  We consider the multicast transmission of a real-time Internet of Things
(IoT) system, where a server transmits time-stamped status updates to multiple
IoT devices. We apply a recently proposed metric, named age of information
(AoI), to capture the timeliness of the information delivery. The AoI is
defined as the time elapsed since the generation of the most recently received
status update. Different from the existing studies that considered either
multicast transmission without hard deadlines or unicast transmission with hard
deadlines, we enforce a hard deadline for the service time of multicast
transmission. This is important for many emerging multicast IoT applications,
where the outdated status updates are useless for IoT devices. Specifically,
the transmission of a status update is terminated when either the hard deadline
expires or a sufficient number of IoT devices successfully receive the status
update. We first calculate the distributions of the service time for all
possible reception outcomes at IoT devices, and then derive a closed-form
expression of the average AoI. Simulations validate the performance analysis,
which reveals that: 1) the multicast transmission with hard deadlines achieves
a lower average AoI than that without hard deadlines; and 2) there exists an
optimal value of the hard deadline that minimizes the average AoI.
