Optimizing Age of Information in UAV-Mounted IRS Assisted Short Packet Systems

In this paper, we investigate an unmanned aerial vehicle (UAV) mounted intelligent reflecting surface (IRS) assisted systems, where an access point (AP) sends short packets to multiple users with the help of an UAV mounted IRS (U-IRS). The maximum average age of information (AoI) among users is minimized by jointly optimizing the time interval allocation, power allocation and UAV trajectory. However, the formulated problem is difficult to solve due to the non-convex objective function and coupled variables. Thus, we propose an alternating optimization (AO) based algorithm by dividing the problem into three sub-problems, which thus can be efficiently solved via the successive convex approximation (SCA) and block coordinate descent (BCD) techniques. Simulation results show that the proposed U-IRS design can significantly improve the AoI performance as compared to the UAV relay design and the packet error rate (PER) minimized U-IRS design. Furthermore, we find that increasing the number of IRS elements has a compromise effect on the AoI performance. On the one hand, it helps to achieve higher beamforming gain for further improving the transmission reliability and thus better compensating for the packet error owing to using short packet. On the other hand, it leads to higher channel eatimation overhead for reaping the beamforming gain, which however introduces unfavorable larger delay. As a result, while the IRS elements number exceeds a certain value, the AoI performance no more improves but degrades instead.