An Overview of Massive MIMO for 5G and 6G

Massive MIMO is a disruptive technology that was born more than a decade ago, and that has been adopted as an integral part of the 5G standard. Without its many advantages, the next generation of communications systems would not be able to meet most of the requirements necessary to be called 5G. However, several challenges had to be solved from its inception until its adoption. Nowadays, with most of them overcome, researchers are turning their attention to novel ways to enhance the technology and have it once more being an essential feature of the next generation of communications systems, e.g., 6G. This work provides a brief but comprehensive overview of the technology known as Massive MIMO, its challenges, available solutions, and novel research directions. Additionally, we provide discussions on the use of Massive MIMO in Terahertz bands and the use of Machine Learning algorithms for channel estimation and signal detection tasks in such systems.