认知无线电网络MAC接入性能分析

认知无线电网络中授权用户享有频谱的优先使用权,一旦授权用户出现认知,用户必须中断正使用的频谱,造成正在传输的MAC帧丢失,认知无线电网络直接采用现有的IEEE802.11 DCF协议,无法解决由于中断引发的认知用户MAC帧丢失问题,当认知用户再次获得频谱接入机会时,将重新竞争接入,导致频谱切换延时较大.为解决该问题,本文改进了IEEE 802.11 DCF协议,提出增加冻结缓存机制,以此解决用户信道切换的延时较大问题.当无线链路出现中断时,改进的DCF协议通过冻结节点,中断当前计时器并缓存当前状态信息,认知用户和基站之间链路一旦恢复则继续传输中断前的MAC帧,从而避免了认知用户的重新竞争接入.仿真结果表明改进的DCF协议在频谱切换时能增大接入吞吐量,减小接入延时,提高了DCF机制在认知网络中的应用性能.     

基于层次分析法的频谱资源分配效果综合评价算法

基于AHP的思想提出一种对频谱资源分配效果进行综合评价的算法:将频谱资源分配的效果作为目标层,以认知用户总满意度、频谱分配公平性及系统经济性3个因子为第1准则层,以频谱资源紧张程度、已分配带宽比例、缺口带宽比例等因素为第2准则层,根据准则层各因素的关联关系和重要性程度得出各因素权重,通过虚拟分配及单位化计算各因素的观测值进行综合评价。空闲矩阵、分配矩阵、干扰矩阵等基础数据基于均匀分布进行构造,分别对频谱资源处于充足和紧张的情况进行模拟,采用文中算法对3种频谱分配方案进行仿真。结果显示:系统资源充足时,两级动态分配方案效果最好,多信道联合优化方案次之,次用户协作方案最差;系统资源匮乏时,两级动态分配方案和多信道联合优化方案,均优于次用户协作方案。     

基于采样协方差矩阵的频谱感知判决门限优化

在认知无线电频谱感知算法中,传统的基于采样协方差矩阵特征值极限分布函数的频谱感知算法难以同时实现高检测概率和低虚警概率.在此基础上,提出了一种基于采样协方差矩阵的频谱感知判决门限优化方法.利用随机矩阵理论,分别得到了采样协方差矩阵最大特征值和最小特征值极限分布函数下的判决门限,并将两个判决门限的加权求和作为最终判决门限.仿真结果表明,在获得较高检测概率的情况下,优化判决门限仍可保持较低的虚警概率.     

一种新的循环谱统计量频谱感知算法

为了提高认知无线电中感知用户对授权用户（主用户）的检测概率,提出一种新的基于循环谱统计量的主用户信号检测算法．该算法利用循环谱离散频域平滑方法,对信号的循环谱在循环频率处建立合理的检测统计量,然后求解得到对应不同虚警概率的判决门限,以使感知用户能检测信噪比更低的授权用户信号．该算法具有检测结构简单,易实现的特点．仿真结果表明,在高斯白噪声信道中,虚警概率为0．1,频域平滑累积次数为255,在信噪比为-16dB的条件下,与Dandawate-Giannakis（DG）方法相比,本文算法的检测概率提高23％．     

测点级声学故障检测中的报警频次建模方法研究

文中围绕测点级声学故障检测方法开展研究,根据区间估计原理建立频带能量超标检测流程,针对随机报警事件干扰提出基于报警事件频次建模的二级阈值检测法,并利用实测数据进行了算法验证.数据分析显示,该方法能够有效检测正常工况下的随机报警事件干扰,降低测点级声学故障检测的虚警率.     

基于碰撞试验的车辆碰撞声信号特征

为使车辆碰撞发生重大交通事故后,能够及时报警和施救,将声音识别技术应用于交通事故的检测和报警。采用声音信号特征提取方法,通过试验搜集车辆典型碰撞声、自然界雷声、喇叭声等多种交通复合声音信号。使用Matlab工具,对采集的车辆正常行驶过程中无干扰、有干扰以及不同车速和不同角度碰撞时的3类声信号特征进行频谱分析。分析表明:发生重大交通事故时的车辆碰撞声信号频率远远高于其余两类状态,完全可以通过提取车辆碰撞声音信号特征来判断是否发生了重大交通事故。     

电子战跳频接收机的互调抑制设计和测试

针对跳频接收机的体制和灵敏度、动态范围、瞬时工作带宽、测频精度等设计参数,通过改变预选器组成、本振频率和本振带宽,分析接收信道的中频选取原则,以实现跳频接收机的互调抑制设计.提出在满足截获概率和虚警概率条件下,接收机跳频工作方式的单音和双音互调抑制性能测试方法.测试结果表明,跳频接收机的互调抑制设计满足通信对抗的技战术使用要求.     

