Concurrent, Scheduled, or Hybrid Transmission Protocol for ISAC

Integrated sensing and communication (ISAC) systems enable communication and sensing functions via three protocols, including ($\text{TP}_{1}$) concurrently using the same time-frequency resources, ($\text{TP}_{2}$) scheduling them independently, or ($\text{TP}_{3}$) a hybrid technique that combines both. Nevertheless, all existing studies rely on the first approach, and none provide a comprehensive performance evaluation of all three protocols. Thus, this paper offers an extensive performance evaluation of these protocols, emphasizing their advantages, limitations, and trade-offs. Specifically, we maximize the sum communication and/or sensing rate for all protocols in a full-duplex ISAC system with multiple users and targets. Numerical results reveal that protocols ($\text{TP}_{1}$) and ($\text{TP}_{3}$) necessitate a sensing metric considering both transmit and receiver beams to ensure adequate sensing performance. In contrast, although protocol ($\text{TP}_{2}$) can utilize more straightforward sensing metrics and avoid communication-sensing interference, it may not achieve high communication performance.