Doctoral Thesis: Massive MIMO in Real Propagation Environments

Sammanfattning: Popular Abstract in English Mobile communication systems are now evolving towards the fifth generation (5G). In the near future, we expect an explosive increase in the number of connected devices, such as phones, tablets, wearable accessories, sensors, connected vehicles and so on. We can imagine that much higher data rates than in today's 4G systems are required. Sounds like the future of communication is specially developed for urban living? The answer is not really! The 5G visions also include bringing the current "4 billion unconnected" population in remote regions into the Internet world. There is also a great interest in "green communications", aiming for less energy consumption and carbon emissions from the ICT (information and communication technology) industry. One of the technologies that has a potential to fulfill these requirements and visions is massive MIMO (massive multiple-input multiple output). Massive MIMO is a relatively new research area that starts around five years ago. Literally, massive MIMO means that a large number of antennas and transceivers are used on the base-station side in a communication system. By having more antennas, we can basically exploit more degrees of freedom in the spatial domain. With a massive number of antennas, we open up the spatial dimension on a much larger scale than ever before. Through spatial signal processing, we gain higher data rates and communicate with more connected devices simultaneously. Thanks to the large spatial degrees of freedom, the communication service can also cover a larger area, which is especially preferable in remote regions. On the other hand, we can also reduce the transmit power radiated from base stations and terminals. This makes massive MIMO a candidate also in the area of "green communications". A lot of theoretical analysis has been done, showing the extraordinary advantages of massive MIMO, both in terms of spectral efficiency and transmit-power efficiency. Despite all these advantages, we face many challenges when bringing massive MIMO from theory to practice. The challenges include 1) whether physical propagation environments give the same advantages as predicted in theoretical studies, and 2) how can system complexity stay low at the same time as the number of transceivers becomes "massive". The PhD thesis covers the concepts of massive MIMO, its pros and cons, and tries to answer the above two questions. Based on conducted channel measurements at Lund University, we focus on massive MIMO propagation behavior, modeling and performance evaluation in real-life environments. The "ideal" world of theory has been connected to the "non-ideal" reality. Not much has been known about massive MIMO behavior in real propagation environments, and whether the claims about massive MIMO also hold there, until the studies in this thesis were done.