Wireless link quality prediction （LQP） is the foundation for proactive operations and is therefore a key technique in Mleviating network performance degradation. However, accurate LQP is difficult ...because of the dynamic nature of wireless environments. A recent study found that fluctuations in intermediate quality links often show dynamics on a sub-second granularity, making the task even more challenging. In order to leverage the intermediate links, as well as fine-tune upper-layer protocols, we propose to use nonparametric modeling in nonlinear time series analysis that predicts short-term link quality online. Unlike existing studies, we do not define any new experimental or hypothetical models, or train models using a set of training data. Functional-coefficient autoregression is employed to predict the link dynamics at high time resolutions. We apply our approach and a local linear regression-based LQP （a typical parametric modeling approach） to both NS-2 simulation and empirical packet traces. The results indicate that the proposed method has much higher prediction accuracy and convergence speed than the local linear regression-based LQP under dynamic network conditions.
As the contention resolution scheme in IEEE 802.11, binary exponential back off (BEB) has long been criticized for its high collision probability in saturation situation. In this paper, a novel ...contention resolution algorithm is designed to overcome its disadvantage. The cause of BEB's poor performance is identified as the overlapped back off intervals of stations and the restoration of contention window to the smallest value upon a successful transmission. Therefore two new concepts, namely back off interval isolation (BII) and improved slow decrease (ISD), are proposed in our enhanced collision avoidance (ECA) algorithm. BII introduces an extra back off interval selection process and stations at different stages have separated back off intervals. With ISD, contention window can only decrease after a certain number of successive successful transmissions. Two dimensional Markov chain is utilized to model the contention resolution process and the theoretical performance of ECA is also deduced. Simulation results show that our algorithm outperforms BEB in both throughput and delay by lowering the collision probability.
This paper proposes an efficient bandwidth allocation scheme to fully utilize the multi-access ability of multimode terminals in heterogeneous networks. To model the fact that the suitability of ...different networks to different traffics varies, which is seldom considered in previous literature, the term "Match-Degree" is introduced and calculated using grey relational analysis. Afterwards, two methods, namely weighted bandwidth factor and cumulative utility function are proposed to integrate the influence of "Match-Degree" into the conventional utility-functions. Moreover, energy consumption and network price are also considered to make our model more comprehensive. The bandwidth allocation problem is formulated as maximizing the overall utility by adjusting the bandwidth allocated to each user. Based on optimization theory, an iterative algorithm is constructed to solve this problem. And numerical results validate the performance enhancement of our scheme compared with the existing ones.
In this letter, a wide stopband quarter‐mode substrate‐integrated waveguide (QMSIW) diplexer is proposed. By using coupling feeding lines and QMSIW cavities, the two passbands of the diplexer can be ...controlled independently by their respective QMSIW bandpass filters. In comparison to other counterparts, the proposed diplexer provides flexible passband specification, compact size, and high isolation. The design exhibits a wide 20‐dB stopband up to 3.2f0, a frequency ratio of 1.6, and a high isolation level larger than 42.2 dB across the passbands.
Directional antennas have received much attention recently in multihop ad hoc networks, due to such benefits as increased transmission range, reduced co-channel interference, improved spatial reuse, ...and reduced susceptibility to jamming. However, existing random access scheduling schemes with directional antennas suffer from the issues of deafness and high probability of collisions. Moreover, such schemes cannot provide throughput and delay guarantees. In this paper, we propose a topology-transparent scheduling algorithm in mobile multihop ad hoc networks with directional antennas, to overcome these deficiencies. The proposed algorithm fully exploits the benefits of directional antennas and utilizes both assigned and unassigned time slots efficiently. We study the performance of our algorithm analytically and by simulation. The results show that the performance of the proposed algorithm, in certain cases, can be comparable to that of some topology-dependent algorithms designed for static networks. Yet, the proposed algorithm also performs well even when the network topology is dynamic, as in mobile networks.
This paper proposes an efficient dynamic traffic allocation scheme for multi-flow distribution in LTE-WLAN heterogeneous networks, where data traffic is split into multiple sub-flows and transmitted ...through different networks simultaneously. The state transition equations of the M/M/1 queuing system are modified to model the parallel transmissions. To make our scheme more practical, the modified model also considers random interference and dynamic changes of available bandwidth, which are often ignored in previous works. In addition, traffic characteristics are taken as key concerns in the formulation of allocation scheme to make it more effective. A novel scheme - layered video streaming allocation (LVSA)- based on traffic characteristics is proposed to increase system capacity, especially in high delay situations. Simulation results indicate that our schemes have a significant improvement in delay performance and system capacity compared with other schemes.
