  In this paper, the design of a multiple-input multiple-output (MIMO) optical
wireless communication (OWC) link based on vertical cavity surface emitting
laser (VCSEL) arrays is systematically carried out with the aim to support data
rates in excess of 1 Tb/s for the backhaul of sixth generation (6G) indoor
wireless networks. The proposed design combines direct current optical
orthogonal frequency division multiplexing (DCO-OFDM) and a spatial
multiplexing MIMO architecture. For such an ultra-high-speed line-of-sight
(LOS) OWC link with low divergence laser beams, maintaining alignment is of
high importance. In this paper, two types of misalignment error between the
transmitter and receiver are distinguished, namely, radial displacement error
and orientation angle error, and they are thoroughly modeled in a unified
analytical framework assuming Gaussian laser beams, resulting in a generalized
misalignment model (GMM). The derived GMM is then extended to MIMO arrays and
the performance of the MIMO-OFDM OWC system is analyzed in terms of the
aggregate data rate. Novel insights are provided into the system performance
based on computer simulations by studying various influential factors such as
beam waist, array configuration and different misalignment errors, which can be
used as guidelines for designing short range Tb/s MIMO OWC systems.
