摘要
载波恢复技术是数字通信系统中一个必不可少的单元，用来纠正信号传输和解调过程中所带来的频偏和相偏。本文首先介绍了经典的最大似然估计(ML)算法，面向判决(DD)算法和简化星座(RC)算法。ML算法是理论上的最佳算法，但是比较复杂，实现起来很困难。DD算法大大的简化了ML算法的复杂度，在实际中得到了广泛的应用，但是在固定的环路滤波带宽的情况下，无法解决收敛速度和稳态方差之间的矛盾。针对这一问题，本文给出了一种自适应调节环路带宽的改进DD算法。经仿真验证，这种改进算法可以很好的解决这一矛盾。然后介绍了适用于高阶QAM的极性判决相位检测算法。这是一种多模式的联合算法，在开始利用了有较大捕获范围的RC算法，在进入跟踪模式后利用DD算法来减小稳态方差。本文在对极性判决算法进行研究的基础上，对其模式转化方法进行了优化，并提出了“极性判决+扫频”的算法，使其捕获范围有了较大的扩展。最后，本文给出了QAM测试仪的设计与实现方案。
The carrier recovery is a necessary part in the digital communication system. It is used to compensate frequency offset and phase jitter brought by transmission and demodulation. Firstly, this paper introduces the maximum likelihood (ML) algorithm, the decision directed (DD) algorithm and the reduced constellation (RC) algorithm. Although ML algorithm is one of the best in the theory, it is too complex to implement. DD algorithm reduces the complexity of ML algorithm and gets widely used in reality. But in the case of constant loop bandwidth, it is impossible to harmonize the conflict of convergence speed and steady-state variance. To solve the problem, this paper proposes a new DD algorithm with adaptive loop bandwidth. And proved by simulation, the new algorithm can solve the problem well. Then this paper introduces the polarity decision algorithm for high-order QAM, which has many states. It uses RC algorithm with wider acquisition range at the beginning, and uses DD algorithm in the tracking status. Based on the research of the polarity decision algorithm, this paper proposes a new status transform method, and a new algorithm of "polarity decision and scan frequency algorithm". In this method we can get wider acquisition range. Finally, this paper gives the scheme of QAM tester's design and implementation.
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