UAV-Enabled Relaying IRS: Joint Deployment, Time Allocation, and Beamforming Optimization

Intelligent reflecting surface (IRS) has garnered significant attention as a promising candidate for 6G wireless communications. However, conventional IRSs generally suffer from limited wireless coverage due to their half-space passive reflections and fixed locations. To enhance reflected signal coverage, we propose in this paper a new architecture of UAV-enabled relaying IRS by mounting an IRS on a UAV and leveraging both passive reflection and active relaying at the same time. In particular, we consider the wireless transmission from a base station (BS) to a remote user aided by a UAV-mounted IRS. Our goal is to jointly optimize the time allocation for the UAV’s active relaying, the IRS’s passive beamforming, the BS’s active beamforming and the location of the UAV/IRS to maximize the end-to-end achievable rate. However, this problem is non-convex and thus challenging to be optimally solved. To address this issue, we first derive the optimal BS active beamforming in closed-form. Next, we introduce an auxiliary variable for the IRS’s passive beamforming to recast the optimization problem as a scalar problem, which is optimally solved by decomposing it into several simplified cases. An alternating projection algorithm is then proposed to recover the IRS passive beamforming from the optimized auxiliary variable. Numerical results show that the UAV-enabled relaying IRS outperforms conventional schemes that rely solely on UAV relaying or IRS reflection and provide some useful insights.