  This paper elaborates on the design challenges, opportunities, and solutions
for New Radio-based access to Unlicensed spectrum (NR-U) by taking into account
the beam-based transmissions and the worldwide regulatory requirements. NR-U
intends to expand the applicability of 5th generation New Radio access
technology to support operation in unlicensed bands by adhering to
Listen-Before-Talk (LBT) requirements for accessing the channel. LBT was
already adopted by different variants of 4th generation Long Term Evolution
(LTE) in unlicensed spectrum, i.e., Licensed-Assisted Access and MulteFire, to
guarantee fair coexistence among different radio access technologies. In the
case of beam-based transmissions, the NR-U coexistence framework is
significantly different as compared to LTE in unlicensed spectrum due to the
use of directional antennas, which enhance the spatial reuse but also
complicate the interference management. In particular, beam-based transmissions
are needed in the unlicensed spectrum at millimeter-wave (mmWave) bands, which
is an attractive candidate for NR-U due to its large amount of allocated
spectrum. As a consequence, some major design principles need to be revisited
to address coexistence for beam-based NR-U. In this paper, different problems
and the potential solutions related to channel access procedures, frame
structure, initial access procedures, re-transmission procedures, and
scheduling schemes are discussed. A simulation evaluation of different
LBT-based channel access procedures for NR-U/Wi-Fi indoor mmWave coexistence
scenarios is also provided.