A novel ultra-wideband (UWB) pulses design algorithm based on modified Hermite polynomials is presented in this paper. Secondary Schmidt orthogonalization method and combination algorithm are ...proposed to generate a class of orthogonal UWB pulses which can be used in multiple access schemes and suppress the interference between multi-users. The constructed pulse series can not only comply with the power spectral emission mask of federal communications commission (FCC), but also optimally utilize the bandwidth and power allowed. Moreover, the flexibility of combination algorithm permits simple modifications to generate new set of pulses which can accommodate other circumstances.
In this work, the Nb-14Si 24Ti-10Cr-2Al-2Hf-0.1Y alloy （at.%） was processed by the liquid-metal-cooled directional solidification （DS） at 1750 ℃ with withdrawal rates of 1.2, 6, 18 mm/min and post ...heat treatment （HT） at 1450 ℃ for 10 h. The microstructures of the direction- ally solidified and heat treated samples were investigated. The results show that the microstructure of directionally solidified alloy mainly consists of petaloid Nbss ＋ Nb5Si3 eutectics and Ti-rich Nbss ＋ Nb5Si3 ＋ Cr2Nb eutectics. With the increase of withdrawal rate, the primary NbsSi3 is eliminated, Nbss ＋ Nb5Si3 eutectic cells turn round and connected with the microstructure refine- ment and Nbss ＋ Nb5Si3 ＋ CrzNb eutectics turn to a river-like morphology. After heat treatment, Nbss ＋ Nb5Si3 ＋ Cr2Nb eutectics disappeared and petaloid Nbss ＋ Nb5Si3 eutectics turn to a spe- cific fiber-mesh structure gradually, which is promoted by higher withdrawal rates. Furthermore, both the volume fraction of Cr2Nb and the content of Cr in Nbss of Nbss ＋ Nb5Si3 eutectics change regularly with the increase of withdrawal rate and heat treatment at 1450 ℃ for 10 h.
The saturated hydrocarbon fraction of a heavily biodegraded crude oil from Liaohe oilfield was analyzed by using comprehensive two-dimensional gas chromatography coupled with time-of-flight mass ...spectrometry （GC×GC-TOFMS）. The ＂humps＂, which was termed as unresolved complex mixtures （UCMs） in conventional gas chromatographic analysis, was separated for identification of the individual compound. Main compounds of UCMs were identified according to the combined information of characteristics of GC×GC chromatogram and time-of-flight mass spectra. It’s found that UCMs are mainly composed of great number of ring compounds with extremely low concentration. The ring compounds are separated according to their boiling point diversity in the first dimension column, then to their polarity or ring number variety in the second dimension column. In the sample there are two groups of UCMs, i.e., the first group compounds with boiling point lower that of C24 and the second group compounds with boiling point higher than that of C24. The first group of UCMs mainly consists of monocyclic, bicyclic and tricyclic ring compounds. The first group of UCMs is mainly composed of saturated hydrocarbons, in which the percentage of ring compounds with signal-to-noise ratio above 100 is about 75% of the total quantity of saturated hydrocarbon, while its mass is over 80% of the total saturated hydrocarbon. The second group of UCMs is mainly composed of tetracyclic and pentacyclic ring compounds, in which the percentage of UCM compounds with signal-to-noise ratio over 100 is about 17% of the total quantity of saturated hydrocarbon, and its mass is about 0.5% of the total saturated hydrocarbon. The results of this study are helpful for heavy oil developing and genesis mechanism understanding.
The scope of hydrogel applications can be greatly expanded by the improvement of mechanical properties. However, enhancement of nanocomposite hydrogels (NC gels) has been severely limited because the ...size of crosslinking nanoparticles is too large, at least in one dimension. Here we report a new strategy to synthesize non-aggregated spherulite nanoparticles, with diameters <5 nm, in aqueous solution, and their enhancement to hydrogel. The stress and stretch ratio at rupture of our NC gel are 430 and 121 KPa with only 40-p.p.m. nanoparticle content. The NC gel containing 200-p.p.m. nanoparticles can revert to 90% of its original size after enduring 100-MPa compressive stress. Our results demonstrate that the suppression of nanoparticle size without aggregation helps to establish a super stretchable and high-toughness hydrogel network at very low inorganic content.
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