Coprime Sensing Reference Signal Design Based on Zadoff-Chu Sequence and Index Modulation for Uplink ISAC System

Uplink sensing is widely studied in integrated sensing and communication (ISAC) systems, where the ISAC signal is transmitted by user equipment (UE), reflected by the target, and finally received by the base station (BS). Uplink sensing has the advantage of minimal modification to the communication infrastructure. The uplink signal transmitted by UE is unknown to BS. The randomness of communication data will lead to a decrease in sensing accuracy. Therefore, the reference signals are applied for uplink sensing. Deterministic sensing subcarriers inserted into the orthogonal frequency division multiplexing (OFDM) signal can serve as sensing reference signals realizing the sensing function. However, reference signals such as the demodulation reference signal (DMRS) and the sounding reference signal (SRS) are often equally spaced in the frequency dimension, resulting in a reduction of the maximum unambiguous range. In addition, the dedicated sensing reference signals would consume communication resources and lead to a decrease in the communication data rate. To address these challenges, this paper proposes the coprime Zadoff-Chu (ZC) sequence and index modulation-based sensing reference signal (ZC-IM-RS) for the uplink ISAC system. The indices and inserted ZC sequences of the coprime ZC-IM-RS are modulated by communication data to carry more communication information. Additionally, the coprime ZC-IM-RS follows the coprime distribution to avoid the reduction in the maximum unambiguous range. In terms of communication, the spectral efficiency of the ISAC signal with coprime ZC-IM-RS is improved. In terms of sensing, the maximum unambiguous range of the proposed coprime ZC-IM-RS is larger than that of the equally spaced reference signal. Moreover, a novel demodulation algorithm and parameter estimation algorithm for the coprime ZC-IM-RS are proposed. The simulation results show that the gain in spectral efficiency reaches 7.3%. The maximum unambiguous range of the coprime ZC-IM-RS is increased by up to 4 times compared to that of the uplink reference signal.