To achieve the expected 1000x data rates under the exponential growth of traffic demand, a large number of BSs or APs will be deployed in 5G wireless systems to support high data rate services and to ...provide seamless coverage. Although such BSs are expected to be small-scale with lower power, the aggregated energy consumption of all BSs would be remarkable, resulting in increased environmental and economic concerns. In existing cellular networks, turning off the underutilized BSs is an efficient approach to conserve energy while preserving the QoS of mobile users. However, in 5G systems with new physical layer techniques and highly heterogeneous network architecture, new challenges arise in the design of BS ON-OFF switching strategies. In this article, we begin with a discussion of the inherent technical challenges of BS ON-OFF switching. We then provide a comprehensive review of recent advances on switching mechanisms in different application scenarios. Finally, we present open research problems and conclude the article.
Hybrid fillers of different geometries are increasingly utilized for the development of functional polymer composites. We herein report the role of HDPE-g-MAH as a compatibilizer for ternary ...composites consisting of HDPE, multi-walled carbon nanotubes and hexagonal boron nitride (BN). Through melt blending, HDPE-g-MAH can reduce the agglomeration of fillers and facilitate the formation of network structure. Due to the synergistic effect, ternary composites have demonstrated significantly higher thermal conductivity than those binary composites, and their maximum increase relative to the matrix is 262%. The mechanical performance and thermal conductivity are explained from perspectives of the morphology and crystallinity of the composites. The rheological properties of both binary and ternary composites have close relationship with their thermal conductivity. Although a high fraction of BN nanosheets can greatly reduce the electrical conductivity of ternary composites, they posed little effect on the electromagnetic interference shielding performance, owing to their electrical insulating nature. This research can provide new clues for the development of functional materials.
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In this paper, we investigate the problem of frame design, resource allocation, and user association in a massive multiple input multiple output (MIMO) heterogeneous network (HetNet) with wireless ...backhaul (WB) and linear processing. The objective is to maximize the sum downlink rate of all users, subject to constraints on data rates of WBs and fairness-aware constraints. Such a problem is formulated as an integer programming problem with both coupled variables and coupled constraints. We first develop a centralized scheme in which we decompose the original problem into two subproblems and iteratively solve them until convergence to achieve a near-optimal solution. We then propose a distributed scheme by formulating a repeated game among all users and prove that the game converges to a Nash Equilibrium. Simulation studies show that the proposed schemes are adaptive to different network scenarios and traffic patterns, and achieve considerable gains over several benchmark schemes.
We investigate the problem of base station (BS) ON-OFF switching, user association, and power control in a heterogeneous network (HetNet) with massive multiple input multiple output (MIMO), aiming to ...turn OFF under-utilized BS's and maximize the system energy efficiency. With a mixed integer programming problem formulation, we first develop a centralized scheme to derive the near optimal BS ON-OFF switching, which is an iterative framework with proven convergence. We further propose two distributed schemes based on game theory, with a bidding game between users and BS's, and a pricing game between wireless service provider and users. Both games are proven to achieve a Nash Equilibrium. Simulation studies demonstrate the efficacy of the proposed schemes.
Massive MIMO is envisioned as a promising technology for 5G wireless networks due to its high potential to improve both spectral and energy efficiency. Although the massive MIMO system is based on ...innovations in the physical layer, the upper layer techniques also play important roles in harvesting the performance gains of massive MIMO. In this article, we begin with an analysis of the benefits and challenges of massive MIMO systems. We then investigate the multi-layer techniques for incorporating massive MIMO in several important network deployment scenarios. We conclude this article with a discussion of open and potential problems for future research.
Wide-bandgap semiconductors exhibiting a bandgap of ∼1.7–1.9 eV have generated great interest recently due to their important applications in tandem solar cells as top cells and emerging indoor ...photovoltaics. However, concerns about the stability and toxicity especially in indoor application limit the choice of these materials. Here we report a new member of this family, germanium monosulfide (GeS); this material displays a wide bandgap of 1.7 eV, nontoxic and earth-abundant constituents, and high stability. We find that the little success of GeS solar cells to date is primarily attributed to the challenge in fabricating high-quality polycrystalline GeS films, wherein the high thermal expansion coefficient (α = 3.1 × 10–5 K–1) combined with high crystallization temperature (375 °C) of GeS induces large tensile strain in the GeS film that peels off GeS from the substrate. By introducing a high-α buffer layer between GeS and substrate, we achieve a high-quality polycrystalline GeS thin film that compactly adheres to substrate with no voids. Solar cells fabricated by these GeS films show a power conversion efficiency of 1.36% under AM 1.5G illumination (100 mW cm–2). The unencapsulated devices are stable when stored in ambient atmosphere for 1500 h. Their efficiencies further increase to 3.6% under indoor illumination of 1000 lux.
