Network densification via deploying dense small cells is one of the dominant evolutions towards future cellular network to increase spectrum efficiency. Packet transmission delay and reliability in the resultant interference-limited heterogeneous cellular network (HCN) are essential performance metrics for system design. By modeling the locations of base stations (BSs) in HCN as superimposed of independent Poisson point processes, we propose an analytical framework to derive the timely throughput of HCN, which captures both the delay and reliability performance. In the analysis, the BS activity and temporal correlation of transmissions are taken into consideration, both of which have significant effect on network performance. The effect of mobility, BS density, and association bias factor is investigated through numerical results, which shows that network performance derived ignoring the temporal correlation of transmissions is optimistic.
Water evaporation as a source of energy to trigger moisture-responsive soft materials is an emerging field in a variety of energy-harvesting devices, which has attracted widespread attention. Here, we design and fabricate bioinspired nacrelike composite film actuators consisting of graphene oxide and sodium alginate, which demonstrate an obvious shrinkage in volume when their state transfers from wet to dry and the contractile stress is up to 42.3 MPa. Based on these features, the film actuators can show rapid and continuous movements under the water gradient. The flipping frequency of the actuators can reach up to 76 rounds min–1, which is much faster than those in previous reports. The film can flap back and forth quickly on water vapor even after loading a cargo that is 9 times its own weight. Moreover, high mobility with multimodal motion including blooming, stretching, folding, and twisting can also be achieved by modulating the shapes of films. Thus, film actuators may hold great potential in many fields, such as microrobots, artificial muscles, and sensors on grounds of their rapid response speed and adjustable motion models.
Heterogeneous backhaul deployment using different wired and wireless technologies is a potential solution to meet the demand in small-cell and ultra-dense networks. Therefore, it is of cardinal importance to evaluate and compare the performance characteristics of various backhaul technologies in order to understand their effect on the network aggregate performance and provide guidelines for system design. In this chapter, we propose relevant backhaul models and study the delay performance of various backhaul technologies with different capabilities and characteristics, including fibre, xDSL, millimetre-wave (mm-wave) and sub-6 GHz. Using these models, we aim to optimize the base station (BS) association so as to minimize the mean network packet delay in a macro-cell network overlaid with small cells. Furthermore, we model and analyse the backhaul deployment cost and show that there exists an optimal gateway density that minimizes the mean backhaul cost per small-cell BS. Numerical results are presented to show the delay performance characteristics of different backhaul solutions. Comparisons between the proposed and traditional BS association policies show the significant effect of backhaul on network performance, which demonstrates the importance of joint system design and optimization for radio access and backhaul networks.
Polarization weight (PW) is a novel construction method for polar codes, which results in universal reliability ordering efficiently and yields imilar performance as other channel dependent construction methods. In this paper, we interpret PW method or algorithm from a rate allocation perspective of channel polarization to show its effectiveness. Take the AWGN channel as an example, we show that the rate allocation for information bits by PW method coincides with the capacities of channel polarization. Furthermore, some properties of PW algorithm, including the universal and fractal of the reliability ordering, are illustrated, which facilitate the implementation in practice. Comprehensive simulation results with various code rates, code lengths and list sizes are also shown to validate the effectiveness of PW method.
Flexible conductors are emerging soft materials for diverse electrical applications. However, it still remains a great challenge to fabricate high-performance soft conductors that are highly conductive, largely stretchable, and rapid room-temperature self-healable. Here, we design and fabricate flexible conductive bilayer composite films composed of healable elastomeric substrates and wrinkled graphenes. The elastomeric substrates, obtained by a facile bulk copolymerization of N-isopropylacrylamide and 2-methoxyethyl acrylate, show fast room-temperature self-healing efficiency of up to 96%, imparted by the reversible hydrogen bonds. Importantly, the substrates also display strong interfacial adhesion crucial to the formation of stable bilayer composite films based on a prestrain route. The synergy between self-healing of the substrates and wrinkled structures of graphene is endowed to the composite films for mechanical and electrical healing. By adjusting the prestrain ratio of the substrates, the composite films could display the tunable stretchability, conductivity, and self-healing. The optimal bilayer composite film exhibits a high conductivity of 126 S cm-1, a large stretchability of 300%, and rapid room-temperature self-healing. Moreover, it is demonstrated that the composite films are strain-sensitive and can be used as strain sensors to monitor stretching deformation and human motion. These prominent demonstrations suggest a great potential of the bilayer composite films in next-generation wearable electronics.
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