  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.
