  In this paper, we consider a downlink (DL) massive multiple-input
multiple-output (MIMO) system, where different users have different mobility
profiles. To support this system, we propose to use a hybrid orthogonal time
frequency space (OTFS)/orthogonal frequency division multiplexing (OFDM)
modulation scheme, where OTFS is applied for high-mobility users and OFDM is
used for low-mobility users. Two precoding designs, namely full zero-forcing
(FZF) precoding and partial zero-forcing (PZF) precoding, are considered and
analyzed in terms of per-user spectral efficiency (SE). With FZF, interference
among users is totally eliminated at the cost of high computational complexity,
while PZF can be used to provide a trade-off between complexity and
performance. To apply PZF precoding, users are grouped into two disjoint groups
according to their mobility profile or channel gain. Then, zero-forcing (ZF) is
utilized for high-mobility or strong channel gain users to completely cancel
the inter-group interference, while maximum ratio transmission (MRT) is applied
for low-mobility users or users with weak channel gain. To shed light on the
system performance, the SE for high-mobility and low-mobility users with a
minimum-mean-square-error (MMSE)-successive interference cancellation (SIC)
detector is investigated. Our numerical results reveal that the PZF precoding
with channel gain grouping can guarantee a similar quality of service for all
users. In addition, with mobility-based grouping, the hybrid OTFS/OFDM
modulation outperforms the conventional OFDM modulation for high-mobility
users.
