OFDM系统中频率同步算法的研究

OFDM是B3G乃至4G系统中的一种关键技术。它的最大优点是对抗频率选择性衰落,同时又提高了频谱利用率。本文首先介绍了多载波传输与OFDM之间的关系以及OFDM系统的基本原理,接着对OFDM系统中的同步技术和载波同步算法进行了分析．最后对基于训练符号的时域相关算法做了详细的研究。     

复杂电磁环境下IEEE802.11g系统中OFDM信号仿真

为了更好地研究动态多场景下传播环境对OFDM系统中信号的影响,对复杂环境下IEEE802.11g中OFDM信号进行了仿真。首先,搭建了高速移动、多径传播以及同频干扰等因素对OFDM信号传输影响的数学模型;其次,在介绍了OFDM发射信号和接收模型的基础上,分析了IEEE802.11g设备面临的复杂信号环境;最后,仿真并分析了复杂环境对OFDM通信系统性能的影响。结果表明高速移动、多径传播以及同频干扰等复杂环境会引起OFDM系统性能的恶化。模型不仅可以研究复杂环境对OFDM信号性能的影响,还可以作为下一步OFDM系统性能提升的验证平台,具有一定的实用性。     

OFDM系统中频率同步算法的研究

OFDM是B3G乃至4G系统中的一种关键技术。它的最大优点是对抗频率选择性衰落,同时又提高了频谱利用率。本文首先介绍了多载波传输与OFDM之间的关系以及OFDM系统的基本原理,接着对OFDM系统中的同步技术和载波同步算法进行了分析,最后对基于训练符号的时域相关算法做了详细的研究。     

基于认知OFDM系统的功率分配算法

在正交频分复用的认知无线电系统中,授权用户会受到认知用户频带内子载波带外功率泄漏的干扰。针对这一问题,提出了一种基于幂函数分布的次优化功率分配方案,通过线性约束的凸优化数值运算,有效降低了最优方案中运算的复杂度。给出了在认知用户发射功率约束条件下的信道吞吐量分析。仿真结果表明,所提出的方案在满足授权用户干扰门限约束条件下,提高了现有功率分配方案的认知用户信道吞吐量。     

认知OFDM系统中基于能耗受限的频谱感知方法

针对传统频谱感知方法没有考虑能耗受限对认知系统的影响,以认知正交频分复用(OFDM)系统为背景,提出一种基于能耗受限的联合优化频谱感知方法。该方法基于传统能量检测方法,以最大化吞吐量性能为目标,将频谱感知问题建模为一个优化问题,联合多个频带选取最优感知时间和判决门限参数,使其在能耗受限的情况下,总吞吐量达到最大。仿真结果表明,与传统使用能耗均分的检测方法相比,该方法具有更大的吞吐量,能够更好地节约能耗。     

基于无线OFDM系统的调制方式保护算法

随着无线通信系统的宽带化,传统的数据加密算法具有很高的计算复杂度,并且没有考虑到物理层调制方式的安全性。针对这一问题,从物理层加密的角度,提出了一种基于无线OFDM系统的调制方式保护算法。在单载波和多载波情况下,分别对所提算法的调制方式保护效果进行了分析,并通过采用典型的认知无线电调制识别方法对加密前后的识别率进行了仿真和比较。理论分析和仿真结果表明,所提算法在不改变原系统固有性能的情况下,具有不错的调制方式保护性能。     

干扰约束下的认知无线电传输链路自适应算法

针对干扰约束下的认知无线电传输链路自适应算法开展研究,基于OFDM的认知用户与主用户共存,认知用户在保证对主用户干扰容限的前提下执行子载波与功率分配,以树形分支贪婪算法为基础,引入功率余量因子和干扰余量因子并通过比较余量因子相对大小来选择子载波,通过减小分支数以降低算法复杂度.仿真结果表明,改进的树形分支贪婪算法获得的...     

Cyclic autocorrelation based blind OFDM detection and identification for cognitive radio

Cognitive radio is considered as a promising technique to increase the utilization of limited spectral resource. The key issue in cognitive radio is to design a reliable spectrum sensing method that is able to detect the signal in the target channel as well as to recognize different signals. In this paper, focusing on classifying different OFDM signals, we propose a two-step detection and identification approach. The key parameters to separate different OFDM signals are the subcarrier spacing and guard interval. A simple but reliable peak detection method is adopted in the first step, while a peak searching method is used to determine the length of guard interval. Simulations are carried out in AWGN to verify the validation of the proposed method. It is shown that our method can satisfy the detection and identification requirement with a low false alarm probability.     

