【作者】 杨子原；

【导师】 袁东风；

【作者基本信息】 山东大学 ， 通信与信息系统， 2009， 硕士

【摘要】 近年来随着无线通信技术的飞速发展,人们对无线通信数据传输速率的要求越来越高。根据香农信息论原理,通信系统对无线频谱资源的需求也相应增长,这导致频谱资源变得越来越紧张,从而成为制约无线通信发展的重大瓶颈。另一方面,已经分配给现有很多无线通信系统的频谱资源却在时间和空间上存在着不同程度的闲置。1999年,Joe Mitola博士提出了认知无线电(Cognitive Radio)这一全新的概念,这种“智能的无线电”结合了认知科学和软件无线电(Software Defined Radio)技术,旨在对空、时、频等各域上的空闲资源(亦称为“频谱空洞”或“白色空间”)进行有效的感知探测和合理的再利用,认知无线电技术也随之成为了当前无线通信研究和开发的新热点。目前,以正交频分复用(OFDM)为代表的多载波调制(MCM)技术已被多种无线通信系统和标准所采用。非连续载波NC-OFDM技术具有灵活的频率选择和功率控制特性,这些特性使之较其他物理层接入技术更加适合用于认知无线电系统中。与此同时,OFDM多载波关键技术自身的一些缺点也成为了其在认知无线电系统中应用的障碍,必须加以克服。在这些缺点中,除了传统的高频偏敏感性、高峰均信噪比等之外,由于旁瓣能量泄漏而带来的带外辐射干扰问题被公认为是将OFDM技术应用于认知无线电系统中的典型难题。本论文针对基于OFDM的认知无线电系统中的带外干扰抑制技术展开讨论,旨在寻求合理高效的方案来解决这一技术难题。论文的基本工作概括如下:1.从建立非连续载波NC-OFDM系统带外辐射干扰的基本数学模型入手,阐述高带外辐射能量在主次用户共存的认知无线电系统中的危害,从而说明了解决该技术难题的紧迫性。2.将奈奎斯特和非奈奎斯特加窗技术应用在基于NC-OFDM的认知无线电系统中,结合上述的数学模型分析和仿真来衡量有效频带之外的辐射干扰水平的高低。同时,利用凸函数优化理论来求取适用于认知无线电频谱池中的最优参数的二次连续多项式(SOCW)窗函数。3.采用对有效频带内不同子载波进行功率控制的措施来达到在给定带外辐射水平的情况下,最大化系统容量,即信息传输速率的问题。论文该部分将针对两种不同的干扰模型(两种不同的场景),提出了区别于传统OFDM注水功率分配的优化模型,同时给出低复杂度的启发式算法。同时,将干扰自适应功率控制算法应用于上述提出的加窗OFDM系统,进一步提高了系统容量。更多还原

【Abstract】 With the great improvements of the wireless communication techniques, much higher data transmitting rates are needed by peoples. Based on Shannon’s Information Theory, the resources of the wireless communication are much more required correspondingly. Currently, The scarcity of frequency resources has become one of the biggest obstacles in wireless communication. On the other hand, large amounts of the wireless resources that have been regulated by many wireless Standards are partly idle on the dimensions of time and frequency. In the year 1999, Dr. Joe Mitola proposed the concept of cognitive radio. Connecting the cognitive science with the software defined radio, this type of agile radio could detect the idle resources in the dimensions of time, frequency or space, and reuse them in high efficiency. As a result, cognitive radio techniques have been becoming the new hot points in both the research and the developing fileds in wireless communication.At present, multicarrier modulation techniques ,such as OFDM, have been adopted by many wireless protocols. Non-Contiguous OFDM(NC-OFDM),with the characteristics of flexible frequency selection and adaptive power loading, is more appropriate for cognitive radio systems as the physical layer access technique. Meanwhile, many shortcomings of OFDM, such as high Carrier frequency offset sensitivity and Peak-to-Average Power Ratio (PAPR), are also some obstacles in OFDM based cognitive radios. Besides, the out-of-band radiation derived from the side-lobe leakage is considered as the one of the biggest difficulties in OFDM based cognitive radios.This thesis will talk about the out-of-band radiation problem in OFDM based cognitive radio systems, and try to find out some feasible methods to overcome this shortage. The main work includes:1. Set up the mathematical model of the out-of-band radiation problem in OFDM based cognitive radios, and explain why the larger side-lobes will increase the inference from the secondary users to the primary users.2. Based on the model proposed afore, the out-of-band radiation power will be evaluated by both theory deduction and computer simulation. Meanwhile, the optimal parameter of the SOCW window in the spectrum pooling system will also be worked out.3. Perform the adaptive power control method to find out the maximal channel capacity of the OFDM based cognitive radio system under the interference restriction of the primary users. In this part, based on the traditional water-filling power loading theory. We will propose several optimal and sub-optimal algorithms under the two different interference scenarios in spectrum pooling. Whereafter, we combine the optimal SOCW window and the adaptive power control algorithms and demonstrate that these two methods will restrain effectively the out-of-band interference from the secondary users to the primary users in OFDM based cognitive radio systems.更多还原