Secrecy and energy efficiency in massive MIMO aided heterogeneous C-RAN: a new look at interference

In this paper, we investigate the potential benefits of the massive multiple-input multiple-output (MIMO) enabled heterogeneous cloud radio access network (C-RAN) in terms of the secrecy and energy efficiency (EE). In this network, both remote radio heads (RRHs) and massive MIMO macrocell base stations are deployed and soft fractional frequency reuse is adopted to mitigate the intertier interference. We first examine the physical layer security by deriving the area ergodic secrecy rate and secrecy outage probability. Our results reveal that the use of massive MIMO and C-RAN can greatly improve the secrecy performance. For C-RAN, a large number of RRHs achieves high area ergodic secrecy rate and low-secrecy outage probability, due to its powerful interference management. We find that for massive MIMO aided macrocells, having more antennas and serving more users improves secrecy performance. Then, we derive the EE of the heterogeneous C-RAN, illustrating that increasing the number of RRHs significantly enhances the network EE. Furthermore, it is indicated that allocating more radio resources to the RRHs can linearly increase the EE of RRH tier and improve the network EE without affecting the EE of the macrocells

A new look at physical layer security, caching, and wireless energy harvesting for heterogeneous ultra-dense networks

Heterogeneous ultra-dense networks enable ultra-high data rates and ultra-low latency through the use of dense sub-6 GHz and millimeter-wave small cells with different antenna configurations. Existing work has widely studied spectral and energy efficiency in such networks and shown that high spectral and energy efficiency can be achieved. This article investigates the benefits of heterogeneous ultra-dense network architecture from the perspectives of three promising technologies, physical layer security, caching, and wireless energy harvesting, and provides an enthusiastic outlook toward application of these technologies in heterogeneous ultra-dense networks. Based on the rationale of each technology, opportunities and challenges are identified to advance the research in this emerging network

Sparse supervised dimension reduction in high dimensional classification

Supervised dimension reduction has proven effective in analyzing data with complex structure. The primary goal is to seek the reduced subspace of minimal dimension which is sufficient for summarizing the data structure of interest. This paper investigates the supervised dimension reduction in high dimensional classification context, and proposes a novel method for estimating the dimension reduction subspace while retaining the ideal classification boundary based on the original dataset. The proposed method combines the techniques of margin based classification and shrinkage estimation, and can estimate the dimension and the directions of the reduced subspace simultaneously. Both theoretical and numerical results indicate that the proposed method is highly competitive against its competitors, especially when the dimension of the covariates exceeds the sample size

Hydrogen Storage Properties of Low-Silica Type X Zeolites

Hydrogen adsorption properties of low-silica type X zeolites (LSX, Si/Al = 1) containing alkali or alkali-earth metal cations (Li+, Ca2+, and Mg2+) have been studied. It was found that the hydrogen adsorption capacities of LSX zeolites at 77 K were determined mainly by the porosity of the zeolite, while at 298 K, the storage capacities depended on both the H2−cation interactions and the porosity. Among the three exchanged zeolites, Li-LSX had the highest H2 capacity of 1.5 wt % at 77 K and 1 atm, and Ca-LSX had the highest capacity of 0.50 wt % at 298 K and 10 MPa. The hydrogen storage in LSX zeolites via spillover was also investigated. Three methods including bridge building with a catalyst, metal doping via incipient wetness impregnation and metal doping via chemical vapor deposition (CVD) were employed to induce hydrogen spillover, and enhance the storage capacities. Thus, the storage capacities were increased to 0.96−1.2 wt % on the Pt-doped zeolites at 298 K and 10 MPa. The differences between the three methods were compared and discussed. Furthermore, 5 and 10 wt % Ni were doped on Ca-LSX zeolite. The 10 wt % Ni-doped Ca-LSX zeolite showed a storage capacity of 1.15 wt % at 100 atm and 298 K. The important volumetric storage capacities of these zeolites were also estimated based on the densities of the densified zeolites. A 21 g/L portion was obtained for Pt-doped Ca-LSX, and 20 g/L was obtained for Ni-doped Ca-LSX, both at 298 K and 10 MPa. The high volumetric capacities were obtained because of the high densities of zeolites which are substantially higher (2−3 times higher) than that of carbons and metal−organic frameworks

