  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). Although multiple studies have considered index modulation
(IM) based OTFS (IM-OTFS) schemes to further improve system performance, a
challenging and open problem is the development of effective IM schemes and
efficient receivers for practical OTFS systems that must operate in the
presence of channel delays and Doppler shifts. In this paper, we propose two
novel block-wise IM schemes for OTFS systems, named delay-IM with OTFS
(DeIM-OTFS) and Doppler-IM with OTFS (DoIM-OTFS), where a block of
delay/Doppler resource bins are activated simultaneously. Based on a maximum
likelihood (ML) detector, we analyze upper bounds on the average bit error
rates for the proposed DeIM-OTFS and DoIM-OTFS schemes, and verify their
performance advantages over the existing IM-OTFS systems. We also develop a
multi-layer joint symbol and activation pattern detection (MLJSAPD) algorithm
and a customized message passing detection (CMPD) algorithm for our proposed
DeIMOTFS and DoIM-OTFS systems with low complexity. Simulation results
demonstrate that our proposed MLJSAPD and CMPD algorithms can achieve desired
performance with robustness to the imperfect channel state information (CSI).
