Index Modulation Aided Fragmented Spectra Centralization in OTFS Systems

By selecting various active-silent combinations, classical frequency-domain index modulation (IM) generates numerous energy-free subcarriers dispersed across subblocks, which we refer to as the fragmented spectra. This letter proposes a novel IM approach aimed at collecting the fragmented spectra in subblocks to form a completely unused frequency band, called the IM-aided fragmented spectra centralization (IM-FSC). Specifically, within each subblock, if the last subcarrier is to be activated, we transfer this active position to the first unactivated subcarrier and employ a distinguishable constellation to modulate the symbol on this subcarrier. Through this approach, each subblock contributes one energy-free subcarrier, and the unused frequency band can be obtained accordingly by combining these fragmented spectra from multiple subblocks. We designate this unoccupied frequency band as zero padding (ZP) and guard interval for interference cancellation and channel estimation in orthogonal time frequency space (OTFS) system. The closed-form expressions of bit error rate (BER) and generalization guidelines are also derived. Simulation results demonstrate the promising communication capability and channel estimation accuracy of the proposed scheme.