  In this article, the new term, namely beam-space multiplexing, is proposed
for the former multi-layer beamforming for 4G TD-LTE in 3GPP releases. We
provide a systematic overview of beam-space multiplexing from engineering and
theoretical perspectives. Firstly, we clarify the fundamental theory of
beam-space multiplexing. Specifically, we provide a comprehensive comparison
with the antenna-space multiplexing in terms of theoretical analysis, channel
state information acquisition, and engineering implementation constraints.
Then, we summarize the key technologies and 3GPP standardization of beam-space
multiplexing in 4G TD-LTE and 5G new radio (NR) in terms of multi-layer
beamforming and massive beamforming, respectively. We also provide system-level
performance evaluation of beam-space multiplexing schemes and field results
from current commercial TD-LTE networks and field trial of 5G. The practical
deployments of 4G TD-LTE and 5G cellular networks demonstrate the superiority
of beam-space multiplexing within the limitations of implementation complexity
and practical deployment scenarios. Finally, the future trends of beam-space
multiplexing in 6G and beyond are discussed, including massive beamforming for
extremely large-scale MIMO (XL-MIMO), low earth orbit (LEO) satellites
communication, data-driven intelligent massive beamforming, and multi-target
spatial signal processing, i.e., joint communication and sensing, positioning,
etc.
