  As a promising technique for high-mobility wireless communications,
orthogonal time frequency space (OTFS) has been proved to enjoy excellent
advantages with respect to traditional orthogonal frequency division
multiplexing (OFDM). However, a challenging problem is to design efficient
systems to further improve the performance. In this paper, we propose a novel
block-wise index modulation (IM) scheme for OTFS systems, named Doppler-IM with
OTFS (DoIM-OTFS), where a block of Doppler resource bins are activated
simultaneously. For practical implementation, we develop a low complexity
customized message passing (CMP) algorithm for our proposed DoIM-OTFS scheme.
Simulation results demonstrate our proposed DoIM-OTFS system outperforms
traditional OTFS system without IM. The proposed CMP algorithm can achieve
desired performance and robustness to the imperfect channel state information
(CSI).
