Extremely Large-Scale MIMO: Fundamentals, Challenges, Solutions, and Future Directions

Extremely large-scale multiple-input-multipleoutput (XL-MIMO) is a promising technology to empower the next-generation communications. However, XL-MIMO, which still in its early stage of research, has been designed with variety hardware and performance analysis schemes. To illustrate differences similarities among these schemes, we comprehensively review existing XL-MIMO designs characteristics this article. Then, thoroughly discuss research status from "channel modeling", "performance analysis", "signal processing". Several challenges are introduced respective solutions provided. We then propose two case studies for hybrid propagation channel modeling effective degrees freedom (EDoF) computations practical scenarios. Using our proposed solutions, perform numerical results investigate EDoF scenarios unparallel surfaces multiple user equipment, respectively. Finally, several future directions.