直接序列扩频通信中扩频码捕获系统的研究

围绕扩频通信中的关键技术-扩频码的捕获展开讨论.针对直接序列扩频通信中的同步速度问题,研究了扩频码的捕获原理;通过状态分析,利用编码序列相位转移图,研究了扩频码的捕获过程;论述了虚警产生的原因、虚警概率和虚警对同步捕获速度的影响.在此基础上,提出了一种带有辅助电路的扩频码捕获系统,该系统可大大降低非相关状态下输入门限检测器的等效噪声电压,从而降低虚警概率,缩短捕获时间,提高通信效率.     

换乘效率对城市轨道交通分担率的影响研究

为评估换乘效率对轨道交通分担率的影响,引入拥挤感知系数、环境修正系数及换乘次数惩罚系数,将客观换乘时间转化为感知换乘时间,并以此为基础构建了轨道交通分担率模型.利用北京市第4次出行调查数据对该模型进行标定,探讨了轨道交通网络换乘效率改善对不同出行距离乘客分担率的影响.结果表明:换乘次数、换乘走行时间、换乘等待时间及换乘环境的改善能够有效地提高轨道交通的分担率,尤其是对中短距离乘客的影响巨大;换乘拥挤是影响中长距离乘客选择轨道交通的重要因素,当换乘通道通过能力及换乘站台容量较小时,拥挤感知系数快速增加,严重降低轨道交通网络的分担率.     

基于自动删除算法的最小选择恒虚警检测器

将自动删除算法和最小选择方法相结合，提出了一种在强干扰目标背景下基于自动删除算法的最小选择恒虚警检测方法,分析了自动删除算法的性能，推导了该方法的平均虚警概率，平均检测概率的解析表达式．该检测方法的排序时间只有自动删除均值检测器的一半，而通过计算机仿真显示在多目标环境中．当强干扰目标较多时，该检测方法优于自动删除均值检测器。     

Price-based power control algorithm in cognitive radio networks based on monotone optimization

This paper considers a price-based power control problem in the cognitive radio networks(CRNs).The primary user(PU) can admit secondary users(SUs) to access if their interference powers are all under the interference power constraint. In order to access the spectrum, the SUs need to pay for their interference power.The PU first decides the price for each SU to maximize its revenue. Then, each SU controls its transmit power to maximize its revenue based on a non-cooperative game. The interaction between the PU and the SUs is modeled as a Stackelberg game. Using the backward induction, a revenue function of the PU is expressed as a non-convex function of the transmit power of the SUs. To find the optimal price for the PU, we rewrite the revenue maximization problem of the PU as a monotone optimization by variable substitution. Based on the monotone optimization, a novel price-based power control algorithm is proposed. Simulation results show the convergence and the effectiveness of the proposed algorithm compared to the non-uniform pricing algorithm.     

直接序列扩频通信中扩频码捕获系统的研究

围绕扩频通信中的关键技术-扩频码的捕获展开讨论．针对直接序列扩频通信中的同步速度问题。研究了扩频码的捕获原理；通过状态分析，利用编码序列相位转移图，研究了扩频码的捕获过程；论述了虚警产生的原因、虚警概率和虚警对同步捕获速度的影响．在此基础上，提出了一种带有辅助电路的扩频码捕获系统，该系统可大大降低非相关状态下输入门限检测器的等效噪声电压，从而降低虚警概率，缩短捕获时间，提高通信效率．     

Reputation-based collaborative spectrum sensing scheme in cognitive radio networks

Collaborative spectrum sensing is proposed to improve the detection performance in cognitive radio (CR) networks. However, most of the current collaborative sensing schemes are vulnerable to the interference of the malicious secondary users (SUs). In this paper we propose a reputation-based collaborative spectrum sensing scheme to improve the security of cooperative sensing by mitigating the impacts of misbehaviors. The fusion center calculates the reputation rating of each SU according to their history reports to weight their sensing results in the proposed scheme. We analyze and evaluate the performance of the proposed scheme and its advantages over previous schemes in expansibility and integrity. Simulation results show that the proposed scheme can minimize the harmful influence from malicious SUs.     

