  In this letter, an orthogonal time frequency space (OTFS) based
non-orthogonal multiple access (NOMA) scheme is investigated for the
coordinated direct and relay transmission system, where a source directly
communicates with a near user with high mobile speed, and it needs the relaying
assistance to serve the far user also having high mobility. Due to the
coexistence of signal superposition coding and multi-domain transformation, the
performance of OTFS-based NOMA is usually challenging to be measured from a
theoretical perspective. To accurately evaluate the system performance of the
proposed scheme, we derive the closed-form expressions for the outage
probability and the outage sum rate by using the Inversion formula and
characteristic function. Numerical results verify the performance superiority
and the effectiveness of the proposed scheme.
