  Optical wireless communication (OWC) emerges as a pivotal solution for
achieving terabit-level aggregate throughput in next-generation wireless
networks. With the mature high-speed transceivers and advanced (de)multiplexing
techniques designed for fiber optics, fiber-coupled OWC can be seamlessly
integrated into existing ultra-high-speed networks such as data centres. In
particular, OWC leveraging spatial mode multiplexing (SMM) and few-mode fiber
(FMF) coupling can significantly increase capacity, though misalignment may
reduce performance. This paper presents a thorough investigation into the
SMM-enabled FMF coupling OWC systems affected by link misalignment,
specifically focusing on systems with intensity modulation with direct
detection (IM/DD) receivers. A theoretical analysis is conducted to assess the
fiber coupling efficiency of the considered system in the presence of both
pointing error and angle of arrival (AOA) fluctuations caused by random device
vibrations. Our model elucidates the dependence of coupling efficiency to the
order of the incident modes, highlighting the critical role of beam properties
in system performance. To mitigate the intermodal crosstalk arising from link
misalignment, we employ zero-forcing beamforming (ZFBF) to enhance the overall
aggregated data rate. Through extensive numerical results, we identify optimal
system configurations encompassing aperture design and mode selection, leading
to a capacity boost exceeding 200%.
