LoS Identification Based Transmit Power Minimization in Cell-Free ISAC System

The cell-free integrated sensing and communication (ISAC) system, which collaborates multiple access points (AP), offers superior sensing and communication performance compared to single base station ISAC system. However, when line-of-sight (LoS) link does not exist between AP and the target, the sensing performance is significantly degraded thus the energy dedicated for sensing is wasted. To address this issue, we propose a two-stage design. In the identification stage, APs transmits omnidirectional beams to identify the LoS link between AP and the target. In the subsequent ISAC stage, if a LoS link exists, AP transmits ISAC signals. Otherwise only communication signals are transmitted. The detection probability of LoS identification is derived. Additionally, we propose a joint time and power allocation algorithm that minimizes transmit power while ensuring the minimum detection probability, communication throughput, sensing signal-to-interference-plus-noise ratio (SINR), and the maximum transmit power requirements. Numerical results demonstrate the proposed two-stage design outperforms state-of-the-art schemes without joint optimization and two-stage design.