Constellation Optimization Assisted OTFS with Index Modulation

OTFS is recognized as a promising modulation technology to enhance and extend coverage for high-mobility communications. In this paper, we propose a Modified-Constellation assisted Orthogonal Time-Frequency-Space with Index Modulation (MCOTFS-IM) scheme to improve BER performance over doubly selective fading channels. The proposed scheme optimizes the positions of QAM constellation points that are close to the origin, such that the Euclidean distance for detecting active or inactive status of a grid point in Delay-Doppler domain can be enlarged. This adjustment offers a more accurate detection rate, thus improving the overall BER across index bits and information bits for index modulation. We further apply the Jacobi preconditioning Conjugate gradient (Jac-CG) method to achieve performance of MMSE detection with relatively low complexity. Simulation results demonstrate that the proposed MCOTFS-IM outperforms OTFS-IM in terms of BER under various system setups.