OFDM系统非平稳相关散射突变信道的表示与估计

In broadband orthogonal frequency-division multiplexing (OFDM) wireless systems, the fading process under extremely high mobility is not expected to fulfill the wide-sense stationarity uncorrelated scattering (WSSUS) assumption. To alleviate this problem, channel estimation is adopted at the receiver as the critical technique to combat the impact brought to the signals by such Non-WSSUS transient channels. However, current channel estimation methods are not designed for such Non-WSSUS channel, thereby seriously hindering the development of the broadband wireless access for extremely high mobility applications. In this project, Non-WSSUS transient channel is studied for OFDM systems. First, the time-varying statistical characteristics of transient channel is investigated and the mechanism which causes the inter-carrier interference (ICI) in transient channel is analyzed; further, a compressive and sparse expression of transient channel is explored; finally, a series of channel estimation theroy is established for applications under extremely high mobility. Specifically, Markov chain and birth-death process based multi-scenario time-varying channel analysis is developed to describe the statistical characteristics of transient channel; local scattering function (LSF) and the generalized channel correlation (GCC) function is introduced as intuitive and useful statistical characterization of transient channels; a generalized LSF is designed and several useful approximations and interpretations for the LSF are given for the subclass of practically relevant doubly underspread non-WSSUS channels; a computationally efficient, nonstationary multiwindow LSF estimator is proposed to yield reliable estimates in the case of doubly underspread channels even for a single channel realization; a time-frequency-space united sparse model is established on the basis of LSF and GCC; time-frequency stochastic channel core vector is extracted from the proposed model; antennas and delay difference detection is designed for the proposed model; real-time tracking and estimation of non-zero entries in time-frequency stochastic channel core vector is achieved by orthogonal matching pursuit (OMP); finally, the CIR matrix is reconstructed based on principal of restricted energy. Simulation system of transient channel estimation is designed for algorithm verification. This project aims at proposing a complete baseband solution for broadband OFDM wireless systems under extremely high mobility based on transient channel estimation, and providing a solid theoretical foundation and technical support for broadband wireless access of high speed railway.

高铁环境下的无线信道具有非广义平稳相关散射突变特性，目前广泛研究的广义平稳非相关散射快衰落信道估计技术不可能实现对此类突变信道的表示与估计，严重制约了下一代移动通信系统对高速移动环境的全覆盖。本项目对OFDM系统非广义平稳相关散射突变信道进行研究，分析其时变统计特性及非平稳衰落过程引起ICI的机理，探索其可压缩稀疏表示方法，建立信道估计理论。利用多场景马尔科夫链生灭过程进行突变信道统计描述和时变分析，构造局部散射和信道相关函数刻画时频独立的平均散射功率，使突变信道退化为局部低度双扩展信道，通过时频随机信道核建立突变信道的可压缩时频空稀疏模型，结合天线数估计与时延检测对等效模型进行补偿，利用群正交匹配实时追踪和估计信道核，以能量有限原则对CIR进行联合重构，实现突变信道的准确估计，并设计相应的快速算法。本项目研究可获得OFDM系统突变信道估计理论与方法，为高铁宽带接入提供理论基础和方法支撑。

作为下一代5G移动通信系统的核心技术之一的OFDM技术在具有优势的同时，需要对无线信道进行信道估计来保障其性能。不同于目前广泛研究的广义平稳非相关散射（WSSUS）快衰落信道具有单一的统计特性，高铁的无线信道呈现出非广义平稳相关散射（Non-WSSUS）的突变特性，这导致以WSSUS为前提的快衰落信道的估计方法不能应用于高铁环境下的无线通信中。针对这一问题，本项目首先研究了高铁场景下Non-WSSUS无线信道的统计特性，提出了基于散射特征的高铁无线信道统计模型，推导了移动端的多普勒功率谱表示式，分析了OFDM系统下Non-WSSUS信道中引起载波间干扰（ICI）的机理和影响ICI的因素。接着，在分析Non-WSSUS信道统计特性的基础上，研究了突变Non-WSSUS信道的可压缩稀疏表达方法，利用信道自相关矩阵的奇异值分解，推导了基于信道核向量的紧凑型Non-WSSUS信道冲激响应（CIR）矩阵压缩表示，紧凑型广义Non-WSSUS信道冲激响应矩阵（G-CIR）的压缩表示，利用稀疏的时延-多普勒函数，得到了OFDM系统中Non-WSSUS信道、突变Non-WSSUS信道、多径马尔科夫（Markov）生灭过程Non-WSSUS信道CIR的稀疏表达。最后，针对上述高铁场景下的突变Non-WSSUS信道，建立了信道估计、ICI消除及功率分配理论，基于信道核向量的紧凑型CIR矩阵压缩表示，提出了等效信道模型中信道核向量估计算法，稀疏化信道核向量的重构算法，非理想载波同步下快衰落信道核向量估计算法。基于Non-WSSUS信道的稀疏时延-多普勒函数，提出了预测-再估计的权重L1最小化Non-WSSUS信道CIR估计算法，粒子滤波辅助非凸稀疏信号恢复算法，并将其应用于突变Non-WSSUS信道的CIR估计中，推导了稀疏重构的误差界和精确恢复条件。基于对Non-WSSUS信道ICI机理的分析，提出了基于循环前缀和迫零均衡技术的联合消除算法，适于高铁信道载波间干扰的自适应相位旋转共轭消除算法，高速环境下信道估计和符号检测联合最优化策略，高速移动环境下OFDM系统的导频序列设计的路径跟踪优化算法，总功率最小的资源分配算法，子载波配对正交频分复用中基于机会解码转发中继的功率最小化算法，并对5G技术进行了研究。本项目的研究成果为高铁宽带无线接入提供参考。