Massive multiple-input multiple-output (MIMO) is currently entering the practical implementation phase, and key implementation issues for this technology have yet to be fully addressed. Crucial among these is the practical problem of frequency synchronisation, which refers to the adjustment of the clock frequency of local nodes to the clock frequency of a reference node by estimating and compensating carrier frequency offset. Existing theoretical studies on massive MIMO generally assume perfect frequency synchronisation; however, the potentially very high complexity of this process poses a major challenge for massive MIMO systems. Therefore, new frequency synchronisation techniques are urgently needed to make the practical implementation of massive MIMO feasible. In this study, the authors provide a comprehensive classification of the existing research efforts along this line, considering different antenna architectures and modulation schemes. They also highlight the key challenges in frequency synchronisation for massive MIMO, and they outline future research directions on this topic.

中文翻译：

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大规模MIMO的频率同步：一项调查

大规模多输入多输出（MIMO）当前正在进入实际实施阶段，并且该技术的关键实施问题尚未完全解决。其中最关键的是频率同步的实际问题，它是指通过估计和补偿载波频率偏移将本地节点的时钟频率调整为参考节点的时钟频率。现有的关于大规模MIMO的理论研究通常假设完美的频率同步；因此，对MIMO的研究并不十分理想。然而，该过程的潜在非常高的复杂性对大规模MIMO系统提出了重大挑战。因此，迫切需要新的频率同步技术以使大规模MIMO的实际实现成为可能。在这个研究中，作者考虑了不同的天线架构和调制方案，对沿这条线的现有研究成果进行了全面分类。他们还强调了大规模MIMO频率同步中的关键挑战，并概述了该主题的未来研究方向。