In this correspondence, we investigate an intelligent reflective surface (IRS) assisted downlink ultra-reliable and low-latency communication (URLLC) system, where an access point (AP) sends short packets to multiple devices with the help of an IRS. Specifically, a performance comparison between the frequency division multiple access (FDMA) and time division multiple access (TDMA) is conducted for the considered system, from the perspective of average age of information (AoI). Aiming to minimize the maximum average AoI among all devices by jointly optimizing the resource allocation and passive beamforming. However, the formulated problem is difficult to solve due to the non-convex objective function and coupled variables. Thus, we propose an alternating optimization based algorithm by dividing the original problem into two sub-problems which can be efficiently solved. Simulation results show that TDMA can achieve lower AoI by exploiting the time-selective passive beamforming of IRS for maximizing the signal to noise ratio (SNR) of each device consecutively. Moreover, it also shows that as the length of information bits becomes sufficiently large as compared to the available bandwidth, the proposed FDMA transmission scheme becomes more favorable instead, due to the more effective utilization of bandwidth.

翻译：本文研究智能反射面（IRS）辅助的下行超可靠低延迟通信（URLLC）系统，其中接入点（AP）借助IRS向多个设备发送短数据包。具体而言，针对所考虑系统，从信息平均年龄（AoI）的角度，对频分多址（FDMA）和时分多址（TDMA）进行了性能比较。目标是通过联合优化资源分配和无源波束赋形，最小化所有设备中的最大平均AoI。然而，由于目标函数的非凸性和变量耦合，所构建的问题难以求解。因此，我们提出一种基于交替优化的算法，将原问题分解为两个可高效求解的子问题。仿真结果表明，TDMA可通过利用IRS的时间选择性无源波束赋形依次最大化每个设备的信噪比（SNR），从而实现更低的AoI。此外，结果表明，当信息比特长度相对于可用带宽足够大时，由于带宽利用效率更高，所提出的FDMA传输方案更为有利。