Joint Waveform and Beamforming Design for RIS-Aided ISAC Systems

A novel dual-function radar communication (DFRC) system is proposed, that achieves a high communication rate, and can flexibly trade-off rate for improved sensing performance. The proposed system is a monostatic multiple-input multiple-output (MIMO) radar and transmits wideband, precoded, orthogonal frequency division multiplexing (OFDM) waveforms from its antennas. The system subcarriers are divided into two groups, i.e., shared and private. On a shared subcarrier, all antennas can transmit simultaneously, while on a private subcarrier only one antenna can transmit at a time. A novel target estimation approach with low complexity is proposed to overcome the coupling of transmitted symbols and radar target parameters in the target echoes, which arises due to the shared use of subcarriers by the transmit antennas. The proposed method first operates on all (shared and private) subcarriers to obtain coarse angle estimates, and then fine-tunes those estimates based on the signal received on the private subcarriers. The resolution of the coarse angle estimates is limited by the physical receive array, while the fine-tuning is enabled by effectively constructing a virtual array that has a larger aperture than the receive array. The precoding matrix is optimally designed to optimize a weighted combination of the beampattern error with respect to a desirable beampattern, and the signal-to-noise ratio at the communication receiver.