  In this work, we study the use of non-orthogonal multiple access (NOMA) and
orthogonal time frequency space (OTFS) modulation in a multiple-input
multiple-output (MIMO) communication network where mobile users (MUs) with
different mobility profiles are grouped into clusters. We consider a downlink
scenario where a base station (BS) communicates with multiple users that have
diverse mobility profiles. High-mobility (HM) users' signals are placed in the
delay-Doppler (DD) domain using OTFS modulation in order to transform their
time-varying channel into a sparse static channel, while low-mobility (LM)
users signals are placed in the time-frequency (TF) domain. Precoding is
adopted at the BS to direct focused beams towards each cluster of users.
Moreover, NOMA spectrum sharing is used in each cluster to allow the
coexistence of a single HM user and multiple LM users within the same resource
block. LM users access disjoint subchannels to ensure their orthogonality. All
users within the same cluster first detect the HM user's signal. Afterward, LM
users suppress the interference from the HM user and detect their own signals.
Closed-form expressions of the detection signal-to-noise ratios (SNRs) are
derived. The numerical results showed that the performance of the proposed
system highly depends on the number of LM users, the number of clusters and the
power allocation factors between HM and LM users.