Due to the high potential to enhance spectral efficiency and spatial reuse, small cell networks, SCNs, have emerged as a promising solution to improve the capacity of mobile communication systems so ...as to satisfy the ever growing demand for high data rate services. However, without proper planning, the dense deployment of SCNs may cause severe interference, resulting in limited capacity. In hotspots with a large number of users, the small cell network is challenged by the extremely high aggregated capacity requirement and may fail to guarantee the quality of service of all users. To leverage the benefits of SCNs and overcome the drawbacks, we propose a cooperative small cell network, CSCN, architecture that jointly utilizes several advanced techniques to enhance the capacity of hotspots. In this article, we first examine the existing solutions for capacity enhancement and hotspots. We then present the basic concept of the proposed CSCN architecture, and discuss the related technical aspects. The high potential of a CSCN in terms of capacity improvement and interference mitigation is demonstrated by a simulation study. Finally, we present several open problems for future research based on the CSCN architecture.
Given the difficulty in manually adjusting the position and posture of the pile body during the pile driving process, the improved Denavit-Hartenberg (D-H) parameter method is used to establish the ...kinematics equation of the mechanical arm, based on the motion characteristics of each mechanism of the mechanical arm of the pile driver, and forward and inverse kinematics analysis is carried out to solve the equation. The mechanical arm of the pile driver is modeled and simulated using the Robotics Toolbox of MATLAB to verify the proposed kinematics model of the mechanical arm of the pile driver. The Monte Carlo method is used to investigate the working space of the mechanical arm of the pile driver, revealing that the arm can extend from the nearest point by 900 mm to the furthest extension of 1800 mm. The actuator’s lowest point allows for a descent of 1000 mm and an ascent of up to 1500 mm. A novel multi-strategy grey wolf optimizer (GWO) algorithm is proposed for robotic arm three-dimensional (3D) path planning, successfully outperforming the basic GWO, ant colony algorithm (ACA), genetic algorithm (GA), and artificial fish swarm algorithm (AFSA) in simulation experiments. Comparative results show that the proposed algorithm efficiently searches for optimal paths, avoiding obstacles with shorter lengths. In robotic arm simulations, the multi-strategy GWO reduces path length by 16.575% and running time by 9.452% compared to the basic GWO algorithm.
In this paper, we investigate the problem of optimal base station (BS) ON-OFF switching and user association in a heterogeneous network (HetNet) with massive MIMO, with the objective to maximize the ...system energy efficiency (EE). The joint BS ON-OFF switching and user association problem is formulated as an integer programming problem. We first develop a centralized scheme, in which we relax the integer constraints and employ a series of Lagrangian dual methods that transform the original problem into a standard linear programming (LP) problem. Due to the special structure of the LP, we prove that the optimal solution to the relaxed LP is also feasible and optimal to the original problem. We then propose a distributed scheme by formulating a repeated bidding game for users and BS's, and prove that the game converges to a Nash Equilibrium (NE). Simulation studies demonstrate that the proposed schemes can achieve considerable gains in EE over several benchmark schemes in all the scenarios considered.
The narrow face of continuous casting mold is the first impact region of the jet from the submerged entry nozzle (SEN), and against the characteristic a new pattern of electromagnetic brake device is ...proposed, which is called Vertical Electromagnetic Brake (V-EMBr) and its magnetic poles can cover the free surface and the frontier region of solidified shell. The effect laws of magnetic flux density, casting speed and the submergence depth of the SEN on flow field in mold with V-EMBr are investigated by numerical simulation method. The results show that with the magnetic flux density increasing, inhibitory effect of electromagnetic force on molten steel from SEN is increased gradually, impact strength of molten steel stream on narrow face is weakened gradually, the vortex center of lower recirculation zone is moved up gradually and the flow velocity of free surface is decreased gradually. These can reduce the fluctuations of free surface and chance of slag effectively, suppress impinging depth of molten steel in lower recirculation zone and float air bubbles and inclusions beneficially. In addition, the V-EMBr technology can also be applied to different casting speed and different submergence depth of the SEN in continuous casting process. The significantly metallurgical effects and flexibility of application range which are generated by V-EMBr is in line with the original intention of the V-EMBr design.
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