Cell-Free Massive MIMO

Distributed multiple-input multiple-output (MIMO) systems hinge on cooperative signal processing techniques to turn interference into desired signal, thereby providing excellent levels of quality of service throughout the network. Specifically, multiple geographically distributed access points jointly and coherently serve all the users in their joint coverage area, implementing interference-suppression mechanisms in transmit precoding and receive combining. The great theoretical benefits of the signal co-processing are known since the early 2000s, but the gains experienced in practical implementations over the last two decades have not yet achieved the desired levels. However, the cell-free massive MIMO technology has recently revisited the distributed MIMO concept. Cell-free massive MIMO is deemed to be a key-enabling technology for the 6G physical layer as fruitfully combines the best of two worlds, namely network ultra-densification and joint coherent signal processing, relying on the now mature massive MIMO technology. Cell-free massive MIMO promises unprecedented practical gains and to succeed where all the prior distributed MIMO systems failed. The aim of this chapter is to comprehensively describe the beauty of distributed MIMO with emphasis on its latest enhanced embodiment, that is cell-free massive MIMO.