  5G New Radio (NR) promises to support diverse services such as enhanced
mobile broadband (eMBB), ultra-reliable low-latency communication (URLLC), and
massive machine-type communication (mMTC). This requires spectrum, most of
which is occupied by 4G Long Term Evolution (LTE). Hence, network operators are
expected to deploy 5G using the existing LTE infrastructure while migrating to
NR. In addition, operators must support legacy LTE devices during the
migration, so LTE and NR systems will coexist for the foreseeable future. In
this article, we address LTE-NR coexistence starting with a review of both
radio access technologies. We then describe the contributions by the 3rd
Generation Partnership Project (3GPP) to solving the coexistence issue and
catalog the major coexistence scenarios. Lastly, we introduce a novel spectrum
sharing scheme that can be applied to the coexistence scenarios under study.
