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