A Novel Interference Modeling Scheme in Cognitive Networks

In this paper,we propose a mathematical model of aggregate co-channel interference over Rayleigh fading in cognitive networks.Unlike the statistical models in the literature that aim at finding the bound or approximation of the interference,the proposed model gives an accurate expression of probability density function(PDF),cumulative distribution function(CDF) and mean and variance of the interference,which takes into account a number of factors,such as spectrum sensing scheme,and spatial distribution of the secondary users(SUs).In particular,we focus on a more general spatial structure where there are two roles of primary users(PUs)and the interfering SUs distributed in the two-dimensional space.The framework developed in this paper is easy to be applied in power control,error evaluation and other applications.     

PERFORMANCE ANALYSIS OF HYBRID DS SFH/SSMA SYSTEMS OVER MULTIPATH FADING CHANNELS

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高速列车转向架构架载荷识别与分布特性

以某型高速列车转向架构架为对象, 研究了高速列车转向架构架载荷识别与分布特性; 分析了基于动应力响应识别构架载荷的原理并基于截断奇异值法对构架载荷进行了反推识别, 采用核密度估计法对构架载荷极值分布特性进行了分析, 基于3σ准则获得了不同出现概率下的构架载荷极值区间, 利用雨流计数法编制了构架载荷的二维载荷谱并基于Goodman方程将二维载荷谱等效转换为一维载荷谱, 基于一维载荷谱分析了各载荷系载荷的累积频次分布规律。研究结果表明: 对于本文研究对象而言, 当截断数目为1时, 载荷识别结果的相对误差最小; 载荷极小值与载荷极大值的概率密度分布整体相对于坐标系的纵坐标轴对称, 涵盖载荷范围越大的载荷系, 其概率密度的极值越低; 齿轮箱载荷系极大值与极小值涵盖的载荷范围最大, 最大载荷达到25 kN, 制动载荷系、侧滚载荷系与横向载荷系次之, 最大载荷达到了15 kN, 浮沉载荷系的最大载荷约为5 kN, 扭转载荷系极值涵盖的范围最小, 最大极值约为3 kN; 随着出现概率的增大, 各载荷系极值区间也逐渐变大; 各载荷系的二维载荷谱均有明显的载荷频次极值, 各载荷系的载荷频次极值均出现在低幅值区域; 对于二维载荷谱等效后的一维载荷谱累积频次分布, 各载荷系总累积频次相当, 齿轮箱载荷系的最大载荷幅值明显大于其他载荷系, 其他载荷系的最大载荷幅值由大到小依次为侧滚载荷系、制动载荷系、浮沉载荷系、横向载荷系和扭转载荷系。      

Multi-channel spectrum sensing in cognitive ad-hoc networks: An energy-efficient manner

Cognitive radio,which is capable of enabling dynamic spectrum access,is a promising technology in future wireless communication.The feasibility of cognitive radio network greatly depends on the energy efciency and reliability of spectrum sensing technology.In this paper,spectrum sensing in cognitive ad-hoc network（CAN）with wide-band dynamic spectrum is considered.A cognitive cluster head（CCH）is set and responsible for dividing the wide-band spectrum into multiple sub-channels;it can either sense sub-channels in a centralized manner,or make use of sensing modules to sense sub-channels in a distributed manner.Then cognitive users（CUs）can get sensing results and access to the available sub-channel.We take the cost of control message into consideration and formulate the energy consumption of CAN in terms of sub-channel sampling rate and whole-band sensing time.We define energy efciency intuitively and solve the energy efciency optimization problem with sensing reliability constraints by constructing a parametric problem and obtain the optimal sampling rate and the wholeband sensing time.Power dissipation model of a practical A/D convertor（ADC）is introduced,and numerical results are given to show the energy efciency performance of two diferent sensing manners.     

A selection scheme for optimum number of cooperative secondary users in spectrum sensing

An optimization scheme for choosing the optimum number of secondary users in cooperative spectrum sensing based on the cyclostationary feature detection with Neyman-Pearson criterion is proposed in this paper. The optimal soft combination test statistic for the cooperative spectrum sensing based on cyclostationary feature detection is derived according to the generalized likelihood ratio test and its corresponding detection performance is deduced. A target function, considering two important parameters as the resource use efficiency and the number of samples employed by each cooperative secondary user in the system design, is constructed to obtain the optimum number of cooperative secondary users. It can be found that the selection scheme is to make a tradeoff between the system complexity of the cognitive radio network and the global sensing performance of the cooperative spectrum sensing.