Emil Björnson and Jiayi Zhang
Linköping University, Beijing Jiaotong University

Signal processing is at the core of the 5G communication technology. The use of large arrays with 64 or more antennas is becoming mainstream and the commercial deployment started in 2019. This technology is known as Massive MIMO (multiple-input multiple-output) and was viewed as science fiction just ten years ago, but with the combination of advanced signal processing and innovative protocols, it is now a reality. Just as the seminal papers on Massive MIMO were published ten years ago, this is likely the time when the new technology components for 6G will be identified. In this tutorial, we will consider two such promising research directions, which might be utilized in the conventional cellular spectrum as well as in mmWave or sub-THz bands.

The first new direction is Cell-free Massive MIMO, which refers to a large-scale distributed antenna system that is made practical by innovative signal processing and radio resource allocation algorithms. Different from cellular communications, each user is served by all or a user-unique subset of the antennas. The system is designed to achieve high spectral and energy efficiency, but under the unusual constraint of being scalable from a computational and cost perspective to enable large network deployments. The main goal is to achieve uniformly good and reliable service via excessive macro-diversity, as compared to the micro-diversity achieved by conventional Massive MIMO with large arrays. We will cover the basic theory as well as the recent algorithmic and implementation developments.

The second new direction is intelligent reflecting surfaces, which are also known as software-controlled meta-surfaces and reconfigurable intelligent surfaces. These are semi-passive surfaces consisting of an array of meta-atoms with reconfigurable properties that can be controlled to reflect an incoming wave in a controllable way. While only the transmitter and receiver can be optimized in conventional wireless communication systems, the addition of intelligent reflecting surfaces enables optimization also of the channels (i.e., the creating of smart radio environments). We will derive the propagation model from physics and tackle difficult issues such as channel estimation and real-time operation.

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