The fifth-generation (5G) of cellular networks and beyond requires massive connectivity, high data rates, and low latency. Millimeter-wave (mmWave) communications is a key 5G enabling technology to meet these requirements thanks to its technical potentials that can be integrated with other 5G enablers such as ultra-dense networks (UDNs) and massive multiple-input-multiple-output (massive MIMO) systems. However, some technical challenges, which are mainly related to specific characteristics of mmWave propagation, must be addressed. All the aforementioned points will be discussed in this article before presenting the different existing architectures of massive MIMO mmWave systems. This survey mainly aims at presenting a comprehensive state-of-the-art review of the channel estimation techniques associated with the different mmWave system architectures. Subsequently, we will provide a comparison among existing solutions in terms of their respective benefits and shortcomings. Finally, some open directions of research are discussed, and challenges that wait to be met are pointed out.

中文翻译：

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毫米波通信系统的信道估计技术：成就和挑战

第五代及以后的蜂窝网络（5G）需要大量的连接，高数据速率和低延迟。毫米波（mmWave）通信是可以满足这些要求的关键5G技术，这归功于它的技术潜力，可以与其他5G支持器（例如超密集网络（UDN）和大规模多输入多输出（大规模） MIMO）系统。但是，必须解决一些技术挑战，这些挑战主要与毫米波传播的特定特性有关。在介绍大型MIMO mmWave系统的不同现有架构之前，将在本文中讨论所有上述要点。这项调查的主要目的是对与不同mmWave系统架构相关的信道估计技术进行全面的技术综述。随后，我们将根据现有解决方案各自的优点和缺点进行比较。最后，讨论了一些开放的研究方向，并指出了有待解决的挑战。