OTFS with dual frequency index modulation

Abstract

The integration of orthogonal time frequency space (OTFS) and index modulation (IM) is an attractive innovation, which can meet both high throughput and low energy consumption in highly mobile scenarios. This paper presents a novel dual frequency index modulation (DFIM) scheme for OTFS to improve spectral efficiency (SE). Exploring the internal relationship between indices, the proposed scheme, termed OTFS-DFIM, turns the classic index mapping process into a cascaded procedure involving partial index mapping and index updating. The differential information between grid points in the delay-Doppler domain and the time-frequency information are utilized to jointly determine the active grid points, which enables the OTFS frame to remain sparse and results in promising bit error rate (BER) performance. We also propose three distribution modes of IM subblocks in the two-dimensional grid and analyze their impact on receiver structure and system performance. The tight closed-form expressions of BER are derived. Additionally, we evaluate existing linear/non-linear equalization algorithms in OTFS and adapt them to the OTFS-IM/OTFS-DFIM system, offering a comprehensive insight on the tradeoffs between complexity and BER performance. Numerical simulations validate the superior performance of the OTFS-DFIM scheme in both SE and BER.