认知无线电OFDM系统中符号内相关频谱感知算法

提出一种利用导频的周期性在符号内进行相关运算的OFDM信号频谱感知算法。该方法首先将接收序列分成两个子序列,利用导频的周期性在每个子序列内进行叠加平均,以降低噪声方差;然后,将两个子序列进行相关运算,降低时变信道对相关值的影响。同时,通过分析相关值的期望与方差,进一步验证了新方法在时变信道下的有效性。仿真结果表明:新频谱感知方法在时变信道下明显优于传统方法,且减少了参与计算的符号个数,同时计算复杂度较低。     

基于OFDM的双门限协作频谱感知仿真

针对频谱感知中单门限能量检测的不足,研究了双门限能量检测,结合OFDM频谱利用率较高的特点,提出了将双门限能量检测与OFDM系统相结合的方法.以OFDM系统为基础,将系统中的子信道作为待感知用户,根据可用子载波数量确定实际感知用户,在子信道中进行双门限能量检测,数据融合中心采用“or”准则进行判决.仿真结果表明,该方法能够较好地识别出主用户,从而控制OFDM系统的数据传输,随着可用子载波个数的增加,可以得到更好的识别性能.     

一种基于OFDM认知无线电网络的次优动态资源分配算法

研究了采用正交频分复用的认知无线电网络中的多用户资源分配问题,包括子载波分配和功率分配。在认知无线电系统中,除了考虑主用户与次用户之间的相互干扰,还要求将主用户对次用户的干扰控制在预设门限之下,因此,系统模型更为复杂。整数约束条件,使得寻求最优解的算法复杂度高,无法用于对实时性要求高的系统。因此,提出了一种寻求次优解的分步式资源分配算法,以降低算法的复杂度。首先采用一种综合考虑了功率限制和对主用户的干扰限制的新型子载波分配方案,然后提出改进的线性注水算法进行功率分配。仿真结果表明,相比于最优分配算法,所提出的算法可以在获得较好的系统容量的同时,有效地降低系统的复杂度,适用于对实时性要求高的系统。     

基于OFDM的认知无线电频谱检测算法研究

频谱检测是认知无线电的重要任务之一,国内外都对此领域展开了广泛研究, 而正交频分复用(Orthogonal Frequency Division Multiplexing, OFDM)技术灵活的选频方案为实现认知无线电系统提供了良好的平台。本文针对OFDM系统的特点,提出了一种新的频谱检测算法,该算法通过对OFDM收发双方的信号建模,以能量检测的方法为基础,充分利用了正交频分复用符号中的未受多经效应污染的循环前缀(Circular Prefix, CP)和空闲时间来提高检测的精度。计算机仿真结果表明：与传统的只利用空闲时间的能量检测方法和利用所有CP符号信息的方法相比,本文中的算法更有效的提高了频谱检测的概率。     

An energy efficient security protocol for IEEE 802.11 WLANs

Security protocols in wired and wireless networks make use of computationally intensive cryptographic primitives and several message exchanges for authenticated key exchange at the session level and data confidentiality and integrity at the packet level. Moreover, changes in connectivity require mobile stations to repeatedly authenticate themselves, thereby expending more energy. In this paper, we propose an energy efficient security protocol for wireless local area networks (WLANs) that employs (a) different cryptographic primitives based on their suitability in terms of energy consumption and security level, (b) different levels of security and types of security services depending on the type of packet in 802.11 WLANs, and (c) a light-weight hashed key chain to reduce the number of expensive authentication transactions due to connectivity losses. We use packet traces from three different networks to compare the performance of the energy efficient security protocol with that of the standard 802.11 WLAN security protocol and show significant reduction in energy consumption.     

