Cell-Free Massive MIMO: A Survey

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.