Integrated sensing and communication (ISAC) has attracted growing interests for enabling the future 6G wireless networks, due to its capability of sharing spectrum and hardware resources between communication and sensing systems. However, existing works on ISAC usually need to modify the communication protocol to cater for the new sensing performance requirement, which may be difficult to implement in practice. In this paper, we study a new intelligent reflecting surface (IRS) aided millimeter-wave (mmWave) ISAC system by exploiting the distinct beam scanning operation in mmWave communications to achieve efficient sensing at the same time. First, we propose a two-phase ISAC protocol aided by a semi-passive IRS, consisting of beam scanning and data transmission. Specifically, in the beam scanning phase, the IRS finds the optimal beam for reflecting signals from the base station to a communication user via its passive elements. Meanwhile, the IRS directly estimates the angle of a nearby target based on echo signals from the target using its equipped active sensing element. Then, in the data transmission phase, the sensing accuracy is further improved by leveraging the data signals via possible IRS beam splitting. Next, we derive the achievable rate of the communication user as well as the Cram\'er-Rao bound and the approximate mean square error of the target angle estimation Finally, extensive simulation results are provided to verify our analysis as well as the effectiveness of the proposed scheme.

翻译：集成感知与通信（ISAC）因能够在通信与感知系统之间共享频谱及硬件资源，已引起对未来第六代无线网络发展的广泛关注。然而，现有关于ISAC的研究通常需要修改通信协议以满足新的感知性能要求，这在实践中可能难以实现。本文通过利用毫米波通信中独特的波束扫描操作，研究了一种新型智能反射面（IRS）辅助的毫米波ISAC系统，以期同时实现高效感知。首先，我们提出了一种由半无源IRS辅助的两阶段ISAC协议，包含波束扫描与数据传输阶段。具体而言，在波束扫描阶段，IRS通过其无源元件找到最优波束，用于将基站的信号反射至通信用户；同时，IRS利用其配备的有源感知元件，基于目标回波信号直接估计邻近目标的角度。随后，在数据传输阶段，通过可能的IRS波束分裂利用数据信号，进一步提升了感知精度。接着，我们推导了通信用户的可达速率、目标角度估计的克拉美-罗界及近似均方误差。最后，大量的仿真结果验证了我们的分析及所提方案的有效性。