A cognitive management framework for spectrum selection

To increase cognitive radio (CR) operation efficiency, there has been an interest in enhancing the awareness level of spectrum utilization. In this context, this paper builds a new cognitive management functional architecture for spectrum selection (SS). It relies on a knowledge manager (KM) retaining a set of advanced statistics that track the suitability of spectral resources to support a set of heterogeneous applications under varying interference conditions. Based on this architecture, a novel proactive strategy is proposed for both SS and spectrum mobility (SM) functionalities. The required interactions between the proposed decision-making processes are described, and their capability to exhibit robustness to unexpected changes in the radio environment is highlighted. The results show that the proposed strategy efficiently exploits the KM support for low loads, while the SM functionality introduces significant gains for high loads with respect to other strategies. Finally, to assess the practicality of the proposed approach, the signaling requirements in the radio interface are evaluated.     

Interference cancellation aided channel estimation for OFDM/OQAM system

Abstract Orthogonal frequency division multiplexing offset-QAM （OFDM/OQAM） is a promising multi- carrier transmission technique for its merits such as operating without cyclic prefix （CP） and flexibility of introducing well time-frequency localized pulse shapes. However, under dispersive channel condition, the intrinsic inter-symbol and inter-carrier interference arises in OFDM/OQAM system. Especially, the surrounding data symbols will cause interferences to the pilot symbols, leading to channel estimation error and degradation of system performance. In this paper, an efficient pilot structure and a channel estimation method are proposed for OFDM/OQAM system. The basic principal is to rebuild the transmitted symbols around the considered pilot position through pre-decision, then calculate and cancel the interference from the received signal. Analysis and simulation results show that the proposed scheme outperforms conventional ones with respect to both channel estimation accuracy and bit error rate （BER） performance while with less pilot consumption.     

Channel access and traffic control for dynamic-spectrum networks with single-transmit, dual-receive radios

Cognitive radios have a great potential to improve spectrum utilization by enabling dynamic spectrum access. A key challenge in operating these radios is how to implement an efficient medium access control (MAC) mechanism that adaptively and efficiently allocates transmission powers and spectrum according to the surrounding environment. In this work, we propose a distributed MAC protocol for operating spectrum-agile radios in a multi-hop ad hoc network. Our protocol is unique in that it exploits the “dual-receive” capability of radios, which is used to overcome channel access problems that are common to multi-channel designs. We conduct theoretical analysis of the protocol and study its performance via simulations. To further improve the system throughput, we propose a framework for joint adaptive load and medium access controls. Simulation results show that the proposed scheme achieves more than 90% of the maximum (global) system throughput that is achieved at saturation, while guaranteeing low collision rates.     

应用MDFT结构的认知无线电频谱感知方法

针对认知无线电系统采用传统正交频分复用（OFDM）技术时,其高频谱泄露带来的邻带间干扰及造成的主用户信号检测性能下降等问题,提出了一种基于改进离散傅里叶变换（MDFT）调制滤波器组的认知无线电（CR）多用户接入频谱检测方案,并推导了CR系统在应用MDFT滤波器组条件下检测概率表达式。理论推导及不同参数下仿真结果表明,与基于OFDM的CR系统相比,MDFT结构的系统比以矩形窗函数为原型滤波器的OFDM系统带外有更高阻带衰减;在较低信噪比情况下,基于3GPP TS 25.104协议标准的多径步行信道采用MDFT滤波器组结构的CR系统频谱感知性能相比于采用OFDM调制方式有了明显提升。     

ML aided context feature extraction for cognitive radio

This paper addresses the estimation of different context features of a primary user network, such as transmitters’ positions, antenna patterns and directions, and propagation model characteristics. It is based on radio signal strength measurements obtained by a sensor network without any prior knowledge about the configuration of the primary transmitters in terms of antenna types or propagation model. A Maximum Likelihood Aided Context Feature Extraction (MLACFE) method is introduced based on applying image processing and a Maximum Likelihood estimation algorithm over the set of measurements to identify the existing transmitters in the scenario and their parameters. The proposed method can provide a quite similar performance than a classical ML method, in terms of average estimation errors while at the same time reducing the computation time in about three orders of magnitude, for the considered case study.     

认知无线网络中基于凸优化的功率分配研究

主要研究认知无线网络中基于OFDM（orthogonal frequency dinsion multiplexing）的功率分配问题。为将认知用户(secondary user,SU)对主用户(primary user,PU)的干扰功率限制在主用户可容忍的范围内,同时最大化认知用户的传输速率,提出了基于凸优化理论的功率分配方案。仿真结果表明,在主用户可容忍的干扰极限和认知用户总功率约束下,该方案能最大化认知用户的传输速率。