【作者】 秦文；

【导师】 彭启琮；

【作者基本信息】 电子科技大学 ， 信号与信息处理， 2008， 博士

【摘要】 正交频分复用(Orthogonal Frequency Division Multiplexing，OFDM)技术作为一项多载波调制(Multi-Carrier Modulation，MCM)技术，在很多领域得到了广泛的应用，其最重要的优点是相比传统的频分复用技术具有更高的频谱利用率。这得益于它利用了子载波之间的正交性，使得不同子载波信号在频谱上相互重叠，却不影响它们各自的接收。但是，这一优点同时也给OFDM技术埋下了一个隐患，即子载波间干扰(Inter-Carrier Interference，ICI)。随着通信要求的不断提高，OFDM系统逐渐向子载波数更多，载波频率更高，移动速度更快的方向发展。这将导致子载波间隔更窄，ICI的程度更严重。此时，OFDM系统的正交性变得十分脆弱，ICI的影响将不可忽视。本文以无线移动环境下的OFDM系统为对象，围绕ICI的消除进行研究。主要内容分别涉及ICI自消除，时变信道估计，系统均衡和多成因ICI的联合消除等方面。具体而言，主要有以下七个方面的工作和创新点：(1)指出目前存在的基于多项式的ICI自消除编码方法并非最优且编码效率缺乏灵活性，提出了基于生成矩阵的可变编码效率的ICI自消除编码方法，并且，在编码效率给定的情况下，通过搜索算法设计出达到ICI消除效果最优的生成矩阵。(2)提出在OFDM系统中的脉冲成形技术应该遵从两个标准：ICI抑制效果和带外功率泄漏。并在滚降系数给定的情况下，给出了最佳脉冲波形的模型，以此设计出了在减少带外功率泄露和抑制ICI干扰两方面均具有很好表现的脉冲波形。(3)提出了基于粒子滤波的时变信道估计方法。首先，利用相邻OFDM符号周期内中间时刻的信道冲击响应进行自回归(Auto-Regressive，AR)建模，并在导频符号辅助情况下，建立起状态空间模型。通过对ICI干扰的统计分析，指出其不符合高斯分布。于是，采用粒子滤波对线性非高斯的状态空间模型进行信道估计。最后，利用三次样条插值逼近出完整的时变信道。(4)将ICI自消除的原理应用在了改善时变信道估计性能上。首先，指出BEM建模下基于导频符号辅助信道估计所存在的问题，即相邻非导频符号产生的ICI干扰，降低了估计的准确度。然后，分析发现子载波所产生的ICI系数具有渐变特性。于是，提出在接收端采用ICI自消除操作以减小ICI项，提高估计效果。(5)提出了OFDM系统中基于叠加训练(SuperimposedTraining)序列的时变信道估计方法。叠加序列具有不占用频谱资源的优点。但是，由于叠加序列方法利用了一阶统计量，则要求被估计值必须恒定，而单纯的时变信道是不可能满足这一要求的。所以，利用BEM建模将一个OFDM符号周期内的时变信道转化为恒定的BEM基系数，便可以实现利用叠加序列估计时变信道。(6)在复指数BEM(CE-BEM)信道建模的基础上，提出了低复杂度的判决反馈均衡器(Decision Feedback Equalizer，DFE)。该均衡器利用估计得到的CE-BEM基系数直接构造DFE，从而避免了恢复时变信道的运算，以及均衡中的大矩阵求逆运算，大大降低了复杂度。(7)提出了对时变信道和相位噪声同时产生ICI的消除方法。由于在时变信道和相位噪声混合的情况下，依然满足平稳窄带特性，因此，采用卡洛BEM(KL-BEM)对混合时变信道和相位噪声进行建模，从而完成了对时变信道和相位噪声的同时估计。更多还原

【Abstract】 Orthogonal frequency division multiplexing (OFDM) is one kind of multi-carrier modulation (MCM), and has been used in many fields. Its top advantage is the much higher frequency spectrum efficiency than the conventional frequency division multiplexing, which benefit from the orthogonality between the sub-carriers. The different sub-carriers overlap each other, and don’t affect the receiving respectively. But the advantage also brings a hidden trouble, i.e., inter-carrier interference (ICI). With the development of the communication requirement, the number of sub-carriers in OFDM becomes bigger and bigger; the carrier frequency becomes higher and the moving speed becomes more rapid. These result in the interval of sub-carriers narrower and the level of ICI more serious. In this case, the orthogonality in OFDM becomes quite fragile. So, the effect of ICI cannot be neglected.This paper researches on ICI cancellation in OFDM system of mobile environment. The contents involve ICI self-cancellation, time-varying channel estimation, system equalization and joint multi-origin ICI cancellation. Concretely, the author’s research and contributions are listed as follows.(1) First, the current ICI self-cancellation coding scheme based on polynomial is not optimal and lack of flexibility in coding efficiency. An ICI self-cancellation coding scheme with variable coding efficiency based on generator matrix is presented. And, at the instance with given coding efficiency, an optimal generator matrix in ICI canceling is designed by a new algorithm.(2) Two standards of designing optimum pulse in OFDM systems are presented: the effect of ICI reducing and the out-of-band leakage power. An optimum pulse model is established with given roll-off factor, and a pulse which has good performance in reducing the out-of-band leakage power and ICI simultaneously, is designed.(3) A time-varying channel estimation scheme based on particle filter is presented here. Firstly, channel impulse responses at the middle time of the adjacent OFDM symbol period are expressed by an AR model. Then, a state space model is established with aid of the pilot symbols, in which the ICI term is pointed out to be non-Gaussian variable after statistical analysis. So, the particle filter is used to estimate the channel in a linear and non-Gaussian state space model. Finally, cubic spline interpolation method is adopted to approximate the whole time-varying channel.(4) The idea of ICI self-cancellation is used to improve the performance of time-varying channel estimation. Firstly, the problem on pilot-aided channel estimation with BEM modeling is pointed out that ICI term of adjacent non-pilot symbols will decrease the accuracy of estimation. And then, these ICI coefficients from the symbols on the adjacent sub-carriers are found to be approximate. So, an ICI self-cancellation operation is presented to reduce the ICI terms at the receiver, in order to improve the effect of estimation.(5) A time-varying channel estimation scheme using superimposed training in OFDM systems is presented. The superimposed training sequence has the advantage of saving spectrum resources. But this scheme requires that the estimated parameter must be constants due to the utilizing of first-order statistics, which is conflict with the feature of time-varying channel. So, the time-varying channel during an OFDM symbol period is transformed to a series of time-invariant BEM coefficients by BEM modeling, which makes it possible to estimate time-varying channel using superimposed training.(6) A low-complexity decision feedback equalizer (DFE) is presented based on CE-BEM modeling. This equalizer directly constructs DFE utilizing the estimated CE-BEM coefficients. It successfully avoids the computation of recovering the time-varying channel from the CE-BEM coefficients and inverting a large-scale matrix in equalization. So, it decreases the computational complexity greatly.(7) A scheme of reducing ICI generated by time-varying channels and phase noise together is presented. When the time-varying channel is mixed with the phase noise, it is still a stationary narrowband random process. So, KL-BEM is adopted to modeling the mixed channels and phase noise, which makes it possible to estimate the time-varying channel and the phase noise jointly.更多还原