Comparative Evaluation of an ISAC Precoding Scheme for OTFS and OFDM Waveforms in Perceptual Mobile Networks

The popularization of the perceptual mobile networks (PMN) concept gives rise to the waveform design for integrated sensing and communications (ISAC). In this work, we propose a precoding scheme for ISAC, which is applicable for either orthogonal time frequency space (OTFS) and orthogonal frequency-division multiplexing (OFDM) waveforms, the two most discussed waveforms in B5G or 6 G PMNs. The essence of the precoding scheme is shaping the envelope of the specific communication symbols according to the expected reference sensing signal, which is derived from point targets and is associated with the time-frequency (TF) or delay-Doppler (DD) grids in OFDM or OTFS blocks/frames, respectively. The proposed precoder provides an adaptive adjustment mechanism between the sensing and communications functions for diverse applications in the PMN. We conduct simulations to demonstrate the trade-off between communications and sensing performances in terms of bit error rate (BER) and detection probability, respectively. The simulation results reveal that the OFDMbased ISAC waveform shows greater robustness on retaining the communication performance than the OTFS-based approach when the system has to lean towards the sensing function. We also identify the potential reason for this observation as the effective channel in OTFS case being spread more widely along the time domain, which gives the precoder fewer degrees of freedom when optimizing the transmit waveform.