  A near-field wideband communication system is investigated in which a base
station (BS) employs an extra-large scale antenna array (ELAA) to serve
multiple users in its near-field region. To facilitate near-field multi-user
beamforming and mitigate the spatial wideband effect, the BS employs a hybrid
beamforming architecture based on true-time delayers (TTDs). In addition to the
conventional fully-connected TTD-based hybrid beamforming architecture, a new
sub-connected architecture is proposed to improve energy efficiency and reduce
hardware requirements. Two wideband beamforming optimization approaches are
proposed to maximize spectral efficiency for both architectures. 1)
Fully-digital approximation (FDA) approach: In this method, the TTD-based
hybrid beamformer is optimized by the block-coordinate descent and penalty
method to approximate the optimal digital beamformer. This approach ensures
convergence to the stationary point of the spectral efficiency maximization
problem. 2) Heuristic two-stage (HTS) approach: In this approach, the analog
and digital beamformers are designed in two stages. In particular, two
low-complexity methods are proposed to design the high-dimensional analog
beamformers based on approximate and exact line-of-sight channels,
respectively. Subsequently, the low-dimensional digital beamformer is optimized
based on the low-dimensional equivalent channels, resulting in reduced
computational complexity and channel estimation complexity. Our numerical
results show that 1) the proposed approach effectively eliminates the spatial
wideband effect, and 2) the proposed sub-connected architecture is more energy
efficient and has fewer hardware constraints on the TTD and system bandwidth
compared to the fully-connected architecture.
