  In this paper, a Full Duplex (FD) eXtremely Large (XL) Multiple-Input
Multiple-Output (MIMO) node equipped with reconfigurable metasurface antennas
at its transmission and reception sides is considered, which is optimized for
simultaneous multi-user communications and sensing in the near-field regime at
THz frequencies. We first present a novel Position Error Bound (PEB) analysis
for the spatial parameters of multiple targets in the vicinity of the FD node,
via the received backscattered data signals, and devise an optimization
framework for its metasurface-based precoder and combiner. Then, we formulate
and solve an optimization problem aiming at the downlink sum-rate maximization,
while simultaneously ensuring a minimum PEB requirement for targets'
localization. Our simulation results for a sub-THz system setup validate the
joint near-field communications and sensing capability of the proposed FD XL
MIMO scheme with metasurfaces antennas, showcasing the interplay of its various
design parameters.
