphires, but the charge transfer between O2− and Fe3+ may have a greater contribution. The change in lattice parameter indicates that Fe3+ is reduced to Fe2+ during reduction annealing, and adjacent Fe2+ and Fe3+ form an Fe2+-Fe3+ ion pair. The absorption caused by intervalence charge transfer of Fe2+-Fe3+ is the essential reason for the grayish-blue appearance of yellow sapphires after reduction annealing. The charge compensation mechanism of Fe2+-Fe3+ in natural sapphire is also discussed, and oxygen vacancy is considered to be the most suitable charge compensator for Fe2+-Fe3+

Effect of Li+Li^+ ions co-doping on luminescence, scintillation properties and defects characteristics of LuAG:Ce ceramics

Monovalent $Li^+$ codoped $Lu_3Al_5O_{12}$:Ce (LuAG:Ce) optical ceramics were fabricated by solid state reaction method and further optimized by an air-annealing process. Optical absorption, radioluminescence spectra and scintillation properties such as light yield, scintillation decay times and afterglow were measured and compared with those of the $Li^+$ free LuAG:Ce ceramic and the commercial LuAG:Ce single crystal samples. Positive effect of $Li^+$ codopant consists mainly in the significant increase of scintillation light yield, acceleration of scintillation decay as well as the decrease of afterglow intensity. With 0.3% Li codoping, the obtained LuAG:Ce,Li ceramic displays a light yield of ∼29200 ph/MeV at 10 μs shaping time, higher than that of the LuAG:Ce single crystal and optical ceramic scintillators ever reported. The partial conversion of the stable $Ce^{3+}$ to $Ce^{4+}$ centers and the shallow and deep traps effect suppression by the $Li^+$ codoping are discussed

Optical, luminescence and scintillation characteristics of non-stoichiometric LuAG:Ce ceramics

Non-stoichiometric $Lu_{3+x}Al_{5}O_{12}:Ce$ ($Lu_{3+x}$AG:Ce, x=1, 2, 3 and 4 %) ceramics were fabricated by solid state reaction method and further optimized by an air-annealing process. Absorption, luminescence spectra and scintillation characteristics such as light yield, scintillation decay times, energy resolution, proportionality and afterglow were measured and compared with those of the latest LuAG:Ce single crystal and stoichiometric LuAG:Ce,Mg ceramic samples. Thanks to the elimination of oxygen vacancies produced in the vacuum sintering process, air-annealing treatment led to a significant decrease of afterglow and a remarkable enhancement of radioluminescence intensity and light yield. The highest light yield was found in annealed 1% Lu3+ rich Lu3+1%AG:Ce ceramic, reaching 14,760 ph/MeV (1 μs shaping time) and 22,400 ph/MeV (10 μs shaping time). Scintillation decays of $(Lu_{3+x}$AG:Ce ceramics consist of both fast (decay time 65–73 ns) and slow (decay time 740–1116 ns) decay components where the relative intensity of the latter is higher (~58%). A decreasing trend in scintillation efficiency was observed with increasing excess of Lu (with higher x values) in the samples. This can be explained by the existence of various electron traps due to LuAl antisite defect and structure disorder at the gain boundaries and interfaces

Towards Bright and Fast Lu3Al5O12:Ce,Mg Optical Ceramics Scintillators

The recent advent of Lu 3 Al 5 O 12 :Ce optical ceramics marks a turning point in scintillator material technology. Because of their lower preparation tem-perature, brightness, and robustness such materials can now compete with single crystals. Their further scintillation effi ciency optimization includes the thorough control of the defects responsible for optical and scintillation losses. The choice of sintering agent appears critical to achieve both high optical transparency and scintillation performance. In this work, the optical investi-gations coupled with X-ray absorption near-edge spectroscopy evidence the benefi cial role of MgO sintering agent. Mg 2+ co-dopants in ceramics drive the partial conversion of Ce 3+ to Ce 4+ . The Ce 4+ center, however, does not impair the scintillation performance due to its capability to positively infl uence the scintillation process. The importance of simultaneous application of such co-doping and annealing treatment is also demonstrated. With 0.3 at% Mg, our ceramics display a light yield of ≈25000 photons/MeV with short 1 µs shaping time, a relative fast component intensity as high as 60%, and very low afterglow. Such performances are better than those of the isostructural single crystals ever reported. We discuss the role of co-doping and annealing in the scintillation mechanism that make such combined treatments a posi-tive strategy for material engineering