  Integrated sensing and communications (ISAC) is a critical enabler for
emerging 6G applications, and at its core lies in the dual-functional waveform
design. While orthogonal frequency division multiplexing (OFDM) has been a
popular basic waveform, its primitive version falls short in sensing due to the
inherent unregulated auto-correlation properties. Furthermore, the sensitivity
to Doppler shift hinders its broader applications in dynamic scenarios. To
address these issues, we propose a superposed index-modulated OFDM (S-IM-OFDM).
The proposed scheme improves the sensing performance without excess power
consumption by translating the energy efficiency of IM-OFDM onto
sensing-oriented signals over OFDM. Also, it maintains excellent communication
performance in time-varying channels by leveraging the sensed parameters to
compensate for Doppler. Compared to conventional OFDM, the proposed S-IM-OFDM
waveform exhibits better sensing capabilities and wider applicability in
dynamic scenarios. Both theoretical analyses and simulations corroborate its
dual benefits.
