Exploiting Double Timescales for Integrated Sensing and Communication with Delay-Doppler Alignment Modulation

For integrated sensing and communication (ISAC) systems, the desired channel variables by communication and sensing tasks vary with different timescales. For sensing, one is mainly interested in the state information (e.g., delays, angles, Doppler frequencies, etc.) of individual multi-path channel components, which evolves much more slowly than the composite channel state information (CSI) required by communications. In this paper, by exploiting the double timescales for sensing and communication, a novel technique termed as delay-Doppler alignment modulation (DDAM) is investigated, which is an appealing technique for ISAC systems, since the sensing result of resolvable multi-paths can be directly exploited for delay-Doppler compensation and path-based beamforming of DDAM. We first show that with perfect CSI, as long as the number of base station (BS) antennas is no smaller than that of resolvable multi-paths, the proposed DDAM is able to transform the time-frequency double selective-fading channel into a simple additive white Gaussian noise (AWGN) channel for inter-symbol interference (ISI)-free communication without requiring the conventional channel equalization or multi-carrier transmission. We then present the DDAM-based signal processing for ISAC, and the resulting communication performance with imperfectly sensed CSI is studied. Simulation results demonstrate that the proposed DDAM-based ISAC can achieve higher communication rate compared to orthogonal frequency division multiplexing (OFDM) and DFT-spread(s)-OFDM, while guaranteeing high sensing performance.