  Orthogonal time frequency space (OTFS) modulation has recently emerged as a
potential 6G candidate waveform which provides improved performance in
high-mobility scenarios. In this paper we investigate the combination of OTFS
with non-orthogonal multiple access (NOMA). Existing equalization and detection
methods for OTFS-NOMA, such as minimum-mean-squared error with successive
interference cancellation (MMSE-SIC), suffer from poor performance.
Additionally, existing iterative methods for single-user OTFS based on
low-complexity iterative least-squares solvers are not directly applicable to
the NOMA scenario due to the presence of multi-user interference (MUI).
Motivated by this, in this paper we propose a low-complexity method for
equalization and detection for OTFS-NOMA. The proposed method uses a novel
reliability zone (RZ) detection scheme which estimates the reliable symbols of
the users and then uses interference cancellation to remove MUI. The thresholds
for the RZ detector are optimized in a greedy manner to further improve
detection performance. In order to optimize these thresholds, we modify the
least squares with QR-factorization (LSQR) algorithm used for channel
equalization to compute the the post-equalization mean-squared error (MSE), and
track the evolution of this MSE throughout the iterative detection process.
Numerical results demonstrate the superiority of the proposed equalization and
detection technique to the existing MMSE-SIC benchmark in terms of symbol error
rate (SER).
