In this paper, the problem of vertical handover in software-defined network (SDN) based heterogeneous networks (HetNets) is studied. In the studied model, HetNets are required to offer diverse services for mobile users. Using an SDN controller, HetNets have the capability of managing users' access and mobility issues but still have the problems of ping-pong effect and service interruption during vertical handover. To solve these problems, a mobility-aware seamless handover method based on multipath transmission control protocol (MPTCP) is proposed. The proposed handover method is executed in the controller of the software-defined HetNets (SDHetNets) and consists of three steps: location prediction, network selection, and handover execution. In particular, the method first predicts the user's location in the next moment with an echo state network (ESN). Given the predicted location, the SDHetNet controller can determine the candidate network set for the handover to preallocate network wireless resources. Second, the target network is selected through fuzzy analytic hierarchical process (FAHP) algorithm, jointly considering user preferences, service requirements, network attributes, and user mobility patterns. Then, seamless handover is realized through the proposed MPTCPbased handover mechanism. Simulations using real-world user trajectory data from Korea Advanced Institute of Science & Technology show that the proposed method can reduce the handover times by 10.85% to 29.12% compared with traditional methods. The proposed method also maintains at least one MPTCP subflow connected during the handover process and achieves a seamless handover. Index Terms-software defined heterogeneous network, seamless handover, echo state network, fuzzy analytic hierarchy process, multipath transmission control protocol.
<p>Benefited from the tightening emissions of air pollutants, a large annual decrease in mixing ratio of SO<sub>2</sub> and a moderate decrease in PM<sub>2.5</sub> can be identified in northern China since 2014. However, a few extreme PM<sub>2.5</sub> pollution events still occur for sometimes during heating seasons, e.g., the 99th percentile value of PM<sub>2.5</sub> concentrations during the heating season in 2018 had exceeded 200 &#181;g m<sup>-3</sup> therein. One unit of percentile value corresponds to approximately 30 hours. To reveal real causes of these extreme PM<sub>2.5</sub> pollution events, we define two technical terms in this study, i.e., 1) secondary particulate species formed in ambient air (conventionally-defined FSPM); 2) formation of secondary particulate matter in the fresh plumes during the initial several minutes (plume-processed FSPM). We also introduce a metric, i.e., PM<sub>2.5</sub>/CO in unit of &#181;g m<sup>-3</sup> / ppm. With these technical terms in mind, we then struggle to dissect real mechanisms causing the severe PM<sub>2.5</sub> pollution event in 11-14 January 2019 across norther China. A staircase function of ratios of PM<sub>2.5</sub>/CO against PM<sub>2.5</sub> rather than a linear increase or decrease with PM<sub>2.5</sub> generally occurred through the event. However, in general, larger ratios of PM<sub>2.5</sub>/CO were indeed observed with larger concentration of PM<sub>2.5</sub>. Regarding frequently observed invariant ratios accompanying with large variations in PM<sub>2.5</sub>, larger ratios are, however, probably not caused by conventionally-defined FSPM in PM<sub>2.5</sub>. Alternatively, our further multiple-technical analysis results confirm plume-processed FSPM, followed by accumulation under poor meteorological conditions, dominatingly resulting in the severe event.&#160;</p>
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