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带宽效率等优点，已成为近年来的研究热点之一，被认为是未来第四代移动通信的核心技术。
然而，要实现这一目标，仍然存在着大量的问题有待解决，而信道均衡技术就是其中之一。当OFDM系统存在较高的载波频偏或者无线通信系统的发射端和接受端之间的相对运动引起的多普勒频移较大时，OFDM系统载波间的正交特性就会遭到破坏，从而造成严重的子载波间干扰(ICI)，并导致系统性能的极大降低。因此本篇论文对OFDM系统中消除ICI的信道均衡技术进行了深入研究。
本文首先介绍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、结构以及特点。
随后，本文对OFDM系统中子载波间干扰(ICI)的产生机理进行了具体分析，并在此基础上研究和分析了几种能有效消除ICI的频域均衡方法和干扰自消除方法。针对干扰自消除方法带宽利用率过低的问题，本文还进一步提出了一种带宽效率较高的改进算法。
最后，针对时变信道下的OFDM系统，本文重点提出了一种基于线性预编码技术的信道均衡方法。我们首先对OFDM系统中引入无冗余线性预编码技术的迫零均衡方法的误码率性能进行了理论分析，并推导出了最优无冗余线性预编码矩阵的设计准则。为了更充分地利用信道的分集增益，本文还进一步给出了含冗余的线性预编码矩阵的构造准则。最后本文还提出了一种基于分块预编码技术的均衡方案，分析表明这种方案具有预编码矩阵设计简单和计算量较低的优点。理论研究和仿真实验表明，在较高信噪比下，本文所提出的上述均衡方法能有效地抑制ICI和显著地改善OFDM系统的误码率性能。
Orthogonal Frequency Division Multiplexing (OFDM) is a high efficiency multi-carrier transmission technology. Currently, because of nice ability against multi-path interference and high bandwidth efficiency, OFDM gets more and more attention and has already been one of key techniques in 4G mobile communication systems.
However, to achieve this goal, there are still a lot of problems to be resolved, and channel equalization technology is one of them. When there is a higher carrier frequency offset or the Doppler frequency shift caused by the relative movement between the transmitter and receiver is larger, the orthogonality between subcarriers will be destroyed, resulting in a serious inter-carrier interference (ICI), and lead to greatly degrade system performance. To mitigate the ICI, this paper makes an in-depth research on the channel equalization problems in the OFDM wireless communication systems.
Firstly, the characteristics and model of wireless channel and the basic principle, structure and characteristics of OFDM systems are introduced.
Then some frequency domain equalization algorithms and ICI self-cancellation schemes are proposed based on analyzing the generating mechanism of ICI in detail. In order to improve bandwidth efficiency, we also propose a modified ICI self-cancellation scheme.
On the other hand, for OFDM systems over time-variant channel, we propose a zero-forcing equalization method based on linear precoding technique. First, we analyze the bit error rate (BER) performance of this system without introducing redundancy and derive the optimal criterion for the design of non-redundant linear precoders. In order to achieve maximum diversity gain, we then derive the criterion in designing the optimal redundant precoders. Finally, we propose a block precoded method. This method can effectively reduce the designing complexity and computing complexity. Theoretic analysis and simulation results demonstrate that in a higher signal-to-noise ratio (SNR), our equalization technologies are capable of mitigating the ICI and improving the BER performance.
引文
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