  Space-time shift keying-aided orthogonal time frequency space
modulation-based multiple access (STSK-OTFS-MA) is proposed for reliable uplink
transmission in high-Doppler scenarios. As a beneficial feature of our
STSK-OTFS-MA system, extra information bits are mapped onto the indices of the
active dispersion matrices, which allows the system to enjoy the joint benefits
of both STSK and OTFS signalling. Due to the fact that both the time-, space-
and DD-domain degrees of freedom are jointly exploited, our STSK-OTFS-MA
achieves increased diversity and coding gains. To mitigate the potentially
excessive detection complexity, the sparse structure of the equivalent
transmitted symbol vector is exploited, resulting in a pair of low-complexity
near-maximum likelihood (ML) multiuser detection algorithms. Explicitly, we
conceive a progressive residual check-based greedy detector (PRCGD) and an
iterative reduced-space check-based detector (IRCD). Then, we derive both the
unconditional single-user pairwise error probability (SU-UPEP) and a tight bit
error ratio (BER) union-bound for our single-user STSK-OTFS-MA system employing
the ML detector. Furthermore, the discrete-input continuous-output memoryless
channel (DCMC) capacity of the proposed system is derived. The optimal
dispersion matrices (DMs) are designed based on the maximum attainable
diversity and coding gain metrics. Finally, it is demonstrated that our
STSK-OTFS-MA system achieves both a lower BER and a higher DCMC capacity than
its conventional spatial modulation (SM) {and its orthogonal frequency-division
multiplexing (OFDM) counterparts. As a benefit, the proposed system strikes a
compelling BER vs. system complexity as well as BER vs. detection complexity
trade-offs.
