【作者】 邹云；

【导师】 郭成安；

【作者基本信息】 大连理工大学 ， 信号与信息处理， 2008， 硕士

【摘要】 正交频分复用(OFDM)技术是多载波传输方案中的一种有效传输方式,由于其具有良好的抗多径干扰能力和较高的频带利用率,已经成为近年来研究的热点之一,被认为是第四代移动通信的核心技术。然而,未来的无线通信系统将工作在高载频、高容量、高移动速度的条件下,信道具有时间和频率双选择性,即广义时变信道。此时,OFDM系统存在较高的载波频率偏移以及由无线通信系统的发射端和接收端的相对运动引起的多普勒频移,OFDM系统子载波间的正交性遭到破坏,造成严重的子载波间干扰(ICI),导致系统性能的大幅下降。信道均衡技术正是解决这一问题的有效手段,本文对OFDM系统中消除ICI的信道均衡技术进行了深入的研究。本文首先讨论了移动通信系统的信道特性以及信道模型,并详细描述了OFDM系统的基本原理和特点。随后,本文对OFDM系统在广义时变信道下ICI的生成机制进行了具体分析,并在此基础上研究和分析了ICI自消方法.针对传统ICI自消方法频带利用率过低的缺陷,通过改变预编码矩阵的结构,提出频带利用率较高的改进方法,并提出串行补偿的改进均衡算法来获得更好的均衡效果。然后,本文重点研究了广义时变信道下OFDM系统的最小均方误差(MMSE)均衡算法。MMSE均衡算法采用统计量代替每一个信号各自的方差,在对多进制调制信号进行均衡时存在一定的误差,而且随着调制进制数的增加,误差逐渐增大。本文提出二次MMSE均衡算法,在第一次均衡后利用均衡结果对每个信号的方差进行独立估计,在此基础上进行二次均衡,这一改进算法获得很大的性能增益。接着,本文结合广义时变信道传输矩阵的特点以及ICI生成机制,采用了迭代的思想并忽略信道矩阵中影响较小的元素对二次MMSE均衡算法进行改进,得到复杂度较低的简化算法。最后,本文研究了频域Turbo均衡算法在OFDM系统中的应用。将上述的迭代方法结合忽略信道矩阵中影响较小元素的思想应用于Turbo均衡算法中,解决了普通的Turbo算法中计算量过大的问题。理论分析和仿真实验表明,本文所提出的上述均衡方法能有效的抑制ICI,使OFDM系统的误码率得到显著降低,并且具有较低的计算复杂度。更多还原

【Abstract】 Orthogonal Frequency Division Multiplexing (OFDM) is a high efficiency multi-carrier transmission technology. It is wildly studied and used in wireless communication because it can effectively combat the inter-symbol interferences and it has high spectral efficiency. OFDM is considered to be one of the most expecting solutions in the 4th generation of communication.However, the future communication is expected to be used with high frequency, high speed and high quantity, there is a higher carrier frequency offset and the Doppler frequency shift caused by the relative movement between the transmitter and the receiver. Therefore, the orthogonality of the subcarriers are destroyed, resulting in a serious inter-carrier interference (ICI), and lead to irreducible performance degradation. To mitigate the ICI, the use of equalizer is considered to be one of the most effective ways. This paper makes an in-depth research on the channel equalization of OFDM systems in wide-sense time-varying fading channels.Firstly, the characteristics and model of wireless channel and basic principle, structure and characteristics of OFDM systems are introduced.Then the generation of mechanism of ICI of OFDM in wide-sense time-varying channels are analyzing in detail. Based on the analysis, ICI self-cancellation scheme is studied. In order to improve the spectral efficiency, the structure of pre-codeing matrix is changed, and combined with the serial equalizing method, the system performance of the ICI self-cancellation is also enhanced.This paper focus on the MMSE equalization of OFDM system in wide-sense time-varying channels. MMSE equalization has weakness when Multi-mode modulation is used, and with the increase of the modulated modes, the effect of the weakness becomes larger. To solve this problem, a so-called Modified MMSE equalization is proposed. For the first MMSE round, the estimation of the input signals are used to estimate the energy of separate input signals, for the second round, the separate energies are used to form the equalizer. This method great increases the symbol error rate (SER). Iterative and diagonal methods are used to reduce the computation complexity of the Modified MMSE equalization.At last, the principle of Turbo equalization of OFDM system is studied. Combined with the iterative and diagonal method, the low complexity solution is proposed. Theoretic analysis and simulation results demonstrate that all the equalization algorithms proposed in this paper are capable of mitigating the ICI and improving the SER performance, also, some of the algorithms reduce the computation complexity.更多还原