  The emerging orthogonal time frequency space (OTFS) modulation technique has
shown its superiority to the current orthogonal frequency division multiplexing
(OFDM) scheme, in terms of its capabilities of exploiting full time-frequency
diversity and coping with channel dynamics. The optimal maximum a posteriori
(MAP) detection is capable of eliminating the negative impacts of the
inter-symbol interference in the delay-Doppler (DD) domain at the expense of a
prohibitively high complexity. To reduce the receiver complexity for OTFS
scheme, this paper proposes a variational Bayes (VB) approach as an
approximation of the optimal MAP detection. Compared to the widely used message
passing algorithm, we prove that the proposed iterative algorithm is guaranteed
to converge to the global optimum of the approximated MAP detector regardless
the resulting factor graph is loopy or not. Simulation results validate the
fast convergence of the proposed VB receiver and also show its promising
performance gain compared to the conventional message passing algorithm.