Hydrogen Storage Properties of N-Doped Microporous Carbon

A N-doped microporous carbon was synthesized by using NaY as a hard template and acetonitrile as the carbon and nitrogen precursor. The hydrogen storage measurements indicated that the N-doped microporous carbon had an 18% higher storage capacity than the pure carbon with a similar surface area. Furthermore, hydrogen storage via spillover was studied on a sample comprising Pt supported on N-doped microporous carbon, and a storage capacity of 1.26 wt % at 298 K and 10 MPa was obtained, showing an enhancement factor of 2.4 by spillover. In addition, the Pt/N-doped microporous carbon exhibited 1.46 times the storage capacity of Pt/microporous carbon. Significantly higher heats of adsorption were obtained on the N-doped microporous carbon samples than that on undoped carbons for both H2 adsorption and adsorption by spillover. The experimental results were consistent with the theoretical calculations from the literature

Increasing Selective CO₂ Adsorption on Amine-Grafted SBA-15 by Increasing Silanol Density

Two template removal methods were employed to create porosity in mesoporous silica SBA-15: ethanol extraction versus conventional high-temperature calcination. The resulting silicas were subjected to amine (3-aminopropyl) grafting and studied for their CO2 adsorption properties. The goal was to significantly increase the surface silanol density, and hence the grafted amine loading, leading directly to increased CO2 adsorption capacity and CO2/N2 selectivity. Thus, the silanol density was increased from 3.4 OH/nm2 for the calcined SBA-15 to 8.5 OH/nm2 for the SBA-15 by solvent extraction. Correspondingly, for these two samples, the grafted amine loading was increased from 2.2 to 3.2 mmol/g, and the CO2 adsorption capacity was increased from 1.05 to 1.6 mmol/g at conditions relevant to CO2 capture (0.15 bar and 25 °C), or a 52% increase. The CO2/N2 selectivity was increased from 46 to 131. The isosteric heats of adsorption, the sorbent stability during cyclic adsorption–desorption, and the (positive) effects of moisture on CO2 adsorption were also investigated and compared

Significantly Increased CO₂ Adsorption Performance of Nanostructured Templated Carbon by Tuning Surface Area and Nitrogen Doping

Carbon dioxide adsorption properties of a series of templated carbon adsorbents with high Brunauer–Emmett–Teller surface areas (1361–3840 m²/g) and with/without nitrogen doping (6–7 wt % N) were systematically studied. Two linear relationships between CO₂ adsorption capacities and surface areas of nitrogen-doped/undoped nanostructured templated carbons were first established. The doped nitrogen was present in the forms of pyridinic nitrogen, pyrrolic/pyridonic nitrogen, quaternary nitrogen, and an oxidized form of nitrogen. The interaction energies with CO₂, as approximated by the isosteric heats of adsorption, were increased from 30 kJ/mol on the undoped carbon to 50 kJ/mol on the N-doped carbon as a result of these nitrogen sites. The increased interactions led to an enhancement in CO₂ adsorption capacity by a factor of 2, while N₂ uptake was not enhanced. The optimized N-doped templated carbon, N-TC-EMC, possessed remarkable CO₂ capacity (4 mmol/g at 1 atm and 298 K) and selectivity (CO₂/N₂ at 1 atm = 14). Postdoping ammonia treatment was found beneficial to CO₂ adsorption. CO₂ performance of N-doped carbon under wet condition and conditions relevant to flue gas, rates of adsorption, and regeneration requirement, which are important for practical applications, were also investigated. The results showed that N-doped templated carbon exhibited all prerequisite attributes for CO₂ capture and storage applications: high CO₂ capacity and CO₂/N₂ selectivity, fast and reversible adsorption, thermal and moisture stabilities, and ease in CO₂ desorption

