  Few-mode multi-core fiber (FM-MCF) based Space-Division Multiplexing (SDM)
systems possess the potential to maximize the number of multiplexed spatial
channels per fiber by harnessing both the space (fiber cores) and mode (optical
mode per core) dimensions. However, to date, no SDM transmissions over
field-deployed FM-MCFs in realistic outdoor settings have been reported, which
contrasts with SDM schemes demonstrated using single-mode multi-core fibers
(SM-MCFs) installed in practical fiber cable ducts. In this paper, we present
the successful demonstration of bidirectional SDM transmission over a 5-km
field-deployed seven ring-core fiber (7-RCF) with a cladding diameter of 178
${\mu}$m, achieving a Spectral Efficiency (SE) of 2$\times$201.6 bit/s/Hz. This
work establishes a new record for the highest SE attained in SDM demonstrations
utilizing field-deployed fiber cables, achieving an approximate 10x increase
compared to the SE of reported field-deployed optical fiber cable transmission
systems. Notably, these results are realized through the utilization of
small-scale modular 4$\times$4 multiple-input multiple-output (MIMO) processing
with a time-domain equalization (TDE) tap number not exceeding 15, maintaining
a complexity per unit capacity comparable to that of MIMO equalization in SDM
demonstrations employing weakly coupled SM-MCF cables. These results underscore
the significant potential for achieving heightened SE and expanding capacity
per individual fiber using SDM techniques in practical applications.
