On the developments and applications of optical microcavities: an overview

Devices based on the optical microcavities which confine light to small volumes by resonant recircu- lation are already indispensable for a wide range of studies and applications. This article provides an overview of the development and application of optical microcavities. We first give a pedagogical introduction to the in- teraction between a two-level system and a quantized electromagnetic field in the cavity, based on the so-called Jaynes-Cummings model, which is basic and important theory model in the cavity quantum electrodynamics, and various quantum phenomena and applications of it. Then, we overview three basic types of the microcavity structures, and also highlight the progress achieved so far in these systems. Based on these three structures, we give an account of three representative applications of optical microcavities, and explain their microcavity requirements and the state of the art for these devices, before outlining the challenges for the future.     

融合网络编码的无线协作中继网络资源分配

综述融合网络编码的无线协作中继网络资源分配的研究进展,总结了已有典型资源分配策略的特点、性能及系统设计要求,分析了网络编码技术(包括数字网络编码和物理层网络编码)的引入对无线协作中继网络资源分配策略设计的影响,给出了一种实用化的系统级跨层优化框架及设计基本原则,并探讨了无线协作中继网络资源分配研究的发展趋势及其走向实用化所亟待解决的关键问题.     

Performance Analysis on Two-Way Relay System with Co-Channel Interference

In this paper, we investigate the performance of two-way relay system with co-channel interference in a Rayleigh fading environment. Due to the mathematical intractability of original expression of signal-to-interference and noise ratio (SINR) induced by the bi-directional forwarding, a couple of effective bounds are derived for important performance metrics such as outage probability and average bit error rate of modulation, with application of diverse physical layer network coding (PNC) schemes of two time slot (2TS) and three time slot (3TS). Some asymptotic solutions are also proposed to intuitively exhibit the trends of performance in high SINR regime. We demonstrate that the performance bounds have practical meaning corresponding to different forwarding capability of the relay. Numeric simulations validate our analysis by showing that the theoretic bounds match well with simulation results. Additionally, the effect of distance between relay and sources is discussed in our interference scenario, as well as the impact of relay’s power allocation factor in 3TS PNC scheme.     

VMIMO-OFDM系统精频偏估计算法的研究

在对多种可应用于VMIMO-OFDM系统的频偏估计方法的性能和效率进行分析的基础上,针对基于VMIMO-OFDM的无线传感器网络中的精频偏估计问题,提出了一种在频域进行频偏估计的算法.同时,针对基于VMIMO-OFDM的无线传感器网络应用于瑞利衰落严重的环境时不同的导频子载波上频偏估计的误差差异较大的问题,提出了一种基于信道统计信息(CSI)加权的精频偏估计算法,该方法对各个子载波上的频偏估计结果进行加权平均,可使估计误差最小化.理论分析和仿真结果均表明,新提出的算法在高斯信道和多径信道中均有更优的性能.     

跨层协作的两级垂直切换判决

在建构网络质量QoN概念的基础上,将跨层协作和两级判决相结合,提出一种新型的垂直切换模型,并给出了详细流程。仿真表明,该算法相对于传统算法而言,在切换时延和服务终端时延等指标上均有显著改善,从而证明了两级判决能优化切换流程,确保业务连续性。     

Mitigating collisions through power-hopping to improve 802.11 performance

In this article, we introduce a power-hopping technique (PH-MAC) that, by alternating between different transmission power levels, aims to deliberately cause packet capture and thereby reduce the impact of collisions in 802.11 WLANs. We first devise an analytical model of the 802.11 protocol with heterogeneous capture probabilities, and show that, depending on the network load, the capture effect can enhance the throughput performance of all nodes. We base the design of PH-MAC on the findings following from this analysis and demonstrate that important performance improvements can be achieved by exploiting the interactions between the MAC and PHY layers to mitigate collisions. Finally, to understand the feasibility of this technique in practical deployments, we present a prototype implementation of PH-MAC which relies on commodity hardware and open-source drivers. We evaluate the performance of this implementation in an indoor testbed under different network conditions in terms of link qualities, network loads and traffic types. The experimental results obtained show that our scheme can provide significant gains over the default 802.11 mechanism in terms of throughput, fairness and delay.     

能量感知非均匀成簇路由优化算法

无线传感器网络(WSN)具有的能量有限,其能量利用效率的高低直接影响着网络的生命周期.为了提高无线传感器网络的能量利用效率,提出了一种能量感知非均匀成簇路由优化算法(Energy Awareness Unequal Clustering Routing Optimization Algorithm,EUCR).该算法通过节点在网络中所处的位置确定各节点的邻居节点,并以局部能量选举簇头,各簇头根据其邻居节点构建非均匀分簇网络.同时该算法在路由阶段考虑了簇头的剩余能量和转发代价.仿真结果表明,EUCR算法能有效提高网络的能量利用效率,并延长网络的生命周期.     

A generic model for optimizing single-hop transmission policy of replenishable sensors

Energy harvesting from the working environment has received increasing attention in the research of wireless sensor networks. Recent developments in this area can be used to replenish the power supply of sensors. However, power management is still a crucial issue for such networks due to the uncertainty of stochastic replenishment. In this paper, we propose a generic mathematical framework to characterize the policy for single hop transmission over a replenishable sensor network. Firstly, we introduce a Markov chain model to describe different modes of energy renewal. Then, we derive the optimal transmission policy for sensors with different energy budgets. Depending on the energy status of a sensor and the reward for successfully transmitting a message, we prove the existence of optimal thresholds that maximize the average reward rate. Our results are quite general since the reward values can be made application-specific for different design objectives. Compared with the unconditional transmit-all policy, which transmits every message as long as the energy storage is positive, the proposed optimal transmission policy is shown to achieve significant gains in the average reward rate.     

Research on Time Synchronization Method Under Arbitrary Network Delay in Wireless Sensor Networks

To cope with the arbitrariness of the network delays, a novel method, referred to as the composite particle filter approach based on variational Bayesian (VB-CPF), is proposed herein to estimate the clock skew and clock offset in wireless sensor networks. VB-CPF is an improvement of the Gaussian mixture kalman particle filter (GMKPF) algorithm. In GMKPF, Expectation-Maximization (EM) algorithm needs to determine the number of mixture components in advance, and it is easy to generate overfitting and underfitting. Variational Bayesian EM (VB-EM) algorithm is introduced in this paper to determine the number of mixture components adaptively according to the observations. Moreover, to solve the problem of data packet loss caused by unreliable links, we propose a robust time synchronization (RTS) method in this paper. RTS establishes an autoregressive model for clock skew, and calculates the clock parameters based on the established autoregressive model in case of packet loss. The final simulation results illustrate that VB-CPF yields much more accurate results relative to GMKPF when the network delays are modeled in terms of an asymmetric Gaussian distribution. Moreover, RTS shows good robustness to the continuous and random dropout of time messages.