  Extremely large-scale multiple-input-multiple-output (XL-MIMO), which offers
vast spatial degrees of freedom, has emerged as a potentially pivotal enabling
technology for the sixth generation (6G) of wireless mobile networks. With its
growing significance, both opportunities and challenges are concurrently
manifesting. This paper presents a comprehensive survey of research on XL-MIMO
wireless systems. In particular, we introduce four XL-MIMO hardware
architectures: uniform linear array (ULA)-based XL-MIMO, uniform planar array
(UPA)-based XL-MIMO utilizing either patch antennas or point antennas, and
continuous aperture (CAP)-based XL-MIMO. We comprehensively analyze and discuss
their characteristics and interrelationships. Following this, we introduce
several electromagnetic characteristics and general distance boundaries in
XL-MIMO. Given the distinct electromagnetic properties of near-field
communications, we present a range of channel models to demonstrate the
benefits of XL-MIMO. We further discuss and summarize signal processing schemes
for XL-MIMO. It is worth noting that the low-complexity signal processing
schemes and deep learning empowered signal processing schemes are reviewed and
highlighted to promote the practical implementation of XL-MIMO. Furthermore, we
explore the interplay between XL-MIMO and other emergent 6G technologies.
Finally, we outline several compelling research directions for future XL-MIMO
wireless communication systems.
