  Orthogonal time-frequency space (OTFS) has been confirmed to take advantage
of full time-frequency diversity to significantly improve error performance in
high-mobility scenarios. We found that the proposed message passing (MP) and
variational Bayes (VB) detectors can achieve approximate maximum a posteriori
(MAP) detection, the interferences cannot be completely eliminated in the
absence of noise. To achieve near-optimal MAP detection, this letter proposes a
novel detection method based on sum-product algorithm (SPA) with low
complexity. Leveraging subtly factorized posteriori probabilities, the obtained
pairwise interactions can effectively avoid enumeration of high-dimensional
variables, thereby making it applicable to fractional Doppler cases. We further
simplify the proposed algorithm in the logarithmic domain so that the message
propagation processing only involves addition. Finally, simulations results
demonstrate the superior error performance gains of our proposed algorithm at
high signal-to-noise ratios (SNRs).