Characteristics of Hydrogen Storage by Spillover on Pt-Doped Carbon and Catalyst-Bridged Metal Organic Framework

Metal dispersion is a crucially important factor for hydrogen spillover storage on metal/carbon materials. For Pt on carbon (Pt/C), dispersion into nearly 2 nm clusters or nanoparticles is necessary to facilitate spillover. On an effective Pt/C spillover sorbent, temperature-programmed desorption (TPD) results reveal the highest hydrogen signal is from the high-energy Pt edges, steps or (110) surfaces, even though the (111) faces are more abundant. Previous theoretical studies showed the high-energy sites (including the 110 face) are by far the most preferred for effective splitting of hydrogen. These are in significantly smaller fractions for larger particles, and thus the larger particles are less efficient. In addition, the rate-limiting step for spillover on effective Pt/C is identified by the susceptibility to isotopic differences, first-order behavior and isolation from catalyzed H2/HD/D2 equilibrium measurements; we conclude it is the spillover step or surface diffusion. We extended our analysis to a review of our previous work, spillover on metal organic frameworks (MOFs). This has been achieved by bridging a commercial H2 dissociation catalyst (Pt/C) to MOFs, large enhancement factors (up to 8) were observed. Unlike Pt/C sorbents, sample-to-sample consistency in storage capacity on the bridged MOF samples is difficult to achieve. Inconsistency in the enhancements by spillover is shown; however, significant enhancement factors are still observed when samples are prepared and activated properly. Common pitfalls (and their consequences) in sample preparation for both Pt/C and bridged MOFs are discussed in detail

Glow Discharge Plasma-Assisted Template Removal of SBA-15 at Ambient Temperature for High Surface Area, High Silanol Density, and Enhanced CO₂ Adsorption Capacity

Glow discharge plasma was successfully applied for effective removal of the organic template P-123 from SBA-15 ordered mesoporous silica at near-room-temperature (below 50 °C) and in a short operation time (2 h). The as-made SBA-15 treated with glow discharge exhibited a larger surface area of 1025 m2 g–1 with larger pores and microspore volume as compared with that of conventional calcination (550 °C and 5 h, 827 m2 g–1). In addition to less structural shrinkage, the plasma-prepared SBA-15 showed significantly increased silanol density from 5.4 to 6.6–7.6 mmol g–1, which led directly to higher amine loading from 1.8 to 3.0 mmol g–1. Consequently, the plasma-treated sample showed 77% more CO2 capacity and 60% higher CO2/N2 selectivity than the conventionally treated sample at 0.15 bar and 25 °C. The advantage of using glow discharge plasma for low-temperature template removal for achieving enhanced performance for CO2 adsorption is clearly demonstrated

Prediction of the Effective Thermal Conductivity of Hollow Sphere Foams

Microscale and mesoscale hollow sphere foam (HSF) materials have attracted tremendous attention in recent decades due to their potential applications. Here, we study the effective thermal conductivity (ETC) of HSFs using an equivalent model, in which hollow spheres are first treated as equivalent solid particles, and then are combined with the ETC models that have been previously developed for solid particle filled composites. Compared with the rule of mixture model and syntactic foam models, this model shows better accuracy in predicting the ETC of HSFs. The theoretical model, together with finite element simulations, is then used to guide the design of HSFs. The results show that smaller size (nanoscale), lower packing fraction, lower shell conductivity, larger shell porosity, longer binder length, and higher interfacial thermal resistance lead to significantly lower ETC, while packing pattern, sphere size distribution, pore size of the porous shell, and binder radius have relatively minor influences. Moreover, size effects are investigated to use the proposed model for microscale and nanoscale problems. Aside from the well-known Knudsen effect, the size effect induced by interfacial thermal resistance should also be considered when the sphere size is smaller than a critical length. Interestingly, the Knudsen effect in the pores of a porous shell is shown to have an insignificant influence on the ETC. This study provides deep understanding of the thermal (and electrical, equivalently) behavior of the HSFs, which will potentially aid future design of novel and multifunctional HSF materials