Towards a fully connected intelligent digital world, 5G and beyond networks experience a new era of Internet of intelligence with connected people and things. This new era brings challenging demands to the network, such as high spectral efficiency, low-latency, high-reliable communication, and high energy efficiency. One of the major technological breakthroughs to cope with these unprecedented demands is the cell-free (CF) massive multiple-input multiple-output (mMIMO) systems. In CF mMIMO, a large number of distributed access points are connected to a central processing unit, and serve a smaller number of users over the same time-frequency resources. The system has shown a great potential in improving the network performance in various perspectives compared to the co-located mMIMO and conventional small-cell systems. Furthermore, the system can be flexibly integrated with various emerging techniques/technologies for 5G and beyond networks to boost the network performance in different perspectives. Despite the substantial reported theoretical gains of CF mMIMO systems, the full picture of a practical scalable deployment of the system is not clear yet. In this paper, we provide a comprehensive survey of different aspects of the CF mMIMO system from the general system model, the detailed system operation, the limitations towards a practically implemented system to the potential of integrating the system with emerging techniques/technologies. Besides, we provide a number of timely open problems and future research directions to fully exploit the CF mMIMO system potential in delivering the anticipated requirements of future wireless networks.

中文翻译：

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无蜂窝大规模 MIMO：调查


面向万物互联的智能数字世界，5G及其他网络将开启人与物互联的智能互联新时代。新时代给网络带来了高频谱效率、低时延、高可靠通信、高能效等具有挑战性的需求。应对这些前所未有的需求的重大技术突破之一是无小区（CF）大规模多输入多输出（mMIMO）系统。在CF mMIMO中，大量分布式接入点连接到中央处理单元，并在相同的时频资源上服务较少数量的用户。与共置mMIMO和传统小基站系统相比，该系统在各个方面都显示出在提高网络性能方面的巨大潜力。此外，该系统可以灵活地与5G及其他网络的各种新兴技术集成，从不同角度提升网络性能。尽管 CF mMIMO 系统在理论上取得了巨大的成果，但该系统实际可扩展部署的整体情况尚不清楚。在本文中，我们对 CF mMIMO 系统的不同方面进行了全面的调查，从一般系统模型、详细系统操作、实际实现系统的局限性到系统与新兴技术/技术集成的潜力。此外，我们还提供了许多及时的开放问题和未来的研究方向，以充分发挥 CF mMIMO 系统在满足未来无线网络的预期要求方面的潜力。